Nighttime NO_x loss and ClNO₂ formation in the residual layer of a polluted region: Insights from field measurements and an iterative box model

The heterogeneous reaction of dinitrogen pentoxide (N₂O₅) on aerosols is an important sink of nitrogen oxides (NO_x) in the polluted boundary layer, and the production of nitryl chloride (ClNO₂) can have significant effects on the atmospheric oxidative capacity. However, the heterogeneous loss of N₂O₅ and the formation of ClNO₂ are still not well quantified, especially in China. In a previous study, we measured ClNO₂ and N₂O₅ concentrations in several air masses at a high-elevation site in Hong Kong, and found the highest levels ever reported at one night. The present study employed an iterative box model to investigate five N₂O₅/ClNO₂-laden nights. We first estimated the N₂O₅ uptake coefficient and ClNO₂ yield and then calculated the relative importance of N₂O₅ heterogeneous reactions to NO_x loss and the accumulated ClNO₂ production over the entire night. The average uptake coefficient was 0.004±0.003, and the average yield was 0.42±0.26. As the air masses aged, the accumulated ClNO₂ reached up to 6.0ppbv, indicating significant production of ClNO₂ in the polluted air from the Pearl River Delta. ClNO₂ formation (N₂O₅+Cl^-), N₂O₅ hydrolysis (N₂O₅+H₂O), and NO₃ reactions with volatile organic compounds (NO₃+VOCs) consumed 23%, 27%, and 47% of the produced NO₃, respectively, as the average for five nights. A significant portion of the NO_x in the air masses (70%±10%) was removed during the night via NO₃ reactions with VOCs (~40%) and N₂O₅ heterogeneous loss (~60%).