Nocturnal atmospheric chemistry of NO and NO over Changzhou in the Yangtze River Delta in China.

Comprehensive observations of the nocturnal atmospheric oxidation of NO and NO were conducted at a suburban site in Changzhou in the YRD using cavity ring-down spectroscopy (CRDS) from 27 May to 24 June, 2019. High concentrations of NO precursors were observed, and the nocturnal production rate of NO was determined to be 1.7 ± 1.2 ppbv/hr. However, the nighttime NO and NO concentrations were relatively low, with maximum values of 17.7 and 304.7 pptv, respectively, illustrating the rapid loss of NO and NO. It was found that NO dominated the nighttime atmospheric oxidation, accounting for 50.7%, while O and OH only contributed 34.1% and 15.2%, respectively. For the reactions of NO with volatile organic compounds (VOCs), styrene was found to account for 60.3%, highlighting its dominant role in the NO reactivity. In general, the contributions of the reactions between NO and VOCs and the NO uptake to NO loss were found to be about 39.5% and 60.5%, respectively, indicating that NO uptake also played an important role in the loss of NO and NO, especially under the high humidity conditions in China. The formation of nitrate at night mainly originated from NO uptake, and the maximum production rate of NO reached 6.5 ppbv/hr. The average NO consumption rate via NO and NO chemistry was found to be 0.4 ppbv/h, accounting for 47.9% of the total NO removal. The predominant roles of NO and NO in nitrate formation and NOx removal in the YRD region was highlighted in this study.