Response of PM_2.5 chemical composition to variations in anthropogenic emissions and meteorological conditions during COVID-19 lockdown

Abstract. PM_2.5 is a primary atmospheric pollutant with various sources and complicated chemical compositions that are influenced by various factors, such as anthropogenic emissions (AE) and meteorological conditions (MC). MC have significant impacts on variations of the atmospheric pollutant; therefore, emission reduction policies and ambient air quality are non-linearly correlated, which hinders accurate assessments of the effectiveness of control measures. The online observations of PM_2.5 and its chemical composition were conducted in Hohhot, China, from December 1, 2019, to February 29, 2020, to investigate PM_2.5 chemical compositions respond to the variation of AE and MC. Moreover, the random forest (RF) model was used to quantify the AE and MC contributions of PM_2.5 and its chemical composition during severe hazes and the COVID-19 pandemic lockdown period. During the clean period, MC contributed -124 % to PM_2.5 concentrations, indicating that MC promoted PM_2.5 dispersion. During severe pollution episodes, MC contributed 49 % to PM_2.5 concentrations, indicating that MC promoted PM_2.5 accumulation. The inorganic aerosols (SO₄^2-, NO₃^-, and NH₄⁺) showed the strongest response to MC. MC had significant contributions to the PM_2.5 (36 %), SO₄^2- (32 %), NO₃^- (29 %), NH₄⁺ (28 %), OC (22 %), and SOC (17 %). 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, SO₄^2-, NO₃^-, NH₄⁺, OC, and SOC variations, respectively. The variations of MC (especially the increase in relative humidity) rapidly generated meteorologically sensitive species (SO₄^2-, NO₃^-, and NH₄⁺), which led to severe winter pollution. This study provides reference for assessing the net benefits of emission reduction measures for PM_2.5 and its chemical compositions.