References

ACP

Atmospheric Chemistry and Physics

ACP

1680-7324

Copernicus GmbH

Göttingen, Germany

10.5194/acp-6-4739-2006

Scavenging of ultrafine particles by rainfall at a boreal site: observations and model estimations

Andronache

¹ Grönholm

² Laakso

² Phillips

³ Venäläinen

⁴

Boston College, Chestnut Hill, Massachusetts, 02467 USA

Department of Physical Sciences, University of Helsinki, P.O. Box 64, 00 014 Helsinki, Finland

Princeton University, Atmospheric and Oceanic Sciences Program, Princeton, New Jersey 08540, USA

Climate and Global Change Finnish Meteorological Institute Erik Palménin aukio 1, P.O. Box 503, 00 101 Helsinki, Finland

23 10 2006

6 12 4739 4754

This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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Values of the scavenging coefficient determined from observations of ultrafine particles (with diameters in the range 10–510 nm) during rain events at a boreal forest site in Southern Finland between 1996 and 2001 were reported by Laakso et al. (2003a). The estimated range of the median values of the scavenging coefficient was [7×10−6–4×10−5] s−1, which is generally higher than model calculations based only on below-cloud processes (Brownian diffusion, interception, and typical phoretic and charge effects). In the present study, in order to interpret these observed data on scavenging coefficients from Laakso et al. (2003a), we use a model that includes below-cloud scavenging processes, mixing of ultrafine particles from the boundary layer (BL) into cloud, followed by cloud condensation nuclei activation and in-cloud removal by rainfall. The range of effective scavenging coefficient predicted by the new model, corresponding to wide ranges of values of its input parameters, are compared with observations. Results show that ultrafine particle removal by rain depends on aerosol size, rainfall intensity, mixing processes between BL and cloud elements, in-cloud scavenged fraction, in-cloud collection efficiency, and in-cloud coagulation with cloud droplets. The scavenging coefficients predicted by the new model are found to be significantly sensitive to the choice of representation of: (1) mixing processes; (2) raindrop size distribution; (3) phoretic effects in aerosol-raindrop collisions; and (4) cloud droplet activation. Implications for future studies of BL ultrafine particles scavenging are discussed.

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