Measurements of Cl2 in coastal urban air were made in Irvine, California from Aug.-Nov., 2005, using negative ion atmospheric pressure chemical ionization with tandem mass spectrometry. Cl2 ranged from <2.5 to 20 ppt with a two month mean of 3.5 ¿ 3.5 ppt (1σ) and an average noon photolysis rate of 44 ppt hr-1 Cl2. Model calculations suggest that Cl2 photochemistry results in an increase of 5--8 ppb in daily maximum O3 levels, relative to a base case scenario without Cl2.