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 southern Indian Ocean at La Re´union island (21.5°S, 55.5°E). BC concentrations obtained at Piton Textor, an altitude site (2150 m) representative of free troposphere, exhibited diurnal patterns and concentrations different from urban locations on the island, with maximum concentrations observed at daytime (~50–150 ng/m3) and minimum levels (~10–70 ng/m3) at night-time. BC diurnal variation is anti-correlated with diurnal ozone measured semi-continuously in parallel during 1998–1999, suggesting possible interaction of ozone and precursors (NOx, VOC, etc.) on carbonaceous aerosols, especially at night-time. Daytime BC enhancement may be explained by 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 photochemical processes, due to low precursor levels, while night-time ozone recovery is mainly driven by dynamical processes from upper tropospheric layers. Night-time BC and ozone in the lower troposphere 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 maximum values during austral winter (~70 ng/m3, 41 ppbv) respectively. Night-time BC and ozone seasonalities 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 African biomass burning season, and a continental origin in austral winter and spring. Winter and spring BC and ozone enhancement corroborate with fire-count maximum peaks observed over Africa and Madagascar, suggesting that the main cause is combustion products long-range transported in stable layers evidenced by thermodynamic analysis using 1996–1999 PTU soundings. These assessments are confirmed by 5-day backtrajectories, which show important seasonal shift in origin of air masses arriving in the lower troposphere of the south-western Indian Ocean.