Reinforcement of Secondary Circulation by Aerosol Feedback and PM 2.5 Vertical Exchange in the Atmospheric Boundary Layer

• Published in: Geophysical research letters Vol. 48; no. 16
• Main Authors: Li, Jixiang, Wu, Min, Li, Yang, Ma, Shuxin, Wang, Zhanxiang, Zhao, Yuan, Lian, Lulu, Song, Shijie, Huang, Tao, Gao, Hong, Tao, Shu, Liu, Junfeng, Mao, Xiaoxuan, Ma, Jianmin
• Format: Journal Article
• Language: English
• Published: 28.08.2021
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2021GL094465

The secondary circulation (SC) plays a vital role in the vertical exchange of meteorological variables and aerosol particles between the atmospheric boundary layer (ABL) and the free atmosphere. We show that, under favorable synoptic‐scale weather conditions aggravating PM 2.5 pollution, heavy particle contaminations reduce wind speed in the low ABL via an aerosol feedback mechanism that perturbs the SCs. We examine the aerosol feedback impact on the reinforcement of SCs subject to heavy PM 2.5 pollution occurring in Beijing and its surroundings in China. Results reveal that episodic and composite heavy PM 2.5 events simulated by the aerosol feedback model runs reduce the ABL wind speeds and enhance the SC. We identify the SC signals by the differences of resultant wind speeds and PM 2.5 concentrations between “feedback” and “nonfeedback” simulations under the presence of a hill topographic forcing, which is conducive to the vertical exchange of PM 2.5 between the ABL and free atmosphere. Vertical exchange of an air pollutant between the atmospheric boundary layer (ABL) and free atmosphere has been a major concern for the transfer of the pollutant out of the ABL. The secondary circulation (SC) provides one of the pathways for the vertical mixing of air pollutants. Following the classical SC formation mechanism in the ABL, in the present study, we demonstrate that heavy PM 2.5 pollution reduces the wind speed and temperature near the surface via an aerosol feedback mechanism, which reinforces the SC. We carried out extensive model simulations of heavy PM 2.5 pollution cases in Beijing and surrounding regions in China, subject to feedback and nonfeedback modeling scenarios. The differences between the feedback and nonfeedback model runs revealed enhanced SCs associated with heavy PM 2.5 contamination. The reinforced SCs could also be discerned in the presence of hill topography. The results fill our knowledge gaps in the understanding of how and to what extent the aerosol feedback could contribute to the change in SCs, in addition to meteorological factors. PM 2.5 pollution tends to decrease near surface wind speed and temperature via an aerosol feedback mechanism Declining wind speed near the surface over heavily polluted areas reinforces the secondary circulation (SC) in the atmospheric boundary layer (ABL) Reinforced SC enhances PM 2.5 vertical mixing between the ABL and free atmosphere