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

Abstract

Abstract. 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 %. 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.

Reply to referee 1

“…These limitations will be discussed in the context of the HUMPPA campaign. This campaign is particularly suitable for this purpose since two other studies have been published that specifically address the temporal evolution of H2O2 using a box model (Crowley et al, 2018) and the contribution of entrainment to the early morning increase of O3 (Ouwersloot et al, 2012) for this particular campaign. Results from HUMPPA will also apply to the other campaigns discussed here.…”

“…Additionally, a potential interference in the HO2 observations by RO2 radicals (Hens et al, 2014) will also affect the H2O2 production rate. Crowley et al (2018) used a box model to study PAA, PAN and H2O2 during HUMPPA, deriving HO2 concentrations that fit the temporal evolution of these species between July 20 and August 12, 2010. Modelled diel averaged HO2 profiles indicate maximum noontime values of 20 pptv roughly 50 % lower than mean observation (35 pptv), which is due to a modelled RO2 interference (see Fig.…”

“…Previous measurements of HO2 reported for DOMINO and HUMPPA are therefore an upper limit as they are affected by a fractional measurement of RO2. Crowley et al (2018) suggest that on average 50 % of the daytime HO2 signal during HUMPPA is due to a RO2 interference caused by high NO additions. During later campaigns (PARADE, HOPE, CYPHEX) the reduction of NO addition resulted in significantly smaller RO2 interferences of the order 12 to 15 % (Mallik et al, 2018).…”

“…during HUMPPA. In a recent modelling study, Crowley et al (2018) determined the contribution of RO2 to the measured HO2 during the daylight hours to be of the order of 50 %. If we correct the HUMPPA data for this potential interference, median daytime HO2 is reduced from 24 pptv (14 to 35 pptv for the 25 -75 % percentiles) to 12 pptv (7 -17.5 pptv).…”

“…interference of the HO2 measurements (Crowley et al 2018). The total uncertainty is calculated by error propagation: and J(H2O2)) yields ± 86 %, ± 62 %, ± 76 %, ± 72 %, and ± 39 % for PARADE, DOMINO, HUMPPA, HOPE and CYPHEX, respectively.…”