Relative humidity and pH dependence of the vapor pressure of ammonium nitrate-nitric acid solutions at 25° C

Quantitative expressions for the ammonia-nitric acid equilibrium product and the partial pressures of ammonia and nitric acid over non-ideal nitric acid-ammonium nitrate solutions are developed. The relative humidity and pH dependence of the equilibrium product and the partial pressures are obtained from free energy thermodynamic data. The thermodynamic predictions show the ammonia-nitric acid equilibrium product is inversely related to relative humidity. The importance of correcting for non-ideality is demonstrated. The assumption of ideality for ammonium nitrate in solution incurs an error of an order of magnitude in the ammonia-nitric acid equilibrium product prediction at the point of deliquescence and of 20% in the relative humidity of deliquescence. The trends indicated by the analysis are consistent with the filter study results of Forrest, Tanner, Spandau. D'Ottavio and Newman ( 1980, Atmospheric Environment 14, 137–144) and Appel, Wall, Tokiwa and Haik (1980, Atmospheric Environment 14, 549–554).