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 (PM_2.5) standard than was first adopted fourteen years ago. This necessitates strengthened knowledge of the magnitude and sources that influence urban PM_2.5 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 PM_2.5) to annual mean PM_2.5 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 PM_2.5. 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 PM_2.5 in most UK cities.