Oscillation cumulative volatile organic compounds on the northern edge of the North China Plain: Impact of mountain-plain breeze.

Under weak weather systems, pollutants circulate on plains and hillsides by means of oscillatory mountain-plain breezes (MPB). To investigate the impact of MPB on ambient volatile organic compounds (VOCs) on the northern edge of the North China Plain (NCP) region, synchronous online VOCs observations were first performed in the southwestern (Beijing, urban area) and northeastern (Xinglong, background area) of the dominant wind directions from September 11, 2020, to October 13, 2020. The results revealed that during the observation period, the average mixing ratios of TVOCs in Beijing and Xinglong were 32.5 ± 21.6 and 14.2 ± 8.6 ppbv, respectively, and a coincident temporal evolution trend of VOCs was observed at the two sites. Combined with the results of the backward trajectory, southerly air transport led to a significant increase in VOCs concentration, and southerly airflow was usually accompanied by apparent MPB circulation. Furthermore, the occurrence of MPB led to a sharp increase (1.5 times) in VOCs concentration in both Beijing and Xinglong. This study indicates that the periodic oscillation of the MPB leads to the large-scale pollution of "garbage out and garbage in", and the VOCs discharged by industry are the key to future control.