Size-resolved particle deposition velocities of sub-100 nm diameter particles over a forest

Despite their importance to accurate prediction of the particle ensemble and associated impacts, few empirical studies have attempted to quantify the dependence of deposition velocities (vd) on friction velocity (u*) and particle diameter for sub-100 nm particles. Data from a pilot study conducted over a Beech forest in Denmark are used here in to test an indirect method of assessing these dependencies where particle fluxes derived using eddy covariance are conditionally sampled based on parameters of the particle size distribution. The data indicate strongly bi-directional fluxes, with almost 13 of all 12-h periods exhibiting upwards fluxes particularly during particle formation events. Vertical velocities for particle ensembles with number geometric mean diameter (GMD) of 20–30, 30–40, 40–50, 50–60 and 60–70 nm have an approximately equal magnitude during deposition and emission periods which may imply that surface uptake does not strongly limit deposition rates of these particles. The results also indicate a marked size dependence of vd. During deposition periods the median vd for particle ensembles with a number GMD of 20–30 nm is 4.5 mm s−1 and decreases to 1.5 mm s−1 for particle ensembles with a GMD of 60–70 nm. The median vd for periods when the GMD was approximately 55 nm is 2.7 mm s−1 and the mean is 4.2 mm s−1, which agrees well with the mean vd of 4.3 mm s−1 for 50 nm particles over a Scots Pine forest at Hyytiälä in central Finland. Also, as in the data from Hyytiälä, the vd for GMD of 50–60 nm increases markedly with u* from 1 to 1.8 mm s−1 at u*<0.4 m s−1, to 7.5 mm s−1 at u*>1 m s−1.