Aerosol characterization in an oceanic context around Reunion Island (AEROMARINE field campaign)

Abstract This article presents the results of the AEROMARINE field campaign which took place between February and April 2019 off the coast of Reunion Island in the South West Indian Ocean basin. The Southern Indian Ocean is of major interest for the study of marine aerosols, their distribution and variability. Six instrumented light plane flights and a ground-based microwave radiometer were used during the field campaign. These measurements were compared with the long-term measurements of the AERONET sun-photometer (based in St Denis, Reunion Island) and various instruments of the high altitude Maido Observatory (2.2km above sea level, Reunion Island). These results were also analyzed using different model outputs: (i) the AROME mesoscale weather forecast model to work on the thermodynamics of the boundary layer, (ii) the FLEXPART-AROME Lagrangian particle dispersion model to assess the geographical and vertical origin of air masses, and (iii) the chemical transport model CAMS (Copernicus Atmosphere Monitoring Service) to work on the aerosol chemical composition of air masses. This allowed to highlight two points: (1) the atmospheric layer above 1.5km is mainly composed of aerosols from the regional background; (2) the local environment (ocean or island) has little impact on the measured concentrations. Marine aerosols emitted locally are mostly measured below 0.5km. The daytime marine aerosol distributions in the free troposphere measured by the aircraft were compared to the aerosol distribution measured at the high altitude Maido Observatory at night when the Observatory is located in the free troposphere. The results indicate that the high altitude site measurements are representative of the concentration of marine aerosols in the free troposphere. We also found that the CAMS reanalyses overestimated the aerosol optical depth in this region. Finally, our study strongly suggests that the AERONET station in St Denis (Reunion Island) can be considered as a representative marine station under the Tropics.