Nighttime NOx loss and ClNO2 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 (N2O5) on aerosols is an important sink of nitrogen oxides (NOx) 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 NOx 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 NOx in the air masses (70%±10%) was removed during the night via NO3 reactions with VOCs (~40%) and N2O5 heterogeneous loss (~60%).