A numerical simulation of kinetic constraints upon achievement of the ammonium nitrate dissociation equilibrium in the troposphere

Abstract

Various mechanisms and associated rate expressions for the reversible evaporation of volatile ammonium salts are presented, and experimental evidence for them discussed. The literature concerned with the existence of stable NH 4 + salt monomers in the gas phase in highlighted. Four kinetic mechanisms are presented: monomer formation; adsorption of one precursor species on the particle surface; biomolecular surface reaction; and transport-limited particle growth. No mechanism emerges as the rate-limiting process for all experimental studies. Rate constant ratios for the first two mechanisms are calculated from the literature and shown to differ greatly from the thermodynamic equilibrium constant. A simple box model is developed to determine the importance of kinetic constraints on aerosol growth and evaporation in the tropospheric boundary layer. No attempt is made to model actual tropospheric processes other than the daytime oxidation of NO 2 and the kinetics of aerosol growth itself, so that model output is representative rather than definitive. Substantial departures from equilibrium are seen, giving gas concentration products ranging from −200 to 5000% of the theoretical value, Ke. At low temperatures and high relative humidities (r.h.) a large proportion of the potential condensable NO 3 − remains in the gas phase. The behaviour is general over a range of initial aerosol loadings, temperatures, r.h. and model formulations. Both pure NH 4NO 3 aerosol and a hypothetical equimolar mixture with (NH 4) 2SO 4, conform to the general case.