Evidence of the complexity of aerosol transport in the lower troposphere on the Namibian coast during AEROCLO-sA

International audienceThe evolution of the vertical distribution and optical properties of aerosols in the free troposphere, above stratocumulus, is analysed for the first time over the Namibian coast, a region where uncertainties on aerosol-cloud coupling in climate simulations are significant. We show the high variability of atmospheric aerosol composition in the lower and middle troposphere during the AEROCLO-sA field campaign (22 August–12 September 2017) around the Henties Bay supersite, using a combination of ground-based, airborne and space-borne lidar measurements. Three distinct periods of 4 to 7 days are observed, associated with increasing aerosol loads (aerosol optical thickness at 550 nm ranging from ~ 0.2 to ~ 0.7), as well as increasing aerosol layer depth and top altitude. Aerosols are observed up to 6 km above mean sea level during the later period. Aerosols transported within the free troposphere are mainly polluted dust (dust mixed with smoke from fires in Angola) for the first 2 periods (22 August–1 September 2017) and smoke (from Angola and South America) for the last part (3–9 September) of the field campaign. Lagrangian back trajectory analyses highlight that the highest aerosol layers (between 5 and 6 km above mean sea level) come from South America (Brazil, Argentina and Paraguay) and reach Henties Bay after 4 to 5 days. They are transported eastward by the mid latitude westerlies and towards Southern Africa by the equatorward moving cut-off low originating within the westerlies. This results in a very complex mixture of aerosols over the coastal regions of Namibia that must be taken into account when investigating aerosols radiative effects above stratocumulus clouds in the south east Atlantic Ocean