Representation of Precipitation Phases and a New Parameterization for Below-Cloud Scavenging in Regional Air Quality modelling

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. To study cloud processes and precipitation chemistry, we examined representation of below-cloud aerosol scavenging of in the current GEM-MACH model, including a revised approach in precipitation phase partitioning and 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). 

Overall, the multi-phase partitioning and Wang et al. (2014) scavenging scheme improve GEM-MACH performance as compared with observations. Including multi-phase approach leads to a decrease on SO₄^2- scavenging and impacts the below-cloud scavenging of SO₂ into the aqueous phase. The impact of the new scheme on wet deposition of NO₃^- and NH₄⁺ varies, with both increases and decreases in wet scavenging, and is more important at specific cloud locations. The two aerosol scavenging rates differ during liquid precipitation in the 0.1-1 µm size range mostly at high precipitation intensity. The two aerosol scavenging schemes diverge for aerosols smaller than 1 µm for solid precipitation at lower intensity (R=0.01 mm/h), while at higher precipitation intensities (R=10 mm/h), the two schemes show larger differences for aerosols larger than 1 µm. The changes on the speciated particles (sulphate, nitrate and ammonium) are consistent with the changes in the wet scavenging, leading to higher modelled concentrations of particulate sulphate in the atmosphere.