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Atmospheric chemistry models generally assume organic aerosol (OA) to be photochemically inert. Recent mechanisms for the oxidation of biogenic isoprene, a major source of secondary organic aerosol (iSOA), produce excessive OA in the absence of subsequent OA reactivity. At the same time, models underestimate atmospheric concentrations of formic and acetic acids for which OA degradation could provide a source. Here we show that the aqueous photooxidation of an isoprene-derived organosulfate (2-methyltriolsulfate or MTS), an important iSOA component, produces formic and acetic acids in high yields and at timescales competitive with deposition. Experimental data are well fit by a kinetic model in which three sequential oxidation reactions of the isoprene organosulfate produce two molar equivalents of formic acid and one of acetic acid. We incorporate this chemistry and that of 2-methyltetrol, another ubiquitous iSOA component, into the GEOS-Chem global atmospheric chemistry model. Simulations show that photooxidation and subsequent revolatilization of this iSOA may account for up to half of total iSOA loss globally, producing 4 Tg a−1 each of formic and acetic acids. This reduces model biases in gas-phase formic acid and total organic aerosol over the Southeast United States in summer by ∼30% and 60% respectively. While our study shows the importance of adding iSOA photochemical sinks into atmospheric models, uncertainties remain that warrant further study. In particular, improved understanding of reaction dependencies on particle characteristics and concentrations of particle-phase OH and other oxidants are needed to better simulate the effects of this chemistry on the atmospheric budgets of organic acids and iSOA.
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This content will become publicly available on December 11, 2025
Process analysis of elevated concentrations of organic acids at Whiteface Mountain, New York
Abstract. Organic acids represent an important class of compounds in the atmosphere, but there is limited research investigating their chemical production, particularly in the northeast United States. To improve our understanding of organic acid sources, a modeling analysis was performed for air masses reaching the summit of Whiteface Mountain (WFM), New York, where measurements of organic acids in cloud water have been collected. The analysis focuses on a pollution event associated with a heat wave that occurred on 1–2 July 2018 that exhibited unusually high concentrations of formic (HCOOH), acetic (CH3COOH), and oxalic (OxAc) acid in cloud water. The gas-phase production of organic acids for this pollution event was modeled using a combination of the regional transport model Weather Research and Forecasting Model with Chemistry (WRF-Chem), which gives information on transport and environmental factors affecting air parcels reaching WFM, and the Lagrangian chemical box model BOXMOX, which allows analysis of chemistry with different chemical mechanisms. Two chemical mechanisms are used in BOXMOX: (1) the Model for Ozone and Related chemical Tracers (MOZART T1) and (2) the Master Chemical Mechanism (MCM) version 3.3.1. The WRF-Chem results show that air parcels sampled during the pollution event at WFM originated in central Missouri, which has strong biogenic emissions of isoprene. Many air parcels were influenced by emissions of nitrogen oxides (NOx) from the Chicago metropolitan area. The gas-phase oxidation of isoprene and its related oxidation products was the major source of HCOOH and CH3COOH, but both mechanisms substantially underproduced both acids compared to observations. A simple gas–aqueous mechanism was included to investigate the role of aqueous chemistry in organic acid production. Aqueous chemistry did not produce more HCOOH or CH3COOH, suggesting missing chemical sources of both acids. However this aqueous chemistry was able to explain the elevated concentrations of OxAc. Anthropogenic NOx emissions from Chicago had little overall impact on the production of all three organic acids. Further studies are required to better constrain gas and aqueous production of low-molecular-weight organic acids.
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- Award ID(s):
- 1945563
- PAR ID:
- 10570962
- Editor(s):
- Russell, Lynn M
- Publisher / Repository:
- European Geophysical Union
- Date Published:
- Journal Name:
- Atmospheric Chemistry and Physics
- Volume:
- 24
- Issue:
- 23
- ISSN:
- 1680-7324
- Page Range / eLocation ID:
- 13693-13713
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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