ISORROPIA-Lite: A Comprehensive Atmospheric Aerosol Thermodynamics Module for Earth System Models
Stylianos Kakavas, Spyros Ν. Pandis, Athanasios Nenes
Abstract
Aerosol simulations especially for Earth System Models require a thermodynamics module with a good compromise between rigor and computational efficiency. We present and evaluate ISORROPIA-lite, an accelerated and simplified version of the widely used ISORROPIA-II v.2.3 aerosol thermodynamics model, expanded to include the effects of water uptake from organics and an updated interface communicating simulation diagnostics and information. ISORROPIA-lite assumes the aerosol is in metastable equilibrium (i.e., salts do not precipitate from supersaturated solutions) and treats the thermodynamics of Na<sup>+</sup>–NH<sub>4</sub><sup>+</sup>–SO<sub>4</sub><sup>2–</sup>–NO<sub>3</sub><sup>–</sup>–Cl<sup>–</sup>–Ca<sup>2+</sup>–K<sup>+</sup>–Mg<sup>2+</sup>–Organics–H<sub>2</sub>O aerosol using binary activity coefficients from precalculated look-up tables. Off-line comparison between ISORROPIA-II and ISORROPIA-lite (without organic water effects) for more than 330,000 atmospherically-relevant states demonstrated that <em>i</em>) ISORROPIA-lite provides virtually identical results with ISORROPIA-II in metastable mode and <em>ii</em>) differences between stable mode ISORROPIA-II and ISORROPIA-lite are less than 25% for the concentrations of the various semivolatile aerosol components and similar to the differences between stable and metastable modes of ISORROPIA-II. Using ISORROPIA-lite reduced computational cost by 35% compared to ISORROPIA-II simulations in stable mode with online calculation of binary activity coefficients. Application of ISORROPIA-lite in the PMCAMx chemical transport model accelerated the 3D simulations by about 10% compared to using ISORROPIA-II in stable mode with changes in the concentrations of the major aerosol components of less than 10%. Simulations considering the effects of the organic aerosol water did not slow down ISORROPIA-lite but increased the concentrations of the inorganic semivolatile components especially at nighttime. Organic water could highly contribute to the total PM<sub>1</sub> water mass and increase the concentrations of fine nitrate and ammonium by as much as 1 μg m<sup>–3</sup> in places where the organic aerosol and RH levels are high.