Impacts of primary emissions and secondary aerosol formation on air pollution in an urban area of China during the COVID-19 lockdown

Abstract

Restrictions on human activities were implemented in China to cope with the outbreak of the Coronavirus Disease 2019 (COVID-19), providing an opportunity to investigate the impacts of anthropogenic emissions on air quality. Intensive real-time measurements were made to compare primary emissions and secondary aerosol formation in Xi’an, China before and during the COVID-19 lockdown. Decreases in mass concentrations of particulate matter (PM) and its components were observed during the lockdown with reductions of 32–51%. The dominant contributor of PM was organic aerosol (OA), and results of a hybrid environmental receptor model indicated OA was composed of four primary OA (POA) factors (hydrocarbon-like OA (HOA), cooking OA (COA), biomass burning OA (BBOA), and coal combustion OA (CCOA)) and two oxygenated OA (OOA) factors (less-oxidized OOA (LO-OOA) and more-oxidized OOA (MO-OOA)). The mass concentrations of OA factors decreased from before to during the lockdown over a range of 17% to 58%, and they were affected by control measures and secondary processes. Correlations of secondary aerosols/ΔCO with O x (NO 2 + O 3 ) and aerosol liquid water content indicated that photochemical oxidation had a greater effect on the formation of nitrate and two OOAs than sulfate; however, aqueous-phase reaction presented a more complex effect on secondary aerosols formation at different relative humidity condition. The formation efficiencies of secondary aerosols were enhanced during the lockdown as the increase of atmospheric oxidation capacity. Analyses of pollution episodes highlighted the importance of OA, especially the LO-OOA, for air pollution during the lockdown.

Variations and Sources of Organic Aerosol in Winter Beijing under Markedly Reduced Anthropogenic Activities During COVID-2019

“… 11 , 17 , 69 Higher O 3 concentration in E2 (45.5 μg/m 3 ) than that in E1 (15.9 μg/m 3 ) could promote ozonation reaction in atmosphere and further more generation of secondary aerosols. Different from some cities in northwest and southwest China and the Yangtze River Delta (YRD) region where aerosol concentration decreased after lockdown, 70 − 75 average PM 2.5 concentration increased from 41.8 μg/m 3 before lockdown to 74.1 μg/m 3 (by 77%) after lockdown in Beijing. It even raised up to the highest (356 μg/m 3 ) in the Lunar New Year’s Eve when fireworks emitted lots of Cl – , K + , and SO 4 2– .…”

“…Similarly, the studies in Lanzhou and Xi’an also found OA from coal and biomass burning remained stable after COVID-19 lockdown. 70 , 71 …”

Measurement report: The importance of biomass burning in light extinction and direct radiative effect of urban aerosol during the COVID-19 lockdown in China

“…The temporal variations of hourly mean b scat , b abs , b ext , and SSA together with PM 2.5 mass concentrations for the entire sampling period are depicted in Figure S8, while a statistics summary of optical and chemical parameters during the normal and COVID-19 lockdown periods is shown in Table 1. The optical coefficients decreased dramatically in accord with the significant reduction of PM 2.5 since stringent control measures on emission sources implemented during the lockdown period (Tian et al, 2021;Zheng et al, 2020). The mean values of b scat , b abs , and b ext during the normal period were 688.1 ± 261.4 Mm -1 , 86.6 ± 43.0 Mm -1 , and 774.7 ± 298.1 Mm -1 , respectively, which is consistent with the values (657.4 ± 436.9 Mm -1 , 104.0 ± 69.6 Mm -1 , and 761.4 ± 506.5 Mm -1 ) reported previously in winter of 2009 in Xi'an (Cao et al, 2012), even though a series of nationwide air quality standards and long-term pollution control policies have been OA (R 2 = 0.94-0.99), indicating an enhanced role of secondary formation in the lockdown.…”

“…The Q-ACSM measured concentrations of non-refractory species in PM 1 (NO 3 -, SO 4 2-, NH 4 + , Cl -, and OA), and OA was further resolved into POA, less-, and more-oxidized oxygenated OA (LO-OOA and MO-OOA). Detail information on Q-ACSM data process and source apportionment of OA can be found in our previous paper (Tian et al, 2021). The Xact 625i quantified hourly element concentrations through X-ray fluorescence analysis, including Si, K, Ca, Cr, Mn, Fe, Zn, As, Se, Ba, Hg, and Pb.…”

“…All instruments were placed at the rooftop of an office building (~ 10 m above the ground) and approximately 30 m from the nearest traffic road. Detailed description of the sampling site can be found in Tian et al (2021). In this study, the sampling campaign consisted of two distinct periods: normal period (1 to 23 January, 2020) and COVID-19 lockdown period (27 January to 7 February, 2020).…”