Comparison of PM2.5 and CO2 Concentrations in Large Cities of China during the COVID-19 Lockdown

ADVANCES IN ATMOSPHERIC SCIENCES, VOL. 39, JUNE 2022, 861–875 • Original Paper • Comparison of PM2.5 and CO2 Concentrations in Large Cities of China during the COVID-19 Lockdown Chuwei LIU1, Zhongwei HUANG1, Jianping HUANG*1,2, Chunsheng LIANG1, Lei DING1, Xinbo LIAN1, Xiaoyue LIU1, Li Zhang1, and Danfeng WANG1 1Collaborative Innovation Center for Western Ecological Safety (CIWES), College of Atmospheric Sciences, Lanzhou University, Lanzhou 730000, China 2CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101, China (Received 20 July 2021; revised 1 November 2021; accepted 18 November 2021) ABSTRACT Estimating the impacts on PM2.5 pollution and CO2 emissions by human activities in different urban regions is important for developing efficient policies. In early 2020, China implemented a lockdown policy to contain the spread of COVID-19, resulting in a significant reduction of human activities. This event presents a convenient opportunity to study the impact of human activities in the transportation and industrial sectors on air pollution. Here, we investigate the variations in air quality attributed to the COVID-19 lockdown policy in the megacities of China by combining in-situ environmental and meteorological datasets, the Suomi-NPP/VIIRS and the CO2 emissions from the Carbon Monitor project. Our study shows that PM2.5 concentrations in the spring of 2020 decreased by 41.87% in the Yangtze River Delta (YRD) and 43.30% in the Pearl River Delta (PRD), respectively, owing to the significant shutdown of traffic and manufacturing industries. However, PM2.5 concentrations in the Beijing-Tianjin-Hebei (BTH) region only decreased by 2.01% because the energy and steel industries were not fully paused. In addition, unfavorable weather conditions contributed to further increases in the PM2.5 concentration. Furthermore, CO2 concentrations were not significantly affected in China during the short-term emission reduction, despite a 19.52% reduction in CO2 emissions compared to the same period in 2019. Our results suggest that concerted efforts from different emission sectors and effective long-term emission reduction strategies are necessary to control air pollution and CO2 emissions. Key words: PM2.5, CO2 emissions, lockdown measures, traffic emission, industrial activity Citation: Liu, C. W., and Coauthors, 2022: Comparison of PM2.5 and CO2 concentrations in large cities of China during the COVID-19 lockdown. Adv. Atmos. Sci., 39(6), 861−875, https://doi.org/10.1007/s00376-021-1281-x. Article Highlights: • During the COVID-19 lockdown in China, The concentrations of PM2.5 in southern (northern) China decreased significantly (slightly). • Both weather conditions and reduced industrial intensity were important reasons for the different changes in PM2.5 concentrations in various urban agglomerations. • The CO2 emissions decreased in China but had little effect on reducing CO2 concentration. • Short-term lockdowns cannot reduce CO2 concentrations in the atmosphere effectively. 1. Introduction Environmental pollution is among the most serious problems threatening China's densely populated cities. Particulate matter smaller than 2.5 μm (PM2.5) is of particular concern due to its serious impact on human health (Tang et al., 2020) and its role in climate change (Guo et al., 2017). The formation of PM2.5 in large cities in China is mainly caused * Corresponding author: Jianping HUANG Email: by anthropogenic emissions related to industrial production, agriculture, transportation, and activities related to daily life. Changes in urban air quality are attainable by various policy implementations, including restrictions on vehicle movement. During the 2008 Beijing Olympics, restrictions on private vehicles were implemented in Beijing, which reduced the mobile source emissions of NOx and NMVOCs by 46% and 57%, respectively (Wang et al., 2010). During the Asia-Pacific Economic Cooperation (APEC) Summit in 2014, Beijing restricted daily vehicle use, postponed the provi- © Institute of Atmospheric Physics/Chinese Academy of Sciences, and Science Press and Springer-Verlag GmbH Germany, part of Springer Nature 2022 862 HUMAN ACTIVITIES ON AIR POLLUTION sion of central heating, and closed construction sites (Liu et al., 2017); consequently, the emissions of various pollutants dropped significantly. Starting in January 2020, Chinese cities adopted a range of policies, including community lockdown and delays in resuming work and reopening schools, to curb the further spread of COVID-19. The traffic volume in densely populated cities declined significantly due to these measures. However, the specific control measures adopted during the spread of COVID-19, the 2008 Beijing Olympic Games, and the APEC meeting differ. In addition to controlling the traffic volume, the industry, construction, and coal burning were also imposed during the Beijing Olympic Games, and the APEC meeting. The city lockdown during COVID-19 generally reduced the traffic volume, while other industries may not have been suspended to a large extent. Therefore, the city lockdown caused by COVID-19 provides a valuable opportunity to assess the contribution of traffic emissions and other factors affecting PM2.5 concentrations in large cities. Overall, the PM2.5 concentration in China has dropped significantly over the last decade (Chu et al., 2021). The PM2.5 concentration in Wuhan, which had the most stringent lockdown measures, experienced the most significant declines (Yao et al., 2021). The substantial reduction in emissions in various sectors (industry, transportation, human activities, etc.) is the main reason for the decline in PM2.5. In addition, the reduction in traffic caused a significant drop in NO2 concentrations. The concentrations of PM2.5 and NO2 showed relatively consistent changes. The reduction in the NOx hinders the formation of secondary aerosols, which is conducive to reduced PM2.5 concentration (Chu et al., 2021). However, the nature of the PM2.5 concentration changes in different regions varies (Pei et al., 2020). Southern cities had a stronger controlling effect on PM2.5 than northern cities (Lu et al., 2021). For large southern cities such as Shanghai, PM2.5 concentration dropped significantly (Chen et al., 2020), while the PM2.5 concentration in the BTH region remained high (Sulaymon et al., 2021). The abnormal changes in the PM2.5 concentration in BTH are partially attributed to unfavorable weather conditions, noting that the pollutants cannot be quickly diffused or removed under conditions of high temperature, high humidity, and low wind speeds, the combination of which keeps the concentration of urban pollutants at high levels (Wang et al., 2020; Sulaymon et al., 2021). Meanwhile, heating, coal burning, and local industrial activities also impact emissions (Nichol et al., 2020; Dai et al., 2021). Therefore, the root cause of the different PM2.5 concentration changes in (...truncated)