Bromine activation (the production of Br in an elevated oxidation state) promotes ozone destruction and mercury removal in the global troposphere and commonly occurs in both springtime polar boundary ...layers, often accompanied by nearly complete ozone destruction. The chemistry and budget of active bromine compounds (e.g., Br2, BrCl, BrO, HOBr) reflect the cycling of Br and affect its environmental impact. Cyanogen bromide (BrCN) has recently been measured by iodide ion high-resolution time-of-flight mass spectrometry (I- CIMS), and trifluoro methoxide ion time-of-flight mass spectrometry (CF3O- CIMS) during the NASA Atmospheric Tomography Mission second, third, and fourth deployments (NASA ATom), and could be a previously unquantified participant in active Br chemistry. BrCN mixing ratios ranged from below the detection limit (1.5 pptv) up to as high as 36 pptv (10 s average) and enhancements were almost exclusively confined to the polar boundary layers in the Arctic winter and in both polar regions during spring and fall. The coincidence of BrCN with active Br chemistry (often observable BrO, BrCl and O3 loss) and high CHBr3/CH2Br2 ratios imply that much of the observed BrCN is from atmospheric Br chemistry rather than a biogenic source. Likely BrCN formation pathways involve the heterogeneous reactions of active Br (Br2, HOBr) with reduced nitrogen compounds, for example hydrogen cyanide (HCN/CN-), on snow, ice, or particle surfaces. Competitive reaction calculations of HOBr reactions with Cl-/Br- and HCN/CN- in solution, as well as box model calculations with bromine chemistry, confirm the viability of this formation channel and show a distinct pH dependence, with BrCN formation favored at higher pH values. Gas-phase loss processes of BrCN due to reaction with radical species are likely quite slow and photolysis is known to be relatively slow (BrCN lifetime of ∼ 4 months in midlatitude summer). These features, and the lack of BrCN enhancements above the polar boundary layer, imply that surface reactions must be the major loss processes. The fate of BrCN determines whether BrCN production fuels or terminates bromine activation. BrCN reactions with other halogens (Br-, HOCl, HOBr) may perpetuate the active Br cycle; however, preliminary laboratory experiments showed that BrCN did not react with aqueous bromide ion (< 0.1 %) to reform Br2. Liquid-phase reactions of BrCN are more likely to convert Br to bromide (Br-) or form a C–Br bonded organic species, as these are the known condensed-phase reactions of BrCN and would therefore constitute a loss of atmospheric active Br. Thus, further study of the chemistry of BrCN will be important for diagnosing polar Br cycling.
Wildfires are increasing in size across the western US, leading to
increases in human smoke exposure and associated negative health impacts.
The impact of biomass burning (BB) smoke, including ...wildfires, on regional
air quality depends on emissions, transport, and chemistry, including
oxidation of emitted BB volatile organic compounds (BBVOCs) by the hydroxyl
radical (OH), nitrate radical (NO3), and ozone (O3). During the
daytime, when light penetrates the plumes, BBVOCs are oxidized mainly by
O3 and OH. In contrast, at night or in optically dense plumes, BBVOCs
are oxidized mainly by O3 and NO3. This work focuses on the
transition between daytime and nighttime oxidation, which has significant
implications for the formation of secondary pollutants and loss of nitrogen
oxides (NOx=NO+NO2) and has been understudied. We present
wildfire plume observations made during FIREX-AQ (Fire Influence on Regional
to Global Environments and Air Quality), a field campaign involving multiple
aircraft, ground, satellite, and mobile platforms that took place in the
United States in the summer of 2019 to study both wildfire and agricultural
burning emissions and atmospheric chemistry. We use observations from two
research aircraft, the NASA DC-8 and the NOAA Twin Otter, with a detailed
chemical box model, including updated phenolic mechanisms, to analyze smoke
sampled during midday, sunset, and nighttime. Aircraft observations suggest
a range of NO3 production rates (0.1–1.5 ppbv h−1) in plumes
transported during both midday and after dark. Modeled initial instantaneous
reactivity toward BBVOCs for NO3, OH, and O3 is 80.1 %, 87.7 %, and 99.6 %, respectively. Initial NO3 reactivity is 10–104
times greater than typical values in forested or urban environments, and
reactions with BBVOCs account for >97 % of NO3 loss in
sunlit plumes (jNO2 up to 4×10-3s-1), while
conventional photochemical NO3 loss through reaction with NO and
photolysis are minor pathways. Alkenes and furans are mostly oxidized by OH
and O3 (11 %–43 %, 54 %–88 % for alkenes; 18 %–55 %, 39 %–76 %, for furans, respectively), but phenolic oxidation is split between
NO3, O3, and OH (26 %–52 %, 22 %–43 %, 16 %–33 %,
respectively). Nitrate radical oxidation accounts for 26 %–52 % of
phenolic chemical loss in sunset plumes and in an optically thick plume.
Nitrocatechol yields varied between 33 % and 45 %, and NO3
chemistry in BB plumes emitted late in the day is responsible for 72 %–92 % (84 % in an optically thick midday plume) of nitrocatechol
formation and controls nitrophenolic formation overall. As a result,
overnight nitrophenolic formation pathways account for 56 %±2 % of
NOx loss by sunrise the following day. In all but one overnight plume
we modeled, there was remaining NOx (13 %–57 %) and BBVOCs
(8 %–72 %) at sunrise.
Recent work has quantified the delay times in measurements of volatile
organic compounds (VOCs) caused by the partitioning between the gas phase
and the surfaces of the inlet tubing and instrument ...itself. In this study we
quantify wall partitioning effects on time responses and transmission of
multifunctional, semivolatile, and intermediate-volatility organic compounds
(S/IVOCs) with saturation concentrations (C∗) between 100 and 104 µg m−3. The instrument delays of several chemical ionization
mass spectrometer (CIMS) instruments increase with decreasing C∗, ranging from
seconds to tens of minutes, except for the NO3- CIMS where it is
always on the order of seconds. Six different tubing materials were tested.
Teflon, including PFA, FEP, and conductive PFA, performs better than metals
and Nafion in terms of both delay time and transmission efficiency.
Analogous to instrument responses, tubing delays increase as C∗ decreases,
from less than a minute to >100 min. The delays caused by Teflon
tubing vs. C∗ can be modeled using the simple chromatography model of Pagonis
et al. (2017). The model can be used to estimate the equivalent absorbing
mass concentration (Cw) of each material, and to estimate delays under
different flow rates and tubing dimensions. We also include time delay
measurements from a series of small polar organic and inorganic analytes in
PFA tubing measured by CIMS. Small polar molecules behave differently than
larger organic ones, with their delays being predicted by their Henry's law
constants instead of their C∗, suggesting the dominance of partitioning to
small amounts of water on sampling surfaces as a result of their polarity
and acidity properties. PFA tubing has the best performance for gas-only
sampling, while conductive PFA appears very promising for sampling S/IVOCs
and particles simultaneously. The observed delays and low transmission both
affect the quality of gas quantification, especially when no direct
calibration is available. Improvements in sampling and instrument response
are needed for fast atmospheric measurements of a wide range of S/IVOCs
(e.g., by aircraft or for eddy covariance). These methods and results are
also useful for more general characterization of surface–gas interactions.
The declining trend in vehicle emissions has underscored the growing significance of volatile organic compound (VOC) emissions from volatile chemical products (VCPs). However, accurately representing ...VOC chemistry in simplified chemical mechanisms remains challenging due to its chemical complexity including speciation and reactivity. Previous studies have predominantly focused on VOCs from fossil fuel sources, leading to an underrepresentation of VOC chemistry from VCP sources. We developed an integrated chemical mechanism, RACM2B-VCP, that is compatible with WRF-Chem and is aimed at enhancing the representation of VOC chemistry, particularly from VCP sources, within the present urban environment. Evaluation against the Air Quality System (AQS) network data demonstrates that our model configured with RACM2B-VCP reproduces both the magnitude and spatial variability of O3 and PM2.5 in Los Angeles. Furthermore, evaluation against comprehensive measurements of O3 and PM2.5 precursors from the Reevaluating the Chemistry of Air Pollutants in California (RECAP-CA) airborne campaign and the Southwest Urban NOx and VOC Experiment (SUNVEx) ground site and mobile laboratory campaign confirm the model's accuracy in representing NOx and many VOCs and highlight remaining biases. Although there exists an underprediction in the total VOC reactivity of observed VOC species, our model with RACM2B-VCP exhibits good agreement for VOC markers emitted from different sectors, including biogenic, fossil fuel, and VCP sources. Through sensitivity analyses, we probe the contributions of VCP and fossil fuel emissions to total VOC reactivity and O3. Our results reveal that 52 % of the VOC reactivity and 35 % of the local enhancement of MDA8 O3 arise from anthropogenic VOC emissions in Los Angeles. Significantly, over 50 % of this anthropogenic fraction of either VOC reactivity or O3 is attributed to VCP emissions. The RACM2B-VCP mechanism created, described, and evaluated in this work is ideally suited for accurately representing ozone for the right reasons in the present urban environment where mobile, biogenic, and VCP VOCs are all important contributors to ozone formation.
Atmospheric HONO mixing ratios in indoor and outdoor environments span a
range of less than a few parts per trillion by volume (pptv) up to tens of
parts per billion by volume (ppbv) in combustion ...plumes. Previous HONO
calibration sources have utilized proton transfer acid displacement from
nitrite salts or solutions, with output that ranges from tens to thousands
of ppbv. Instrument calibrations have thus required large dilution flows to
obtain atmospherically relevant mixing ratios. Here we present a simple
universal source to reach very low HONO calibration mixing ratios using a
nitrite-coated reaction device with the addition of humid air and/or HCl
from a permeation device. The calibration source developed in this work can
generate HONO across the atmospherically relevant range and has high purity
(> 90 %), reproducibility, and tunability. Mixing ratios at
the tens of pptv level are easily reached with reasonable dilution flows.
The calibration source can be assembled to start producing stable HONO
mixing ratios (relative standard error, RSE ≤ 2 %) within 2 h, with output
concentrations varying ≤ 25 % following simulated transport or
complete disassembly of the instrument and with ≤ 10 % under ideal
conditions. The simplicity of this source makes it highly versatile for
field and lab experiments. The platform facilitates a new level of accuracy
in established instrumentation, as well as intercomparison studies to
identify systematic HONO measurement bias and interferences.
Iodide chemical ionization mass spectrometry (CIMS) is a common analytical
tool used in both laboratory and field experiments to measure a large suite
of atmospherically relevant compounds. Here, we ...describe a systematic ion
molecule reactor (IMR) temperature dependence of iodide CIMS analyte
sensitivity for a wide range of analytes in laboratory experiments. Weakly
bound iodide clusters, such as HCl, HONO, HCOOH, HCN, phenol, 2-nitrophenol,
and acyl peroxynitrate (PAN) detected via the peroxy radical cluster, all
exhibit strong IMR temperature dependence of sensitivity ranging from −3.4 % ∘C−1
to 5.9 % ∘C−1 (from 37 to 47 ∘C). Strongly
bound iodide clusters, such as Br2, N2O5, ClNO2, and PAN
detected via the carboxylate anion, all exhibit little to no IMR temperature
dependence ranging from 0.2 % ∘C−1 to −0.9 % ∘C−1 (from 37 to 47 ∘C). The IMR temperature relationships of weakly bound clusters
provide an estimate of net reaction enthalpy, and comparison with database
values indicates that these clusters are in thermal equilibrium. Ground site
HCOOH data collected in the summer of 2021 in Pasadena (CA) are corrected
and show a reversal in the diel cycle, emphasizing the importance of this
correction (35±6 % during the day, -26±2 % at night).
Finally, we recommend two approaches to minimize this effect in the field,
namely heating or cooling the IMR; the latter technique has the added benefit of
improving absolute sensitivity.
We present a comparison of fast-response instruments installed onboard the NASA DC-8 aircraft that measured nitrogen oxides (NO and NO2), nitrous acid (HONO), total reactive odd nitrogen (measured ...both as the total (NOy) and from the sum of individually measured species (ΣNOy)), and carbon monoxide (CO) in the troposphere during the 2019 Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) campaign. By targeting smoke from summertime wildfires, prescribed fires, and agricultural burns across the continental United States, FIREX-AQ provided a unique opportunity to investigate measurement accuracy in concentrated plumes where hundreds of species coexist. Here, we compare NO measurements by chemiluminescence (CL) and laser-induced fluorescence (LIF); NO2 measurements by CL, LIF, and cavity-enhanced spectroscopy (CES); HONO measurements by CES and iodide-adduct chemical ionization mass spectrometry (CIMS); and CO measurements by tunable diode laser absorption spectrometry (TDLAS) and integrated cavity output spectroscopy (ICOS). Additionally, total NOy measurements using the CL instrument were compared withΣNOy (= NO + NO2 + HONO + nitric acid (HNO3) + acyl peroxy nitrates (APNs) + submicrometer particulate nitrate (pNO3)). Other NOy species were not included in ΣNOy as they either contributed minimally to it (e.g., C1–C5 alkyl nitrates, nitryl chloride (ClNO2), dinitrogen pentoxide (N2O5)) or were not measured during FIREX-AQ (e.g., higher oxidized alkyl nitrates, nitrate (NO3), non-acyl peroxynitrates, coarse-mode aerosol nitrate). The aircraft instrument intercomparisons demonstrate the following points: (1) NO measurements by CL and LIF agreed well within instrument uncertainties but with potentially reduced time response for the CL instrument; (2) NO2 measurements by LIF and CES agreed well within instrument uncertainties, but CL NO2 was on average 10 % higher; (3) CES and CIMS HONO measurements were highly correlated in each fire plume transect, but the correlation slope of CES vs. CIMS for all 1 Hz data during FIREX-AQ was 1.8, which we attribute to a reduction in the CIMS sensitivity to HONO in high-temperature environments; (4) NOy budget closure was demonstrated for all flights within the combined instrument uncertainties of 25 %. However, we used a fluid dynamic flow model to estimate that average pNO3 sampling fraction through the NOy inlet in smoke was variable from one flight to another and ranged between 0.36 and 0.99, meaning that approximately 0 %–24 % on average of the total measured NOy in smoke may have been unaccounted for and may be due to unmeasured species such as organic nitrates; (5) CO measurements by ICOS and TDLAS agreed well within combined instrument uncertainties, but with a systematic offset that averaged 2.87 ppbv; and (6) integrating smoke plumes followed by fitting the integrated values of each plume improved the correlation between independent measurements.
We present the development of a chemical ionization mass spectrometer ion source specifically designed for in situ measurements of trace gases in
the upper troposphere and lower stratosphere. The ion ...source utilizes a
commercially available photoionization krypton lamp, primarily emitting
photons in the vacuum ultraviolet (VUV) region at wavelengths of 124 and 117 nm (corresponding to energies of 10 and 10.6 eV, respectively), coupled to a
commercially available Vocus proton transfer reaction mass spectrometer. The
VUV ion source can produce both negative and positive reagent ions; however,
here we primarily focus on generating iodide anions (I−). The
instrument's drift tube (also known as ion–molecule reactor) operates at
pressures between 2 and 10 mbar, which facilitates ambient sampling at
atmospheric pressures as low as 50 mbar. The low operating pressure reduces
secondary ion chemistry that can occur in iodide chemical ionization mass spectrometry (CIMS). It also allows the
addition of water vapor to the drift tube to exceed typical ambient humidity
by more than 1 order of magnitude, significantly reducing ambient humidity
dependence of sensitivities. An additional benefit of this ion source and
drift tube is a 10- to 100-fold reduction in nitrogen consumed during
operation relative to standard I− ion sources, resulting in
significantly reduced instrument weight and operational costs. In iodide
mode, sensitivities of 76 cps ppt−1 for nitric acid, 35 cps ppt−1 for Br2
and 8.9 cps ppt−1 for Cl2 were achieved. Lastly, we demonstrate that this
ion source can generate benzene (C6H6+) and ammonium
(NH4+) reagent ions to expand the number of detected atmospheric
trace gases.
Isocyanic acid (HNCO), an acidic gas found in tobacco smoke, urban environments, and biomass-burning-affected regions, has been linked to adverse health outcomes. Gasoline- and diesel-powered engines ...and biomass burning are known to emit HNCO and hypothesized to emit precursors such as amides that can photochemically react to produce HNCO in the atmosphere. Increasingly, diesel engines in developed countries like the United States are required to use selective catalytic reduction (SCR) systems to reduce tailpipe emissions of oxides of nitrogen. SCR chemistry is known to produce HNCO as an intermediate product, and SCR systems have been implicated as an atmospheric source of HNCO. In this work, we measure HNCO emissions from an SCR system-equipped diesel engine and, in combination with earlier data, use a three-dimensional chemical transport model (CTM) to simulate the ambient concentrations and source/pathway contributions to HNCO in an urban environment. Engine tests were conducted at three different engine loads, using two different fuels and at multiple operating points. HNCO was measured using an acetate chemical ionization mass spectrometer. The diesel engine was found to emit primary HNCO (3–90 mg kg fuel−1) but we did not find any evidence that the SCR system or other aftertreatment devices (i.e., oxidation catalyst and particle filter) produced or enhanced HNCO emissions. The CTM predictions compared well with the only available observational datasets for HNCO in urban areas but underpredicted the contribution from secondary processes. The comparison implied that diesel-powered engines were the largest source of HNCO in urban areas. The CTM also predicted that daily-averaged concentrations of HNCO reached a maximum of ∼ 110 pptv but were an order of magnitude lower than the 1 ppbv level that could be associated with physiological effects in humans. Precursor contributions from other combustion sources (gasoline and biomass burning) and wintertime conditions could enhance HNCO concentrations but need to be explored in future work.
Extensive airborne measurements of non-methane organic gases (NMOGs), methane, nitrogen oxides, reduced nitrogen species, and aerosol emissions from US wild and prescribed fires were conducted during ...the 2019 NOAA/NASA Fire Influence on Regional to Global Environments and Air Quality campaign (FIREX-AQ). Here, we report the atmospheric enhancement ratios (ERs) and inferred emission factors (EFs) for compounds measured on board the NASA DC-8 research aircraft for nine wildfires and one prescribed fire, which encompass a range of vegetation types.
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