Below-Cloud Scavenging of Aerosol Particles by Snow at an Urban Site in Finland

MIKHAIL PARAMONOV (1), Aki Virkkula (1), Tiia Grönholm (1), Sabine Göke (1), Lauri Laakso (1,2)

(1) University of Helsinki, Helsinki, Finland (2) North-West University, Republic of South Africa

     Abstract Number: 263
     Last modified: May 3, 2010

Abstract
Below-cloud scavenging of aerosol particles by snow is an important mechanism of wet deposition in mid-latitude, polar and mountainous regions. It is a more complicated process when compared to rain scavenging due to the variety of frozen precipitation types and their physical properties. The study presents an analysis of below-cloud snow scavenging of aerosol particles in the 0.01 to 1 micro-metre size range and 0.1 to 1.6 mm/hr snowfall rate range for an urban environment. The calculated mean scavenging coefficients were in a good agreement with, and not significantly different from, 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 of 0.09 to 0.3 micro-metres in diameter. It was also shown that snow is a better scavenger of aerosol particles than rain per equivalent water content. Snow was found to scavenge aerosol particles more efficiently when it was mixed with other types of frozen precipitation such as snow mixed with rain, ice pellets and snow showers. An increase in scavenging efficiency of snow was also observed when temperatures were slightly above 0°C. Relative humidity was deemed as the most important meteorological parameter affecting the efficiency of snow scavenging, where an 8% increase in RH resulted in one order of magnitude 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. Cases where snow did not produce a significant scavenging effect on aerosol concentrations were shown to be attributed to non-uniform precipitation around the measurement site with the use of the radar data.