<p>Here we present a field measurement of ClNO<sub>2</sub> (nitryl chloride) and N<sub>2</sub>O<sub>5</sub> (dinitrogen pentoxide) by a state-of-the-art instrument at a regional site in Pearl River Delta during a photochemical pollution season from Sept. 26<sup>th</sup> to Nov. 17<sup>th</sup>, 2019. Three patterns of air masses are sampled during this campaign, including the dominating air masses from north and northeast urban regions (Type A), the southeast coast (Type B) and the South China Sea (Type C). The concentration of ClNO<sub>2</sub> and N<sub>2</sub>O<sub>5</sub> were observed much higher in Type A and B than those in Type C, indicated the urban nighttime chemistry is more active than the background marine regions. Two key parameters that regulating ClNO<sub>2</sub> formations, N<sub>2</sub>O<sub>5</sub> uptake coefficient and ClNO<sub>2</sub> production yield, were estimated by measured parameters, and the performance of the previously derived parameterizations were assessed. We find the ClNO<sub>2</sub> formation was limited by the N<sub>2</sub>O<sub>5</sub> uptake rather than N<sub>2</sub>O<sub>5</sub> source at this site. By examining the relationship of particulate chloride and other species, we implied that anthropogenic emissions (e.g., biomass burning) rather than sea salt particles dominate the origin of particulate chloride, despite the site is only about 100 km away from the ocean. Model simulations showed the chloride radical liberated by ClNO<sub>2</sub> photolysis during the next day had a small increase in concentrations of OH, HO<sub>2</sub> and RO<sub>2</sub> radicals, as well as minor contributions to RO<sub>2</sub> radical and O<sub>3</sub> formation (<5 %, on daytime average) in all the three types of air masses. Relative higher contributions were observed in Type A and B. The overall low contributions of ClNO<sub>2</sub> to ozone pollution are consistent with those reported recently from wintertime observations in China (included Shanghai, Beijing, Wangdu and Mt. Tai). This may be attributed to: (1) Relative low particle mass concentration limited ClNO<sub>2</sub> formation; (2) Other reactions channels had larger radical formation rate during the ozone pollution episodes and weakened the ClNO<sub>2</sub> contribution indirectly. The results provided scientific insights into the role of nighttime chemistry in photochemical pollution under various scenarios in coastal areas.</p>