Modelling below-cloud scavenging of size resolved particles in GEM-MACHv3.1

. Below-cloud scavenging is the process of aerosol removal from the atmosphere between cloud-base and the ground by precipitation (e.g. rain or snow), and affects aerosol number/mass concentrations, lifetime and distributions. An accurate representation of precipitation phases is important in treating below-cloud scavenging as the efficiency of aerosol scavenging differs significantly between liquid and solid precipitation. The impact of 10 different representations of below-cloud scavenging on existing model biases (Makar et al, 2018), was examined through implementing a new aerosol below-cloud scavenging scheme (from Wang et al., 2014) and comparing with the GEM-MACH’s existing scavenging scheme, based on Slinn (1984). Further, the current GEM-MACH employs a single-phase precipitation for below-cloud scavenging: total precipitation is treated as either liquid or solid depending on a fixed environment temperature threshold. Here, we consider co-existing liquid and solid 15 precipitation phases as they are predicted by the GEM microphysics. GEM-MACH simulations are compared with - observed precipitation samples, with a focus on the particulate base cation NH 4+ , acidic anions NO 3- , SO 4= , HSO 3- in precipitation, and ambient particulate sulfate, ammonium and nitrate. Overall, the precipitation-phase partitioning and Wang et al. (2014) scavenging scheme improve GEM-MACH performance relative to earlier approaches. Including multi-phase approach leads to a decrease in SO 42- scavenging 20 and impacts the below-cloud scavenging of SO 2 into the aqueous phase over the domain. Sulphate biases improved from +46% to -5% relative to Alberta Precipitation Quality Monitoring Program wet sulphate observations. At Canadian Air and Precipitation Monitoring Network stations the biases became more negative, from -10% to -30