It’s a recipe for choking smog: Burn tons of fossil fuels. Pump those chemicals into the air, where they react on surfaces of buildings and roads. The result is the creation of photochemical smog-forming chlorine atoms, UC Irvine scientists led by Barbara Finlayson-Pitts (pictured) report in a new study. Under extreme circumstances, this previously unknown chemistry could account for up to 40 parts per billion of ozone – nearly half of California’s legal limit on outdoor air pollution. The reaction is not included in computer models used to predict air pollution levels and the effectiveness of ozone control strategies that can cost billions of dollars. Study results appear the week of July 20 in the Proceedings of the National Academy of Sciences.