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  • Title: Investigating the evolution of summertime secondary atmospheric pollutants in urban Beijing.
    Author: Ji D, Gao W, Zhang J, Morino Y, Zhou L, Yu P, Li Y, Sun J, Ge B, Tang G, Sun Y, Wang Y.
    Journal: Sci Total Environ; 2016 Dec 01; 572():289-300. PubMed ID: 27505262.
    Abstract:
    Understanding the formation of tropospheric ozone (O3) and secondary particulates is essential for controlling secondary pollution in megacities. Intensive observations were conducted to investigate the evolution of O3, nitrate (NO3-), sulfate (SO42-) and oxygenated organic aerosols ((OOAs), a proxy for secondary organic aerosols) and the interactions between O3, NOx oxidation products (NOz) and OOA in urban Beijing in August 2012. The O3 concentrations exhibited similar variations at both the urban and urban background sites in Beijing. Regarding the O3 profile, the O3 concentrations increased with increasing altitude. The peaks in O3 on the days exceeding the 1h or 8h O3 standards (polluted days) were substantially wider than those on normal days. Significant increases in the NOz mixing ratio (i.e., NOy - NOx) were observed between the morning and early afternoon, which were consistent with the increasing oxidant level. A discernable NO3- peak was also observed in the morning on the polluted days, and this peak was attributed to vertical mixing and strong photochemical production. In addition, a SO42- peak at 18:00 was likely caused by a combination of local generation and regional transport. The OOA concentration cycle exhibited two peaks at approximately 10:00 and 19:00. The OOA concentrations were correlated well with SO42- ([OOA]=0.55×[SO42-]+2.1, r2=0.69) because they both originated from secondary transformations that were dependent on the ambient oxidization level and relative humidity. However, the slope between OOA and SO42- was only 0.35, which was smaller than the slope observed for all of the OOA and SO42- data, when the RH ranged from 40 to 50%. In addition, a photochemical episode was selected for analysis. The results showed that regional transport played an important role in the evolution of the investigated secondary pollutants. The measured OOA and Ox concentrations were well correlated at the daily scale, whereas the hourly OOA and Ox concentrations were insignificantly correlated in urban Beijing. The synoptic situation and the differences in the VOC oxidation contributing to O3 and SOAs may have resulted in the differences among the correlations between OOA and Ox at different time scale. We calculated OOA production rates using the photochemical age (defined as -log10(NOx/NOy)) in urban plumes. The CO-normalized OOA concentration increased with increasing photochemical age, with production rates ranging from 1.1 to 8.5μgm-3ppm-1h-1 for the plume from the NCP.
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