SOME PHYSICAL FACTORS INFLUENCING SCAVENGING RATIOS

A better understanding of the causes of scavenging ratio variation is required if one is to use them more precisely for predicting the chemical concentrations in precipitations. For this purpose, a set of 82 rain and associated aerosol samples were obtained during a 1-year sampling program in Paris. Scavenging ratios are calculated for C1, SO4(2-), Na, K, Mg, Ca, Zn, Al, Si and Fe. First, it is shown that scavenging ratios enable us to describe the scavenging processes correctly, even on an urban site: classification of air masses according to back-trajectories indicates similar long range transport impact on both aerosol and precipitation. Scavenging ratios are then shown to vary with the volume of rain: this is interpreted as a dilution effect related to the depletion of the aerosol source. On the other hand, average rain-rate has little effect, if any, on the scavenging parameter. Finally, the role of the cloud mass type (stratiform or convective) is investigated, in relation with differences in nucleation capabilities.