Modelling the dispersion of particle number concentrations in the West Midlands, UK

Ultrafine particles (UFPs with a diameter less than 100 nm) are tiny and respirable particles. Because of their small sizes, UFPs can penetrate cells and tissue, accumulate in lungs, and cause health effects. Although UFPs are currently not regulated in the same way as mass concentrations for large particles such as PM_2.5 and PM₁₀, the 2021 WHO global air quality guidelines have highlighted the pressing issue of UFPs with a good practice statement. Particle number concentrations (PNC) are the most common measure for UFPs with tiny mass. UFPs often dominate the total ambient PNC in urban environments. There is a strong need to quantify the PNC in the ambient air through measurements and modelling. This study simulates the dispersion of particle number concentrations in the West Midlands (WM), UK using the local scale ADMS-Urban model, which is an advanced quasi-Gaussian plume dispersion modelling system. ADMS-Urban implements a physics-based approach to represent the characteristics of the atmospheric boundary layer. It can represent a variety of source types (such as road and grid emissions) occurring in urban environments and requires a range of input data. Grid sources of PNC for SNAP (Selected Nomenclature for Air Pollution) sectors across the WM were obtained from TNO. Road sources were derived based on the local traffic activity maps (from Transport for West Midlands and Birmingham City Council) and PNC emission factors available in the literature. Meteorological data for Birmingham Airport was used to drive the dispersion. Particle number was used as a passive scalar, with no inclusion of aerosol microphysics. Background data from the rural Chilbolton air quality site was downloaded from Defra UK-Air website. Advanced canyon and urban canopy parameters were derived based on the building data and road network shapefiles using ArcGIS tools. The model was run on the University of Birmingham’s BlueBEAR HPC. Model evaluation was conducted by comparing the modelled (from a receptor run) and measured data at the Birmingham Air Quality Supersite. Overall, the model performed well. Based on the modelling output from a contour run, street scale resolution maps for annual PNC were generated, which could be linked to local population and health data for potential epidemiological studies.