Long-term aerosol particle flux observations. Part II: Particle size statistics and deposition velocities

Abstract

Aerosol particle dry deposition is driven by meteorological conditions as well as by strong functional dependence on particle size. The current study aims to understand seasonal differences in size-integrated deposition velocities over a pine forest. Long period of measurements was used to study dependence of particle deposition velocities on size from 10 nm to 1 μm. Particle fluxes have been obtained by eddy covariance (EC) technique applied together with condensational particle counter. Size differentiation is performed by means of statistical analysis and modelling approach utilising the concurrent particle size spectra measurements. Theoretical particle deposition model including Brownian diffusion, interception and turbophoresis mechanisms was applied to analyse the results. In addition, empirical representation of collection velocity was considered. The empirically determined power factor for deposition dependence on Schmidt number in Brownian diffusion regime was observed to be larger in absolute value than expected from theory. Minimum of deposition velocity occurred at around 200 nm and steep increase at larger sizes was observed. Winter-time observations were dominated by bi-modal size distributions and deposition velocities were qualitatively well described by model while the quantitative difference of 30% remained on seasonal average level. It appeared that larger than 200 nm particles contributed to elevated average deposition values in winter. In autumn, significant deviation between modelled and measured deposition velocities was observed. We also showed that, in case of size-integrated flux measurements, using geometric mean diameter to characterise the particle size distribution can lead to biased interpretation of size dependence of deposition velocities. Further research is needed to understand seasonally different driving variables for deposition velocities preferably involving direct size-resolved flux measurements.