With special regard to the class of substances in precipitation that potentially originate from both gaseous and particulate precursors, the process of precipitation scavenging is discussed. A model relating daily average ground level scavenging ratios (W) to nucleation scavenging and in-cloud chemical transformation is introduced and is used as guidance in the regression analysis of 3 years of SO4= and NO3- scavenging-ratio observations made at six locations in eastern Canada. It was found that W of SO4=-, NO3=-, and SO4=-bearing particles is inversely proportional to the one-third power of the precipitation amount in the event. The best regression model explained 41% of the variance in log W for SO4=. It included the effects of location, precipitation amount, precipitation type, and in-cloud SO2 oxidation. The last effect accounted for 50% of the variance explained. The analysis predicts that, on average, in-cloud SO2 oxidation accounts for 42--79% of the SO4= observed in rain and with the exception of one site, less than 20% of the SO4= observed in snow. These results are consistent with a mechanism of SO2 oxidation involving photochemically produced H2O2. A similar analysis for NO3- supports the hypothesis that throughout the year much of the NO3- in precipitation originates from in-cloud NO2 oxidation. It suggests that, depending on location, oxidation of 0.5--1.2 ppbv of NO2 is sufficient to explain observations. One possible mechanism of oxidation is the reaction with O3 to form NO3 and hence soluble N2O5. These results, combined with observations in the eastern United States, indicate that in eastern North America SO2 oxidation within a rain storm becomes an increasingly important source of rain SO4= as latitude increases. Scavenging ratios are useful tools for investigating in-cloud conversion and for parameterizing precipitation removal. However, they are limited to describing the average characteristic of an ensemble of storms. They cannot be used with confidence to analyze individual events.