Relationships among aerosol constituents from Asia and the North Pacific during PEM‐West A

Abstract

Aerosol particle samples collected from Asia and the North Pacific were analyzed to investigate the relationships among atmospheric sea salt, mineral aerosol, biogenic emissions (methanesulfonate (MSA)), and several anthropogenic substances (sulfate, nitrate, and various trace elements). These studies specifically focused on the sources for aerosol SO4= and on the long‐range transport of continental materials to the North Pacific. Ground‐based aerosol sampling was conducted at four coastal‐continental sites: Hong Kong, Taiwan, Okinawa, and Cheju; and at three remote Pacific islands, Shemya, Midway, and Oahu. Non‐sea‐salt (nss) SO4= and MSA were uncorrelated at the East Asian sites presumably because pollution sources overwhelm the biogenic emissions of nss SO4=. At the coastal‐continental sites, marine biogenic emissions accounted for only 10 to <5% of the total nss SO4=. In contrast, over the ocean the concentrations of nss SO4= and MSA were correlated (Midway r = 0.70; Oahu r = 0.59), and higher percentages of biogenic nss SO4= occurred, 55 and 70% at Oahu and Midway, respectively. The concentrations of nss SO4= and NO3− were correlated at Cheju, Hong Kong, Taiwan, Okinawa, Midway, and Oahu, indicating some similarities in their sources and the processes governing their transport; however, differences in the nss SO4=/NO3− ratios among sites suggest regional differences in the pollution component of the aerosol. At Shemya the concentrations of MSA during the summer (100 ng m−3 or more) are about 2 orders of magnitude higher than those in winter. The dimethylsulfide‐derived fraction of the nss SO4= is highest in the summer when the monthly median nss SO4=/MSA ratios range from 2.7 to 4.5, i.e., comparable to the ratios observed over Antarctica and other high‐latitude locations. However, the monthly median nss SO4=/MSA ratios increase, reaching 50 to 200 in the winter as productivity nearly ceases, and the biogenic fraction of nss SO4= at Shemya decreases dramatically; this suggests a strong seasonal pollution component to the sulfate aerosol. The meteorological conditions favoring the long‐range transport of Asian dust to the North Pacific also lead to transport of anthropogenic materials. At Oahu the correlation between NO3− and Al (dust) was highly significant (r = 0.75; p < 0.001), while the correlations between nitrate and Al at the continental sites were low. These differences indicate that the composition of the air sampled at the coastal‐continental stations may be quite different from the air transported to the remote ocean. This phenomenon also appeared to affect the relationship between nss SO4= and antimony. The correlations between nss SO4= and Sb were weak at the Asian sites but strong at the open ocean sites where the nss SO4=/Sb ratios were higher than those over the continent.