Long-range impacts of biomass burning on PM_2.5: a case study of the UK with a globally nested model

Abstract. Open biomass burning impacts air quality through direct emissions of fine particulate matter (PM_2.5) and its role in secondary PM_2.5 formation. Here the interest is in the long distance and cumulative influences of biomass burning on annual mean concentrations of PM_2.5 in a country far removed from major biomass burning regions: the UK. A novel, globally nested setup of the EMEP4UK atmospheric chemistry transport model is used to isolate contributions to UK PM_2.5 from global biomass burning activity. Long-range influences are found to be considerable, with 0.99 μg m^-3 of UK-averaged PM_2.5 in 2019 being conditional on biomass burning emissions. Of this, 97 % and 73 % are associated with biomass burning outside the UK and outside the model's European domain, respectively – notably from Russia, Asia and boreal North America – which highlights the importance of boundary conditions on regional modelling setups. The simulations suggest some influences of biomass burning have lags of several weeks. The long-range component is enhanced by the role of biomass burning in secondary aerosol formation (58 % of PM_2.5 conditional on biomass burning), of which 55 % is organic; the inorganic component (mainly NH₄NO₃) derives from increased oxidation of local emissions, which may be mitigated through local emissions reductions. The PM_2.5 conditional on biomass burning is highly policy relevant for the UK, constituting (for 2019) 20 % of the current WHO target and 10 % of the contribution from all sources. This relative contribution is likely to increase as anthropogenic PM_2.5 declines and as climate change increases northern-hemispheric extratropical biomass burning.