Ground-based investigation of HOx and ozone chemistry in biomass burning plumes in rural Idaho

Ozone (O-3), a potent greenhouse gas that is detrimental to human health, is typically found in elevated concentrations within biomass burning (BB) smoke plumes. The radical species OH, HO2, and RO2 (known collectively as ROx) have central roles in the formation of secondary pollutants including O-3 but are poorly characterized for BB plumes. We present measurements of total peroxy radical concentrations ([XO2] + [RO2]) and additional trace-gas and particulate matter measurements from McCall, Idaho, during August 2018. There were five distinct periods in which BB smoke impacted this site. During BB events, O-3 concentrations were enhanced, evident by ozone enhancement ratios (Delta O-3/Delta CO) that ranged up to 0.06 ppbv ppbv(-1). [XO2] was similarly elevated during some BB events. Overall, instantaneous ozone production rates (P (O-3)) were minimally impacted by the presence of smoke as [NOx] enhancements were minimal Measured XO2 concentrations were compared to zero-dimensional box modeling results to evaluate the Master Chemical Mechanism (MCM) and GEOS-Chem mechanisms overall and during periods of BB influence. The models consistently overestimated XO2 with the base MCM and GEOS -Chem XO2 predictions high by an average of 28 % and 20 %, respectively. One period of BB influence had distinct measured enhancements of 15 pptv XO2 that were not reflected in the model output, likely due to the presence of unmeasured HOx sources. To the best of our knowledge, this is the first BB study featuring peroxy radical measurements.

Automated summary

Ozone and total peroxy radical concentrations increase significantly during biomass burning smoke events, but ozone production rates are minimally impacted by smoke presence. Model predictions overestimated peroxy radical concentrations, indicating potential sources of discrepancy in modeling mechanisms.