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), Ca2++CO3=, and NH4++SO4=. Both WSOC and SO4= show a dominant accumulation mode, with a geometric mean mode diameter Dg of 0.41 and 0.34 &mgr;m and geometric standard deviation &sgr;sg of 0.31 and 0.33, respectively. This mode makes up >70% of the total mass of both species. In comparison, Ca2+ and CO3= show a prominent coarse mode with Dg of 2.98 and 1.76 &mgr;m and &sgr;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 &bgr;, direct back scatter coefficient &bgr;&pgr;, 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 NH4++SO4= has the largest &bgr;, &bgr;&pgr;, and ϕ (median 1760¿10-8 m-1, 79¿10-8 m-1 sr-1, and 6.9 m2g-1, respectively). it is followed by WSOC with 1470¿10-8 m-1, 30¿10-8 m-1 sr-1, and 5.3 m2g-1 for &bgr;, &bgr;&pgr;, and ϕ, respectively. Ca2++CO3= has only about 10% of the &bgr; and ϕ values but 20% of the &bgr;&pgr; of NH4++SO4=, respectively. For both NH4++SO4= and WSOC, the mass scattering efficiency ϕ is inversely related to their Dg between the size range of 0.2--0.45 &mgr;m. ¿ 2000 American Geophysical Union