Near-Explicit Multiphase Modeling of Halogen Chemistry in a Mixed Urban and Maritime Coastal Area

Abstract

In the present study, detailed air-parcel model simulations have been carried out that focus on multiphase halogen chemistry and its impact on the air quality in polluted coastal areas under sea breeze conditions. To achieve this, an advanced halogen multiphase chemistry mechanism is developed, containing 846 gas-phase reactions, 61 uptake processes, and 442 aqueous-phase reactions. Simulations with and without cloud occurrence are performed and compared to control simulations without added marine multiphase chemistry. Overall, the simulations demonstrate the importance of marine multiphase chemistry for air quality in polluted coastal areas. In detail, the simulations indicate significant influence of Cl atoms on oxidation of volatile organic compounds, especially alkanes, alkenes, nonoxidized aromatic compounds, and alcohols. The NO3 radical is pointed out to be an important daytime oxidant contributing up to 29% on the average dimethyl sulfide daytime oxidation flux. Compared to the control simulations, ozone production rates decrease by 29 and 22%, depending on whether the air parcel is influenced by cloud chemistry or not. The NO and NO2 concentrations decrease by 15 and 10% and by 19 and 13% in the cloud-free and cloud simulations, respectively. Halogen nitrate hydrolysis leads to an increase of particulate nitrate of up to 5.6% in the cloud-free simulation. In contrast, particulate nitrate decreases by 6.6% in the cloud simulation, which is related to an increased sulfate formation, resulting in more gas-phase nitric acid. In-cloud sulfate formation increased by 9.1% when halogen multiphase chemistry is considered.