Introduction
Aerosol particles are a key component of the atmospheric chemical environment
as they have climate, human health, and ecosystem effects (Pöschl, 2005;
IPCC, 2013). Measuring aerosol particle chemical composition is a challenging
endeavor that has been the subject of a great deal of innovation in the past
few decades (Jayne et al., 2000; Weber et al., 2001; Jimenez et al., 2009;
Hallquist et al., 2009). The calibration of these instruments has evolved to
better detect speciated composition. Still, there is a need for fundamental
mass-based calibration techniques to place aerosol particle measurements
firmly in the context of other atmospheric chemical observations.
Nitrogen (N) compounds are major constituents of atmospheric aerosol and play
a significant role in atmospheric chemistry, radiative balance, air quality,
and N deposition in both terrestrial and aquatic ecosystems (Neff et al.,
2002; Liao et al., 2003; Forster et al., 2007; Cornell, 2010; Xu and Penner,
2012; Park et al., 2014; Fuzzi et al., 2015). The relative contribution of N
compounds, specifically particulate nitrate, to total atmospheric particle
mass is expected to increase in the coming century due to a projected
reduction in SO2 and increasing NH3 (Bauer et al., 2007;
Bellouin et al., 2011; Hauglustaine et al., 2014; Li et al., 2015), and
already dominates in some urban and agricultural environments (Haywood et
al., 2008; Vieno et al., 2016). Excluding N species in deposition studies
contributes to uncertainty in regional and global nitrogen budgets used to
evaluate ecological, biogeochemical, and climate impacts (Jickells et al.,
2013; Cornell, 2010; Cape et al., 2011). Measuring individual N species,
classes of N compounds, or total N is challenging, and laboratory and field
data are limited. For example, while there are a number of methods to measure
inorganic N species, particulate organic N is more difficult to quantify with
fewer sampling and measurement methods currently available for such a variety
of compounds (Lin et al., 2010; Farmer et al., 2010; Lee et al., 2016).
Measuring the total N mass of atmospheric particles will improve our
understanding of their role in nitrogen cycles associated with sources such
as agriculture or wildfires and processes such as photochemical oxidation.
Several techniques exist to measure total reactive nitrogen (Nr),
defined here as all atmospheric nitrogen excluding N2 and N2O,
which includes both gas (e.g., total odd nitrogen (NOy), NH3,
amines, nitriles, nitrates) and particle-phase species (e.g., inorganic
and organic N compounds). An established rapid-response robust technique
for measuring Nr involves thermal and catalytic conversion to nitric
oxide (NO) with detection by ozone (O3) chemiluminescence. The catalyst
material, temperature, and sampling methods dictate the efficiency, time
resolution, and speciation of measurements (Winer et al., 1974; Williams et
al., 1998; Dunlea et al., 2007; Schwab et al., 2007; Benedict et al., 2017).
The chemiluminescence detection technique has been used to measure NOx
(NO + NO2; Parrish and Fehsenfeld, 2000), total gas-phase Nr
(e.g., Hardy and Knarr, 1982; Horstman, 1982), individual reactive nitrogen
components (e.g., NH3; Breitenbach and Shelef, 1973; Saylor et al.,
2010), or subsets of nitrogen compounds by removal of selected compounds
using filters or denuders upstream (Prenni et al., 2014). Marx et al. (2012)
completed the only study to explicitly report quantitative conversion of
particle-bound Nr for a limited number of species; however the results
show a range of conversion efficiencies (78–142 %). Several other
studies assume at least some (nonquantitative) particle conversion across
their catalysts (Fahey et al., 1985, 1986; Prenni et al., 2014). To our
knowledge, no study selectively isolates particle-phase reactive nitrogen to
assess the particle-phase contribution to total nitrogen signals from
individual sources or in their atmospheric measurement. Here we characterize
the particulate Nr conversion in our converter consisting of heated
platinum and molybdenum catalysts followed by rapid chemiluminescence
detection using common inorganic atmospheric Nr species including
(NH4)2SO4, NH4Cl, NaNO3, NH4NO3, and
(NH4)2C2O4. The application of the converter coupled
with NO-O3 chemiluminescence, hereafter referred to as the Nr
system, to quantitatively convert and measure the sum of Nr particle
mass was evaluated using mass concentrations determined using traditional
particle instrument calibration methods.
Organic carbon species are major constituents of aerosol particles (Jimenez
et al., 2009) and are responsible for some of the more important climate and
health impacts of particles (Pöschl, 2005). Calibration of measurement
systems for organic carbon species is a challenging task since there are
thousands of possible compounds of differing sizes, functional groups, and
therefore volatilities (Jimenez et al., 2016; Murphy, 2016a, b). A
comprehensive mass-based technique for organic aerosol species would be a
highly desirable addition to the current measurement technology.
Theoretically, the high-temperature platinum catalyst in our system should
convert carbon species to carbon dioxide (CO2) in the presence of
air. Conversion of volatile organic compounds (VOCs) to CO2 on
high-temperature precious metal catalysts is a well-developed technique (see
for example the Pt catalyst used in Veres et al., 2010). Total organic carbon
measurements using similar catalysts (e.g., palladium on alumina) followed by reduction to methane have been used previously
(Roberts et al., 1998; Maris et al., 2003). Platinum-based catalysts are
widely used and have been shown to be more efficient than palladium in
oxidation studies (Schwartz et al., 1971; Kamal et al., 2016). Here we
characterize the conversion efficiency of particle-phase organic carbon
across our Pt catalyst by direct measurements using a LI-COR nondispersive
infrared (NDIR) CO2 analyzer. The current converter design coupled
with both NO and CO2 detectors allows simultaneous measurements of
Nr and total carbon (Cy).
Many traditional particle instrument calibration methods involve
measurements of particle properties by inertial, gravitational, diffusional,
electrical (e.g., sizing), thermal, or optical measurement devices (Chen et
al., 2011). Generally, direct mass concentration calibration techniques
involve offline analysis of filters or semi-real time measurements (e.g.,
PILS combined with ion chromatography). More rapid techniques directly
measure number concentrations and particle sizes. However, these methods
often require knowledge of aerosol properties (e.g., composition, shape,
density, refractive index) and sampling parameters (e.g., volumetric flow
rate, pressure, temperature, relative humidity) in order to determine mass
concentrations. The Nr system is an alternative that directly measures
mass traced back to gas-phase calibration standards instead of relying on
particle size, shape, or refractive index.
In order to demonstrate the application of the Nr system to directly
measure particle mass to calibrate particle mass measurement
instrumentation, we compare mass concentrations measured by a new approach
of directly coupling a particle-into-liquid sampler to the electrospray
ionization source of a quadrupole mass spectrometer (PILS–ESI/MS) for
online mass analysis of water-soluble aerosols. The PILS is an established technique developed to efficiently collect
the water-soluble fraction of aerosol (Weber et al., 2001; Orsini et al.,
2003; Sorooshian et al., 2006). Here, we couple the PILS with an
independently calibrated electrospray interface followed by mass
spectrometric detection to obtain online mass measurements of
single-component, laboratory-generated, Nr-containing aerosol that can
be directly compared to the calibration obtained with the Nr system.
In this work, we present the converter setup, system methodology, and
evaluate the particle-conversion efficiency of a custom Nr system for
several atmospherically relevant Nr-containing particles. The
conversion efficiency of the Nr catalyst was evaluated by comparing the
Nr mass signal with the mass calculated from instrument calibration
techniques that measure the particle number size distributions of
laboratory-generated aerosols of known composition. We then show the
quantitative conversion of organic carbon across the instrument's platinum
catalyst followed by CO2 detection. Finally we compare particle mass
directly measured using the PILS–ESI/MS with that measured using the Nr
instrument. The primary objectives are to characterize particle conversion
in the Nr system, and to investigate the capabilities of the Nr system as a calibration instrument that directly measures particle mass
concentration.
Diagram of the custom-built platinum
catalyst system for the total reactive nitrogen instrument (Nr)
operated at 750 ∘C. The outlet flow is followed by a molybdenum
oxide catalyst before the custom NO-O3 chemiluminescent
instrument.
Instrument characterization
Nr gas-phase conversion efficiency
We verified the efficiency of conversion of a range of gas-phase Nr
compounds in this catalyst system using calibrated gas mixtures or standard
streams and auxiliary analysis methods as described in Sect. 2.2.1. The
conversion efficiencies are summarized in Table 1 and range from 95 to
110 %. The values were based on the ratios of the Nr measured
as NO to the expected values specified by each calibration method. The
uncertainties in the measured conversion efficiencies are in the propagated
errors in each calibration method, and in all cases the range encompasses
100 % conversion. For example, the largest uncertainty in the
NH3 conversion efficiency was the NH3 UV absorption cross
section at 184.9 nm (value of
4.4 ± 0.3 × 10-18 cm2 taken from Neuman et al.,
2003). It is possible that there were Nr compounds in the standard
stream aside from NH3 that were responsible for the result being
> 100 %. However, the fact that the determination was above
100 % for both a permeation source and a gas-phase mixture (3.1 ppmv in
N2) implies that the UV absorption cross section is high by
5–10 % or that there were contaminants in both calibration sources.
NH3 is one of the more important reactive nitrogen species in the
atmosphere–biosphere system and is thermodynamically one of the more
difficult to convert. Compounds considered NOy species, such as
nitric acid, acetyl peroxynitrates, and alkyl nitrates, were not studied in
this work (aside from NO2) since they are known to be converted at
high efficiency on precious metal (Fahey et al., 1986) or molybdenum oxide
(Winer et al., 1974) catalysts. The resulting uncertainties in the
Nr measurement are estimated to be ±10 % based on
comparisons of measured NO signals to individual Nr compound
calibrations.
The signal resulting from particles only. (a) Real-time
Nr-measured (black) and CPC-derived (red) aerosol mass
concentrations (µg m-3) from an atomized solution of
NaNO3. (b) Time response of the Nr signal (ppbv)
shown in black (left axis), and the CPC signal (particles cm-3) shown
in red (right axis), as particle sizes of (NH4)2SO4 are
selectively changed. The dashed vertical lines and labels indicate the
singly charged particle diameter selected with the DMA.
N2O is a potential interference that is discussed in Sect. 2.2.1,
though in this instance the conversion efficiency upper limit determined for
this instrument is a negligible interference in the Nr measurements
in ambient air or ZA matrices, and likewise will not be significant in
biomass burning sources given that N2O enhancements in fresh
biomass smoke are generally not observed or contribute minimally to total
nitrogen (Griffith et al., 1991). N2O emissions from other sources
(e.g., natural and anthropogenic agricultural sources, fossil fuel combustion,
or animal waste) can be significant; therefore the interference from
N2O conversion must be considered. O3 is another
potential source of gas-phase interference due to the decomposition of
O3 to O2 + O, followed by reaction of O with
N2O at high temperature to form NO. However, the NO production in
the O + N2O reaction is an approximately 20 % channel with
a net rate constant of approximately
1 × 10-15 cm3 molecule-1 s-1 at
750 ∘C (NIST, 2017). If all the O atoms from 70 ppbv of
O3 were available for reaction with an ambient level of
N2O (340 ppbv), then the 0.1 s residence time in the converter
would result in approximately 28 pptv of NO, an upper limit that is
generally a negligible amount in almost any atmospheric context except in
remote regions.
Nr particle measurements
Nr system setup and response
The atomizer output was diluted with particle-free nitrogen and ultrapure
ZA; therefore, the Nr measurement should theoretically be
attributed to particles only since no detectable gas-phase nitrogen is added
to the sample stream. However, equilibration within the sample lines may
result in outgassing and formation of gas-phase compounds affecting total
Nr detection. Figure 3a shows the initial response of the Nr
system in cleaned inlets for NaNO3. The Nr mass signal tracks the
CPC-derived aerosol mass features closely as the aerosol source
concentrations fluctuate. Additionally, as different particle sizes are
selected by the DMA for (NH4)2SO4 (Fig. 3b), changes in
the total Nr response are fast and precisely track the changes in the
CPC signal. The potential gas-phase constituents equilibrating in the lines
from aerosols in this study include HNO3, HCl, and NH3. If these
compounds formed before reaching the Nr catalyst it is likely
adsorption and desorption from inlets and tubing surfaces would occur (e.g.,
Neuman et al., 1999; Yokelson et al., 2003). As an example, the presence of
NH3 in Fig. 3b (or HNO3 in nitrate-containing particles) would
be indicated by a delayed and lengthened rise or fall in the Nr response
with sudden changes to the input concentrations. However, the total Nr
response precisely tracks the CPC signal on rapid timescales (a few seconds),
suggesting that gas-phase NH3 was not present in significant
quantities. In experiments at exceptionally high aerosol loadings of
(NH4)2C2O4 (up to several parts per million volume of total Nr, i.e., several thousand micrograms per cubic meter) Nr signal “tailing” was
observed, suggesting that NH3 was scavenging to the walls of the inlet
before the heated quartz tubing.
Calculated mass from particles size-selected by the DMA and
corrected for multiply charged particles using SMPS-derived size
distributions compared to aerosol mass concentrations (µg m-3)
measured as Nr for (a) NaNO3,
(b) (NH4)2SO4, (c) NH4Cl, and
(d) (NH4)2C2O4. The particle size is designated
by the color plot (error bars indicate ±1 SD) and the 1 : 1 line is
shown in black with 20 % error indicated by the grey shading.
Marx et al. (2012) reported calculated conversion efficiencies in air
sampled from a small chamber for NaNO3, NH4NO3, and
(NH4)2SO4 to be 78, 142, and 91 %, respectively. The
authors suggested the overestimation of NH4NO3 was a result of its
semi-volatile properties under ambient conditions that led to the formation
of gaseous NH3 and HNO3 in the chamber. For these reasons, we
limit the background artifacts and volatilization effects that may have
occurred during chamber filling and sampling in Marx et al. (2012) by
sampling immediately following solution atomization through conductive
tubing at relatively high sample flow rates. Additionally, we use a DMA to
size-select the atomized polydisperse aerosol to evaluate the particle
conversion efficiency at several different diameters (100–600 nm in 50 nm
increments) to investigate the volatilization effects and conversion
efficiencies of smaller particles for the extended list of
Nr-containing aerosols studied in our work.
Challenges using the DMA/SMPS to determine Nr-particle conversion
efficiency
The voltage scanning (SMPS) function of the DMA and number concentration
measurements by the CPC are a conventional method to determine particle size
distributions, and for calculating particle mass from total volume and
density, assuming spherical particles. For the total nitrogen measurements,
the total particle-bound Nr mixing ratios were retrieved and converted
to mass concentrations for each corresponding salt. Figure 4a–d show the
SMPS-calculated vs. Nr-measured mass concentrations (µg m-3)
for particles of different composition and diameter. The plots show that a
strong correlation (R2 > 0.98) and good agreement was
obtained for smaller particles (50–200 nm) with slopes ranging from 0.86 to 0.97, while for larger particles (≥ 250 nm) the mass-calculated
values from the SMPS-derived distributions were sometimes as much as
> 50 % too high. The R2 for all particles including ≥ 250 nm ranged from 0.71 to 0.85 with slopes of 1.08–1.36.
Correlation plots of mass concentrations measured as Nr
for (a) NaNO3, (b) (NH4)2SO4,
(c) NH4Cl, and (d) (NH4)2C2O4
versus mass concentrations calculated using CPC number concentrations with
UHSAS size distributions. Particle sizes (nm) are indicated by the color plot
and the 1:1 line is shown in dashed black. The solid lines are orthogonal
distance regression fits. The slope (uncertainty) and R2 is shown.
For larger particles, we used a UHSAS to determine the size distribution of
multiply charged species exiting the DMA. The SMPS inversion-derived size
distributions were generally broader than the UHSAS size distributions,
though agreement improved at increased scan times. Small differences in the
size distribution recovered from the voltage scans at larger diameters
(> 200 nm) affected the mass distribution considerably because
particle mass scales with diameter cubed. A possible explanation is that we
are not correctly accounting for the delay time from the DMA exit to the
CPC; therefore the particle counts did not correspond to the correct size
designated from voltage scanning and this likely skewed the size
distribution relative to the true distribution (Collins et al., 2002).
Methods for limiting these effects exist (Russell et al., 1995; Collins et
al., 2002), including slower voltage scan rates. However, our results
demonstrate the added challenges in particle mass determination using
estimated size distributions from the SMPS method. For the remaining
discussion, we measure the size distributions directly using the UHSAS with
particle concentration measurements (by either the CPC or UHSAS) to evaluate
the Nr particle conversion in the Nr system.
Determining Nr-particle conversion efficiency using a DMA and
UHSAS
For the aerosol mass concentrations (µg m-3) calculated using
UHSAS particle size distributions, we refer to these values as UHSAS-calculated mass. Comparisons of the mass directly measured as Nr versus
UHSAS-calculated mass concentrations for atomized solutions of NaNO3,
(NH4)2SO4, NH4Cl, and (NH4)2C2O4 are
shown in Fig. 5 with orthogonal distance regression lines with slopes that
range from 0.910 to 1.06 for concentrations from ∼ 0 to 70 µg m-3. The instruments are highly correlated (R2 = 0.90–0.99) and the fits indicate that for the salts tested there is
quantitative conversion of particulate nitrogen, to within the combined
uncertainties of the methods, independent of diameter (range: 100–600 nm). More detailed particle conversion efficiencies by size are shown in
Table 2 for each aerosol tested. On average across all size ranges the
results indicate 97 ± 7, 101 ± 5, 100 ± 10,
and 93 ± 5 % particle conversion efficiencies for NaNO3,
(NH4)2SO4, NH4Cl, and (NH4)2C2O4,
respectively. The largest deviation from the one-to-one line occurred for
(NH4)2C2O4, which may imply some ammonia loss, though
the agreement is generally still within 10 % for most particle sizes.
For the case of NH4NO3, the UHSAS-measured size distribution
peaked at significantly lower diameters than expected based on the DMA size
selection. This difference has been reported previously (Cai et al., 2008;
Womack et al., 2017), though to a lesser extent (∼ 8 %) than
observed here (up to 30 %). Possible explanations for these differences
could include vaporization and/or evaporation effects, residual water in the
particles, surface effects, or differences in electrical mobility diameter
and geometric diameter due to non-sphericity as discussed in DeCarlo et
al. (2004). For these reasons, we made no attempt to characterize
NH4NO3 behavior in either the DMA or UHSAS and refer to Sect. 4
for mass concentration comparisons of polydisperse aerosol measured using
separate mass measurement techniques (both the Nr system and
PILS–ESI/MS). It is worth noting that NH4NO3 is one of the more
volatile compounds included in this study and it is reasonable to expect
similar particle conversion efficiencies in the Nr system catalysts
for NH4NO3 as the other species tested (Table 2).
Particle conversion efficiencies (%) with uncertainties (1 standard deviation) in parentheses. The sizing accuracy is
∼ ±2.5 % using NIST-traceable PSLs for 150–500 nm spheres as
our calibration standard.
Diameter
NaNO3
(NH4)2SO4
NH4Cl
(NH4)2C2O4
(nm)
100
88.4(18.3)
100.6(3.0)
89.2(5.9)
91.0(3.5)
150
94.0(10.9)
96.5(2.5)
93.4(4.7)
89.0(6.6)
200
98.6(4.0)
98.8(4.8)
93.6(4.2)
90.2(5.1)
250
101(3)
100(3)
98.3(3.7)
94.7(5.6)
300
104(6)
102(9)
101(3)
97.0(6.2)
350
102(6)
101(9)
98.5(5.2)
101(13)
400
103(8)
100(8)
100(6)
94.7(7.4)
450
95.1(4.5)
110(4)
103(6)
–
500
103(15)
109(17)
124(11)
96.3(7.6)
600
83.2(8.7)
91.9(5.5)
–
82.5(8.4)
Average
97.3(7.1)
101(5)
100(10)
92.9(5.4)
Carbon conversion efficiency of Pt catalyst
The high-temperature platinum catalyst should quantitatively convert carbon-containing species to CO2 in the presence of air. Therefore, the
addition of a CO2 analyzer to the system as described in Sect. 2.1.2
allows for simultaneous measurements of Nr and Cy. Gas-phase
carbon conversion across similar precious metals has been studied previously
(e.g., Veres et al., 2010). The efficient conversion of gas-phase C compounds
in our catalyst system was confirmed using a CO standard in air, and a
combination CO2, CO, and CH4 standards in air. The following discussion
focuses on the conversion of particle-phase organic compounds (OC). The
efficient conversion of Nr-containing particles was demonstrated in
Sect. 3.2.2 for the range of N oxidation states and should extend to other
Nr-containing particles; we expect that the resulting Nr and
Cy signals from each detector will be in proportion by dividing the
result by the number of carbon and nitrogen atoms in the parent molecule to
give the standard concentration on a molar basis. Polydisperse particulate
OC was generated from the solution following an N2 purge to eliminate
carbonate from the solution. Aerosol particles from solutions of anthranilic
acid (C7H7NO2, 2-aminobenzoic acid, Sigma Aldrich), threonine
(C4H9NO3, 2-amino-3-hydroxybutanoic acid, Sigma Aldrich),
tryptophan (C11H12N2O2, 2-amino-3-indolylpropanoic acid,
Sigma Aldrich), and quinine (C20H24N2O2, Sigma Aldrich)
were tested. These compounds were chosen based on their water solubility to
avoid the use of organic solvents. An example of the Nr and Cy
response is shown in Fig. 6 for threonine (see Fig. S1 in the Supplement for additional
compounds). The relative difference between the Nr and Cy measured
concentrations (up to several hundred parts per billion volume) is less than 10 %, which is
within the propagated uncertainties of the CO2 calibration standards
and both detection methods. We conclude that the Nr catalyst with a
CO2 detector in parallel can be used as a total carbon measurement
system and would be useful to establish instrument calibrations for
carbon-containing aerosol. The system is currently limited to calibration of
compounds in ZA matrices because ambient levels of the common
gas-phase carbon compounds CO2, CO, and CH4 are high.
An example of the quantitative conversion of atomized polydisperse
threonine (C4H9NO3) to NO and CO2 measured using
NO-O3 chemiluminescence and a LI-COR-6251, respectively. The
measured total Cy (red) is divided by the number of C atoms in
threonine (four).
Nr measurements of biomass burning emissions
As an example of both gas and particle measurements using the Nr
system, we follow with a brief discussion of N emissions from biomass
burning. The primary gaseous N compounds in biomass burning plumes include
NO, NO2, N2, NH3, and to a lesser extent HCN,
CH3CN, HONO, HNCO (Lobert et al., 1990, 1991; Kuhlbusch et al.,
1991; McMeeking et al., 2009; Burling et al., 2010; Stockwell et al., 2014,
2015), and other Nr-containing gases. Figure 7 shows results
obtained from a representative fire (Fire 047) from the Fire Influence on
Regional and Global environments Experiment (FIREX) 2016 Missoula Fire Lab
study (https://www.esrl.noaa.gov/csd/projects/firex/, last access:
6 May 2018). Figure 7a shows the co-measured Nr and NO
concentrations (ppmv). The majority of the Nr system's response is
due to the sum of gas-phase Nr constituents that were measured by a
FTIR spectrometer (Selimovic et al., 2018), an H3O+ chemical
ionization mass spectrometer (Koss et al., 2018), and a broadband
cavity-enhanced extinction spectrometer (Min et al., 2016) (Fig. 7b). At the
beginning of the burn (before 10:23 MST) the average relative percent
difference between the total nitrogen signal and the sum of individually
measured gas-phase compounds is ∼ 16 %, which is less than the
combined error of the individual measurements. There is greater disagreement
shown in Fig. 7c (difference is up to ∼ 1 ppmv; up to ∼ 50 %
relative percent difference) during other stages of the fire. The modified
combustion efficiency (MCE) is a measure to estimate the relative
contribution of flaming and smoldering combustion that occurred over the
course of a fire, where the MCE is defined as the ratio of ΔCO2/(ΔCO2+ΔCO) (Yokelson et al.,
1996). A higher MCE value (approaching 0.99) designates relatively pure
flaming combustion (more complete oxidation), and a lower MCE (∼ 0.75–0.84) designates more smoldering combustion. We have shown in our
laboratory experiments that there is quantitative Nr particle
conversion across the Nr catalyst; therefore, it is possible that
the residual signals are due to particulate N-containing compounds.
Particulate ammonium may contribute to the excess Nr signal
measured during periods dominated by smoldering combustion
(MCE < 0.90). The oxidized N-containing gas-phase species are
relatively more abundant during the initial part of the fire; thus
particulate nitrate could account for some Nr signal during the
flaming-dominated stages as shown in Fig. 7. By confirming particulate
Nr conversion in this system, it is possible that a total N budget
can be reconstructed for additional laboratory fires measured during the
FIREX laboratory study where individual particle-phase Nr data are
available.
Time series for Fire Sciences Lab 2016 measurements of emissions from
a subalpine fir canopy sample (Fire 047). (a) Total reactive
nitrogen (Nr, red) and nitric oxide (NO, blue) measurements.
(b) Comparison of the difference (Nr-NO, gold) with the
sum of the measured gas-phase Nr species (purple). The sum of
individually measured gas-phase species in order of abundance include
NH3, HNCO, HCN, HONO, NO2, CH3NO2,
and 40 minor organic nitrogen species. NO2 and HONO were measured
with a broadband cavity-enhanced extinction spectrometer, HCN and NH3
were measured with a FTIR spectrometer, and all remaining organic species were measured
with a
H3O+ CIMS. (c) Residual Nr (black) in parts
per million volume
with modified combustion efficiency overlaid (MCE, red).
Application to calibrate the PILS–ESI/MS
Here we demonstrate the capability of the total nitrogen system as an
independent calibration method for other aerosol measurement systems.
Nr measurements of laboratory-generated single-component inorganic and
organic aerosol particles were used to characterize the PILS–ESI/MS. The
strength of using the Nr system to calibrate the PILS–ESI/MS and other
aerosol mass instruments is that it is a direct method to calibrate the
entire coupled online system. The current calibration approach for nearly
all detectors used with the PILS involves liquid-phase standards to
calibrate the detection method independently from the PILS.
The inorganic salts selected for the comparison between the Nr and the
PILS–ESI/MS instruments all contained N atoms, either in the cation, anion,
or both. The total Nr measured as NO (ppbv) included all the N atoms
atomized from the single-component solution. Dividing the total Nr
measurement by the number of N atoms in the parent molecule gives the
standard concentration (ppbv) of the corresponding anion (e.g., Cl-,
NO3-, SO42-, C2O42-). The mixing
ratios (ppbv) are converted to micrograms per cubic meter from the molecular
weight of the corresponding anion. We refer to these mass concentrations as
“X measured as equivalent Nr” in the remainder of the text, where
X is the corresponding anion of the aerosol particle. The anion mass
calculated in this way was only necessary when comparing directly to
PILS–ESI/MS measurements of ammonium salts of nitrate, sulfate, chloride,
and oxalate.
Nr and PILS–ESI/MS mass concentration comparisons
To compare to the calibration approaches using liquid-phase standards
described in Sect. 2.1.3 for the PILS–ESI/MS, we performed particle mass
comparisons using these methods with anion-specific mass concentrations
derived from the Nr measurement system. A single-component aerosol
was used to minimize complex matrix effects including ion suppression and
enhancement common in ESI.
An example of the Nr system and PILS–ESI/MS co-sampling a laboratory-generated polydisperse aerosol stream is shown in Fig. 8. Here we did not
size-select aerosols but measured all particle sizes below a 2.5 µm
cutoff (URG cyclone, Chapel Hill, NC). There are two reasons for this
experimental setup: (1) generating a sufficient aerosol mass concentration
to calibrate the PILS–ES/MS was challenging because it requires a minimum
flow of 11 L min-1, while the DMA output flow is < 1 L min-1; therefore the DMA aerosol flow required a large
dilution. (2) Conventionally, the PILS instrument samples with a cyclone with a 1 or 2.5 µm cutoff, which is similar to other mass measurement instruments
including the aerosol mass spectrometer (AMS) and filter collection.
Figure 8 shows the aerosol nitrate (blue) trace from NaNO3
particles measured by the PILS–ESI/MS shifted in time to account for the
system delay time so that it aligns with the relatively steady concentration
periods with the Nr trace (black). The PILS–ESI/MS had a response
time of roughly 4–5 min in its current configuration. Several stages in the
PILS system included mixing volumes (e.g., syringe pumps and mixing vessels)
that prevented rapid response to rapidly changing concentrations and smeared
the response. For instrument comparisons 60 s data were averaged and
compared during periods with relatively steady concentrations (generally
lasting 5–10 min). Examples of PILS–ESI/MS traces aligned such that the initial
response of both instruments coincides and is shown in Fig. S2.
The PILS–ESI/MS-measured aerosol nitrate mass (blue) and the nitrate
measured as Nr (black) (µg m-3) for an atomized
solution of NaNO3 (polydisperse). The PILS–ESI/MS trace is shifted
to account for the delayed response and the instrument time constant.
The correlation plot of PILS–ESI/MS to equivalent anion mass measured as
Nr for each aerosol type (NaNO3, (NH4)2SO4,
NH4Cl, and NH4NO3) is shown in Fig. 9a–d. The
concentrations ranged from ∼ 10 to 120 µg m-3 and
the standard linear regression fits for each aerosol type are included in
Fig. 9, and were highly correlated with a R2 = 0.99. For
(NH4)2SO4, the concentration exceeded the linear dynamic
range of the PILS–ESI/MS for sulfate (see Fig. S2a; > 130 µg m-3) as determined by liquid-standard calibration curves. The
linear range of ESI is limited at high concentrations due to limited surface
sites available for ionization (Tang et al., 2004). For this reason values
outside the linear dynamic range of the PILS–ESI/MS (> 130 µg m-3) for sulfate were excluded from the linear regression
fit. NH4NO3 shows a similar, less pronounced trend; however, it
is still included in the regression plot as it was difficult to isolate
whether this was analyte suppression during electrospray ionization or a
linear dynamic range issue. Based on the regression fits in Fig. 9, the
difference between the PILS–ESI/MS and Nr system for each inorganic
component is less than 6 %. The uncertainty in the ESI signal varies by
compound and averaging time; however from the tests described here the
maximum uncertainty is estimated at ∼ 15 %. Combining this
uncertainty with the uncertainty in the ESI calibrations (maximum ±10 %), the air and liquid flow rate (both ∼ ±4 %)
and dilution (∼ ±5 %) in quadrature gives a total
maximum uncertainty associated with mass measurements of ±20 %. Thus,
while the slope of the correlations of the two instruments (based on 60 s
averages during periods with constant concentrations) shows a relative
difference of less than ∼ 6 %, the uncertainty in the
PILS/ESI measurement of single-component aerosols is closer to
∼ 20 % and could be greater if the transmission and
ionization efficiencies of the ESI differ from the efficiencies present
during calibration periods. This uncertainty is greater than the uncertainty
(±10 %) reported for the PILS–IC instrument for ionic species in
Weber et al. (2001) but lower than the AMS uncertainty for nitrate (33 %)
and sulfate (35 %) estimated by Bahreini et al. (2009), though the AMS has
a much faster time response.
Scatter plots of PILS–ESI/MS measured versus equivalent anion mass
measured as Nr for salts NaNO3 (blue),
NH4NO3 (gold), (NH4)2SO4 (red), and
NH4Cl (magenta). The data are 60 s averages and only include times
when the atomized aerosol output was relatively constant (i.e., not when
concentrations were rising or falling). The slope (1σ) and R2
are
shown.
The Nr-measured (black) CPC number with UHSAS size (blue)
calculated, UHSAS number and size (red) calculated, and PILS–ESI/MS-measured (green) aerosol concentrations (µg m-3) for anions of DMA
size-selected aerosol for salts of (a) NaNO3,
(b) (NH4)2SO4, (c) NH4Cl, and
(d) (NH4)2C2O4. The PILS–ESI/MS traces were
shifted in time several minutes early to account for the delayed instrument
response time.
Even though greater aerosol particle mass could be produced by directly
sampling the polydisperse output of the atomizer, our analysis also included
measurements using the DMA size-selected output. During these tests the flow
was divided between the Nr system, CPC, UHSAS, and PILS–ESI/MS
with a large dilution flow that resulted in turbulent mixing
(Re > 4000). The CPC and UHSAS particle number
concentrations showed improved agreement with turbulent mixing compared to
earlier differences of up to 10 % at high concentrations discussed in
Sect. 2.2.2 and were within a few percent of each other. Examples of the
real-time temporal profiles for these measurements are shown in Fig. 10a–d
with the PILS–ESI/MS time offset by several minutes to account for its
delayed response. The calculated and measured aerosol mass time traces in
Fig. 10 show agreement for all measurement techniques tested in this study.
The figures indicate that the PILS–ESI/MS was not given sufficient time to
rise to a steady constant concentration for the first diameter selected. This
is confirmed in Fig. 10b when 200 nm particles were size
selected twice in succession, with the first selection lasting only
∼ 2 min before flushing with water quickly followed by a longer period
of sampling at the same diameter. The PILS–ESI/MS concentration during this
longer sampling period does reach the expected concentration as indicated by
the Nr (black) and CPC (blue) concentrations. The time series of
oxalate in Fig. 10d shows agreement for the equivalent Nr- and
PILS–ESI/MS-measured mass, indicating these same calibration methods are
effective for organic compounds, although the UHSAS was not sampling during
this experiment. We conclude that the PILS–ESI/MS quantitatively measures
single-component inorganic aerosol for a range of sizes; however, the low
particle throughput hindered
our ability to evaluate the quantitative abilities of the PILS–ESI/MS system
for particles < 200 nm diameter.
These results establish the quantitative abilities of this novel
configuration (PILS–ESI/MS) for sampling simple single-component laboratory-generated aerosol. However, our current ESI/MS calibration methods are
sensitive to the experimental conditions, which must be precisely maintained
during ESI calibrations and throughout the entire sampling period. Changes
in flow rate, interface positioning, or solvent composition have significant
impacts on both the transmission and ionization efficiency, ultimately
effecting pre-determined ESI calibration factors. In general, PILS
characterization has been limited to theoretical predictions or experimental
comparisons that involve coupling the PILS with a mass analyzer (e.g., IC;
Orsini et al., 2003; Sorooshian et al., 2006). Here we show experimentally
that the Nr system can be used as a mass calibration method for pure
Nr-containing polydisperse aerosol.
Summary and conclusions
We report the successful application of a total reactive nitrogen (Nr)
system for conversion of gas-phase and particle-bound Nr compounds. The
Nr system was tested using laboratory-generated aerosol from solutions
of (NH4)2SO4, NH4Cl, NaNO3, and
(NH4)2C2O4. The particle conversion efficiency of
each compound was calculated at each size-selected diameter by the
ratio of the concentration measured as Nr to mass concentrations
calculated from number concentration and size distribution measurements
using a CPC and UHSAS. Overall, the particle conversion efficiency for a
selection of Nr-containing aerosols ranged from 93 to 101 % with an
overall estimated uncertainty of ∼ 10 %. The Nr particles tested span the range of N oxidation states, and therefore we are
confident these results extend to other Nr-containing particles. Most
catalyst-based Nr systems measure total gas-phase Nr-only,
individual Nr compounds (e.g., NH3), or ignore the contribution of
particulate Nr to total signal completely. However, it is useful to
measure the total unspeciated Nr signal, which includes both gases and
particles, to improve our understanding of total N emissions and their
deposition, loss, and availability in ecosystems (e.g., McCalley and Sparks,
2009). We have presented a rapid, robust measurement technique that
quantitatively measures particle Nr mass that allows for accurately
interpreting ambient measurements and allows improved mass closure of the
N budget to be constructed for the 2016 Fire Sciences Laboratory
measurements of wildfire emissions. Future applications of this custom
system aim to distinguish gas- and particle-phase nitrogen contributions from
total measured Nr signal using upstream filters and denuders.
Additional characterization tests showed the platinum catalyst in the
Nr system quantitatively converts both gaseous- and particulate-organic
carbon (OC) to CO2 to within the propagated uncertainties of each
detection method (±10 % each). The resulting Nr and Cy
signals from each detector are in proportion with the number of carbon and
nitrogen atoms in the parent molecule. In order for this to be a reliable
total particulate carbon measurement system under ambient conditions, a
highly accurate and precise CO2 measurement system is imperative to
measure the signal above ambient CO2, CO, and CH4 backgrounds.
Alternatively, ambient gas-phase constituents could be effectively
eliminated from the sampling matrix. For these reasons, the application of
the system is currently limited to calibration of single-component OC-
and/or Nr-containing particles.
After establishing efficient conversion of Nr particles, we
experimentally demonstrated that the Nr conversion technique can be
used to calibrate aerosol particle mass measurement methods when sampling
pure Nr-containing polydisperse aerosol. The Nr equivalent mass
measurements of pure atomized polydisperse aerosol showed an agreement
within ±6 % with the PILS–ESI/MS measurements of the corresponding
anion for the salts (NH4)2SO4, NH4Cl, NaNO3, and
NH4NO3. There is a clear advantage to calibrating the entire
PILS–ESI/MS system altogether as this avoids complications arising from
calibrating the ESI/MS and PILS independently. We conclude that the Nr
system is an effective measurement technique that can be used to directly
calibrate aerosol mass measurement instruments. With this direct mass
calibration method, complications that arise due to optical (e.g., refractive
index) and physical properties (e.g., morphologies) in particle number
calibration methods are avoided. Additionally, this method is an online
technique that provides a rapid measurement of particle mass unlike offline
mass measurement methods such as filter analyses. The Nr converter
described followed by NO and CO2 detection constitute a viable new approach for
calibrating aerosol mass instrumentation for both N-containing and organic
carbon particles.