Atmospheric Chemistry and Physics Discussions
www.atmos-chem-phys-discuss.net
1680-7367
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6
3
2006
10.5194/acpd-6-3801-2006
http://www.atmos-chem-phys-discuss.net/6/3801/2006/
http://www.atmos-chem-phys-discuss.net/6/3801/2006/acpd-6-3801-2006.html
http://www.atmos-chem-phys-discuss.net/6/3801/2006/acpd-6-3801-2006.pdf
3801
3844
2006-05-12
Scavenging of ultrafine particles by rainfall at a boreal site: observations and model estimations
C. Andronache
T. Grönholm
L. Laakso
V. Phillips
A. Venäläinen
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, 00 560 Helsinki P.O. Box 503, 00 101 Helsinki, Finland
Values of the scavenging coefficient were 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. The estimated range
of values of the scavenging coefficient was [7×10<sup>−6</sup>–4×10<sup>−5</sup>] s<sup>−1</sup>, which is generally
higher than model calculations based only on below-cloud processes (Brownian diffusion,
interception, and typical charge effects). A new 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, is presented. The
effective scavenging coefficients estimated from this new model have values comparable with those obtained
from 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.
Implications for the treatment of scavenging of BL ultrafine particles in numerical models are discussed.