Recent reductions in UK PM2.5: The role of NO2 and transboundary sources

Fine particulate matter (PM_2.5) remains the UK's most harmful air pollutant, contributing to approximately 29,000 premature deaths annually. This study investigates the drivers of recent UK PM_2.5 improvements, with particular focus on the role of European transboundary pollution and atmospheric chemistry controlling secondary inorganic aerosol formation.

Analysis of UK Automatic Urban and Rural Network (AURN) monitoring data from 2016-2024 demonstrates a 25% reduction in annual average PM_2.5 concentrations, with a pronounced step-change across 2019 to 2020 that has been sustained through 2024. In 2024, 98.6% of monitoring sites achieved the UK's 2040 target of 10 μg/m³, compared to just 60.5% in 2019. Wind sector analysis demonstrates that the highest PM_2.5 concentrations and largest reductions are associated with easterly and south-easterly air masses originating from continental Europe, with median concentrations under easterly flow declining by approximately 5 μg/m³ between the 2016-2019 and 2020-2024 periods.

To quantify regional source contributions, we employed Simplified Quantitative Transport Bias Analysis (SQTBA) using HYSPLIT 72-hour back-trajectories generated every 3 hours at 11 urban and rural background sites across the UK. This approach accounts for meteorological transport and dispersion effects on observed concentrations. Results indicate that the highest PM_2.5 concentrations are associated with air masses from central and eastern Europe, particularly Germany, Belgium, Netherlands and Poland. Comparison between 2016-2019 and 2020-2024 periods reveals that the largest reductions in PM_2.5 are associated with these same European source regions, particularly during winter and spring when secondary inorganic aerosol formation is most efficient. European contributions to UK PM_2.5 declined from approximately 2 μg/m³ to 1 μg/m³ between the two periods, representing a 50% reduction in the transboundary component.

Measurements from UK rural monitoring networks (NAMN, AGANet) and high-temporal-resolution supersites demonstrate that ammonium nitrate concentrations have declined by 44-54% since 2016, closely tracking observed PM_2.5 reductions. Thermodynamic modeling using ISORROPIA-II at two contrasting UK sites highlights that ammonium-nitrate formation throughout the study period is “NOx limited”, meaning ammonium-nitrate concentrations are more sensitive to changes in NOx than ammonia. This regime means that NOx emission reductions associated with COVID-19 and vehicle fleet turnover will have had a pronounced effect on ammonium-nitrate in the UK and Europe.

This work suggests that recent UK PM_2.5 improvements result from both domestic emission controls and reductions in transboundary sources from Europe. Due to ammonium-nitrates sensitivity to NOx over ammonia, NOx controls emerge as the primary driver of the recent PM_2.5 reductions observed in the UK. Overall, this highlights the benefits of NOx emissions reductions on human health.