key: cord-0717452-japwd7mx
authors: Yao, Lan; Li, Weiyue; Du, Yi
title: Impact of COVID-19 on air quality in the Yangtze River Delta, China
date: 2021-07-27
journal: Environ Monit Assess
DOI: 10.1007/s10661-021-09342-1
sha: 1dedfa6f05314870d53f9f63b4ae9c1acca62cd5
doc_id: 717452
cord_uid: japwd7mx

To investigate the effect of nationwide restrictions due to COVID-19 on air quality in the Yangtze River Delta (YRD), China, we defined four periods named period I (January 1 to 23, 2020), period II (January 24 to February 23), period III (February 24 to April 7), and period IV (April 8 to May 31), which indicated normal period, lockdown period, regional work resumption period, and nationwide work resumption period, respectively. Hourly PM(2.5), PM(10), NO(2), SO(2), CO, and O(3) in 41 cities in the YRD region were analyzed. Compared to period I, NO(2) decreased by 58% during period II and increased in periods III and IV. SO(2) remained constant during the four periods (7–8 μg/m(3)). Higher PM(2.5) concentration was monitored during period II (41 μg/m(3)) when compared to period III (35 μg/m(3)), which was resulted from the enhanced secondary formation. Spatial distribution analysis further indicated that PM(2.5) in the northern YRD during period II was higher than that during period III, whereas PM(2.5) in the southern YRD in the period II was similar to that in period III. The results demonstrated that PM(2.5) shows a nonlinear response to the reduction of its precursors, and this phenomenon varies in different areas. Compared to periods I (36 μg/m(3)) and III (64 μg/m(3)), relatively higher O(3) during period II (64 μg/m(3)) was probably resulted from less NO emission and hence weakened NO titration effect. The study suggested that coordinated and balanced measures are needed to improve air quality. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s10661-021-09342-1.

Page 2 of 13 2020). To control the movement of its population to avoid infection after the 2020 Festival, China implemented nationwide restrictions. For example, the traditional 7-day festival was extended to 18 days (from January 24 to February 10). Besides, 14 days of quarantine was required after people return to their working and living cities and schools closed. Thus, COVID-19 caused large-scale and prolonged shutdowns in urban areas, leading to a large reduction of the primary emission. The effectiveness of anthropogenic emission reduction on air quality has been a concerning issue for researchers. Previous studies investigated the role of short-term pollutant emission control measures in air quality improvement during the 2008 Beijing Olympics (8-24 August, Wang et al., 2010; Witte et al., 2009) , the 2010 Guangzhou Asian Games (12-27 November, Liu et al., 2013; Tao et al., 2015; Yao et al., 2013) , the 2014 Asia-Pacific Economic Cooperation (APEC) meeting (5-11 November, Guo et al., 2016; Tang et al., 2015; Xu et al., 2019) , and the 7-day festival (Huang et al., 2012; Jiang et al., 2015; Yao et al., 2019) . The COVID-19 provided a unique chance to test the sensitivity of primary emission reduction on air quality Zhang et al., 2020; Zheng et al., 2020) . Zheng et al. investigated the variations in chemical compositions, source contributions, and regional transport of PM 2.5 from January 23 to February 22, 2020, compared with the same period in 2019 . Zhang et al. reported NO x emission reduction and recovery during COVID-19 in East China from January 1 to March 12, 2020 . Huang et al. investigated the haze formation mechanism in the case of a reduction of primary emissions in East China from January to February 2020 . As the COVID-19 infection continues, studies of its impact on air quality on longer time scales are desirable.

The YRD region is severely affected by the epidemic. For example, the Shanghai government listed Hubei Province and another 19 cities as severely affected areas in China, among which 9 cities are located in the YRD region (https:// www. shaim eiba. com/ baike/ sheng huo/ 18068. html). This study investigated the effects of COVID-19 on air quality based on 5 months of hourly concentration of PM 2.5 , PM 10 , NO 2 , SO 2 , CO, and O 3 in 41 cities in the YRD region. Spatial-temporal variation and daily pattern of air pollutants and comparison of changes in 2019 were discussed.

Hourly concentrations of PM 2.5 , PM 10 , NO 2 , SO 2 , CO, and O 3 in 41 cities of the YRD from January 1, 2020 to May 31, 2020 were obtained from the National Urban Air Quality Real-Time Publishing Platform (http:// 106. 37. 208. 233: 20035/) of China National Environmental Monitoring Centre. Each city was equipped with several air quality automatic monitoring stations (urban assessing stations). Some cities with special geographical location were additionally equipped with one regional assessing stations (marked as Duizhaodian on the Platform). Hourly concentrations of pollutants in each city were averaged from all the monitoring stations except the regional assessing stations (Duizhaodian). Hourly mean concentrations of pollutants of YRD region were averaged from those of the 41 cities. The 41 cities include 13 cities in Jiangsu Province, 11 cities in Zhejiang Province, 16 cities in Anhui Province, and Shanghai.

To investigate the effects of reduced anthropogenic activities as COVID-19 on air quality, we divided the whole study period as four specific periods based on three important time nodes. On January 23, Wuhan, the first place of the outbreak of coronavirus, declared unprecedented traffic restrictions, including suspending the city's public transport and all outbound flights and trains. This event marked the beginning of lockdown in China. Shanghai, the leading city of the YRD region, announced that work resumption should not be earlier than February 9. It needs to be emphasized that most people have left the city before January 23 to return to their hometown for the Chinese Spring Festival (started from January 24), called as "Chunyun." Home quarantine is required for 14 days after they return to Shanghai before work resumption. Thus, the lockdown period lasted from January 24 to February 23. From February 24, massive resumption of work and production in urban areas were started in the YRD region. On 00:00, April 8, Wuhan started lifting outbound travel restrictions, which indicated ending lockdown all over China. Thus, the four periods Page 3 of 13 523 were from January 1 to 23, January 24 to February 23, February 24 to April 7, and April 8 to May 31, which were sequentially numbered as I, II, III, and IV, respectively, as seen in Fig. 1 .

In period I, a few daily cases were reported and scarce containment measure was taken, which was considered as a normal period. On the contrary, the COVID-19 outbreak occurred during period II, and the most rigorous control measurements were implemented to reduce population mobility. In period III, almost no locally transmitted infections were recorded and the reported cases were sourced from virus carriers traveling from abroad. People started to resume work and normal life in YRD during this period. The domestic coronavirus is effectively under control in China during period IV, and this period represents a nationwide resumption of work and production. In summary, period I, period II, period III, and period IV represent prelockdown period, lockdown period, period of ending lockdown on urban scale, and period of ending lockdown on a national scale, respectively.

The original meteorological data is from the ERA5 reanalysis datasets (ERA5 hourly data on single levels from 1979 to present), which is provided by the European Centre for Medium-Range Weather Forecasts (ECMWF). Variables including precipitation, total column water vapor, 10-m u-component of wind, and 10-m v-component of wind were used in the present work, with horizontal resolution of 0.25° × 0.25°. The ERA5 atmospheric reanalysis was reported to have improved performance (Graham et al., 2019) .

The average concentrations of air pollutants in YRD during the four periods are depicted in Fig. 2 . NO 2 , a primary pollutant from vehicle emission, presented the highest concentration during period I and significantly decreased during period II and then increased during periods III and IV, with concentrations of 39, 16, 29, and 26 μg/m 3 during the four sequential periods, respectively. Independent-sample T test was applied to exam the significant difference in the mean value of NO 2 during the four periods. Results show that there are significant differences in the average NO 2 concentrations during period I, period II, and period III (both P < 0.01), and no significant difference in periods III and IV (P > 0.05). Compared to periods I and III, NO 2 decreased by 58% during period II and increased by 81% during period III. However, PM 2.5 , mainly formed from secondary formation, did not exhibit such a variation trend. Concentrations of PM 2.5 during the four periods were 65, 41, 35, and 33 μg/m 3 , respectively. T test indicated that PM 2.5 concentration in period II was significantly (P < 0.01) higher than that in periods III and IV. Despite lower primary emission of pollutants in period II, higher PM 2.5 indicated enhanced secondary formation (e.g. sulfate and nitrate) during period II, which was agreed well with the previous study (Chang et al., 2020; Huang et al., 2020) . Huang et al.

(2020) measured major compositions of PM 2.5 (e.g., sulfate, nitrate, ammonium, and organic matter) during the COVID-2019 lockdown and provided evidence that the heavy haze pollution in East China during the lockdown was triggered by enhancements of secondary pollution. Chang et al. (2020) observed fast formation of secondary inorganic (mostly nitrate) aerosols in Shanghai during the 2020 Chinese Spring Festival (lockdown period) and highlighted that regional transport facilitated nitrate formation. Unlike NO 2 , SO 2 , a primary pollutant mainly from industrial emission, exhibited no significant variation, with concentrations of 8, 7, 7, and 8 μg/m 3 during the four periods. The latest study focusing on severe haze events characterized by high PM 2.5 concentration in China during COVID-19 outbreak proposed that large emission reduction in transportation and a slight reduction in industrial would not help avoid severe air pollution in China . PM 10 displayed a similar variation pattern to NO 2 , with concentrations of 79, 51, 59, and 61 μg/m 3 during periods I, II, III, and IV, respectively. Mass ratio of PM 2.5 /PM 10 during the four periods was 82%, 81%, 61%, and 55%, respectively. A higher PM 2.5 /PM 10 ratio in period II was consistent with the enhanced secondary formation during this period. CO concentrations were 0.952, 0.682, 0.633, and 0.625 mg/m 3 . Notably, O 3 exhibited a different variation trend from gaseous pollutants and particulate matters. Concentrations of O 3 were 36, 64, 64, and 93 μg/m 3 during the four periods. Compared to period I, O 3 increased by 77.8% during period II (64 μg/m 3 ). Huang et al. (2020) explained that large decreases in NO x emissions increased O 3 and nighttime NO 3 radical formation, leading to increased atmospheric oxidizing capacity, which further facilitated the formation of secondary particulate matter.

Spatial-temporal variation Figure 3 shows the daily variation of NO 2 , PM 2.5 , PM 10 , SO 2 , CO, and O 3 in YRD during the four periods. Hourly NO 2 displayed a bimodal distribution during the four periods, with a morning peak and a nighttime peak. NO 2 at the nighttime peak (47 μg/m 3 ) was higher than that at the morning peak (40 μg/m 3 ) before the COVID-19 outbreak (period I), whereas NO 2 concentrations at the morning peak and nighttime peak were similar after the COVID-19 outbreak (periods II, III, and IV). PM 2.5 and PM 10 presented a bimodal distribution during the four periods. Concentrations of PM 2.5 and PM 10 at the nighttime peak were higher than that at the morning peak during period II. The gap between nighttime PM 2.5 concentration during period II and period III was greater than that in the daytime. However, PM 2.5 /PM 10 ratios in the day (07:00-18:00, 86%, 86%, 62%, and 58%, respectively) were higher than that at night (19:00-06:00, 81%, 76%, 57%, and 49%, respectively) during the four periods, indicating stronger secondary formation in daytime. SO 2 exhibited a unimodal distribution during the four periods. However, the peak hour when SO 2 showing the highest concentration differed. Compared to period II, the peak hour of SO 2 came earlier during periods III and IV. The results agreed well with the previous study conducted in Wuhan that the lockdown due to the COVID-19 outbreak modified the diurnal variation patterns of PM 2.5 sources . CO presented a similar daily variation to PM 2.5 , with relatively higher concentration during period II, compared to periods III and IV. O 3 exhibited similar daily variation during the four the periods, with a large peak at 15:00. The average O 3 concentrations during periods II and III were similar. However, nighttime O 3 during period II was higher than that in period III. A considerable level of nocturnal O 3 favors nocturnal NO 3 radical formation during period II (Sun et al., 2018) . Spatial variation of NO 2 , PM 2.5 , PM 10 , SO 2 , CO, and O 3 in YRD during the four periods is depicted in Fig. 4 . Nanjing, Hangzhou, and Hefei were provincial capital cities of Anhui, Jiangsu, and Zhejiang, respectively. Higher concentrations of NO 2 were observed in central YRD, including Shanghai, the provincial capital cities, and the adjacent cities during the four periods. In period I, NO 2 in central YRD were at the level of 40-60 μg/m 3 . NO 2 concentrations in 33 cites (80%) decreased to less than 20 μg/m 3 during period II, among which included NO 2 in the southernmost three cities in YRD dropping to less than 10 μg/m 3 . After the resumption of work and production, NO 2 in the central YRD increased to 30-40 μg/m 3 . However, PM 2.5 exhibited a very different spatial distribution with NO 2 in the YRD. The highest average PM 2.5 concentrations (110-120 μg/m 3 ) were observed in the northern YRD, and PM 2.5 concentrations in the YRD shows a decreasing trend from north to south during period I. Higher PM 2.5 concentration was also measured in the northern part of the YRD during the period II, despite lower NO 2 in this area. PM 2.5 in northern YRD during period II was higher than that in periods III and IV, whereas PM 2.5 concentrations in southern YRD during periods II, III, and IV were similar. The spatial distribution of PM 10 in the YRD was similar to PM 2.5 , with a higher concentration in the northern YRD than that in the southern YRD during the four periods. However, increased PM 10 in the southern YRD during periods III and IV was observed when compared to that in period II, which was different from PM 2.5 . The difference in the spatial and temporal distribution of PM 2.5 and PM 10 indicated that road dust and construction dust are the main sources of PM 10 in urban YRD.

Concentrations of SO 2 in the YRD displayed no significant difference in spatial variation during the four periods. SO 2 in most cities in the YRD were less than 10 μg/m 3 , with SO 2 of a few cities ranging from 10 to 20 μg/m 3 . This SO 2 level was still higher when compared to Barcelona, Spain (1.0-2.6 μg/m 3 ), and lower than that in India (10-30 μg/m 3 ) during the COVID-19 epidemic (Sharma et al., 2020; Singh & Chauhan, 2020; Tobias et al., 2020) . CO exhibited similar spatial variation to PM 2.5 during the periods I and II, with a higher concentration in the northern YRD and lower concentration in the southern YRD.

Unlike NO 2 , O 3 in the YRD shows no significant difference in spatial distribution during periods I, II, and III. O 3 in the northern YRD seems higher than that in the southern YRD during the period IV. According to media reports (https:// www. sohu. com/a/ 37074 6933_ 440288), Wenzhou, the southernmost city in the YRD, became the second worst-hit area next to Hubei Province, of which the capital city is Wuhan. Thus, NO 2 concentration in Wenzhou was very low (7 μg/m 3 ) during the period II due to the most rigorous control measures, whereas O 3 concentration (70 μg/m 3 ) in Wenzhou during the period II was at a higher level (Fig. 4) . A similar O 3 increase was reported in Milan, Italy, during the lockdown (Collivignarelli et al., 2020) . The elevated O 3 concentration during the lockdown was probably due to less NO emission and the weakened NO titration effect (Tan et al., 2009) .

Period I represents a normal period, and period II represents the most rigorous control period due to the novel coronavirus. Percent changes (a positive value means an increase and a negative value means decrease) of the air pollutants in the YRD during period II when compared to the period I are mapped in Fig. 5 . NO 2 in the most YRD cities decreased by more than 50% (average 58.2%) during the period II and NO 2 in the southern YRD even reduced up to 70-80%. PM 2.5 and PM 10 in the most YRD cities decreased by 30-40% during period II, with the average proportion of 37% and 35%, respectively. Averagely, the proportion of O 3 increase in the YRD was 81% during period II and that was 101-155% in the provincial capital cities. Analysis of meteorological conditions Time series of daily average of precipitation, total column water vapor, and wind speed in the YRD region during the study period in 2020 are in Fig. S1 . It seems that precipitation was evenly distributed and wind speed was similar during the study period in 2020. Total column water vapor shows an increasing trend from January to May, 2020. Independent-sample T test was applied to exam the significant difference of meteorological parameters during the four periods. There was no significant difference (P ≫ 0.05) in precipitation between the four periods, as well as wind speed. Significant differences were obtained for the total column water vapor in the four periods, verifying an increasing trend of total column water vapor. The speed and direction of the horizontal wind at a height of 10 m above the ground surface in the YRD region during the four periods in 2020 are depicted in Fig. 6 . Wind fields during period I and period II were similar, with the prevailing wind from the north and northeast. Compared to period I and period II, the predominant wind direction changed in period III and period IV, with the prevailing wind from the east, southeast, and south.

For comparison, average concentrations of pollutants in the four periods in 2020 with that in 2019 are listed in Table 1 . Time series of hourly concentrations of Table 1 , concentrations of pollutants, except O 3 , generally decreased during the four periods in 2020 when compared to 2019. However, a larger reduction of pollutants was observed during period II, which was closely related to the strict lockdown policy in period II. For example, NO 2 during the numbered four periods in 2020 decreased by 18.2%, 45.1%, 24.8%, and 3.9%, respectively, when compared to that in 2019. Similarly, PM 10 during the four periods in 2020 decreased by 22.9%, 35.1%, 29%, and 2.8%, respectively. In contrast to other pollutants, O 3 , a typical secondary pollutant formed from photochemical reaction, presented an increasing trend during the study period in 2020, compared to 2019. Further, a larger increase in O 3 was observed in period II in 2020 (32.3%), which provided evidence that O 3 increased due to reduction of NO x emission during period II. SO 2 in period II in 2020 was similar to that in 2019. In period IV, concentrations of pollutants in 2020, except O 3 , were similar to that in 2019, which was directly associated with the intensive resumption of work and production on national scale. Figure 7 shows percentage changes in the concentrations of pollutants, electricity consumption, and highway transportation in YRD in 2020 when compared to the corresponding monthly average in 2019. The monthly average concentrations of air pollutants in YRD were downloaded from Yangtze River Delta regional air quality forecast business platform. Compared to 2019, the six pollutants indicating air quality in the YRD decreased in 2020 except O 3 . For temporal comparison, a larger reduction of pollutants in the YRD was observed in the first three months of 2020, especially in February. For example, NO 2 , PM 2.5 , PM 10 , SO 2 , and CO decreased by 37%, 38%, 34%, 19%, and 26% in February, 2020, respectively. Similar results were reported in Wuhan in central China, which was attributed to emission reduction (92.0%) rather than meteorological conditions during the lockdown . O 3 (8-h maximum) in the YRD presented an increasing trend in 2020 when compared to 2019. O 3 (8-h maximum) increased by 14%, 17%, and 18% in January, February, and April, respectively.

Seventy-three percent of the total freight transport in China was achieved by highway transportation in 2019. Compared to 2019, reduction (1.4-35.2%) of highway freight transportation in the YRD was recorded from January to May in 2020, particularly in February (Fig. 7b) . For example, highway freight transportation in Shanghai, Jiangsu, Zhejiang, and Anhui decreased by 27. 2%, 35.2%, 21.4%, and 19.2% in February 2020, respectively, and started to resume since March. Compared to highway freight transportation, a larger reduction of highway passenger transportation was recorded in 2020 than that in 2019. No significant resumption of highway passenger transportation was observed in the YRD from March in 2020. For example, highway passenger transportation reduced by 50.6-60.3% in February, and that was 37.9-70.8% in May 2020, compared to the same period in 2019. Total electricity consumption is closely related to industrial production because electricity consumption from industry accounts for a dominant proportion. The electricity consumption from industry accounted for 69% of the total electricity consumption in China, with service, residential, and agriculture proportion of 16%, 14%, and 1%, respectively (National Energy Administration, 2020). From Fig. 7c , compared to 2019, the total electricity consumption of Zhejiang and Anhui reduced in February in 2020, which was resulted from large reduction of electricity consumption from industry. For example, industrial electricity consumption of Zhejiang and Anhui reduced by 32.4% and 50.6%, respectively, which was related to the lockdown policy. The total electricity consumption in the YRD during the study period in 2020 slightly reduced when compared to that in 2019.

In summary, the air quality in the YRD region greatly improved during the study period in 2020 due to the countermeasures to interrupt the transmission of COVID-19 when compared with the same period in 2019. However, increasing O 3 is worthy of attention.

This lockdown in China due to the novel coronavirus outbreak provided a unique chance to test the sensitivity of primary emission reduction on air quality. Our results show that NO 2 significantly decreased and SO 2 almost remain unchanged in the YRD region during the period of the COVID-19 outbreak when the most rigorous control measures were implemented. Under the circumstances, PM 2.5 in the northern YRD areas still exhibited higher concentrations when compared to the case in which NO 2 increased, implying enhanced secondary formation. The results demonstrated that PM 2.5 shows a nonlinear response to the reduction of its precursors and this phenomenon varies from region to region. O 3 increase was also recorded during the COVID-19 outbreak, which was probably related to the weakened NO titration effect. Further researches need to probe into the nonlinear response mechanism and regulation principle of PM 2.5 , O 3 , and their precursors.

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Acknowledgements Great appreciation to Dr. Yafei Yan for