ISORROPIA II: a computationally efficient thermodynamic equilibrium model for K+–Ca2+–Mg2+–NH4+–Na+–SO42−–NO3−–Cl−–H2O aerosols
This study presents ISORROPIA II, a thermo- dynamic equilibrium model for the K+Ca2+Mg2+NH+ Na+SO24 NO 4 sive evaluation of its performance is conducted against water uptake measurements for laboratory aerosol and predictions of the SCAPE2 thermodynamic module over a wide range of atmospherically relevant conditions. The two models agree well, to within 13% for aerosol water content and total PM mass, 16% for aerosol nitrate and6%for aerosol chloride and ammonium. Largest discrepancies were found under condi- tions of low RH, primarily from differences in the treatment ofwater uptake and solid state composition. In terms of com- putational speed, ISORROPIA II was more than an order of magnitude faster than SCAPE2, with robust and rapid con- vergence under all conditions. The addition of crustal species does not slow down the thermodynamic calculations (com- pared to the older ISORROPIA code) because of optimiza- tions in the activity coefficient calculation algorithm. Based on its computational rigor and performance, ISORROPIA II appears to be a highly attractive alternative for use in large scale air quality and atmospheric transport models.