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.)
