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

Abstract

Total suspended particulate (TSP) samples were collected during wintertime from November 24, 1998 to February 12, 1999 in Beijing. Ionic species including Cl⁻, NO₃⁻, SO₄²⁻, Na⁺, NH₄⁺, K⁺, Mg²⁺ and Ca²⁺ were determined by Ion Chromatography (IC). The sum average concentration of all the determined ions accounted for 18.9% of the TSP concentration, and SO₄²⁻ appeared the dominant ion with an average concentration of 30.84 µg m⁻³; the sum mass concentration of SO₄²⁻, NO₃⁻, Ca²⁺ and NH₄⁺ 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 NH₄⁺/SO₄²⁻ molar ratio was low (0.58). It indicated that sulfate may present as CaSO₄ and (NH₄)₂SO₄·CaSO₄·2H₂O. When TSP concentration (average 0.186 mg m⁻³) was relatively low, the NH₄⁺/SO₄²⁻ molar ratio was 1.94, close to the theoretical ratio of 2 of (NH₄)₂SO₄. Under this condition (NH₄)₂SO₄ is expected to exist as the major form of sulfate. When the TSP concentration level was medium (average 0.35 mg m⁻³), the NH₄⁺/SO₄²⁻ molar ratio appeared an average value (1.27), (NH₄)₂SO₄, (NH₄)₂SO₄·CaSO₄·2H₂O and CaSO₄ are expected to be present in those aerosol particles. Meteorological conditions including wind speed and wind direction were related to the TSP concentration level.