Clouds play an active role in the processing and cycling of chemicals in the atmosphere. Gases and aerosols can enter cloud droplets through absorption/condensation (of soluble gases) and activation and impact scavenging (of aerosol particles). Once inside the cloud droplets these tracers can dissolve, dissociate, and undergo chemical reactions. For example, it is believed that aqueous phase chemistry in cloud is the highest contributor to sulphate aerosol production. Some of the aqueous-phase tracers will be removed from the atmosphere when precipitation forms and reaches the ground. However, the majority of clouds are non-precipitating, and upon cloud dissipation and evaporation, the tracers, physically and chemically altered, will be released to the atmosphere. Updrafts and downdrafts in convective clouds are also efficient ways of redistributing atmospheric tracers in the vertical. It is therefore important to represent these cloud-related physical and chemical processes when modelling the transport and transformation of atmospheric chemical tracers, particularly aerosols.

A new multiple-pollutant (unified) regional air-quality modelling system, AURAMS, with size- and chemical-composition-resolved aerosols is being developed at the Meteorological Service of Canada. In the current version of AURAMS, many of the cloud processes mentioned above are represented. They include nucleation (activation) scavenging of aerosols, aqueous-phase chemistry (both mass transfer and oxidation), and wet deposition. Initial evaluations of AURAMS are being carried out for two summertime multi-day simulations in August 1988 and August 1989 over eastern North-America (during the Eulerian Model Evaluation Field Study period). This presentation will focus on the impact of the various cloud processes on modelled regional aerosols and an evaluation against the available precipitation-chemistry data for the period.