Insights into HO x and RO x chemistry in the boreal forest via measurement of peroxyacetic acid, peroxyacetic nitric anhydride (PAN) and hydrogen peroxide

Crowley, John N.; Pouvesle, Nicolas; Phillips, Gavin J.; Axinte, Raoul; Fischer, Horst; Petäjä, Tuukka; Nölscher, Anke; Williams, Jonathan; Hens, Korbinian; Harder, Hartwig; Martinez-Harder, Monica; Novelli, Anna; Kubistin, Dagmar; Bohn, Birger; Lelieveld, Jos

Unlike many oxidised atmospheric trace gases, which have numerous production pathways, peroxyacetic acid (PAA) and PAN are formed almost exclusively in gas-phase reactions involving the hydroperoxy radical (inline-formulaHO2), the acetyl peroxy radical (inline-formulaCH3C(O)O2) and inline-formulaNO2 and are not believed to be directly emitted in significant amounts by vegetation. As the self-reaction of inline-formulaHO2 is the main photochemical route to hydrogen peroxide (inline-formulaH2O2), simultaneous observation of PAA, PAN and inline-formulaH2O2 can provide insight into the inline-formulaHO2 budget. We present an analysis of observations taken during a summertime campaign in a boreal forest that, in addition to natural conditions, was temporarily impacted by two biomass-burning plumes. The observations were analysed using an expression based on a steady-state assumption using relative PAA-to-PAN mixing ratios to derive inline-formulaHO2 concentrations. The steady-state approach generated inline-formulaHO2 concentrations that were generally in reasonable agreement with measurements but sometimes overestimated those observed by factors of 2 or more. We also used a chemically simple, constrained box model to analyse the formation and reaction of radicals that define the observed mixing ratios of PAA and inline-formulaH2O2. After nudging the simulation towards observations by adding extra, photochemical sources of inline-formulaHO2 and inline-formulaCH3C(O)inline-formulaO2, the box model replicated the observations of PAA, inline-formulaH2O2, ROOH and OH throughout the campaign, including the biomass-burning-influenced episodes during which significantly higher levels of many oxidized trace gases were observed. A dominant fraction of inline-formulaCH3inline-formulaO2 radical generation was found to arise via reactions of the inline-formulaCH3C(O)inline-formulaO2 radical. The model indicates that organic peroxy radicals were present at night in high concentrations that sometimes exceeded those predicted for daytime, and initially divergent measured and modelled inline-formulaHO2 concentrations and daily concentration profiles are reconciled when organic peroxy radicals are detected (as inline-formulaHO2) at an efficiency of 35 %. Organic peroxy radicals are found to play an important role in the recycling of OH radicals subsequent to their loss via reactions with volatile organic compounds.

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Crowley, John N. / Pouvesle, Nicolas / Phillips, Gavin J. / et al: Insights into HOx and ROx chemistry in the boreal forest via measurement of peroxyacetic acid, peroxyacetic nitric anhydride (PAN) and hydrogen peroxide. 2018. Copernicus Publications.

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