Abstract. The STRAP (Synergie Transdisciplinaire pour Répondre aux Aléas liés aux Panaches volcaniques) campaign was conducted over the entire year of 2015 to investigate the volcanic plumes of Piton de La Fournaise (La Réu-nion, France). For the first time, measurements at the local (near the vent) and at the regional scales were conducted around the island. The STRAP 2015 campaign has become possible thanks to strong cross-disciplinary collaboration between volcanologists and meteorologists. The main observations during four eruptive periods (85 days) are summarised. They include the estimates of SO 2 , CO 2 and H 2 O emissions, the altitude of the plume at the vent and over different areas of La Réunion Island, the evolution of the SO 2 concentration, the aerosol size distribution and the aerosol extinction profile. A climatology of the volcanic plume dispersion is also reported. Simulations and measurements show that the plumes formed by weak eruptions have a stronger interaction with the surface of the island. Strong SO 2 mixing ratio and particle concentrations above 1000 ppb and 50 000 cm −3 respectively are frequently measured over a distance of 20 km from Piton de la Fournaise. The measured aerosol size distribution shows the predominance of small particles in the volcanic plume. Several cases of strong nucleation of sulfuric acid have been observed within the plume and at the distal site of the Maïdo observatory. The STRAP 2015 campaign provides a unique set of multi-disciplinary data that can now be used by modellers to improve the numerical parameterisations of the physical and chemical evolution of the volcanic plumes.

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<p><strong>Abstract.</strong> The STRAP (Synergie Transdisciplinaire pour Répondre aux Aléas liés aux Panaches volcaniques) campaign was conducted in 2015 to investigate the volcanic plumes of Piton de La Fournaise (La Réunion, France). For the first time, measurements at the local (near the vent) and at the regional scales around the island were conducted. The STRAP 2015 campaign has become possible thanks to a strong cross-disciplinary collaboration between volcanologists and meteorologists. The main observations during four eruptive periods (85 days) are summarized. They include the estimates of SO<sub>2</sub>, CO<sub>2</sub> and H<sub>2</sub>O emissions, the altitude of the plume at the vent and over different areas of La Réunion Island, the evolution of the SO<sub>2</sub> concentration, the aerosol size distribution, and the aerosol extinction profile. A climatology of the volcanic plume dispersion is also reported. Simulations and measurements showed that the plume formed by weak eruption has a stronger interaction with the surface of the island. Strong SO<sub>2</sub> and particles concentrations above 1000 ppb and 50 000 cm<sup>−3</sup>, respectively, are frequently measured over 20 km of distance from the Piton de la Fournaise. The measured aerosol size distribution shows the predominance of small particles in the volcanic plume. A particular emphasis is placed on the gas-particle conversion with several cases of strong nucleation of sulfuric acid observed within the plume and at the distal site of the Maïdo observatory. The STRAP 2015 campaign gave a unique set of multi-disciplinary data that can now be used by modellers to improve the numerical paramameterizations of the physical and chemical evolution of the volcanic plumes.</p>

Black carbon (BC) was monitored during 1997-1999 in the lower troposphere of the southern Indian Ocean at La Réunion 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 (NO x , 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.