Below-cloud scavenging of aerosol particles by snow at an urban site in Finland

Below-cloud scavenging of aerosol particles by snow is an important mechanism of wet deposition in mid-latitude, polar and mountainous regions. The study presents an analysis of below-cloud snow scavenging of aerosol particles in the 0.01-1 mu m size range and 0.1-1.2 mm h(-1) snowfall rate range for an urban environment. The calculated mean scavenging coefficients were in good agreement with those previously reported for a rural background site in Finland. The variation of scavenging coefficients across the size distribution clearly exhibited a Greenfield gap for particles 0.06-0.3 mu m in diameter. It was shown that snow is a better scavenger of aerosol particles than rain per equivalent water content. The relative humidity (RH) was deemed the most important meteorological parameter affecting the efficiency of snow scavenging, where an increase in the RH clearly resulted in an increase in below-cloud scavenging coefficient values. A new parameterization equation was developed for scavenging coefficients with respect to both particle diameter and relative humidity for snowfall intensities up to 1.2 mm h(-1).