Climatology of tropospheric ozone and water vapour over Chennai: a study based on MOZAIC measurements over India
Résumé
Climatology of tropospheric ozone (O3) and water vapour (H2O) 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 AirCraft (MOZAIC) data during 1996-2001. The seasonal trends in mixing ratio of O3 are discussed in view of annual changes in the transport pattern, rainfall, and its relationship with ambient H2O. The vertical distributions of both O3 and H2O show large seasonal variations. The observed high mixing ratios of O3 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 O3 from July to September coincides with the summer monsoon circulation over Indian subcontinent. The surface level mixing ratio of O3 was observed to be below 30 ppbv throughout the year. On the other hand, monthly rainfall and vertical distributions of H2O show just opposite seasonal variations to that of O3 mixing ratio. Chennai is a coastal city; therefore, the seasonal change in surface wind flow causes large variability in H2O concentrations of 6-30 g/kg in the lower troposphere. More or less similar annual patterns of mixing ratios of O3 and H2O are repeated during different years of 1996-1998, except for some small-scale differences. High level of O3 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 O3, 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 O3 were observed over Chennai due to transport of air from forest fire regions of Indonesia during October 1997.
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