Cloud processing of gases and aerosols in a regional air quality model (AURAMS)

Abstract

The representation of cloud chemistry and scavenging processes in a new multiple-pollutant (unified) regional air-quality modelling system, AURAMS, is described. Aqueous-phase chemistry and scavenging processes are coupled explicitly with microphysical fields from the meteorological driver model and the size- and chemical-composition-resolved aerosols in AURAMS. The impact of aqueous-phase oxidation on regional aerosols (primarily sulphate), both in terms of mass and size distribution, is examined based on model simulations of a 1-week period over eastern North America. It is shown that aqueous-phase oxidation contributes about 30% to 40% of the total atmospheric sulphate production in this case. Cloud chemistry is also shown to modify the aerosol size distribution, which in turn can either enhance or reduce aerosol scattering efficiency in different geographic regions depending on where on the aerosol size spectrum the mass is added. The study also indicates that precipitation evaporation can be an important process in terms of tracer redistribution in the vertical. Whether and how to treat tracer release from precipitation evaporation can have a significant impact on model predictions of near-surface ambient tracer concentrations and wet deposition fluxes. Comparison between observations and AURAMS predictions shows that modelling cloud processing of gas and aerosols depends critically on the meteorological driver model's ability to predict cloud microphysics fields. In this case, the model underpredicted precipitation amount for the study period, which contributed to the underestimation of wet deposition and in turn may also have impacted the modelled ambient tracer concentrations.