To eliminate the spread of a novel coronavirus breaking out in the end of 2019 (COVID-19), the Chinese government has implemented a nationwide lockdown policy after the Chinese lunar New Year of 2020, resulting in a sharp reduction in air pollutant emissions. To investigate the impact of the lockdown on aerosol chemistry, the number fraction, size distribution and formation process of oxalic acid (C 2 ) containing particles and its precursors were studied using a single particle aerosol mass spectrometer (SPAMS) at the urban site of Liaocheng in the North China Plain (NCP). Our results showed that five air pollutants (i.e., PM 2.5 , PM 10 , SO 2 , NO 2 , and CO) decreased by 30.0-59.8% during the lockdown compared to those before the lockdown, while O 3 increased by 63.9% during the lockdown mainly due to the inefficient titration effect of O 3 via NO reduction. The increased O 3 concentration can boost the atmospheric oxidizing capacity and further enhance the formation of secondary organic aerosols, thereby significantly enhancing the C 2 particles and its precursors as observed during the lockdown. Before the lockdown, C 2 particles was significantly originated from biomass burning emissions and their subsequent aqueous-phase oxidation. The hourly variation patterns and correlation analysis before the lockdown suggested that relative humidity (RH) and aerosol liquid water content (ALWC) played a key role in the formation of C 2 particles and the increased aerosol acidity can promote the conversion of precursors such as glyoxal (Gly) and methyglyoxal (mGly) into C 2 particles in the aqueous phase. RH and ALWC decreased sharply but O 3 concentration and solar radiation increased remarkably during the lockdown, the O 3 -dominated photochemical pathways played an important role in the formation of C 2 particles in which aerosol acidity was ineffective. Our study indicated that air pollution treatment sponges on a joint-control and balanced strategy for controlling numerous pollutants.

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The COVID-19 event triggered global attention which broke out at the end of 2019. To investigate the impact of the COVID-19 pandemic prevention and control actions on the chemical composition, size distribution, and mixing state of individual particles, real-time individual particles in the urban atmosphere of the Northern China were analyzed using single particle aerosol mass spectrometry (SPAMS) during January 16 to February 4, 2020. The results showed that the concentrations of PM 2.5 , NO x , and CO were lower during DP (during the pandemic) than those during BP (before the pandemic), while O 3 concentration increased by about 40.9% during DP due to a lower concentration of NO 2 restraining the decomposition of O 3 via the reaction of NO with O 3 . The number count of carbonaceous particles during DP decreased by 20.2% compared to that during BP due to the sharp reduction of factory production and vehicular transportation during DP. Dust particles during DP exhibited weaker 23 Na + , 56 Fe + , and 79 PO 3signals than those during BP, suggesting that dust particles during DP were mostly derived from mineral dust rather than industrial sources. The total particles during DP peaked at a larger size than those during BP, due to the higher fraction of secondary inorganic ions through the enhanced heterogeneous aqueous oxidation. The unscaled size distribution of total particles peaked at 0.50 µm during BP and at 0.66-0.70 µm during DP, suggesting that particles remained for a long time in the atmosphere and went through a strong aging process during DP. The single particles during DP were more aged than those during BP, owing to the stronger atmospheric oxidizing capacity during DP.

The Chinese government issued an unprecedentedly strict lockdown policy to control the spread of the novel coronavirus disease 2019 (COVID-19), significantly mitigating air pollution because of the dramatic reduction of industrial and traffic emissions. To explore the impact of COVID-19 lockdown (LCD) on organic aerosols, the mixing states and evolution processes of amine-containing particles were studied using a single particle aerosol mass spectrometer from January to March 2020 in Liaocheng, which is a seriously polluted city in North China. The counts and percentages of amine-containing particles in total obtained particles during the pre-LCD (547832, 29.8 %) were higher than those during the LCD (283983, 20.7 %) and post-LCD (102026, 18.4 %), mainly due to the reduced emission strength of amines and suppressed gas-to-particle partitioning of amines during the LCD and post-LCD. 74 (C 2 H 5 ) 2 NH 2 + was the most abundant amine marker, which accounted for 98.2 %, 98.4 %, and 96.7 % of all amine-containing particles during the pre-LCD, LCD, and post-LCD, respectively. Correlation analysis and temporal variations indicated that the gas-to-particle partitioning of amines was facilitated by the stronger acidic environment and lower temperature, while the effect of RH and aerosol liquid water content was minor. The A-OC particles were the most abundant type (accounting for ∼40 %) throughout the observation period. The temporal profiles and correlation analysis suggested that the impact of the increased O 3 on the amines and their oxidation products (e.g., trimethylamine oxide) was minor. The identified particle types, correlation analysis, and the potential source contribution function results implied that the amine-containing particles were mainly derived from local and surrounding sources during the LCD, while those were mainly affected by long-range transport during the pre-LCD and post-LCD. Our results could deepen the comprehension of the sources and atmospheric processing of amines in the urban area of North China during the COVID-19 outbreak.

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