SEASONAL AND LONG-TERM VARIATIONS OF AEROSOL CONTENT IN THE ATMOSPHERIC MIXING REGION AT A TROPICAL STATION ON THE ARABIAN SEA-COAST

Altitude profiles of aerosol number density in the atmospheric mixing region obtained using a bistatic CW lidar at Trivandrum (8.55 degrees N, 77 degrees E), a station situated on the West coast, near the south ern tip of the Indian peninsula, during the period 1989 to 1994 showed a systematic seasonal dependence. The number density shows a peak at similar to 300-400 m which is quite prominent during the local summer period and becomes less discernable during the North-East monsoon peri od. The mixing region aerosol optical depth estimated using these aero sol number density profiles shows a seasonal variation with two maxima ; one during the winter to summer transition period and the other duri ng the South-West monsoon period. A long term increasing trend is also observed in the aerosol optical depth from 1989 to 1994. A comparison of the mixing region aerosol optical depth with the total aerosol opt ical depth obtained using a solar radiometer has shown that this incre asing trend is not confined to the mixing region alone but extends to higher altitudes also. This study also shows that under quiescent cond itions, similar to 30% on average of the total aerosol optical depth, at 0.5 mu m wavelength, is contributed by the aerosols in the altitude region below 1 km. (C) 1998 Elsevier Science Ltd. Ail rights reserved .