Seasonal and long term variations of aerosol content in the atmospheric mixing region at a tropical station on the Arabian sea-coast

Abstract

Altitude profiles of aerosol number density in the atmospheric mixing region obtained using a bistatic CW lidar at Trivandrum (8.55°N, 77°E), a station situated on the West coast, near the southern tip of the Indian peninsula, during the period 1989 to 1994 showed a systematic seasonal dependence. The number density shows a peak at ^~300-400 m which is quite prominent during the local summer period and becomes less discernable during the North-East monsoon period. The mixing region aerosol optical depth estimated using these aerosol number density profiles shows a seasonal variation with two maxima; one during the winter to summer transition period and the other during 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 optical depth obtained using a solar radiometer has shown that this increasing trend is not confined to the mixing region alone but extends to higher altitudes also. This study also shows that under quiescent conditions, ^~30% on average of the total aerosol optical depth, at 0.5 μm wavelength, is contributed by the aerosols in the altitude region below 1 km.