Think heatwaves are bad? Scientists say they're now making the air more toxic too

Hotter summers caused by climate change could make the air more polluted, despite years of efforts to clean it, according to a new study. Researchers found that heatwaves can trigger natural emissions from trees and soil that sharply increase harmful ozone levels.

The study suggests that rising temperatures could make it harder for countries to improve air quality, even as pollution from vehicles, factories and power plants continues to fall.

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Researchers from Fudan University, working with scientists from Duke University and the University of California, Irvine (UCI), studied what happened during China's record-breaking heatwave in the summer of 2022.

Heatwave caused ozone levels to rise

China has spent more than a decade reducing pollution through stricter rules on industries, power plants and vehicles. As a result, fine particle pollution has fallen steadily.

However, during the summer of 2022, ozone pollution rose sharply instead of continuing its downward trend.

The researchers found that the country's record heatwave played a key role.

During the summer, average temperatures increased from around 73°F to 77°F, while one location recorded an extreme 115°F.

The Yangtze River Basin experienced some of the worst conditions, with temperatures reaching around 11°F above normal during the hottest days.

At the same time, severe drought dried out the soil, with some areas losing more than half of their soil moisture.

Across the basin, daily ozone levels increased by 21% compared with the average recorded in 2020 and 2021.

By comparison, between 2013 and 2019, ozone levels had increased by only about 1.9 parts per billion (ppb) each year.

Trees and soil worked together

The researchers found that extreme heat caused trees and shrubs to release much larger amounts of terpenoids, a group of highly reactive carbon compounds.

One of these compounds, isoprene, increased by more than 130% in the worst-hit areas.

At the same time, hot and dry soil released more nitrogen oxides.

Scientists had studied these two natural processes separately for years, but this study shows how they work together during heatwaves to create much more ozone pollution.

Satellite observations also showed that formaldehyde levels across the Yangtze River Basin increased by around 31%, closely matching the researchers' model.

How more ozone formed

The researchers explained that as terpenoids break down, they produce highly reactive particles called peroxy radicals.

These radicals react with nitric oxide released from the soil and convert it into nitrogen dioxide. Sunlight then breaks the nitrogen dioxide apart, creating fresh ozone.

This process allows ozone to build up much faster than scientists had previously understood.

The researchers also found that although China has reduced nitrogen oxide emissions from vehicles and industries under its Clean Air Action Plan, nitrogen dioxide levels still increased during the 2022 heatwave.

The study suggests that the extra nitrogen dioxide came mainly from the soil rather than from human activities.

Fine particle pollution also increased

The same natural process also increased secondary organic aerosols, tiny particles that form from terpenoids.

In the worst-affected areas, these particles increased by up to four micrograms per cubic metre.

This meant both ozone pollution and fine particle pollution became worse at the same time.

Scientists identified heat thresholds
The study also found that pollution increased rapidly after certain temperatures were crossed.

Fine particle pollution rose by around 50% once temperatures exceeded 86°F.

Ozone pollution increased much more sharply after temperatures crossed 95°F.

Although fine particle pollution continued to decline across much of China during 2022, some cities in Sichuan and Chongqing recorded increases in both ozone and particle pollution.

Researchers warn pollution strategies may need to change

The researchers said they were surprised by the strength of the natural chemical reactions during heatwaves.

“The vegetation and the soil are essentially conspiring during heatwaves – the trees pump out these reactive compounds that supercharge the atmosphere’s oxidation capacity, which then grabs the nitrogen coming out of the soil and turns it into ozone much faster than we thought possible,” the researchers noted.

“If we don’t account for this in our pollution control strategies, we could be chasing our tails as the climate continues to warm.”

Tree planting may also need careful planning

The study says that while planting more trees helps remove carbon dioxide from the atmosphere, it can also increase terpenoid emissions during extreme heat.

The researchers say future forest planning should consider atmospheric chemistry alongside climate goals so that efforts to tackle global warming do not unintentionally worsen air pollution.

Problem could become bigger as the planet warms
The team also modelled a future climate in which temperatures rise by around 9°F, in line with a high-emissions scenario.

Under those conditions, the impact of this natural feedback nearly doubled.

The researchers say future air pollution policies should take into account emissions from both human activities and natural sources, as climate change is expected to make heatwaves more frequent and intense.