Nighttime NO x loss and ClNO 2 formation in the residual layer of a polluted region: Insights from field measurements and an iterative box model

Abstract The heterogeneous reaction of dinitrogen pentoxide (N 2 O 5 ) on aerosols is an important sink of nitrogen oxides (NO x ) in the polluted boundary layer, and the production of nitryl chloride (ClNO 2 ) can have significant effects on the atmospheric oxidative capacity. However, the heterogeneous loss of N 2 O 5 and the formation of ClNO 2 are still not well quantified, especially in China. In a previous study, we measured ClNO 2 and N 2 O 5 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 N 2 O 5 /ClNO 2 -laden nights. We first estimated the N 2 O 5 uptake coefficient and ClNO 2 yield and then calculated the relative importance of N 2 O 5 heterogeneous reactions to NO x loss and the accumulated ClNO 2 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 ClNO 2 reached up to 6.0 ppbv, indicating significant production of ClNO 2 in the polluted air from the Pearl River Delta. ClNO 2 formation (N 2 O 5  + Cl − ), N 2 O 5 hydrolysis (N 2 O 5  + H 2 O), and NO 3 reactions with volatile organic compounds (NO 3  + VOCs) consumed 23%, 27%, and 47% of the produced NO 3 , 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 NO 3 reactions with VOCs (~ 40%) and N 2 O 5 heterogeneous loss (~ 60%).