Measurement report of the change of PM2.5 composition during the COVID-19 lockdown in urban Xi'an: Enhanced secondary formation and oxidation

https://doi.org/10.1016/j.scitotenv.2021.148126Get rights and content

Highlights

  • Sulfate and MO-OOA formations were enhanced during the COVID-19 lockdown.

  • Organosulfurs (OS) and organic nitrate (ON) increased during the lockdown.

  • The increases of OS and ON were much efficient in the nighttime.

Abstract

Enhanced secondary aerosol formation was observed during the COVID-19 lockdown in Xi'an, especially for polluted episodes. More oxidized‑oxygenated organic aerosol (MO-OOA) and sulfate showed the dominant enhancements, especially in large particle-mode. Meanwhile, relative humidity (RH) showed a positive promotion on the formation of sulfate and MO-OOA during the lockdown, but had no obvious correlation with less oxidized‑oxygenated organic aerosol (LO-OOA) or nitrate. Organosulfurs (OS) displayed a higher contribution (~58%) than inorganic sulfate to total sulfate enhancement in the polluted episode during the lockdown. Although the total nitrate (TN) decreased during the lockdown ascribing to a larger reduction of inorganic nitrate, organic nitrate (ON) showed an obvious increase from pre-lockdown (0.5 ± 0.6 μg m−3 and 1 ± 2% of TN) to lockdown (5.3 ± 3.1 μg m−3 and 17 ± 9% of TN) in the polluted case (P < 0.05). In addition, RH also displayed a positive promotion on the formation of ON and OS, and the increases of both OS and ON were much efficient in the nighttime than in the daytime. These results suggest that higher RH and stagnant meteorology might facilitate the sulfate and MO-OOA enhancement, especially in the nighttime, which dominated the secondary aerosol enhancement in haze pollution during the lockdown.

Introduction

In late 2019 and early 2020, a novel coronavirus disease named COVID-19 was identified and broke out rapidly all around the world (Chen et al., 2020; Huang et al., 2020; Qi et al., 2020), which has infected near 40 million people and killed more than 1000,000 people as of 15 October 2020 (https://covid19.who.int/). In order to prevent its rapid spread, restrict lockdown and control measures were implemented in many countries and cities (Abdullah et al., 2020; Sun et al., 2020; Tobías et al., 2020). The Chinese central government firstly enacted national lockdown policies and a set of strict measures against COVID-19 in January 2020 (Wang et al., 2020), in which the city lockdown was imposed in Wuhan on 23 January, one day before the New Year Holiday, and then other cities in the following 1–2 days (Tian et al., 2020; Wu et al., 2020). Industrial operations, travels and even restaurants were all limited except essential activities such as procurement of food and medical treatment. As a result, human activities, energy production, and industrial coal and oil combustion reduced drastically, which have significantly affected the economy as well as emissions of air pollutants (Shi and Brasseur, 2020; Zhu et al., 2020), providing a unique chance to evaluate the response of air quality to dramatical emissions changes.

Emissions of primary pollutants decreased largely during the lockdown period due to significantly reduced transportation and industrial activity, and substantial decreases of the air quality index (AQI) and PM2.5 concentration were also observed in many cities in China (Bao and Zhang, 2020; Fan et al., 2020; Wang et al., 2020). For example, an approximate 65% decrease in tropospheric nitrogen oxides (NO2) compared to 2019 over the eastern China region was observed through the Sentinel 5P satellite (Huang et al., 2020). Reductions of approximate 37–57% of volatile organic compounds (VOCs) and 27–46% of PM2.5 concentrations compared to the period before lockdown were also observed over the YRD Region (Li et al., 2020), suggesting the improvement of air quality due to the national restriction policies in those cities. However, severe haze events over eastern and northern China were still observed during the COVID-19 lockdown, despite large reductions in anthropogenic emissions (Chang et al., 2020; Huang et al., 2020). Further studies show that enhanced secondary pollution was the main cause of the haze events occurred during the COVID-19 lockdown, and meteorological conditions such as stagnant air, unfavorable wind direction play vital roles in these haze formation. Large reduction of NOx increased ozone (O3) concentration in urban air and thus enhanced the atmospheric oxidizing capacity, which facilitated the secondary aerosol formation and offset the reduction in primary emissions (Huang et al., 2020; Le et al., 2020). These results indicate the variations of COVID-19 lockdown impacts among different regions and the complex coordination between primary and secondary pollutants for haze formation.

As one of the most important megacities in Northwest China, Xi'an has experienced rapid urban development and suffered severe air pollution in recent years, especially during winter with unfavorable meteorological conditions (Elser et al., 2016; Zhong et al., 2020). The implementation of a series of control measures against COVID-19 outbreak in Xi'an provides an opportunity to assess the impacts of human activities and anthropogenic emissions on haze pollution. In this study, based on high-resolution measurements using a soot particle long-time-of-flight AMS (SP-LToF-AMS), we compared the chemical composition of non-refractory PM2.5 (NR-PM2.5) and organic aerosol (OA) sources during the COVID-19 lockdown with those during pre-lockdown in early 2020 and the same period in New Year Holiday of 2019 (NYH-19). Comparisons of primary and secondary aerosol as well as the OA oxidation state were elucidated to gain clues on the impacts of COVID-19 lockdown on air quality in Xi'an.

Section snippets

Sampling

The observation site was located in downtown Xi'an with surrounding residential, traffic, and commercial areas. Instruments were set up at the campus of the Institute of Earth Environment, Chinese Academy of Sciences (34°23′N, 108°89′, ~15 m a.g.l) and the measurements were conducted during pre-lockdown (from 18 January to 23 January 2020), COVID-19 lockdown that was also the New Year Holiday in 2020 (24 January to 31 January 2020, NYH-20), note that ~13 h data on 27–28 January were not

The changes of NR-PM2.5 species and OA sources during the COVID-19 lockdown

Due to the strict measures that reduce social activities and anthropogenic emissions, the gaseous pollutants, including SO2, NO2 and CO, decreased during COVID-19 lockdown compared to pre-lockdown. NO2 declined the most from 57.0 ± 17.7 μg m−3 during pre-lockdown to 28.5 ± 10.7 μg m−3 during lockdown, with a decrease ratio of 50% (P < 0.05) (Fig. 1 and Table S3). In contrast, the average concentration of O3 increased from 35.3 ± 24.6 μg m−3 during pre-lockdown to 53.6 ± 26.9 μg m−3 during

Conclusion

The COVID-19 lockdown implemented in many countries to prevent its rapid spread has dramatically affected the economic activities and air quality. Here we conducted high-resolution characterization of NR-PM2.5 using a SP-LToF-AMS in Xi'an. Aerosol composition and chemistry during the COVID-19 lockdown were investigated and compared with those during pre-lockdown and NYH-19. The results show that the secondary formation of sulfate and MO-OOA were largely enhanced during the COVID-19 lockdown,

CRediT authorship contribution statement

Jing Duan: Methodology, Data curation, Formal analysis, Writing – original draft, Writing – review & editing. Ru-Jin Huang: Conceptualization, Validation, Data curation, Writing – review & editing, Project administration, Supervision, Funding acquisition. Yunhua Chang: Writing – review & editing. Haobin Zhong: Data curation, Writing – review & editing. Yifang Gu: Writing – review & editing. Chunshui Lin: Writing – review & editing. Thorsten Hoffmann: Writing – review & editing. Colin O'Dowd:

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This work was supported by the Strategic Priority Research Program of Chinese Academy of Sciences (No. XDB40000000), the National Natural Science Foundation of China (NSFC) under Grant No. 41925015, 41877408, and 91644219, the Chinese Academy of Sciences (no. ZDBS-LY-DQC001), the National Key Research and Development Program of China (No. 2017YFC0212701), and the Cross Innovative Team fund from the State Key Laboratory of Loess and Quaternary Geology (No. SKLLQGTD1801). The associated data are

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