Ultra-clean marine boundary layers over the Southeast Atlantic during the African biomass burning transport season
Abstract
Aerosol particles generated from biomass burning in southern Africa frequently overlie the Southeast Atlantic stratocumulus cloud deck. In addition to inducing potentially important direct and semi-direct radiative effects, these particles may also generate indirect effects if they entrain into the cloud-topped marine boundary layer (MBL) from the free troposphere. The deployment of an ARM Mobile Facility (AMF) to Ascension Island (ASI) as part of the Layered Atlantic Smoke Interactions with Clouds (LASIC) campaign is providing an invaluable platform to probe the coupling between meteorology, aerosol particles and clouds in this system. Observations from LASIC show that the months (June - October) typically associated with the strongest biomass burning aerosol signal in the free troposphere feature the days with the highest daily median surface aerosol number concentrations over a year's worth of data. Interestingly, however, some of these months also feature the lowest daily median number concentrations, with ultra-clean (daily median < 50 cm-3 in the accumulation mode) events lasting between a few days to over a week. We start by characterizing the finer scale behavior of aerosol number concentrations, size distributions and optical properties from ASI during both the ultra-clean and most polluted days, compared to monthly means, from July - November 2016. These results are compared to a period (December 2016 - April 2017) with less pronounced day-to-day aerosol variability and no ultra-clean events at the daily scale. Then, using a combination of soundings and ground-based remote sensing from ASI, trajectories, meteorological and aerosol reanalysis and satellite products, we begin to diagnose the key processes leading to ultra-clean conditions in the SE Atlantic MBL during the biomass burning season. Understanding the drivers of very low aerosol number concentrations in a region and season heavily impacted by smoke will help inform continued efforts to constrain the broader climatic impacts of these particles.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFM.A11C1895P
- Keywords:
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- 0321 Cloud/radiation interaction;
- ATMOSPHERIC COMPOSITION AND STRUCTURE;
- 3311 Clouds and aerosols;
- ATMOSPHERIC PROCESSES;
- 3355 Regional modeling;
- ATMOSPHERIC PROCESSES;
- 3360 Remote sensing;
- ATMOSPHERIC PROCESSES