Thermodynamics of carbonates and hydrates related to heterogeneous reactions involving mineral aerosol

Carbonates that are widely present in mineral aerosol may interact with gas‐phase species through reactions of the form XCO3 + 2HY + nH2O ↔ XY2.nH2O + CO2 + H2O (R1), where X = Ca or Mg, Y = NO3 or Cl, and n is the hydration number of the XY2 salt. Laboratory investigations of R1 (X = Ca, Y = NO3) indicate that nitric acid is irreversibly acquired by CaCO3 to form Ca(NO3)2 in idealized environments that do not contain CO2. However, CO2 is present in the atmosphere and could drive R1 in reverse. In the bulk, the XY2 salts are known to exist as stable hydrates, yet an expression for the variation of the deliquescence relative humidity (DRH) with temperature (T) for hydrates has not been reported. One goal of this work is to determine the thermodynamically preferred state governed by R1 for tropospheric conditions. A related objective is to derive an equation for predicting DRH(T) for hydrates. Equilibrium concentrations of HCl and HNO3 as functions of RH and T were determined for R1 and compared with ambient measurements from regions where high dust loadings are common. An equation for DRH(T) for hydrates was derived by building on previous work for anhydrous salts. Predictions of DRH(T) for hydrates agree with measurements and indicate that DRH(T) is markedly different for hydrated and anhydrous forms of certain salts. For R1 with stable‐hydrated XY2, the forward direction is thermodynamically preferred for tropospheric conditions. These reactions scavenge HCl and HNO3 from the gas phase. For R1 with anhydrous XY2, the forward direction is prohibited under some tropospheric conditions. The behavior of the chemical system in this situation is dependent on solid‐phase nucleation kinetics. R1 containing aqueous‐ionized CaCl2 can proceed in reverse under some tropospheric conditions, and thereby release HCl into the gas phase. Our work suggests that hydrated states of XY2 should be considered when modeling R1 in low RH situations.