Impact of traffic on black carbon aerosol concentration at la Réunion Island (Southern Indian Ocean)

To gain information on particle pollution by mobile sources, 3 experiments were conducted during the 1996–1998 period at Saint-Denis, the biggest urban site of La Réunion island (21.5°S; 55.5°E), situated in the Indian Ocean. Black Carbon (BC) concentrations were recorded with an Aethalometer which show high levels whatever the season (daily average: 270–650 ng m−3). At this site, a marked diurnal BC concentration variation is also evidenced in accordance with the observed traffic pattern. Measured daytime BC concentrations are 2–4 times greater than nighttime values. We checked that neither MBL height obtained by radio soundings nor wind speed or direction could explain satisfactorily the BC variations. A comparison with BC concentrations measured at other more remote sites of the island (Sainte-Rose and the altitude site Piton Textor) suggests that the background concentrations of the island are of the order 50 ngC m−3. These background values are almost never encountered in the main city (range: 80–2800 ngC m−3). We show that due to a singular convergence of parameters (topography of the island, road network, movement of population, quality of fuel), the city of Saint-Denis appears as polluted as continental European big cities.