Reactive Aldehyde Chemistry explains the Missing Source of Hydroxyl Radicals

Abstract Hydroxyl radicals (OH) drive the removal of most primary pollutants and greenhouse gases in the atmosphere, determine the tropospheric oxidation capacity, and regulate air quality and climate. However, the state-of-the-art mechanisms still underestimate the observed high OH concentrations at the low nitrogen oxide and high volatile organic compounds regime, indicating a missing OH source. The missing mechanism would bias the understanding of tropospheric air pollution and the lifetime of greenhouse gases. People attempted but failed to find the mechanism to explain high OH radicals in the past decades fully. Here, with a retrospective meta-analysis of a comprehensive warm-season field observation dataset and theoretical study, we find the reactive aldehyde chemistry, especially the autoxidation of carbonyl RC(O)O2 radicals derived from higher aldehydes, is a noteworthy OH regeneration mechanism that overwhelms the contribution of isoprene autoxidation mechanism, the latter is proved to largely contribute to the missing OH source under high isoprene condition. The R(CO)O2 radicals undergo fast H-migration, confirmed by the quantum chemical calculations, to produce unsaturated hydroperoxyl-carbonyls that generate OH radicals by fast photolysis. The new chemistry can explain almost all unknown OH sources with significant contributions in areas rich in both natural and anthropogenic emissions. It would be further enhanced on a larger spatiotemporal scale under a future low nitrogen oxide society.