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.

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