INFLUENCE OF UNDERLYING SURFACE ROUGHNESS TO THE DEPOSITION OF AIRBORNE PARTICLES IN NORTHERN WINTER CONDITIONS

摘要

The surface roughness length is one of a few most important parameters determining the deposition velocity of airborne particles. In this context the mid-latitude flat rural areas could be divided into two substantially different categories: (1) forest and (2) open areas. The roughness difference is most dramatic, if the snow cover is thick enough to cover the grass and other small roughness elements at open sites. The typical roughness length of boreal and sub-boreal forest is 0.5 -1 m, whereas the roughness length of snow surface does not usually exceed 0.001 m (except due to snow blown up by the strong wind). Consequently, the atmospheric resistance to the deposition is much larger over the open areas and therefore lower dry deposition velocities are expected. The early experimental studies (see McMahon and Denison, 1979) support the above considerations despite the large scatter of results. Several model runs (Ruijgrok et al., 1995) show, that the deposition velocity of particles over the forested land can be tens of times higher than over the open one. The difference depends on particle size, being clearly larger for particles sized several micrometers than for the sub-micron ones. Donat and Ruck (1999) provided the wind tunnel simulations of deposition of sub-micron particles onto the ground below the trees and found the deposition fluxes 20 - 60 % (depending on tree crown shape) larger than onto the open surface in similar conditions. Although several field and wind tunnel experiments have been carried out recently, the main attention is paid to the deposition of small particles in the nearly neutral stratification. The present study is focused to the deposition of mineral particles sized about 10 μm as average. Such particles occur typically in the fly ashes of thermal power plants and can contain remarkable amounts of sulphates, alkaline oxides and radionucleides. A natural mdeium-term collector -snow cover-is used to trace the deposition fluxes. The role of dry and wet deposition is discussed.