How the UK’s recent heatwaves affect air pollution

Record hot temperatures in the UK are causing significant changes to levels of harmful air pollution, according to new data from last month’s heatwave.

In mid-July, the temperature in the UK exceeded 40°C for the first time since records began during a record-breaking hot spell.

Researchers from the National Centre for Atmospheric Science identified two significant changes in air pollution triggered by the heatwave event.

Drawing on a national network of air pollution monitoring sites, they found harmful ground-level ozone levels rose sharply, exceeding World Health Organisation guidelines.

Surprisingly, they also found small particulate matter was mostly made-up of organic material – particles that are potentially more toxic than non-organic material when breathed in.

Air pollution levels are closely linked to the weather, and hot spells often arrive hand-in-hand with poor air quality as the sun turns up the heat on a melting pot of airborne chemicals.

The heatwave led to dangerously high levels of ground-based ozone across the country, with particularly high concentrations in the South East, according to analysis led by Professor James Lee from the National Centre for Atmospheric Science and University of York.

Along the South East coast, monitoring stations recorded ozone levels that were nearly double the exposure limits recommended by the World Health Organisation, reporting around 200μg ozone per cubic metre.

Unlike many pollutants, ozone is rarely emitted directly by human activities. Instead, it’s formed by a reaction that takes place in the atmosphere.

Heatwaves are known to trigger ozone pollution events because sunlight reacts with precursor pollutants already in the air – such as nitrogen oxides from vehicle exhausts and volatile organic compounds – to produce ground-layer ozone.

Unlike other types of air pollution, ozone concentrations were typically higher in rural areas compared to urban areas.

‘Although both nitrogen dioxide and volatile organic compounds are more common in urban areas, they take a long time to react to form ozone. By the time the reaction has taken place, the air has moved to a different area,’ explains Professor Lee.

At ground level, ozone can cause shortness of breath, asthma attacks and increase the risk of respiratory infection and disease.

Dr Grant Forster, who operates the Weybourne Atmospheric Observatory on the north Norfolk coast, warns that we need to be aware of the impact of heatwaves in future.

‘Due to the effects of climate change, we can expect to see heat waves more often in the future, leading to a higher frequency of dangerous ozone pollution events across the UK,’ said Dr Forster.

However, researchers caution that it will not be straightforward to predict how ozone levels will respond to other changes – such as emissions of nitrogen oxides and volatile organic compounds.

Researchers at the University of Manchester took a more detailed look at small particle pollution in Manchester during July’s heatwave.

‘We were surprised by the different levels of pollution we saw during the heatwave,’ said Dr James Allan, National Centre for Atmospheric Science and University of Manchester. ‘It was certainly not typical for a summer day in Manchester.’

Although the overall concentrations of small particulate matter were not significantly different to a normal day in Manchester, the type of small particles detected were.

‘The small pollution particles we saw were almost all organic material and black carbon, which is surprising,’ explained Dr Allan.

‘There are two possible explanations for this,’ Dr Allan continues, ‘Firstly, it’s possible that continental wildfires were contributing burnt material, but secondly, the sun’s heat is likely to have been triggering chemical reactions in the atmosphere, leading to new organic particle formation.’

Photo by Nick Fewings