The Independent Impacts of PM 2.5 Dropping on the Physical and Chemical Properties of Atmosphere over North China Plain in Summer during 2015–2019

Great changes occurred in the physical and chemical properties of the atmosphere in the North China Plain (NCP) in summer caused by PM 2.5 dropping from 58 μg/m 3 in 2015 to 36.0 μg/m 3 in 2019. In this study, we first applied the WRF-Chem model to quantify the impact of PM 2.5 reduction on shortwave radiation reaching the ground (SWDOWN), planetary boundary layer height (PBLH), and the surface concentration of air pollutants (represented by CO). Simulation results obtained an increase of 15.0% in daytime SWDOWN and 9.9% in daytime PBLH, and a decrease of −5.0% in daytime CO concentration. These changes were induced by the varied PM 2.5 levels. Moreover, the variation in SWDOWN further led to a rise in the NO 2 photolysis rate ( J NO 2 ) over this region, by 1.82 × 10 −4 ~1.91 × 10 −4 s −1 per year. Afterwards, we employed MCM chemical box model to explore how the J NO 2 increase and the precursor decrease (CO, VOCs, and NO x ) influenced O 3 and HO x radicals. The results revealed that the photolysis rate ( J ) increase would individually cause a change on daytime surface O 3 , OH, and HO 2 radicals by +9.0%, +18.9%, and +23.7%; the corresponding change induced by the precursor decrease was −2.5%, +1.9%, and −2.3%. At the same time, the integrated impacts of the change in J and precursors cause an increase of +6.3%, +21.1%, and +20.9% for daytime surface O 3 , OH, and HO 2 . Generally, the atmospheric oxidation capacity significantly enhanced during summer in NCP due to the PM 2.5 dropping in recent years. This research can help understand atmosphere changes caused by PM 2.5 reduction comprehensively.