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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. 

Abstract. Organic aerosols generated from the smoldering combustion of woodcritically impact air quality and health for billions of people worldwide;yet, the links between the chemical components and the optical or biologicaleffects of woodsmoke aerosol (WSA) are still poorly understood. In thiswork, an untargeted analysis of the molecular composition of smoldering WSA,generated in a controlled environment from nine types of heartwood fuels(African mahogany, birch, cherry, maple, pine, poplar, red oak, redwood, andwalnut), identified several hundred compounds using gas chromatography massspectrometry (GC-MS) and nano-electrospray high-resolution mass spectrometry(HRMS) with tandem multistage mass spectrometry (MSn). The effects ofWSA on cell toxicity as well as gene expression dependent on the aryl hydrocarbon receptor (AhR) and estrogen receptor(ER) were characterized with cellular assays, andthe visible mass absorption coefficients (MACvis) of WSA were measuredwith ultraviolet–visible spectroscopy. The WSAs studied in this work have significantlevels of biological and toxicological activity, with exposure levels inboth an outdoor and indoor environment similar to or greater than those ofother toxicants. A correlation between the HRMS molecular composition andaerosol properties found that phenolic compounds from the oxidativedecomposition of lignin are the main drivers of aerosol effects, while thecellulose decomposition products play a secondary role; e.g., levoglucosanis anticorrelated with multiple effects. Polycyclic aromatic hydrocarbons(PAHs) are not expected to form at the combustion temperature in this work,nor were they observed above the detection limit; thus, biological and opticalproperties of the smoldering WSA are not attributed to PAHs. Syringylcompounds tend to correlate with cell toxicity, while the more conjugatedmolecules (including several compounds assigned to dimers) have higher AhRactivity and MACvis. The negative correlation between cell toxicity andAhR activity suggests that the toxicity of smoldering WSA to cells is notmediated by the AhR. Both mass-normalized biological outcomes have astatistically significant dependence on the degree of combustion of thewood. In addition, our observations support the fact that the visible lightabsorption of WSA is at least partially due to charge transfer effects inaerosols, as previously suggested. Finally, MACvis has no correlationwith toxicity or receptor signaling, suggesting that key chromophores inthis work are not biologically active on the endpoints tested.