Diagnosing domestic and transboundary sources of fine particulate matter (PM2.5) in UK cities using GEOS-Chem

The UK is set to impose a stricter ambient annual mean fine particulate matter (PM2.5) standard than was first adopted fourteen years ago. This necessitates strengthened knowledge of the magnitude and sources that influence urban PM2.5 in UK cities to ensure compliance and improve public health. Here, we use a regional-scale chemical transport model (GEOS-Chem), validated with national ground-based observations, to quantify the influence of specific sources within and transported to the mid-sized UK city Leicester. Of the sources targeted, we find that agricultural emissions of ammonia (NH3) make the largest contribution (3.7 μg m−3 or 38 % of PM2.5) to annual mean PM2.5 in Leicester. Another important contributor is long-range transport of pollution from continental Europe accounting for 1.8 μg m−3 or 19 % of total annual mean PM2.5. City sources are a much smaller portion (0.2 μg m−3; 2 %). We also apply GEOS-Chem to the much larger cities Birmingham and London to find that agricultural emissions of NH3 have a greater influence than city sources for Birmingham (32 % agriculture, 19 % city) and London (25 % agriculture, 13 % city). The portion from continental Europe is 16 % for Birmingham and 28 % for London. Action plans aimed at national agricultural sources of NH3 and strengthened supranational agreements would be most effective at alleviating PM2.5 in most UK cities.