Ozone trends in the vertical structure of Upper Troposphere and Lower stratosphere over the Indian monsoon region

Ozone trends in the Upper Troposphere and Lower Stratosphere over the Indian region are investigated using three satellite data sets namely Halogen Occultation Experiment (1993-2005), Stratospheric Aerosol and Gas Experiment (1993-2005) II, and Aura Microwave Limb Sounder (MLS, 2005-2011). Estimated ozone trends using multi-variate regression analysis are compared with trends at two Indian ozonesonde stations (Delhi, 28 degrees N, 77 degrees E and Pune, 18 degrees N, 73 degrees E), and a 3-D Chemical Transport Model (CTM, SLIMCAT) for the 1993-2005 time period. Overall, all the observational data sets and model simulations indicate significant increasing trend in the upper troposphere (0-2.5 %/year). In the lower stratosphere, estimated trends are slightly positive up to 30 mb and are negative between 30 and 10 mb. Increasing trends in the upper troposphere is probably due to increasing trends in the tropospheric ozone precursor gases (e. g. CO, NOx, NMHCs). Here, we argue that these contrasting ozone-trend profiles might be partially responsible for insignificant long-term trends in the tropical total column ozone. On seasonal scale, positive trends are observed during all the seasons in the upper troposphere while structure of trend profile varies in lower stratosphere. Seasonal variations of ozone trends and its linkages with stratospheric intrusions and increasing trends in lightning flashes in the troposphere are also discussed.