MASS SIZE DISTRIBUTION AND CHEMICAL-COMPOSITION OF AEROSOLS AT THE SILENT VALLEY, INDIA

Field observational programmes were undertaken at the Silent Valley, a core zone of the Nilgiri Biosphere Reserve, in three different seasons during 1989-90 to study the mass size distributions of atmospheric aerosols and some of their water soluble components (SO4, NO3, K and Ca). The average concentration of total suspended particulates was 37 mug/m3 which is far below the values reported from urban and non-urban regions in the country. Atmospheric aerosols, SO4, K and Ca showed bimodal distributions, whereas NO3 showed unimodal distribution in all the three seasons. Although the aerosols showed bimodal distribution, the contribution of submicron particles was higher than that of coarse particles. This is the pecularity of forest region which was not observed in other parts of the country where coarse particles dominate over the submicron particles. Another special feature observed was the mass size distribution of K aerosols which showed bimodal distribution with a prominent peak in the submicron size range. It has been observed from the mass size distributions of aerosols and some of their chemical components that most of them are in submicron size range, acidic in nature, and can cause acidification on precipitation in the region.