Study of Ozone and NO2 over Gadanki - a rural site in South India

We have studied long-term changes in tropospheric NO2 over South India using ground-based observations, and GOME and OMI satellite data. We have found that unlike urban regions, the region between Eastern and Western Ghat mountain ranges experiences statistically significant decreasing trend. There are few ground-based observatories to verify satellite based trends for rural regions. However, using a past study and recent measurements we show a statistically significant decrease in NOX and O-3 mixing ratio over a rural location (Gadanki; 13.48A degrees N, 79.18A degrees E) in South India. In the ground-based records of surface NOX, the concentration during 2010-11 is found to be lower by 0.9 ppbv which is nearly 60 % of the values observed during 1994-95. Small but statistically significant decrease in noon-time peak ozone concentration is also observed. Noon-time peak ozone concentration has decreased from 34 A +/- 13 ppbv during 1993-96 to 30 A +/- 15 ppbv during 2010-11. NOX mixing ratios are very low over Gadanki. In spite of low NOX values (0.5 to 2 ppbv during 2010-11), ozone mixing ratios are not significantly low compared to many cities with high NOX. The monthly mean ozone mixing ratio varies from 9 ppbv to 37 ppbv with high values during Spring and low values during late Summer. Using a box-model, we show that presence of VOCs is also very important in addition to NOX in determining ozone levels in rural environment and to explain its seasonal cycle.