Measurements of Cl<sub>2</sub> in coastal urban air were made in Irvine, California from Aug.-Nov., 2005, using negative ion atmospheric pressure chemical ionization with tandem mass spectrometry. Cl<sub>2</sub> 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<sup>−1</sup> Cl<sub>2</sub>. Model calculations suggest that Cl<sub>2</sub> photochemistry results in an increase of 5–8 ppb in daily maximum O<sub>3</sub> levels, relative to a base case scenario without Cl<sub>2</sub>.