MEASUREMENTS AND MODEL PREDICTIONS OF HYDROXYL RADICAL, PEROXY RADICALS, AND NITROUS ACID: EVIDENCE OF MISSING SOURCES OR SINKS IN FORESTED AND INDOOR ENVIRONMENTS

Abstract

The hydroxyl radical (OH) is the dominant oxidant in the atmosphere, controlling the lifetimes of volatile organic compounds (VOCs) and contributing to the formation of ozone (O3) and secondary organic aerosols (SOA). The oxidation of VOCs by OH propagates a radical cycle between OH, the hydroperoxy radical (HO2), and organic peroxy radical (RO2). Due to its importance in the atmosphere, there is a substantial body of literature investigating ROX (≡ OH + HO2 + RO2) chemistry in the outdoor environment, but few measurements occurring indoors, where people spend the majority of their time. This work includes the first measurements of OH and HO2 in a residential environment during use of a gas stove. Emissions from the gas stove quickly create a complex indoor atmosphere with unexpectedly high concentrations of OH and HO2, suggesting missing radical sources in indoor spaces. In more pristine regions, there has been significant disagreement between measurements and modeled concentrations of ROX. While progress has been made in improving OH agreement in deciduous forests, HO2 and RO2 model disagreement persists. Measurements of radicals spanning three forested campaigns are compared through a chemical coordinate analysis and suggest that newly proposed radical loss pathways still cannot account for the discrepancies between model and measurements. Finally, measurements of radical species and their major precursors in a coniferous forest emphasize the importance of nitrous acid (HONO) on ROX concentrations. HONO photolyzes to iv form OH and NO, producing a substantial amount of total ROX. HONO concentrations are particularly difficult to model, as there are numerous sources of varying importance in different environments. While common chemical mechanisms only include gas-phase reactions of HONO, heterogeneous reactions were added in order to characterize sources of HONO in the remote, forested environment. Overall, this work discusses missing sources and sinks of radicals and their major precursor, HONO, in indoor spaces, deciduous, and coniferous forests, addressing gaps in our understanding of radical chemistry that impact the formation of secondary air pollution.