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. Unlike many oxidised atmospheric trace gases, which have numerous productionpathways, peroxyacetic acid (PAA) and PAN are formed almost exclusively ingas-phase reactions involving the hydroperoxy radical ( HO2 ), theacetyl peroxy radical ( CH3C(O)O2 ) and NO2 and are notbelieved to be directly emitted in significant amounts by vegetation. As theself-reaction of HO2 is the main photochemical route to hydrogenperoxide ( H2O2 ), simultaneous observation of PAA, PAN and H2O2 can provide insight into the HO2 budget. We presentan analysis of observations taken during a summertime campaign in a borealforest that, in addition to natural conditions, was temporarily impacted bytwo biomass-burning plumes. The observations were analysed using anexpression based on a steady-state assumption using relative PAA-to-PANmixing ratios to derive HO2 concentrations. The steady-state approachgenerated HO2 concentrations that were generally in reasonableagreement with measurements but sometimes overestimated those observed byfactors of 2 or more. We also used a chemically simple, constrained box modelto analyse the formation and reaction of radicals that define the observedmixing ratios of PAA and H2O2 . After nudging the simulationtowards observations by adding extra, photochemical sources of HO2 and CH3 C(O) O2 , the box model replicated the observations ofPAA, H2O2 , ROOH and OH throughout the campaign, including thebiomass-burning-influenced episodes during which significantly higher levelsof many oxidized trace gases were observed. A dominant fraction of CH3 O2 radical generation was found to arise via reactions ofthe CH3 C(O) O2 radical. The model indicates that organicperoxy radicals were present at night in high concentrations that sometimesexceeded those predicted for daytime, and initially divergent measured andmodelled HO2 concentrations and daily concentration profiles arereconciled when organic peroxy radicals are detected (as HO2 ) at anefficiency of 35 %. Organic peroxy radicals are found to play animportant role in the recycling of OH radicals subsequent to their loss viareactions with volatile organic compounds.

atmospheric chemistry and physics

Insights into HO&amp;lt;sub&amp;gt;&amp;lt;i&amp;gt;x&amp;lt;/i&amp;gt;&amp;lt;/sub&amp;gt; and RO&amp;lt;sub&amp;gt;&amp;lt;i&amp;gt;x&amp;lt;/i&amp;gt;&amp;lt;/sub&amp;gt; chemistry in the boreal forest via measurement of peroxyacetic acid, peroxyacetic nitric anhydride (PAN) and hydrogen peroxide