Marine boundary layer peroxy radical chemistry during the AEROSOLS99 campaign: Measurements and analysis

During the AEROSOLS99 cruise over the Atlantic and Southern Indian Oceans, peroxy radicals (RO2* = HO2 + Sigma RO2, where R is an organic group) were continuously measured. Regular diurnal variations were observed with maximu m mixing ratios of 30 to 55 parts per trillion by volume around noon. The l atitudinal distribution of RO2* over the ocean seems to remain uniform unde r similar conditions of solar insolation. The composition of the marine bou ndary layer reflects long-range transport of air from the African continent . The influence of biomass burning over remote areas of the marine boundary layer is confirmed, although the atmosphere over the Southern Hemisphere w as often pristine, with rapid destruction of ozone observed during the dayl ight hours. Observed RO2* concentrations are in reasonable agreement with b ox model calculations, but cannot account for the highest diurnal variation s of ozone observed. Possible explanations include halogen chemistry and no cturnal entrainment Of O-3 from the layers above, but available data cannot readily determine their relative contributions.