Abundant chlorine-containing oxygenated organic molecules (Cl-OOMs) were formed from the reactions between Cl atoms and aromatics, and Cl-addition was identified as a non-negligible pathway for the formation of Cl-OOMs. Furthermore, many ambient Cl-OOMs potentially derived from Cl atoms and aromatics were measured in suburban Shanghai and most of them have adverse health effects. These findings provide critical insights into the formation mechanisms of Cl-OOMs in polluted settings.