Earth's radiation budget is affected by new particle formation (NPF) and the growth of these nanometre-scale particles to larger sizes where they can directly scatter light or act as cloud condensation nuclei (CCN). Large uncertainties remain in the magnitude and spatiotemporal distribution of nucleation (less than 10 nm diameter) and Aitken (10–60 nm diameter) mode particles. Acquiring size-distribution measurements of these particles over large regions of the free troposphere is most easily accomplished with research aircraft.
We report on the design and performance of an airborne instrument, the nucleation mode aerosol size spectrometer (NMASS), which provides size-selected aerosol concentration measurements that can be differenced to identify aerosol properties and processes or inverted to obtain a full size distribution between 3 and 60 nm. By maintaining constant downstream pressure the instrument operates reliably over a large range of ambient pressures and during rapid changes in altitude, making it ideal for aircraft measurements from the boundary layer to the stratosphere.
We describe the modifications, operating principles, extensive calibrations, and laboratory and in-flight performance of two NMASS instruments operated in parallel as a 10-channel battery of condensation particle counters (CPCs) in the NASA Atmospheric Tomography Mission (ATom) to investigate NPF and growth to cloud-active sizes in the remote free troposphere. An inversion technique to obtain size distributions from the discrete concentrations of each NMASS channel is described and evaluated.
Concentrations measured by the two NMASS instruments flying in parallel are self-consistent and also consistent with measurements made with an optical particle counter. Extensive laboratory calibrations with a range of particle sizes and compositions show repeatability of the response function of the instrument to within 5–8 % and no sensitivity in sizing performance to particle composition. Particle number, surface area, and volume concentrations from the data inversion are determined to better than 20 % for typical particle size distributions. The excellent performance of the NMASS systems provides a strong analytical foundation to explore NPF around the globe in the ATom dataset.
Particles play important roles in chemical and physical processes in the
atmosphere: they provide sites for heterogeneous reactions (Ravishankara,
1997), they serve as nuclei for the formation of clouds, and they directly
and indirectly affect the Earth's radiation budget (Solomon and IPCC Working
Group Science, 2007). Many primary particles, those directly emitted into
the atmosphere in the solid or liquid phase, affect the radiation budget by
acting as cloud condensation nuclei (CCN) or directly scattering or
absorbing sunlight. However, secondary particles, those formed by nucleation
from the gas phase in the atmosphere, often dominate both aerosol–cloud and
aerosol–radiation interactions (Kulmala et al., 2004). By number, the
majority of the particles present in the troposphere in most environments
have diameters
Once secondary particles have grown to diameters greater than about 50 nm, they often serve as CCN under conditions of water supersaturation common in the lower troposphere (Seinfeld and Pandis, 2006). However, the uncertainty in the contribution of secondary particles to global CCN abundance is very high, with recent estimates ranging from 5 % (Wang and Penner, 2009) to 60 % (Yu and Luo, 2009). This uncertainty stems, at least in part, from poorly constrained new particle formation (NPF) mechanisms in the free troposphere. These mechanisms determine not only the nucleation rate (which may only be of minor importance, Westervelt et al., 2014), but more importantly the spatiotemporal distribution of freshly nucleated particles, which directly affects the number and distribution of secondary CCN (e.g. Merikanto et al., 2009). Measurements of the spatio-temporal distribution of nucleation mode aerosol in the atmosphere can be used to infer the contribution of different NPF mechanisms and condensable vapours to formation and growth of these particles (Yu et al., 2010; Kazil et al., 2010) . Understanding how much gas-phase species from anthropogenic origins contribute to these processes in comparison to species that have natural origins and may have been present in the pre-industrial era will enable us to better constrain aerosol–cloud interactions in the pre-industrial atmosphere (Carslaw et al., 2017). Measuring newly formed particles and their growth in pristine areas of today's atmosphere can help us understand the contribution of these processes to the Earth's pre-industrial radiation budget, and therefore improve our estimates of aerosol radiative forcing.
Since new particles form at initial diameters around 1 nm (Kulmala et al.,
2000) they must undergo significant growth to become CCN, increasing their
diameter by
We are measuring the global distributions of aerosols on the Atmospheric
Tomography (ATom) mission (
One of the goals of ATom is to map the spatial distribution of newly formed particles, as well as those large enough to act as CCN. These measurements are being used to constrain NPF mechanisms used in global chemistry-climate models, and to evaluate loss and growth mechanisms that influence the abundance and spatial distribution of cloud-active particles. These tasks require accurate and precise measurements of the aerosol size distribution spanning 3–1000 nm in diameter, which in turn require a coordinated and inter-calibrated set of in situ instruments onboard the aircraft. In this paper, we describe the operating principles, calibration, and laboratory and in-flight performance of the NMASS instruments used to measure the size distributions of the nucleation and Aitken modes during ATom. The optical particle counters used to measure the accumulation-mode aerosol size distribution are described in detail by Kupc et al. (2018). The inlet, sampling system, altitude-dependent corrections for diffusional losses, and methodology to combine the different instruments are described by Brock et al. (2018), along with comparisons between different instruments for measuring aerosol size distribution and abundance during ATom.
Plumes and layers of nucleation and Aitken mode aerosol of vertical
thickness around 100 m have frequently been observed in the free troposphere
(Kupiszewski et al., 2013; Schröder et al., 2000; Petzold et al., 1999),
and yet questions remain about the ultimate fate of the associated particles
in the atmosphere, especially whether they grow and are transported to sizes
and locations, respectively, to have significant effects on the radiation
budget. Since modern, large research aircraft generally operate at airspeeds
above 100 m s
While the NMASS instrument has been used on research aircraft since 1999, in the stratosphere (Borrmann et al., 2010; Lee et al., 2003), and troposphere (Brock et al., 2000; Schröder et al., 2000; Petzold et al., 1999), a comprehensive description of the instrument and its uncertainties has not been published. In the following sections, we describe the principle of operation of the NMASS, laboratory studies describing its sensitivity to particle number concentration, size, composition, and the numerical inversion to produce a size distribution from the discrete CPC measurements. Finally, we describe the operation of two NMASS instruments sampling in parallel during the ATom mission, which together provide 10 channels of 1 Hz size discrimination between 3 and 60 nm.
Schematic of the NMASS layout and flow system.
The NMASS is comprised of five parallel CPCs operating at an internal pressure of 120 hPa (Fig. 1). Each CPC detects particles above a different minimum size, determined by the maximum vapour supersaturation encountered by the particles. Operated in parallel, the CPCs provide continuous concentrations of particles in five different cumulative size classes between 3 and 60 nm. Knowing the response function of each CPC, numerical inversion techniques can then be applied to recover a size distribution from the continuous concentrations while taking into account the non-ideal response function of each channel.
Sample air enters the NMASS instrument through a pressure-reducing orifice (Sect. 3.4). Sample pressure is maintained at using a pressure controller upstream of a pump. Total flow through all five CPC modules is maintained at a constant value by adjusting a bypass flow using a solenoid control valve. With pressure and flow kept constant, the supersaturation in each CPC is determined by the absolute temperature of the saturator and by the difference in temperature between the saturator and the condenser. The five CPCs are set to different minimum detection sizes by varying this temperature difference while the saturator temperature of each unit is held constant. The sizing limits are constrained by diffusion losses within the instrument and practical limits to the degree of thermal control required for nucleating large particles at relatively low supersaturations.
The design of this instrument owes much to previous efforts to study and
improve the performance of CPCs. In particular Saros et al. (1996),
Wiedensohler et al. (1994), and Mcdermott et al. (1991) have demonstrated
that the supersaturation within a CPC can be effectively manipulated to
control its detection efficiency as a function of particle diameter (see
McMurry, 2000, for a history of CPC development). The differences in
detection efficiencies among different, individual CPCs have been used to
determine the concentrations of particles over one or two size ranges,
particularly to identify the presence of an ultrafine (
The NMASS instrument operates the five embedded CPCs using a single integrated
data acquisition and control system, flow and temperature regulation systems
and power supplies, and an external pump and pressure controller. The
physical layout of the instrument is constrained by space and weight
limitations for the operation on stratospheric aircraft (it was designed for
use on the NASA ER-2 high-altitude research aircraft), the dissipation of
heat, the need to limit particle losses due to diffusion, minimization of
electronic noise, and accessibility to components for maintenance and
repair. Because the instrument was originally designed for autonomous
operation in wing-mounted aircraft pods, there is no integral display or
user interface. The instrument has a mass of 35 kg in flight-ready
configuration and requires an external 8 kg pump. Dimensions are
approximately 720 mm long by 360 mm wide by 390 mm high. The instrument
consumes
The Wilson et al. (1983) CPC designed for the NASA ER-2 and WB-57 high-altitude research aircraft provided the concept for confining the aerosol to the centre streamline in the condenser and established a condenser geometry that functioned at pressures from 400 to 40 hPa. These features were incorporated in the NMASS CPCs which have been operated at pressures from 60 to 120 hPa depending on the altitude range of the particular aircraft.
Schematic of a CPC unit of the NMASS. The sample flow is split between aerosol flow and sheath flow. Aerosol flow is selected from the centre of the sample flow stream and passed through a capillary to reduce losses. Sheath flow is passed through a filter to remove particles, over a warm Fluorinert bath to pick up the vapour and then into the condenser. In the condenser, Fluorinert vapour in the sheath flow diffuses into the aerosol flow and condenses onto particles, growing them to optically detectable sizes. The grown particles then pass through the optics block, consisting of a laser aligned with a lens, beam-block, and detector. The measured flow across the capillary and the counts in the optical detector are used to calculate the concentration of the particles. Dimensions are given in the Supplement, Table S1.
Kelvin diameter
In the NMASS, air enters a single inlet through a pressure-reducing orifice
and is then carried to each of the CPCs (Fig. 1). The flow entering each CPC
is split into two branches (Fig. 2). The first branch passes through a
filter (Model DIF-BK40, Headline Filters Ltd., Aylesford, UK) to remove
particles, and then through a saturator controlled at 39
The most commonly used working fluid for CPCs is
The supersaturation reached in the condenser of each CPC of the NMASS is a function of the heat and mass transfer within the condenser, which depends on pressure and flow rate as well as temperature. The dependence of the maximum supersaturation on pressure is complex and difficult to model (Stolzenburg and McMurry, 1991), especially since not all needed thermodynamic properties of Fluorinert such as mass diffusivity in air are known. Since the response of each CPC varies with ambient pressure, allowing the instrument pressure to vary with altitude on aircraft campaigns would require a large number of calibrations for the different pressure conditions, and parameterizations to characterize how the instrument response varies with pressure. To simplify calibration and avoid the related uncertainties with this parameterization, the CPCs within the NMASS are maintained at a constant internal pressure.
As the internal pressure of the NMASS must be below ambient pressure at
all times, the choice of this pressure depends on the minimum anticipated
ambient pressure. Diffusional and inertial particle losses are enhanced at
low pressure, so it is desirable to use the highest practical instrument
pressure. The NMASS was originally designed for operation on stratospheric
research aircraft such as NASA's ER-2 and WB-57F; when operated in the
stratosphere an internal pressure of 60 hPa is used. For lower altitude
measurements, aboard tropospheric aircraft such as the NASA-DC8 and NOAA's
WP-3D, an internal pressure of 100 or 120 hPa has been used. Internal
pressure is maintained by sampling through a thin-plate orifice at the inlet
to the instrument and controlling the downstream pressure using a pressure
controller and external pump. The response of each CPC unit depends on the
flow rate as well as the pressure, so the volumetric flow rate through the
CPCs must be kept constant. As mass flow through the orifice changes
with ambient pressure, this requires an active flow control system. As the
aircraft ascends, flow through the orifice becomes insufficient for the CPCs,
so a larger orifice must be switched into its place. The NMASS is thus operated
with two different sized orifices, 500 and 750
Total flow through the NMASS instrument is controlled automatically by adjusting a proportional control valve (Model 248A, MKS Instruments Inc., Andover, Massachusetts, USA) that regulates flow through a bypass line, as shown in Fig. 1. This flow circuit maintains a nearly constant volumetric flow through the CPCs even as changes in upstream pressure alter the volumetric flow downstream of the orifice. The flow through each CPC is determined by the pressure drop across the filter in the saturator (see Fig. 1) and the proportional control valve. The pressure drop across each capillary is continuously measured during operation, as shown in Fig. 1. Calibrations were done to relate these pressure drops to a volumetric flow, and it is these flows that are then used to determine the concentration in each channel from the number of particles counted.
Determining a size distribution by differencing parallel instruments requires
precise control of the instrument response. At relatively low values of
supersaturation, small changes in temperature difference between the
saturator and condenser can produce large excursions in supersaturation, and
thus
Diagram of the calibration set up used to characterize the NMASS counting efficiency as a function of particle diameter. The calibration includes different aerosol types such as limonene, ammonium sulfate, and dioctyl sebacate particles to test the instrument sensitivity to particle composition.
The counting efficiency of each NMASS channel as a function of diameter can
be varied by changing the temperature difference between the saturator and
condenser, but also varies with internal pressure. We chose to operate the
NMASS at
The Fluorinert droplets nucleated and grown in the condensers are detected
with 5 simple optical particle counters which use near-IR laser diodes as the
light source and a forward scattering geometry. The optics blocks and laser
diode electronics are modified versions of the Model 3760/3010 detector (TSI
Inc., St. Paul, MN, USA). The scattered light is detected with a photodiode.
Custom electronics correct for shifting of the baseline voltage from the
photodiode circuit as concentration increases. Analog pulses are converted to
TTL-level signals within a shielded enclosure, then routed to 32 bit
counter and timer circuits. Total counts in each channel are accumulated over a
time period determined by software (typically 1 s). The fraction of time in
each second in which particles are occupying the laser beam and the system
cannot process another particle (the dead time) is measured using five
additional 32 bit counters and timers. Corrections are made for dead time by
dividing the number of counts by the fraction of measurement time for which
the detector electronics are not busy processing. This correction allows
ambient concentrations up to
Laboratory studies were used to determine the counting efficiencies of each
NMASS channel as a function of particle diameter. Aerosols were produced with
three different methods: (1) limonene ozonolysis, (2) atomization of ammonium
sulfate, and (3) atomization of di-2-ethylhexyl (dioctyl) sebacate. These
methods produce particles of widely differing composition that can help
identify any composition-dependent sizing effects. The dependence of the
counting efficiency with size was studied by placing a Boltzmann steady-state
charge distribution on the generated particles with a Po-210 neutralizer and
passing them through a nano-DMA to select particles of a single electrical
mobility (Fig. 4). The fraction of doubly charged particles is small for
particles with diameters
Counting efficiency of the two NMASSs in the settings used for the
ATom mission (downstream pressure at 120 hPa, saturator temperatures of both
instruments are set to 39
We used a nano-DMA column (Model 3085, TSI Inc., St. Paul, MN, USA) in a
custom-built DMA system with non-recirculating sheath flow. By varying the
sheath flow between 3 and 16 L min
Two
The efficiency of each NMASS CPC is taken as the ratio of the standard
temperature and pressure (STP, taken as 273.16 K and 1013 hPa)
concentration measured in the NMASS to that measured in the reference CPC.
The concentration of the NMASS and reference CPCs is calculated as the number
of pulses counted by the instrument per unit of time divided by the flow
rate, corrected for dead time. This concentration is corrected for pressure
and temperature to get the STP concentration. The uncertainty in the NMASS
downstream pressure at 120 hPa is
Condenser temperatures, 50 % cut-off diameters
(
Counting efficiency of NMASS 1 against particle diameter for
diameters between 65 and 650 nm using atomized ammonium sulfate particles.
The decay of counting efficiency with particle diameter at diameters above
150 nm is mainly caused by particle impaction on the orifice at the
instrument inlet. At particle diameters above 70 nm, all 5 channels (shown
by the different colours and symbols in the legend) have the same counting
efficiency. Particle counting efficiency is 100 % at 109 nm and drops to
50 % at 546 nm. At diameters
The counting efficiency for each channel as a function of diameter for
particles produced from limonene ozonolysis for the settings used during ATom
are shown in Fig. 5. Fits to the response curves are included to guide the
eye. Condenser temperatures and diameters at which each channel detects 10,
50, and 90 % of the particles are given in Table 1. For particles
The repeatability of each CPC's response function is limited by temperature,
pressure and flow control. The variation in 50 % detection efficiency
diameter with respect to temperature difference between saturator and
condenser is shown in Fig. 3. The saturator temperature was held constant at
34.8
The gradient of the Kelvin curve is a determining factor in the NMASS
detection efficiency stability. If the curve is too steep, any small thermal
instability will cause a large variation in
For a given temperature difference between saturator and condenser, the
measured
Sensitivity of the NMASS to the composition of the aerosol sample was tested
by calibrating with ammonium sulfate and dioctyl sebacate particles
generated using an atomizer, and comparing this to the calibration with
limonene ozonolysis particles. These three compositions were chosen because
they can be produced using a flow tube reactor or an atomizer, and represent
a range of different particle compositions similar to those found in the
atmosphere. Limonene ozonolysis products represent particles nucleated and
grown with low volatility organic compounds, dioctyl sebacate particles are
liquid organic droplets, and ammonium sulfate is representative of aged
atmospheric sulfate particles fully neutralized by ammonia. Calibrations
with these different compositions show no statistically significant variation
between counting efficiency curves (Fig. 7). Note that sensitivity to
composition was only done for particles
At the highest diameter settings (lowest condenser to saturator temperature
difference), the supersaturation is at its lowest and thus more
sensitive to any fluctuations in flow and temperature. In each of the NMASS
instruments the highest diameter channel exhibits small fluctuations in
cut-off diameter, most likely due to non-uniformities in supersaturation
through the condenser block and being on the steepest part of the Kelvin
curve here (see Fig. 3). NMASS 2 channel 5 shows the largest drift in
Counting efficiency of NMASS 1 as a function of particle diameter for particles of different chemical composition: limonene ozonolysis products (diamonds), atomized ammonium sulfate (stars), and dioctyl sebacate (circles). Only three channels are shown here because it was not possible to produce atomized particles small enough for the two channels with the smallest cut-off sizes by atomizing ammonium sulfate or dioctyl sebacate. Counting efficiencies fall with decreasing particle diameter as particles become smaller than the Kelvin activation diameter of each channel. There is no statistically significant sensitivity of counting efficiency to particle composition.
Calibrations of NMASS 2 channel 5 in detail. Calibrations on three separate days are shown with fitted logistic functions to guide the eye. This shows instabilities in the supersaturation of the condenser of this channel, with the 50 % diameter varying between 36.5 and 39.1 nm. The black dotted line shows the fit to all data, which is used for inverting the NMASS data.
Each of the NMASS channels,
Three-mode lognormal descriptions of representative aerosol
size distribution cases giving model number concentration
Because of the nonlinear inversion, it is not possible to directly calculate
how uncertainties propagate from concentration and sizing errors through to
the final size distribution. Instead, we use a Monte Carlo technique to
calculate the range in variation of the number, surface area, and volume
concentrations for several representative size distributions characteristic
of the ATom project. To perform this analysis, eight size distributions that
represent a range of those encountered during the ATom flights were simulated
using a three-mode lognormal distribution (Table 2). These model
distributions were then used to calculate the expected instrument response;
that is, the concentrations that would have been measured by each channel of
the NMASS were calculated from the known response functions. These
concentrations were then each independently and randomly adjusted by a value
falling within the concentration uncertainty as represented by one standard
deviation of a Gaussian distribution. Further, the response functions of each
channel were similarly independently and randomly adjusted in diameter space
using the observed variation in each channel's response during calibration
(Table 1). The “perturbed” concentration measurements and response
functions were then inverted to recover
Set-up for checking the NMASS inversion against an SMPS. Atomized
ammonium sulfate is dried and passed to a DMA, which selects a narrow range
of particle sizes. A
Values of these statistics for the eight test size distributions are given in
Table 2. The mean magnitudes of
Note that, for particles with diameters lying between the response curves of
the first channels of each NMASS instrument (i.e. between
Comparison of SMPS and NMASS inversions for size distributions
produced by atomized ammonium sulfate particles size selected by a DMA:
panel
The system of two NMASSs was compared with an SMPS, the standard technique
for ground-based measurements of nucleation-mode particle size distributions.
Aerosols were generated by atomizing ammonium sulfate and dried with a
silica gel diffusion drier before entering a custom-built DMA with
recirculating sheath flow. The DMA was used to select a narrow size range of
particles, and then the sample flow was split between the two NMASSs and an
SMPS as shown in Fig. 9. The SMPS is made up of a TSI 3085 nano-DMA column
and a TSI 3022A CPC. A short section of tube with an inner diameter of
1.59 mm is used after the first DMA to generate turbulence and ensure the
sample flow is well mixed before it is split between the instruments. As
the internal pump of the CPC is unable to keep stable flow at
0.3 L min
The first DMA was used to select particles of a given size, which were then sent to the NMASS and SMPS. The SMPS scanned for 10 min between 4 and 200 nm, and the average NMASS concentrations were calculated over this time. Concentrations from the SMPS and NMASS were inverted using the Twomey–Markowski method described in Sect. 4.1. DMA performance and the suitability of this method for inverting the SMPS data are discussed in the Supplement.
Flows, temperatures, and pressures measured by both NMASSs over the
total course of an ATom flight. Ambient pressure is shown in grey, coloured
lines in panel
Comparisons of inverted size distributions from the SMPS and NMASS
instruments are shown in Fig. 10, with a 20 nm peak selected by the DMA in
the upper panel, and 32 nm in the lower panel. For the 20 nm peak, the SMPS
peak appears at 19.9 nm and the NMASS peak at 21.2 nm, indicating a 6 %
discrepancy. The largest uncertainties on the SMPS sizing come from the
voltage and the time lag correction, which at 20 nm gives a total sizing
uncertainty of
The effect of variation in sizing of NMASS 2 channel 5 is examined by recalculating the inversion with the range of calibrations from multiple days (Fig. 11). The resulting differences in the inverted size distribution move the peak between 33.7 and 37.8 nm. We expect this additional diameter uncertainty of about 12 % to show up between 27 and 60 nm, where the measurement from NMASS 2 channel 5 plays a determining role in the particle sizing.
Two NMASS instruments have recently been flown on a NASA DC-8 in the boundary
layer and free troposphere on the ATom (Prather et al., 2017), repeatedly
profiling between
The stability of the temperatures, flows, and pressures within the NMASSs is
critical to maintaining constant instrument response during flight. In
Fig. 11, we show key parameters for instrument stability: the CPC temperature
difference, total CPC flow and instrument pressure. The greatest temperature
instability is in NMASS 2 channel 5, where the temperature difference
fluctuates by 0.46
Example data taken on the NASA DC-8 aircraft during ATom in February
2017. The top of panel
The pressure control system with two orifices, described in Sect. 2.4,
maintains a total volume flow rate of 122–144 cm
For operation on ATom, the sample flow is passed through a large diameter Nafion™ dryer before entering the NMASSs. This reduced the relative humidity to below 20 %. This ensures that particles measured in the NMASS are classified consistently by dry diameter, and avoids potential problems of particle losses associated with water vapour condensation during flow expansion in the orifice or effects of water vapour on the performance of the CPC working fluid.
Example data from the NMASS measuring during ATom in February 2017 are shown in Fig. 12. Concentrations of the 10 NMASS channels, along with the total concentration of particles with diameter between 63 and 1000 nm measured by an ultra-high sensitivity aerosol spectrometer (Kupc et al., 2018) and the inverted size distribution are shown in Fig. 12a. From about 01:41 to 01:46 the large difference between the concentrations in channels 1 and 2 indicates recent or active new particle formation, since particles between 3 and 7 nm have a relatively short lifetime in the atmosphere and so must have been formed recently. Channels 1 and 2 vary independently of channels 3, 4, and 5 between 01:41 and 01:45, indicating two distinct modes of particles present. As concentrations in channels 1 and 2 become very similar after 01:48 and concentrations in channels 1–4 become very similar after 01:58, this indicates that most particles are now larger than 7 nm, and then larger than 28 nm.
Average size distributions for 1 min of data each are shown in Fig. 12b–d. These illustrate recent new particle formation (panel b), more aged recently formed particles (panel c) and older Aitken mode particles (panel d).
The stable, reproducible characteristics of the NMASS, demonstrated in this
paper, allow measurements of fast time-response, size-selected aerosol
concentrations in flight over rapidly changing ambient pressure from the
boundary layer to the stratosphere. For the ATom mission, two NMASS
instruments were modified and extensively calibrated and tested in the
laboratory with a range of particle sizes. The response function of each of
10 CPC channels was determined, and the repeatability of the
Performance in flight shows that temperatures, pressures, and flows remain
within acceptable bounds except for pressures
The two NMASS instruments flying on the ATom mission are providing a high-quality, contiguous tropospheric dataset of nucleation- and Aitken-mode size distributions with global coverage of the Pacific and Atlantic Ocean basins and seasonal variation. These data will be used to evaluate the dominant mechanisms of atmospheric new particle formation and the contribution of nucleated particles to the global distribution of cloud-active particles and, through model sensitivity studies, their subsequent influence on radiative forcing.
Calibration, laboratory testing, and in-flight data are publicly available
at the Oak Ridge National Laboratory Distributed Active Archive Center (Williamson et al.,
2018). Processed and quality-controlled data for the ATom mission are publicly available at the
ATom data archive:
The supplement related to this article is available online at:
All authors contributed substantially to the work presented in this paper. CB, JW, DG, and JMR designed, built, programmed, and tested the NMASS instruments, which were then modified by CW and CB. CW and AK calibrated the NMASSs and collected data during ATom-1 and ATom-2 missions. FE and RM made the orifice changer system and other instrument parts. CW prepared the manuscript with contributions from all authors.
The authors declare that they have no conflict of interest.
This publication's contents do not necessarily represent the official views of the respective granting agencies. The use or mention of commercial products or services does not represent an endorsement by the authors or by any agency.
The authors acknowledge support by NASA's Earth System Science Pathfinder Program under award NNH15AB12I and by NOAA's Health of the Atmosphere and Atmospheric Chemistry, Carbon Cycle, and Climate Programs. Agnieszka Kupc is supported by the Austrian Science Fund FWF's Erwin Schrodinger Fellowship J-3613. We would like to thank Bernadett Weinzierl, Maximilian Dollner, T. Paul Bui, and Glenn S. Diskin for access to their preliminary data. Jose Jimenez and Pedro Campuzano-Jost kindly loaned us a TSI 3776 CPC and Paul Ziemann an electrometer. Finally, we would like to thank David Fahey, Karl Froyd, Daniel Murphy, Steven Ciciora, and Daniel Law for insightful discussions. Edited by: Eric C. Apel Reviewed by: three anonymous referees