Atmospheric dust aerosol over the Tibetan Plateau

Cascade impactor and bulk samples of atmospheric aerosol particles were collected at three sites on the Tibetan Plateau: Udaoliang (September 1993 to May 1994) and Lhasa and Gongga (May 1998). The average dust concentration at these ground-based sites was 82 μg m^-3; this is lower than the lowest average concentration observed over the nine major Chinese deserts (arithmetic mean value of 270 μg m^-3), and it also is lower than that over the Loess Plateau (170 μg m^-3). The elemental mass-particle size distributions for several major dust-derived elements (Al, Ca, Fe, K, Mn, Si, and Ti) were approximately lognormal, but those for Al, Fe, and Ti differed from Ca, K, and Si. Dry deposition velocities calculated from the observed size distributions and a two-layer deposition model averaged 2.7 cm^-1 for Al, Fe, and Ti. For Ca, K, and Si, which appear to be more strongly affected by local Tibetan sources, the average calculated dry deposition velocity was 4.3 cm s^-1. Wet deposition fluxes estimated from scavenging ratios indicate that less than 10% of the total deposition on the Tibetan Plateau is attributable to wet removal during the 9 months for which data are available. The yearly mean dust deposition rate (local plus remote and dry plus wet) is estimated to be of the order of ∼100 g m^-2 yr^-1, which is lower than that onto the Chinese desert regions (320 g m^-2 yr^-1) or the Loess Plateau (250 g m^-2 yr^-1), suggesting that the sources upwind of China, and the Tibetan Plateau itself, are not major sources for Asian dust.