Ozone in the remote marine boundary layer: A possible role for halogens

On the spring 1995 cruise of the National Oceanic and Atmospheric Administration research vessel Malcolm Baldrige , we measured very large diurnal variations in ozone concentrations in the marine boundary layer. Average diurnal variations of about 32% of the mean were observed over the tropical Indian Ocean. We simulated these observations with the Model of Chemistry in Clouds and Aerosols, a photochemical box model with detailed aerosol chemistry. The model was constrained with photolysis rates, humidity, aerosol concentrations, NO, CO, and O 3 specified by shipboard observations and ozonesondes. Conventional homogeneous chemistry, where ozone photolysis to O( 1 D ) and HO x chemistry dominate ozone destruction, can account for a diurnal variation of only about 12%. On wet sea‐salt aerosols (at humidities above the deliquescence point), absorption of HOBr leads to release of BrCl and Br 2 , which photolyze to produce Br atoms that may provide an additional photochemical ozone sink. After 8 days of simulation, these Br atoms reach a peak concentration of 1.2×10 7 cm −3 at noon and destroy ozone through a catalytic cycle involving BrO and HOBr. Reactive Br lost to HBr can be absorbed into the aerosol phase and reactivated. The model predicts a diurnal variation in O 3 of 22% with aerosol‐derived Br reaction explaining much, but not all, of the observed photochemical loss. The lifetime of ozone under these conditions is short, about 2 days. These results indicate that halogens play an important role in oxidation processes and the ozone budget in parts of the remote marine boundary layer.