The importance of carbon and mineral dust to seasonal aerosol properties in the Nepal Himalaya

Though recent studies suggest that aerosols may influence the Asian monsoons, limited knowledge exists regarding aerosol impacts on regional climate and air quality. To improve understanding of aerosols in the Himalaya, mass concentrations (PM10 and PM2.5), chemical composition, and wavelength dependent aerosol optical depth (delta(lambda)) were measured from December 1998 through October 2000 at sites on the edge of the Kathmandu valley (Nagarkot) and in the remote Himalaya (Langtang). Though highly variable, aerosol concentrations peaked in February to May with average PM2.5 = 59 +/- 61 mug/m(3) and aerosol optical depth delta(500 nm) = 0.37 +/- 0.25 at Nagarkot. With arrival of southerly flow and monsoon rains, PM2.5 dropped to 8 +/- 7 mug/m(3) in June through September. With the cessation of summer rains, aerosol concentrations began steadily increasing with average PM2.5 = 25 +/- 14 mug/m(3) during October to January. During all seasons, a large PM2.5/PM10 ratio (0.77 +/- 0.21), the predominance of particulate organic material (48 +/- 28% of PM2.5), and an ionic composition dominated by SO42-, NH4+, and NO3- (16 +/- 14% of PM2.5) all implicated combustion sources. Nonetheless, a large fraction of the aerosol was insoluble and non-carbonaceous (33 +/- 27%), particularly during the peak season when it reached 45%. Elemental analysis of a filter subset showed large concentrations of Ca, Si, Al, and Fe, indicating that dust was responsible for much of the remaining material. Though an influence of the Kathmandu valley was likely, evidence supported long-range transport of desert dust to the Himalaya from and regions ranging from India and the Middle East to perhaps as far as the Sahara. (C) 2003 Elsevier Science Ltd. All rights reserved.