UPDATED: March 3, 2017.
Photo location was corrected in caption: The city depicted is Denver, not Boulder, Colo.
![Ground-level ozone in the form of smog, shown here in Denver, Colorado, is a growing problem in the western United States and elsewhere around the world. (Image credit: National Renewable Energy Laboratory, Colorado State University ) Ground-level ozone in the form of smog, shown here in Denver, Colorado, is a growing problem in the western United States and elsewhere around the world.](/sites/default/files/styles/landscape_width_1275/public/legacy/image/2019/Jun/PHOTO%20-%20boulder%20smog%20-%20030117%20-%20NERL%20-%201120x534%20-%20LANDSCAPE.jpg?itok=qvZn37Gn)
Ground-level ozone in the form of smog, shown here in Denver, Colorado, is a growing problem in the western United States and elsewhere around the world. (Image credit: National Renewable Energy Laboratory, Colorado State University )