Production and Scavenging Mechanism of OH Radical in Atmospheric Water Droplets and Natural Water

Photochemical production rate of liquid phase OH radical was measured by irradiation of artificial sunlight to water samples and then trapping of generated OH radical to benzene followed by HPLC determination of phenol, which was generated by the reaction of OH radical with benzene. Scavenging rate was also determined by similar manner through the reaction of benzene with the radical. Analytical instrument, which automatically determines the production or scavenging rate of liquid phase OH radical, was developed in this study. Four water samples per hour can be subject to the analysis using this analyzer. Using the auto analyzer of OH radical, we determined production and scavenging rates of OH radical in rain, dew, drinking water and river waters in Hiroshima Prefecture and the Seto Inland Sea waters, western Japan. Photochemical production rate of OH radical was 1 nM/h to hundreds nM/h (for summer time noon condition at 34 \deg N) for rain, dew (formed on Teflon sheet), drinking water (natural mineral water) and seawater, whereas the rate was μ M/h levels for dew (formed on pine tree needles), river and drinking water (tap water). Major producer of aqueous OH radical was found to be fluorescent organic matter, nitrite, nitrate, and hypochlorite. Fluorescent organic matter that is composed of yet uncharacterized number of organic compounds contributed to most of OH production in rain, river and seawaters studied whereas nitrite, nitrate and hypochlorite were major contributors (ave. 99, 83 and 99%) for the radical production in dew, natural mineral water and tap water, respectively. Scavenging rate constant of OH radical in rain and dew was also estimated to be the order of 10⁵/s and thus the lifetime of OH radical was in the order of microseconds. Assuming all the OH radicals were generated by aqueous photochemical reactions, the steady state concentration of OH radical in rain and dew was about 1\times10^-15M for both rain and dew waters. Major sink of OH radical was dissolved organic matter and nitrite in rain and dew, respectively.