Chemical composition of aerosol and snow in the high Himalaya during the summer monsoon season

Aerosol and surface snow samples were collected from Hidden Valley in the Dhaulagiri region of western Nepal during the summer monsoon of 1994. Temporal variations of major ion (Na+ NH4+, K+, Mg2+, Ca2+, Cl−, NO3− and SO42− concentrations in the aerosol samples are clearly related to the influx of monsoon air masses. Snow was enriched in NH4+, and NO3−, while Na+/Cl− ratios were lower in the snow compared to the aerosol. A large part of this is explained by the difference in the air masses represented by aerosol and snow chemistry. Snow chemistry in general represented stronger southerly monsoon circulation, which resulted in precipitation events in Hidden Valley, whereas aerosol chemistry represented weaker monsoon or local circulation as the sampling was not conducted during rainy and foggy weather. Enrichment off NH4+ and NO3− in snow is attributed to their biogenic and agricultural sources from villages to the south and east of Hidden Valley. In addition, scavenging of HNO3 present in the air could also have contributed to the enrichment of NO3− in the snow. A lower Na+/Cl−atio in snow is attributed to scavenging of HCl present locally and/or due to less fractionation of monsoon air masses during more intense circulation and shorter travel time. The observed differences in the chemistry of the two media due to the influence of monsoon versus local air masses supports the concept of using glaciochemical records from that region to interpret monsoon variations in the past. Although the aerosol samples show excess cations, our data suggest the presence of acidic gases in the air locally. The overall major soluble ion concentrations of the aerosol are comparable or lower than those measured at several other remote tropospheric sites. Our results further support the concept that high elevation mountain sites in the Himalayas can be used to investigate the composition and the evolution of the remote continental troposphere.