Aqueous phase photochemical formation of hydrogen peroxide in authentic cloud waters

Abstract

Authentic cloud water samples absorb ultraviolet radiation and thereby initiate formation of peroxides (primarily hydrogen peroxide (H 2 O 2 )). This aqueous phase photoformation of H 2 O 2 is a widespread phenomenon in the terrestrial troposphere; midday equinox‐normalized H 2 O 2 photoformation rates ranged up to 3.0 μ M /h for 36 different cloud water samples collected from sites in New York, North Carolina, Ontario, Virginia, and Washington. By comparison, previously published field studies have shown that over the eastern United States and Canada, approximately 100% of winter cloud water samples, 58–100% of spring and fall samples, and 3–7% of summer samples had peroxide concentrations less than 5 μ M. . Previously published model estimates of aqueous phase H 2 O 2 formation rates in cloud drops from gas‐to‐drop transfer of H 2 O 2 and HO 2 • range from 1–10 μ M /h for all but the most pristine regions of the troposphere. Based on a comparison of these published field and modeling results with the measured aqueous phase H 2 O 2 photoformation rates reported here, we conclude that aqueous phase photochemistry is a significant and in some cases probably dominant source of H 2 O 2 to tropospheric cloud drops. Theory predicts and experiments confirm that the initial rate of aqueous phase H 2 O 2 photoformation is linearly dependent on solar actinic flux. The chromophores responsible for aqueous phase peroxide photoformation have not been identified, but there is evidence that organic chromophores are responsible for the H 2 O 2 photoformation reported here.