3.1. Impact of COVID-19 on Human Activity Observed by NTL
shows the average NTL radiance for every province from December 2019 to March 2020. During January and February 2020, COVID-19 broke out across China, and most provinces implemented strict quarantine policies, resulting in the majority of people spending their time at home. The average artificial NTL radiance levels during this period are generally lower than December 2019 in most provinces, except for Inner Mongolia, Xinjiang, Xizang and Qinghai, where the crisis is not severe, with 194, 76, 1, and 18 cases, respectively. Additionally, the NTL of these provinces prior to the outbreak is not bright compared to the other provinces. With the reopening of most provinces in March 2020, the average NTL brightness has increased in comparison to January and February. However, exceptions to this have been observed in Hubei province, with its capital city Wuhan being the center of the outbreak and several other provinces near it that contain larger numbers of confirmed cases, such as Hunan (1019 cases), Jiangxi (937 cases), and Guangdong (1585 cases).
shows NTL radiance of mainland China for the same season during 2018/2019 and 2019/2020, which are December 2018–March 2019 and December 2019–March 2020. As the blue bars show in Figure 1
, March and January 2019 exhibit overall higher radiance levels than the other two months in the control: December 2018 and February 2019. However, this has obviously changed since the outbreak of COVID-19, as the orange bars show. Because the lockdown policy was officially implemented in January 2020 and lasted almost two months, the mean radiance value in both January and February has clearly decreased. Although some recovery was observed in March 2020, the value is still lower than that of December 2019. One possible explanation is that the reopening measure has been slowly carried out at varying paces in different provinces of China since March. In addition, it is clear to see that the mean values for January, February, and March 2020 are all lower than those for 2019.
shows the differences of NOP in the three NTL categories between the first three months (January, February and March) of 2020 and December 2019 (the former minus the latter), in each province and all of China. Because of the quarantine policies conducted in most provinces, people stopped going shopping, getting together, and working in the office, which mostly takes place in commercial centers, or so-called Central Business Districts (CBDs) at night. Instead, most people stayed at home in the residential areas. These changes in human activity are reflected in the statistical results of the NTL categorization.
For the 5–20 nW·· category, indicative of residential areas, 20 provinces have higher NOP in all three of the studied months of 2020 than in December 2019. Only Xingjiang in January, and Chongqing, Heilongjiang, Jilin, Liaoning, and Xinjiang in March have decreasing numbers larger than 10,000. All the negative differences are smaller than 10,000 in February 2020, when the crisis was most rigorous. These spatiotemporal patterns are due to the stay-at-home policies and more people spending their nights at home.
For the 20–40 nW·· group, which reflects the anthropogenic NTL of transportation facilities and public infrastructures, the numbers increased in January 2020 and decreased in February 2020 and March 2020 for most provinces. The quarantine policies were implemented at the end of January 2020 and the transportation and public facilities were busier in the first three weeks than in December 2019 due to the approaching Spring Festival. The corresponding areas are therefore brighter in January 2020, on average. Negative values are found in most provinces for February 2020 and March 2020 because the virus spread rapidly, and the public and transportation infrastructures were not utilized as frequently as usual.
For the commercial-center pixels, with NTL brightness higher than 40 nW··, their numbers decreased significantly during all three months, compared to before in most provinces. People tended to avoid going to shopping and entertainment centers due to the quarantine policies and fear of getting infected. Although most cities have reopened and loosened their restrictive policies, people are still cautious to gather in crowds, and most shopping malls are closing earlier in the evenings. There are positive values in Henan, Shaanxi, and Tianjin but the increases are all under 500 pixels, which can be due to differences in seasonal variations, sensor’s condition, or snowfall.
We further investigate the NTL of Wuhan City and the results are shown in Figure 5
. Figure 5
a,b are the NTL images of Wuhan before and during the lockdown, and Figure 5
c is the difference between Figure 5
a,b. The regions in the yellow and green circles are the Jianghan and Guanggu commercial centers, respectively, which are two of the most prosperous and crowded areas in Wuhan. We can observe that these regions are dimmer during the lockdown, in February 2020, compared to December 2019. The differences also show mostly negative values in these two circles. Contrastingly, residential areas outside of the commercial centers are brighter in January, and the differences are obviously positive in those regions. However, the pixels on highways and main roads are not clearly getting brighter or dimmer, they tend to show a mixed texture. This phenomenon can also be verified in the difference map (Figure 5
d is the NOP difference in the three categories between the first three months of 2020 and December 2019 (the former minus latter). For all the months during the city lockdown, there are significantly more pixels in the residential category and fewer pixels in the commercial centers than before. The number of transportation and public facilities lights decreased in January and March but increased in February. Besides the explanations that have already been discussed, in February, large amounts of food, living, and medical resources were transported to Wuhan from all over China and some other countries around the world; national and private medical teams volunteered to support and traveled to Wuhan through highways and airports; temporary hospitals such as Huoshenshan and Leishenshan were built rapidly. Consequently, the NOP in this category was higher during February than December.
3.2. Impact of COVID-19 on Air Quality
The main air pollution sources in China are power plants, industrial facilities, automobiles, biomass burning, and fossil fuels used in homes and factories for heating [32
]. Figure 6
is the daily average AQI time series of China from 1 January 2020 to 26 March 2020. A significant decreasing trend can be seen through the data with a p-value smaller than 0.001. With the breakout of COVID-19, Chinese central and local governments executed strict policies on restricting the production of non-essential industries, traveling by private motor vehicles, which are among the primary sources of air pollutant emissions. As a consequence, the total air quality improved and the AQI decreased in this period. A valley point can be observed on 15 February 2020, nearly a week after most cities in China ordered shutdown policies around 7–10 February 2020. Although the production and travel restrictions orders were applied, the dispersion and deposition of air pollutants is not an instantaneous procedure but needs a period of time depending on the atmospheric conditions, including wind speed and directions, temperature, air pressure, etc. [35
], as well as geographic characteristics such as vegetation and mountains [36
]. Therefore, the time series shows a time lag of several days for the atmospheric response.
is the monthly average AQI of each province and the whole of mainland China (lower right corner) from January to March 2020. The air pollution of most provinces was less severe in February and March than January, especially for the Northeast, Northwest, east coast, and central China. The reduction in many non-essential industries and private vehicles are the main reasons for better air quality. Although Wuhan began its lockdown on 23 January 2020, all other cities were still open until the first week of February. The pollutant emissions from the industries and vehicles did not change much during January. Exceptions can be found in Xinjiang, Qinghai, Gansu, Yunnan, and Guizhou where the air quality became worse in February and March. For these provinces, the number of confirmed COVID-19 cases was relatively small, totaling 78, 18, 139, 184, and 147, respectively, as of 20 April 2020. Therefore, the quarantine policies were not as strict as in other areas and many industries kept working during the study period. Moreover, the winter season, with its dry air conditions, is not conducive to the dispersion of air pollutants [35
]. The total air quality condition of China also showed improvement over the three months.