This paper presents the first observational results from an Indian station on the long-term changes in surface ozone (O3)—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 O3 shows a slow increase of ~ 0.1 ppb year−1 and a faster increase of 0.4 ppb year−1 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−1), and the overall increase from 1973 to 2012 is ~ 10 ppb. The increase in day time O3 (peak O3) is estimated as 0.42 ppb year−1 during 1997–2012 and 2.93 ppb year−1 during 2006–2012. Interestingly, the long-term trend in O3 showed seasonal dependence which is more pronounced during O3 peaking seasons (winter/summer). The observed trends were analysed in the light of the changes in NO2, a major outcome of anthropogenic activities and methane which has both natural and anthropogenic sources and also meteorological parameters. Surface O3 and NO x exhibited positive association, but with varying rate of increase of O3 for NO x < 4 and > 4 ppb. Methane, a precursor of O3 also showed increase in tune with O3. Unlike many other high-latitude locations, meteorology plays a significant role in the long-term trends in O3 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 O3 (TCO) also showed an increase of 0.03 DU year−1 during 1996–2005 which enhanced to 0.12 DU year−1 after 2005. Both surface O3 and satellite-retrieved TCO were positively correlated with daily maximum temperature, increasing at the rate of 1.54 ppb °C−1 and 1.9 DU °C−1, respectively, on yearly basis. Surface O3 is found to be negatively correlated with water vapour content (ρ v) at this tropical site, but at higher levels of ρ v, O3 shows a positive trend.
Environmental Science and Pollution Research – Springer Journals
Published: Mar 14, 2018
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