Soot particles and their impacts on the mass cycle in the Tibetan atmosphere

Atmospheric aerosol particles in urban and mountain areas around Lhasa city (29.65°N, 91.13°E) in the Tibetan Plateau were collected in the summers of 1998 and 1999. The particles were analyzed with electron microscopes and an energy dispersive X-ray spectrometer. Individual particle morphology, elemental composition and mixture of sulfate and nitrate were investigated. In the urban area, soot particles emitted from vegetation burning were dominant. These particles were characterized by chain or aggregate morphologies, and an elemental composition of potassium and sulfur. Such particles were frequently detected in mountain areas out of the city, where they formed droplets acting as condensation nuclei. Quantitative estimation indicated that sulfur was accumulated onto the soot particles during their dispersion from the urban area to mountain areas. Sulfate and nitrate detections indicated that soot particles collected in the urban area did not contain nitrate and BaCl2-reactive sulfate, which revealed that the combination of sulfur and potassium in the particles was not K2SO4. In contrast, the particles dispersed to mountain areas contained BaCl2-reactive sulfate and some contained nitrate, suggesting that soot particles emitted from the urban area could increase the buffering capacity of aerosol particles and enhance the formation of particulate sulfate through heterogeneous conversion in the Tibetan atmosphere.