Impact of traffic on black carbon aerosol concentration at la Reunion 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 Reunion island (21.5 degrees S; 55.5 degrees 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. (C) 2000 Elsevier Science Ltd. All rights reserved.