Rapid declines in UK PM2.5 reflect coordinated reductions in regional nitrate and continental influence

Daniel Bryant , John Halfacre, Massimo Vieno, Eiko Nemitz, Lucy V Brown, Shona E Wilde, Alastair Lewis, Sarah Moller

Fine particulate matter (PM2.5) is a major environmental risk factor for premature mortality globally. Although PM2.5 concentrations have declined across Europe since the 1990s, recent progress has shown signs of slowing, raising concerns over the achievability of increasingly stringent ambient air quality targets. Ambient PM2.5 concentrations across the UK underwent a rapid and sustained decline beginning in 2019-2020, with annual mean concentrations decreasing by approximately 2.4 µg m-3 (23.8 %) and remaining persistently lower through to 2024. Using long-term observations of aerosol chemical composition, we demonstrate that these reductions were primarily due to a reduction in ammonium nitrate, a major component part of ambient PM2.5. Atmospheric transport pathways analysis and source attribution modelling show that the locations and periods with the largest PM2.5 improvements in the UK were those influenced by air masses that had been substantially influenced by continental European emissions. There was an estimated absolute reduction in transboundary PM2.5 of 0.9 [IQR: 0.65-1.15] µg m−3, equivalent to approximately 43% of the national PM2.5 decline. Aerosol thermodynamic analysis indicates that nitrate formation across the UK is predominantly nitric acid limited. Regional reductions in the precursor nitrogen oxide (NOx) emissions effectively suppresses nitrate aerosol formation in the UK’s ammonia-rich atmosphere. Whilst NOx emission reduction policies across Europe have been motivated to reduce ambient NO2 concentrations, they have also delivered substantial co-benefits for PM2.5 that may have been underappreciated. The preferential selection of future non-combustion decarbonisation pathways (e.g. electrification over biofuels) with lower NOx emissions, has substantial potential to further reduce PM2.5 across Europe and create additional health co-benefits across large geographic areas.