Variability of NO2 in different environments at a moderately polluted island over the southwestern Indian Ocean

This work presents the temporal evolution and spatial distribution of nitrogen dioxide (NO2) measured with the help of passive diffusion tubes in different environments: rural, sub-urban, urban, at La Reunion Island in December 1997 and November 2000 (austral spring), respectively. NO2 concentration exhibits notable enhancement from 1997 to 2000. For instance, its mean concentration was similar to 16 and 20 mug/m(3) in December 1997 and November 2000, respectively, at La Reunion Island, especially in inhabited regions due to increase of local anthropogenic activities, mainly traffic circulation, in the absence of house beating and biomass burning. We also observe a net increase of pollutant levels by a factor two in rural/remote areas within 3 years, which is tied to the extension of inhabited areas and air-flows from enhanced local sources. The increase of atmospheric pollutants is mainly due to enhancement of anthropogenic activities (traffic) since the 1990s at La Reunion Island, which results from a constant increase of population and consequently, from a higher number of vehicles in circulation. Importantly, in cities like Saint-Denis, where traffic network has been subject to notable improvement (e.g. single to double lanes, etc.), pollutant levels exhibit little variability as in temperate continental latitudes. Semi-continuous NO2 concentration measured in parallel with ozone in November 2000 also showed that daytime photochemical ozone production is influenced by pollutant and precursor (NO2) levels at Saint-Denis, while nighttime ozone increase is mainly tied to dynamical processes. The November 2000 ozone diurnal pattern is notably different from that observed in September 1995, during which a daytime ozone loss was observed, pointing out that atmospheric conditions have been subject to notable changes, i.e., clean to moderately polluted, within a few years at La Reunion Island. The results presented here point out that the. source enhancement effect' is superimposed on dynamical and photochemical processes to influence pollutants variability and hence atmospheric chemistry, in a quite 'pristine' tropical oceanic region. (C) 2003 Elsevier B.V. All rights reserved.