Uptake of the HO₂ radical by water: Molecular dynamcs calculations and their implications for atmospheric modeling

The mass accommodation coefficient α of the HO₂ radical in aqueous aerosols remains largely uncertain in atmospheric modeling. Therefore, in the present paper, a molecular dynamics computer simulation was performed to evaluate the HO₂ α in liquid water. The calculations yielded an α near unity, which defines a possible upper bound of the uptake coefficient γ. Implications for a large γ in the tropospheric atmosphere are discussed using model calculations for two typical cases: remote marine air and polluted urban air. It is suggested that the concentrations of HO₂ and other related species are quite sensitive to γ in the range of 0.2-1, particularly in the case of polluted urban air. In particular, the maximum diurnal HO₂ concentration is reduced by 73% in marine air and by 31% in urban air, when γ = 1 is incorporated.