Atmospheric selenium: Geographical distribution and ocean to atmosphere flux in the Pacific

Abstract

Approximately 700 weekly aerosol samples from seven island sites in the North Pacific have been analyzed for their selenium content. A much more limited set of selenium aerosol analysis is also presented for two South Pacific island sites. Selenium concentrations (0.25±0.16 ng m−3) in the high‐latitude North Pacific atmosphere were a factor of 2 greater and much more variable than those found at mid‐North Pacific basin sites. Aerosol concentrations at five sites in the mid‐ and southwestern North Pacific were uniformly low, with mean concentrations of 0.10–0.13 ng Se m−3. Seasonal trends in the data were not evident at these locations. Near the equator, in the productive upwelling regime of the central North Pacific, mean particulate selenium concentrations were a factor of 3–4 greater than those observed in oligotrophic midocean regions. Effects of the 1982–1983 El Niño Southern Oscillation were also apparent in the aerosol record in this region. Consideration of aerosol selenium, nitrate, and non‐sea‐salt sulfate data from the Pacific suggests that source functions of aerosol selenium and excess sulfate may be similar, while removal mechanisms of selenium and nitrate may be analogous. The geographical distribution of aerosol selenium appears to be more closely related to primary productivity of the surrounding waters rather than to transport of continentally derived material into the central ocean basin. We estimate an ocean‐to‐atmosphere vapor phase selenium flux of 5–8×109 g Se yr−1. The anomalous enrichment of aerosol selenium which is observed in the marine atmosphere may be explained by the gas‐to‐particle conversion of this naturally produced vapor phase.