Seasonal variations in aerosol concentrations and compositions in the Nepal Himalaya

This paper summarizes a 15-month long study (September 1996–November 1997) on water soluble components (Na+, NH4+, K+, Mg2+, Ca2+, Cl−, NO3−, and SO42−) in atmospheric aerosols conducted at a remote Himalayan site and a rural Middle-Mountain site in Nepal. Most of the species sampled behave similarly in terms of seasonal and short-term variations. The aerosol concentrations were low during the second half of the monsoon and post-monsoon seasons and gradually increased during the winter season. Aerosol concentrations in some samples were up to 20 times higher during the pre-monsoon and early-monsoon seasons than they were in post-monsoon. The seasonal variations are clearly related to variations in atmospheric circulation. The high concentrations during the pre-monsoon season are attributed to regional scale valley wind systems and the maximum concentrations at both sites coincide with maxima in the local wind speed record. Occasional shifts in the large-scale circulation resulted in dramatic fluctuations in aerosol concentrations during the pre-monsoon and early monsoon seasons. In the middle of May the large-scale circulation changed from westerly to south-easterly monsoon in the Himalaya. The summer monsoon circulation brought considerable amounts of pollution to the sites from southeast. Precipitation played an important role in short-term (few samples, e.g., during the late winter at the Himalayan site) to longer-term (several samples, e.g., after mid-June at both sites) reduction in the aerosol concentrations, while the lack of precipitation in the pre-monsoon and early monsoon supported a gradual build-up of pollutants in the atmosphere. During the pre-monsoon and early monsoon, when concentrations were elevated at both sites, they were often higher at the remote Himalayan site than at the middle mountain site. A pollution layer in the monsoon circulation due to dry convective rise of pollutants at distant sources and horizontal upper air transport by monsoon flow could have caused these elevated concentrations