Specification of the mutual deliquescence region in ISORROPIA, a thermodynamic equilibrium model for inorganic atmospheric aerosols

The transition of a unicomponent aerosol from the solid to the liquid phase as relative humidity (RH) increases occurs at a discrete RH known as the deliquescence relative humidity (DRH). When two or more species are present, however, a liquid phase may appear at a lower RH known as the mutual deliquescence relative humidity (MDRH). In the region between the MDRH and the highest DRH of the individual components, known as the mutual deliquescence region (MDR), the system exists as either a completely deliquesced solution, or as a liquid solution in equilibrium with one or more solid phases. Development of the appropriate thermodynamic solution is a computationally intense problem, yet necessary when accurate predictions of aerosol composition are required.; The computationally efficient thermodynamic equilibrium model ISORROPIA is developed further to calculate the aerosol composition in a mutual deliquescence region (MDR). As ISORROPIA was designed to be as computationally efficient as possible, the specification of the system in the MDR was originally greatly simplified, and assumed to equal the weighted average of the purely solid and completely deliquesced thermodynamic solutions. The current project adds a new alternative to the model for calculating the composition in the MDR. In this alternative, the Gibbs free energy of the possible states is calculated and the case with the lowest Gibbs free energy is selected. Calculating the composition of each of the potential cases increases the computational time slightly for solutions that lie within the MDR, but allows for a thermodynamically consistent solution. Work is presented for aerosol sulfate-ammonium systems.