Photoformation Rate of OH radical in Water-Extract of Atmospheric Aerosol and Water-Soluble Gases Collected in Higashi-Hiroshima, Japan

The hydroxyl radical (?OH) photoformation rate (ROH), chemical composition and primary generation sources of ?OH in the atmospheric water-extract of aerosol (WEA) and water-soluble gaseous (WSG) fractions were studied at Higashi-Hiroshima from June 2008 to June 2010. Among the major anions being present in the WSG samples, the SO42- concentration was highest (9.8±2.9 nmol m-3), followed by NO3- and Cl-, while for WEA fractions the mean concentration of SO42- was 81±33 nmol m-3. The concentration of H2O2 was 0.2±0.3 and 0.9±2.3 nmol m-3 in the WSG and WEA, respectively. Using a solar simulator, the extracted sample was irradiated and the ROH was determined and normalized based on the air-volume, which revealed values of 1.4 and 0.5 nmol h-1 m-3 for the WEA and WSG, respectively. Linear relationship between irradiation time and production of ?OH in both fractions suggested high capacity or similar efficiency of ?OH generation. The contribution of NO3-, NO2- and H2O2 to the formation of ?OH via photolysis were 4.5, 0.7 and 1.2%, respectively, for the WSG fraction, while in the WEA fraction, NO3-, NO2- and H2O2 contributed 8.9, 1.2 and 2.5%, respectively. The photo-Fenton reaction contributed 42% of the total ?OH formation in the WSG fraction, while unknown sources that might be humic-like substances (HULIS) accounted for the remaining 52%.