Variability of carbonaceous aerosols, ozone and radon at Piton Textor, a mountain site on Re´union 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.