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 (PM2.5) and its role in secondary PM2.5 formation. Here the interest is in the long distance and cumulative influences of biomass burning on annual mean concentrations of PM2.5 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 PM2.5 from global biomass burning activity. Long-range influences are found to be considerable, with 0.99 μg m-3 of UK-averaged PM2.5 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 PM2.5 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 PM2.5 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 PM2.5 declines and as climate change increases northern-hemispheric extratropical biomass burning.