1Boston College, Chestnut Hill, Massachusetts, 02467, USA
2Department of Physical Sciences, University of Helsinki, P.O. Box 64, 00 014 Helsinki, Finland
3Princeton University, Atmospheric and Oceanic Sciences Program, Princeton, New Jersey, 08540, USA
4Climate and Global Change Finnish Meteorological Institute Erik Palménin aukio 1, 00 560 Helsinki P.O. Box 503, 00 101 Helsinki, Finland
Abstract. 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 [7×10−6–4×10−5] s−1, 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.