INTRODUCTION Emissions of NH 3 in the UK have fallen by 19% since 1990. Further decreases of 10% are forecast by the year 2010. Much larger decreases in emissions of SO 2 and NO x have occurred in the UK (by 88% and 43% respectively in the last 35 years) Further decreases of 55% and 38% are forecast by the year 2020. As a result of these changes, levels of acid deposition and nitrogen deposition have decreased. However the relative contribution of ammonia to nitrogen deposition and to acid

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... ition (resulting from in-soil oxidation of ammonia) is increasing. In addition to efforts to nationally monitor the levels of ammonia concentration (Sutton et al., 2001; Tang et al., 2001) and deposition of reduced nitrogen (Fowler et al., 2005), numerical models have been developed to estimate nationally the concentrations of ammonia and ammonium aerosol and the deposition of reduced nitrogen. Models have the advantage that they are able to give good spatial coverage, where measurement data may not be available, as well as the ability to simulate future emissions scenarios. Below we present the results of the Fine Resolution Atmospheric Multi-pollutant Exchange model (FRAME) developed by