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 difference 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 H2O2 or O3. Sensitivity studies showed that the mass accommodation effect has a significant effect on the nitrate concentration of small drops for mass accommodation coefficients ranging between 0.1 and 1.
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