A fresh crop of data

Record hot temperatures in the UK over the summer caused significant changes to the levels of harmful pollutants such as ozone and particulate matter, prompting a number of novel observations by the National Centre for Atmospheric Science (NCAS).

A field of ripened wheat in Drinkstone, Suffolk, during the July 2022 heatwave.

In mid-July, the temperature in the UK exceeded 40°C for the first time since records began. NCAS researchers 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 WHO guidelines.

Surprisingly, they also found that the bulk of the small particulate matter observed was mostly made-up of organic material – particles that are potentially more toxic than non-organic material when inhaled – in addition to black carbon.

James Allan from NCAS said he suspected wildfires and heatwaves to be behind this.
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.

Analysing air pollution data over Monday 18 July and Tuesday 19July, researchers at NCAS offered explanations for how the heat wave affected air quality in urban areas.


Ozone events and their origins

One notable occurrence was that of 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.

Along the South East coast, monitoring stations recorded ozone levels that were nearly double the exposure limits recommended by the WHO, reporting around 200μg/m3.
In comparison, the annual mean concentration of ozone last year was around 70μg in rural areas, and the WHO guideline limit is 100μg.

The highest ozone levels were recorded on 19 July in Sibton, St Osyth and Weybourne.
Ozone is rarely emitted directly by human activity, and is more often formed by reactions in the atmosphere. Heatwaves are known to trigger this because sunlight reacts with precursor pollutants already in the air – such as nitrogen oxides (from vehicle exhausts) and VOCs – to produce ground-layer ozone.

To make matters worse, slow moving air during hot spells can lead to a build-up of precursor pollutants, and faster evaporation can increase the rate of VOC emissions.
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, warned that we need to be aware of the impact of heatwaves in future, which will be more frequent due to the effects of climate change, and this will lead to a higher frequency of dangerous ozone pollution events.

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.


Particle pollution

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,” says Dr James Allan. “It was certainly not typical for a summer day in Manchester.”

Dr Allan operates the Manchester Air Quality Supersite, a specialist monitoring station near the city centre. Here, they look at highly detailed pollution metrics – not only tracking the amount of pollution particles, like small particulate matter – but also measuring the size and chemical make-up of any particles they find.

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,” he said. “There are two possible explanations for this,” he continued.

“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.”

Organic matter is likely to be more toxic than non-organic matter when breathed in.
Over both heatwave days, NCAS researchers based at the University of Manchester also reported very high numbers of larger, coarse pollution particles.

They analysed the large particle pollution samples to reveal a chemical make-up including silicon, iron, calcium and aluminium – ingredients that suggest the particles were dust from the ground.

Although dust particles are usually too large to reach deep into our lungs, they are still known to cause irritation, especially for people with respiratory conditions.

Dust particles are to be expected in hot weather events, according to Dr Allan, who said that dry ground, as well as wind and vehicles, can contribute to dust in the air.

“Given that we can expect more extreme heat events in the future, it is essential that we understand what the implications are for air quality, as well as the issues caused directly by heat,” he said.