A mathematical model for simultaneous gas diffusion, aerosol diffusion and evaporation is presented to predict performance of the annular denuder system. Approximating the aerosol size distribution by a lognormal function and using the statistical method of moment, the evolution of the three aerosol moments and the mass balance equations for the two associated gases in the annular denuder system are obtained. The governing equations were cast into dimensionless form and the performance of the model was validated by the existing data available in the literature. The effects of governing dimensionless groups on the model performance were evaluated in the sensitivity analysis. The model was then applied for sampling nitric acid gas using a typical range of atmospheric aerosol and gas properties to derive the performance equation of the annular denuder system. The methodologies presented herein are helpful in providing design and operational criteria of the annular denuder system for a real sampling of atmospheric reactive gases that minimizes measurement artifacts.