Title: Open cells exhibit weaker entrainment of free-tropospheric biomass burning aerosol into the south-east Atlantic boundary layer

Abstract

This work presents synergistic satellite, airborne and surface-based observations of a pocket of open cells (POC) in the remote south-east Atlantic. The observations were obtained over and upwind of Ascension Island during the CLouds and Aerosol Radiative Impacts and Forcing (CLARIFY) and the Layered Smoke Interacting with Clouds (LASIC) field experiments. A novel aspect of this case study is that an extensive free-tropospheric biomass burning aerosol plume that had been transported from the African continent was observed to be in contact with the boundary layer inversion over the POC and the surrounding closed cellular cloud regime. The in situ measurements show marked contrasts in the boundary layer thermodynamic structure, cloud properties, precipitation and aerosol conditions between the open cells and surrounding overcast cloud field. The data demonstrate that the overlying biomass burning aerosol was mixing down into the boundary layer in the stratocumulus cloud downwind of the POC, with elevated carbon monoxide, black carbon mass loadings and accumulation-mode aerosol concentrations measured beneath the trade-wind inversion. The stratocumulus cloud in this region was moderately polluted and exhibited very little precipitation falling below cloud base. A rapid transition to actively precipitating cumulus clouds and detrained stratiform remnants in the form of thinmore » quiescent veil clouds was observed across the boundary into and deep within the POC. The subcloud layer in the POC was much cleaner than that in the stratocumulus region. The clouds in the POC formed within an ultra-clean layer (accumulation-mode aerosol concentrations of approximately a few cm^-3) in the upper region of the boundary layer, which was likely to have been formed via efficient collision–coalescence and sedimentation processes. Enhanced Aitken-mode aerosol concentrations were also observed intermittently in this ultra-clean layer, suggesting that new particle formation was taking place. Across the boundary layer inversion and immediately above the ultra-clean layer, accumulation-mode aerosol concentrations were ~1000 cm^-3. Importantly, the air mass in the POC showed no evidence of elevated carbon monoxide over and above typical background conditions at this location and time of year. As carbon monoxide is a good tracer for biomass burning aerosol that is not readily removed by cloud processing and precipitation, it demonstrates that the open cellular convection in the POC is not able to entrain large quantities of the free-tropospheric aerosol that was sitting directly on top of the boundary layer inversion. This suggests that the structure of the mesoscale cellular convection may play an important role in regulating the transport of aerosol from the free troposphere down into the marine boundary layer. We then develop a climatology of open cellular cloud conditions in the south-east Atlantic from 19 years of September Moderate Resolution Imaging Spectroradiometer (MODIS) Terra imagery. This shows that the maxima in open cell frequency (> 0.25) occurs far offshore and in a region where subsiding biomass burning aerosol plumes may often come into contact with the underlying boundary layer cloud. If the results from the observational case study applied more broadly, then the apparent low susceptibility of open cells to free-tropospheric intrusions of additional cloud condensation nuclei could have some important consequences for aerosol–cloud interactions in the region.« less

Authors:

Abel, Steven J.  ^[1];

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• Search DOE PAGES for ORCID "0000-0002-1330-4199"

Barrett, Paul A.  ^[1];

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• Search DOE PAGES for ORCID "0000-0002-3763-0909"

Zuidema, Paquita  ^[2];

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• Search DOE PAGES for ORCID "0000-0003-4719-372X"

Zhang, Jianhao  ^[2];

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• Search DOE PAGES for ORCID "0000-0001-6988-2935"

Christensen, Matt  ^[3];

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• Search DOE PAGES for ORCID "0000-0002-4273-6644"

Peers, Fanny  ^[4];

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• Search DOE PAGES for ORCID "0000-0002-2796-8738"

Taylor, Jonathan W.  ^[5];

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• Search DOE PAGES for ORCID "0000-0002-2120-186X"

Crawford, Ian  ^[5];

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• Search DOE PAGES for ORCID "0000-0003-4433-7310"

Bower, Keith N.  ^[5];

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• Search DOE PAGES for ORCID "0000-0002-9802-3264"

Flynn, Michael ^[5]

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+ Show Author Affiliations

1. Met Office, Exeter (United Kingdom)
2. Univ. of Miami, Miami, FL (United States). Rosenstiel School of Marine and Atmospheric Science
3. Univ. of Oxford (United Kingdom)
4. Univ. of Exeter, Exeter (United Kingdom)
5. Univ. of Manchester (United Kingdom)

Publication Date:

Mon Apr 06 00:00:00 EDT 2020

Research Org.:

Univ. of Miami, FL (United States)

Sponsoring Org.:

Office of Science (SC), Biological and Environmental Research (BER). Earth and Environmental Systems Science Division

OSTI Identifier:

1609084

Grant/Contract Number:  

SC0018272

Resource Type:

Accepted Manuscript

Journal Name:

Atmospheric Chemistry and Physics (Online)

Additional Journal Information:

Journal Name: Atmospheric Chemistry and Physics (Online); Journal Volume: 20; Journal Issue: 7; Related Information: S. Abel, P. Barrett, P. Zuidema, J. Zhang, M. Christensen, F. Peers, J. Taylor, I. Crawford, K. Bower, and M. Flynn, 2020: Open cells can decrease the mixing of free-tropospheric biomass burning aerosol into the south-east Atlantic boundary layer. Atmos. Chem. Phys., 20, p. 4059-4084, doi:10.5194/acp-20-4059-2020; Journal ID: ISSN 1680-7324

Publisher:

European Geosciences Union

Country of Publication:

United States

Language:

English

Subject:

54 ENVIRONMENTAL SCIENCES