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Climatology of tropospheric ozone (O) and water vapour (HO) over a metro city of Chennai in southern India has been presented based on the Measurements of OZone and water vapour by Airbus In-service Air Craft (MOZAIC) data during 1996-2001. The seasonal trends in mixing ratio of O are discussed in view of annual changes in the transport pattern, rainfall, and its relationship with ambient HO. The vertical distributions of both O and HO show large seasonal variations. The observed high mixing ratios of O in winter and spring months can be attributed to long-range transport of pollutants by the northwesterly winds associated with winter monsoon circulation. The lowest mixing ratio of 15 ppbv of O from July to September coincides with the summer monsoon circulation over Indian subcontinent. The surface level mixing ratio of O was observed to be below 30 ppbv throughout the year. On the other hand, monthly rainfall and vertical distributions of HO show just opposite seasonal variations to that of O mixing ratio. Chennai is a coastal city; therefore, the seasonal change in surface wind flow causes large variability in HO concentrations of 6-30 g/kg in the lower troposphere. More or less similar annual patterns of mixing ratios of O and HO are repeated during different years of 1996-1998, except for some small-scale differences. High level of O mixing ratio, particularly during the early winter months of year 1997, seems unusual as we do not see such trends for other year of measurements over Chennai. The case studies of profiles of O, relative humidity (RH), and temperature in view of long-range transport over Chennai during the different seasons have been also discussed. Significant enhancements in tropospheric O were observed over Chennai due to transport of air from forest fire regions of Indonesia during October 1997. Copyright © 2010 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

The influence of local meteorology and convection activities on the vertical distribution of carbon monoxide (CO) over Chennai in southern India was investigated by analysing the measurements of ozone aboard airbus in-service aircraft observations during the years 2012–2013. The seasonal variation of CO in the free troposphere was observed to be different and less pronounced than that in the planetary boundary layer (PBL). The near surface mixing ratio of CO was the highest ( 190 ± 68 ppbv ) during winter, while enhanced values ( 117 ± 11 ppbv ) in the free troposphere were observed during post-monsoon. The mixing ratios were the lowest throughout the troposphere during the monsoon. In the PBL, the mixing ratios of CO showed a decline with an increase in wind speed and were the highest ( > 200 ppbv ) under stagnant conditions ( 1 - 2 m s - 1 ) during winter. The higher CO in the lower free troposphere during the pre-monsoon period is attributed to the stronger biomass burning emissions. In the middle–upper troposphere, higher levels of CO during post-monsoon are due to the enhanced vertical mixing of regional emissions associated with weaker wind shears and frequent convection activities. Overall, the contrasting effects of stronger CO emissions can be observed in winter/pre-monsoon, while the efficient vertical mixing during the monsoon/post-monsoon season governs the observed seasonality of CO. The model for ozone and related chemical tracers, version 4 (MOZART-4) provides a reasonable representation of the convection effect on the CO mixing ratio. This study highlights a need to conduct more observations, especially of aircraft-borne instruments, to understand the effects of regional-scale emissions and dynamics in the middle–upper tropospheric chemistry over South Asia. [ABSTRACT FROM AUTHOR]