Fogwater had been collected from 1992 to 1993 at the midslope of Mt. Oyama (a.s.l. 1252 m) at the southwest of the Kanto Plains. The concentrations of five species of aldehydes, formaldehyde, acetalde-hyde, glyoxal, 2-oxopropanal, and hydroxyacetaldehyde, in fogwater were measured. The maximum value of the total concentration of these aldehydes was 187 μ;M (April 1992). The maximum concentration of glyoxal, whose concentration was the highest among these aldehyde, was 97 μ;M (June 1992). Much aldehydes were contained in highly polluted fogwater and the aldehyde concentrations in fogwater increased with the decrease of pH. When the lowest altitude of the upslope fog rose up to the sampling station, the total concentration of aldehydes increased. The fraction of aldehyde with a high Henry's law constant increased with the elevation of the lowest altitude of fog. Aldehyde concentrations in fogwater are affected not only by liquid water content and air pollution but also by the lowest altitude of fog.
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