A woman pushes a bicycle as haze shrouds a street in the Indonesian island of Sumatra. Beawiharta/Reuters

Greenhouse gases emitted in cities near the equator have the biggest impact on ozone levels worldwide, a new study suggests.

China and the U.S. may be the two greatest contributors to greenhouse gas emissions, but when it comes to global ozone levels, the bigger threat comes from smaller polluters near the equator. That’s according to a new study in Nature Geoscience that suggests that where pollution is emitted on the globe matters more than how much the world pollutes overall. As such, the authors conclude, nations in South, East, and Southeast Asia may be doing more harm to their northern and southern neighbors even if they emit less pollution overall.

The key to this finding lies in the way ozone is created from emissions. The greenhouse gas forms when sunlight reacts with pollutants from cars and factories, including nitrogen oxide and volatile organic compounds. Though ozone in the upper atmosphere protects us from harmful UV rays, the kind that hangs closer to the ground makes up the smog that chokes cities. (Recent studies also suggest that dirty air is increasingly expanding into some rural areas.)

"Since 1980 or so, emissions have grown globally, but they've also shifted toward the equator," says West, an environmental scientist at the University of North Carolina, Chapel Hill, and a coauthor of the study. Prior to this period, air pollution climbed predominantly in the global north as the economies of the United States, the United Kingdom, Russia, and China grew. But beginning in the late 20th century, lower-income countries closer to the equator — particularly those throughout South and Southeast Asia — started growing their populations and economies, especially in urban areas. As these cities industrialized, air pollution began increasing in these places at a faster pace than their neighbors.

Scientist have long known that emissions near the equator have a larger effect on ozone production than those farther north or south, due to factors like warmer temperatures and more intense sunlight. But West says he was surprised to find that the shift was such a major contributor to rising global ozone levels. In fact, his study indicates that location is, by far, the most important factor.

To reach this conclusion, West and his colleagues simulated the total amount of ozone in the atmosphere between 1980 and 2010. They then singled out factors contributing to those levels, including the shift in emissions toward the equator and the overall magnitude of emissions. They found that more than half of the global ozone increase could be attributed to the shift in where emissions were coming from. By comparison, only a quarter of the ozone could be attributed to the overall increase of global emissions.

Today, air pollution is rising fastest in cities in some of the world’s poorest countries. Though West’s research doesn’t include data beyond 2010, he says Africa and Latin America, both with regions that sit in the tropics and have experienced rapid urban growth, may play an increasingly significant role if their emissions levels continue to rise.

“Each world region has its own incentive to take action to solve their pollution problem,” West says. Chinese cities are frequently bathed in smog, with days so bad that schools and offices have had to shut down. And based on the current emission levels in the U.S., cities can face as much as a 70 percent increase in unhealthy ozone levels during the summer. But he adds that pollution-curbing policies also need to focus on regions that “have a global influence that may have not been appreciated before.”

“The ozone that we breathe is a mixture of what was produced from our own emissions and those that come as our background air pollution,” which is driven by emissions around the world, West adds. So this is as much a priority for the countries near the equator as it is for those farther away.

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