A modeling study of halogen chemistryʹs role in marine boundary layer ozone

This study reports the impact of reactive halogen species on diurnal ozone changes in the marine boundary layer (MBL). A photochemical box model has been used to simulate the mixing ratios of ozone, constrained with airborne field observations of Lagrangian flights operated during the ACE 1 campaign, 1995. The model includes bromine, chlorine, and iodine chemistry as well as aqueous-phase chemistry in sea-salt aerosols. Diurnal ozone changes in the MBL depend on photochemical production and loss, entrainment from the buffer layer (BuL), and deposition to the sea surface. Model calculations are compared with ozone observed over the Southern Pacific Ocean near Cape Grim, Tasmania. The model simulates well the observed ozone, reproducing the diurnal cycle in ozone mixing ratios. Based on budget analysis, ozone production in the MBL mainly results from the transport of BuL ozone, while ozone destruction is mainly controlled by a photochemical destruction pathway, a O(1D)+HO2 reaction. When the halogen chemistry is included to the model, a slight decrease of ozone mixing ratios and time shifting in a maximum period of time are revealed. The time shifting mainly results from reactive bromine and chlorine species (e.g., Br, BrO, Cl, and ClO), which show peaks after sunrise. On diurnal average, total loss rate by reactive halogen species is 1.2×10−2 ppbv h−1. Model results confirm that the halogen chemistry can significantly affect ozone levels during early morning after sunrise.