| 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. |