Response of PM 2.5 chemical composition to the emission reduction and meteorological variation during the COVID-19 lockdown.

PM _2.5 is a main atmospheric pollutant with various sources and complex chemical compositions, which are influenced by various factors, such as anthropogenic emissions (AE) and meteorological conditions (MC). MC have a significant impacts on variations in atmospheric pollutant; therefore, emission reduction policies and ambient air quality are non-linearly correlated, which hinders the accurate assessment of the effectiveness of control measures. In this study, we conducted online observations of PM _2.5 and its chemical composition in Hohhot, China, from December 1, 2019, to February 29, 2020, to investigate how the chemical compositions of PM _2.5 respond to the variations in AE and MC. Moreover, the random forest (RF) model was used to quantify the contributions of AE and MC to PM _2.5 and its chemical composition during severe hazes and the COVID-19 pandemic lockdown period. During the clean period, MC reduced PM _2.5 concentrations by 124%, while MC incresed PM _2.5 concentrations by 49% during severe pollution episode. Inorganic aerosols (SO ₄ ^2- , NO ₃ ^- , and NH ₄ ⁺ ) showed the strongest response to MC. MC significantly contributed to PM _2.5 (36%), SO ₄ ^2- (32%), NO ₃ ^- (29%), NH ₄ ⁺ (28%), OC (22%), and SOC (17%) levels during pollution episodes. From the pre-lockdown to lockdown period, AE (MC) contributed 52% (48%), 81% (19%), 48% (52%), 68% (32%), 59% (41%), and 288% (-188%) to the PM _2.5 , SO ₄ ^2- , NO ₃ ^- , NH ₄ ⁺ , OC, and SOC reductions, respectively. The variations in MC (especially the increase in relative humidity) rapidly generated meteorologically sensitive species (SO ₄ ^2- , NO ₃ ^- , and NH ₄ ⁺ ), which led to severe winter pollution. This study provides a reference for assessing the net benefits of emission reduction measures for PM _2.5 and its chemical compositions.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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