Excerpt from the linked summary^1 about research published in City and Environment Interactions:^2

>First author Dr Jamie Kelly, who conducted the research while based at UCL Geography before moving to the Centre for Research on Energy and Clean Air, said: “We were surprised to find how pervasive the contribution of agricultural emissions of ammonia to particulate pollution really is.

>“Particulate pollution across the UK is dominated by aerosols formed from rural agricultural emissions of ammonia. This influence extended from rural areas to mid-sized cities like Leicester, large cities like Birmingham and, for the UK, anomalously large cities like London.

>“This is because ammonia and aerosol particles can stay suspended in the atmosphere for days to a few weeks and so be transported long distances.”

>This kind of fine particulate pollution can have serious health effects, with estimates saying it may contribute to between 29,000 and 99,000 additional premature deaths each year in the UK.

> 

>The team ran multiple simulations with different pollution sources turned on and off, to see how each source contributed to the spread of particulate pollution.

>They found that UK agriculture contributed 38% of the particulate pollution in Leicester and 32% in Birmingham. Even in large cities like London, agriculture contributed 25% of the city’s pollution.

>Cities only contributed between about 13-24% to their own pollution, mostly from traffic, energy production, industry, and furnaces in commercial and residential locations.

>Senior author Dr Eloise Marais (UCL Geography) said: “Our work has identified that addressing urban air pollution doesn’t only require local solutions like ultra-low emission or clean air zones, but also national-scale measures that reduce ammonia emissions from rural agriculture.

>“Such actions have potentially large health benefits, as the fine particulate matter pollution formed from ammonia is a leading global health risk.”

^1 University College London, 31 Mar. 2023, https://www.ucl.ac.uk/news/2023/mar/farms-found-be-biggest-particulate-pollution-source-citie

^2 Jamie M. Kelly et al. Diagnosing domestic and transboundary sources of fine particulate matter (PM2.5) in UK cities using GEOS-Chem. City and Environment Interactions 18, 100100 (2023). https://doi.org/10.1016/j.cacint.2023.100100