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

Abstract

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.