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

摘要

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 N2O5and the formation of ClNO2are still not well quantified, especially in China. In a previous study, we measured ClNO2and N2O5concentrations 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 N2O5uptake coefficient and ClNO2yield and then calculated the relative importance of N2O5heterogeneous reactions to NOxloss and the accumulated ClNO2production 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 ClNO2reached up to 6.0ppbv, indicating significant production of ClNO2in the polluted air from the Pearl River Delta. ClNO2formation (N2O5+Cl−), N2O5hydrolysis (N2O5+H2O), and NO3reactions 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 NOxin the air masses (70%±10%) was removed during the night via NO3reactions with VOCs (~40%) and N2O5heterogeneous loss (~60%).