Farms found to be the biggest particulate pollution source for cities

Between 25% and 38% of air pollution that could harm human health in UK cities is the result of agriculture, more than produced by the city itself, while pollution drifting in from continental Europe is a sizable source as well, finds a new study led by UCL researchers.

The researchers studied three cities, Leicester, Birmingham and London, for the paper published in City and Environment Interactions . They modelled the spread of fine particulate matter pollution (which can be inhaled and harm our health) across the country using a computer simulation, then verified their results using data from pollution monitors distributed throughout 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.

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."

Cities only contributed between about 13-24% to their own pollution, mostly from traffic, energy production, industry, and furnaces in commercial and residential locations. Pollution drifting in from continental Europe contributes between about 16% and 28% to UK city pollution, more so in the southeast where they’re closest to the continent. Other identified sources included international shipping and dust from sources like construction and road traffic and from natural desert dust.

The researchers used data about the source of emissions from existing national pollution monitoring networks and used the GEOS-Chem computer model to simulate how pollutants like ammonia, sulphur dioxide and nitrogen oxides interact and migrate. The team also used data from a series of commercial low-cost pollution monitors around Leicester to check their simulations.

Ammonia emissions released from fertilisers and animal waste is the biggest driver of fine particulate matter pollution, particles smaller than 2.5 micrometres in size. Though ammonia itself is a short-lived gas, when it combines with other pollutants such as nitrogen oxides or sulphur dioxide, it can create fine particulate matter and last for days and travel great 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.

Efforts to reduce traffic pollution in urban areas have been adopted around the world, including in cities throughout the UK. London’s ultra-low emission zone is one effort to reduce pollution levels within the city, while both Leicester and Birmingham likewise have their own action plans to reduce air pollution.

Because of these efforts, urban-emitted air pollution has generally been declining over the last decade, while farm emissions largely have not.

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."

Possible plans to reduce agricultural emissions could include disincentivising the use of urea-based fertilisers, using slurry covers, better methods of applying manure to fields, installing scrubbers to livestock housing to clean the air before it enters the outdoors, and using low nitrogen feed for livestock.

Dr Marais said: "The UK government needs to develop policies that encourage the agricultural sector to implement these strategies. Emissions of ammonia from agriculture can be minimised by adopting a range of readily available, cost-effective technologies."

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