Turbulent Transport of Particles to Snow-Covered Surfaces.

The dry deposition process for particles to a partly snow-covered surface has been investigated using the eddy correlation method. To measure particle-number fluxes, PMS ASASP-300 and CSASP-100-HV optical counters were used along with a sonic anemometer as sensors. For particles in two size ranges, 0.15-0.30 mu m and 0.5-1.0 mum, overall average dry deposition velocities for 29 half-hour runs were found to be 0.034 +/- 0.014 cm s^{-1} and 0.021 +/- 0.005 cm s^{-1} , respectively. The average deposition velocities were close to results from prior wind-tunnel and theoretical investigations. These results were also comparable with those reported by other authors over grass. The normalized average surface deposition velocity in nearly neutral and stable conditions for particles with diameters of 0.15 -0.30 μm and 0.5-1.0 mu m appeared to be 0.006 and 0.002, respectively. It is shown that the quasilaminar layer resistance is the dominating component of the total resistance to the snow -covered surface in stable conditions. Analysis of errors associated with estimates of the dry deposition velocities was also performed in this research. The probable error of the dry deposition velocity for each half-hour run was found to be 100% or larger. This demonstrates that it is necessary to average a number of independent runs in order to obtain reliable estimates of particle dry deposition velocity. It was shown that relatively large sampling rates reduced the effects of counting noise on deposition measurements of 0.5-1.0 μm particles. Small correlation coefficients between vertical velocity and the particle concentration were found even after corrections for the effects of counting noise. Normalized spectra of particles showed some similarity to those of temperature in the lower frequency range. Normalized standard deviations of particles were much larger than those of temperature and water vapor.