Metropolitan status is obtained by any city if its agglomeration index and population core exceed 50,000 or more18. The world’s first COVID-19 lockdown was imposed in the city of Wuhan on January 21, 2020 and the last in Singapore on April 7, 202019. Given this and the spatial heterogeneity, we selected the Asia-Pacific metropolitan cities, especially in middle and low income countries. These cities are listed as the main metropolitan cities16 and most of them are among the first in the list of the most polluted cities. “Spatial heterogeneity is defined either as the variation in space in the distribution of a pattern of points, or as the variation of a qualitative or quantitative value of a surface pattern”20. Table 1 presents the socio-economic characteristics of the selected cities. Analysis showed that the highest levels of AOD were found in Calcutta, India (2.86) on January 14, 2019, while the lowest levels of AOD were seen in Ho Chi Minh, Vietnam (0.013) on March 21, 2019. Likewise, the highest levels of AOD were found in Beijing, China (26% change) on February 7, 2020, while the lowest levels of AOD were recorded in Ho Chi Minh (0.01% change) on March 21, 2020. According to the analysis, the top four cities for ODA in 2019 were Calcutta, Beijing, Busan and Tokyo, however, the situation changed slightly in 2020 during the lockdown period. During the lockdown, the highest levels of AOD were found in Beijing, Calcutta, Delhi and Dhaka (Fig. 1).
The lowest levels were observed in the cities of Ho Chi Minh, Delhi, Taipei and Wuhan in 2019. On the contrary, Ho Chi Minh, Tehran, Bangkok and Shanghai were respectively leading the cities with the lowest level, from January to May 2020. Busan The cities of, Calcutta and Tokyo were positively impacted during the lockdown and reaped more environmental benefits than other metropolitan cities (Fig. 1).
NO results2 has shown in an interesting way a mixture of environmental spatial heterogeneity. A maximum level of NO2 was observed in Shanghai, China, which was (588.953 Âµmol / m2), on March 17, 2019 when the minimum level of NO2 was recorded in Calcutta (5.06 Âµmol / m2), May 2, 2019. In addition, the maximum level of NO2 was observed in the city of Beijing (20%) on January 17, 2020, compared to 2019, which then declined to nearly 95% during the lockdown period. The minimum level in Kathmandu, Nepal showed a change of almost 85%, on May 9, 2020 compared to 2019. Highest NO2 the issuing cities in 2019 were Beijing, Wuhan, Shanghai and Delhi; while during the 2020 lockdown, the main cities were Beijing, Shanghai, Tokyo and Seoul (Fig. 2). The lowest amount of NO2 was recorded in Calcutta, Colombo, Kathmandu and Karachi in 2019; while in 2020 these three cities (Kathmandu, Colombo and Tehran) were on the list of the lowest levels. Almost 50% NO2 a reduction in emissions was observed in the city of Wuhan (Fig. 3).
Considering another important air pollutant, CO, Beijing was the highest city with 98.97 Ã 103 Âµmol / m2 CO emissions on January 3, 2019 and the lowest was in the Iranian city of Tehran with CO emissions (24.21 Ã 103 Âµmol / m2) recorded on January 2, 2019. Taking into account the results of this analysis, CO emissions have rather increased in Beijing (10%) according to the observations made on January 28, 2020 which decreased to nearly 65% ââin March 2020 (period of confinement). In addition, the lowest CO emissions remained in Tehran with (21% variation) on March 22, 2020 compared to the previous year. The highest CO emitting city in 2019 was Beijing, followed by Wuhan, Shanghai and Delhi, and during the 2020 lockdown, Beijing was followed by Wuhan, Shanghai and Calcutta (Fig. 4). The lowest CO levels in 2019 were from Tehran, Kathmandu, Bangkok and Colombo, and during the 2020 lockdown were from Tehran, Colombo, Karachi and Wuhan.
Figure S1 shows the results of SO2 emissions from T20 cities. The maximum level of SO2 was observed in Kathmandu (1362.43 Ã 103 Âµmol / m2) on April 12, 2019, and the minimum level was observed in Beijing (124.12 Ã 103 Âµmol / m2) on January 23, 2019. Similarly, the maximum level of SO2 was recorded in Dhaka and Bangladesh (with an increase of 42%) on April 22, 2020, and the minimum level of SO2 in Seoul which was (down 87%) on January 17, 2020. In 2019, the highest levels of SO2 were in Kathmandu, followed by Calcutta, Dhaka and Singapore, and during the 2020 lockdown was Dhaka (42%), followed by Calcutta (23%), Hong Kong (21%) and Beijing (19%) (Fig. S1. ). SO2 was lowest in Beijing, Delhi, Wuhan and Bangkok in 2019, which was a normal year compared to 2020. Seoul, Beijing, Busan and Shanghai were among the four cities with the most SO emissions2 emissions during the 2020 lockdown.
Air pollution is closely linked to the mobility of people and other modes of transport22. We considered six modes of mobility, that is, public transit, parks, residential, retail, grocery shopping and the workplace (Figure 5). According to Google mobility data23, mobility in central Pakistan has declined by 75% due to nationwide lockdown. The minimum mobility (-38%) in Seoul was observed on February 25, 2020, however, mobility in parks peaked (158%) on April 30, 2020. While the minimum mobility in parks was observed at Delhi on April 16, 2020 (- 98%). In the residential sector, maximum mobility was recorded in Singapore on May 1, 2020 (55%) and minimum mobility in the residential sector was documented in Calcutta on March 11, 2020 (-3%). In the public transport sector, maximum mobility was observed in Colombo on March 10, 2020 (12%). And minimal mobility was recorded in Manila on April 10, 2020 (-90%). According to Google mobility data, 90% of trips were reduced until mid-April 2020 in the Asia-Pacific region and it should be noted that most of the world’s populated cities are in Asia-Pacific.