Atmospheric aerosol concentration level and chemical characteristics of water-soluble ionic species in wintertime in Beijing, China

摘要

Total suspended particulate (TSP) samples were collected during wintertime from November 24, 1998 to February 12, 1999 in Beijing. Ionic species including Cl~-, NO_3~-, SO_4~(2-), Na~+, K~+, Mg~(2+) and Ca~(2+) were determined by Ion Chromatography (IC). The sum average concentration of all the determined ions accounted for 18.9% of the TSP concentration, and SO_4~(2-) appeared the dominant ion with an average concentration of 30.84 μg m~(-3); the sum mass concentration of SO_4~(2-), NO_3~-, Ca~(2+) and NH_4~+ accounted for about 83.2% of all the eight ions measured. The study indicated that the chemical form of sulfate and ammonium varies with TSP concentration levels. During heavy pollution periods, the average TSP concentration was 0.66 mg m~(-3), and the NH_4~+/SO_4~(2-) molar ratio was low (0.58). It indicated that sulfate may present as CaSO_4 and (NH_4)_2SO_4·CaSO_4·2H_2O. When TSP concentration (average 0.186 mg m~(-3)) was relatively low, the NH_4~+/SO_4~(2-) molar ratio was 1.94, close to the theoretical ratio of 2 of (NH_4)_2SO_4. Under this condition (NH_4)_2SO_4 is expected to exist as the major form of sulfate. When the TSP concentration level was medium (average 0.35 mg m~(-3)), the NH_4~+/SO_4~(2-) molar ratio appeared an average value (1.27), (NH_4)_2SO_4, (NH_4)_2SO_4·CaSO_4·2H_2O and CaSO_4 are expected to be present in those aerosol particles. Meteorological conditions including wind speed and wind direction were related to the TSP concentration level.