Investigating the evolution of summertime secondary atmospheric pollutants in urban Beijing

Understanding the formation of tropospheric ozone (O-3) and secondary particulates is essential for controlling secondary pollution in megacities. Intensive observations were conducted to investigate the evolution of O-3, nitrate (NO3-), sulfate (SO42-) and oxygenated organic aerosols ((OOAs), a proxy for secondary organic aerosols) and the interactions between O-3, NOx oxidation products (NOz) and OOA in urban Beijing in August 2012. The O-3 concentrations exhibited similar variations at both the urban and urban background sites in Beijing. Regarding the O-3 profile, the O-3 concentrations increased with increasing altitude. The peaks in O-3 on the days exceeding the 1 h or 8 h O-3 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 SO(4)(2-)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 x [SO42-] + 2.1, r(2) = 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 O-x concentrations were well correlated at the daily scale, whereas the hourly OOA and O-x concentrations were insignificantly correlated in urban Beijing. The synoptic situation and the differences in the VOC oxidation contributing to O-3 and SOAs may have resulted in the differences among the correlations between OOA and O-x at different time scale. We calculated OOA production rates using the photochemical age (defined as -log(10)(NOx/NOy)) in urban plumes. The CO-normalized OOA concentration increased with increasing photochemical age, with production rates ranging from 1.1 to 8.5 mu g m(-3) ppm(-1) h(-1) for the plume from the NCP. (C) 2016 Elsevier B.V. All rights reserved.