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

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Abstract
Concentrations of aerosol NO3− and non‐sea‐salt (nss) SO4= 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 NO3− concentrations were relatively uniform, ranging between 0.18 and 0.35 μg/m3 STP; mean nss SO4= ranged from 0.37 to 1.20 μg/m3 STP. At most stations there was no consistent correlation between NO3− and nss SO4=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 SO4= concentrations were associated with high mineral aerosol concentrations; this suggests that the SO4= 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 SO4= concentrations; evidence suggests that the SO4= was not derived from the soil material itself. At Fanning Island, NO3− concentrations showed a strong seasonal variation; there is evidence that the high NO3− concentrations are attributable to transport from the continents. In contrast, nss SO4= 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 NO3− data from the tropics suggest that there is a relatively uniform background concentration of about 0.10 μg/m3 STP; this concentration is close to that recently predicted by models that assume only lightning and the stratosphere as sources for NOx.