Comparing the ISORROPIA and EQSAM Aerosol Thermodynamic Options in CAMx

The Comprehensive Air quality Model with extensions (CAMx) has been employing ISORROPIA for inorganic aerosol thermodynamic calculations. Recently, the Equilibrium Simplified Aerosol Model (EQSAM) was added to CAMx as an alternative to ISORROPIA for computing concentrations of the inorganic ions: sulfate, nitrate, ammonium, sodium and chloride. By design, EQSAM has more streamlined algorithms than ISORROPIA, which may lead to different model concentrations and faster run times. We apply CAMx using both thermodynamic equilibrium modules for simulations of the continental US with 12 km grid resolution for winter and summer months. Model predictions of inorganic ions by both algorithms are compared and model performance is evaluated against ambient data collected by speciated PM monitoring networks. The overall model run times are also compared. Our purpose is to introduce EQSAM as a newly available option in CAMx and provide information relevant to choosing between the two available aerosol thermodynamics modules in CAMx taking into consideration the scheme’s performance attributes as well as the requirements of each model application.