Generalized local fractions – a method for the calculation of sensitivities to emissions from multiple sources for chemically active species, illustrated using the EMEP MSC-W model (rv5.5)

Abstract. This paper presents an extension of the original Local Fraction methodology, to allow the tracking of the sensitivity of chemically active air pollutants to emission sources. The generalized Local Fractions are defined as the linear sensitivities of chemical species to source emission changes, as propagated through the full set of non-linear chemical transformations. The method allows to track simultaneously sensitivities from hundreds of sources (typically countries or emission sectors) in a single simulation. The current work describes how the non-linear chemical transformations are taken into account in a rigorous manner, while validating the implementation of the method in the European Monitoring and Evaluation Programme (EMEP) Meteorological Synthesizing Centre – West (MSC-W) chemistry-transport model by examples. While effectively producing the same results as a direct 'brute force' method, where the impact of emission reductions of each source has to be computed in a separate scenario simulation, the generalized Local Fractions are an order of magnitude more computationally efficient when large numbers of scenarios are considered.