Fast Nocturnal Heterogeneous Chemistry in a Coastal Background Atmosphere and Its Implications for Daytime Photochemistry
Ying‐Bo Niu, Bo Zhu, Ling‐Yan He, Zhe Wang, Xiao‐Yu Lin, Meng‐Xue Tang, Xiaofeng Huang
Abstract
Abstract Nitryl chloride (ClNO 2 ) 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 N 2 O 5 and ClNO 2 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 ClNO 2 at night, while N 2 O 5 was generally low. The maximum concentrations of N 2 O 5 and ClNO 2 were 1.4 and 1.7 ppbv, respectively, associated with a transported urban plume with high levels of O 3 and NO 2 . The average product of N 2 O 5 uptake coefficient and ClNO 2 yield (γ × f = 0.02, with average γ of 0.027 and f of 0.71) was roughly twice those in urban areas in China, indicating active N 2 O 5 heterogenous chemistry and effective ClNO 2 formation in the background atmosphere. The production rate of Cl radicals ( p Cl) from the photolysis of ClNO 2 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, NO 2 produced by ClNO 2 photolysis in the morning accounted for approximately 10% of the NO 2 increase within several hours after sunrise. Thus, high nocturnal ClNO 2 production and its following daytime photochemistry should be considered seriously in regional air quality modeling in the PRD and regions with similar conditions.