Isotopic Evidence Suggests a High Contribution of Hypohalous Acids to Sulfate Formation in the Coastal Marine Boundary Layer.

A potential impact of reactive halogens on climate and air quality is through oxidation of sulfur dioxide by hypohalous acids (HOX with X = Cl, Br, or I) to produce sulfate aerosols. This sulfate formation mechanism is typically not included in climate and air quality models, largely due to the limited observational evidence supporting its significance. In this study, we measured the oxygen isotopic composition of sulfate in aerosol particles as well as the concentrations of hypohalous acids during a field campaign in coastal Hong Kong. The oxygen isotopic signatures suggest that hypohalous acids account for more sulfate production than the traditional oxidant hydrogen peroxide (H2O2) in both clean air masses from the South China Sea and polluted air masses from the southeast coast of China. Throughout the campaign, the contribution of hypohalous acids to sulfate production is estimated to range from 30 to 76%. Among the hypohalous acids, the contribution of HOI to sulfate production is largest in clean air masses, whereas the contribution of HOI and HOCl are comparable in polluted air masses. Our results highlight the need for future measurements of HOI in the coastal marine boundary layer.