Spatial Disparities in NTL Changes
The NTL radiance maps of major cities in South Korea are presented in Figures. 2–7, which show the spatial disparities in NTL changes during the implementation of the social distancing policy. The NTL images were selected two weeks or more after the application of the social distancing policy and when there was minimum cloud cover over the cities. All major cities had an extensive decrease in NTL radiance from the city center to peripheral areas during the implementation of different levels of the policy. NTL changed the most in Seoul, with 18.15% during the end of level 4 and the end of the “return to normalcy” period.
The NTL radiance maps of Seoul during different social distancing policy levels are provided in Figure. 2, showing spatial disparities in NTL changes. Figure. 2a shows the NTL radiance map of Seoul at level 1 social distancing on 6 May 2020, Figure. 2b shows level 2 on 3 November 2020, Figure. 2c shows level 2.5 on 28 December 2020, Figure. 2d shows level 4 on 12 October 2021, Figure. 2e shows the return to normalcy on 1 December 2021 and Figure. 2f shows the reinforcement of the policy on 27 December 2021. The spatial disparities in NTL changes during Figure. 2a to Figure. 2b were nonsignificant. However, NTL radiance throughout Seoul decreased during Figure. 2c, Figure. 2d and Figure. 2f. The average NTL radiance value also decreased with the increase in the level of social distancing. In Figures. 2a-f, the NTL radiance was 652.99, 638.83, 568.67, 598.70, 788.87, and 686.83 nW·cm-2·sr-1, respectively. When level 1 was implemented, various facilities were mostly still open. Therefore, the activity level of people throughout Seoul was still high, affecting NTL radiance. However, during levels 2, 3, and 4, the facilities had to be closed at 22:00, limiting human activity during the night and lowering NTL radiance.
Figure. 3 shows the NTL radiance map of Busan. Figure. 3a shows at the time of level 1 social distancing on 10 May 2020, Figure. 3b shows level 1.5 on 2 March 2021, Figure. 3c shows level 2 on 1 October 2020, Figure. 3d shows level 2.5 on 3 January 2021, Figure. 3e shows level 4 on 22 September 2021, Figure. 3f shows the return to normalcy on 1 December 2021 and Figure. 3g shows the reinforcement of social distancing on 29 December 2021. During Figures. 3a-g, the NTL radiance was 519.75, 535.46, 513.35, 511.68, 491.16, 597.64, and 563.97 nW·cm-2·sr-1, respectively.
Figure. 2. NTL radiance map over Seoul during various social distancing policy levels. (a) NTL radiance map of Seoul on 6 May 2020. (b) NTL radiance map of Seoul on 3 November 2020. (c) NTL radiance map of Seoul on 28 December 2020. (d) NTL radiance map of Seoul on 12 October 2021. (e) NTL radiance map of Seoul on 1 December 2021. (f) NTL radiance map of Seoul on 27 December 2021.
Figure. 4 shows the NTL radiance map of Incheon. Figure. 4a shows at the time of level 1 social distancing on 5 May 2020, Figure. 4b shows level 2 on 4 September 2020, Figure. 4c shows level 2.5 on 25 December 2020, Figure. 4d shows level 4 on 22 September 2021, Figure. 4e shows the return to normalcy on 1 December 2021 and Figure. 4f shows the reinforcement of social distancing on 29 December 2021. During Figures. 4a-f, the NTL radiance was 605.83, 542.26, 528.87, 576.04, 665.22, and 634.21 nW·cm-2·sr-1, respectively.
Figure. 5 shows the NTL radiance map of Gyeonggi. Figure. 5a shows at the time of level 1 social distancing on 6 May 2020, Figure. 5b shows level 2 on 2 March 2021, Figure. 5c shows level 2.5 on 3 December 2020, Figure. 5d shows level 4 on 19 September 2021, Figure. 5e shows the return to normalcy on 1 December 2021 and Figure. 5f shows the reinforcement of social distancing on 27 December 2021. During Figures. 5a-f, the NTL radiance was 496.40, 418.80, 469.00, 457.9, 477.10, and 399.3 nW·cm-2·sr-1, respectively.
Figure. 3. NTL radiance map over Busan during various social distancing policy levels. (a) NTL radiance map of Busan on 10 May 2020. (b) NTL radiance map of Busan on 2 March 2021. (c) NTL radiance map of Busan on 1 October 2020. (d) NTL radiance map of Busan on 3 January 2021. (e) NTL radiance map of Busan on 22 September 2021. (f) NTL radiance map of Busan on 1 December 2021. (g) NTL radiance map of Busan on 29 December 2021.
Figure. 6 shows the NTL radiance map of Daejeon. Figure. 6a shows at the time of level 1 social distancing on 12 May 2020, Figure. 6b shows level 2 on 25 December 2020, Figure. 6c shows level 3 on 8 August 2021, Figure. 6d shows level 4 on 22 September 2021, Figure. 6e shows the return to normalcy on 1 December 2021, and Figure. 6f shows the reinforcement of social distancing on 26 December 2021. During Figures. 6a-f, the NTL radiance was 518.67, 454.22, 448.63, 409.91, 485.31, and 492.36 nW·cm-2·sr-1, respectively.
Figure. 7 shows the NTL radiance map of Daegu. Figure. 7a shows at the time of level 1 social distancing on 12 May 2020, Figure. 7b shows level 1.5 on 2 March 2021, Figure. 7c shows level 2 on 25 December 2020, Figure. 7d shows level 3 on 22 September 2021, Figure. 7e shows the return to normalcy on 4 December 2021 and Figure. 7f shows the reinforcement of social distancing on 26 December 2021. During Figures. 7a-f, the NTL radiance was 610.57, 699.17, 660.85, 588.67, 803.11, and 727.56 nW·cm-2·sr-1, respectively. Therefore, there were substantial NTL radiance changes during the implementation of different social distancing policy levels in cities in South Korea. The changes can be observed in areas with high NTL radiance from red to orange or yellow.
Figure. 4. NTL radiance map over Incheon during various social distancing policy levels. (a) NTL radiance map of Incheon on 5 May 2020. (b) NTL radiance map of Incheon on 4 September 2020. (c) NTL radiance map of Incheon on 25 December 2020. (d) NTL radiance map of Incheon on 22 September 2021. (e) NTL radiance map of Incheon on 1 December 2021. (f) NTL radiance map of Incheon on 29 December 2021.
Figure. 5. NTL radiance map over Gyeonggi during various social distancing policy levels. (a) NTL radiance map of Gyeonggi on 6 May 2020. (b) NTL radiance map of Gyeonggi on 2 March 2021. (c) NTL radiance map of Gyeonggi on 3 December 2020. (d) NTL radiance map of Gyeonggi on 19 September 2021. (e) NTL radiance map of Gyeonggi on 1 December 2021. (f) NTL radiance map of Gyeonggi on 27 December 2021.
Figure. 6. NTL radiance map over Daejeon during various social distancing policy levels. (a) NTL radiance map of Daejeon on 12 May 2020. (b) NTL radiance map of Daejeon on 25 December 2020 (c) NTL radiance map of Daejeon on 8 August 2021. (d) NTL radiance map of Daejeon on 22 September 2021. (e) NTL radiance map of Daejeon on 1 December 2021. (f) NTL radiance map of Daejeon on 26 December 2021.
Figure. 7. NTL radiance map over Daegu during various social distancing policy levels. (a) NTL radiance map of Daegu on 12 May 2020. (b) NTL radiance map of Daegu on 2 March 2021 (c) NTL radiance map of Daegu on 25 December. (d) NTL radiance map of Daegu on 22 September 2021. (e) NTL radiance map of Daegu on 4 December 2021. (f) NTL radiance map of Daegu on 26 December 2021.
Time Series of NTL Changes
The temporal variations in the daily NTL radiance of major cities in South Korea are shown in Figures. 8–10. We calculated the average NTL radiance inside the city boundary. In addition, we added daily confirmed COVID-19 cases (blue line) to observe the effectiveness of the Korean government’s implementation of social distancing rules throughout the COVID-19 pandemic. The green dots represent the daily NTL radiance (nW·cm-2·sr-1) variation generated from Black Marble products (VNP46A2) from 5 January 2019 until 27 June 2022, and the red line is the seven-day moving average. We also added the timeline since the first case of a variant of concern was detected and started to spread in South Korea.
The time series of NTL radiance changes in major cities in South Korea were similar following the change in social distancing rules in each city. The increase in the level of social distancing due to the escalation in the number of daily confirmed cases caused a gradual decrease in the NTL values in all major cities. This can be seen in 2021, when the increase in the level of social distancing from level 2 to 4 caused a decrease in the NTL values in Seoul, Gyeonggi, Incheon, Busan, and Daejeon. Likewise, easing the social distancing level led to an increase in NTL values. This can be seen when the social distancing rules were changed from level 4 to “return to normalcy” in November 2021 in all major cities, especially the city of Seoul, which experienced a change of 18% compared to the end of October 2021. Other cities, such as Daegu, experienced an increase in the NTL value of 14.43%, followed by Busan at 12%, Daejeon at 11.62%, Gyeonggi at 9.28%, and Incheon at 8.77%. In 2022, when Omicron caused a substantial increase in the number of daily confirmed cases in all cities in South Korea, the government implemented only “adjusted social distancing.” The policy was not as strict as level 4. As a result, the observed NTL values were relatively stable in all cities. Then, the NTL values remained stable even though the government began to implement a “return to normalcy,” unlike in 2021, which caused a substantial increase in the NTL values.
For further analysis, we selected Seoul(Figure. 8a) during the implementation of social distancing policy level 1 from April to 22 August 2020. The NTL radiance values were still high, approximately 650 nW·cm-2·sr¬-1. Because of the rising number of confirmed COVID-19 cases, the Seoul authority raised the policy to level 2 until 11 October 2020. The NTL radiance values were lower than those during level 1, approximately 650 nW·cm-2·sr¬-1. The policy managed to reduce the number of cases.
Then, green box in Figure. 8a shows on 12 October to 18 November 2020, the level of social distancing was lowered to level 1. An ease in social distancing affected the NTL radiance values, which rose to approximately 630 nW·cm-2·sr¬-1. The Seoul authority raised the policy to level 1.5 due to rising numbers of confirmed COVID-19 cases, which reached 100 cases. However, the NTL radiance tended to increase and did not suppress the number of daily confirmed cases. Therefore, the Seoul government raised the social distancing level to 2.5 and managed to reduce the number of cases. During this period, the first cases of COVID-19 variants of concern were found in South Korea and included Alpha, Beta, and Gamma. To control the spread and number of daily confirmed cases, the Seoul government lowered and maintained social distancing at level 2 and managed to maintain the number of confirmed cases under 200 per day. During this period, NTL radiance showed a downward trend caused by restrictions on many aspects of people’s lives.
Delta was detected in South Korea as a COVID-19 variant of concern. This variant has the ability to spread more easily than prior variants, thus increasing the number of daily confirmed cases. To counter the spread, the social distancing policy was raised to level 4 in Seoul. As a result, Delta cases reached 1,000 in Seoul, forcing the government to adjust the policy, prohibiting gatherings of more than five people and more than three people after 18:00. This policy caused the NTL radiance to continue to decline to approximately 600 nW·cm-2·sr¬-1.
Purple box in Figure. 8a shows the restriction on business hours, which had been maintained until 22:00, was relaxed until 00:00, and the restrictions on COVID-19 were gradually eased/lifted. During this period, NTL radiance was observed to be as high as 100 to 200 nW·cm-2·sr¬-1 in all cities. Unfortunately, at this time, a subvariant of Omicron with a low mortality rate but increased contagiousness started to spread, which led to a rapid increase in the number of confirmed cases. Quarantine authorities once again limited business hours to 21:00 to control the extremely high number of confirmed cases of COVID-19 due to the Omicron strain, but the infection trend of confirmed cases of COVID-19 had already gone out of control. Despite the restrictions on business hours, the number of infected people per day continued to exceed 1,000.
In response, the government of South Korea started the “With Corona” policy for a gradual daily recovery in November 2021, considering that the fatality rate of COVID-19 was lower than in the early days and people were fatigued. However, NTL radiance did not show a great difference compared to the previous radiance even in November when the business hours restrictions were lifted. There seemed to be a tendency for people to limit nighttime activities on their own without government intervention and remain vigilant for fear of being infected with COVID-19.
The social distancing policy regulated the closing hours of shops, restaurants, and public facilities, and the time series of NTL radiance changes were associated with human activity. Notably, the Suomi-NPP VIIRS NTL data were captured at 00:00 daily. Hence, it is reasonable to assume that most people were asleep at that time. Nevertheless, we did find changes in NTL radiance. Therefore, we could assume that there were still people working or playing outside at that time.
Figures. 8–10 show that the implementation of different social distancing policy levels highly affected the NTL radiance in major cities of South Korea because of the rules that limited the number of people who could gather and the closing of various facilities. Although the policy measures were eased for people who had been fully vaccinated, they still affected NTL radiance. The reason is that people in South Korea were already fully vaccinated by the end of 2021, when COVID-19 variants of concern began to spread, according to ourworldindata.org.
Figure. 8. The time series NTL changes during the implementation of different levels of social distancing rules. (a) Time series of Seoul from January 2020 to June 2022. (b) Time series of Busan from January 2020 to June 2022. The green dots represent the NTL radiance (nW·cm-2·sr-1), the red line is the seven-day moving average, and the blue line is the number of daily confirmed cases. The variants of concern that spread in South Korea are indicated by hollow arrows.
Figure. 9. The time series NTL changes during the implementation of different levels of social distancing rules. (a) Time series of Incheon from January 2020 to June 2022. (b) Time series of Gyeonggi from January 2020 to June 2022. The green dots represent the NTL radiance (nW·cm-2·sr-1), the red line is the seven-day moving average, and the blue line is the number of daily confirmed cases. The variants of concern that spread in South Korea are indicated by hollow arrows.
Figure. 10. The time series NTL changes during the implementation of different levels of social distancing rules. (a) Time series of Daejeon from January 2020 to June 2022. (b) Time series of Daegu from January 2020 to June 2022. The green dots represent the NTL radiance (nW·cm-2·sr-1), the red line is the seven-day moving average, and the blue line is the number of daily confirmed cases. The variants of concern that spread in South Korea are indicated by hollow arrow.