Measurement of photochemically formed hydroxyl radical in rain and dew waters

Photochemical formation of hydroxyl radical (OH radical) was studied using rain (n=22) and dew (n=9) waters collected during June to September, 1997 in Higashi-Hiroshima city, Hiroshima prefecture. The mean photochemical formation rates of OH radical for rain and dew waters were 0.36 mu M h(-1) and 0.83 mu M h(-1) (1 M=1 mol dm(-3)), respectively, at noon, clear sky conditions in Higashi-Hiroshima city (34 degree north) on May 01. Photoformation of OH radical was detected in all the samples studied, except for one rain water, and the OH radical formation kinetics appeared to be linear. Effect of filtration (0.45 mu m glass fiber filter) was studied to elucidate the formation mechanisms of OH radical. Comparison between filtered and unfiltered samples indicated that the particles (>0.45 mu m) present in rain and dew waters caused small change (less than 22%) in OH radical photoformation. Furthermore, OH radical formation rates from nitrate (NO3-) and nitrite (NO2-) ion photolyses were estimated, based on the photolysis rate constants (J(NO3-) = 2.43 x 10(-7) s(-1): J(NO2-) = 2.81 x 10(-5) s(-1): J(HNO2) = 3.30 x 10(-4) s(-1)), nitrate and nitrite ion concentrations, and pK(a) of nitrous acid in those samples. The estimated OH radical formation rates from nitrate ion photolysis accounted for only small percentage of OH radical observed in rain and dew waters. Nitrous acid photolysis accounted for majority of OH radical formed in dew waters (mean=99.3%), while on the average 11.2% of OH radical was from nitrous acid photolysis in rain waters. For rain waters, it was speculated that major sources of OH radical were reaction between reduced iron (Fe(II)) and hydrogen peroxide (HOOH), "Fenton's reaction" and photoformation from dissolved organic compounds.