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Enhanced photochemical formation of secondary organic aerosols during the COVID-19 lockdown in Northern China.

ABSTRACT: 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 (C₂) 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, PM₁₀, SO₂, NO₂, and CO) decreased by 30.0-59.8% during the lockdown compared to those before the lockdown, while O₃ increased by 63.9% during the lockdown mainly due to the inefficient titration effect of O₃ via NO reduction. The increased O₃ concentration can boost the atmospheric oxidizing capacity and further enhance the formation of secondary organic aerosols, thereby significantly enhancing the C₂ particles and its precursors as observed during the lockdown. Before the lockdown, C₂ particles were 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 C₂ particles and the increased aerosol acidity can promote the conversion of precursors such as glyoxal (Gly) and methyglyoxal (mGly) into C₂ particles in the aqueous phase. RH and ALWC decreased sharply but O₃ concentration and solar radiation increased remarkably during the lockdown, the O₃-dominated photochemical pathways played an important role in the formation of C₂ 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.

SUBMITTER: Meng J

PROVIDER: S-EPMC7666554 | biostudies-literature | 2020 Nov

REPOSITORIES: biostudies-literature

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Enhanced photochemical formation of secondary organic aerosols during the COVID-19 lockdown in Northern China.

Meng Jingjing J Li Zheng Z Zhou Ruiwen R Chen Min M Li Yuanyuan Y Yi Yanan Y Ding Zhijian Z Li Hongji H Yan Li L Hou Zhanfang Z Wang Gehui G

The Science of the total environment 20201114

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<sub>2</sub>) containing particles and its precursors were studied using a single particle aerosol ...[more]

PMID: 33223177

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Project description:The molecular characteristics and formation mechanism of biogenic secondary organic aerosols (BSOAs) in the forested atmosphere are poorly known. Here, we report the temporal variations in and formation processes of BSOA tracers derived from isoprene, monoterpenes, and β caryophyllene in PM2.5 samples collected at the foot of Mt. Huang (483 m a. s. l) in East China during the summer of 2019 with a 3 h time resolution. The concentrations of nearly all of the detected species, including organic carbon (OC), elemental carbon (EC), levoglucosan, and SIA (sum of SO42-, NO3-, and NH4+), were higher at night (19:00-7:00 of the next day) than in the daytime (7:00-19:00). In addition, air pollutants that accumulated by the dynamic transport of the mountain breeze at night were also a crucial reason for the higher BSOA tracers. Most of the BSOA tracers exhibited higher concentrations at night than in the daytime and peaked at 1:00 to 4:00 or 4:00 to 7:00. Those BSOA tracers presented strong correlations with O3 in the daytime rather than at night, indicating that BSOAs in the daytime were primarily derived from the photo-oxidation of BVOCs with O3. The close correlations of BSOA tracers with SO42- and particle acidity (pHis) suggest that BSOAs were primarily derived from the acid-catalyzed aqueous-phase oxidation. Considering the higher relative humidity and LWC concentration at night, the promoted aqueous oxidation was the essential reason for the higher concentrations of BSOA tracers at night. Moreover, levoglucosan exhibited a robust correlation with BSOA tracers, especially β-caryophyllinic acid, suggesting that biomass burning from long-distance transport exerted a significant impact on BSOA formation. Based on a tracer-based method, the estimated concentrations of secondary organic carbon (SOC) derived from isoprene, monoterpenes, and β caryophyllene at night (0.90 ± 0.57 µgC m-3) were higher than those (0.53 ± 0.34 µgC m-3) in the daytime, accounting for 14.5 ± 8.5% and 12.2 ± 5.0% of OC, respectively. Our results reveal that the BSOA formation at the foot of Mt. Huang was promoted by the mountain-valley breezes and anthropogenic pollutants from long-range transport.

Dataset Information

Enhanced photochemical formation of secondary organic aerosols during the COVID-19 lockdown in Northern China.

Publications

Enhanced photochemical formation of secondary organic aerosols during the COVID-19 lockdown in Northern China.

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