Measurements of peroxides in cloudwater and rain

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Abstract
Measurements of total peroxide content are reported for 190 cloud water samples from nonsupercooled stratiform clouds, and for 54 rain samples, collected at several nonurban sites in the eastern United States. Observed aqueous peroxide concentrations ranged from less than 0.1 μM to over 100 μM, with rain samples, precipitating cloudwater samples, and nonprecipitating cloudwater samples all exhibiting median values of less than 10 μM and similar concentration distributions. Specific analyses for H2O2 on 48 of the samples showed that in the majority of samples tested, H2O2 was the sole contributor to the total peroxide concentration, but in some samples, substantial signal was observed after selective destruction of H2O2, possibly indicating the presence of organic peroxides. Calculation of the total potential partial pressure of H2O2 for the samples collected in nonprecipitating clouds, using the measured aqueous concentrations and cloud liquid water content values, indicates a median potential partial pressure of H2O2 of 0.1 ppbv, and a maximum value of approximately 1 ppbv, Gaseous SO2 and aqueous peroxide were rarely simultaneously present in substantial concentrations; generally one reactant was present in great excess over the other, consistent with the occurrence of the aqueous phase reaction of peroxide with S(IV) in which one or the other species is the limiting reagent. Evaluation of the rates of S(IV) oxidation by peroxide and ozone, under conditions representative of the clouds sampled in this study, indicates that the former reaction, but not the latter, can produce substantial acidification in such clouds.