Fast Nocturnal Heterogeneous Chemistry in a Coastal Background Atmosphere and Its Implications for Daytime Photochemistry

Nitryl chloride (ClNO2) is an important chlorine reservoir in the atmosphere that affects the oxidation capacity by producing first- and second-generation radicals, but it has been rarely observed in the world. Here, online measurements of both N2O5 and ClNO2 at a coastal background site in the Pearl River Delta (PRD) in southern China, a region with high atmospheric oxidation capacity in the daytime, are presented. We observed continuous accumulation of elevated ClNO2 at night, while N2O5 was generally low. The maximum concentrations of N2O5 and ClNO2 were 1.4 and 1.7 ppbv, respectively, associated with a transported urban plume with high levels of O-3 and NO2. The average product of N2O5 uptake coefficient and ClNO2 yield (gamma x f = 0.02, with average gamma of 0.027 and f of 0.71) was roughly twice those in urban areas in China, indicating active N2O5 heterogenous chemistry and effective ClNO2 formation in the background atmosphere. The production rate of Cl radicals (pCl) from the photolysis of ClNO2 was estimated to be an order of magnitude higher than those at coastal and rural sites in the literature. Such levels of Cl radical production could have considerable contribution to the atmospheric oxidation capacity after sunrise. In addition, NO2 produced by ClNO2 photolysis in the morning accounted for approximately 10% of the NO2 increase within several hours after sunrise. Thus, high nocturnal ClNO2 production and its following daytime photochemistry should be considered seriously in regional air quality modeling in the PRD and regions with similar conditions.