Field measurements of the dissociation of ammonium nitrate and ammonium chloride aerosols

Abstract

Atmospheric concentrations of HNO 3, HCl and NH 3 have been measured at Essex, UK and Cabauw, The Netherlands, using both filter pack (Essex) and denuder (Cabauw) sampling methods and integration times of 3, 12 and 24 h. Comparisons of the concentration products [HNO 3] [NH 3] and [HCl] [NH 3] with the predictions of equilibrium chemical thermodynamics for a non-interactive external mixture of NH 4Cl and NH 4NO 3 show generally good agreement at temperatures above ca 5°C, and indicate that both aerosol salts are normally present at these sites. The ratio [HCl] [HNO 3] agrees well with theory at lower temperatures ( < ~ 5°C), but shows a significant excess of HNO 3 at higher temperatures. Reasons for deviations of measurements from theoretical predictions are discussed, although no firm conclusions are reached.