A chemical reaction long assumed to be unimportant in urban air quality may be a significant source of ozone, the major component of smog.
Hydroxyl (OH) radicals, among the most reactive natural chemicals in the atmosphere, help cleanse the air of some noxious pollutants. In many cases, and especially in urban environments, ozone results from that cleansing, says Amitabha Sinha, a physical chemist at the University of California, San Diego.
Previous studies suggested that the majority of the atmosphere's hydroxyl radicals are produced when ultraviolet radiation cleaves a molecule of ozone to produce a single oxygen atom, which in turn reacts with water vapor in the air. Now, lab tests by Sinha and colleagues Shuping Li and Jamie Matthews hint that reactions involving nitrogen dioxide and light may in some cases produce substantial quantities of hydroxyl radicals, and therefore more ozone.