Ultra-Clean Marine Boundary Layers over the Southeast Atlantic

Aerosol number concentrations within the remote marine boundary layer (MBL) often exhibit significant spatio-temporal variability. A better understanding of the underlying processes is crucial for understanding and modeling of cloud-aerosol-radiation interactions. We focus on one aspect of this variability in our study- days with exceptionally low accumulation mode (100 - 1000 nm) particle concentrations. Our analysis centers on data from the deployment of an ARM Mobile Facility to Ascension Island from June 2016 to October 2017 as part of the Layered Atlantic Smoke Interactions with Clouds (LASIC) campaign. These observations show that ultra-clean conditions near the surface are common in the southeast Atlantic. Interestingly, they also occur exclusively in the period associated with significant influence from overlying African biomass burning aerosol layers (July - October). We then explore whether signatures of biomass burning smoke remain during these ultra-clean periods and how this fits into larger patterns of aerosol variability in the region. Point observations from LASIC are further contextualized with HYSPLIT back trajectories, satellite observations of cloud properties and MERRA-2 aerosol reanalysis to determine the relative importance and time scales of aerosol sources and sinks in generating the conditions observed at ASI. We conclude with some broader thoughts about why ultra-clean conditions might matter for aerosol-cloud-radiation interactions in an area heavily influenced by biomass burning smoke.