Measurements of peroxides in cloudwater and rain

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 H 2 O 2 on 48 of the samples showed that in the majority of samples tested, H 2 O 2 was the sole contributor to the total peroxide concentration, but in some samples, substantial signal was observed after selective destruction of H 2 O 2 , possibly indicating the presence of organic peroxides. Calculation of the total potential partial pressure of H 2 O 2 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 H 2 O 2 of 0.1 ppbv, and a maximum value of approximately 1 ppbv, Gaseous SO 2 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.