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We present a fast method for calculating the effects of ammonia emission reduction scenarios on reduced nitrogen components over the Netherlands. The method combines the EMEP4NL model, which is a full atmospheric chemical transport model (ACTM), with the SHERPA tool, which is an approximate representation of a full ACTM. The SHERPA tool uses a geographically weighted regression method to derive source-receptor relations, based on a limited number of "training" runs of the ACTM (here, EMEP4NL). The training runs consist of a base run of the year of interest plus uniform emission reduction runs for the primary pollutants of interest. Once the source-receptor relations are derived, scenario studies can be performed at a much lower computational cost with SHERPA (i.e. order 104–105 more efficient) as compared to a full run with the EMEP4NL model. Here, we study five specific ammonia emission reduction scenarios, in which a 30% emission reduction is enforced for specific regions in The Netherlands, Germany and Belgium. Our analysis shows that the EMEP4NL model and the SHERPA tool yield similar results in terms of the reduction of the yearly average concentration and deposition of reduced nitrogen components. Because of its computational efficiency and ability to adequately reproduce the results of the EMEP4NL model, the SHERPA tool is relevant for policy making. It can provide predictions of the effects of many different local emission reduction measures in a computationally efficient manner. • We present a method to perform fast ammonia emission reduction studies with a focus on reduced nitrogen components. • An atmospheric chemical transport model is used to train a simplified model. • We validate our results for small, arbitrarily-shaped areas in The Netherlands, Belgium and Germany. • Our method leads to a factor 104 reduction in computation time and adequate accuracy compared to the full model. • It enables policy makers to efficiently study many different local emission reduction scenarios in an efficient manner. [ABSTRACT FROM AUTHOR]