Acid-base balance in aerosol components of the Asia-Pacific region

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Abstract
We have estimated for remote areas the potential extent of neutralization and solubilization of alkaline aerosol by sulfuric acid through atmospheric uptake of acidic sulfur gases by dust particles. Aerosol components were resolved from time sequence measurements of elemental composition in Nuclepore filter samples from 5 locations in the Asia-Pacific region, using factor analysis and multiple linear regression applied to data sets of 66–229 steps of 2.4–4.0 hours each, representing 526 analyses for 7–12 elements. Each component was evaluated for chemical equivalent concentrations of sulfur and alkaline mineral elements. Two or three factors accounted for 87–97% of the variance of each set, consistent with a model of linear mixing of components for most observed variability and resulting in 12 components that may represent a broad range of aging, from recently generated soil dust to particles exposed to trace SO2 or other sulfur gases for a week or longer. Virtually no aerosol sulfur was found without being associated with soil elements. All 12 components contained statistically significant amounts of both sulfur and alkaline elements in relative proportions that suggest two rate limitations on acidic gas uptake: neutralization of rapidly reacting bases, mainly CaCO3, and gradual neutralization of slowly reacting bases, mainly clay minerals, by H2SO4 after SO2 uptake. After each stage of titration in the atmosphere, further gas uptake may be inhibited by an acidic particle surface. The resolved aerosol components show cases approaching both stoichiometric limits, but no case of excess sulfur beyond the second limit, consistent with these being practical limits in the remote areas investigated. If mineral aerosols are solubilized by strongly acidic surface solutions at ambient relative humidities, as suggested by these results, subsequent deposition to ocean, fresh water, or land surfaces may be more important inputs of nutrient or toxic metal ions than deposition of insoluble mineral particles or of wet or dry deposition of acids followed by dissolution of particles in seawater or leaching of soils. DOI: 10.1111/j.1600-0889.1989.tb00311.x