Long-Range Transport of Continental Aerosols over the Atlantic Ocean and Their Effects on Cloud Structures

Timothy J. Garrett Department of Atmospheric Sciences, University of Washington Seattle, Washington

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Peter V. Hobbs Department of Atmospheric Sciences, University of Washington Seattle, Washington

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Abstract

Airborne measurements of aerosols and cloud microstructures were made in the vicinity of the Azores Islands. Two dichotomous cases are examined: a clean marine air mass and a continentally influenced air mass. The clean marine air mass had relatively low Aitken nucleus (CN) and cloud condensation nucleus (CCN) concentrations, while the continentally influenced air mass had high CN and CCN concentrations. Also, black carbon, sulfate, and SO2 concentrations were significantly higher in the continentally influenced air mass. The continentally influenced air mass had a monomodal aerosol number distribution with a peak at about 0.05-µm radius, whereas the clean marine air mass had a bimodal aerosol number distribution with peaks at about 0.02 and 0.08µm radius.The formation of the aerosol mode at 0.08µm is attributed to aerosols left behind when drizzle drops (which grow efficiently by the collision-coalescence mechanism in clean marine air) evaporate. Stratocumulus clouds in the continentally influenced air mass had a mean cloud droplet number concentration about 160% higher, and a cloud droplet effective radius about 27% lower, than for stratocumulus clouds in the clean marine air mass.

These measurements demonstrate that polluted continental air can affect aerosol concentrations and cloud microstructures up to at least 1500 km from land.

Abstract

Airborne measurements of aerosols and cloud microstructures were made in the vicinity of the Azores Islands. Two dichotomous cases are examined: a clean marine air mass and a continentally influenced air mass. The clean marine air mass had relatively low Aitken nucleus (CN) and cloud condensation nucleus (CCN) concentrations, while the continentally influenced air mass had high CN and CCN concentrations. Also, black carbon, sulfate, and SO2 concentrations were significantly higher in the continentally influenced air mass. The continentally influenced air mass had a monomodal aerosol number distribution with a peak at about 0.05-µm radius, whereas the clean marine air mass had a bimodal aerosol number distribution with peaks at about 0.02 and 0.08µm radius.The formation of the aerosol mode at 0.08µm is attributed to aerosols left behind when drizzle drops (which grow efficiently by the collision-coalescence mechanism in clean marine air) evaporate. Stratocumulus clouds in the continentally influenced air mass had a mean cloud droplet number concentration about 160% higher, and a cloud droplet effective radius about 27% lower, than for stratocumulus clouds in the clean marine air mass.

These measurements demonstrate that polluted continental air can affect aerosol concentrations and cloud microstructures up to at least 1500 km from land.

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