An update on findings from the Layered Atlantic Smoke Interactions with Clouds (LASIC) campaign in the remote southeast Atlantic

Description

From June 1, 2016 through October 31, 2017, the DOE ARM Mobile Facility 1 characterized the aerosol and cloud structure during two biomass-burning aerosol seasons to unprecedented detail over Ascension Island (14W, 8S), in the remote southeast Atlantic. The site is subject to the outflow of biomass-burning aerosol from continental Africa, over 1500 km away, from approximately July to November and is located within warm ocean waters that encourage deep boundary layers. Smoke reaches the surface more often than not, at times reaching black carbon mass concentrations similar to those near fire sources on land, alternating occasionally with very clean surface conditions. The surface aerosol loading is not necessarily well-correlated with that in the free-troposphere, indicating distinct aerosol transport pathways. Aerosol layering is apparent in micropulse lidar data, with free-tropospheric aerosol, when present, typically in contact with the uppermost cloud layer. Filter-based estimates indicate more shortwave absorption and lower single-scattering albedos earlier in the boreal summer. The event with the highest aerosol loading is explained by a direct transport within the boundary layer of smoke from the continent, but this transport pathway is otherwise atypical. The boundary layer is deeper when smoke is present near the surface and more well-mixed, with a stronger diurnal cycle in potential temperature. Non-precipitating low cloud fraction decreases when smoke is present in the boundary layer. In summary the new observations indicate an aerosol-cloud regime that is extensive and with the potential to demonstrate unanticipated aerosol-cloud interactions.