Anthropogenic secondary organic aerosol (ASOA), formed from anthropogenic emissions of organic compounds, constitutes a substantial fraction of the mass of submicron aerosol in populated areas around ...the world and contributes to poor air quality and premature mortality. However, the precursor sources of ASOA are poorly understood, and there are large uncertainties in the health benefits that might accrue from reducing anthropogenic organic emissions. We show that the production of ASOA in 11 urban areas on three continents is strongly correlated with the reactivity of specific anthropogenic volatile organic compounds. The differences in ASOA production across different cities can be explained by differences in the emissions of aromatics and intermediate- and semi-volatile organic compounds, indicating the importance of controlling these ASOA precursors. With an improved model representation of ASOA driven by the observations, we attribute 340 000 PM2.5-related premature deaths per year to ASOA, which is over an order of magnitude higher than prior studies. A sensitivity case with a more recently proposed model for attributing mortality to PM2.5 (the Global Exposure Mortality Model) results in up to 900 000 deaths. A limitation of this study is the extrapolation from cities with detailed studies and regions where detailed emission inventories are available to other regions where uncertainties in emissions are larger. In addition to further development of institutional air quality management infrastructure, comprehensive air quality campaigns in the countries in South and Central America, Africa, South Asia, and the Middle East are needed for further progress in this area.
Volatile organic compounds (VOCs) emitted from biomass burning impact air quality and climate. Laboratory studies have shown that the variability in VOC speciation is largely driven by changes in ...combustion conditions and is only modestly impacted by fuel type. Here, we report that emissions of VOCs measured in ambient smoke emitted from western US wildfires can be parameterized by high- and low-temperature pyrolysis VOC profiles and are consistent with previous observations from laboratory simulated fires. This is demonstrated using positive matrix factorization (PMF) constrained by high- and low-temperature factors using VOC measurements obtained with a proton-transfer reaction time-of-flight mass spectrometer (PTR-ToF-MS) on board the NASA DC-8 during the FIREX-AQ (Fire Influence on Regional and Global Environments and Air Quality) project in 2019. A linear combination of high- and low-temperature factors described more than 70% of the variability of VOC emissions of long-lived VOCs in all sampled wildfire plumes. An additional factor attributable to atmospheric aging was required to parameterize short-lived and secondarily produced VOCs. The relative contribution of the PMF-derived high-temperature factor for a given fire plume was strongly correlated with the fire radiative power (FRP) at the estimated time of emission detected by satellite measurements. By combining the FRP with the fraction of the high-temperature PMF factor, the emission ratios (ERs) of VOCs to carbon monoxide (CO) in fresh wildfires were estimated and agree well with measured ERs (r 2 = 0.80–0.93).
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Cooking is a source of volatile organic compounds (VOCs), which degrade air quality. Cooking VOCs have been investigated in laboratory and indoor studies, but the contribution of cooking to the ...spatial and temporal variability in urban VOCs is uncertain. In this study, a proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS) is used to identify and quantify cooking emission in Las Vegas, NV, with supplemental data from Los Angeles, CA, and Boulder, CO. Mobile laboratory data show that long-chain aldehydes, such as octanal and nonanal, are significantly enhanced in restaurant plumes and regionally enhanced in areas of Las Vegas with high restaurant densities. Correlation analyses show that long-chain fatty acids are also associated with cooking emissions and that the relative VOC enhancements observed in regions with dense restaurant activity are very similar to the distribution of VOCs observed in laboratory cooking studies. Positive matrix factorization (PMF) is used to quantify cooking emissions from ground site measurements and to compare the magnitude of cooking with other important urban sources, such as volatile chemical products and fossil fuel emissions. PMF shows that cooking may account for as much as 20 % of the total anthropogenic VOC emissions observed by PTR-ToF-MS. In contrast, emissions estimated from county-level inventories report that cooking accounts for less than 1 % of urban VOCs. Current emissions inventories do not fully account for the emission rates of long-chain aldehydes reported here; thus, further work is likely needed to improve model representations of important aldehyde sources, such as commercial and residential cooking.
We deployed an extractive electrospray ionization
time-of-flight mass spectrometer (EESI-MS) for airborne measurements of
biomass burning aerosol during the Fire Influence on Regional to Global
...Environments and Air Quality (FIREX-AQ) study onboard the NASA DC-8 research
aircraft. Through optimization of the electrospray working solution, active
control of the electrospray region pressure, and precise control of
electrospray capillary position, we achieved 1 Hz quantitative measurements
of aerosol nitrocatechol and levoglucosan concentrations up to pressure
altitudes of 7 km. The EESI-MS response to levoglucosan and nitrocatechol was
calibrated for each flight, with flight-to-flight calibration variability of
60 % (1σ). Laboratory measurements showed no aerosol size
dependence in EESI-MS sensitivity below particle geometric diameters of 400 nm, covering 82 % of accumulation-mode aerosol mass during FIREX-AQ. We
also present a first in-field intercomparison of EESI-MS with a chemical
analysis of aerosol online proton-transfer-reaction mass spectrometer
(CHARON PTR-MS) and a high-resolution Aerodyne aerosol mass spectrometer
(AMS). EESI-MS and CHARON PTR-MS levoglucosan concentrations were well
correlated, with a regression slope of 0.94 (R2=0.77). AMS
levoglucosan-equivalent concentrations and EESI-MS levoglucosan showed
a greater difference, with a regression slope of 1.36 (R2=0.96),
likely indicating the contribution of other compounds to the AMS
levoglucosan-equivalent measurement. The total EESI-MS signal showed correlation
(R2=0.9) with total organic aerosol measured by AMS, and the
EESI-MS bulk organic aerosol sensitivity was 60 % of the sensitivity to
levoglucosan standards.
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Isoprene NO3 Oxidation Products from the RO2 + HO2 Pathway Schwantes, Rebecca H; Teng, Alexander P; Nguyen, Tran B ...
The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory,
10/2015, Volume:
119, Issue:
40
Journal Article
Peer reviewed
We describe the products of the reaction of the hydroperoxy radical (HO2) with the alkylperoxy radical formed following addition of the nitrate radical (NO3) and O2 to isoprene. NO3 adds ...preferentially to the C1 position of isoprene (>6 times more favorably than addition to C4), followed by the addition of O2 to produce a suite of nitrooxy alkylperoxy radicals (RO2). At an RO2 lifetime of ∼30 s, δ-nitrooxy and β-nitrooxy alkylperoxy radicals are present in similar amounts. Gas-phase product yields from the RO2 + HO2 pathway are identified as 0.75–0.78 isoprene nitrooxy hydroperoxide (INP), 0.22 methyl vinyl ketone (MVK) + formaldehyde (CH2O) + hydroxyl radical (OH) + nitrogen dioxide (NO2), and 0–0.03 methacrolein (MACR) + CH2O + OH + NO2. We further examined the photochemistry of INP and identified propanone nitrate (PROPNN) and isoprene nitrooxy hydroxyepoxide (INHE) as the main products. INHE undergoes similar heterogeneous chemistry as isoprene dihydroxy epoxide (IEPOX), likely contributing to atmospheric secondary organic aerosol formation.
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The evolution of organic aerosol (OA) and aerosol size distributions within smoke plumes is uncertain due to the variability in rates of coagulation and OA condensation/evaporation between different ...smoke plumes and at different locations within a single plume. We use aircraft data from the FIREX-AQ campaign to evaluate differences in evolving aerosol size distributions, OA, and oxygen to carbon ratios (O:C) between and within smoke plumes during the first several hours of aging as a function of smoke concentration. The observations show that the median particle diameter increases faster in smoke of a higher initial OA concentration (1000 µg m.sup.-3 ), with diameter growth of over 100 nm in 8 h - despite generally having a net decrease in OA enhancement ratios - than smoke of a lower initial OA concentration (<100 µg m.sup.-3 ), which had net increases in OA. Observations of OA and O:C suggest that evaporation and/or secondary OA formation was greater in less concentrated smoke prior to the first measurement (5-57 min after emission). We simulate the size changes due to coagulation and dilution and adjust for OA condensation/evaporation based on the observed changes in OA. We found that coagulation explains the majority of the diameter growth, with OA evaporation/condensation having a relatively minor impact. We found that mixing between the core and edges of the plume generally occurred on timescales of hours, slow enough to maintain differences in aging between core and edge but too fast to ignore the role of mixing for most of our cases.
The impact of biomass burning (BB) on the atmospheric burden of volatile organic compounds (VOCs) is highly uncertain. Here we apply the GEOS-Chem
chemical transport model (CTM) to constrain ...BB emissions in the western USA at ∼ 25 km resolution. Across three BB emission inventories
widely used in CTMs, the inventory–inventory comparison suggests that the totals of 14 modeled BB VOC emissions in the western USA agree with each
other within 30 %–40 %. However, emissions for individual VOCs can differ by a factor of 1–5, driven by the regionally averaged emission
ratios (ERs, reflecting both assigned ERs for specific biome and vegetation classifications) across the three inventories. We further evaluate GEOS-Chem
simulations with aircraft observations made during WE-CAN (Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption and Nitrogen) and
FIREX-AQ (Fire Influence on Regional to Global Environments and Air Quality) field campaigns. Despite being driven by different global BB
inventories or applying various injection height assumptions, the model–observation comparison suggests that GEOS-Chem simulations underpredict
observed vertical profiles by a factor of 3–7. The model shows small to no bias for most species in low-/no-smoke conditions. We thus attribute the
negative model biases mostly to underestimated BB emissions in these inventories. Tripling BB emissions in the model reproduces observed vertical
profiles for primary compounds, i.e., CO, propane, benzene, and toluene. However, it shows no to less significant improvements for oxygenated
VOCs, particularly for formaldehyde, formic acid, acetic acid, and lumped ≥ C3 aldehydes, suggesting the model is missing secondary
sources of these compounds in BB-impacted environments. The underestimation of primary BB emissions in inventories is likely attributable to
underpredicted amounts of effective dry matter burned, rather than errors in fire detection, injection height, or ERs, as constrained by aircraft
and ground measurements. We cannot rule out potential sub-grid uncertainties (i.e., not being able to fully resolve fire plumes) in the nested
GEOS-Chem which could explain the negative model bias partially, though back-of-the-envelope calculation and evaluation using longer-term ground
measurements help support the argument of the dry matter burned underestimation. The total ERs of the 14 BB VOCs implemented in GEOS-Chem only
account for half of the total 161 measured VOCs (∼ 75 versus 150 ppb ppm−1). This reveals a significant amount of missing reactive
organic carbon in widely used BB emission inventories. Considering both uncertainties in effective dry matter burned (× 3) and unmodeled
VOCs (× 2), we infer that BB contributed to 10 % in 2019 and 45 % in 2018 (240 and 2040 Gg C) of the total VOC primary
emission flux in the western USA during these two fire seasons, compared to only 1 %–10 % in the standard GEOS-Chem.
Biomass burning (BB) is a major source of reactive organic carbon
into the atmosphere. Once in the atmosphere, these organic BB emissions, in
both the gas and particle phases, are subject to ...atmospheric oxidation,
though the nature and impact of the chemical transformations are not
currently well constrained. Here we describe experiments carried out as part
of the FIREX FireLab campaign, in which smoke from the combustion of fuels
typical of the western United States was sampled into an environmental chamber and
exposed to high concentrations of OH, to simulate the equivalent of up to
2 d of atmospheric oxidation. The evolution of the organic mixture was
monitored using three real-time time-of-flight mass spectrometric
instruments (a proton transfer reaction mass spectrometer, an iodide
chemical ionization mass spectrometer, and an aerosol mass spectrometer),
providing measurements of both individual species and ensemble properties of
the mixture. The combined measurements from these instruments achieve a
reasonable degree of carbon closure (within 15 %–35 %), indicating that most
of the reactive organic carbon is measured by these instruments. Consistent
with our previous studies of the oxidation of individual organic species,
atmospheric oxidation of the complex organic mixture leads to the formation
of species that on average are smaller and more oxidized than those in the
unoxidized emissions. In addition, the comparison of mass spectra from the
different fuels indicates that the oxidative evolution of BB emissions
proceeds largely independent of fuel type, with different fresh smoke
mixtures ultimately converging into a common, aged distribution of gas-phase
compounds. This distribution is characterized by high concentrations of
several small, volatile oxygenates, formed from fragmentation reactions, as
well as a complex pool of many minor oxidized species and secondary organic
aerosol, likely formed via functionalization processes.
Gas-phase low volatility organic compounds (LVOC), produced from oxidation of isoprene 4-hydroxy-3-hydroperoxide (4,3-ISOPOOH) under low-NO conditions, were observed during the FIXCIT chamber study. ...Decreases in LVOC directly correspond to appearance and growth in secondary organic aerosol (SOA) of consistent elemental composition, indicating that LVOC condense (at OA below 1 μg m–3). This represents the first simultaneous measurement of condensing low volatility species from isoprene oxidation in both the gas and particle phases. The SOA formation in this study is separate from previously described isoprene epoxydiol (IEPOX) uptake. Assigning all condensing LVOC signals to 4,3-ISOPOOH oxidation in the chamber study implies a wall-loss corrected non-IEPOX SOA mass yield of ∼4%. By contrast to monoterpene oxidation, in which extremely low volatility VOC (ELVOC) constitute the organic aerosol, in the isoprene system LVOC with saturation concentrations from 10–2 to 10 μg m–3 are the main constituents. These LVOC may be important for the growth of nanoparticles in environments with low OA concentrations. LVOC observed in the chamber were also observed in the atmosphere during SOAS-2013 in the Southeastern United States, with the expected diurnal cycle. This previously uncharacterized aerosol formation pathway could account for ∼5.0 Tg yr–1 of SOA production, or 3.3% of global SOA.
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Volatile chemical products (VCP) are an increasingly important source of hydrocarbon and oxygenated volatile organic compound (OVOC) emissions to the atmosphere, and these emissions are likely to ...play an important role as anthropogenic precursors for secondary organic aerosol (SOA). While the SOA from VCP hydrocarbons is often accounted for in models, the formation, evolution, and properties of SOA from VCP OVOCs remain uncertain. We use environmental chamber data and a kinetic model to develop SOA parameters for 10 OVOCs representing glycols, glycol ethers, esters, oxygenated aromatics, and amines. Model simulations suggest that the SOA mass yields for these OVOCs are of the same magnitude as widely studied SOA precursors (e.g., long-chain alkanes, monoterpenes, and single-ring aromatics), and these yields exhibit a linear correlation with the carbon number of the precursor. When combined with emissions inventories for two megacities in the United States (US) and a US-wide inventory, we find that VCP VOCs react with OH to form 0.8–2.5× as much SOA, by mass, as mobile sources. Hydrocarbons (terpenes, branched and cyclic alkanes) and OVOCs (terpenoids, glycols, glycol ethers) make up 60–75 and 25–40% of the SOA arising from VCP use, respectively. This work contributes to the growing body of knowledge focused on studying VCP VOC contributions to urban air pollution.
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