Air Pollution Change during the Lockdown and Unlock Periods
Figure 2a shows the mean NO2 before, during and after lockdown in India in 2020. The analyses performed for the pre-lockdown period show that Delhi, Ahmedabad, Surat, Chennai, Dhanbad, Tata, Visakhapatnam, Hyderabad and Kolkata are the hotspots of pollution (shown as the circles in the figure). The primary sources of NO2 are vehicular and industrial emissions, and other anthropogenic activities such as agriculture, biomass burning and aquaculture (Garg et al. 2002; Beirle et al. 2003). These cities show about 7–8 × 1015molec./cm2 of NO2, whereas the rural regions (other than the urban areas) have concentrations of about 0–2 × 1015molec./cm2. During the lockdown period, there was a significant reduction in the number of vehicles on roads, and as a result, the NO2 concentration is highly reduced in India. However, a few mining and Industrial activities were still running during the lockdown period with some restrictions (Ray and Subramanian 2020). The industrial regions continued to contribute similar amounts of NO2 during the lockdown. The overall concentration also decreased to very low values over the whole country (0–3 × 1015 molec./cm2) except for IGP, where it is about 4–5 × 1015 molec./cm2 during the unlock period.
Figure 2b and c show the concentrations of NO2 during the same periods of 2019 together with the climatology (2015–18) of NO2, respectively. There are high values of NO2 during the pre-lockdown period of 2020 compared to that of 2019 and the average concentration during 2015–18 amounts to 1–2 × 1015molec./cm2 (regions marked in black circles). However, there is a reduction in NO2 in these regions during the lockdown, and it again increased by 1–2 × 1015 molec./cm2 during the unlock phases as compared to that in the previous years.
The tropospheric O3 (Fig. 3) concentration is about 35–40 DU in the northern India before the lockdown period (March 2020) and it increased to 42.5–47.5 DU during the lockdown. Peninsular region shows about 30–32.5 DU during the pre-lockdown and it increased to 35–37.5 DU in the lockdown. The IGP has the highest levels of O3 during the lockdown, as the pollution is very high there. However, ozone decreased substantially during the unlock period, about 30–35 DU in peninsular India (lowest during the unlock period) and 40–42.5 DU in the hilly regions. The NE and IGP regions also exhibit a significant decrease in O3 during the unlock phase (40–45 DU), making a difference of about 5–15 DU from the lockdown to the pre-lockdown period. Furthermore, in general, ozone shows high values in northern India as compared to that in the south, due to the severe pollution in the north.
We have also compared the tropospheric O3 and NO2 during the pre-lockdown, lockdown and unlock periods in 2020 to those in 2019, as illustrated in Fig. 4. A significant increase in NO2 is observed over the industrial regions of central India (e.g., Chhota-Nagpur and Jamshedpur) and cities (e.g., Delhi and Mumbai) in the pre-lockdown period. However, these regions show lower NO2 during the lockdown (1–1.5 × 1015 molec./cm2), but it again increased (about 1–1.5 × 1015 molec./cm2) during the unlock period. Cities of Delhi, Mumbai, Ahmedabad, Hyderabad and Kolkata also show similar changes in NO2. The NO2 concentrations were higher during the pre-lockdown and it suddenly decreased during the lockdown, and eventually increased to much higher levels in the unlock phases. The tropospheric O3 shows a significant increase over NW prior to the lockdown (3–5 DU) and a decrease by 2–5 DU during lockdown, except for the hilly regions, as compared to that in 2019. In the unlock phase, O3 increased from that of the lockdown by 2–4 DU, but still was lower by 0–2 DU from the previous year (2019) values. The NO2 and O3 concentrations show a similar pattern of reciprocal changes in most regions.
Figure SM2 shows the percent change in NO2 and O3 during the pre-lockdown, lockdown and unlock periods compared to those in 2019. As discussed above, the NO2 concentrations show high reduction during lockdown (10–20%) except in the hilly regions, where pollution is also very small. Conversely, the NO2 concentrations increased by 40–50% in most cities and northern India during the unlock phases, although some areas in the south still show a reduction of about 10%. Similarly, O3 concentrations show negative change during the lockdown phase in most regions, except in Kashmir (about 5–10%). This reduction in ozone spread to almost all regions across India during the unlock periods, with values up to 10%, analogous to the changes in NO2.
Regional Progression of NO2 and O3
Table 1 lists the regional changes in ozone and NO2 during each lockdown phase as a percent difference from the previous year (2019). Figure 1 shows the topographic divisions of India, which are used to differentiate the changes in the trace gases during the period of study. A clear reduction in NO2 is observed during the lockdown in the central, IGP and peninsular regions, whereas the other regions show an overall increase from the previous year. The NO2 concentrations in the industrial regions are mostly around 5–7 × 1015 molec./cm2 during the lockdown. Air quality improved over IGP during the lockdown (6%) and worsened during the unlock (18%) in terms of NO2 concentrations. This sharp increase in NO2 during the unlock period suggests the rise in pollution as soon as the restrictions were eased. In addition, the hilly regions show NO2 concentration of about 0–1 × 1015 molec./cm2 during the entire lockdown period. The NE regions show 3–4 × 1015 molec./cm2 during the lockdown, which significantly reduced in the subsequent unlock phases. The central India that includes the major industrial areas (e.g., Jamshedpur and Chandrapur) also shows a decline in NO2 concentration during the lockdown with a decrease of about 5.3% from the previous year values. Nevertheless, the concentration over peninsular region shows a consistent increase during the lockdown (4%), although the NW region exhibits a mixed response of positive and negative drifts from the previous year concentrations.
During the lockdown, high concentrations of O3 were found in IGP (45–47.5 DU), while the rest of the regions show lower concentrations of about 35–40 DU. We also observe the highest O3 in IGP compared to the other regions during the lockdown. Most regions show a decrease in O3 concentrations with respect to that in 2019 in the pre-lockdown period. The highest decrease is observed in NE (-17.3%) and the lowest in the hilly region (+ 3%) during the lockdown. The O3 concentrations are much smaller than the previous year values for the same period, although O3 started to increase in all regions as soon as the lockdown restrictions were relaxed (Table 1). This is evident from the increase of O3 concentrations from the beginning of unlock phase, for which the hilly regions show the largest reduction (2.51%) from the 2019 levels.
Since the regional averages are estimated over large areas, it is difficult to find changes in the distribution of a pollutant with respect to industrial activities and transport sectors. Therefore, we examine the change in NO2 and O3 concentrations in the major cities of India before (March), during (April–May) and after (June–September) the lockdown in 2020 (Table 1). Our results show a decrease in NO2 over the major cities. However, the areas with heavy traffic demonstrate a substantial increase in O3 concentrations during the lockdown (e.g., Delhi). The NO2 concentrations are relatively lower in many cities even before the lockdown because of the local lockdown imposed in a few places (e.g., Kerala, Pune and Mumbai). Therefore, the difference in NO2 concentration with the pre-lockdown (March) is very small, for which the smallest change is observed at Ahmedabad (-1%) and the largest at Lucknow (-15.74%). On the other hand, the highest reduction in NO2 is observed in Delhi (36%) and the smallest in Kolkata (6%) in the lockdown period. During the unlock period, the NO2 concentrations in all cities show the same levels as observed during the pre-lockdown; indicating decrease in air quality and the impact of ease in restrictions. Meanwhile, the ozone concentrations show a similar pattern to the changes in NO2, as its concentration decreased in most cities. The minimum and maximum decrease in O3 during the lockdown are observed in Delhi (2%) and Kolkata (18.73%), respectively.
Air Quality in 2020 Compared to that during 2015–2020
Figure 5 shows the monthly mean of NO2 and O3 for the period 2015–2020. The analyses show a general increase in tropospheric O3 from January to June and a gradual decrease thereafter. The NO2 concentrations peak during March–April–May and show the minimum in June–July–August. A record decrease in O3 is observed during the lockdown in all regions except in the peninsular India, with its lowest values in April 2015. As soon as the country entered the unlock phase, O3 in all regions started to increase, particularly from October onwards, which is higher than that during the same months of previous years. The monthly mean NO2 (April–May) also shows lower values in the lockdown period in all regions. All regions except NE show significant decrease in April 2020 as compared to that in 2019. However, NO2 exhibits slightly higher values in May 2020 to that of previous year. During the unlock phase, NO2 increased noticeably in all regions and it shows the highest concentrations in NEI, NW, IGP and hilly regions during June–September 2020, in comparison to previous year values.
Comparison of Daily NO2 in 2020 with that of 2019 and 2021
Figure 6 shows the difference (in percent) of NO2 during the pre-lockdown, lockdown and unlock periods of 2020 with the corresponding time periods in 2021. Figure SM3 shows the mean NO2 distribution over India during the pre-lockdown, lockdown and unlock periods of last 3 years (2019–2021). An increase of 43, 60, 15, 38, 16 and 31% in NO2 prior to the lockdown period of 2020 (pre-lockdown) is observed in IGP, Central, Hilly, North East, North West and Peninsular India, respectively. This suggests increase in pollution during the same period of 2021 without any lockdown. India was hit by the largest wave of COVID-19 in February 2021 and the government imposed lockdown in different states again during this period. For instance, Maharashtra had 4 phases of lockdown from April to June 2021, during which the schools and public places remained closed, public gatherings were restricted and offices remained shut. Several states, like Tamil Nadu, Kerala, Karnataka, Rajasthan, Bihar, UP and Odisha enforced complete lockdown in the same period. In contrast, several other states, like Punjab, Chandigarh, Andhra Pradesh, Arunachal Pradesh and Nagaland had partial lockdown. However, most states started to lift the restrictions and moved to the unlock phase from 15 June 2021 onwards. This situation caused the increase in NO2 in some states, and the decrease in the states with complete lockdown.
Comparing the lockdown phases of 2020 with the same period in 2021, there is again a clear increase in the amount of NO2. We find an increase in NO2 during lockdown and unlock periods of 2020, at about24, 21, 17, 31, 8 and 5.5% during lockdown (25 March – 31 May) and 20, 11, 7.5, 12.5, 2.5 and 8% during unlock (01 June – 30 September) in IGP, Central, Hilly, NE, NW and Peninsular India, respectively. We also observe a rapid increase in NO2 over IGP and Central India during the period. Figure 6 (bottom panel) shows the daily mean NO2 concentrations in these regions compared to those in 2019 and 2021. The results show very high levels of NO2 over Central India and IGP during the pre-lockdown period of 2021; it should be noted that the second wave of COVID-19 also occurred during the same period in India. Local restrictions were imposed from 15 April to 15 June, and therefore, significant decrease in NO2 is apparent until July 2021. The NO2 concentrations again increased during the unlock period and they outcompeted the overall NO2 concentrations observed in 2019 and 2020.
Figure 7 shows the CPCB ground-based NO2 and O3 measurements from 2019 to 2021 during the pre-lockdown, lockdown and unlock periods in India. The measurements from CPCB show an overall reduction of NO2 in all cities during the lockdown. Among the cities, except Kolkata, NO2 levels increased during the same period of lockdown in 2021 compared to that in 2020. For example, NO2 shows high values (40.61 ug/m3) in 2019, which reduced significantly in 2020 (22.15 ug/m3) due to the lockdown restrictions in Delhi. However, the NO2 concentrations increased to 30.18 ug/m3 during the same period in 2021 because of the relaxation of restrictions. On the other hand, NO2 concentrations are lower in 2021 (7.75 ug/m3) than those in 2020 (11.88 ug/m3) during the lockdown in Kolkata. A similar change in NO2 concentrations is observed during the unlock period in all cities, except Delhi and Kolkata, where NO2 concentrations are higher in 2021 and lower in 2019 with respect to the values in 2020. Table SM1 shows the change (in %) in surface O3 and NO2 in different cities of India.
Conversely, the ground-level ozone shows a significant increase during the lockdown period as compared to the ozone levels in pre-lockdown and unlock periods. Mahato et al. (2020) also reported an increase (10%) of O3 over the industrialized and transport dominated regions of India because of the slowdown in atmospheric titration of NO2. The increased ozone in April–August can also be due to the higher solar radiation during the period in the Indian sub-continent (Gorai et al. 2017).
Assessment of Different Studies on Air Pollution in India and the World
Studies Using the CPCB Data in India
Several studies have been performed using the CPCB NO2 and O3 concentrations during the lockdown (before and after 25 March 2000) in India. For example, Mahato et al. (2020) found a decrease of 53% in NO2 and an increase of 1% of O3 in Delhi for the period from 3 March to 14 April 2020. Sharma et al. (2020a, b) found a decrease of 18% for NO2 and an increase of 17% in O3 in India from 10 March to 17 May 2020. In a similar study, Kumari and Toshniwal (2020) reported a decrease of 60 and 78% of NO2 and an increase of 27 and 30% of O3 in Delhi and Mumbai, respectively, during the period 01 March – 15 April 2020. Singh et al. (2020) studied the air quality in 134 sites of India and found a significant decline in NO2 (30–70%). They also found a marginal increase in ground-level O3 over Delhi. Kumari and Toshniwal (2020) found a decrease in NO2 and SO2 during the lockdown period (50–80%) in Delhi, Mumbai and Singrauli. Navinya et al. (2020) studied the air quality across 17 cities (e.g., Delhi, Bangalore and Ahmedabad) in India and reported a decline of NO2 in Bangalore (86%), Delhi (70%) and Ahmedabad (67%). Our analysis with CPCB measurements in the respective cities during the lockdown period also shows comparable changes in NO2 and O3, although small variations due to the differences in time periods are there. However, our study is more comprehensive as it discusses the NO2 and O3 distribution together for the whole India and during the complete pre-lockdown, lockdown and unlock periods.
Comparison with Satellite Measurements in India
Sathe et al. (2021) used satellite derived tropospheric column NO2 during the lockdown period (25 March – 17 May) and reported a decrease of about 46–61% over selected cities in India (e.g., Bangalore, Chennai, Delhi and Kolkata). Shehzad et al. (2020) observed a reduction of about 40 and 50% of NO2 at Mumbai and Delhi, respectively, from 01 January to 05 April 2020, using the Sentinel-5P observations. Kumar (2020) used the OMI data and found a notable decrease in NO2 in the period of 29 February – 30 May 2020 (with a difference of 45% from the average of 2017–2019) in six megacities in India. Prakash et al. (2021) studied NO2 concentrations across India using the TROPOMI measurements and found a decrease of about 35–43% during the lockdown period compared to that in previous years in the megacities (Delhi, Mumbai and Bangalore). Siddiqui et al. (2020) also used TROPOMI to examine the changes of NO2 during the lockdown (20 March 2020 – 3 May 2020) and observed a noticeable reduction in metro cities (34% in Kolkata, 33% in Chennai, 70% in Bangalore and 57% in Mumbai). We also find comparable results, as there are changes of around -40% in NO2 and 5–10% in ozone at different regions and cities of India during the lockdown.
Global Analysis of Pollution Changes during COVID-19 Periods
Nakada and Urban (2020) used surface and satellite data, and observed an increase of 30% in O3 and a decrease of 54.3% in NO2 over Sao Paulo state in Brazil during the period 25 February – 20 April 2020. Xu et al. (2020) used ground-based measurements and observed -52.8 and + 3.6% differences in NO2 and O3, respectively, over Central China between 2020 and the average for the period 2017–2019. Berman and Ebisu (2020) reported a decrease of 25.5% of NO2 during 08 January – 21 April 2020 with respect to the average of 2017–2019 in the USA. Similar results were also found for ozone and NO2 by Kerimray et al. (2020) for Almaty and Baldasano (2020) for Barcelona. A model study by Menut et al. (2020) reported 30–50% reduction in NO2 during March 2020 in Europe when simulating conditions with and without the lockdown restrictions. Sicard et al. (2020) reported an increase in O3 (17% in Europe and 36% in Wuhan) and reduction (56%) of NO2 (European cities and Wuhan) during lockdown. Adams (2020) used the Ministry of Environment air pollution data and found a significant reduction in NOx and a small slump of ground level O3 in Ontario, Canada, from 3 January to 6 February 2020. Otmani et al. (2020) used ground-based measurements at two sites in Morocco and found reduction (96%) in NO2 during the lockdown period (11 March – 02 April 2020). Tobías et al. (2020) analyzed ground-based observations and found 45–51% reduction in NO2 and 33–57% increase in O3 in Barcelona during the lockdown (16 February – 30 March 2020). Our analysis with satellite and ground-based measurements show comparable results, as most regions show a reduction of about 40–50% in NO2 and an increase of 5–10% in tropospheric O3 during lockdown. However, it should be noted that the changes in tropospheric O3 and total column ozone will be different in these conditions and are not directly comparable.
Limitation of the Study
There is a significant number of studies on lockdown related air quality changes, but we have selected only O3 and NO2 for analysis here for comparison. In addition, since satellite and ground-based measurements are not directly comparable, a separate discussion is presented for the two analyses. However, qualitative comparison is possible in such instances to mutually cross-check the validity and accuracy of the measurements (e.g., both data show increase in O3 and decrease in NO2 during the lockdown). Therefore, caution must be practiced during the comparison studies on these trace gases for lockdown and unlock periods.