A model of ammonia/ammonium conversion and deposition in a hill cap cloud

Abstract

An aqueous-phase cloud-chemistry model is used to investigate the conversion of ammonia gas to ammonium aerosol in an air stream as it passes through a hill cap cloud. The sensitivity of the conversion to the scavenging of soluble gases such as SO2 and HNO3, and to the sulphur chemistry occurring in the cloud, is investigated. The subsequent effect of conversion on the pattern of deposition of reduced nitrogen (NHx) to the surface before, during and after processing by the cap cloud is also considered. The fraction of ammonia present in the inflowing air stream converted to ammonium depends critically on the ratio of ammonia to the concentration of all other species present in solution with which it is capable of forming stable ammonium salts. If sulphate, generated by the oxidation of S(IV) in solution by ozone (and hydrogen peroxide when available), is the only such species, ammonium production is directly linked to sulphate production. When aqueous ammonia is in deficit to acidic species (e.g. HSO), at least 85% of the input ammonia is converted to ammonium. When ammonia is in excess, the fraction of ammonia converted to ammonium depends upon the degree of this excess. It is calculated that conversion of ammonia to ammonium changes patterns of deposition of reduced nitrogen to the surface significantly. Within the cap cloud the efficient deposition of ammonium in cloud droplets (compared with the less efficient deposition of submicron aerosol) dominates over the dry deposition of ammonia gas, most of which is absorbed into the aqueous phase of the cloud. This generates a total reduced-nitrogen flux within the cloud that is typically several times larger than the pre-cloud flux. To the lee of the cap cloud, deposition of reduced nitrogen is also predicted to be much lower than pre-cloud values. This lowered deposition results firstly from the depletion of gas-phase ammonia, and secondly from the inefficient deposition of aerosol particles which individually carry an increased loading of ammonium after passage through the cap cloud (and so collectively constitute a larger proportion of the total loading of reduced nitrogen). This conversion enables the NHx loading of processed air streams to be transported over much longer distances before being deposited. The depleted concentrations of ammonia may enable further emissions of ammonia into the air stream as it passes over recently fertilized terrain, thus replenishing some of the NH, lost over cloud-covered hills.