The scavenging of nitrogen compounds by clouds and precipitation:: Part II. The effects of cloud microphysical parameterization on model predictions of nitric acid scavenging by clouds

Model predictions of nitric acid scavenging by an air parcel model and a two-dimensional dynamic cloud model with detailed microphysics are compared with corresponding predictions for parameterized microphysics for the same initial conditions. In general the gas uptake proceeds slower in the model with parameterized microphysics than in the model with detailed microphysics and, the larger the drop size, results in a slower mass transfer. This behaviour can be explained by the inverse drop-radius dependency of the mass transfer coefficient and the time delays were a direct result of the microphysical parameterization. With the air parcel model for the scavenging of nitric acid, time delays in the range of several seconds to several minutes for the model with parameterized microphysics have been found in comparison with the predictions of the model with detailed microphysics. This may not seem significant. However it has an impact on the scavenging behaviour of rapidly developing clouds, as the results from the two-dimensional model show. Here the parameterized solution predicted approximately a 50% lower rainfall rate, scavenging efficiency and wet-deposition efficiency of nitric acid than the detailed model. The results of the present study suggest that parameterized cloud models tend to underestimate the scavenging of gases and their wet deposition.