Abstract
Large wildfires markedly alter regional atmospheric composition, but chemical complexity challenges model predictions of downwind impacts. Here, we elucidate key facets of gas-phase photochemistry and assess novel chemical processes via a case study of the 2013 California Rim Fire plume. Airborne in situ observations, acquired during the NASA Studies of Emissions, Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS) mission, illustrate the evolution of volatile organic compounds (VOC), oxidants, and reactive nitrogen over 12 hours of atmospheric aging. Measurements show rapid formation of ozone and peroxyacyl nitrates (PNs), sustained peroxide production, and prolonged enhancements in oxygenated VOC and nitrogen oxides (NOx). Measurements and Lagrangian trajectories constrain a 0-D puff model that approximates plume photochemical history and provides a framework for evaluating key processes. Simulations examine the effects of 1) previously-unmeasured reactive VOC identified in recent laboratory studies, and 2) emissions and secondary production of nitrous acid (HONO). Inclusion of estimated unmeasured VOC leads to a 250 % increase in OH reactivity and a 70 % increase in radical production via oxygenated VOC photolysis. HONO amplifies radical cycling and serves as a downwind NOx source, although two different HONO production mechanisms (particulate nitrate photolysis and heterogeneous NO2 conversion) exhibit markedly different effects on ozone, NOx, and PNs. Analysis of radical initiation rates suggests that oxygenated VOC photolysis is a major radical source, exceeding HONO photolysis when averaged over the first 2 hours of aging. Ozone production chemistry transitions from VOC-sensitive to NOx-sensitive within the first hour of plume aging, with both peroxide and organic nitrate formation contributing significantly to radical termination. To simulate smoke plume chemistry accurately, models should simultaneously account for the full reactive VOC pool and all relevant oxidant sources.
Highlights
55 Biomass burning accounts for at least 30% of global emissions of non-methane volatile organic compounds (VOC) (Akagi et al, 2011; Andreae, 2019; Yokelson et al, 2008)
Normalized excess mixing ratios (NEMRs) for these gases peak downwind of the Rim Fire; for example, the highest propane NEMR occurs at an age of 4 h and is 50% higher than the initial value
Emission ratios (Liu et al, 2017; Permar et al, 2021) that are not observed. These VOC are minor contributors to plume photochemistry; this comparison underscores the challenge of accounting for background variability in Lagrangian or pseudo-Lagrangian simulations
Summary
55 Biomass burning accounts for at least 30% of global emissions of non-methane volatile organic compounds (VOC) (Akagi et al, 2011; Andreae, 2019; Yokelson et al, 2008). 60 compilations are incomplete, and it is estimated that previously “unidentified” VOC account for ~50% of total pyrogenic VOC mass (Akagi et al, 2011; Yokelson et al, 2013; Gilman et al, 2015). With extended VOC inventories from recent laboratory work (Hatch et al., 70 2017; Koss et al, 2018), several studies have begun characterizing the impacts of previously unidentified VOC. Analyzing a series of laboratory burns, Coggon et al (2019) estimate that species included in the Master Chemical Mechanism (MCMv3.3.1) account for ~60% of the primary hydroxyl radical (OH) reactivity measured via Proton Transfer Time-of-flight Mass Spectrometry (PTR-MS). Decker et al (2019) estimate 75 that the MCM accounts for ~30% of the observed nitrate radical (NO3) reactivity in the same laboratory experiments
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