Oxidation State of Selenium for Inferring Sources of Sulfate in the Atmospheric Aerosols

Source emissions of sulfate aerosols as well as other pollutants over the Asian Continent have been increasing due to rapid development of industrial activities in recent years. In winter, sulfate aerosols are usually transported eastward to downwind areas by westerly winds to directly affect the environment of the downwind areas including Japan and its surrounding ocean.

Concentrations of non-sea-salt sulfate (nss-SO₄^2-) and selenium (Se) in aerosols show a good correlation to each other, suggesting that they would be derived from the same sources, e.g., fossil fuel combustion. Selenium in fossil fuels is emitted by combustion into the atmosphere as Se(0) or Se(IV), and the latter is supposed to be oxidized into Se(VI) during the transport and deposition process. Therefore, the Se(VI)/Se(IV) concentration ratio should be a good indicator of the distance or duration of transport of aerosols in the aerial oxidation or reduction environment.

We performed observations to confirm the idea and to understand what determines the Se(VI)/Se(IV) ratio in aerosols. Consequently, a positive correlation was found between Se(VI)/Se(IV) ratio and oxidation-reduction potential in the aerosols. This fact suggested that the selenium^,s oxidation state reflects oxidation-reduction condition in the atmospheric aerosols.

As a field experiment, aerosol samples were collected in winter in 1999 at two sites facing the Sea of Japan, and analyzed for nss-SO₄^2-, Se(IV), and Se(VI) to infer emission sources. Episodes of a high nss-SO₄^2- concentration were observed several times. On the basis of the wind conditions as well as the Se/S nss-SO₄^2- and Se(VI)/Se(IV) ratio, these episodes are classified into three types as emission source, i.e., continental-coal, local-coal and local-petroleum types.