Numerical studies of the properties of low-level, warm stratiform clouds and precipitation and their interaction with aerosol particles and gaseous species

A one-dimensional model with size-resolved cloud microphysics has been fully coupled with size-resolved aqueous-phase chemistry. The model, driven by prescribed dynamics, has been used to study the properties of low-level, warm stratiform clouds and precipitation and their interaction with aerosol and gas pollutants.; The model shows how precipitation is inhibited when a precipitating marine cloud moves over polluted regions with higher concentrations of cloud condensation nuclei (CCN). The model also shows that precipitation can be initiated in a non-precipitating cloud which has a high concentration of small droplets by seeding giant CCN (>1 mum in radius) into the cloud. Giant CCN can either enhance or inhibit precipitation depending on their concentration. The role of giant CCN on precipitation formation is only important when both CCN and giant CCN (especially those larger than 5 mum in radius) concentrations are high.; In the model simulations activation processes remove most of the aerosol mass within the cloud layer despite very low supersaturation. Impaction scavenging inside the cloud layer removes little aerosol mass, but can remove as much as 50% of the number of aerosols during a period of a few hours. Weak precipitation having a rate less than 0.1 mm hr-1 can remove 50-80% of below-cloud aerosols, both in number and mass, in 4 hours. Scavenging coefficients for bulk aerosol mass have a strong dependence on precipitation intensity, but not for bulk aerosol number. Precipitation which consists of a larger number of small droplets (for a given total mass) can remove aerosols more effectively due to the larger total droplet surface area.; Aqueous-phase chemistry can effectively transfer mass from the gas phase to the particulate phase and thus can modify CCN and cloud properties. The removal of SO2, NH3 and H2O2 by cloud droplets and by precipitation strongly depends on the gaseous relative concentrations. Scavenging coefficients have a high correlation with the total droplet surface area, regardless of the size of droplets. Scavenging coefficients also have positive correlations with precipitation rate, especially when the precipitation has a mean droplet radius of 20 mum or larger. Using the Henry's law equilibrium approach for below-cloud scavenging can sometimes cause large errors in modeling soluble gaseous species concentrations.