Observation of nitrogen oxide-influenced chlorine chemistry and source analysis of Cl2 in the Yangtze River Delta, China

Abstract

Atmospheric chlorine (Cl)- and nitrogen (N)-containing species such as Cl2, ClNO2, and N2O5 participate actively in atmospheric chemistry by triggering radical reactions that lead to the formation of secondary pollutants. Despite of their importance in atmospheric chemistry, the measurements of these species are scarce, due to analytical challenges. Herein, we report observation results of Cl2, ClNO2, and N2O5 from measurements using high-resolution time-of-flight chemical ionization mass spectrometry (HR-ToF-CIMS) in Shanghai and Changzhou in the highly industrialized and populated Yangtze River Delta (YRD) of East China. In Changzhou, the maximum concentrations of N2O5, ClNO2, and Cl2, were 0.4, 1.3, and 0.5 ppbv (1-min averages), respectively, while they were 0.5, 5.7, and 1.1 ppbv, respectively, in Shanghai. Much higher ClNO2 levels were found in Shanghai compared to Changzhou, although the concentrations of the precursor (N2O5) are similar, indicating different heterogenous N2O5 uptake and Cl activation efficiency at these two sites. The diurnal variation of Cl2 in Shanghai shows a strong daytime peak, which is contradictory to its fast lost by photolysis during daytime; the diurnal pattern of Cl2 in Changzhou shows a prominent night peak and a much smaller daytime peak than that in Shanghai. The heterogeneous uptake of ClNO2 and the photolysis of O3 on chloride-containing aerosols are believed to contribute to efficient Cl2 formation at night and day, respectively. The estimated heterogeneous uptake coefficient of O3 is 0.004 ± 0.007, with a maximum of 0.034. Further analysis on these heterogeneous reactions with the help of aerosol composition data (chloride, liquid water content, pH etc.) was also performed to ascertain the role of the secondary formation of Cl2 in daytime Shanghai. Through simple estimation methods, the maximum daytime production rate of Cl2 was estimated to be 0.95 ppbv h−1, while daytime photolysis of Cl2 contributed to 98% of Cl formation. Our results indicate that there is a large secondary source of Cl2 in the YRD region during the day, which can contribute substantially to Cl formation and requires more investigations.