Selenite and selenate in clouds at a high-altitude mountain location in central Japan

Abstract

The atmosphere is a temporal reservoir of selenium (Se). Understanding the chemical reactions and speciation of Se in primary emissions is crucial because atmospheric deposition is an important source of both Se contamination and micronutrients for terrestrial (agricultural) systems. In this study, the concentrations of selenite and selenate in acid cloud water (pH 4.3 average), including non-precipitating and precipitating clouds, were analyzed at a site near the summit of Mt. Norikura (2770 m a.s.l.), central Japan. The soluble Se in cloud water has a positive correlation with that of non-sea-salt sulfate, implying that the Se was incorporated in ammonium sulfate. The ammonium sulfate was generated from fossil fuel combustions in industrial and urban areas of southwest Japan and China. The concentrations of selenite and selenate in cloud water gradually decreased with a rise in acidity (pH) in approximately the same proportion. The coexistence of selenite and selenate perhaps indicates an oxidation-reduction equilibrium for the HSeO3−–SeO42− couple. For the precipitating cloud water, the selenite was the dominant species in all cloud events. This may be attributed to scavenging of local coal-combustion aerosols from southwest Japan by the rain droplets (washout). The selenite/selenate ratios of clouds as well as precipitating clouds were not correlated with gaseous O3 concentrations and remained stable throughout both day and night. In addition, laboratory experiments with Se–hydrogen peroxide (H2O2) solutions mimicking the cloud water conditions indicated that selenious acids (HSeO3−) cannot be oxidized with H2O2. The observational and experimental results suggest that the Se oxidation states in acid clouds are not caused by oxidation by the possible oxidants for SO2 such as H2O2 and O3.