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1) To examine the effect of human activities and release of pollutants at the earth's surface on the charged species of the troposphere, a first steady state ion composition model is indigenously developed. The model suggests that ion composition could be perturbed as a result of increasing emissions of parent neutral species including alkaline compounds, and that in particular, the relative atmospheric abundance of pyridinated water cluster ions is increasing dramatically and should further increase in the future.
2) The first ever satellite evidence of a systematic and substantial increasing trend in tropospheric ozone over the past two decades over both the marine and continental regions in tropics has been reported which is confirmed by the model study. Results indicate an increase in tropospheric ozone by 23% over Indian subcontinent in the past two decade that means a radiative forcing of climate may be up by 0.4-0.7 Wm -2 which is substantial 3) It has been demonstrated using the 2-D modelling approach that the changes from anthropogenic activities are less in magnitude but permanent in nature whereas changes due to variations in solar activity are comparatively higher but non-permanent. A cooling of the order of 10 ° K in the mesosphere and 8-12 ° K in the stratosphere is projected for the double-CO 2 scenario which is expected in 2050. 4) A three-dimensional model has been used to evaluate the measured ozone distribution during the INDOEX (Indian Ocean Experiment) period which reproduces most of the observed features. Results indicate the low surface ozone values (~20 ppb), which increases in the mid-troposphere ( 40-60 ppb) followed by a steep gradient (~120 ppb) near the tropopause over the oceanic region. The low value of O 3 is characterized by the low concentration of NO x and CO in the deep ocean. 5) Impact of Volcanic Eruption on Atmospheric Composition: It has been reported by us that the total ozone amount has declined by 4-7 DU and the density of ultra-fine charged aerosol has increased several folds over the Indian region due to volcanically induced perturbations which injected large amount of SO 2 directly into the tropical stratosphere. 6) Reconstruction of Past Chemical Climate: In a relatively new approach, simulations were made to reconstruct the atmospheric composition and thermal structure of the palaeo-periods extending from last ice age to the present using interactive chemical model. Model results revealed a cooling of troposphere and a warming by about 10 to 15 ° k in the upper stratosphere as compared to present during the last ice age. 7) Assessment Work on Temperature Trend in the Middle Atmosphere: A comprehensive study to assess the long term trend in the temperature structure of the middle atmosphere is taken up.
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