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 (H2O2)). This aqueous phase photoformation of H2O2 is a widespread phenomenon in the terrestrial troposphere; midday equinox‐normalized H2O2 photoformation rates ranged up to 3.0 μM/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 μM.. Previously published model estimates of aqueous phase H2O2 formation rates in cloud drops from gas‐to‐drop transfer of H2O2 and HO2• range from 1–10 μM/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 H2O2 photoformation rates reported here, we conclude that aqueous phase photochemistry is a significant and in some cases probably dominant source of H2O2 to tropospheric cloud drops. Theory predicts and experiments confirm that the initial rate of aqueous phase H2O2 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 H2O2 photoformation reported here.

Fogs and Air Quality on the Southern California Coast

“…Anion concentrations were dominated by VA, 1990/1991, Anastasio et al, 1994and 10.1 in Angiola, CA, 2000/2001, Herckes et al, 2002b.…”

Evaluating the triplet state photoreactivity of dissolved organic matter isolated by chromatography and ultrafiltration using an alkylphenol probe molecule

“…Evidence in the literature suggests that the indirect photolysis of TMP occurs by both ROS ( 1 O 2 and peroxyl radicals) (Faust and Hoigne 1987;Faust and Allen 1992;Allen and Faust 1994;Anastasio et al 1994) and non-ROS (Tratnyek and Hoigne 1994;Canonica et al 1995;Canonica and Freiburghaus 2001;Aguer et al 2005, Halladja et al 2007 pathways. To determine which of these pathways is dominant in our study, we performed three scavenging experiments that resulted in different TMP degradation rates (Fig.…”

Optical properties of humic-like substances (HULIS) in biomass-burning aerosols

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