UK fine particle levels fell sharply after 2019, mainly from less ammonium nitrate

Why it matters

Nitrogen oxide controls were mainly aimed at nitrogen dioxide, but this study suggests they also cut fine particles. Because ammonium nitrate travels far, European cuts can help the UK and UK cuts can help neighbours. The authors argue that choosing low-NOx routes to net zero, such as electrification, could add further health benefits.

What they did

The authors combined UK and European monitoring data on PM2.5, NO2 and aerosol chemistry. They traced air-mass paths with back trajectories and linked them to measured PM2.5 to estimate the share arriving from continental Europe. They used a thermodynamic model on measurements from two rural UK sites to see which ingredient limits nitrate formation. They also ran the EMEP4UK chemical transport model with 30% cuts to UK NOx or ammonia emissions, under 2015 and 2022 conditions. This is a preprint and has not been peer reviewed.

Key findings

• Median annual UK PM2.5 fell from about 10.1 µg m-3 in 2015-2019 to 7.7 µg m-3 in 2020-2024, a reduction of 23.8 %.
• Rural particulate nitrate fell 43.2 % between 2017 and 2024, and nitrate made up 43-53 % of mean total secondary inorganic aerosol mass.
• Across 14 sites, the transboundary PM2.5 increment fell by 0.87 µg m-3, about 43 % of the mean PM2.5 decline. The number of European-influenced air masses did not change.
• At the two rural sites, nitrate formation was limited by nitric acid for 88.3 % of observations at Chilbolton and 86.0 % at Auchencorth Moss.
• The model showed PM2.5 was more sensitive to ammonia cuts under 2015 conditions but to NOx cuts under 2022 conditions across most of the UK.

Limitations

• The thermodynamic analysis uses only two UK rural sites, plus a comparison with one Dutch site, so it cannot show one uniform regime across northwestern Europe.
• The attribution to European NOx cuts rests on trajectory analysis, trends and modelling that fit together. It is not a direct measurement of cause, and the work is a non-peer-reviewed preprint.
• The model tests cut only UK land-based emissions, leaving out offshore sources such as the North Sea, and European composition data were available at few sites (none in Belgium).

Glossary

• PM2.5: Airborne particles smaller than 2.5 micrometres across, linked to health harm.
• Ammonium nitrate: A particle formed when nitric acid gas reacts with ammonia; it can travel long distances.
• Nitric acid limited: A condition where nitrate particles respond mainly to the amount of nitric acid, not to ammonia, because ammonia is in excess.
• SQTBA: A method that links measured pollution at a site to the areas that air masses passed over on the way there.