HR: 0800h
AN: A51E-0828    [Abstracts]
TI: Thermodynamics of Carbonates and Hydrates
AU: * Kelly, J T
EM: jimkelly@ucdavis.edu
AF: University of California Department of Mechanical and Aeronautical Engineering, One Shields Avenue, Davis, CA 95616 United States
AU: Wexler, A S
EM: aswexler@ucdavis.edu
AF: University of California Department of Mechanical and Aeronautical Engineering, One Shields Avenue, Davis, CA 95616 United States
AB: Carbonates are widely present in mineral aerosol and may interact with gas-phase species through reactions of the form: XCO$_{3}$ + 2HY = X(Y)$_{2}$ + CO$_{2}$ + H$_{2}$O (R1), where X = Ca or Mg and Y = NO$_{3}$ or Cl. Laboratory investigations of R1 (X = Ca, Y = NO$_{3}$) indicated that, in idealized N$_{2}$ environments, HNO$_{3}$ is irreversibly taken up by CaCO$_{3}$ to form Ca(NO$_{3}$)$_{2}$. The idealized experimental environments did not contain CO$_{2}$, which is present in the atmosphere and would tend to drive R1 in the reverse direction. In the bulk, the X(Y)$_{2}$ salts are known to exist in stable hydrated forms under conditions relevant to the atmosphere. Although the phase state of the particular X(Y)$_{2}$ product influences the equilibrium disposition of R1 and is important to a number of air quality issues, an equation capturing the temperature (T) dependence of the deliquescence relative humidity (DRH) of hydrated salts has not been reported to date. One goal of this work is to determine the thermodynamically preferred state governed by R1 under conditions relevant to the atmosphere. A related objective is to derive an equation for predicting DRH(T) for hydrated forms of X(Y)$_{2}$. Using principles of thermodynamics, equilibrium concentrations of HNO$_{3}$ and HCl as functions of RH and T were determined for reactions of the form of R1 (X = Ca or Mg, Y = NO$_{3}$ or Cl). These concentrations were compared with ambient measurements of HNO$_{3}$ and HCl to determine the possibility of R1 proceeding in the reverse direction under atmospheric conditions. An equation for DRH(T) for hydrated salts was derived by building on previous work for anhydrous salts. Predictions of DRH(T) for hydrates agree well with available measurements and indicate that DRH(T) is markedly different for the hydrated and anhydrous forms of some salts. For cases of R1 with X(Y)$_{2}$ in the stable hydrated form, the forward direction of the reaction is thermodynamically preferred for atmospheric conditions. For cases of R1 with anhydrous X(Y)$_{2}$, the reverse direction is preferred under some low RH scenarios. Our work suggests that hydrated states of X(Y)$_{2}$ should be considered when modeling heterogeneous reactions of the form of R1 in low RH situations.
DE: 0305 Aerosols and particles (0345, 4801)
SC: Atmospheric Sciences [A]
MN: 2004 AGU Fall Meeting