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 urba n site of La Reunion island (21.5 degrees S; 55.5 degrees E), situated in t he Indian Ocean. Black Carbon (BC) concentrations were recorded with an Aet halometer which show high levels whatever the season (daily average: 270-65 0 ng m(-3)). At this site, a marked diurnal BC concentration variation is a lso evidenced in accordance with the observed traffic pattern. Measured day time BC concentrations are 2-4 times greater than nighttime values. We chec ked that neither MBL height obtained by radio soundings nor wind speed or d irection 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 conc entrations of the island are of the order 50 ngC m(-3). These background va lues 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 ci ty of Saint-Denis appears as polluted as continental European big cities. ( C) 2000 Elsevier Science Ltd. All rights reserved.