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

Black carbon (BC) was monitored during 1997-1999 in the lower troposphere o f the southern Indian Ocean at La Reunion island (21.5 & 55.5 E). BC concen trations obtained at Piton Textor. an altitude site (2150 in) representativ e of free troposphere, exhibited diurnal patterns and concentrations differ ent from urban locations on the island. with maximum concentrations observe d at daytime (similar to 50-150 ng/m(3)) and minimum levels (similar to 10- 70 ng/m(3)) at night-time. BC diurnal variation is anti-correlated with diu rnal ozone measured semi-continuously in parallel during 1998-1999. suggest ing possible interaction of ozone and precursors (NOx, VOC, etc.) on carbon aceous aerosols. especially at night-time. Daytime BC enhancement may be ex plained by dynamical processes. due to updraught of air masses from lower l evels to the troposphere, while at night-time, this process is reversed. Da ytime ozone depletion is governed by photochemical processes, due to low pr ecursor levels. while night-time ozone recovery is mainly driven by dynamic al 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 (similar to 15 ng/m(3), 22 ppbv), secondary peaks in autumn and spring (similar to 35 ng/m(3), 36 ppbv) and maximum values durin g austral winter (similar to 10-70 ng/m(3), 41 ppbv) respectively. Night-ti me BC and ozone seasonalities are concordant with night-time radon seasonal trend in the lower troposphere, indicating that sampled air masses have ma inly a marine origin in summer. off the African biomass burning season, and a continental origin in austral winter and spring. Winter and spring BC an d ozone enhancement corroborate with fire-count maximum peaks observed over Africa and Madagascar. suggesting that the main cause is combustion produc ts long-range transported in stable layers evidenced by thermodynamic analy sis using 1996-1999 PTU soundings, These assessments are confirmed by 5-day backtrajectories. which show important seasonal shift in origin of air mas ses arriving in the lower troposphere of the south-western Indian Ocean.