The Hull Acid Rain Model (HARM) is extensively used in the UK to help in formulating strategies for reducing the emissions of acidifying pollutants. Here we describe the development of a 10 km version of HARM, which incorporates both software development and a new representation of the dry deposition of reduced N. Output from the new model for the UK is compared with S and N deposition data available from the UK's monitoring networks for 1992–1994 using linear regression. The model is able to reproduce the overall patterns of deposition for S, oxidised and reduced N. Although improved from earlier model versions, HARM still appears to underestimate dry deposition of reduced N. We describe the implications of the changes to HARM for assessing current policy commitments to reduce emissions of acidifying pollutants through changes in critical loads (CL) exceedance.