Aerosol source apportionment modelling using a coupled regional–urban scale system

Recent air quality studies point towards the importance of distinguishing aerosol sources and their chemical composition in relation to the toxicity of particulate matter (PM). While aerosol source apportionment datasets are becoming increasingly available, model evaluations remain scarce. In this study, results from the regional-scale European Monitoring and Evaluation Programme (EMEP) Meteorological Synthesizing Centre – West (MSC-W) and coupled urban EMEP (uEMEP) Gaussian plume downscaling system are evaluated against three European positive-matrix-factorization (PMF) source apportionment datasets. These datasets are based on 28 predominantly urban measurement sites, spanning the years 2013 to 2018. In our analysis, special attention is paid to the impact of urban downscaling to 250 m resolution as well as to the role of primary and secondary organic aerosol. Results show that the model performance varies considerably between PMF factors, which may be explained in part by the ambiguity involved in the matching to modelled species and to uncertainties in the PMF analysis itself. Nevertheless, common model strengths and weaknesses can be identified. For example, model strengths relate to the ability to describe temporal variations of individual PMF factor concentrations while weaknesses relate to the apparent discrepancies in some of the underlying emission distributions. While downscaling generally improves results for road traffic and residential heating, it can also enhance existing biases, with overall model performance for these components remaining poor. Downscaling of residential heating is further found to be sensitive to the treatment of condensable wood burning emissions.