Photoformation of OH radical in water-extract of atmospheric aerosols and aqueous solution of water-soluble gases collected in Higashi-Hiroshima, Japan

Abstract

Chemical composition, hydroxyl radical (•OH) photoformation rate (ROH) and primary generation sources of •OH in water-extract of aerosol (WEA) and aqueous solution of water-soluble gases (WSG) in the atmosphere were studied at Higashi-Hiroshima, Japan from June 2008 to June 2010. Among the major anions present in the WSG samples, the SO42- concentration was highest (9.4 ± 4.0 nmol m-3), followed by NO3- and Cl-, while the mean concentration of SO42- in the WEA fraction was 81 ± 33 nmol m-3. The initial concentrations of H2O2 were 0.2 ± 0.3 and 0.9 ± 2.3 nmol m-3 in the WSG and WEA, respectively. The extracted sample was irradiated using a solar simulator, after which the ROH was determined and normalized against the sun-light intensity of a clear sky midday on May 1 at 34°N, which revealed values of 1.4 and 0.5 nmol h-1 m-3 for the WEA and WSG, respectively. The contribution of NO3-, NO2- and H2O2 to the formation of •OH via photolysis was 4.5, 0.7 and 1.2%, respectively, for the WSG fraction, while in the WEA fraction, the contribution was 8.9, 1.2 and 2.5%, respectively. Other contribution sources of •OH could include the photo-Fenton reaction and fluorescent matter such as humic-like substances (HULIS). The photo-Fenton reaction was estimated to potentially contribute up to 42% of the total •OH formation in the WSG fraction, while the fluorescent matter may account for the remaining 52%.