From diesel cars to wood-burning stoves, and industrial emissions to Saharan dust, small particles in the air are associated with a host of health problems, including lung disease, heart disease, and increased susceptibility to bacterial infections. Understanding where this pollution comes from is vital if we want to clean our air.

Back in April 2014, satellite pictures showed that a Saharan dust plume was heading towards the UK. At the same time the air quality index – a measure of a variety of pollutants, including small particles, ozone, nitrogen oxides and sulphur dioxide – dropped markedly, and yellowish dust deposits were seen in southern England. Many journalists appear to have assumed that the Saharan dust was the cause of the poor air quality, and the story was reported widely.

Since then Eiko Nemitz from the Centre for Ecology & Hydrology near Edinburgh, UK, and colleagues have analysed particulate measurements from four sites across the UK and used an atmospheric chemistry transport model to ascertain the type of pollution and its source during the spring 2014 event.

Most air-quality monitoring stations don't provide information about the chemical composition of pollutants, but the sites at Auchencorth Moss and Chilbolton do. By looking at the data from these two sites, Nemitz and his colleagues were able to see that ammonium nitrate dominated during this episode.

"Ammonium nitrate is a chemical component of particulate matter that is not emitted to the atmosphere in the form of particles, but is produced chemically in the atmosphere from the precursor gases ammonia and nitric acid," said Nemitz. "More than 80% of the ammonia in the UK derives from farming activity (livestock wastes and fertilizer emissions), whilst the nitric acid is produced through oxidation of nitrogen oxides emitted from traffic and industry."

So where did the ammonium nitrate come from on this occasion? The dominant meteorological feature at the time of the event was an area of high pressure over Europe, with relatively low surface temperatures. "This limited the evaporation of ammonium nitrate into ammonia and nitric acid, enhancing the lifetime of particle nitrate and the potential influence of its long-range transport," write Nemitz and his colleagues in Environmental Research Letters (ERL).

Using their atmospheric chemistry transport model, the scientists were able to show that the most likely source of the ammonium nitrate on this occasion was continental Europe, along with some further emissions in the UK. Meanwhile, the majority of the Saharan dust plume stayed aloft at a higher level in the atmosphere. This fits with what is known about the usual methods of transporting Saharan dust to the UK.

"Saharan dust episodes in the UK require long southerly trajectories preceded by relatively high wind speeds across the Sahara," said Nemitz. "These conditions are relatively rare, which is why Saharan dust episodes in the UK get media coverage. For ammonium nitrate to build up at ground level requires cool temperatures and relatively slow moving air masses, as experienced in the UK during this episode." The latter is more common, and ammonium nitrate pollution episodes occur across the UK at some point during most years.

In this case the media hastiness meant that many people believed the pollution episode was a natural and unavoidable event – the Saharan dust plume. Instead Nemitz and his colleagues feel it is important that everyone is aware of these anthropogenically-derived pollution events, so that people understand and are willing to implement policies such as emissions targets, efficient application of fertilizers and management of livestock wastes, in order to combat this kind of air pollution.

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