Reducing aerosol pollution could lead to an unexpected rise in ozone

In 2013, China unveiled an ambitious plan to improve its air quality. The Chinese government implemented strict policies aimed at reducing fine particulate matter (PM 2.5), a type of pollution responsible for about 1 million deaths in the country each year. Thus far, the nation’s efforts have been largely successful; by 2020, PM 2.5 levels dropped by nearly 40%. But new research suggests that by solving one pollution problem, China may have inadvertently caused another (Nat. Geosci. 2022, DOI: 10.1038/s41561-022-00972-9).

China’s aggressive tactics for reducing PM 2.5 may have contributed to an increase in ozone, which can lead to a number of environmental and human health effects. According to the new study, ozone levels—which have risen in eastern China since 2013 (Environ. Sci. Technol. Lett. 2020, DOI: 10.1021/acs.estlett.0c00171)—are heavily dictated by the amount of particulates in the air.

But the effect isn’t unique to these regions. Using a model that simulates the chemistry and transport of pollutants, the researchers estimated that by 2014, 21% of the Northern Hemisphere’s population was living under similar atmospheric conditions. “Historically, we’ve thought about particulate matter pollution and ozone pollution as being separate problems,” says the study’s lead author Mathew Evans, an atmospheric chemist at the University of York. “What this shows is that, actually, they’re often inextricably linked.”

In the 1950s, scientists discovered that nitrogen oxides (NO_x) and volatile organic compounds (VOCs) can react to produce ozone. Depending on the ratio of NO_x to VOCs in the air, policymakers in many countries target either one or both of these compounds to keep local ozone concentrations under control. Particulates, meanwhile, are usually regulated separately, often by focusing on their emissions at the source, such as from power plants and automobiles.

However, scientists have recently proposed that PM 2.5 can actually inhibit ozone by absorbing peroxy radicals, chemical intermediates formed during ozone’s production (Proc. Natl. Acad. Sci. U.S.A. 2018, DOI: 10.1073/pnas.1812168116). When Evans and colleagues included this chemistry in their model, they found that reducing PM 2.5 can cause ozone to increase by up to 30%, especially over heavily populated regions in China and India.

These findings “provide further evidence that aerosols may be the dominant factor in determining photochemical [ozone] production in regions with high levels of PM 2.5,” says Qi Ying, an environmental engineer at Texas A&M University, who was not involved in the study.

Ying says there are still uncertainties in the model, though. For example, it’s unclear how different aerosol compositions or environmental conditions affect the uptake of peroxy radicals by particulates. Evans notes that this is the next stage of research, but regardless, the takeaway is clear: “You have to think about air quality in this more holistic sort of way,” he says. “The tales we tell ourselves in textbooks sometimes have to be changed.”