Size distribution and estimated optical properties of carbonate, water soluble organic carbon, and sulfate in aerosols at a remote high altitude site in Western China

Abstract

Measurements at the GAW station in western China reveal the levels and size distributions of chemical components in aerosols. The results indicate similarly high levels of three components, water-soluble organic carbon (WSOC), Ca 2+ +CO 3 (=), and NH 4 + +SO 4 (=). Both WSOC and SO 4 (=) show a dominant accumulation mode, with a geometric mean mode diameter D(g) of 0.41 and 0.34 μm and geometric standard deviation σ(sg) of 0.31 and 0.33, respectively. This mode makes up >70% of the total mass of both species. In comparison, Ca 2+ and CO 3 (=) show a prominent coarse mode with D(g) of 2.98 and 1.76 μm and σ(sg) of 0.29 and 0.22, respectively, that accounts for >60% of the mass. Based on these characteristics, estimates of the volume scatter coefficient β, direct back scatter coefficient β(π), and mass scattering efficiency φ for each component were made, assuming external mixing and optical and growth characteristics of corresponding pure chemical compounds. The results show that NH 4 + +SO 4 (=) has the largest β, β(π), and φ (median 1760x10 -8 m -1, 79x10 -8 m -1 sr -1, and 6.9 m 2 g -1, respectively). It is followed by WSOC with 1470x10 -8 m -1, 30x10 -8 m -1 sr -1, and 5.3 m 2 g -1 for β, β(π), and φ, respectively. Ca 2+ CO 3 (=) has only about 10% of the β and φ values but 20% of the β(π) of NH 4 + + SO 4 (=), respectively. For both NH 4 + +SO 4 (=) and WSOC, the mass scattering efficiency φ is inversely related to their D(g) between the size range of 0.2-0.45 μm.