Aerosol and Precipitation Chemistry at a Remote Himalayan Site in Nepal
Abstract:As a part of a year long aerosol measurement program in the Nepal Himalayas, a series of 12 h samples of aerosol and event-based samples of precipitation were collected in postmonsoon, 1996; premonsoon, 1997; and monsoon, 1997. Results show that diurnal variations in the regional valley wind system are responsible for variations in the major ion (Na+, NH4+, K+, Mg2+, Ca2+, Cl-, NO3-, and SO4-2) concentrations in the aerosol. On time scales greater than a day, changes in atmospheric circulation and associated changes in the precipitation regime have an important effect on aerosol ion concentrations.
Periods of high major ion concentration in the aerosol generally coincide with similar periods in precipitation chemistry, although a linear relationship between concentrations in these 2 media is not robust due to limited data. High scavenging ratio values are found for all species except SO42-. Nitrate, Cl-, and NH4+ are enriched in the precipitation compared to the aerosol, implying the presence of gaseous acidic species (HNO3 and HCl) and gaseous NH3 in the air. Ammonium was also enriched with respect to SO42- in aerosol during a dry episode in the monsoon season. This may represent relatively local sources of NH3 (from neighboring villages) and was not scavenged due to the lack of precipitation.
Empirical orthogonal function (EOF) analysis clearly shows the presence of 2 dominant pollutant transport mechanisms for the premonsoon and monsoon seasons (i.e., valley wind system and monsoon circulation). Although physically different, these 2 transport mechanisms follow similar transport pathways and transport aerosol into the Himalayas from similar source regions. Further, EOF analysis suggests a southerly maritime signal in the aerosol during monsoon and a more distant westerly maritime signal during premonsoon. Our results support the potential for using glaciochemical records from the Himalayas to investigate variations in the strength of past monsoon circulation and westerly disturbances.
Document Type: Research Article
Affiliations: 1: Department of Hydrology and Meteorology, Babar Mahal, Kathmandu, Nepal 2: Climate Change Research Center, Institute for the Study of Earth, Oceans, and the Space, University of New Hampshire, Durham, New Hampshire
Publication date: April 1, 2002