Size distributions of particulate sulfate, nitrate, and ammonium at a coastal site in Hong Kong

Size distributions of particulate sulfate, ammonium, and nitrate were measured at a coastal site in Hong Kong over 22 days using a MOUDI cascade impactor. Sulfate, ammonium, and nitrate were trimodally distributed in the size range of 0.052–10 μm. Sulfate and ammonium were the dominant species in fine particles (Dp<1.8 μm), co-existing in the condensation (0.20 μm) mode and droplet (0.57 μm) modes. The droplet mode dominated the distributions of these two ions. Higher concentrations of droplet mode sulfate were associated with high relative humidity and low clouds. Transport of aerosols from southern China by strong northerly wind also increased the sulfate concentration in Hong Kong. Mode diameters of coarse sulfate and ammonium were about 4–6 μm. Nitrate was mainly distributed in the coarse particles with mode diameter of 3.95±0.69 μm. Coarse mode nitrate was formed by the reaction of gas-phase nitric acid with sea-salt or soil particles. Little fine mode nitrate was found.