Effect of biomass burning, convective venting, and transport on tropospheric ozone over the Indian Ocean: Reunion Island field observations

Relationships between vertical distribution of tropospheric ozone at Reunion Island (21 degrees S-55 degrees E), satellite (NOAA advanced very high resolution radiometer) observations of fires, smoke plumes, and convective events in southeastern Africa and Madagascar, and analyses of meteorological situations (European Centre for Medium-Range Weather Forecasts) are presented. This study is based on 7 years (1992 to 1999) of 2-monthly PTU-O-3 radiosoundings at Reunion. Results show, for the first time, that during 1995 tropospheric ozone content rose above average and that this year should be set apart as atypical. Stratospheric contributions are also ruled out using an identification method based on considerations of ozone, humidity, vertical stability, and meteorological conditions. The seasonal variation of ozone profiles during typical years and without the stratospheric contribution suggests that ozone contamination from biomass burning is a maximum during October in the whole free troposphere. During August, before the deep convection period, but already within the fire period, only the middle troposphere is contaminated by ozone inputs. By contrast, through November to December, well within the deep convection period, all the higher troposphere is contaminated. The comprehensive study of the observations in 1993, taken as a typical year, highlights the roles of convection and transport in contamination of remote oceanic regions. August contamination of the middle troposphere by about 70 ppbv of ozone is contrasted to October enhancement of the whole free troposphere by about 100 ppbv of ozone after the, spreading of deep convective events. Fire satellite data further indicate that column integrated contamination level mainly depends on biomass burning intensity. Through August to October the fourfold increase of ozone concentration is comparable with the fivefold augmentation of fires. The redistribution of ozone with altitude depends on the convection intensity near source regions in accordance with convection detection and backtrajectory analysis.