Atmospheric chemistry - Rethinking ozone production

More than a hundred million people live in cities that fail to meet international standards for air quality. Efforts to improve conditions in these urban areas have usually focused on reducing emissions of reactive hydrocarbons (such as unburned gasoline vapors), nitrogen oxide free radicals (NO and NO_2, together known as NO_x), and primary and secondary sources of particulate matter (such as diesel smoke and sulfur dioxide). Control strategies have changed over time in response to evolving understanding about atmospheric photochemistry and the impact of urban emissions on air quality downwind of cities (1, 2). The results reported by Li et al. on page 1657 of this issue (3) may require another rethinking of these control strategies.