In this paper, we present the design, development, and characteristics of the novel aerosol mass spectrometer ERICA (ERC Instrument
for Chemical composition of Aerosols; ERC – European Research Council) and
selected results from the first airborne field deployment. The instrument
combines two well-established methods of real-time in situ measurements of
fine particle chemical composition. The first method is the laser desorption and ionization technique, or laser ablation technique, for single-particle mass spectrometry (here with a frequency-quadrupled Nd:YAG laser at λ= 266 nm). The second method is a combination of thermal particle
desorption, also called flash vaporization, and electron impact ionization
(like the Aerodyne aerosol mass spectrometer). The same aerosol sample flow
is analyzed using both methods simultaneously, each using time-of-flight
mass spectrometry. By means of the laser ablation, single particles are
qualitatively analyzed (including the refractory components), while the flash vaporization and electron impact ionization technique provides quantitative information on the non-refractory components (i.e., particulate sulfate, nitrate, ammonia, organics, and chloride) of small particle ensembles. These
techniques are implemented in two consecutive instrument stages within a
common sample inlet and a common vacuum chamber. At its front end, the
sample air containing the aerosol particles is continuously injected via an
aerodynamic lens. All particles which are not ablated by the Nd:YAG laser in the first instrument stage continue their flight until they reach the second instrument stage and impact on the vaporizer surface (operated at 600 ∘C). The ERICA is capable of detecting single particles with
vacuum aerodynamic diameters (dva) between ∼ 180 and 3170 nm (d50 cutoff). The chemical characterization of single particles is achieved by recording cations and anions with a bipolar time-of-flight mass spectrometer. For the measurement of non-refractory components, the particle size range extends from approximately 120 to 3500 nm (d50 cutoff; dva), and the cations are detected with a time-of-flight mass
spectrometer. The compact dimensions of the instrument are such that the
ERICA can be deployed on aircraft, at ground stations, or in mobile laboratories.
To characterize the focused detection lasers, the ablation laser, and the
particle beam, comprehensive laboratory experiments were conducted. During
its first deployments the instrument was fully automated and operated during 11 research flights on the Russian high-altitude research aircraft M-55
Geophysica from ground pressure and temperature to 20 km altitude at 55 hPa and
ambient temperatures as low as -86 ∘C. In this paper, we show
that the ERICA is capable of measuring reliably under such conditions.
Introduction
Beyond the experimental determination of physical aerosol properties,
detailed measurements of the chemical composition of aerosol particles are
essential for studies in the context of urban pollution, health effects,
cloud formation, radiative transfer in the atmosphere, and climate change
(Fuzzi et al., 2015). Chemical
composition measurements can provide information on the aerosol source
– natural or anthropogenic – and on the state of chemical and physical
processing of the particles while aging during transport
(IPCC, 2013; Seinfeld and Pandis, 2016).
Besides offline methods, which involve particle collection on suitable
substrates by impactors or filter samplers followed by subsequent laboratory
analyses (Elmes and Gasparon, 2017), in situ real-time
measurements adopting aerosol particle mass spectrometry have become a
widespread, established tool. For the implementation of aerosol mass
spectrometry, two complementary measurement techniques are commonly used.
The first method uses a pulsed laser to vaporize and ionize individual
submicron- to micrometer-sized particles by laser desorption and ionization
(LDI; Suess and Prather, 1999) for single-particle mass spectrometry
(SPMS). The resulting ions are extracted into a time-of-flight mass
spectrometer. In terms of the deliverables, with this method single-particle
mass spectra of both refractory and non-refractory components of soot, salt,
mineral dust, and meteoric dust particles, as well as metal-containing
particles, can be detected. The second method is based on thermal
desorption and electron impact ionization (TD-EI), which allows quantitative
measurement of non-refractory species (sulfate, nitrate, ammonium, chloride,
and organic compounds) in ensembles of particles (Drewnick
et al., 2005). While the latter method provides quantitative mass
concentrations of non-refractory components, the mass spectrometer signals
of the LDI method can only be used for the identification of the ions themselves
and not for determination of absolute mass concentrations. Within certain
limitations this may become possible if the data of other instruments are
included in the analysis (e.g., Ault et al., 2009; Healy et al., 2012;
Gunsch et al., 2018; Köllner et al., 2021). Details on the
methodologies, limitations, and considerations of the inherent experimental
errors of these measuring techniques can be found in
Kulkarni et al. (2011) and the references therein.
For single-particle analysis by the LDI method, a time-of-flight mass
spectrometer (TOFMS) is a suitable choice because in this way a full
bipolar mass spectrum of a single particle can be recorded (Hinz et
al., 1996). The trigger signal for firing the laser pulse that causes the
ionization of the particle can be used as the trigger of the TOFMS. Thereby,
the ions are separated from neutral molecules in less than a microsecond,
preventing further reactions between ions and molecules as for example in an
ion trap mass spectrometer (Fachinger et al., 2017). For
the TD-EI technique (Aerodyne aerosol mass spectrometer – AMS), a quadrupole mass spectrometer was used
in the beginning (Jayne et al., 2000) until it was replaced
by a TOFMS (Drewnick et al., 2005; DeCarlo et al., 2006). The advantages of
the TOFMS are a higher m/z resolution, higher sensitivity, and thereby lower
detection limits compared to the quadrupole technique
(DeCarlo et al., 2006). Additionally, the TOFMS also makes it
possible to perform single-particle analysis using the thermal desorption
technique, provided there is an optical triggering of the detected particles
(Cross et al., 2009; Freutel et al., 2013). Furthermore, TOFMSs are compact and rugged.
Compact and mobile online instruments based on the LDI or the TD-EI method
have been deployed on research aircraft to measure particle chemical
composition at high temporal and spatial resolutions. PALMS
(Particle Analysis by Laser Mass Spectrometry; Murphy et
al., 1998) has been operated at altitudes of up to 20 km. Other
aircraft-based, online single-particle laser ablation aerosol mass
spectrometers, which are operated at lower altitudes, include the A-ATOFMS
(aircraft aerosol time-of-flight mass spectrometer; Pratt
et al., 2009), the ALABAMA (Aircraft-based Laser ABlation Aerosol MAss
spectrometer; Brands et al., 2011; Clemen et al., 2020), and miniSPLAT
(miniaturized version of the Single Particle Laser Ablation
Time-of-flight Mass Spectrometer; Zelenyuk et al., 2015). Instruments
utilizing the TD-EI technique have been deployed on research aircraft using
a C-ToF-MS (compact time-of-flight mass spectrometer from Tofwerk AG,
Switzerland), e.g., by Bahreini et al. (2009), Morgan et al. (2010),
Schmale et al. (2010), Brito et al. (2018), Schulz et al. (2018), and
Haslett et al. (2019), while a mAMS (mini aerosol mass spectrometer) was used for example by Vu et al. (2016) and Goetz et al. (2018). An HR-ToF-MS
(high-resolution time-of-flight mass spectrometer) was adapted for aircraft
use, for example, by Dunlea et al. (2007) and Willis et al. (2016). However, as these references show, for aircraft-borne measurements of aerosol chemical
composition usually only one of the two mass spectrometry methods is
implemented on a single aircraft mostly as a consequence of limitations in
weight and space. Although several aerosol instruments can be operated
simultaneously at one location during ground-based measurements or in a
laboratory environment (e.g., Möhler et al., 2008; Dall'Osto et al.,
2012; Roth et al., 2016), up to now two different aerosol mass
spectrometers have rarely been available on the same aircraft (e.g., Murphy et al.,
2006a; Toon et al., 2016; Froyd et al., 2019; Schneider et al., 2019; Hodzic
et al., 2020; Guo et al., 2021; Köllner et al., 2021). Since the two
techniques deliver complementary information on the aerosol composition and
also cover slightly different size ranges, a single instrument implementing
both methodologies in one apparatus has obvious advantages, provided that it
is sufficiently small and light. The temporal resolution of the ablation
laser (for other reasons see Sect. 2.3) limits the number
of particles detected (e.g., Su et al., 2004). The addition
of a TD-EI unit largely enhances the data yield for the particle analysis by
complementary information. Furthermore, the opportunities for measurements
at high altitudes are rare, so an aerosol instrument which provides a
high information output is advantageous.
The subject of this paper is the ERICA (ERC Instrument for Chemical composition
of Aerosols; ERC – European Research Council), which has been developed in
our laboratories at the Johannes Gutenberg University and the Max Planck
Institute for Chemistry in Mainz. It is a hybrid instrument implementing
both of the aforementioned particle desorption and ionization methods in one
single fully automated apparatus. The adopted techniques for automating the
operation (including pressure and temperature control), details on the
electronic hardware, the mechanical adaption, the inlet system, the
electrical distribution, and the remote control, are detailed in a
separate paper by Dragoneas et al. (2022).
The ERICA was deployed for the first time during the aircraft field
campaigns of the StratoClim project (Stratospheric and upper tropospheric
processes for better climate predictions; Brunamonti et al., 2018; Bucci et
al., 2020) in August and September 2016 at the Kalamata International
Airport (KLX; 37.07∘ N, 22.03∘ E; Kalamata, Greece) and
during July and August 2017 at the Tribhuvan International Airport (KTM;
27.70∘ N, 85.36∘ E; Kathmandu, Nepal). Although the
instrument was initially designed for implementation on the Russian high-altitude research aircraft M-55 Geophysica (Borrmann et al., 1995; Stefanutti et
al., 1999) and operation in the low particle number density environment of
the upper troposphere and lower stratosphere (up to 20 km altitude), the
ERICA can be integrated into suitable racks to be implemented onto other
research aircraft such as NASA's DC-8 (Schneider et al., 2021),
DLR's HALO (Deutsches Zentrum für Luft- und Raumfahrt (DLR), High Altitude and LOng Range Research Aircraft (HALO); https://halo-research.de/, last access: 28 March 2022), or the NSF–NCAR HIAPER (National Science Foundation (NSF), National
Center for Atmospheric Research (NCAR), High-Performance Instrumented Airborne Platform for Environmental Research (HIAPER); Laursen et al.,
2006). Furthermore, the ERICA can be used for a variety of ground-based
stationary or mobile applications. In this paper we show the design of
the ERICA and results from laboratory characterization measurements, as well as
results selected for a proof-of-concept demonstration from the field
campaign in Kathmandu, Nepal. The instrumental design and characterization
are presented here in some detail (in particular in the Supplement) in order
to support potential design efforts of other groups and to provide
benchmark tests and values.
Since the two instrument components share a single vacuum system, weight is
saved due to common components like pumps, power supply units, and the vacuum
chamber. Furthermore, the mechanical components of the ERICA are designed to
operate under the demanding conditions like thermal stress and vibrations
aboard an aircraft. The final design of the compact instrument was
implemented in an aircraft rack (Dragoneas et al., 2022) of 60 cm × 74 cm × 140 cm (height × width × length) with a total weight of 200 kg.
In addition, field deployments with research aircraft at high altitudes are
rare, so as much information as possible – with as many instruments as
possible – should be collected. Thus, a compact design is crucial for
implementation on such aircraft, and therefore a combination of two
measurement methods into one apparatus is a major advantage. To visualize
the orientation of the major components, a three-dimensional drawing of the
instrument body as well as a
photograph of the instrument mounted in the M-55 Geophysica rack for the StratoClim campaign is provided in Sect. S1.1 in the Supplement.
Instrument descriptionGeneral principle and design of the ERICA
The principal configuration of the ERICA with its inlet system, the LDI
section (denominated ERICA-LAMS), and the TD-EI section (denominated ERICA-AMS) is shown in Fig. 1 and is described
in the following. During aircraft operation, the sample airflow is provided
by a constant-pressure inlet (CPI; Molleker et al., 2020)
serving as a critical orifice at the instrument's front end (see Sect. 2.2). The particles are focused in the aerodynamic
lens (ADL) into a narrow beam and accelerated into the vacuum chamber, where
they first reach the optical particle detection units (PDU1 and PDU2 in
Fig. 1) of the ERICA-LAMS. Here, optical particle
detection and sizing are realized via a particle flight time measurement by
means of light scattering. For this purpose, two parallel continuous-wave
laser beams (Gaussian beam shape) are directed onto the particle beam. The
light scattered from the passing individual particles is focused by
ellipsoidal reflectors onto photomultiplier tubes (PMTs). The time elapsing
between the two light-scattering signals is used to derive the particles'
vacuum aerodynamic diameter dva (for a definition see Jimenez et
al., 2003b, a; DeCarlo et al., 2004) by involving a calibration
(Brands et al., 2011). This time is also used
to determine the point in time the particle reaches the ablation spot of the
ERICA-LAMS. If well positioned and timed, the particle is desorbed and
ionized during the LDI process by a triggered 266 nm UV pulse (Gaussian beam
shape) from a frequency-quadrupled Nd:YAG laser. The resulting cations and
anions are extracted into a bipolar time-of-flight mass spectrometer
(B-ToF-MS) and detected by micro-channel plates (MCPs).
Overview of the ERICA setup. (ADL – aerodynamic lens, LD – laser
diode, EP – extraction plates, MCP – micro-channel plate, PDU – particle
detection unit, PMT – photomultiplier tube, PS – pumping stage, SU –
shutter unit, TMP – turbomolecular pump). The additional backing pump
(MD1) for the TMPs is not shown. The detection laser beams and the ablation
laser beam enter the vacuum chamber perpendicularly to the plane of drawing.
The constant pressure inlet (not shown) is located upstream of the main
valve.
It is assumed that a large fraction of the sampled particles will not
generate a single-particle spectrum. The major reasons for this effect are as follows:
first, the particles are not ablated because the laser is firing at a
limited maximum repetition rate of 8 pulses s-1. During the idle time
of the Nd:YAG laser, particles remain unablated, even if they are
successfully detected by the units PDU1 and PDU2. This actually is by far
the largest fraction of the sampled particles emerging from the ADL. If, for
example, the ambient number density of particles with diameters above the
optical detection limit is 100 cm-3 (NTP, normal temperature and pressure, 20 ∘C and 1013 hPa), then, at most only 5.4 % (8 shots s-1 and sampling volumetric flow rate of 1.48 cm3 s-1) of the detectable particles are hit by the
laser. Second, the particles are too small for optical detection. Third,
particles for which the calculation of the trigger failed continue their
travel towards the ERICA-AMS vaporizer. Fourth, particles that primarily
consist of materials that are transparent at a UV wavelength of 266 nm, such
as pure sulfuric acid, are hard to ablate (Murphy et
al., 2007). We selected a UV laser with 266 nm wavelength due to smaller
dimensions of the laser and the fact that chemical substances show less
fragmentation compared to ablation with shorter wavelengths
(Thomson et al., 1997). In general, however, it is also
possible to implement excimer lasers operating at shorter wavelengths to
ablate pure sulfuric acid droplets. Also, pure sulfuric acid is detected by
the ERICA-AMS. Thus, even most particles amenable to laser ablation, which
pass through the ablation region, remain undestroyed. Another reason why a
spectrum is not triggered over a signal threshold for recording is a low
number of generated ions during the LDI process. These unablated particles
pass through the ablation region of the ERICA-LAMS and enter the
continuously operating the ERICA-AMS. The ERICA-AMS is based, in analogy to the
Aerodyne AMS, on the TD-EI method. A filament
provides the electrons for ionization of the vapor molecules emanating from
the vaporizer. The resulting cations are extracted into the C-ToF-MS and
eventually detected by its MCPs.
The particle size range within the 50 % cutoff in detection efficiency
(d50) of the ERICA-LAMS is between 180 and 3170 nm (see Sect. 3.2.2). The signal-to-noise ratio of optical
particle detection is sufficient for particle time-of-flight calibration
between 80 and 5000 nm (see Sect. S4 in the Supplement). For the
ERICA-AMS, the detectable particle size range is determined by the
transmission and focusing properties of the aerodynamic lens. For the ADL
used in our instrument, Xu et al. (2017), who used this lens
in combination with an ACSM (aerosol chemical speciation monitor),
determined a transmission range from ∼ 120 to 3500 nm. We
assume that the detectable particle size range of the ERICA-AMS matches this
transmission range. The design details of the ERICA-AMS are the same as
those of the Aerodyne AMS and are well described in the literature (e.g.,
Jayne et al., 2000; Jimenez et al., 2003c; Drewnick et al., 2005;
Canagaratna et al., 2007). A fundamental difference from the commercial
Aerodyne AMS is the use of a simple shutter mechanic instead of a chopper to
block the particle beam for the reference background measurement.
Aerosol particle inlet and vacuum system
A continuous flow of sampled air containing particles enters the instrument
via a critical orifice at the sample inlet (see Fig. 1). For measurements at ambient ground pressure, a pinhole (diameter of 100 µm) maintains a volumetric flow rate ΦERICA of 1.48 cm3 s-1. However, in order to achieve a constant pressure in the ADL (pADL= 4.5 hPa), the mass flow rate needs to be kept constant during flight operations with largely varying ambient pressures (for the M-55 Geophysica ranging from ground
pressure to 50 hPa). If pADL is not kept constant, the
transmission of the particles through the inlet into the vacuum system
becomes altitude-dependent (Zhang et al., 2002). For this
purpose, a newly developed, automatically controlled compressible rubber
O-ring setup, the so-called CPI, is deployed (Molleker et
al., 2020). As the ADL the intermediate pressure lens IPL-013 (Peck et al.,
2016; Xu et al., 2017) was integrated to focus the particles into a beam
with sufficiently small divergence, i.e., less than the diameter of the
vaporizer element at a distance of 55 cm downstream of the exit of the ADL.
The lens itself contains six apertures (excluding the first critical
orifice) with decreasing diameters (from 5.0 mm down to 2.9 mm), and the
exiting particles are accelerated to velocities of up to 200 m s-1. The inner end of the ADL tube protrudes from a holder plate through a radially
sealed feedthrough and is attached to a ball joint inside the first pumping
stage of the vacuum chamber. Four fine threaded screws, two of them with
scale, enable the operator to tilt the lens precisely in two dimensions in
order to adjust the particle flight direction so that it becomes aligned with
the vaporizer of the ERICA-AMS. By means of this design, the particle beam
remained stable during flights even in the presence of vibrations caused by
turbulence in the convective anvil outflows of tropical cumulonimbus at 12
to 18 km altitude.
The vacuum chamber was purchased from Aeromegt GmbH (Germany) and is a
modified design of a LAAPTOF (laser ablation aerosol
particle time-of-flight mass spectrometer; Gemayel et al., 2016). During
mobile operation on aircraft, two diaphragm pumps (model MD 1 VARIO-SP,
Vacuubrand GmbH + Co KG, Germany; pumping rate of 5 × 102 cm3 s-1) yield 3 mbar for the backing pressure of the four-stage turbomolecular pump (TMP1). As in the Aeromegt LAAPTOF, the four-stage turbomolecular pump (see Fig. 1;
SplitFlow 270, Pfeiffer Vacuum GmbH, Germany) is utilized for pumping the
entire single-particle mass spectrometer (ERICA-LAMS part). Its first
pumping stage (PS1) operates at a rate of 3.0 × 104 cm3 s-1. The second pumping stage (PS2; see Fig. 1) reduces the pressure of the chamber,
containing PDU1, down to 3 × 10-4 mbar
(pumping rate of 1.55 × 105 cm3 s-1). A pinhole of a
1.8 mm opening diameter placed perpendicularly to the particle beam separates
PS2 from the third pumping stage (PS3). For the particle detection unit
PDU2, PS3 provides a vacuum pressure of 8 × 10-7 mbar with a
pumping rate of 1.55 × 105 cm3 s-1. The fourth
pumping stage (PS4) is attached to the chamber of the B-ToF-MS, which is
maintained at a pressure of 4 × 10-7 mbar (pumping rate of
2.0 × 105 cm3 s-1). The particle detection unit
PDU2 and the mass spectrometer chamber are connected through a centered
4 mm aperture.
The shutter unit (SU) separates the ERICA-LAMS mass spectrometer chamber
from the ERICA-AMS ionizer vacuum chamber (see Fig. 1). The latter is separated from the SU by an orifice of 7 mm in diameter. The turbomolecular pump TMP2 (see Fig. 1; model HiPace® 80, Pfeiffer Vacuum GmbH, Germany; pumping rate of
6.7 × 104 cm3 s-1) is attached to the ionizer
chamber, keeping it at a pressure of 1 × 10-7 mbar. The turbomolecular pump TMP3 (model HiPace® 30, Pfeiffer Vacuum GmbH, Germany) provides a pumping rate of 2.2 × 104 cm3 s-1 in the C-ToF-MS such that here the operational pressure is 2 × 10-7 mbar. Both HiPace® pumps, TMP2 and
TMP3, are backed by the third pumping stage (PS3) of the SplitFlow pump. A
schematic of the distribution of the pumps and the vacuum connections
between the pumps is shown in Sect. S1.2 in the Supplement.
ERICA-LAMS
The ERICA-LAMS is based on the commercial LAAPTOF (Gemayel et al., 2016;
Marsden et al., 2016). However, it has been thoroughly modified, so only the
vacuum chamber (including the four-stage TMP), the ADL adjustment mechanics,
and the B-ToF-MS remain. The setup of the optical single-particle
detection module for the ERICA-LAMS consists of the two particle detection units
PDU1 and PDU2 (see Fig. 1), based on the design of
the ALABAMA (Brands et al., 2011; Clemen et al., 2020). Each of these
particle detection units (PDU1 and PDU2) contains a continuous-wave laser
(LD1 and LD2), an ellipsoidal reflector, and a PMT (PMT1 and PMT2). By that,
each particle passing both laser beams causes two light-scattering
signals. The distance from the exit of the ADL to the focal point of the
first ellipsoidal reflector (i.e., the first particle detection point) is
58.8 mm; the distance between the first and second detection point is 66.5 mm. A scheme of the geometry with dimensions of the ERICA is provided in Sect. S1.3 in the Supplement. The laser sources are 150 mW UV-laser diodes operating at a wavelength of 405 nm (model SF-AW210 distributed by
InsaneWare-Deluxe, Germany) mounted in a heat sink.
The continuous-wave laser light is focused by a plano-convex lens with a
focal length of 4.02 mm to a 1/e2 radius w0 of
30 µm (see Sect. 3.2.1). To reduce optical
disturbances like diffraction fringes, the laser beam passes through a
baffle of four apertures before the beam enters the detection region.
Finally, approximately 40 mW of light illuminates the particle detection
region. Each PDU is individually mounted on a disjoined micro XY translation
stage (1 µm precision, model MKT 30-D10-EP by OWIS GmbH, Germany), and thus, they can be tilted in two dimensions for adjusting the laser foci onto the particle beam. An O-ring around the tube holding the four aperture rings provides the vacuum seal at the pivot point.
In order to focus the light scattered by the individual particles on a
detector, ellipsoidal reflectors (model E50NV-01 anti-fingerprint coated, Optiforms,
Inc., Temecula, CA, USA) were used. A detailed description of the
ellipsoidal-reflector setup can be found in Sect. S1.4 in the Supplement. A
plano-convex lens collimates the reflected scattered light towards the
sensitive area of the PMT (model H10721-210, Hamamatsu Photonics K.K.,
Japan). This design collects a maximum of 70 % of the total scattered
light from a spherical particle (100 nm), according to model calculations
adopting Mie theory and using the geometry of the detection unit except for
the pinholes (which cause losses). The acquired PMT signals are processed by
a custom-built electronic board, hereafter referred to as the trigger card
(TC) following the design from the ALABAMA (Brands et al., 2011; Clemen
et al., 2020).
The ablation laser is triggered by the TC that measures the particle flight
time between the two PMTs and computes the precise time of the particle arrival
at the “ablation spot” by multiplying the particle flight time between
PDU1 and PDU2 by a factor (pulse generator multiplier), considering the
geometry of the instrument (see Sect. S1.3 in the Supplement). The
triggering of the ablation laser considers the time span of 145 µs
between triggering the laser flash lamps and the Q-switch. The precise
values for this timing are set experimentally. Also, the TC triggers the
high-voltage switches for the ion extraction.
As a consequence of the ablation laser pulse, the material of an aerosol
particle is vaporized and ionized in a single step by a multi-photon process
(Suess and Prather, 1999). For the LDI, a frequency-quadrupled Nd:YAG
laser (model Ultra 50, Quantel, France) generates 6 ns long pulses with 266 nm wavelength and typical values of around 4 mJ for the pulse energy. The co-emitted light from the laser at wavelengths of 1064 and 532 nm is not
filtered by a wavelength separator inside the laser head in order to
minimize the number of optical elements in the light path before the
ablation spot.
Schematic of the ablation laser unit of the ERICA-LAMS and
corresponding optical dimensions (z0: focal length; w0,dia: laser beam focus 1/e2 diameter). The particle beam is pointing perpendicularly to the plane of the drawing. The dichroic mirrors are labeled as DM1 and DM2.
As shown in Fig. 2, the emitted laser beam is
oriented orthogonally to the particle flight axis and focused onto the
particle beam by a plano-convex lens (anti-reflection-coated model L-11612,
Laseroptik GmbH, Germany). From the laser head, the beam is directed towards
the mass spectrometer chamber by the dichroic mirror DM1 (see
Fig. 2; model G340722000, Qioptiq Photonics GmbH
& Co. KG, Germany). This mirror also separates the UV light from the
light at the other wavelengths (1064 and 532 nm) by reflecting
> 99.5 % of the 266 nm light while only 12.6 % of the light
at other wavelengths is reflected towards the ablation spot. The laser beam,
now mostly consisting of UV light, enters and exits the vacuum chamber
through uncoated and 3∘ tilted quartz glass windows in order to
reduce back-reflections towards the laser head. The exiting beam is directed
by a second dichroic mirror DM2 through an attenuating UV-absorbing glass
filter (model UG11, Qioptiq Photonics GmbH & Co. KG, Germany) to an
optical energy meter (EnergyMax™-USB, model J-25MB-LE,
Coherent, Inc., USA) by which the energy of each pulse can be measured such
that the laser pulse energy is detected and stored. The focal length of the
lens (f= 76 mm) is such that a high UV light intensity is centered at
the ablation spot (see Fig. 1). This spot is
located at the center between the extraction plates (EPs) of the B-ToF-MS
(from Tofwerk AG, Switzerland). For adjusting the beam waist of the UV laser
to the ablation spot, the dichroic mirror DM1 is mounted on a holder, which
allows tilting the mirror with 2 degrees of freedom. The minimum beam at
the ablation spot, which can be obtained with this setup, has a
1/e2 diameter w0,dia of 250 µm (see Sect. 3.2.1). For this fine adjustment, the focusing lens
can also be moved in the direction towards the vacuum chamber. By means of
this setup, the diameter of the laser beam at the location of the particle
beam can be enlarged from the minimum of 250 µm up to approximately
740 µm so that the energy density at the ablation spot can be reduced in a controlled way (Brands et al., 2011).
Considering a nearly Gaussian beam shape, as measured and confirmed by the
fitting method in Sect. 3.2.1, the power density
available to ablate the particle depends on the location of the particle
within the laser beam. After each pulse the laser has to idle for at least
120 ms in order to keep the output energy constant; this fact limits the
repetition rate for the ERICA-LAMS to 8 pulses s-1 (instead of the
nominal 10 pulses s-1 according to the manufacturer's specification).
The maximum repetition rate of the ablation laser, along with factors such
as particle losses in the ADL, particle beam divergence, particle and laser
beam alignment, and the sensitivity of the particle detection units, limits
the number of particles analyzed (Su et al., 2004; Zelenyuk and Imre,
2005; Brands et al., 2011; Marsden et al., 2016; Clemen et al., 2020), which
affects the spatial resolution for measurements from a fast-flying aircraft.
For the analysis of the single particles, the ions generated by the laser
pulse are accelerated into the B-ToF-MS using an electric extraction field
in the ablation region. The acceleration field between the EP is turned on
only for the short time interval of 2 µs, which is long enough for
sufficient ion extraction. For this purpose, fast solid-state high-voltage
transistor switches (model HTS 61-03-C, Behlke Power Electronics GmbH,
Germany) are triggered by the TC and switch within 18 ns about 1.2 µs
before the Q-switch actually fires the laser. During the time when no
particles are detected by PDU1 and PDU2 or the ablation laser is in its idle
time, the EPs are connected to ground. Upon connection to ground, the
electric field decays with an RC time constant of approximately 10 ms. The
high-voltage (HV) switch was implemented since the electric extraction fields cause
charged aerosol particles to deviate from their straight flight direction
(e.g., Chen et al., 2020; Clemen et al., 2020), and as a result, they
might not hit the vaporizer in the ERICA-AMS part. In order to also reduce
particle deflection caused by an electric field forming outside the ion
optics, in addition the particle flight path through the ERICA-LAMS part is
shielded by grounded plates. Inside the time-of-flight mass spectrometers,
reflectrons (see Fig. 1) serve to enlarge the ion
flight path and to increase the mass resolution RMS to up to 700 (see Sect. 3.2.4).
The generated ion signal is picked up by MCPs (model MCP 40/12/10/8 D 46:1,
Photonis USA, Inc., Sturbridge, MA, USA), amplified, and collected by a
digital oscilloscope (model PicoScope 6404C, Pico Technology, UK). The
oscilloscope features four channels with 8 bit vertical resolution and a
maximum sampling performance of 5 gigasamples per second (GS s-1). The
time resolution is set to 1.6 ns per sample. Each of the two MCP outputs,
for the anions and cations, is conditioned and sampled simultaneously by two
separate channels (two channels for cations and two channels for anions) of
different input voltage ranges (full range for cations 200 mV and 4 V,
respectively, and for anions 100 mV and 4 V, respectively), an approach for extending the dynamic range of the analog-to-digital conversion
(Brands et al., 2011). A graphic user
interface was developed for the control of the oscilloscope and the fast
export of raw data to binary files. These files are converted to a format
that is compatible with the in-house-developed evaluation software CRISP
(Concise Retrieval of Information from Single Particles) by Klimach (2012) for a posteriori analysis. In each file the bipolar mass spectrum,
the time of ablation (time stamp), and the particle flight time
(“upcounts”) between PDU1 and PDU2 are stored.
ERICA-AMS
All particles which are not ablated in the ERICA-LAMS (see Sect. 2.3) continue their flight towards the ERICA-AMS
instrument part. The design of the ERICA-AMS is the same as the design of
the commercial Aerodyne AMS, which is described in the literature
(Drewnick et al., 2005; Canagaratna et al., 2007). However, a major
difference to the commercial AMS is the use of the SU in the ERICA-AMS
instead of a chopper and a longer particle flight path between the ADL and
the vaporizer (see below). In the ERICA-AMS, quantification is given in the
same way as in the commercial AMS since the shutter performs the same
function as the chopper. The vaporizer, ionizer, and ion optics, as well as
the C-ToF-MS, are identical to those in the commercial Aerodyne C-ToF-MS,
ToF-ACSM, and mAMS. The details are described in
Drewnick et al. (2005), Canagaratna et al. (2007), and
Fröhlich et al. (2013).
In the ERICA-AMS, non-refractory components are thermally desorbed by a
tungsten vaporizer (surface diameter of 3.8 mm) operating at a temperature
of approximately 600 ∘C. The vapor molecules and fragments are
ionized by electrons (impact energy of 70 eV) continuously emitted by a
filament (emission current of 1.6 mA). This vaporization and ion generation
unit was manufactured by Aerodyne (Aerodyne Research Inc., Billerica, MA,
USA). The generated ions (cations) are extracted through an electrostatic
lens stack into the C-ToF-MS. The extraction path is perpendicular to the
aerosol particle flight path (orthogonal extraction). The ions are extracted
in batches with a frequency of 50 kHz. The trigger pulse for ion extraction
defines the starting time for the time-of-flight mass spectrometric ion
analysis (Drewnick et al., 2005; Canagaratna et al., 2007). After passing
through the C-ToF-MS, the ions impinge on the MCP (model MCP 40/12/10/8 D 46:1, Photonis USA Inc., Sturbridge, MA, USA) and generate a signal, which
is amplified and collected by the data acquisition card (DAQ card; model
ADQ1600 USB3, Teledyne Signal Processing Devices Sweden AB, Sweden). The DAQ
card serves for both the generation of periodic trigger pulses for ion
extraction and the acquisition of ion-generated signals from the MCPs. This
device samples at 1.6 GS s-1 with a high vertical resolution of 14 bit. Multiple consecutive spectra are processed at a hardware level over a
time period of user-selectable length (typically 400 ms) and are streamed
via a USB 3.0 connection as one averaged raw spectrum to the main control
computer.
For quantitative aerosol composition measurements, the background signal,
which originates from air molecules and residual vapor molecules inside the
chamber, has to be subtracted from the aerosol sampling signal. For this
purpose, the SU is used to periodically block the particle beam. The SU
consists of a C-shaped surface made of metal, which is mounted on the shaft
of a high-vacuum magnetically coupled feedthrough (Pfeiffer Vacuum GmbH,
Germany). The shaft periodically rotates the shutter by 90∘ into
and back out of the particle beam path. In this way, the particle stream to
the vaporizer is blocked and permitted, respectively, for adjustable time
periods. In the commercial Aerodyne AMS (Canagaratna et al., 2007), the
particle beam is periodically blocked by a chopper inside the low-vacuum
stage. By means of the chopper it is possible to distinguish between
different vacuum aerodynamic particle sizes, as the particle flight elapsed
from its pass through the chopper until its arrival at the vaporizer is
size-dependent. The distance between the chopper and the vaporizer and the
corresponding flight time need to be long enough to achieve such
size-resolved sampling. In the design of the ERICA-AMS, the distance from
the shutter to the vaporizer is very short. This would not be the case if a
chopper were mounted directly behind the ball joint of the ADL. However, by
periodically blocking the particle beam with a chopper at this position, the
detection frequency of the ERICA-LAMS would have been reduced accordingly. Thus,
a simple shutter has been implemented, and the particle size information can
only be provided by the PDU of the ERICA-LAMS (see Sect. S4 in the
Supplement).
Based on experience from flight operation and laboratory experiments, one
measurement cycle has been selected to have a length of 10 s, which
corresponds to 25 measured averaged raw spectra. Out of these, 12 spectra
were recorded with the shutter open (4.8 s), 11 with the shutter closed (4.4 s) for background measurement, and 2 during the movement of the shutter.
Since the exact position of the shutter during the acquisition of the latter
is not known, these 2 spectra are discarded and not used for data
evaluation. These open–closed cycles can be adjusted in the acquisition
software (“TofDAQRec” by Tofwerk AG, Switzerland). The collected data are
evaluated by the software “Tofware” from Tofwerk AG (Fröhlich et
al., 2013; Stark et al., 2015; Timonen et al., 2016).
Influence of the ERICA-LAMS on the ERICA-AMS
The assembly of the two instrument parts, i.e., the ERICA-LAMS and the
ERICA-AMS, in a serial configuration might lead to interactions. On the one
hand, it can safely be assumed that the ERICA-LAMS is largely unaffected by
the ERICA-AMS presence and operation. On the other hand, particles which are
ablated or distracted in the ERICA-LAMS are excluded from the total mass
measured by the ERICA-AMS.
The first loss mechanism for particles to be analyzed by the ERICA-AMS is
the ablation of the particles in the ERICA-LAMS. The impact of this
instrument-induced loss depends on the number concentration of particles
within the sampled aerosols and cannot be compensated for. Two examples
illustrate this for different conditions:
In pristine conditions, like the summertime Arctic boundary layer, particle number concentrations rarely exceed 5 cm-3
(Köllner et al., 2017) in the size
range (see Sect. 3.2.2) relevant to our instrument.
For the typical sampling volumetric flow rate (ΦERICA) of 1.48 cm3 s-1, a maximum of around 7 particles s-1 would be
detected by the ERICA-LAMS. Even with the ablation laser being
restricted to a maximum of 8 shots s-1, theoretically this can result in a 100 % loss for the ERICA-AMS since all particles would be ablated and ionized assuming a hit rate (HR; for a definition see Sect. 3.2.3) of 100 %. On the other hand, small particles (dva< 100 nm; see Sect. 3.2.2) cannot be detected sufficiently by the detection units and will not lead to any losses at the ERICA-AMS. Furthermore, in practice, the HR is particle-size-dependent and, for all particle sizes, lower than unity. Thus, the parameter HR is not applicable for estimating the losses of the non-ablated particles. The value of the HR might not be lower than unity because of the failure of the laser pulse hitting the targeted particle but because of the ionization efficiency within the LDI process. Thus, at such low ambient particle concentrations, the quantitative results of the ERICA-AMS measurements must be viewed
critically. In addition, possible measurement strategies, such as including
periods of short inactivity for the ERICA-LAMS, can be adopted. Further
studies and additional instrumentation (size distributions) need to be
considered to quantify the ERICA-AMS results at low particle concentrations.
During the first field deployment (see Sect. 4),
usually around 100 particles s-1 was detected by the PDUs during
ambient aerosol measurements in the planetary boundary layer. Considering
ΦERICA, 8 laser shots s-1 and a maximum HR of 100 %,
about 5.4 % of the particles are ablated and thus will not reach the
vaporizer. For the same reasons as those discussed above, this is a
conservative estimate and the actual losses cannot be determined. However,
the losses (in mass) are small considering the commonly assumed uncertainty
of 30 % in AMS instruments (Bahreini et al., 2009). By
calculation, 30 % losses for the particle numbers equal 27 particles s-1 (∼ 18 particles cm-3). In the upper troposphere and lower stratosphere (UTLS; > 15 km), we measured a particle detection rate of between 5 and 800 particles s-1. Thus, for such measurements, losses for the mass concentration of up to 100 % have to be
considered and the uncertainty of 30 % has to be adapted.
Another loss mechanism is the deflection of charged particles caused by the
temporarily applied electrical field between the high-voltage extraction
plates of the ERICA-LAMS. This will lead to losses which are impossible to
be compensated for because typically the charge distribution of ambient
aerosol particles is not known. Therefore, measures have been taken in order
to minimize these losses as much as possible. As described in Sect. 2.3, the high voltage (HV) for ion extraction is only
applied shortly before a particle is ablated. The deflection caused by the
electric field is dependent on the particle size and charge; the resulting
losses consequently depend on the dimensions and shape of the vaporizer,
meaning that not all deflected charged particles are lost. The HV switch
unit was specially designed to keep the deflection losses to a minimum. The
HV is applied for 10 ms per shot, resulting in a duty cycle of 8 %,
assuming the laser is shooting 8 times s-1.
Instrument characterizationCharacterization of the particle beamDetermination of efficiencies for optical particle detection and particle mass measurements
Knowing the particle beam properties at the PDUs, the ablation spot, and the
vaporizer is essential for interpreting and evaluating measured data. For
proper detection of the sampled particles, a sufficient overlap of the
particle beam with the laser beams and the vaporizer is required. The
optical particle detection efficiency of the PDUs was determined by
comparison of count rates of the individual detection units (PDU1 and PDU2)
with those of either a condensation particle counter (CPC) or an optical
particle counter (OPC) as the reference device (see Sect. S3 in the Supplement).
In this way, the particle numbers or, indirectly, the mass concentrations
measured by the ERICA-AMS can be associated with the number concentration of
the sample airflow. The measured polystyrene latex (PSL) particle sizes and
the respective measurement setups are shown in Sect. S3 in the Supplement.
To determine the size-dependent and ADL-position-dependent optical detection
efficiency DEPDU at the detection units with PSL particles (see Table S5 in the Supplement), the ADL was tilted in steps and DEPDU was measured at different ADL positions xpos while the position of the
detection laser was kept constant. Hereafter, this procedure is referred to
as the “ADL position scan”. This approach, which is similar to the method
reported by, e.g., Marsden et al. (2016) and Clemen et al. (2020), is described by Molleker et al. (2020). DEPDU was determined for each lens position xpos according to Eq. (1).
DEPDUxpos=cts‾Detxposc‾ref⋅ΦERICA
Here, cts‾Det is the averaged value of the number of particles per second counted by each PDU over 30 s, ΦERICA is the volume flow into the ERICA, and c‾ref is the value of the number of
particles per volume unit averaged over 30 s at the reference device.
A typical result of an ADL position scan for PSL particles at PDU1 and PDU2
is shown in the Supplement (Sect. S5.4, Fig. S13). The curve fit to the ADL
position scan can be described as a convolution integral of a rectangular
top-hat function of the effective detection laser width 2reff,L, since the scattered light is only detected above a certain intensity threshold, and a 2-D Gaussian distribution function representing the particle beam cross section. The effective laser beam radius reff,L is the laser beam radius wherein a particle is registered. The convolution is described by Eq. (2) according to Molleker et al. (2020):
DEPSLxpos=12⋅erfxpos+reff,L-x02σ-erfxpos-reff,L-x02σ⋅Ascan
The variable σ is a measure for the particle beam width, i.e., the
particle beam radius, and x0 corresponds to the value of xpos at the peak value. This x0 value is also called the modal value of the ADL position scan. The parameter Ascan is a scaling parameter of the peak value of the ADL position scan and accounts for losses, e.g., ADL transmission efficiency values smaller than unity. Equation (2) is used as a curve-fit function for determining the values of the parameters reff,L, x0, σ, and Ascan. A plateau, such as the one shown in Fig. S13a in the Supplement,
indicates a narrow particle beam with respect to the effective laser width
for the respective measurement.
For the measurements of particles with sizes from 218 to 834 nm, it was
assumed that the particle losses between PDU1 and PDU2 are negligible.
Therefore, the curve fitting for both detection units was performed
simultaneously for each particle size with both data sets (PDU1 and PDU2) by
a comprehensive analysis, which allows us to combine two data sets into one single
common curve-fitting procedure. In the following, this procedure is
referred to as “combined curve fitting”. During this combined
curve-fitting procedure, the variable Ascan was linked for both PDUs by determining one Ascan value for PDU1 and PDU2 simultaneously. Thus, only one value for Ascan per measured particle size was obtained.
For the evaluation of the measurement with PSL particles of 108 nm in size,
a different approach was chosen because losses between PDU1 and PDU2 seemed
reasonable due to the particle beam divergence (Huffman et
al., 2005). Therefore, the evaluation was carried out without the combined
curve-fitting procedure and, thus, individually for the measurements at PDU1
and PDU2. Due to the mathematical relation between the variables reff,L and Ascan during the curve fitting, it was not possible to determine
both variables at the same time. Therefore, reff,L was calculated
separately and kept constant during the curve fitting. Considering the
size dependence of the scattered light intensity based on Mie scattering,
reff,L,108nm was estimated for the measurement with PSL particles of a size of 108 nm, adopting suitable software routines following
Bohren and Huffman (1998). The value of reff,L,218nm,
determined for the measurements of particles with sizes of 218 nm, was used
as base for the estimation. The result of the calculations showed that a
particle of 108 nm scatters the same amount of light as a particle of
218 nm, when it is closer to the focus by a factor of 0.955. Thus,
reff,L,108nm=0.955⋅reff,L,218nm was used as a
curve-fit constant for the evaluation of the measurement with PSL particles
of 108 nm (see Sect. S5.1.1 in the Supplement). Since this calculation is
based on a Gaussian laser beam profile, it can only be seen as an
approximation and especially since the outer parts of the laser beam might
deviate from a Gaussian profile due to diffraction and reflection in the
laser beam setup.
In addition to the particle detection efficiency for PSL particles, the
optical particle detection efficiencies of particle counting at both PDUs
were determined according to Eq. (1) for ammonium nitrate (AN) particles between 91 and 814 nm in size (see Sect. S3 in the
Supplement). Besides the singly charged, the doubly charged particles have
to be considered when using a differential mobility analyzer (DMA) for size
selection out of a polydisperse aerosol. For this, a newly developed,
iterative method was adopted and is described in detail in Sect. S5.2 in the
Supplement. Briefly, the curve-fit function of Eq. (2) was extended by a second term for the doubly
charged particles and two weighing factors to account for the fractions of
the particle charges (see Eq. S15 in the Supplement). As for the
measurements with PSL particles, the parameters reff,L, σ,
x0, and Ascan could be determined by a combined curve-fitting procedure (for exceptions see Sect. S5.2 in the Supplement).
Simultaneously to the measurements with AN particles at the detection units
PDU1 and PDU2 of the ERICA-LAMS, the mean mass concentration of AN was
measured with the ERICA-AMS, similarly to the approach described in
Liu et al. (2007). The efficiency with which particle mass
concentrations were measured with the ERICA-AMS was determined. While this
quantity is equivalent to the “collection efficiency” (CE; e.g.,
Canagaratna et al., 2007; Matthew et al., 2008; Drewnick et al., 2015) in
AMS measurements, we define it as “particle mass detection efficiency” for
consistency with the ERICA-LAMS discussion. As a reference, we used the CPC
to obtain the mean particle number concentration and calculated the input
mass concentration. The curve-fitting evaluation method applied afterwards
also accounts for the doubly charged particle fraction and is described in
detail in Sect. S5.2 in the Supplement. By the curve-fitting procedure, the
parameters reff,V (effective vaporizer radius), σ, x0, and Ascan could be determined (see Sect. S5.2 in the Supplement for definitions and exceptions). All these parameters, reff,L, reff,V, σ, x0, and Ascan, are essential for adjustment procedures
of the instrument and to interpret the obtained laboratory and field mass
spectra. Furthermore, the determined parameters are used in Sect. 3.1.2 to characterize the particle beam and in Sect. 3.2.2 and 3.3.2 to
determine the optical particle detection efficiency and the particle mass
detection efficiency, respectively.
Overall, the parameters serve as a means for the evaluation of the
performance of the instrument.
Particle beam properties
The parameters reff,L, reff,V, σ, x0, and Ascan were determined by the curve-fitting functions (Eqs. 2 and S15 and S17 in the Supplement) and are thus in the dimension relative to the ADL position xpos as read out
on the micrometer adjustment screw (see Sect. S1.3 in the Supplement).
Below, the parameters were rescaled, using the intercept theorem, to the
dimension of the particle beam at the specific position (PDU1, PDU2,
ablation spot, and ERICA-AMS vaporizer).
The particle beam diameter wpart
(1/e diameter) as a function of particle size dva for PSL (squares) and AN (circles)
particles measured at the detection units PDU1 (red, left ordinate) and PDU2
(blue, left ordinate) and for AN particles measured at the ERICA-AMS
vaporizer (right ordinate, black). The reference values for number
concentrations were obtained from the experimental setup with either the CPC
or the OPC (Setup B or Setup C, respectively; see Fig. S8 in the Supplement). The
AN particle beam diameter at the ablation spot (brown triangles, left
ordinate) and the ERICA-AMS vaporizer (green triangles, right ordinate) were
calculated by extrapolation of the measurement at PDU2. The uncertainty in
PSL particle size is given by NIST certificates and converted to
dva. The uncertainty in AN particle size
dva is estimated to be 3 % (Hings, 2006).
These uncertainties for PSL and AN particle sizes are the same for Figs. 3
to 7 and 12. The uncertainties in the particle
beam diameters result from the curve fittings (1 standard deviation).
The curve fittings yield the standard deviation σ, which is
proportional to the particle beam 1e radius at each
detector (PDU or vaporizer). The particle beam diameter wpart is
defined as 2σ, i.e., the 1e diameter of the
Gaussian distribution function. In Fig. 3, wpart is displayed as a function of the particle size dva at various locations within the instrument. The particle beam diameter wpart is
approximately 0.1 mm at PDU1 and 0.2 mm at PDU2 for particle sizes above
400 nm. For PSL particles of 108 nm in size, the wpart values are 5 times (7 times) wider at PDU1 (PDU2). The measurements with the OPC for
larger diameters indicate a trend for wpart from 0.10 to 0.18 mm. For AN particles of 335 nm in size, a minimum of wpart was found, as the corresponding values for wpart at PDU1 and PDU2 are 0.04 and 0.03 mm, respectively. At the vaporizer, the largest value for wpart of
2.2 mm was measured for AN particles of 91 nm in size, which is narrower
than the width of the vaporizers' physical cross-sectional diameter of
3.8 mm. Thus, by adjusting the ADL properly, all investigated AN particles
larger than 91 nm can be collected by the vaporizer. The overall curve
shapes at each PDU depict a “V”, where the smaller and the larger
particles show a larger wpart than particles of 335 nm in size. Smaller
particles can be deflected by collisions with residual gas molecules, and
larger particles are over-focused by the ADL due to their inertia (Zhang
et al., 2002; Peck et al., 2016). Considering the geometry of the
instrument, wpart at the ablation spot and at the ERICA-AMS
vaporizer can also be extrapolated from the respective wpart for AN at PDU2. The longer travel distance for the particles and the particle beam
divergence (Huffman et al., 2005) result in a 3.3-fold-broader wpart for AN particles at the vaporizer than at PDU2. The
calculation yields a maximum wpart of 0.48 mm at the ablation spot, a value which is approximately 2 times the ablation laser beam diameter w0,dia (see overlap parameter determination below in this section), and wpart of 1.07 mm at the vaporizer (both for AN particles of 548 nm in size).
In the following, the overlap of the particle beam with the detection laser
focus is discussed. Considering an optical laser beam diameter w0,dia of 60 µm of the PDUs (see Sect. 3.2.1), the
particle beam diameter wpart is wider by a factor of 2 to 3 (PSL,
dva> 400 nm). However, the laser intensity of a Gaussian
beam provides intensities larger than zero also for radial distances above
w0 and the scattered light might be sufficient for particles to be
detected. The maximum distance from the laser axis where particles can be
detected is represented by the parameter reff,L and not w0. Figure 4 shows the effective laser beam radii reff,L and reff,V as a function of the particle size dva.
Overall, for PSL particles, reff,L is between 0.1 and 0.4 mm. The shape of the curve of the effective laser beam radius depends on the
response function of the scattered light intensity as a function of size,
where an increase to larger sizes is expected. For the measurements with
PSL particles of 108 nm and AN particles of 91 and 138 nm in size, this
is inevitable since the values of reff are calculated based on the Mie scattering according to a rough estimation (see Sect. S5.1 in the
Supplement). For larger particles or the measurements with the OPC as the
reference device, an increase in reff,L with particle size would be expected. Due to the fact that the OPC measurements were performed with
various PMT threshold values (see Sect. S3 in the Supplement), reff,L appears lower than the CPC reference measurements, and thus, reff,L for particle sizes above 834 nm is underestimated in Fig. 4. The AN measurement results do not agree with the results of the
measurements with PSL particles, possibly due to a different refractive
index of AN as compared to that of PSL. The vaporizer width determined by
the ADL position scans, i.e., reff,V, agrees with the vaporizer's
physical dimension of a 1.9 mm radius.
The effective detection laser radius reff,L as a function of particle size dva determined for PDU1 (red, left
ordinate) and PDU2 (blue, left ordinate) with PSL (squares) and AN (circles)
particles and the effective vaporizer radius reff,V as a function of particle size dva for the ERICA-AMS vaporizer (right ordinate, black) determined with AN particles. CPC and OPC measurements are as for Fig. 3. The physical vaporizer radius is marked
by a dashed gray line. The uncertainties in the effective radii result from
the curve fittings (1 standard deviation). The uncertainty in
reff,L for the PSL measurement with particle size of 108 nm was estimated to be 0.002 mm (PDU1) and 0.004 mm (PDU2), and the uncertainties in reff,L for the AN measurements with particle sizes of 138 and 91 nm are conservatively estimated to be 0.009 mm at PDU1 and 0.014 mm at PDU2. These values are the approximated maximum uncertainties in reff,L in the considered
size range of 213 to 814 nm at PDU1 and PDU2. For the measurement with AN
particles of 91 nm in diameter, the uncertainty in reff,V was estimated to be 0.08 mm.
The overlap parameters Sdetect,L, Sdetect,V, and Sablation as a function of particle size dva for PSL (squares) and AN (circles) particles. Sdetect,L was determined for PSL and AN particles at PDU1 (red) and PDU2 (blue). Sdetect,V was determined for AN particles at the ERICA-AMS vaporizer (black). Sablation was calculated for AN particles at the laser ablation spot (brown). CPC and OPC measurements are as for Fig. 3. The horizontal dashed gray line illustrates where the ratio equals 1. The uncertainties in Sdetect,L, Sdetect,V, and Sablation result from the curve-fitting values (1 standard deviation).
To determine the overlap of the particle beam with the detection laser beam,
the particle beam diameter wpart is compared to the effective laser diameter deff,L=2reff,L. Therefore, the overlap parameter Sdetect,L=wpart/deff,L
was calculated for different particle sizes at the PDUs as the maximum
possible overlap of wpart and deff,L for each measurement at lens position xpos=x0. The parameter Sdetect,V=wpart/deff,V (with deff,V=2reff,V) expresses the overlap of the particle beam with the effective vaporizer width. Both are shown in
Fig. 5. The horizontal gray line marks an overlap
parameter of 1. All investigated particle sizes below that line are detected
sufficiently well within 1σ of the particle beam width. That is the
case, within their uncertainties, for all measurements except for PSL
particles of 108 nm in size. The reason for this is a large wpart for the smallest particles resulting from a large particle divergence caused by the small particle inertia for this size (Zhang et al.,
2002). The values of Sdetect,L of the measurements with the OPC are overestimated, since the resulting values of reff,L are underestimated
due to the varying threshold during the measurements (see Sect. S3 in the
Supplement). However, the values are below a ratio of 1. It has to be
remarked that a value above 1 does not indicate impossible particle
detection by the PDUs but just a reduced detection efficiency. As shown in
Sect. S4 in the Supplement, the PDUs can detect particles in a size range
between 80 and 5145 nm, although not with such efficiency as in the
size range between ∼ 180 and 3170 nm (see Sect. 3.2.2).
An overlap parameter Sablation can also be determined for the overlap of the particle beam and the ablation laser spot by dividing the particle beam diameter wpart, exemplarily for AN particles, at the ablation laser spot (see brown curve in Fig. 3) by the
determined optical laser beam waist w0,dia of 250 µm
(Sablation=wpartw0,dia).
The determination of the parameter w0,dia is shown in Sect. 3.2.1. In Fig. 5, Sablation is plotted versus the particle size dva. The calculated fraction of the illuminated area of the UV ablation laser spot is between 0.23 (at dva=335 nm) and 1.91 (at dva=548 nm). Although the particle beam is larger than the ablation laser beam waist
diameter for most particle sizes, it is possible to ablate particles and
measure them with the mass spectrometer. This indicates again that
w0,dia is not the most meaningful measure for the overlap. It also leads to the conclusion that particles can experience largely different
laser intensities depending on the position of the particle within the
ablation laser beam. At least, Sablation smaller than 1 indicates that 1σ of the particle beam is within the w0,dia of the ablation laser spot.
All the data shown for the parameters Sdetect,L, Sdetect,V, and Sablation are the maximum possible values of the respective particle sizes obtained when performing the ADL adjustment separately for each particle size.
ERICA-LAMS characterizationDetection and ablation laser beam widths
For characterization of the laser beams of the PDUs and the ablation laser,
a razor blade was moved stepwise perpendicularly into the respective laser
beam (with steps of 0.01 mm). These characterization experiments were
performed in a separate measurement setup. The remaining energy was measured
using a bolometer (high-sensitivity thermal sensor model 3A, Ophir Optronics
Solutions Ltd.) in the case of the diode lasers and by an energy meter (model
EnergyMax™-USB, J-25MB-LE, Coherent, Inc., USA) for the pulsed
UV ablation laser. The results of the measurements are provided in Sect. S2
in the Supplement.
To measure the beam waist radius w0of the detection laser in two
dimensions (x and y), the razor blade was positioned directly at the focal point. Curve fits of the Gaussian error function (Eq. 3) were applied to all data sets, with P0 for the power offset of the fitted curve, Pmax the maximum power, pos0 the central position of the Gaussian distribution, pos the
horizontal position of the blade (i.e., the independent variable), and
w0 the beam 1/e2 radius of the Gaussian intensity
profile (Skinner and Whitcher, 1972; Araújo et al., 2009).
Ppos=P0+Pmax2⋅1-erf2pos-pos0w0
It was found that the laser spot has an oval cross-sectional shape with the
dimensions of w0= (30.3 ± 1.2) µm and w0= (20.0 ± 0.9) µm (measurement in the x and y directions, respectively). Thus, the 1/e2 diameter (w0,dia=2w0) can be determined for the x direction as w0,dia= (60.6 ± 2.4) µm and for the y direction as w0,dia= (40.0 ± 1.8) µm. The average irradiance over the beam cross section (1/e2 of intensity) of the laser can be estimated as 2.1 × 103 W cm-2. Since the detection units are identical in
construction, this measurement represents both detection units.
The procedure of the characterization of the ablation laser beam is similar
to the one adopted for the detection lasers. Here, however, a
cross-sectional scan is performed at eight different positions along the
laser beam's optical axis. To evaluate the whole beam waist, the
1/e2 radii w were plotted versus the position of the razor
blade from the lens zpos. To determine the focal length z0, the Rayleigh range zR, and the beam waist radius w0 at the axial position zpos, the curve fit of the Gaussian near-field equation (Eq. 4; Siegman, 1986) was applied:
wzpos=w0⋅1+zpos-z0zR2.
From exposures on photosensitive paper, the laser beam profile appeared
radially symmetrical, and this measurement was performed only in one orientation.
The curve fitting results in a Rayleigh range zR of 7.5 mm, a focal length z0 of 76.4 mm, and a beam waist radius w0 of 125 µm. Thus, the beam waist diameter w0,dia is approximately 250 µm,
resulting in an average irradiance over the beam cross section (1/e2 of intensity) of the laser of 1.36 × 109 W cm-2. It has
to be mentioned that particles can encounter very different laser irradiance
depending on their trajectory through the Gaussian profile since the
detection and the ablation laser beam waists are much larger than the
diameters of the sampled particles (Marsden et al., 2018).
The ablation laser beam waist radius and energy density are sufficient for
particle ablation, and the measured values are comparable to those of other
single-particle mass spectrometers, like the ALABAMA (Köllner, 2019)
and A-ATOFMS (Su et al., 2004).
Optical particle detection efficiency
We determined the optical detection efficiencies for PSL and AN particles at
PDU1 and PDU2 for two cases: the largest possible, i.e., the maximum, detection
efficiency DEmax and the detection efficiency for the set ADL
position (xpos= 10.55 mm) during the deployment in Kathmandu, Nepal (KTM), DEKTM. Both DEmax and DEKTM combine the
optical detection efficiency measurements with PSL and AN particles
described in Sect. 3.1.1. Section S5.6 in the Supplement provides a listing
of all relevant equations.
The parameter DEmax was determined for each measurement. For this, the determined set of parameters (reff,L, σ, x0, and Ascan) of each curve fitting, was re-inserted into the respective Eq. (2) or Eq. (S15). For the maximum possible detection
efficiency DEmax, the variable xpos equals the modal value of
the ADL position scan x0, thereby compensating for the size-dependent
particle beam shift (see Sect. S5.7 in the Supplement). To obtain the
DEmax values in practice, the ADL has to be readjusted for each particle size.
Maximum detection efficiency DEmax
as a function of particle size dva for PSL (squares)
and AN (circles) particles measured at PDU1 (red) and PDU2 (blue). CPC and
OPC measurements are as for Fig. 3. The estimated d50 (50 % of the maximum) values of the optical detection are marked by vertical dashed gray lines. The uncertainties in DEmax reflect the
conservatively estimated value of 10 %.
Figure 6 presents the largest possible, i.e., the maximum, detection
efficiency DEmax at ADL position x0 as a function of the
particle size dva. The values of DEmax for PSL particles with
particle sizes larger than 200 nm is above 0.60, reaching the value of 1 for
particle sizes of 834 nm at PDU1. The parameter d50 is typically used
to characterize the detection limits of single-particle counting devices.
The parameter d50 is defined as 50 % of the maximum DEmax value. Here, the low d50 value of the optical particle detection is
between the particle sizes 108 and 218 nm. The upper d50 value lies
slightly above a particle size of 3150 nm. Interpolations or extrapolations
for the measurements with PSL particles are used to estimate the d50
values. We found 180 nm as the lower and 3170 nm as the upper d50
value. At PDU2, the DEmax is lower, which can be explained by the broader particle beam at PDU2 compared to PDU1. The curve progression of the particle measurements up to particle sizes of 1000 nm follows the expected response function of the light scattering, especially the decreasing
DEmax at small particle sizes. The decreasing DEmax values for
large particles and be explained by the reduced transmission of the ADL due
to particles losses by inertial impaction.
Detection efficiency DEKTM as a
function of particle size dva experimentally
determined for PSL (squares, panel a) and AN (circles, panel b) particles measured at the detection units PDU1 (red) and PDU2 (blue) for the ADL setting during field deployment in Kathmandu, Nepal. The estimated d50 values (PDU1) are marked by vertical red
lines. The uncertainties in DEKTM reflect
the conservatively estimated value of 10 %.
Due to the size-dependent particle beam shift, and thus the DEmax for
various particle sizes is found at various lens settings, a compromise for
all particle sizes has to be found to adjust the ADL. To choose the optimum
ADL position, AN particles with various sizes were measured with the
ERICA-AMS at different ADL positions. The position that yields the highest
mass concentration signal as compromise for all sizes is defined as the best
ADL position. We found xpos= 10.55 mm as the optimum ADL position, which was subsequently applied during the field deployment in Kathmandu, Nepal (KTM). Figure 7 shows the optical detection
efficiency during field deployment in KTM DEKTM as a function of the particle size dva at this specific ADL position. The calculations of the parameter DEKTM are based on Eqs. (2) or (S15) and are shown in Sect. S5.6 in the Supplement. Here, besides xpos= 10.55 mm, all other parameter values of the singly charged fraction were adopted from the curve-fitting results of the individual measurements. In Fig. 7a, the detection efficiency
DEKTM of PSL particles is plotted as a function of the particle size dva. The graph shows an increase with particle size up to a maximum for DEKTM of 0.74 for a particle size of 410 nm. By interpolation, the lower d50 value at PDU1 is 190 nm and the upper d50 value is 745 nm. Due to the relatively low maximum DEKTM value for PSL
measurements at PDU2 (0.53) compared to PDU1, the d50 values found at
PDU2 (160 and 750 nm) are misleading. In Fig. 7b
it can be seen that d50 exhibits a pronounced difference for particles with
optical properties other than PSL such as AN. Except for the measurement
with particle sizes of 213 nm at PDU1, all AN particle measurements
(Fig. 7b) result in a DEKTM larger than
0.40 and reach their maximum here for particle sizes of 335 nm (PDU2) and
548 nm (PDU1), both having values around 0.86. Here, solely d50 can
be determined for the measurement with AN particles at PDU1 to 270 nm.
The measurements demonstrated in this section have shown that detection
efficiency varies with particle size and type. The efficiency of the optical
detection strongly depends on the adjustment of the instrument as well as
the optical and the aerodynamic properties of the particle.
Hit rate
Another relevant parameter to describe the performance of a single-particle
laser ablation mass spectrometer is the hit rate HR. The definition of
HR (see Eq. 5), also called ablation efficiency,
is the number of acquired spectra Nspectra, i.e., particles
successfully ionized by the ablation laser and recorded by the oscilloscope,
divided by the number of laser shots Nshots, i.e., attempts to ablate particles (Su et al., 2004):
HR=NspectraNshots.
This definition is largely independent from ambient particle number
concentration and the idle time of the laser but rather reflects the
adjustment of the instrument. For each particle for which a laser shot is
triggered, the aerodynamic particle size is determined by the TC. With the
ERICA-LAMS, HR values of up to 1 (not shown) could be achieved in the
laboratory for PSL particles of a certain size after optimizing the PMT
thresholds and the pulse generator multiplier (see Sect. 2.3) value for the corresponding particle size. To
assess on the smallest detectable particle size, the detection units PDU1
and PDU2 were optimized for the following experiment for PSL particles of
218 nm size.
The hit rate HR (black, left ordinate); the number of
spectra Nspectra (blue, right ordinate, log scale);
and the number of detected particles, i.e., ablation laser shots
Nshots (red, right ordinate, log scale), as a function
of particle size dva (logarithmic bin size) for
ambient urban aerosol. Only the spectra with size information within the
calibrated size range were processed (see Sect. S4 in the Supplement).
Uncertainties in HR, Nshots, and
Nspectra are based on counting statistics.
To determine the hit rate for ambient aerosol, ambient air from outside the
laboratory was sampled. Only spectra of particles with diameters in the
range of calibration (see Sect. S4 in the Supplement) were considered. The
ablation laser was adjusted to the maximum HR for ambient aerosol, by varying
the pulse generator multiplier and adjusting DM1 (Fig. 2). The average ablation laser pulse energy was 3.2 mJ. Figure 8 shows the HR of the described
experiment as a function of the particle size dva. Furthermore,
Nspectra and Nshots are plotted as a function of particle size. In the size range from 100 to 1000 nm, HR values of more than 10 % are
achieved. At the particle sizes between 200 and 300 nm, at approximately
230 nm, a maximum of 0.52 was found. The reason for the maximum at this
particular particle size might be the selected optimization in the
adjustment of the detection and ablation units. Particles are detected by
the PDU as soon as their scattered light is sufficiently intense. This might
be earlier for larger particles due to the higher reff,L, and thus the timing might not be optimal for all particle sizes. In addition, a large
particle beam divergence (see Sect. S5.7 in the Supplement) can lead to a
low HR for small particles (dva< 200 nm) as well as for
large ones (dva> 400 nm). This curve progression reflects
the experimentally determined particle beam width wpart and the overlap parameter Sablation (see Fig. 5 in Sect. 3.1.2). Furthermore, the HR is less than unity over
all sizes, which may be due to the ionization efficiency of particle
components in the LDI process. Besides the particle size, HR also depends
on the particle shape and the chemical composition of the particle
(Su et al., 2004) as well as on the laser intensity of the
ablation laser (Brands et al., 2011).
Single-particle mass spectraSingle-particle mass spectra from laboratory tests
To study mass spectra of different chemical compounds, solutions of sodium
chloride (NaCl), ammonium nitrate (AN; NH4NO3), benz[a]anthracene
(BaA; C18H12), and a gold-sphere suspension were nebulized.
Details on the experimental setup, as well as on the properties of the
studied particles, are provided in Sect. S3 in the Supplement. If not
mentioned separately, all mass spectra were processed by the evaluation
software CRISP (Klimach, 2012). During this processing, the
mass-to-charge ratio (m/z) of all spectra is calibrated and each peak area
is integrated over 25 signal acquisition samples before and after the
determined m/z peak center. In the resulting so-called stick spectra, a
stick reflects the ion peak area in units of millivolts of sample of the
specific m/z. To determine the ion peak area threshold of the ERICA-LAMS,
i.e., minimum peak that can be detected, the data set of the first field
campaign (see Sect. 4) was used. The ion peak area
threshold is defined as the ion peak area at m/z on which during ambient
measurements typically no signals occur (m/z 2 to m/z 6 for cations,
m/z 2 to m/z 11 for anions). To determine the ion peak area threshold,
the normalized cumulative signal intensity distributions for each usually
unoccupied m/z were made and the overall 99 % threshold was determined
(Köllner et al., 2017). Below this ion peak area threshold, 99 % of the baseline noise is present (Köllner et al., 2017). The result for
cations and anions is an ion peak area threshold value of 7 mV sample.
Exemplary stick mass spectra (m/z) of four laboratory-generated
single particles as measured by the ERICA-LAMS. Left: cations; right: anions.
(a) NaCl particle; (b) AN particle; (c) benz[a]anthracene (BaA) particle; (d) gold particle (note the abscissa for panel d is up to m/z 400; the anion shows
no peak above the ion peak area threshold of 7 mV sample).
As an example, Fig. 9a presents a bipolar ion mass
spectrum of a single sodium chloride particle as detected by the ERICA-LAMS
during laboratory measurements. Other pure substance spectra are shown in
Fig. 9b for a single AN particle. The spectral
patterns detected by the ERICA-LAMS are comparable and in good agreement
with results produced by other established single-particle mass
spectrometers, e.g., the ALABAMA (Brands et al., 2011; Köllner et al.,
2017), ATOFMS (Gard et al., 1997; Gross et al., 2000; Liu et al., 2000),
and a modified LAAPTOF (Ramisetty et al., 2018). Also for
ambient stratospheric particles, Schneider et al. (2021) have
shown that spectra from the ERICA-LAMS and ALABAMA are comparable.
We further investigated BaA particles, as BaA has been identified as a
component of soot (Lima et al., 2005). A characteristic example
of their mass spectra is shown in Fig. 9c. Therein,
the Cn and the CnHm pattern is clearly visible in both the
cation and the anion spectra, being indicative of polycyclic aromatic
hydrocarbons (PAHs; e.g., Hinz et al., 1999). Also,
the molecular peak at m/z 228 appears in the spectrum
(C18H12+). This observation is consistent with the typical
performance of mass spectrometers employing lasers with a wavelength of
266 nm, which results in less fragmentation as compared to those with a
wavelength of 193 nm (Thomson et al., 1997). The four examples
shown here demonstrate that the ERICA-LAMS provides valid single-particle
mass spectra that are comparable to those of other instruments in the
literature.
It is noteworthy that an important prerequisite for the later application of the
ERICA during airborne measurements was the capability to detect the presence
of gold particles in the sampled aerosols. Gold can be used as a marker for
self-contamination. By plating the sampling inlet with gold, it can safely
be assumed that if gold-containing particles are found, this indicates that
they have removed material from the inlet (Dragoneas et al.,
2022). To test the instrument's capability of measuring gold particles,
dispersions of gold spheres (dva= 3860 nm) were used. A typical bipolar spectrum is displayed in Fig. 9d. In
addition to the signal on m/z 197 from the Au+ cation, the peak of the
Au2+ cation on m/z 394 was consistently present, providing a
good indication that actual gold particles were detected, even in the
absence of an isotopic pattern or specific anion signal. The Na+,
K+, and Ca+ signals in the spectra can be attributed to the
residual buffer solution of the gold-particle dispersion. The identification
of particle types for which the evidence is based on hardly ionizable
substances, such as gold, is only possible if the content of well-ionizable
substances is moderate (Reilly et al., 2000), since
otherwise no Au signal might be obtained.
Mass spectral resolution
The mass spectral resolution RMS is a measure for the mass separation performance of the mass spectrometer and is defined as
RMS=ΔMM. The parameter ΔM is defined
as the full width at half maximum of M, i.e., the m/z value. Thus, a higher value of RMS indicates a better separation of the m/z peaks in the mass spectra. Appropriate separation is particularly necessary for the identification of neighboring nominal masses like m/z 39 and m/z 40 (for K+ and Ca+) as well as for signals caused by isotopes, e.g., elements such as tin and lead. In Fig. 10, details of two different
raw cation spectra from two ambient aerosol particles are presented. Here,
the output voltage signal of the digitizer is displayed as a function of the
digitizer sample number (1.6 ns per sample). The particles of the presented
spectra were recorded during the StratoClim campaign (July and August 2017)
at ground level at the airport of Kathmandu, Nepal. The signal intensities
correspond to the isotopic abundance of tin (Fig. 10a) and lead (Fig. 10b). The occurrence of both species can be expected in a polluted environment as in Kathmandu, Nepal. Out of these mass spectra, RMS of the ERICA-LAMS can be estimated to be 200 for cations at m/z 120 (Fig. 10a) and 700 at m/z 200 (Fig. 10b). For anion spectra we found an RMS of about 600 at both m/z 100 and m/z 200. The RMS values
of other single-particle mass spectrometers are comparable to the ones
presented here. Brands (2009) states for the ALABAMA a resolution of
200 for cations of m/z 108 and of 600 for anions of m/z 120. The
resolution of the A-ATOFMS (at m/z 100) is 500 for cations and 800 for anions (Pratt et al., 2009). Without any specific m/z value,
Gemayel et al. (2016) state for the LAAPTOF an RMS
of above 600 for both polarities.
Details of cation raw spectra (voltage output versus ion flight
time in the B-ToF-MS) of two ambient single particles at the airport of
Kathmandu, Nepal. (a) Tin isotopic pattern (dva= 277 nm). (b) Lead isotopic pattern (dva= 311 nm).
ERICA-AMS characterizationMass spectral resolution and data preparation
The ERICA-AMS mainly adopts elements of the commercial AMS from Aerodyne
(see Sect. 2.1). The observed mass resolution of 800
at m/z 200 during ambient aerosol sampling (see Sect. S6 in the
Supplement) is comparable with that of commercial C-ToF-MS instruments
(Drewnick et al., 2005). The conversion of the ion flight
time to an m/z is done using predefined calibration peaks. We use the peaks
for CH+, O2+, SO2+,182W+,
184W+, and 186W+, species for which the exact m/z
ratio is known and which occur in every spectrum due to their existence in
the vacuum background or outgassing of the heated tungsten filament. The
wide range of covered m/z values allows us to fit a relation of the three-parameter
time of flight to m/z, which is then valid for the whole
spectrum. The common Ar+ peak is not used because in measurements
shortly after evacuating the chamber, the residual organic peak at the same
nominal mass of m/z 40 can disturb the determination of the peak center.
The software integrates the signal at each particular m/z ratio to
generate a stick spectrum. The signal occurring between the m/z peaks is
used to estimate a baseline, which is subtracted during this integration.
Stick spectra are generated for measurements with open and closed shutters to
subtract the instrument background signal from the aerosol measurement
signal in order to obtain the aerosol contribution only. The difference
between the total and the background signal results in the aerosol signal.
The open–closed cycle is set to 10 s (see Sect. 2.4). A so-called “fragmentation table” is used to attribute the individual m/z peaks to certain species (air, organics, nitrate, sulfate, ammonium, and
chloride; Allan et al., 2004). The fragmentation table can be manually
adapted to compensate for instrument-specific deviations. Along with the
particles, a small fraction of the gaseous components are measured, which
still exhibit the most dominant peaks at m/z 28 (N2), m/z 30
(O2), and m/z 40 (Ar) in the mass spectrum (see
Fig. 11). A more detailed description of the evaluation procedure can be found in, e.g., Allan et al. (2004) and Fröhlich et al. (2013).
Example of an ambient aerosol average spectrum collected during the
field campaign in Kathmandu, Nepal (averaged over the entire campaign
period). Cumulative species (air, organics, nitrate, sulfate, ammonium, and
chloride) colored according to their fraction in the applied fragmentation
table.
Particle mass detection efficiency
Similarly to the determination of the optical detection efficiencies for PSL
and AN particles at PDU1 and PDU2 (see Sect. 3.2.2),
the particle mass detection efficiency for AN particles was determined at
the ERICA-AMS vaporizer for two cases: DEmax and DEKTM. Like with the optical detection efficiency, DEmax and DEKTM
combine the particle mass detection efficiency measurements with AN
particles described in Sect. 3.1.1 (see also Sect. S5.6 in the Supplement).
The parameter DEmax was determined for each measurement at the
ERICA-AMS vaporizer by re-inserting the determined set of parameters
(reff,V, σ, x0, and Ascan) of each curve fitting in Eq. (S17).
Maximum detection efficiency DEmax
(non-filled markers) and the detection efficiency DEKTM (filled markers; ADL setting during
field deployment in Kathmandu, Nepal) as a function of particle size
dva experimentally determined for AN particles
measured at the ERICA-AMS vaporizer. The d50 values of the AMS measurement lie outside the applied particle range. The uncertainties in DEmax and DEKTM reflect the conservatively estimated value of 10 %.
Figure 12 presents the maximum possible particle mass detection efficiency DEmax at ADL position x0 as a function of
the particle size dva. The DEmax values found for the measurements at the ERICA-AMS vaporizer are not comparable in absolute terms with the DEmax values found for the AN measurements at PDU1 and PDU2 (Fig. 7) since the measurements at the position of
the ERICA-AMS vaporizer are analogous to an ionization efficiency (IE)
calibration measurement (see Sect. 3.3.3). During
this IE calibration, among other losses, the transmission losses in the ADL
are compensated for. However, this measurement on the ERICA-AMS vaporizer
demonstrates that the decreasing DEmax values for smaller sizes at the PDUs are caused not by losses in the ADL but by the inability to detect small particles by adopted optical means. No d50 value could be determined for the measurements on the vaporizer. Even though the data point at 91 nm indicates a lower d50 cutoff, we assume that the particle size range in which the ERICA-AMS can measure is between ∼ 120 and 3500 nm, as specified by Xu et al. (2017) for the ADL type used here.
Figure 12 also shows the particle mass detection efficiency during field deployment in KTM DEKTM as a function of the
particle size dva at the ADL position xpos= 10.55 mm. The calculations of the parameter DEKTM are based on Eq. (S17) and are
shown in Sect. S5.6 in the Supplement. For the measurements at the
vaporizer, no d50 values can be determined because the results are
above 50 % of their maximum DEKTM values over the entire size range. The DEKTM at the vaporizer is 1 due to the normalization by the IE calibration, as explained above (see also Sect. 3.3.3).
Overall, the AMS part shows a fairly stable efficiency around 1 for the
examined size range after calibration with AN particles of 483 nm in size.
This is highly desirable to ensure the quantitative measurement of the AMS.
Ionization efficiency
By means of a calibration with a test aerosol of AN, the IE can be
determined and the peak areas obtained from integration can be converted
into a quantitative measure of the aerosol mass concentration of the
atmosphere. In order to determine the IE of the ERICA-AMS, in a first step
the average signal of a single ion must be measured. This is done by
considering single mass spectrum extractions. The assumption is that a
rarely occupied m/z signal has a very low probability of experiencing the
arrival of two ions in the same extraction. The peak area of these m/z
signals, averaged over multiple events where the signal is above the noise
threshold, then represents the average single-ion signal (SIS). The SIS is
given in units of millivolt nanoseconds (mV ns) and depends on multiple factors, mostly
the type and condition of the MCP detector, the applied high voltages and
the resulting field strengths, the temperature, and the gain of the signal
amplifier. After voltage adjustment of the MCP a SIS of around
0.8 mV ns was obtained.
The IE is determined with AN particles applying Setup B as described in
Sect. S3 in the Supplement (Fig. S8). The so-created monodisperse aerosol
is sampled by the instrument as well as by a CPC for reference. This
mass-based approach is similar to the one described in
Drewnick et al. (2005) and considers the transmission
efficiency through the ADL and the possible losses due to particle beam
divergence. As a reference zero, a measurement through a filter is
performed. The IE calibration factor in Tofware is then adjusted so that
the nitrate signal equals the nitrate mass load determined by the CPC. To
calculate the mass load from the CPC data, several corrections have to be
applied. For instance, doubly charged particles of a larger size are also
transmitted through the DMA due to the same electrical mobility, which will
also contribute to the mass load. To reduce this effect, we choose a rather
large particle size of 483 nm for the calibrations so that the
corresponding larger-sized particles of 814 nm are not generated by the
nebulizer in a high quantity. By measuring the concentration of singly
charged 814 nm particles and calculating the charge ratio generated by the neutralizer
according to Tigges et al. (2015), we correct for the
effect of doubly charged 814 nm particles (see Sect. S5.3 in the
Supplement). In addition the Jayne shape factor has to be applied
(Jayne et al., 2000). The IE is usually given for nitrate and
is strongly dependent on the flux of electrons for ionization. The ERICA
achieves an IE of 2000 ions pg-1, or 2.05 × 10-7 ions per molecule. This is lower than reported for the Aerodyne AMS
(Canagaratna et al., 2007),
partly due to operation at a lower filament emission current of 1.6 mA.
Other test aerosol species can be used to determine a species-dependent
relative ionization efficiency (RIE). The RIE of ammonium RIENH4 and the RIE for sulfate RIESO4 were determined by independent measurements
of AN particles and ammonium sulfate particles according to
Canagaratna et al. (2007). An averaged RIENH4 of 4.4 and RIESO4 of 0.97 were calculated. The default RIE values of the organic compounds (RIEorg=1.4) and for chloride (RIEChl=1.3) and for nitrate (RIENO3=1.1) were adopted from Canagaratna et al. (2007).
With the IE and RIE values, the ion count signal can be converted into an
aerosol mass. Together with the known flow into the instrument (ΦERICA= 1.48 cm3 s-1), the mass concentration of
the particulate matter is calculated (Canagaratna et al., 2007). Due
to the installed constant pressure inlet (Molleker et al.,
2020), which keeps the pressure in the ADL constant, the volumetric flow
into the instrument increases with decreasing ambient pressure. With the
assumption of a stable instrument temperature, this leads to a constant mass
flow or normal flow (NTP, 20 ∘C and
1013 hPa). Thus, the dimension of the measurement result is mass per normal
volume.
Detection limits
Several methods can be used to determine the detection limit (DL) for the
species measured by an AMS as described by Drewnick et
al. (2009). One approach is the calculation based on the ion-counting
statistics during a measurement with the shutter closed (closed signal),
denoted as DLstat. The most common approach is a measurement of the signal noise during a measurement of filtered air, denoted as DLfilter.
Especially during in-flight measurements, this filter-based method cannot be
representative of the whole flight due to changing vacuum, temperature, and
instrument background conditions. Thus, for field measurements a detection
limit DLspline was calculated from the closed signal after applying a spline-based detrending method comparable to
Schulz et al. (2018) and Reitz (2011). In each case the DL is defined as 3 times the standard deviation of the respective signal. The detection limits of all species are given in Table 1 for each method. The statistical
approach as well as the filter-based method is based on a long-term filter
measurement in the lab, while DLspline was determined from the measurements during the StratoClim 2017 campaign. The differences are reasonable because DLstat does not consider interferences with other species, especially water and air, whereas DLspline was measured under
different conditions regarding pumping time and consequently instrument
background. The detection limits are slightly higher than reported for other
airborne instruments (e.g., Schulz et al.,
2018) due to not only a different time basis but also a rather strong air beam
signal in our instrument (see Sect. 3.3.5).
Detection limits of the species measured by the ERICA-AMS
determined with several methods. DLstat and
DLfilter measured under lab conditions.
DLspline measured during the StratoClim field
campaign. The limits are given for one measurement cycle (10 s) and are
expected to reduce with longer averaging times t proportionally
to 1/t.
SpeciesDLstatDLfilterDLspline(µg m-3)(µg m-3)(µg m-3)Chloride0.130.240.09Ammonium0.050.40.73Nitrate0.110.120.12Organics0.180.520.5Sulfate0.00370.060.13Air beam and water signal
The ADL is supposed to focus particles into a narrow beam into the vacuum
chamber while the air molecules are strongly diverging after the end of the
lens. However, some of the air is also propagating towards the ion source
and generates ions at m/z ratios of 14 (N+), 16 (O+), 28
(N2+), 32 (O2+), 40 (Ar+), and 44 (CO2+)
as well as the corresponding isotopes. This signal, the so-called “air beam”
signal, can on one hand be used for diagnostic purposes but on the other
hand introduces uncertainties into measuring particle signals at the
corresponding m/z. An air beam signal as small as possible is thus
desirable, e.g., to reduce the detection limit of aerosol species. In the
ERICA-AMS, we experienced a rather strong air beam signal of around
2.9 × 106 ions s-1 (see Fig. 11). This is larger than reported by Canagaratna et al. (2007) (1.5× 106
to 2.5 × 106 ions s-1), with a 5-fold higher IE value
at the same time. We found out that the reason lies in the assembly of the
ERICA. Since the front part of the instrument was optimized for laser
ablation mass spectrometry, a rather large conical skimmer with an inner
diameter of 1.9 mm was built in after the ADL for the separation of air and
particles. While this causes no problem for the laser ablation part, it
leads to a substantial transfer of air molecules towards the following
stages of the vacuum chamber. For improvement, a newly designed skimmer with
an opening of 1 mm and a channel of 21.5 mm length was implemented in order
to reduce the air beam signal by a factor of 6.7, resulting in 4.4 × 105 ions s-1. Since this skimmer was implemented in 2019, earlier campaigns, like StratoClim 2017, were conducted with the large air beam signal. Additionally, interferences of particle signals with the signal of residual water influence the detection limit of ammonium. Here, the background water vapor in the vacuum plays a role. We experience an intense water signal of 2.5 × 106 up to 1 × 107 ions s-1 depending on instrument temperature and pumping time. This water signal occurs independently of the shutter position and thus does not directly relate to the air beam streaming into the instrument but to the background vacuum conditions.
First aircraft-borne measurements
The first field deployment of the ERICA was during an aircraft field
campaign as part of the StratoClim project. The main objective of the
StratoClim project was to produce more reliable predictions of regional and
global climate change through a better understanding of key microphysical,
chemical, and dynamical processes in the UTLS of the Asian monsoon (Rex et
al., 2016; http://stratoclim.org, last access: 21 February 2022). During the
two aircraft field campaigns (43 flight hours), over 150 000 single-particle
mass spectra were recorded and the ERICA-AMS provided reliable data for
about 31.2 h. By means of a satellite communication link to the
operators (Dragoneas et al., 2022), the time of data losses could
be kept low with 29 min for the ERICA-AMS and 39 min for the
ERICA-LAMS. The first aircraft campaign took place in Kalamata, Greece, in
August and September 2016 and the second in Kathmandu, Nepal, in July and
August 2017. The high-altitude research aircraft M-55 Geophysica served as platform
for these campaigns. During its first deployments, the instrument was
fully automated and operated during 11 research flights from ground pressure and
temperature to 20 km altitude at 55 hPa and ambient temperatures as low
as -86 ∘C. It was the first time that bipolar single-particle mass spectra were measured at altitudes above 16 km. Also, the
ERICA-AMS was the first AMS-type mass spectrometer that was successfully
deployed to measure at such high altitudes. The analyses of the research
flight data presented in this study serve to provide a proof of concept for
the ERICA, as well as to document its operational reliability and performance,
without the purpose to provide details on the results connected with the
scientific objectives. Detailed results from the aircraft field campaigns
can be found, for example, in Höpfner et al. (2019),
Schneider et al. (2021), and Appel et al. (2022). In the following, data
examples from the second aircraft campaign of StratoClim 2017 in Kathmandu
(KTM) are shown.
Exemplary single-particle stick spectrum recorded during StratoClim
2017 demonstrates the feasibility of identifying metallic isotopes. (a) Cations; (b) anions. This particle containing heavy metal and sulfate was measured at an altitude of 20 402 m (29 July 2017, 06:09:34 UTC, dva= 602 nm).
A selected bipolar single-particle mass spectrum containing heavy metal
signatures is presented in Fig. 13. The mass spectrum
shows signals of light metals like sodium, magnesium, aluminum, and calcium,
showing that the ERICA-LAMS is able to identify metals by their isotopic
patterns. Furthermore, sulfate fragment ions and heavy metal ions of
chromium, iron, molybdenum, and tungsten are present. The identification of
iron, molybdenum, and tungsten was done by comparing the signal intensity
patterns with those of the natural abundance of the isotopes of the
elements. The presence of molybdenum could be confirmed by signals for
MoO+, which has the same isotopic ratio as Mo+. This particular
mass spectrum was recorded at an altitude of ∼ 20 km
(a.m.s.l.) on 29 July 2017. Attributing this single particle to a certain
source is difficult. However, an anthropogenic source as an exhaust of an
aircraft engine, in which tungsten–molybdenum alloys are in use
(Guan et al., 2011), is conceivable due to its heavy metal signals.
We use the hit rate (HR; see Sect. 3.2.3 for
definition and limitations of the HR) as a function of altitude to determine
whether the ERICA-LAMS can measure over the entire altitude range sampled.
The parameter HR is instrument specific and independent of both the
aircraft residence time and ambient particle number concentration. Figure 14 shows the HR vertical profile for the
entire second aircraft campaign in 500 m bins. Here, the HR values are
between 0.1 and 0.3 over the entire altitude range. At maximum altitude, the
HR is 0.24. These results demonstrate that single-particle mass spectra can
be recorded both on the ground and at altitudes up to more than 20 km.
Variations in HR values may be due to differences in aerosol composition,
size, and shape at different altitudes (Su et al., 2004; Brands et al.,
2011). In addition to the HR, the number of recorded single-particle mass
spectra Nspectra and the number of ablation laser shots Nshots also show that mass spectra can be recorded in all sampled altitude ranges (up to 20.5 km; Fig. 14). However, Nspectra and
Nshots depend on the residence time of the aircraft at the respective flight altitude, which was long at altitudes above 15 km and also below 5 km.
Vertical profile of the hit rate HR (black, bottom abscissa), the number of recorded spectra Nspectra (blue, top abscissa), and number of ablation laser shots Nshots (red, top abscissa) for the entire second aircraft campaign in 500 m bins. Uncertainties in HR, Nspectra, and Nshots are based on counting statistics.
After demonstrating that it is possible to measure with the ERICA at flight
altitudes up to about 20 km, in the following we show that aerosol species
known in the literature can be identified with both the ERICA-LAMS and the
ERICA-AMS. The evaluation of the data was carried out separately for the
ERICA-LAMS and the ERICA-AMS. For the ERICA-AMS, the species reported in
Sect. 3.3.1 were quantified. To determine specific
particle types of the single particles, the ERICA-LAMS data set was
processed with the software CRISP (Klimach, 2012) using the k-means
clustering algorithm as described in Roth et al. (2016). In this processing, all single-particle mass spectra were pre-sorted
into a predefined number of so-called clusters and then manually combined
into meaningful particle types. With this approach, two particle types (in
addition to other particle types not included in this publication) well
described in the literature were found: a meteoric-material-containing
(e.g., Schneider et al., 2021) and an elemental carbon (EC)-containing particle type (e.g., Pratt and Prather, 2010).
To identify the sulfate-containing particle type, the ERICA-LAMS data set
was filtered for single-particle spectra that contained sulfate marker
signals at m/z-96 (SO4-) or m/z-97 (HSO4-) or both
markers. Since these sulfate marker signals are also found in the meteoric-material-containing particle spectra, by this approach, the meteoric-material-containing particle type is a subtype of the sulfate-containing
particle type. In the following, first, we focus on the aerosol composition
at high altitudes (> 10 km), considering particulate sulfate as
well as the meteoric-material-containing particle type.
Vertical profile (flight on 4 August 2017) of (a) the particle number fraction of sulfate-containing single particles (black; ERICA-LAMS), (b) the mass fraction of sulfate (red; ERICA-AMS), and (c) the particle number fraction of meteoric material-containing single particles (gray; ERICA-LAMS). The vertical resolution is in altitude bins of 500 m. The uncertainties in the particle number fraction are calculated from counting statistics. The uncertainty in the mass fraction is based on the background measurement and was propagated for the mass fraction. The dashed horizontal blue line marks the cold-point tropopause (CPT).
Figure 15a shows the vertical profile of the particle
number fraction of the sulfate-containing single particles. It has to be
noted that the ERICA-LAMS is capable of measuring sulfate species of
non-refractory and refractory types but cannot distinguish between both
types. A particle number fraction is the fraction of a particle type out of
all mass spectra recorded in the respective altitude bin (bin size 500 m).
In the vertical profile of the research flight of 4 August 2017, a large number fraction of about 0.6 of the sulfate-containing single particles can be seen between 10 and 17 km (ERICA-LAMS), which increases with higher altitudes up to a maximum value of 1.
Non-refractory sulfate (Canagaratna et al., 2007)
measured by the ERICA-AMS consists mainly of pure sulfuric acid in the
stratosphere (Murphy et al., 2014). The mass
fraction is the calculated fraction of the mass concentration of sulfate
over the total mass concentration determined by the ERICA-AMS for each
altitude bin. In Fig. 15b, the vertical profile of
the sulfate mass fraction is depicted. The profile shows an enhancement,
above the cold-point tropopause (CPT; 17 km), at altitudes starting at
17.5 km. At 20 km altitude, the non-refractory aerosol sulfate mass fraction is 1. A high sulfate mass fraction can be expected due to the proximity of the Junge layer, where the aerosol particles mainly consist of pure sulfuric acid (Junge and Manson, 1961; Murphy et al., 2006b). Since no other species, such as nitrate or organics, were observed by the ERICA-AMS in
significant amounts at this altitude, the convective and radiatively driven
vertical transport within the Asian monsoon anticyclone
(AMA; Ploeger et al., 2015) does not play as much of
a role here anymore, as further detailed below.
As identified and described by Murphy et al. (1998) and
Cziczo et al. (2001), the meteoric-material-containing
particle type is characterized by a high abundance of magnesium (Mg+,
isotopes at m/z 24, m/z 25, and m/z 26) and iron (Fe+, isotopes
at m/z 56 and m/z 54) signals in the cation spectrum and of sulfate
(HSO4- at m/z-97) in the anion spectrum. The occurrence of the
described characteristic signals in the single-particle mass spectra of the
ERICA-LAMS and the dominant presence of the meteoric-material-containing
particle type at high altitudes (> 17 km) were already described
by Schneider et al. (2021). The mean spectrum can be found in Sect. S7 in the Supplement. Figure 15c exemplarily shows the abundance of meteoric
material in the vertical profile of the research flight on 4 August 2017 in the particle number fraction of the meteoric-material-containing particle type. The particle number fraction is larger than 0.6 above 19.5 km and reaches its maximum of 0.8 at the maximum flight altitude of the research flight. The increase in particle number fraction of the described meteoric particle type at high altitudes is also described for measurements with other mass spectrometers, like PALMS (Murphy et al., 2014) and the ALABAMA (Schneider et al., 2021). Furthermore, similar particle number fraction values of up to 0.6 were also reported for a similar particle type recorded in the mid-latitude stratosphere by Murphy et al. (2014).
The demonstrated results of the meteoric-material-containing particle type
can be considered an indication of the reliable operation of the ERICA-LAMS
at high altitudes such as up to 20 km.
The measurements of the two instrument parts, the ERICA-LAMS and ERICA-AMS, were
evaluated separately, and the derived results complement each other. Pure
sulfuric acid cannot be ablated with the frequency-quadrupled Nd:YAG laser
(wavelength 266 nm) used in the ERICA-LAMS because light of this wavelength
is not efficiently absorbed by the particles (Murphy, 2007).
Vice versa, the meteoric particles consist of refractory components that can
be detected by the ERICA-LAMS but not by the ERICA-AMS. The analyses
presented here as examples show that the ERICA can be used by means of the two complementary
measurement methods to measure
aerosol components, such as sulfuric acid and meteoric material, that are
significantly present in the stratosphere.
The results can also be used to show that the aerosol composition and mixing
state between 10 and 17 km differ from those above 17 km. For this, the
mass fraction of sulfate (ERICA-AMS) and the number fraction of
sulfate-containing single-particle spectra (ERICA-LAMS) were examined
(Fig. 15). Below 17 km, the number fraction of
sulfate-containing single-particle spectra is stable at around 0.6 and the mass
fraction of sulfate in the non-refractory aerosol is less than 0.2. This
indicates that many particles contain sulfate, although typically only in a small
mass fraction (about one-third on average), because they are internally mixed with nitrate and organics. Above 17 km, with increasing altitude, the sulfate
mass fraction and the particle number fraction of sulfate-containing single
particles both increase up to 1. The observed change in the mass fraction is
stronger compared to the increase in the number fraction of
sulfate-containing single particles. Since the two measurement methods
not only provide different views on the aerosol but also have different
limitations, this observation must be interpreted with care. A possible
interpretation for the increasing sulfate mass fraction could be that within
the internally mixed aerosol of particles containing a refractory core, e.g., of meteoric dust, and a sulfuric acid coating (Murphy et al., 2014), the coating grows as a consequence of further condensation. However, since the ERICA-LAMS is not capable of measuring pure sulfuric acid particles (Murphy,
2007), it is also possible that partial external mixing of the internally
mixed particles with sulfuric acid particles causes this observation.
Data from the research flight on 8 August 2017 during StratoClim,
Nepal. The vertical resolution is in altitude bins of 500 m. The horizontal blue
line marks the cold-point tropopause (CPT). The Asian tropopause
aerosol layer (ATAL), the free troposphere (FT) and the boundary layer (BL)
are indicated. (a) The mean mass spectrum of 340 EC-containing single particles. (b) The vertical profile of the particle number fraction of EC-containing single particles (ERICA-LAMS). The uncertainty in the particle number fraction is calculated from counting statistics. (c) The vertical profile of the median total mass concentration Ctotal (NTP; ERICA-AMS). The interquartile range of the median total mass concentration Ctotal is shaded in gray.
As described above, the EC particle type was identified using the k-means
clustering for the data set. The EC particle type is characterized by a
Cn+ pattern in the cation and a Cn- pattern in the
anion spectrum (e.g., Hinz et al., 2005). Figure 16a shows the mean spectrum of the recorded EC-particle-type mass spectra (total number 389) during the StratoClim research
flight of 8 August 2017. Here, the described signal pattern is evident in both polarities. Figure 16b displays the vertical
distribution of the particle number fraction of all EC-containing particles
in the research flight (vertical bin size 500 m). As expected, the particle
number fraction of EC is enhanced in the lowest 6 km with a value of around
0.05. EC is created as primary aerosol by combustion processes as part of
soot at low altitudes (Turpin et al., 1991; Seinfeld and Pandis, 2016).
Combustion is a common source of air pollution in Nepal (Saud and Paudel,
2018; Sadavarte et al., 2019). Field measurements with the established
single-particle mass spectrometer A-ATOFMS, which is comparable to the ERICA,
were conducted in the USA. Pratt and Prather (2010) also found a
stable EC particle number fraction of around 0.05 in the altitude range
of 1 to 6 km. This comparison with the A-ATOFMS shows that the ERICA
provides credible results at low altitudes. We observed another enhancement
of the EC particle number fraction in the altitude range between 7 and 15 km
and assume that the occurrence of EC-containing particles in this altitude
range can be caused either by local emitters, such as aircraft
(Liu et al., 2017), or by vertical transport, such as the
convective outflow of the Asian monsoon (Garny and Randel,
2016). Above 16 km, the EC particle number fraction is very low, ranging
around 0.01.
Pure soot is a refractory compound and, consequently, cannot be detected by
the ERICA-AMS (Canagaratna et al., 2007). On the other hand, the ERICA-AMS is capable of providing quantitative mass concentration of the non-refractory components of ambient aerosol and thus is well suited for the identification of particle layers by quantitative means. The total ERICA-AMS mass concentration Ctotal is defined as the sum over all non-refractory aerosol species. Figure 16c depicts the vertical profile of Ctotal for the research flight on 8 August 2017. An enhancement in the total mass concentration is clearly evident for altitudes from ground level to approximately 3.5 km and can be associated with anthropogenic emissions at the ground. This layer can be seen as the boundary layer. In the boundary layer, we found during the flight (monsoon season measurement) a maximum Ctotal of 6.9 µg m-3 at an altitude of 2 km. At ground level, a Ctotal of 4.8 µg m-3 was found for this flight. Pre-monsoon season PM2.5filter measurements (April 2015) in the Kathmandu valley show typical Ctotal values of between 30.0 and 207.4 µg m-3 (Islam et al., 2020) at ground level. Due to particle-scavenging processes,
Ctotal is lower during the monsoon season
(Hyvärinen et al., 2011). The second enhancement (at altitudes between 15.5 and 19.5 km) with a maximum of 2.8 µg m-3 can be associated with the Asian tropopause aerosol layer
(ATAL; e.g., Vernier et al., 2011; Höpfner et al., 2019). In the free
troposphere (at altitudes between 4 and 16 km), Ctotal goes down to approximately 1 µg m-3.
The results from the non-refractory Ctotal can be discussed together with the particle number fraction of the refractory EC particle type to provide complementary information about the sampled aerosol particles.
Within the boundary layer, as measured by the ERICA-AMS, Ctotal
decreases whereas the EC particle number fraction is stable, as in the free
troposphere. This indicates, within the limitations of the applied methods,
that the EC particle type is well mixed within the boundary layer and in the
free troposphere, although Ctotal changes. In the ATAL (> 16 km), EC particles seem to play a minor role in the composition of the aerosol, while for the convective outflow levels (< 16 km), the data
suggest an increase in the EC particle number fraction as a result of
detrainment. (This StratoClim flight on 8 August 2017 was performed at a time
of high convective activity and in the presence of large cloud systems above
the Himalayan foothills.) An example of single-particle information, which the
ERICA-LAMS is capable of delivering, is provided in Sect. S8 of the
Supplement. Due to the lack of a chopper, no particle size information can
be determined by the ERICA-AMS.
Overall, the studies presented here confirm that the ERICA can be adopted
for aircraft missions from ground level up to an altitude of 20 km and
operates reliably under demanding field conditions. A more comprehensive
evaluation of the collected data will be conducted in further studies.
Summary and outlook
In this study we present a novel aerosol mass spectrometer combining the LDI
technique (ERICA-LAMS; quadrupled Nd:YAG laser at λ= 266 nm) with the TD-EI technique (ERICA-AMS; vaporizer operated at a temperature of 600 ∘C, electron impact energy of 70 eV). These techniques are
implemented in two consecutive instrument stages that are connected in
series within a common vacuum chamber. The use of a common vacuum chamber
and other components for both measurement techniques minimizes the weight and
volume of the instrument. The resulting compact dimensions enable the
instrument to be deployed on aircraft, at ground stations, and in mobile
laboratories. By that, the same aerosol sample can be investigated with two
different physical methods. The chemical characterization of single
particles is achieved by recording bipolar mass spectra with a B-ToF-MS. By
deploying both methods, complementary chemical information can be obtained.
By means of the LDI technique, single particles consisting of refractory or
non-refractory components are qualitatively analyzed, while the TD-EI
technique provides quantitative information on the non-refractory components
(i.e., particulate sulfate, nitrate, ammonium, organics, and chloride) of
small particle ensembles. The cations generated by the TD-EI technique are
detected with a C-ToF-MS.
Comprehensive laboratory measurements with PSL and AN test aerosol were
conducted to characterize the key instrumental parameters. Focused laser
beams of the PDUs and the ablation laser beams as well as the particle beam
were investigated. In order to determine the particle beam characteristic
parameters, ADL position scans with particles of various sizes were
performed. The parameters presented in this publication are as follows: the PDU and
ablation laser beam waist radii (w0,dia), the particle beam width (wpart), the effective detection radius of the PDUs (reff,L) and of the vaporizer (reff,V), and the particle beam overlap parameters (Sdetect,L, Sdetect,V, and Sablation), each as a function of
particle size. Extensive information about the beam characteristics was
obtained and shows the performance of the ERICA. Here, 1σ overlap of
the particle beam with the detection laser spot for particle sizes between
213 and 3150 nm was found. The installed ADL is described in the
literature (Peck et al., 2016; Xu et al., 2017) and covers a particle
size range of ∼ 120 to 3500 nm (d50). We found that
the particle beam hits the vaporizer completely even at sizes as low as 91 nm. The evaluation of the particle beam shift resulted in two cases of the optical particle detection efficiency due to a non-concentric focusing of all particle sizes: the maximum optical detection efficiency (DEmax) that theoretically can be achieved and the optical detection efficiency during the field campaign in Kathmandu (DEKTM). The characterization shows that DEmax at the PDUs reaches a value of up to 1.00 compared to a
reference instrument in a laboratory setup and shows an optical detectable
size range of 180 to 3170 nm (d50) for PSL particles. During the
field campaign in Nepal the optical particle detection efficiency DEKTM reached up to 0.86. We found d50 values for the DEKTM of 190
and 745 nm for PSL particles (at PDU1). Particle time-of-flight calibration
was performed for particle sizes between 80 and 5145 nm. The evaluation
of scattered light intensities for particle size determination is also
conceivable but has not been implemented yet.
The capabilities of the ERICA were tested in field and laboratory
experiments. After the adjustment preparation procedure as conducted before
any field campaign, a ground-based field experiment was conducted to
determine the size-resolved HR of the ERICA-LAMS. The result was a maximum
HR of 0.52 for a particle size of around 230 nm. The outcome of this
experiment reflects the results of the particle beam characterization
measurements. In addition, we measured pure chemical substances from
solutions or suspensions in order to validate that ERICA-LAMS raw mass
spectra can be m/z calibrated by the software CRISP correctly. Besides sodium chloride, ammonium nitrate, and benz[a]anthracene, gold spheres were
sampled. All substances could be identified by their specific marker peaks
in the mass spectra after CRISP processing. Furthermore, mass spectra
resolution RMS values of 200 for m/z 120, 700 for m/z 200 (both cations), and about 600 for the anion spectra were determined and are
comparable to similar single-particle mass spectrometers. For the ERICA-AMS,
RMS was determined by the evaluation software Tofware to be 800 for m/z 200, which is also comparable to other C-ToF-MSs. The conversion of the ion time of flight into a mass spectrum is based on six predefined
calibration peaks. A major difference from a commercial AMS instrument is
that the ERICA-AMS features a shutter instead of a chopper. By means of the
shutter, the background signal (shutter closed) can be determined and then
subtracted from the “shutter open” signal. The fragmentation table
implemented in Tofware allows the determination of various species, such
as organics, nitrate, sulfate, ammonium, and chloride. By means of an IE
calibration, the determined sample signal can be turned into an aerosol mass
concentration. The IE calibration procedure was conducted with monodisperse
AN particles using a CPC as a reference device and yielded
2.05 × 10-7 ions per molecule. For the detection limits, results for five aerosol particle species were obtained and presented for three different methods. Also, for the StratoClim 2017 campaign an air beam signal of 2.9 × 106 ions s-1 and a water signal of between
2.5 × 106 and 1 × 107 ions s-1 were found.
Subsequent modification of a skimmer reduced the air beam by a factor of 6.7
for future instrument deployments. The losses in mass due to particles being
ablated and hence not contributing to the ERICA-AMS signal were determined to be
low and within the AMS's measurement uncertainties of 30 % for most
atmospheric conditions. However, for low particle concentrations the losses
have to be considered. To quantify these losses, the operation of the
ERICA-LAMS part would need to be paused, at least intermittently, to enable
undisturbed quantitative measurements by the ERICA-AMS. This procedure can
be implemented in the automated mode. With a similar mode, it would be
possible to investigate the fraction of charged ambient particles by
switching the HV switch on and off at defined intervals.
The two aircraft field campaigns as part of the StratoClim project in 2016
and 2017 were the first field deployments of the ERICA. This was the first
time an AMS-type mass spectrometer was deployed above 16 km, as well as the
first time bipolar single-particle mass spectra were recorded at these altitudes.
Mass spectra examples from high altitudes presented here agree with spectra
presented in the literature and show that the ERICA delivered reasonable data
even under field conditions during autonomous operation aboard a research
aircraft. For the ERICA-LAMS, the meteoric-material-containing particle
type and, for the ERICA-AMS, the sulfate species are used for a proof of
concept of the operation at stratospheric altitudes. For low altitudes, down
to ground level, the EC particle type and total mass concentration serve as
examples of the capabilities of the ERICA-LAMS and ERICA-AMS, respectively.
The vertical profiles of these species and additionally of the HR show a
reasonable instrument performance over the entire altitude range from ground
level up to 20 km. In this study, we also show that the ERICA-LAMS and ERICA-AMS
can provide complementary information about the sampled aerosol. Some
limitations of one ionization method can be partially compensated for by the
other.
Although the ERICA-LAMS and ERICA-AMS combination was developed for the
aircraft deployment within the ATAL and the combination has been shown to
perform reliably in field campaigns, in the future, modifications could be
made to the instrument to address other scientific questions. One
modification might be the implementation of another laser type such as an
excimer laser for measurements in the lower stratosphere
(Murphy et al., 2007). While this is possible for the
ERICA as well, space and weight limitations inherent in the implementation
prevented the use of an excimer laser setup on the M-55 Geophysica. However, the light at the longer ablation laser wavelength generates less fragmentation in the mass spectra (Thomson et al., 1997). Furthermore, the mass spectra recorded with the ERICA are to a higher degree comparable with
instruments like the A-ATOFMS (Gard et al., 1997) and the
ALABAMA (Brands et al., 2011), which also operate
with an ablation laser at a wavelength of 266 nm.
In another upcoming further development, an additional single-particle mode
for the ERICA-AMS will be added, which will be based on optical particle
detection. As with the ERICA-LAMS, a single particle is optically detected by the PDUs,
and by means of the TC the point in time is calculated when the particle
hits the vaporizer. For the same point in time, the data acquisition card is
triggered, and the single-particle mass spectrum is recorded. For the ERICA
this mode is called the optically triggered AMS (OT-AMS) mode. With the method
of the OT-AMS mode, it is possible to quantify the non-refractory components
of single particles when the ablation laser is in idle mode. This method is
similar to the procedure with a light-scattering probe on the AMS (Cross
et al., 2007; Freutel et al., 2013). In addition, the size information of
the measured single particle is obtained by means of the particle flight
time between the two PDUs. One possible future investigation by means of the
OT-AMS mode is the ablation laser's effect on the particles that are only
partly ablated and where the residuals reach the vaporizer of the ERICA-AMS.
This investigation is only possible with a unique feature, the serial
configuration of the SMPS and AMS, as in the OT-AMS mode. A method has to be
developed to ensure the linkage of the results to the very same particle.
Such a procedure needs more implementations and further laboratory studies.
The presented examples of field measurements showed that the instrument has
already been successfully operated during the aircraft campaign of the
StratoClim project. The evaluation of the data is ongoing and will be
presented in further publications. Furthermore, the ERICA was successfully
deployed during the ND-MAX/ECLIF 2 (NASA/DLR-Multidisciplinary Airborne
eXperiments/Emission and CLimate Impact of alternative Fuel; Voigt et al.,
2021) field campaign in January to February 2018
(Schneider et al., 2021) and
during the ACCLIP (Asian summer monsoon Chemical and Climate Impact Project)
test phase in January and February 2020. The main campaign will be set up in
July to August 2022 based in South Korea
(https://www.eol.ucar.edu/field_projects/acclip, last access:
21 February 2022).
Data availability
Data can be accessed by contacting the corresponding author Stephan Borrmann (stephan.borrmann@mpic.de).
The supplement related to this article is available online at: https://doi.org/10.5194/amt-15-2889-2022-supplement.
Author contributions
SB provided the instrumental concept and an initial design in his ERC Advanced Grant proposal. SB, FD, and JS initiated the instrumental design and accompanied its development and characterization. FH and TK designed the detection units. OA, TB, AD, AH, and SM developed the
instrument. OA, AD, AH, and SM performed the described measurements in the
field and in the lab. OA and AH evaluated the data. The lens scan evaluation
method was developed by TK. HCC initiated and accompanied the implementation
of the HV switch and the electric shielding of the ion optic as an essential
improvement. AH, together with SB, OA, AD, FK, and SM, drafted the
manuscript. All co-authors provided detailed comments on the manuscript.
Competing interests
At least one of the (co-)authors is a member of the editorial board of Atmospheric Measurement Techniques. The peer-review process was guided by an independent editor, and the authors also have no other competing interests to declare.
Disclaimer
Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Acknowledgements
We gratefully thank the workshops of the Max Planck Institute for Chemistry
and of the Institute for Atmospheric Physics (Johannes Gutenberg University Mainz) and
Tofwerk AG, in particular Christian Gurk, Helmut Schreiber, Bastian Meckel, David Göttert, Sebastian Best, Joachim Sody, and Urs Rohner, for the essential support. The help of Mike Cubison for customizing Tofware is gratefully acknowledged. Special thanks are due to Wen Xu and Philip Croteau from Aerodyne Research Inc. for the
specification measurements of the ADL deployed. We would like to express our
gratitude to Fred Stroh for his extraordinary commitment to the realization of the field campaigns and to Markus Rex for managing the entire StratoClim
project. Our special thanks are extended to the crew of MDB (Myasishchev
Design Bureau) and the M-55 Geophysica pilots. We would like to thank the Hellenic Air Force and the
Hellenic Civil Aviation Authority for their co-operation in the organization
of the first aircraft campaign of the StratoClim project. Especially, we
would like to give credit to Konstantinos Chinis (wing commander),
Alexandros Kefalas (wing deputy commander), Ioannis Kitsios (MRO director),
and the personnel of the 120 Air Training Wing in Kalamata, Greece, for
providing the best possible support during the field campaign that took
place at their air base in August and September 2016. We extend our sincere
thanks to the officials of the Nepalese government authorities, research institutions involved, and Tribhuvan Airport as well as of the German embassy in Nepal for
their extraordinary support and hospitality, which made the StratoClim field
campaigns and our research possible. We thank the
reviewers for their detailed and helpful suggestions to improve the
manuscript.
Financial support
This research has been supported by FP7 Ideas: European Research Council (EXCATRO – In-situ experiments on the chemical composition of high altitude aerosols and clouds in the tropical upper troposphere and lower stratosphere (grant no. 321040) and STRATOCLIM – Stratospheric and upper tropospheric processes for better climate predictions (grant no. 603557)) and the Bundesministerium für Bildung und Forschung (joint ROMIC project SPITFIRE (grant no. 01LG1205A)).The article processing charges for this open-access publication were covered by the Max Planck Society.
Review statement
This paper was edited by Troy Thornberry and reviewed by Nicholas Marsden and two anonymous referees.
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