Chemical composition of aerosol, sea fog, and rainwater in the marine boundary layer of the northwestern North Pacific and its marginal seas

Abstract

Samples of aerosol, sea fog, and rainwater were collected during a research cruise in the northwestern North Pacific, the Sea of Okhotsk, and the Sea of Japan in the summer of 1998. High concentrations of NO3−, nss‐SO42− and NH4+ in aerosol over the Sea of Japan suggest that anthropogenic substances were transported to this region. Although the Sea of Okhotsk was covered with a clean marine air mass, the concentration of nss‐SO42− was comparatively high in the aerosol samples. This nss‐SO42− is probably of marine biogenic origin. The pH values of fogwater samples were measured to be <3.0 over the Sea of Okhotsk and the Sea of Japan. The concentrations of NO3− and NH4+ in the fogwater collected over the Sea of Japan were higher than those in the other regions, suggesting that the sea fog scavenged anthropogenic substances. The concentration of nss‐SO42− in fogwater over the Sea of Okhotsk was equivalent to that over the Sea of Japan, probably because nss‐SO42− and SO2 of marine biogenic origin were scavenged by the sea fog over the Sea of Okhotsk. The pH values of rainwater samples ranged from 6.1 to 7.2 during the cruise, and acidification of the rain was not significant. The concentrations of nss‐Ca2+ in the rainwater were higher than those of the fogwater. This suggests that the rain‐scavenged continental CaCO3 may have existed above the lower marine boundary layer, where sea fog appeared. Comparisons of the composition of aerosol and fogwater indicated that coarse particles, such as sea salts predominantly act as condensation nuclei of sea fog droplets rather than fine particles such as (NH4)2SO4.