Dry and wet deposition are removal mechanisms of atmospheric aerosol particles. Historically, there are very scarce scientic publications reporting experimentally determined dry
deposition values for the ultra-fine size range. The physics of deposition is studied both using micrometeorological field measurements conducted at SMEAR II site in Hyytiälä, Southern
Finland and by modeling approaches. Dry deposition velocity depends mainly on particle size and magnitude of the atmospheric surface layer turbulence. We present experimentally determined dry deposition velocity (vd) as a function of particle size for the ultra- fine aerosol size range (10 - 150 nm) using relaxed eddy accumulation and eddy-covariance (EC) methods accompanied by particle number size distribution measurements. The highest vd was found
for 10 nm particles and in all size classes vd increased with increasing friction velocity.
By combining two-layer (above and sub-canopy) EC measurements and a new multi-layer canopy deposition model, we addressed how dry deposition is distributed within the forest canopy and between the canopy and the underlying ground. According to the measurements, about 20 - 30 % of particles penetrated the canopy and deposited on the forest floor. The model results showed that turbophoresis, when accounted for at the leaf scale in vertically resolved models, could increase vd for 0.1 - 2 nm particles and explain why the observations
over forests generally do not support the pronounced minimum of deposition velocity for particles of that size. The developed multi-layer model was further used to study the effect of canopy structure (leaf-area shape and density) on vd.
Scavenging coefficients for rain and snow deposition were calculated based on measurements of particle size distribution and precipitation. Parameterizations for both rain and snow wet deposition were derived for example to be applied in air quality and global models. Also a model including both in-cloud and below cloud wet deposition was developed and compared to the field measurements. Both snow and rain scavenging efficiency increased with increasing
precipitation intensity. We also found, that the effectiveness of snow scavenging depends on the crystal or snow flake structure and the air relative humidity. Wet deposition was found to be an order of magnitude more effective "air cleaner" compared to dry deposition.Tutkimuksessa selvitettiin sekä kokeellisten ja teoreettisten menetelmien avulla pienhiukkasten märkä- ja kuivalaskeumaa boreaaliseen metsään
