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<p>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 (HO₂), the acetyl peroxy radical (CH₃C(O)O₂) and NO₂ and are not believed to be directly emitted in significant amounts by vegetation. As the self-reaction of HO₂ is the main photochemical route to hydrogen peroxide (H₂O₂), simultaneous observation of PAA, PAN and H₂O₂ can provide insight into the HO₂ 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 HO₂ concentrations. The steady-state approach generated HO₂ 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 H₂O₂. After nudging the simulation towards observations by adding extra, photochemical sources of HO₂ and CH₃C(O)O₂, the box model replicated the observations of PAA, H₂O₂, 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 CH₃O₂ radical generation was found to arise via reactions of the CH₃C(O)O₂ 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 HO₂ concentrations and daily concentration profiles are reconciled when organic peroxy radicals are detected (as HO₂) at an efficiency of 35&thinsp;%. 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.</p>