Chemical effect of carbonaceous aerosols on the diurnal cycle of MBL ozone at a tropical site: measurements and simulations

During Late austral summer and winter 1998, black carbon (BC) aerosols were monitored with an Aethalometer at 2 sites of La Reunion island (Indian Ocean): Saint-Denis, the main city and Sainte-Rose, a quite uninhabited region situated at the east coast. BC concentration data at Saint-Denis show a marked diurnal cycle, which may be primarily attributed to traffic. The background data found at night-time display average BC concentrations, ranging from about 80 to 250 ng/m(3) whereas during the day, BC concentrations increase by a factor of at least 4. in comparison, BC concentrations vary in the range of 10 to 60 ng/m(3) at Sainte-Rose. Ozone concentration was also measured at Saint-Denis using a Dasibi photometer and found to be at significant levels(means: 16.5-23 ppbv in April and 28.5-34 ppbv in September). A noticeable increase of ozone concentrations during the day points out the build-up of pollutants enhancing photochemical transformations. However, during traffic pollution peaks, ozone concentration displays systematic depletion. The comparison of ozone and BC measurements at both seasons points to some possible effects of heterogeneous interaction of ozone and its precursors with BC particles. These interactions were also simulated with a OD lime-dependent chemistry model using conditions of a polluted site. The measured ozone concentration characteristics (mean concentration and range of variation) are well simulated in the presence of BC, Our model results show that at La Reunion Island adsorption of ozone and its precursors onto BC aerosol particles could be one of the important steps determining ozone concentration characteristics, especially in absence of photochemistry during night-time.