








key: cord-327628-fl8dyahe
authors: Yuan, Qi; Qi, Bing; Hu, Deyun; Wang, Junjiao; Zhang, Jian; Yang, Huanqiang; Zhang, Shanshan; Liu, Lei; Xu, Liang; Li, Weijun
title: Spatiotemporal variations and reduction of air pollutants during the COVID-19 pandemic in a megacity of Yangtze River Delta in China
date: 2020-08-20
journal: Sci Total Environ
DOI: 10.1016/j.scitotenv.2020.141820
sha: 
doc_id: 327628
cord_uid: fl8dyahe

Abstract In recent decades, air pollution has become an important environmental problem in the megacities of eastern China. How to control air pollution in megacities is still a challenging issue because of the complex pollutant sources, atmospheric chemistry, and meteorology. There is substantial uncertainty in accurately identifying the contributions of transport and local emissions to the air quality in megacities. The COVID-19 outbreak has prompted a nationwide public lockdown period and provides a valuable opportunity for understanding the sources and factors of air pollutants. The three-month period of continuous field observations for aerosol particles and gaseous pollutants, which extended from January 2020 to March 2020, covered urban, urban-industry, and suburban areas in the typical megacity of Hangzhou in the Yangtze River Delta in eastern China. In general, the concentrations of PM2.5–10, PM2.5, NOx, SO2, and CO reduced 58%, 47%, 83%, 11% and 30%, respectively, in the megacity during the COVID-Lock period. The reduction proportions of PM2.5 and CO were generally higher in urban and urban-industry areas than those in suburban areas. NOx exhibited the greatest reduction (>80%) among all the air pollutants, and the reduction was similar in the urban, urban-industry, and suburban areas. O3 increased 102%–125% during the COVID-Lock period. The daytime elevation of the planetary boundary layer height can reduce 30% of the PM10, PM2.5, NOx and CO concentrations on the ground in Hangzhou. During the long-range transport events, air pollutants on the regional scale likely contribute 40%–90% of the fine particles in the Hangzhou urban area. The findings highlight the future control and model forecasting of air pollutants in Hangzhou and similar megacities in eastern China.

. A study from the WRF-Chem model showed that long-range transport contributed 30%-85% of PM 2.5 in Hangzhou (Ni et al., 2018) . The WRF-Chem model combined with field observations showed that local emissions contributed more than 50% of PM 2.5 in Hangzhou (Shu et al., 2019) . The WRF/CMAQ model results indicated that local emission sources accounted for 15.8%, 68.6%, 48.3% and 59.2% of the overall concentrations of SO 2 , NO 2 , PM 2.5 and PM 10 , respectively, in Hangzhou . In addition to the model work, there is still a lack of field observation data for quantifying the contribution of local emissions and transport in Hangzhou (Sun et al., 2018; Zhang et al., 2020c) .

A novel coronavirus named "COVID-19" accidently appeared in Wuhan city in late December, 2019 Wu et al., 2020) and its high infectivity strongly threatened human health Xu et al., 2020) . The Chinese government took powerful timely actions to control the spread of the virus, such as the highest epidemic emergency response, community lockdown, traffic restrictions, and factory shutdowns. These effective actions stopped people from gathering and successfully reduced the risk of the novel coronavirus infection. Note that this public health emergency not only locked down people in the community but stopped the operation of the public traffic system and industry machine, which caused a short-term regional reduction of air pollutant emissions from vehicles and some industries in China (Huang et al., 2020b; Shi and Brasseur, 2020;  The strictest nationwide restrictions for preventing the COVID-19 spread greatly reduced the primary emissions and weakened the regional transport effect of air pollutants due to the large-scale regional decrease in the primary pollution (Huang et al., 2020b; Zhang et al., 2020b) . Consequently, the pandemic incident becomes a special atmospheric research hotspot due to the dramatic reduction in anthropogenic emissions (Bauwens et al., 2020; Le et al., 2020; Sun et al., 2020) . This special period offers us a unique opportunity to accurately assess how the reduction in local emissions influences the air quality in megacities .

In this study, we systematically analyzed the continuous three-month data of particulate and gaseous pollutants at one urban site, three urban-industry sites and six suburban sites in Hangzhou. To better understand the variation and reduction in air pollutants, we classified the measurement periods into pre-COVID, Chinese New

Year (CNY), COVID-19 lockdown (COVID-Lock), COVID-Recover-I, and COVID-Recover-II from 1 January, 2020 to 31 March, 2020. We found a significant reduction in PM 2.5-10 , PM 2.5 , NO x , SO 2 , CO and an increase in O 3 during the COVID-Lock period. This study preliminarily reveals the contribution of transport and local emissions in a typical urban area in the YRD. The findings provide a reference for future air pollution control in the megacities in eastern China.

1 and Table S1 ). The urban and urban-industry sites are located in the east low plain area, and the suburban sites are mostly located in the middle and west mountain areas ( Fig. 1) . The urban site is located in the city center with the largest population density and more than 2 million vehicles (Table S1 ). The urban-industry sites are located in the industry districts with a total of more than 2600 enterprises and nearly 1 million vehicles (Table S1 ). Ecotourism is one of the major domains in the suburban area due to the better ecological environment and air quality of this area. Fig. 1 shows that the major stationary emissions of air pollutants (i.e., PM 10 , PM 2.5 , NO x , and SO 2 ) in Hangzhou distribute in the urban-industry area. As a result, we determined that the air quality in the urban and urban-industry areas are worse than that in the suburban area (Table S1 ). These monitoring sites, which comprise different locations and environments, can represent the different air quality in Hangzhou city. Visibility (Vis) was detected by a visibility sensor (PW10, Vaisala, Netherlands).

Our field observations were conducted from 1 January, 2020 to 31 March, 2020. The highest epidemic emergency response in Zhejiang province began on 23 January.

CNY vacation (24 January to 3 February, 2020): There was very low traffic flow (TPI=0.13) and few industries maintained normal operation due to the novel coronavirus epidemic, but large amounts of fireworks were burned in the suburban area. According to the official bans, fireworks were only allowed in the suburban areas in megacities during the CNY. The traffic system resumed, and the average TPI was 0.25 in the urban area (Fig. S1 ).

Nearly 100% of enterprises resumed operation during this period according to the statistics of the Hangzhou Development Commission.

The epidemic emergency response in Zhejiang province was downgraded beginning on 2 March. The industry operation returned to normal, and the traffic system was further recovered (TPI=0.9).

Concentration-weighted trajectory (CWT) analysis, which is based on the backward trajectory, can be employed to identify the transport event and estimate the contribution of air pollutants from different regions . In this study, 72-hour air mass backward trajectories with one-hour resolution were calculated with an ending height of 500 m above ground level based on the meteorological data sets from the Nation Oceanic Atmospheric Administration (NOAA)

During the observation period, the daily average mass concentrations of PM 10 , concentrations of PM 2.5 , NO x , and CO at the urban site were higher than those at the urban-industry and suburban sites (Table S2) , which suggests that vehicle emissions was the major source of air pollutants in the urban area. The weather conditions were mostly stable with low wind speeds (<3 m/s) and high RH (~77%) during the observation period (Fig. 2) .

To evaluate the reduction in air pollutants during the COVID-Lock period, the temporal variations in the six air pollutants (i.e., PM 2.5-10 , PM 2.5 , NO x , SO 2 , CO, and During the COVID-Lock period, the average PM 2.5-10 mass concentrations at the urban site, urban-industry site, and suburban site were 8.5 μg/m 3 , 11.5 μg/m 3 , and 7.0 μg/m 3 , respectively, which decreased 54%, 62%, and 58%, respectively, compared with those during the pre-COVID period (Figs. 3a and S2a). Coarse particles (PM 2.5-10 )

in the megacities of eastern China are mainly dominated by man-made fugitive dust from construction activities and roads, natural dust from the ground, growth of secondary aerosol particles, and industrial activities in urban areas (Guo et al., 2020) .

A higher reduction proportion of the coarse particles occurred at the urban-industry sites than that at the urban and suburban sites, which suggests that man-made dust During the COVID-Lock period, the average SO 2 concentrations at the urban site, urban-industry site, and suburban site were 3.9 μg/m 3 , 6.2 μg/m 3 , and 4.0 μg/m 3 , respectively, which decreased 22%, 9%, and 9%, respectively, compared with those during the pre-COVID period (Figs. 3d and S2d). SO 2 control was effective on normal days, and the average reduction proportion of SO 2 was the lowest among all the air pollutants. Emissions of industrial and coal-fired power plants are the major sources of SO 2 in Zhejiang province . We noticed that the slight J o u r n a l P r e -p r o o f Journal Pre-proof increase in SO 2 concentrations occurred during the COVID-Recover-II period due to the higher production intensity than that during the pre-COVID period, which made up the production shortage during the COVID-Lock period in China. (Ding et al., 2013; Zhang et al., 2018) . In addition, the reduction of PM 2.5 is in favor of the O 3 formation ; this mechanism promotes the increase in O 3 during the COVID-Lock period.

In general, the lockdown actions during the COVID-Lock period achieved obvious positive effectiveness for the air quality improvement in Hangzhou. We

Journal Pre-proof discover that all the air pollutants, with the exception of O 3 , decreased during the COVID-Lock period and the reduction proportion of each pollutant was generally higher in urban and urban-industry areas than the suburban areas. NO x exhibited the highest reduction with a maximum decrease of ~80%. O 3 is the only air pollutant that exhibited an obvious increase more than 1 time during the COVID-Lock period.

During the COVID-Lock period, a continuously low traffic flow was maintained and few industries maintained normal production. Based on the minimum continuous local and regional emissions in the YRD during the COVID-Lock period, it is an opportunity to generally evaluate the role of the planetary boundary layer (PBL) in the local air pollutants in Hangzhou. Fig. 4 shows that PM 2.5-10 , PM 2.5 , NO x and CO all exhibited a significant decrease since sunrise between 7:00~8:00 in the morning.

Mass concentrations of these air pollutants reached the lowest values and the highest temperature occurred from 14:00~15:00 at all the observation sites ( Fig. 4a-d) . The PBL height is an important meteorological factor that affects the mixing, transport, accumulation and dilution of air pollutants Tang et al., 2007) . There is usually negative feedback between the PBL height and particle concentrations (Petäjä et al., 2016) , and a reduced PBL height is favorable for the accumulation of air pollutants in the weak turbulent diffusion conditions . The reduction rate of air pollutants by the elevation of the PBL height with an increase in temperature can be calculated via the mass concentration at sunrise (~8:00) divided by the difference between the mass concentration at sunrise (~8:00) and the minimum values at noon (12:00-14:00) (dashed line in Fig. 4a-d) .

Based on this analysis, we can estimate the air quality impacted by the elevation of the PBL height in the daytime in the urban area, urban-industry area and suburban J o u r n a l P r e -p r o o f Journal Pre-proof area: a reduction of 47%, 41%, and 41%, respectively, for PM 2.5-10 ; a reduction of 8%, 20%, and 27%, respectively, for PM 2.5 ; a reduction of 42%, 42%, and 41%, respectively, for NO x ; and a reduction of 18%, 20% and 16%, respectively, for CO ( Fig. 4e) . Overall, the daytime elevation of the PBL height can dilute approximately ~30% of PM 2.5-10 , PM 2.5 , NO x and CO concentrations on the ground in Hangzhou.

However, we observed the opposite diurnal trends of SO 2 and O 3 in Hangzhou ( Fig. S4) , and a similar diurnal pattern has been observed in Hangzhou by Ji et al. (2018) . The reason for these trends should be that power plants, as the major contributors of SO 2 emissions, mainly operated during the daytime for the electricity consumption of household and industrial activities. O 3 formation mainly occurred in the daytime due to the photochemical cycle in the atmosphere (Xue et al., 2014) .

Consequently 

In this study, a total of six transport events from the CWT analysis were identified during the observation period: two northern long-range transport events during the pre-COVID period (event 1: 1/12 13:00 to 1/15 14:00, event 2: 1/20 15:00 to 1/22

Marked reductions of PM 2.5-10 , PM 2.5 , NO x , SO 2 and CO were observed in Hangzhou during the COVID-Lock period, with an average reduction of 58%, 47%, 83%, 11% and 30%, respectively. Conversely, O 3 increased more than 1 time with the lockdown of traffic and industry. The response of air pollutants to the lockdown actions was more significant in the urban and urban-industry areas, where traffic and industry were the major sources of air pollutants. NO x was the most sensitive air pollutant in response to the reduction in the traffic and industry emissions. The PBL exhibited a more obvious dilution effect for the particle pollutants in the suburban area. We present an approximate calculation that transport contributes 40%-90% of fine particles in the urban area of Hangzhou during the long-range transport events.

Based on the three-month continuous observations, this study can serve as a general reference for future air pollution control. Cooperative control of aerosol particles, NO x and ozone will be an important issue for air quality improvement. It is important to strengthen the coordinated interregional control to reduce the transport effects.

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. 

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The authors are grateful to Ms. Jiani Xu and Ms. Yating Bao for their assistance with data collection. This work was supported by the Zhejiang Provincial Natural Science

