Chatrapatty Bhugwant

Simultaneous measurements of black carbon, PM10, ozone and NOx variability at a locally polluted island in the southern tropics

This paper shows a comparative study of particle and surface ozone concentration measurements undertaken simultaneously at two distinct semi-urban locations distant by 4 km at Saint-Denis, the main city of La Réunion island (21.5° S, 55.5° E) during austral autumn (May 2000). Black carbon (BC) particles measured at La Réunion University, the first site situated in the suburbs of Saint-Denis, show straight-forward anti-correlation with ozone, especially during pollution peaks (∼ 650 ng/m3 and 15 ppbv, for BC and ozone respectively) and at night-time (∼90 ng/m3 and 18.5 ppbv, for BC and ozone respectively). NOx (NO and NO2) and PM10 particles were also measured in parallel with ozone at Lislet Geoffroy college, a second site situated closer to the city centre. NOx and PM10 particles are anti-correlated with ozone, with noticeable ozone destruction during peak hours (mean ∼6 and 9 ppbv at 7 a.m. and 8 p.m. respectively) when NOx and PM10 concentrations exhibit maximum values. We observe a net daytime ozone creation (∼19 ppbv, ΔO3 ∼ +4.5 ppbv), following both photochemical and dynamical processes. At night-time however, ozone recovers (mean ∼11 ppbv) when anthropogenic activities are lower ([BC] ∼100 ng/m3). BC and PM10 concentration variation obtained during an experiment at the second site shows that the main origin of particles is anthropogenic emission (vehicles), which in turn influences directly ozone variability. Saint-Denis BC and ozone concentrations are also compared to measurements obtained during early autumn (March 2000) at Sainte-Rose (third site), a quite remote oceanic location. Contrarily to Saint-Denis observations, a net daytime ozone loss (∼14.5 ppbv at 4 p.m.) is noticed at Sainte-Rose while ozone recovers (∼17 ppbv) at night-time, with however a lower amplitude than at Saint-Denis. Preliminary results presented here are handful data sets for modelling and which may contribute to a better comprehension of ozone variability in relatively polluted areas.

Diurnal and seasonal variation of carbonaceous aerosols at a remote MBL site of La Réunion island

In this study, we characterise the variability of black carbon (BC) obtained at Sainte-Rose, a tropical marine site of La Reunion island (21.5°S, 55°E) from one and a half-year (1998–1999) semi-continuous ground-based measurements. BC diurnal variation shows low concentrations during night-time (mean ∼10–20 ng/m3, following the season) and moderate levels at daytime (mean ∼35 ng/m3), while BC peaks are observed at the beginning and end of the day. Morning (mean ∼50 ng/m3 at 0700 h) and evening (mean ∼70 ng/m3 at 1800 h) BC peaks are thought to be due to local pollution induced by anthropogenic activities, most probably car exhausts from nearby roads. Night-time BC exhibits a seasonal variation too, with maximum levels observed during autumn and winter (∼20 ng/m3) and lowest values measured during spring and summer (∼10 ng/m3). BC seasonality suggests a local-to-regional anthropogenic origin of carbonaceous aerosols following the season. These suggestions are confirmed by wind sector and BC concentrations analysis at Sainte-Rose, which show that daytime BC aerosols originate mainly from inland while night-time BC concentrations originate from northeasterly (regional inhabited areas) to southeasterly (marine) regions, following the season. Wind sector analysis also points out important seasonal shift in origin of night-time air samples. These assumptions are confirmed by trajectory analysis indicating a spatial shift in the origin of air samples following the season. Finally, it is shown that during austral spring and summer, low carbonaceous aerosol concentrations encountered at this site are weakly subjected to scavenging processes. Results point out the potential of ground-based measurements to assess the origin and causes of carbonaceous aerosol variability at remote locations under favourable conditions (minimum local anthropogenic inputs and meteorology).

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 Réunion 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/m3 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/m3 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 0D time-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 Réunion 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.

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

Abstract This work presents the temporal evolution and spatial distribution of nitrogen dioxide (NO 2 ) 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. NO 2 concentration exhibits notable enhancement from 1997 to 2000. For instance, its mean concentration was ∼16 and 20 μg/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 heating 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 NO 2 concentration measured in parallel with ozone in November 2000 also showed that daytime photochemical ozone production is influenced by pollutant and precursor (NO 2 ) 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.

Variability of carbonaceous aerosols, ozone and radon at Piton Textor, a mountain site on Réunion island (south-western Indian Ocean)

Black carbon (BC) was monitored during 1997–1999 in the lower troposphere of the southernIndian Ocean at La Reéunion island (21.5°S, 55.5°E). BC concentrations obtained at PitonTextor, an altitude site (2150 m) representative of free troposphere, exhibited diurnal patternsand concentrations different from urban locations on the island, with maximum concentrationsobserved at daytime (~50–150 ng/m3) and minimum levels (~10–70 ng/m3) at night-time. BCdiurnal variation is anti-correlated with diurnal ozone measured semi-continuously in parallelduring 1998–1999, suggesting possible interaction of ozone and precursors (NOx, VOC, etc.)on carbonaceous aerosols, especially at night-time. Daytime BC enhancement may be explainedby dynamical processes, due to updraught of air masses from lower levels to the troposphere,while at night-time, this process is reversed. Daytime ozone depletion is governed by photochemicalprocesses, due to low precursor levels, while night-time ozone recovery is mainly driven bydynamical processes from upper tropospheric layers. Night-time BC and ozone in the lowertroposphere show a marked seasonal pattern too, with minimum levels during austral summer(~15 ng/m3, 22 ppbv), secondary peaks in autumn and spring (~35 ng/m3, 36 ppbv) and maximumvalues during austral winter (~70 ng/m3, 41 ppbv) respectively. Night-time BC and ozoneseasonalities are concordant with night-time radon seasonal trend in the lower troposphere,indicating that sampled air masses have mainly a marine origin in summer, off the Africanbiomass burning season, and a continental origin in austral winter and spring. Winter andspring BC and ozone enhancement corroborate with fire-count maximum peaks observed overAfrica and Madagascar, suggesting that the main cause is combustion products long-rangetransported in stable layers evidenced by thermodynamic analysis using 1996–1999 PTU soundings.These assessments are confirmed by 5-day backtrajectories, which show important seasonalshift in origin of air masses arriving in the lower troposphere of the south-western Indian Ocean.

Seasonal variation of tropospheric aerosols by ground based measurements at La Reunion island (20.8/spl deg/ S, 55.5/spl deg/ E)

The seasonal variation of aerosols is studied at La Reunion island by ground based measurements from the marine boundary layer (MBL, about 2000 m above sea level) up to the upper troposphere. La Reunion island is located at 20.8 S and 55.5 E. In this paper, with the help of different indicators, it is shown that the whole troposphere is largely influenced by biomass burning during the dry season (August-November). Measured background concentrations of black carbon and ozone are low (20 ng/m/sup 3/ and 40 ppbv respectively) whereas during the biomass burning season, they are multiplied by a factor of at least 2 in magnitude.