18 Jan 2023
18 Jan 2023
Status: this preprint is currently under review for the journal ACP.

Measurement Report: Enhanced contribution of photooxidation to dicarboxylic acids in urban aerosols during the COVID-19 lockdown in Jinan, East China

Jingjing Meng1,2, Yachen Wang1, Yuanyuan Li1, Tonglin Huang1, Zhifei Wang3, Yiqiu Wang4, Min Chen1, Zhanfang Hou1, Kimitaka Kawamura5, and Pingqing Fu2 Jingjing Meng et al.
  • 1School of Geography and Environment, Liaocheng University, Liaocheng 252000, China
  • 2Institute of Surface-Earth System Science, School of Earth System Science, Tianjin University, Tianjin 300072, China
  • 3Jinan Environmental Monitoring Center of Shandong Province, Jinan 250101, China
  • 4Liaocheng Environmental Information and Monitoring Center, Liaocheng, 252000, China
  • 5Chubu Institute for Advanced Studies, Chubu University, Kasugai 487-8501, Japan

Abstract. To curb the spread of a novel coronavirus pandemic (COVID-19), a preventive lockdown (LCD) policy was first implemented across China in early 2020, resulting in a substantial drop-off in anthropogenic pollutant emissions and thus the amelioration of air quality. Unexpectedly, several haze events driven by enhanced secondary organic aerosols (SOA) still took place in the eastern China during the LCD. To investigate the effect of LCD measures on the formation and evolutionary process of SOA, PM2.5 samples were collected before and during the LCD in Jinan, East China. The samples were measured for dicarboxylic acids (diacids) and related compounds, water-soluble inorganic ions, carbonaceous species, as well as the stable carbon isotopic compositions (δ13C) of major diacids. Our results show that despite the sharp decrease of primary pollutants (e.g., CO, SO2, NO2, and element carbon) during the LCD, the O3 concentration, proportion of secondary inorganic aerosols, concentration levels, and relative abundance of diacid homologues in water-soluble organic compounds (WSOC) were still 2–4 times higher than those before the LCD. The ratios of oxalic acid (C2) to diacids (C2/diacids) and to total detected organic components were higher during the LCD than those before the LCD, suggesting the more aged organic aerosols during the LCD under the clearer sky conditions. The temporal changes, diurnal variations in major diacids, and their higher concentrations and contributions during the LCD than before the LCD are mainly due to the enhanced photochemical oxidation by the higher O3 and the stronger solar radiation during the LCD. Interestingly, compound-specific stable carbon isotope ratios (δ13C) of C2 and other major diacids show higher values in the nighttime than the daytime before the LCD, which indicate a significant contribution of organic acids via aqueous phase oxidation at night. Source apportionments using the molecular characteristics of organic compounds and positive matrix factorization (PMF) model suggest that the aqueous oxidation (45.2 %) and coal combustion (16.7 %) were the major sources before the LCD but the photochemical oxidation lunched by the higher O3 concentration (48.8 %) and aqueous oxidation (17.7 %) were the dominant source during the LCD. The increased δ13C values of oxalic acid and other major organic acids along with the high ratios of C2/Gly, C2/mGly, and C2/diacids before and during the LCD confirm an isotopic fractionation effect during the precursor oxidation processes. Furthermore, more positive δ13C values of diacids are observed in the daytime than the nighttime during the LCD, which suggest an enhanced photochemical oxidation in the urban atmosphere during this period.

Jingjing Meng et al.

Status: open (until 01 Mar 2023)

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  • RC1: 'Comment on acp-2022-830', Anonymous Referee #1, 30 Jan 2023 reply

Jingjing Meng et al.

Data sets

Enhanced contribution of photooxidation to dicarboxylic acids in urban aerosols during the COVID-19 lockdown in Jinan, East China Jingjing, Meng

Jingjing Meng et al.


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Short summary
This study investigated the effect of the COVID-19 lockdown (LCD) measures on the formation and evolutionary process of secondary organic aerosol (SOA) from field observations. Results demonstrate that the more aged organic aerosols is observed during the LCD due to the enhanced photochemical oxidation under the clearer sky conditions. Our study also found that diacarboxylic acids during the LCD were derived from the O3-dominated photochemical pathways.