Dynamical nature of tropospheric ozone over a tropical location in Peninsular India: Role of transport and water vapour.

This paper deals with the temporal changes in the altitude distribution of tropospheric ozone (O 3) based on the measurements by balloon-borne O 3 -sondes during 2011–2014, conducted at the tropical, coastal site Thiruvananthapuram on the south west coast of India. This is the first study from this region addressing the highly dynamic nature of tropospheric O 3 profiles (in terms of their vertical structure and short-term changes) and attempting to categorise them based on 121 in-situ measured O 3 profiles. The tropospheric O 3 profiles could be categorised into four major groups namely (i) those with steady O 3 mixing ratio (ii) with increasing mixing ratio, (iii) with mid-tropospheric enhancement and (iv) with multiple layers/laminar nature. The causative mechanisms of these different categories were examined. The observed differences in the tropospheric O 3 distribution are attributed to meteorological conditions in particular the synoptic scale systems, long range transport, intrusion from stratosphere and photochemistry, most importantly, the effect of water vapour content. Water vapour and O 3 showed complex dependence with positive and negative association depending on the precursor levels and availability of water vapour. The altitudinal changes in O 3 also exhibited close association with those of potential temperature and equivalent potential temperature. An analysis of the seasonal characteristics of vertical distribution of tropospheric O 3 also carried out along with the altitude-dependent seasonal behaviour. In general, the total column O 3 estimated by the integration of O 3 -sonde retrieved profiles differed by about ±10% with those retrieved by satellite-based measurements. The TCO contributes to about 16% (34 DU) of the total column O 3 , with minimum of 9% in October and maximum of 27% on March. In general, the OMI retrievals under-estimates the O 3 -sonde derived TCO by 5–10 DU in all the seasons. • Dynamic nature of tropospheric O 3 profiles observations based on O 3 -sonde. • Dependence of O 3 on water vapour shows positive and negative correlation. • Amount of water vapour and precursors decide nature of water vapour dependence of O 3. • The seasonal variation of tropospheric O 3 is altitude-dependent. • Tropospheric column O 3 contributes about 16% of the total column O 3. [ABSTRACT FROM AUTHOR]