Size-resolved sulfate and ammonium measurements in marine boundary layer over the North and South Pacific

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Marine background levels of non-sea-salt- (nss-) SO42− (5.0–9.7 neq m−3), NH4+ (2.1–4.4 neq m−3) and elemental carbon (EC) (40–80 ngC m−3) in aerosol samples were measured over the equatorial and South Pacific during a cruise by the R/V Hakuho-maru from November 2001 to March 2002. High concentrations of nss-SO42− (47–94 neq m−3), NH4+ (35–94 neq m−3) and EC (130–460 ngC m−3) were found in the western North Pacific near the coast of the Asian continent under the influence of the Asian winter monsoon. Particle size distributions of ionic components showed that the equivalent concentrations of nss-SO42− were balanced with those of NH4+ in the size range of 0.06<D<0.22 μm, whereas the concentration ratios of NH4+ to nss-SO42− in the size range of D>0.22 μm were decreased with increase in particle size. We estimated the source contributions of those aerosol components in the marine background air over the equatorial and South Pacific. Biomass burning accounted for the large fraction (80–98% in weight) of EC and the minor fraction (2–4% in weight) of nss-SO42−. Marine biogenic source accounted for several tens percents of NH4+ and nss-SO42−. In the accumulation mode, 70% of particle number existed in the size range of 0.1<D<0.2 μm. In the size rage of 0.06<D<0.22 μm, the dominant aerosol component of (NH4)2SO4 would be mainly derived from the marine biogenic sources.
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