THE SCAVENGING OF NITRATE BY CLOUDS AND PRECIPITATION

A model with spectral microphysics was developed to describe the scavenging of nitrate aerosol particles and HNO3 gas. This model was incorporated into the dynamic framework of an entraining air parcel model with which we computed the uptake of nitrate by cloud drops whose size distribution changes with time because of condensation, collision-coalescence and break-up. Significant differences were found between the scavenging behavior of nitrate and our former results on the scavenging behavior of sulfate. These reflect the following chemical and microphysical differences between the two systems: (1) nitrate particles occur in a larger size range than sulfate particles. (2) HNO3 has a much greater solubility than SO2 and is taken up irreversibly inside the drops in contrast to SO2. (3) nitric acid in the cloud water is formed directly on uptake of HNO3 gas whereas on uptake of SO2 sulfuric acid is formed only after the reaction with oxidizing agents such as e.g., H2O2 or O-3. (4) nitrate resulting from uptake of HNO3 is confined mainly to small drops, whereas sulfate resulting from uptake of SO2 is most concentrated in the largest, oldest drops, which have had the greatest time for reaction. Sensitivity studies showed that the nitrate concentration of small drops is significantly affected by the mass accommodation coefficient.