Particulate sulfate and nitrate in the boundary layer over the North Pacific Ocean

Abstract

Concentrations of aerosol NO 3 − and non‐sea‐salt (nss) SO 4 = have been measured in samples collected weekly between January 1981 and March 1982 at seven stations in the Sea‐Air Exchange Program Asian Dust Network in the North Pacific between 4°N and 53°N. Mean NO 3 − concentrations were relatively uniform, ranging between 0.18 and 0.35 μg/m 3 STP; mean nss SO 4 = ranged from 0.37 to 1.20 μg/m 3 STP. At most stations there was no consistent correlation between NO 3 − and nss SO 4 = or between these species and mineral aerosol. At Midway these species did covary in a clear and consistent manner; this correlation appears to be forced by large‐scale meteorological factors. In some cases, high nss SO 4 = concentrations were associated with high mineral aerosol concentrations; this suggests that the SO 4 = in these episodes was transported from the continents. A particularly impressive event occurred in May 1981, when much of the North Pacific region was affected by extremely high nss SO 4 = concentrations; evidence suggests that the SO 4 = was not derived from the soil material itself. At Fanning Island, NO 3 − concentrations showed a strong seasonal variation; there is evidence that the high NO 3 − concentrations are attributable to transport from the continents. In contrast, nss SO 4 = concentrations at Fanning were relatively constant throughout the year, and they were consistent with recent estimates of the oceanic emission rates of sulfur as dimethyl sulfide. The NO 3 − data from the tropics suggest that there is a relatively uniform background concentration of about 0.10 μg/m 3 STP; this concentration is close to that recently predicted by models that assume only lightning and the stratosphere as sources for NO x .