Characteristics of air quality changes in Quanzhou during the COVID-19 epidemic

: On January 8, 2020, there was an outbreak of coronavirus disease 19 (COVID-2019), and on January 24, 2020, Fujian Province issued the "Fujian Provincial Emergency Plan for Public Health Emergencies", which restricted crowd gathering activities such as markets, greatly reducing transportation and industry. The difference between air quality before and after the epidemic was studied, and the results show that the mass concentrations of SO 2 , NO 2 , CO, PM 10 , and PM 2.5 in Quanzhou City in February 2020 all decreased in January, with decline rates of 5.8–8.8%, 34.7–46.7%, 14.9–25.1%, 9.4–15.9%, and 5.4– 14.3%; O 3 did not show the same change pattern. NO 2 , CO, O 3 , PM 10 , and PM 2.5 decreased compared with the same period in 2018 and 2019, of which SO 2 and NO 2 declined the most, and CO and O 3 declined the least. The spatial distribution map of air quality in Quanzhou City in the first half of 2020 was obtained by the Arcgis anti-distance weighting method, and the results showed that the Qingyuanshan area had the best air quality in Quanzhou and was the least affected by the epidemic; the air quality in Tushan Street and Jintoupu changed the most, showing the most obvious impact of the epidemic. The changes in air quality in Quanzhou during the epidemic period show that traffic and industrial pollution sources were more likely to be affected by the epidemic, and the sources of domestic pollution were relatively stable.


Introduction
In order to stop the spread of coronavirus disease 2019 (COVID-19), several cities in China took strict measures to control transportation, gathering of people, and industrial and commercial activities. The significant decline in transportation and industrial and commercial activities directly reduced emissions from anthropogenic air pollution sources, resulting in significant improvements in air quality. Compared with the first quarter of 2019, China's carbon emissions in the first quarter of 2020 decreased by 9.8% points, with the largest decrease in the transportation sector reaching 43.4%, and the PM 2.5 concentration decreased by 12.6% year-on-year [1] . A significant reduction in the emissions of transportation and industrial pollutants directly affects the composition of the atmosphere. Some scholars have found that during the 2020 epidemic, the average air quality in cities nationwide significantly improved, with Air Quality Index(AQI) and PM 2.5 decreasing by 36% and 48%, respectively [2] . Compared to the period when there was no epidemic, the reduction in pollutant emissions during the epidemic period in Xianyang City resulted in 19.3%, 26.0%, 13.4%, 60.1%, and 9.1% decreases in the concentrations of PM 2.5 , PM 10 , SO 2 , NO 2 , and CO, respectively. During the epidemic control period, pollutant emissions generated by enterprises and residents in Xuzhou City were significantly reduced. After epidemic control, the air quality compliance rate increased to 36.3 percentage points. While there was an increase in the concentration of O 3 -8h, the remaining six pollutants decreased by varying degrees [4] . During the outbreak, air quality changed throughout the country.
By analysing the changes in six types of air pollutants in the urban area of Quanzhou before and after the implementation of COVID-19 control measures and making comparisons with the same period in previous years, this study explored the impact of COVID-19 on the concentration of air pollutants and the correlation between air pollutants and time, space, and meteorological factors. The aim was to provide a scientific basis for screening and environmental management of key pollution sources in Quanzhou City.

Study area
Quanzhou City is located at 117°25'-119°05'E, 24°30'-25°56'N, and is 153 km wide from east to west and 157 km long from south to north. The total land area of the city is 11015 m 2 [16] , located in the southeast coastal area of Fujian Province, on the west bank of the Taiwan Strait. It is adjacent to Xiamen, Jinjiang, and Nan'an. Quanzhou is a modern industrial and trade port city, one of the first national historical and cultural cities, the first East Asian cultural capital, and a World Heritage Site. In 2020, the city's gross regional product (GDP) reached 1015.866 billion yuan, an increase of 2.9% over the previous year based on comparable prices. The total economic output remained the highest in the province for 22 consecutive years, with Quanzhou's industrial investment added value of 512.069 billion yuan and the contribution rate of industry to economic growth reaching 53.5%. The geotectonic structure of Quanzhou City is located southeast of the fold system in South China and in the central and southern sections of the volcanic fault depression belt in eastern Fujian. It is located at a low latitude, faces the ocean to the east, and has a subtropical marine monsoon climate.
COVID-19 broke out in early 2020 and peaked from February to April. Hence, the first half of 2020 was divided into six one-month time periods, and the pollutant changes in each month were analysed. Using the same method, the pollutant concentration changes in 2018 and the first half of 2019 were used as air quality benchmarks.

Data sources and research methods
The concentration data of air pollutants in Quanzhou City were obtained from China Environmental Monitoring Station (http://www.cnemc.cn/) air quality release data; meteorological data of temperature (T), humidity (RH), air pressure (p), wind direction (WD), wind speed (WS), rainfall (RF), and visibility (Vis) were obtained from the National Meteorological Science Data Center (http://data.cma.cn/). The air pollutant and meteorological data regarding the 2018-2020 SO 2 , NO 2 , O 3 , CO, PM 10 , and PM 2.5 were analysed using SPSS statistical analysis software. Data of six types of pollutants were analysed. Simultaneously, the inverse distance weight method of the grid analysis bar in the Arctoolbox toolbox in Arcgis was used, and the AQI was interpolated from January to June 2020 to obtain the spatial and temporal changes in air quality in Quanzhou City during the epidemic period in 2020.

Changes in SO2 and NO2 concentrations in the air before and after the outbreak
Fujian Province launched a first-level response on 24 January 2020 to control the epidemic. Traffic travel was first restricted during the epidemic control process, and vehicle travel was significantly reduced. Motor vehicles are important contributors to atmospheric SO 2 and NO 2 . A reduction in traffic pollution sources directly reduces the contribution rates of SO 2 and NO 2 to the atmosphere. The urban area of Quanzhou contains the Beifeng Industrial Zone, Xiyuan Industrial Zone, Xiayang Industrial Zone, Qunshi Industrial Zone, Chengzhou Industrial Zone, Renfeng Industrial Zone, Houmao Industrial Zone, Chengzhou Industrial Zone, Baogai Industrial Zone, Xunmei Industrial Zone, and Nanpu Industrial Zone, which are the main sources of industrial pollution in the urban area of Quanzhou. After the pandemic, various nonessential factories ceased to operate. The shutdown of factories directly reduced atmospheric pollution sources, and the contribution of industrial pollution sources to NOx is even greater than that of traffic pollution sources. In 2000, the total national NOx emissions were 12.1 × 10 6 t, of which the contribution rates of power plants, industry, and road mobile sources were 38%, 26%, and 25%, respectively. By 2005, the total NOx emissions rose to 19.1 × 10 6 t, with an annual growth rate of approximately 10%. The contribution rates of power plants, industries, and road-moving sources were 43%, 29%, and 20%, respectively.
The average concentration of SO 2 and NO 2 in Quanzhou City during the first half of 2018 to 2020 is shown in Figure 1. . It can be seen that during the same period of 2019, when epidemic control was not implemented, the average concentration of SO 2 did not significantly decrease compared to 2018, and even more than half of the decrease showed a negative growth trend. During the same period in 2020, after the implementation of epidemic control measures, the average concentration of SO 2 decreased significantly in 2018, with a maximum reduction of 66.7%, and there was no negative increase in the decrease. During the period from January to June 2020, the average concentration of SO 2 in February was the lowest. February was the most severe period for epidemic prevention and control. At this time, the traffic and crowd flows reached a minimum, and the SO 2 emission rate also reached a minimum; therefore, the average concentration of SO 2 was reduced to a minimum.
Meanwhile, compared to the first half of 2018, the average NO 2 concentration in Tushan Street in the first half of 2020 decreased by 14.5-35.5%. The average concentration of NO 2 in Jintoupu decreased by 20.5-30.5%. The average concentration of Wan'an NO 2 decreased by 1-61.0%. The average NO 2 concentration in Qingyuan Mountain decreased by 34.0-57.9%. In the first half of 2019, the average NO 2 concentration in Tushan Street decreased by 0.3-20.5%. The average concentration of NO 2 in Jintoupu decreased by 16.2% to 5.9%. The average concentration of Wan'an NO 2 decreased by 52.3% to 17.3%. The average NO 2 concentration in Qingyuan Mountain decreased by 6.6-52.1%. Compared to SO 2 , the average concentration of NO 2 in 2020 had no significant overall change compared to the average concentration in 2018. However, in February and March, the average concentration of NO 2 showed a significant downward trend, with a significant decrease in February compared to January. Among the four monitoring points, the maximum decrease was 46.7%, and the minimum was 34.9%. In February and March, when epidemic prevention and control were most stringent, various factories were shut down, thereby reducing the contribution rate of atmospheric NO 2 . Therefore, the average NO 2 concentration showed a downward trend compared with the same period in February and March 2018. During the same period in 2019, no control was implemented, and there was no significant change in the average concentration of NO 2 compared to that in 2018. After February, the average concentration of NO 2 increased, and the main reason for the decrease and increase was that some factories were shut down during the Spring Festival; afterwards, people returned to the city for work, and the factory officially resumed operation.
It can be seen that after the implementation of epidemic prevention and control measures, the reduction of travel rates and the shutdown of factories have an important impact on traffic and industrial pollution.

Changes in concentrations of CO, O3, PM10
, and PM2.5 in the air before and after the outbreak CO in the atmosphere originates from natural and anthropogenic sources, including methane decomposition, ocean emissions, forest and grassland fires, and volcanic emissions. Anthropogenic sources include emissions from internal combustion engine machinery and the incomplete combustion of various carbonaceous substances in industrial production [7] . As shown in Figure 2, the average concentration of CO in 2020 was lower than those in 2018 and 2019. The characteristic change in its concentration was similar to those of SO 2 and NO 2 , with a significant decrease in February. Compared to January, the decrease was between 14.9% and 21.1% but lower than the change in SO 2 and NO 2 during the same period. There are more natural sources of CO than human sources, and the change in CO was less affected by the epidemic.
There are two main types of ozone production in the atmosphere: the direct input of ozone into the recipient region, known as the direct ozone contribution of the region, and the photochemical reaction of the input precursor pollutants (NO X and VOC S ) to generate ozone, known as the regional ozone precursor contribution [8] . The pollution of O 3 is mainly caused by meteorological factors such as high temperature, low humidity, low WS, and high concentrations of precursor pollutant emissions [9] . As shown in Figure 2, during the 2020 pandemic, the average concentration of O 3 decreased to a certain extent; however, in February, the average concentration of O 3 increased compared to that in January, with an increase of 18.0% to 25.3%. This is because the production of O 3 is affected by NO X and VOC S . The significant reduction in NO X emissions during the epidemic period and the corresponding reduction in the titrating effect on O 3 (NO+O 3 → NO 2 +O 2 ) may reduce the consumption of O 3 and increase the O 3 content in the atmosphere [10] . The overall decrease may be due to a reduction in VOC S emissions during the epidemic period, resulting in a reduction in the O 3 produced by photochemical reactions.
As shown in Figure 2, in February 2020, the average concentration of PM 2.5 at the four monitoring points decreased slightly compared to January, with a decrease of 5.4% to 14.3%. The average concentration of PM 10 at Tushan Street, Jintoupu, and Wan'an monitoring points also decreased slightly, with a decrease of 9.5% to 15.9%. Compared with January, the average concentration of PM 10 in Qingyuan Mountain increased slightly, with an increase of 6.8%; however, its value in February was still lower than that at the other three monitoring points. The reason for the increase in the average concentration of PM 10 in Qingyuan Mountain may be that it is a scenic area with no residents or factories and was therefore only slightly affected by the epidemic. Therefore, the average concentration of PM 10 increased slightly but was lower than the average concentration of PM 10 in February 2018 and 2019, and the change was still within the normal range. In April, prevention and control measures were lifted, and the resumption of work and production began. At this time, the average concentration of PM 10 reached its highest value throughout the year and then began to decline continuously, whereas the average concentration of PM 2.5 showed a downward trend. This occurred because PM 10 and PM 2.5 are affected by seasonal changes. The high mass concentration of PM 2.5 in autumn and winter was mainly due to weak solar radiation, a lower boundary layer, frequent calm winds, and difficulty in the diffusion of pollutants. At the same time, less rainfall in autumn and winter is conducive to the accumulation of pollutants and formation of secondary aerosols. In spring and summer, the solar radiation is relatively strong, and the boundary layer thickness is higher. In addition, rainfall scavenges pollutants, resulting in lower concentrations of PM 2.5 [11] . Overall, the average concentrations of PM 10 and PM 2.5 in 2020 were lower than those in 2018 and 2019, and their change characteristics were similar to those of SO 2 and NO 2 ; however, their decline was not as significant as that of primary pollutants such as SO 2 and NO 2 . The reason for this difference in concentration variation may be related to the conversion of gaseous precursors of PM 2.5 and PM 10 . The contribution rate of conventional particulate pollution sources such as fugitive dust, coal smoke dust, and construction cement dust to PM 2.5 is significantly lower than that of PM 10 , whereas motor vehicle exhaust dust, sulphate, nitrate, and fuel dust are significantly higher [12] . It can be seen that PM 10 and PM 2.5 not only have direct sources in the environment but also are converted into gaseous precursors such as SO 2 and NO 2 . Multiple sources contribute to the insensitivity of PM 10 and PM 2.5 to epidemic control. (c) and (f) 2020

Impact analysis of meteorological factors
During the first half of 2020, compared to the same period in 2019 and 2018, the average concentrations of SO 2 and NO 2 decreased significantly, while the average concentrations of CO, O 3 , PM10, and PM2.5 decreased to a certain extent. In addition to reducing emissions from pollution sources, meteorological factors also have a significant impact on pollutant concentrations. In the atmosphere of the Quanzhou urban area, the concentrations of PM 10 and PM 2.5 , with significant seasonal variation, were significantly higher in spring and winter than in summer and autumn. It has been speculated that they are significantly affected by meteorological conditions [15] . Figure 3 shows the meteorological data, such as temperature, humidity, and rainfall, from 2018 to 2020. It can be seen that the rainfall in 2020 is significantly higher than that in 2018 and 2019. The average rainfall in Quanzhou in 2020 was 211.48 mm, while the rainfall in 2018 and 2019 was 94.55 mm and 121.3 mm, respectively. Studies have shown that moderate and strong rainfall has significant removal effects on SO 2 , PM 10 , and PM 2.5 in the atmosphere, with removal efficiencies of 14.3% to 50.0%, 20.2% to 68.8%, and 20.0% to 74.0%, respectively [13] . After the P4 period in 2020, work and production resumed, epidemic prevention and control were lifted, and residents returned to normal life. However, the overall average concentrations of NO 2 , SO 2 , PM 10 , and PM 2.5 , during the P5 and P6 periods in 2020 were still lower than those in 2019 and 2018. This situation may have been caused by higher precipitation dates and amounts in 2020 than in 2018 and 2019.

Spatial distribution characteristics of air quality in Quanzhou City
Using the data from four monitoring points in Quanzhou City, such as Tushan Street, Jintoupu, Wan'an, and Qingyuan Mountain, and using the inverse distance weighting method in Arcgis, the spatiotemporal distribution of air quality in the first half of 2020 in Quanzhou City was obtained, which was used to explore the changes in air quality and the distribution characteristics of heavily polluted areas ( Figure 4). From January to June, the pollution was mostly concentrated in the Tushan Street and Jintoupu areas. Tushan Street is located in Licheng District, whereas Jintoupu is located in Fengze District. Both of these places are located in the downtown area, with numerous residential and living areas nearby. There are multiple transportation trunk lines running through them, resulting in a huge source of pollutant emissions. The air quality of Qingyuan Mountain was always optimal. On the one hand, Qingyuan Mountain is a scenic area with no residents, no factories, and fewer transportation routes, resulting in lower pollutant emissions. On the other hand, Qingyuan Mountain has a high forest coverage rate of over 80%, with many shrubs and trees. The surface characteristics and wettability of plant leaves allow them to attach to and adhere to a large number of air particles and have a certain absorption and purification ability for gaseous pollutants such as sulphur dioxide and hydrogen fluoride, as well as heavy metals such as lead and cadmium [14] . They help purify the original pollutants from scenic areas, reduce the concentration of pollutants, and improve air quality.
Epidemic control was conducted in February and March after the outbreak. The figure shows that the overall air quality improved significantly, with the AQI decreasing from 50 to 37 in January, 35 to 30 in February, and 41 to 36 in March. During February, Wan'an had the best air quality, which may be due to its proximity to the bay, which is conducive to the spread of gaseous pollutants under the influence of sea winds. In April, when the prevention and control measures were lifted, the air quality index increased slightly, but its value was lower than that in January. This may be due to the sudden increase in personnel flow and business opening after the closure, leading to an increase in pollutant emissions in a short period of time; however, the level of social activity was still insufficient before the outbreak of the epidemic, so the air quality was slightly better than that in January. As the epidemic gradually eased, crowd activity and circulation became more frequent and gradually returned to normal. After April, the air quality index gradually increased, reaching a pre-outbreak value in June. The changes in the air quality index in Quanzhou City from January to June complied with the epidemic control measures, clearly indicating that epidemic control has a direct correlation with changes in air quality.  (1) Owing to the impact of epidemic control in 2020, the air quality in the Quanzhou urban area has significantly changed. There was a significant decrease in SO 2 , with an average decrease of 48.9% compared to 2018, whereas the average decrease in NO 2 was only 10.8%. The slight decrease in the concentrations of pollutants such as PM 10 , PM 2.5 , and CO indicates that traffic pollution sources are the most sensitive to epidemic control, whereas large industrial pollution sources such as power and steel plants are less affected by the epidemic. In addition to being affected by emissions from pollution sources, O 3 is also affected by meteorological conditions. Although there are changes in the concentration during epidemic prevention and control, it does not decrease with strict epidemic prevention and control measures; the increase in concentration after the release of epidemic prevention and control measures indicates that O 3 pollution control is complex and involves multiple factors.

Conclusion
(2) During the epidemic prevention and control period, the changes in air quality in different areas of Quanzhou City varied. There are no residential areas, industrial areas, or transportation trunk lines on Qingyuan Mountain, which has the lowest pollution source emissions, numerous internal vegetation, and strong air pollutant purification efficiency. Therefore, the air quality was the best among the four monitoring points and was less affected by the epidemic, without significant changes. Wan'an is located at the mouth of the Luoyang River, close to the estuarine wetland ecological protection area, with open terrain and high WS, which is conducive to the diffusion of pollutants. The air quality is second only to that of the Qingyuan Mountains. Tushan Street and Jintoupu are located in a central urban area with numerous sources of traffic and domestic pollution, the worst air quality, the greatest impact of the epidemic, and the largest fluctuations in air quality. Therefore, the control of air pollution in the Quanzhou urban area should focus on the urban areas.

Fund project:
Natural Science Foundation of Fujian Province (2021J01973); Doctoral Research Launch Project of Quanzhou Normal University (H21007).