Abstract: Non-methane volatile organic compounds (NMVOC) are emitted into the Earth’s atmosphere by varied biogenic and anthropogenic sources. Though the concentrations of these compounds are minute, they exert an outsized influence on atmospheric composition, primarily through their oxidation chemistry. This chemistry leads to the formation of key secondary species including tropospheric ozone, a harmful pollutant, and secondary organic aerosol (SOA), a key component of atmospheric particulate matter with implications for climate and air quality. Oxidation chemistry is a dense web of interconnected reactions. The branch points in this web are reactive intermediates: short-lived, open-shell species that often have several chemical removal pathways available to them. Peroxy radicals, RO2, formed from the reaction of alkyl radicals and molecular oxygen, are key intermediates in the atmospheric oxidation process. In the atmosphere, RO2 generally have 4 pathways available to them: 1) reaction with NO, 2) reaction with HO2, 3) isomerization, and 4) self- or cross-reactions. The relative importance of these pathways is often determinative of the ultimate outcomes of oxidation chemistry, affecting the extent to which oxidation results in the formation of secondary species, including ozone and SOA. Our group uses a variety of model-informed experimental approaches to investigate the fates of atmospheric peroxy radicals and their impacts on atmospheric composition. In this talk, I will focus on a series of recent, unconventional environmental chamber experiments designed to probe the product distributions that arise from isomerizations and self- or cross-reactions or single isomers of RO2. These pathways remain uncertain, and have only recently been appreciated as important in the formation of secondary organic aerosol. Studying these pathways has proven challenging in traditional environmental chamber experiments due to coupling of oxidant generation with generation of HO2 or NO. To circumvent this, we use direct photolytic approaches to the generation of RO2. Here, an organic precursor with a photolabile functional group is introduced into an environmental chamber and photolyzed under UV lamps, yielding a single-isomer alkyl radical, which then reacts with O2 to form RO2. This simplifies downstream chemistry, and removes the need for an oxidant, giving greater control over experimental conditions and allowing for experiments where pathways 3 and 4 are the dominant fates of RO2. In parallel, we use a modified version of the Framework for 0D Atmospheric Modeling (F0AM) to select experimental conditions, tuning competition between different RO2 fates. By examining product distributions and kinetics, we examine the role of reactivity conditions and RO2 structure in determining RO2 fate and its impacts on downstream product formation and atmospheric composition. Further, I will discuss several computationally-informed environmental chamber experiments that are focused on finding so-called “uncanonical” reactions of RO2, that is, reactions that involve molecular processes not typically considered for simple RO2 radicals. Using automated reaction mechanism generation, we identify several novel reactions of functionalized RO2. We perform carefully designed environmental chamber experiments with the goal of identifying products that are signatures of these unconventional pathways. The results are highly suggestive of these previously unexplored pathways, but also illustrate the extent to which a mechanistic understanding of atmospheric oxidation remains incomplete, motivating future work in this area.
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