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 (HO 2 ), the acetyl peroxy radical (CH 3 C(O)O 2 ) and NO 2 and are not believed to be directly emitted in significant amounts by vegetation. As the self-reaction of HO 2 is the main photochemical route to hydrogen peroxide (H 2 O 2 ), simultaneous observation of PAA, PAN and H 2 O 2 can provide insight into the HO 2 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 HO 2 concentrations. The steady-state approach generated HO 2 concentrations that were generally in reasonable agreement with measurements but sometimes overestimated those observed by factors of two 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, H 2 O 2 . After nudging the simulation towards observations by adding extra, photochemical sources of HO 2 and CH 3 C(O)O 2 , the box model replicated the observations of PAA, H 2 O 2 , ROOH and OH throughout the campaign, including the biomass-burning influenced episodes during which significantly higher levels of many oxidized trace gases were observed. The model indicates that organic peroxy radicals were present at night in high concentrations that sometimes exceeded those predicted for daytime. A dominant fraction of CH 3 O 2 radical generation was found to arise via reactions of the CH 3 C(O)O 2 radical. Initially divergent measured and modelled HO 2 concentrations and daily concentration profiles are reconciled when these organic peroxy radicals are detected (as HO 2 ) at an efficiency of 35 %. The 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.