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Isoprene emitted by vegetation is an important precursor of secondary organic aerosol (SOA), but the mechanism and yields are uncertain. Aerosol is prevailingly aqueous under the humid conditions typical of isoprene-emitting regions. Here we develop an aqueous-phase mechanism for isoprene SOA formation coupled to a detailed gas-phase isoprene oxidation scheme. The mechanism is based on aerosol reactive uptake coefficients (γ) for water-soluble isoprene oxidation products, including sensitivity to aerosol acidity and nucleophile concentrations. We apply this mechanism to simulation of aircraft (SEAC4RS) and ground-based (SOAS) observations over the southeast US in summer 2013 using the GEOS-Chem chemical transport model. Emissions of nitrogen oxides (NOx ≡ NO + NO2) over the southeast US are such that the peroxy radicals produced from isoprene oxidation (ISOPO2) react significantly with both NO (high-NOx pathway) and HO2 (low-NOx pathway), leading to different suites of isoprene SOA precursors. We find a mean SOA mass yield of 3.3 % from isoprene oxidation, consistent with the observed relationship of total fine organic aerosol (OA) and formaldehyde (a product of isoprene oxidation). Isoprene SOA production is mainly contributed by two immediate gas-phase precursors, isoprene epoxydiols (IEPOX, 58 % of isoprene SOA) from the low-NOx pathway and glyoxal (28 %) from both low- and high-NOx pathways. This speciation is consistent with observations of IEPOX SOA from SOAS and SEAC4RS. Observations show a strong relationship between IEPOX SOA and sulfate aerosol that we explain as due to the effect of sulfate on aerosol acidity and volume. Isoprene SOA concentrations increase as NOx emissions decrease (favoring the low-NOx pathway for isoprene oxidation), but decrease more strongly as SO2 emissions decrease (due to the effect of sulfate on aerosol acidity and volume). The US Environmental Protection Agency (EPA) projects 2013–2025 decreases in anthropogenic emissions of 34 % for NOx (leading to a 7 % increase in isoprene SOA) and 48 % for SO2 (35 % decrease in isoprene SOA). Reducing SO2 emissions decreases sulfate and isoprene SOA by a similar magnitude, representing a factor of 2 co-benefit for PM2.5 from SO2 emission controls.
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Understanding the Recent Stagnation in PM 2.5 Concentrations Across the United States: A Seasonal Composition Perspective
Abstract Long‐term declines in concentrations of fine particulate matter (PM2.5) in the United States (U.S.) have been disrupted in recent years, with recent trends stagnating or reversing. In this study, we analyze surface observations of PM2.5composition from 2002 to 2022 to identify the chemical components driving this shift. We find that PM2.5concentrations plateau across seasons and regions in the contiguous U.S. since 2016, even after excluding estimated wildfire impacts, suggesting that the rise in wildfire activity alone does not account for these trends. The stagnation is primarily driven by a slowdown in the reduction of sulfate and a non‐significant increase in organic aerosols. In the Eastern and Central U.S., sulfate concentrations generally mirror decreasing anthropogenic SO2emissions, except in winter, where chemical feedbacks related to oxidant limitations weaken the response of sulfate. We find that nitrate and NO2concentrations decrease slower than anthropogenic nitrogen oxides (NOx) emissions, particularly in fall and winter, suggesting a potential overestimate in the decrease of NOxemissions in the U.S. Environmental Protection Agency National Emission Inventory (NEI) and/or an increasing role of natural and non‐U.S. sources. In the Southeast, the decline in organic aerosol concentrations has stalled since 2015, possibly due to weaker decreases in sulfate‐induced secondary organic aerosol (SOA) formation from isoprene, combined with increases in monoterpene‐derived SOA as the climate warms. Despite continued decreases in the NEI black carbon (BC) emissions, BC concentrations have stagnated since 2015, even after removing the estimated influence of wildfire smoke, indicating a possible underestimate in emissions.
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- Award ID(s):
- 2023670
- PAR ID:
- 10614181
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Atmospheres
- Volume:
- 130
- Issue:
- 12
- ISSN:
- 2169-897X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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