Below-cloud scavenging of aerosol particles by precipitation in a typical valley city, northwestern China

To fill the blank information for aerosol precipitation-scavenging research in north-west of China, the aerosol particle and raindrop size distributions were measured simultaneously during 1 September 2012 to 31 August 2013 in urban Lanzhou. The scavenging coefficients of thunderstorm and non-thunderstorm rain and snow events were studied and presented on the basis of nine selected precipitation cases including 3 snow and 6 rain events. The variation of scavenging coefficients of snowfall across the size distribution clearly exhibited a trough of lower values for particles of 1000 nm-2000 nm in diameter, while the particles smaller than 500 nm were scavenged efficiently by non-thunderstorm rain, and thunderstorm rain more effectively scavenged the particles in 500-1000 nm. The snow scavenging coefficients varied between 3.11 × 10−7 s−1 and 1.18 × 10−3 s−1 in the 10-10,000 nm size range. The scavenging coefficients of thunderstorm (non-thunderstorm) rain were between 8.25 × 10−7 s−1 (7.48 × 10−6 s−1) and 1.23 × 10−3 s−1 (7.46 × 10−4 s−1). Additionally, the number of particles in 10-50 nm was more sensitive to duration of snow, while snowfall intensity was more responsible for particle number concentrations in 50-100 nm and 100-1000 nm. The longer period of precipitation with lower raindrop velocity can more effectively scavenge the particles in the size range of 10-50 nm.