Air pollution transport in the Himalayas of Nepal and Tibet during the 1995–1996 dry season

To investigate if air pollution generated in the Himalayan foothills reached higher elevations, a sequence of surface meteorological and condensation nucleus (CN) measurements were made between October 1995 and May 1996. Measurements were made in the Kathmandu valley of the foothills, in the Dudh Kosi valley of the eastern Himalayas and at the base of Mt. Everest in Nepal and Tibet. The Kathmandu valley measurements revealed a semi-diurnal variation of CN and moisture in the mountain-valley wind system and a reduction of both in strong afternoon convection we call the “Kathmandu chimney”. The air was polluted at all times in the valley. The Dudh Kosi measurements revealed a diurnal variation in CN and moisture; both were drawn into the valley in the strong afternoon valley wind and cleaner and drier air flowed into the valley in the nighttime mountain wind. The air was unpolluted in the mornings and frequently polluted in the afternoon and evenings; some of the evening pollution was from local sources. The CN concentrations in Tibet were smaller than in Nepal at the same elevation due dilution in the deep afternoon convective boundary layer in Tibet; there also may be a “Tibetan chimney”. Evidence is presented for trans-Himalayan pollution transport: a 7-h pollution episode originating in Nepal was advected into Tibet and a 18-h episode originating in Tibet was advected into Nepal.