China spent a decade cleaning its air, then heat turned trees and soil into ozone makers

For more than a decade, China has tried to reduce air pollution from vehicles, factories, and power generation. 

But the extreme heat of 2022 exposed another problem: Very high temperatures can make trees and soil contribute to smog.

What happened?

According to Earth.com, a study by researchers at Fudan University, working with teams at Duke and UC Irvine, linked China's severe 2022 heat wave to a major increase in ozone pollution.

In the Yangtze River Basin, the area the researchers examined, daily ozone levels during the hottest part of the event were 21% higher than the 2020-2021 average.

China's long-running clean-air campaign has generally pushed human-caused pollution downward, but the heat shifted natural emissions in the other direction. As conditions got hotter, plants released much larger amounts of terpenoids, especially isoprene, while dry soil gave off more nitrogen oxides, speeding up the reactions that produce ground-level ozone.

"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.

Published in the journal Environmental Science and Ecotechnology, the study found that isoprene emissions increased by more than 130% in some of the hardest-hit regions, helping explain why ozone rose instead of falling.

Why does it matter?

Ground-level ozone is a major health hazard linked to breathing problems and other serious health risks, especially for children, older adults, and people with asthma. The same heat wave also increased secondary organic aerosol, a form of fine-particle pollution. Some communities were dealing with worsening ozone and particle pollution at the same time.

The results highlight a wider climate problem: Cleaning up pollution from cars, factories, and power plants may still fall short if hotter summers keep boosting pollution from natural sources.

Earth.com reported that aerosol pollution jumped once temperatures rose past about 86 degrees Fahrenheit, while ozone increased more sharply after roughly 95 degrees.

That could lead to more unhealthy air days, reduce the payoff from pollution-control efforts, and make it harder for communities to plan climate-friendly tree-planting projects. Forests help store carbon, but the study suggests that large greening efforts may also need to factor in local air chemistry. 

What are people saying?

The researchers said the scale of the feedback was an unexpected warning sign as global temperatures continue to rise. They also cautioned that current estimates may be conservative, meaning the soil-related effect could be even stronger than existing models suggest.

The team added, "If we don't account for this in our pollution control strategies, we could be chasing our tails as the climate continues to warm." 

Future clean-air strategies may need to consider not just direct human pollution, but also the way extreme heat alters natural atmospheric chemistry.

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