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Discussion papers
https://doi.org/10.5194/acp-2019-628
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-628
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 15 Jul 2019

Submitted as: research article | 15 Jul 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Ultra-clean and smoky marine boundary layers frequently occur in the same season over the southeast Atlantic

Sam Pennypacker, Michael Diamond, and Robert Wood Sam Pennypacker et al.
  • Department of Atmospheric Sciences, University of Washington, Seattle, WA USA

Abstract. We study forty-one days with daily median surface accumulation mode aerosol particle concentrations below 50 cm−3 (ultra-clean conditions) observed at Ascension Island (7.9° S, 14.4° W) between June 2016 and October 2017 as part of the Layered Atlantic Smoke Interactions with Clouds (LASIC) campaign. Interestingly, these days occur during a period of great relevance for aerosol-cloud-radiation interactions, the southeast Atlantic (SEATL) biomass-burning season (approximately June–October). That means that these critical months can feature both the highest surface aerosol numbers, from smoke intrusion into the marine boundary layer, as well as the lowest. While carbon monoxide and refractory black carbon concentrations on ultra-clean days do not approach those on days with heavy smoke, they also frequently exceed background concentrations calculated in the non-burning season from December 2016–April 2017. This is evidence that even what become ultra-clean boundary layers can make contact with and entrain from an overlying SEATL smoke layer before undergoing a process of rapid aerosol removal. Because many ultra-clean and polluted boundary layers observed at Ascension Island follow similar isobaric back-trajectories, the variability in this entrainment is likely closely tied to the variability in the overlying smoke rather than large-scale horizontal circulation through the boundary layer. Finally, surface drizzle rates, frequencies and accumulation – as well as retrievals of liquid water path – all consistently tend toward higher values on ultra-clean days. This implicates enhanced coalescence scavenging in low clouds as the key driver of ultra-clean events in the southeast Atlantic marine boundary layer. These enhancements occur against and are likely mediated by the backdrop of a seasonal increase in daily mean cloud fraction and daily median liquid water path over ASI, peaking in September and October in both LASIC years. Therefore the seasonality in ultra-clean day occurrence seems directly linked to the seasonality in SEATL cloud properties. These results highlight the importance of two-way aerosol-cloud interactions in the region.

Sam Pennypacker et al.
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Status: open (until 27 Sep 2019)
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Sam Pennypacker et al.
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Short summary
Using observations from instruments deployed to a small island in the Southeast Atlantic, we study days where the atmospheric concentrations of particles near the surface are exceptionally low. We still find evidence of some mixing with smoke on some of these days. Heavier drizzle from shallow clouds compared to other days is most likely responsible for clearing out most of the particles.
Using observations from instruments deployed to a small island in the Southeast Atlantic, we...
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