The aim of this article is to propose an appropriate framework for investigating the validity of usual parameterizations of below-cloud scavenging by falling raindrops. We recall the underlying models and we study an adimensionalized form of the exact model in a simplified case (monodisperse raindrops and passive tracer) by defining some key parameters. We show that these parameters control the validity of the parameterization that we recover through asymptotic expansion. Some numerical tests for species HNO3 and SO2 illustrate these results.