Decadal changes in surface ozone at the tropical station Thiruvananthapuram (8.542° N, 76.858° E), India: effects of anthropogenic activities and meteorological variability.

This paper presents the first observational results from an Indian station on the long-term changes in surface ozone (O)-a major environmental pollutant and green house gas-over a period of about 40 years. It is based on the in situ measurements carried out during 1973-1975, 1983-1985, 1997-1998 and 2004-2014 at the tropical coastal station, Thiruvananthapuram. From 1973 to 1997, surface O shows a slow increase of ~ 0.1 ppb year and a faster increase of 0.4 ppb year afterwards till 2009 after which it showed a levelling off till 2012 followed by a minor decrease. The highest rate of increase is observed during 2005 to 2009 (2 ppb year), and the overall increase from 1973 to 2012 is ~ 10 ppb. The increase in day time O (peak O) is estimated as 0.42 ppb year during 1997-2012 and 2.93 ppb year during 2006-2012. Interestingly, the long-term trend in O showed seasonal dependence which is more pronounced during O peaking seasons (winter/summer). The observed trends were analysed in the light of the changes in NO, a major outcome of anthropogenic activities and methane which has both natural and anthropogenic sources and also meteorological parameters. Surface O and NO exhibited positive association, but with varying rate of increase of O for NO < 4 and > 4 ppb. Methane, a precursor of O also showed increase in tune with O. Unlike many other high-latitude locations, meteorology plays a significant role in the long-term trends in O at this tropical site with water vapour abundance and strong solar irradiance which favour photochemistry. A comparison with the corresponding changes in the satellite-retrieved tropospheric column O (TCO) also showed an increase of 0.03 DU year during 1996-2005 which enhanced to 0.12 DU year after 2005. Both surface O and satellite-retrieved TCO were positively correlated with daily maximum temperature, increasing at the rate of 1.54 ppb °C and 1.9 DU °C, respectively, on yearly basis. Surface O is found to be negatively correlated with water vapour content (ρ) at this tropical site, but at higher levels of ρ, O shows a positive trend.