A computationally efficient hybrid approach for dynamic gas/aerosol transfer in air quality models

Dynamic mass transfer methods have been developed to better describe the interaction of the aerosol population with semi-volatile species such as nitrate, ammonia, and chloride. Unfortunately, these dynamic methods are computationally expensive. Assumptions are often made to reduce the computational cost of explicit dynamic calculations, including instantaneous equilibrium and/or use of bulk-aerosol composition. A novel approach to the modeling of the mass transfer of semi-volatile species is presented. A hybrid method is developed that utilizes equilibrium assumptions for the fine aerosol mode (particles with diameters less than 1 μm) and the dynamic approach for the coarse aerosol mode. A comparison among three methods (equilibrium, dynamic, and hybrid) is made for varying conditions of aerosol acidity, dry and wet particles, and marine and urban environments. Results show that the hybrid method maintains most of the predictive ability of the dynamic approach and is 50 times more computationally efficient for our test scenario. Sensitivity of the hybrid method to the equilibrium cut-off diameter and to the frequency of the evaluation of the equilibrium portion of the aerosol distribution is also discussed.