Modeling urban coastal chemistry

Experiments have demonstrated that a significant yield of chlorine (Cl ₂) gas is produced when mixtures of ozone and sodium chloride particles above their deliquescence point are irradiated at 254 nm. Computational simulations of the system illustrate that previously known physical and chemical processes fail to reproduce the observed Cl₂ formation. A methodological analysis of the system supports an overall mechanism initiated by the formation of a relatively stable complex of the hydroxyl radical and chloride ion at the gas-liquid interface as the source of chlorine generation. A rate expression and kinetic parameters for the overall reaction are determined for subsequent application to coastal urban photochemical model simulations. Sensitivity studies underscore the importance of accurately modeling chlorine decomposition processes in alkaline solution. Recommended aqueous-phase rate constants are drawn from a literature evaluation.; A photochemical modeling study is performed to determine whether urban photochemical models simulating sea-salt particle chemistry can predict observed chlorine levels and how such chlorine levels affect ozone formation. An urban airshed model is augmented with current sea-spray generation functions, a detailed gas-phase chlorine chemistry mechanism and several heterogeneous/multiphase chemical reactions considered key processes leading to reactive chlorine formation. Modeling results adequately reproduce regional sea-salt particle concentrations. Chlorine levels in the model are predicted an order of magnitude below observed values, albeit 30 times better than previous studies. The results suggest that inclusion of sea-salt derived chlorine chemistry, using observed or low-predicted coastal Cl₂ concentrations, may increase morning ozone predictions by as much as ∼12 ppb in coastal regions and by ∼4 ppb to the peak domain ozone in the afternoon. An emissions inventory of anthropogenic sources of chlorine is recommended as these may enhance ozone formation even further by emitting chlorine gases directly into polluted regions.; Air quality models consider the formation and deposition of nitric acid (HNO₃) on surfaces to be an irreversible sink of atmospheric nitrogen oxides (NO_x) and therefore an effective termination step in the ozone formation cycle. However, experimental evidence suggests that the reaction of gaseous nitric oxide with nitric acid on surfaces may convert HNO ₃ to photochemically active NO_x. A simulation of this surface-mediated renoxification process has been performed for the South Coast Air Basin of California. Peak ozone concentrations are predicted closer to observed values in the Central Los Angeles area, a region regularly underpredicted by base case models. These results suggest that inclusion of renoxification processes may be a key to resolving long-standing shortcomings of air quality models.