Aerosol loading in the atmospheric boundary layer at Trivandrum coast and in the adjoining oceanic environments during the FFP and IFP of the INDOEX

During the First Field Phase (FFP-98) and the Intense Field Phase (IFP-99) of the INDOEX, characteristics of atmospheric aerosols near the surface and in the atmospheric boundary layer have been studied at Trivandrum using a high volume sampler, a low pressure impactor and a CW lidar, The data obtai ned from these measurements along with the mass concentrations measured on cruises are used to study the spatial distribution of atmospheric aerosols and the advection of continental aerosols in the oceanic environments. Duri ng IFP-99 a significant increase in aerosol loading was observed near the s urface and in the atmospheric boundary layer. The daytime mass concentratio ns (m) measured at Trivandrum are comparable with the measurements conducte d on cruise in the adjoining oceanic regions. The nighttime aerosol mass co ncentrations at Trivandrum are relatively high in January. The day-night co ntrast in decreases during March. A large build-up in m was observed at Tri vandrum associated with the changes in circulation pattern (land breeze-sea breeze), This is attributed to horizontal convergence. The aerosol size di stribution measured using the impactor, in general, is trimodal, In Februar y and March the sub-micron mode shifts towards smaller size regime (< 0.02 mum), The large particle mode (between 0.1 and 1 mum) is rather broad and g ets split into two at times. The gross form of near surface aerosol size di stribution can be approximated to a power law, with the size index in the r ange 3.7 to 4.4. This size index shows a decrease during IFP-99 period, ind icative of a relative increase in larger size particles. The mixing region aerosol optical depth also is relatively high during IFP-99, The mass conce ntration measured along the West Coast on the cruise shows an increase duri ng IFP-99. Along the coast, as the ship sails from Coa towards south, the a erosol mass concentration first decreases and then increases as the ship ap proaches Cochin, This increased concentration remains steady up to the sout hern tip of the Indian peninsula. A significant decrease in m was observed in the ITCZ region, The aerosol concentration south of ITCZ was less than t hat in the North (in the Arabian Sea region). An increased concentration of aerosols was observed in the mid-Arabian Sea oceanic environments, which w as attributed to advecting aerosols due West and North.