Metropolitan status is achieved by any city if its agglomeration index and core population exceed 50,000 or above18. The first COVID-19 lockdown in the world was imposed in Wuhan city on 21st January 2020 and the latest was in Singapore, on 7th April 202019. Keeping that in view and spatial heterogeneity, we have selected the metropolitan cities of the Asia-Pacific especially from middle- and low-income countries. These cities are listed as top metropolitan cities16 and most of them are among top in the list of most polluted cities. “Spatial heterogeneity is defined either as the variation in space in distribution of a point pattern, or variation of a qualitative or quantitative value of a surface pattern”20. Table 1 shows the socio-economic characteristics of the selected cities. The analysis showed that the highest levels of AOD were found in Calcutta, India (2.86) on 14th January 2019 whereas the lowest levels of AOD  were observed in Ho Chi Minh, Vietnam (0.013) on 21st March 2019. Likewise, the highest levels of AOD were found in Beijing, China (26% change) on 7th February 2020 while the lowest levels of AOD were recorded in Ho Chi Minh (0.01% change) on 21st March 2020. According to the analysis, top four cities for AOD in 2019 were Calcutta, Beijing, Busan, and Tokyo, however, the situation was slightly changed in 2020 during the lockdown period. During lockdown highest levels of AOD were found in Beijing, Calcutta, Delhi, and Dhaka (Fig. 1).

Table 1 Demographic characteristics of the selected cities21.
Figure 1
figure1

Comparison of daily changes in AOD concentration of the cities in 2019 and 2020.

The lowest levels were observed in Ho Chi Minh, Delhi, Taipei, and Wuhan cities in 2019. On the contrary, Ho Chi Minh, Tehran, Bangkok, and Shanghai were the top among lowest level cities respectively, from January to May 2020. Busan, Calcutta, and Tokyo cities were positively impacted during the lockdown and reaped more environmental benefits than other metropolitan cities (Fig. 1).

Results of NO2 interestingly showed a mix of environmental spatial heterogeneity. A maximum level of NO2 was observed in Shanghai, China which was (588.953 µmol/m2), on 17th March 2019 whereas the minimum level of NO2 was recorded in Calcutta (5.06 µmol/m2), on 2nd May 2019. Moreover, the maximum level of NO2 was observed in Beijing city (20%), on 17th January 2020, as compared to 2019, which then decreased to almost 95% during the lockdown period. The minimum level in Kathmandu, Nepal showed a change of almost 85%, on 9th May 2020 as compared to the 2019. The topmost NO2 emitting cities in 2019 were Beijing, Wuhan, Shanghai, and Delhi; whereas during the lockdown of 2020 top cities were Beijing, Shanghai, Tokyo, and Seoul (Fig. 2). The lowest amount of NO2 was recorded in Calcutta, Colombo, Kathmandu, and Karachi in 2019; whereas in 2020 these three cities (Kathmandu, Colombo and Tehran) made to the list of lowest levels. Almost 50% NO2 emission reduction was observed in Wuhan city (Fig. 3).

Figure 2
figure2

Spatial patterns indicating a drastic drop in average NO2 concentration in March 2019 and 2020 in the Asia-Pacific region. Maps were created through Google Earth Engine (https://earthengine.google.com/V.2020) by a co-author, Sawaid Abbas.

Figure 3
figure3

Periodic changes of NO2 concentration of the cities in 2019 and 2020 during the P1 (Jan 1–Jan 22), P2 (Jan 23–Feb 22), P3 (Feb 23–Mar 22), P4 (Mar 23–Apr 22) and P5 (Apr 23–May 22). The black and red dotted lines indicate mean NO2 concentration (Jan 1–May 22) in 2019 and 2020 and the dots indicate outliers, respectively.

Considering another important air pollutant CO, Beijing was the highest city with 98.97 × 103 µmol/m2 CO emissions on 3rd January 2019 and the lowest was in Tehran city of Iran with CO emissions (24.21 × 103 µmol/m2) recorded on 2nd January 2019. Given the results of this analysis, CO emissions were rather increased in Beijing (10%) as per observations made on 28th January 2020 which decreased to almost 65% during March, 2020 (lockdown period). Furthermore, the lowest CO emissions remained in Tehran with (21% change) on 22nd March 2020 as compared to the previous year. The top most CO emitting city in 2019 was Beijing followed by Wuhan, Shanghai, and Delhi, and during the lockdown of 2020 Beijing was followed by Wuhan, Shanghai, and Calcutta (Fig. 4). The lowest levels of CO in 2019 were from Tehran, Kathmandu, Bangkok, and Colombo, and during the lockdown of 2020 were from Tehran, Colombo, Karachi, and Wuhan.

Figure 4
figure4

Comparison of daily changes in CO concentration of the cities in 2019 and 2020.

Figure S1 showed the results of SO2 emissions from the T20 cities. The maximum level of SO2 was observed in Kathmandu (1362.43 × 103 µmol/m2) on 12th April 2019, and the minimum level was observed in Beijing (124.12 × 103 µmol/m2) on 23rd January 2019. Similarly, the maximum level of SO2 was recorded in Dhaka and Bangladesh (with a 42% increase) on 22nd April 2020, and the minimum level of SO2 in Seoul which was (87% decrease), on 17th January 2020. During 2019, the highest levels of SO2 were in Kathmandu, followed by Calcutta, Dhaka, and Singapore, and during the lockdown of 2020 was Dhaka (42%), followed by Calcutta (23%), Hong Kong (21%), and Beijing(19%) cities (Fig. S1). SO2 was lowest in Beijing, Delhi, Wuhan, and Bangkok during 2019 which was a normal year than 2020. Seoul, Beijing, Busan, and Shanghai were among the top four cities that emitted the lowest SO2 emission during the lockdown of 2020.

Air pollution is robustly interconnected with the mobility of people and other modes of transportation22. We considered six modes of mobility, i.e., transit, parks, residential, retail, grocery, and workplace (Fig. 5). According to Google mobility data23, mobility in the urban center in Pakistan went down by 75% as a result of nation-wide lockdown. The minimum mobility (-38%) in Seoul was observed on 25th February 2020, however, mobility in the parks reached its maximum (158%) on 30th April 2020. While minimum mobility in parks was noted in Delhi on 16th April 2020 (-98%). In the residential sector, maximum mobility was recorded in Singapore on 1st May 2020 (55%) and minimum mobility in the residential sector was documented in Calcutta on 11th March 2020 (-3%). In the transit sector, maximum mobility was observed in Colombo on 10th March 2020 (12%). And minimum mobility was recorded in Manila on 10th April, 2020 (-90%). According to Google mobility data, 90% movement was reduced till mid-April, 2020 in the Asia-Pacific region and it is worth to note that most of the populated cities of the world are in Asia-Pacific.

Figure 5
figure5

Patterns of changes in mobility in different cities due to lockdown implementation. The anomalies are computed with respect to base period from 1st January to 14th February 2020.

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