Formation of nitrate and non-sea-salt sulfate on coarse particles

The formation of nitrate and sulfate on coarse particles and its relationships with meteorological conditions and relative abundance of sea-salt and soil particles were studied. Size distributions of particulate SO2−4, NO−3, NH+4, Cl−, Na+, K+, Mg2+ and Ca2+ were measured using a MOUDI cascade impactor for 11 days in July to December 1997 at a coastal site in Hong Kong. Most of the nitrate and a small fraction of sulfate were distributed in the coarse mode. Since Hong Kong is influenced by both marine and continental aerosols, the distribution of coarse mode nitrate and sulfate depends on the weather conditions. Evidence of nitrate formation on both sea-salt and soil compounds was found in this study. Chloride depletion of sea-salt by both nitrate and sulfate formation was also observed. When Hong Kong was under prevailing easterly wind accompanied with high relative humidity, Na+ was the dominant cation species and significant chloride depletion (74–88%) from coarse mode sea-salt aerosols was observed. Nitrate accounted for 65% of chloride depletion. During a heavily polluted period when there were high concentrations of both fine (18 μg m−3) and coarse mode (3 μg m−3) sulfate, significant amount of sulfate was associated with sea-salt particles and accounted for 11–29% of the chloride depletion. At least 16% of chloride depletion was not accounted for by nitrate and sulfate alone in this episode. When Hong Kong was influenced by a northeasterly monsoon, Ca2+ was the dominant species in the coarse mode. Soil particles compete with sea-salt particles for acidic gases. Chloride depletion was below 50% and most of the nitrate was found to be associated with Ca2+ in soil particles.