An assessment of the impact of a nation-wide lockdown on air pollution – a remote sensing perspective over India

The nation-wide lockdown imposed over India from 25th March 2020 onwards, in response to the COVID-19 pandemic, placed severe restrictions upon the industrial and transport sectors, which together form a significant chunk of 15 anthropogenic emissions of pollutants into the atmosphere. Atmospheric concentrations of Nitrogen dioxide (NO2), carbon monoxide (CO) and aerosol optical depth (AOD) for lockdown and pre-lockdown periods were investigated with observations from Aura/ OMI, Terra/ MOPITT, Sentinel-5p/TROPOMI and Aqua-Terra/ MODIS satellite sensors. Mean NO2 levels over India during the lockdown period showed a dip of 17% as compared to pre-lockdown period and a decrease of 18% against the 5-year average. Over New Delhi in particular, there was a sharp decrease of 62% in NO2 levels as compared to 2019 and 20 a decline by 54% relative to the preceding 5-year period (2015 2019). Aerosol levels reduced over the country by ~24% from the 5-year mean levels, with a marked reduction over the Indo-Gangetic plains region. An increase in CO levels was noticeable, probably due to its longer life-time as compared to NO2 and aerosols. This study also reports the rate of change of NO2, CO and AOD, indicating increase/decrease in pollutant emissions over the different states of India.


Introduction
Following the outbreak of the novel corona virus  and its declaration by the World Health Organization (WHO) to be a public health emergency of international concern, several countries across the globe imposed national lockdowns to contain the pandemic (Tian et al., 2020). India confirmed its first COVID-19 case on 30 th January 2020 with an exponential increase to 360 cases by 22 nd March 2020. In an attempt to restrict the pandemic, the Indian government called for a 'Janata 30 Curfew' on 22 nd March 2020, followed by imposition of a nation-wide lockdown in phase I (25 th March -14 th April) and phase II (15 th April -3 rd May) which has been further extended (Gettleman and Schultz, Kai, 2020;UN news, 2020). Under the lockdown, 1.30 billion citizens were advised to stay in-doors, transport and industrial production suspended and only essential https://doi.org/10.5194/acp-2020-621 Preprint. Discussion started: 13 July 2020 c Author(s) 2020. CC BY 4.0 License. services were permitted. During this 40-day period, economic activities were greatly affected and there was a fall in net energy consumption by 30% (https://www.ppac.gov.in/). The prolonged cessation of industrial and vehicular activities during 35 COVID-19 lockdown in china caused significant reduction in air pollutants over its 3 cities Fan et al., 2020).
In recent times air pollution has arisen as an environmental issue of major concern worldwide, particularly for developing countries, and may extend from local to global scale (Fang et al., 2009). Emissions of primary pollutants comprising particulate matter (aerosols) and gases (nitrogen dioxide / NO2 and carbon monoxide / CO) play a vital role on environment and human 40 health . Earth-atmospheric radiation budget is greatly affected by the aerosols through scattering and absorption of incoming solar radiation, which further influences the formation of clouds and precipitation (Ramachandran et al., 2013). The size of aerosols can affect formation of clouds and thus aerosols can influence the Indian monsoon (David et al., 2018). Exposure to NO2 has been correlated with an increased rate of morbidity and subsequently increased rate of mortality (WHO, 2013). Global emissions of NOx (NO, NO2) are primarily due to anthropogenic activities such as combustion of fossil 45 fuel and industrial activities, while natural sources include soils and lightning. Among these, combustion is the principal process through which trace gases and aerosols are emitted into the atmosphere. Rapid industrialization and exhaust from vehicles are major sources of air pollution in urban regions. The ambient air quality is largely determined by the abundance of these primary pollutants in the atmosphere (Nishanth et al., 2014). Increase in the concentration levels of trace gases has been a challenging environmental issue in urban and industrial areas. According to the National Ambient Air Quality Monitoring 50 Program, emissions of NO2 over the Indian region are second only to particulate matter emissions and are higher than sulfur dioxide and CO (Gaur et al., 2014). CO acts as both a greenhouse gas and a precursor for ozone chemistry in the lower atmosphere. During the lockdown period in India, ground-based in-situ analysis evidenced strong decline in air pollutants (Mohato and Ghosh, 2020).

55
In spite of the various nationwide lockdowns which may have led to suspension of ground-based measurements, remote sensing data offered a reliable and accurate method to measure air pollution with un-interrupted observations. In the present study we attempted to assess the impact of lockdown on air quality over India by examining remotely sensed NO2, CO, aerosol optical depth data from Aura/Ozone Monitoring Instrument (OMI), Terra/ Measurements of Pollution In the Troposphere (MOPITT), Sentinel-5p/Tropospheric Monitoring Instrument (TROPOMI) and Aqua-Terra/Moderate Resolution Imaging 60 Spectroradiometer (MODIS) respectively. The spatio-temporal variations in these pollutants are examined and short-term climate change due to short-term climate forcing agents during the lockdown period is investigated.  The COVID-19 lockdown over India was implemented in several phases, viz. phase I (21 days from 25 th March to 14 th April 80 2020), phase II (19 days from 15 th April to 3 rd May 2020), phase III (14 days from 4 th May to 17 th May 2020) and phase IV (14 days 18 th May to 31 st May 2020). The present study focuses on the air quality over India, its individual states, and state capitals during the lockdown period. Analysis of satellite based observations of NO2, CO and AOD was carried out for lockdown period as well as pre-lockdown period from 2015-2020. Short-term climatological mean was computed during 2015-2020 for the months corresponding to lockdown period to assess the temporal changes of pollutants in the atmosphere. We 85 have focused our analysis for the first two phases of lockdown in which the industrial and transport sectors were brought to a near standstill. Figure 1 shows the data handling and execution strategy followed in this study. The regional increase/decrease in pollutant emissions over the country and individual states were analyzed. https://doi.org/10.5194/acp-2020-621 Preprint. Discussion started: 13 July 2020 c Author(s) 2020. CC BY 4.0 License.

Impact of lock down on NO2 variations over Indian region
Tropospheric columnar NO2 product from Aura/OMI which is 30% cloud screened and available at 25km spatial resolution has been analyzed for 2015-2020. Temporally averaged concentrations of NO2 for the duration of lockdown period (25 th March to 3 rd May 2020), phase I and phase II of lockdown, and the corresponding period of the previous year (2019) are shown in 95 Figure 2a-c, along with the differences in concentration levels between different periods. In 2019, noticeably high concentrations of NO2 were there over Delhi and eastern part of India. In sharp contrast to this, much lower levels of NO2 over New Delhi, and a significant reduction in ambient levels throughout the country are observed during 2020 lockdown period.
The mean NO2 concentration over the country during the entire duration of lockdown, phase-I and phase-II period in 2020 (2019) are 2.01×10 15 (2.42×10 15 ) molecules cm -2 , 2.03×10 15 (2.45×10 15 ) molecules cm -2 and 1.99×10 15 (2.40×10 15 ) molecules 100 cm -2 respectively, with a reduction of 17 % during the lockdown period as compared to the previous year. However, an increase in NO2 levels is observed in the north, north-east and western parts of India, which may possibly be due to seasonal change. Figure 2d shows the weekly variations of NO2 during the lockdown period. NO2 life time is shorter at day time due to reactive photochemical processes in presence of sunlight and longer at night (Richter et al., 2004). Satellite measurements from Aura/OMI are able to detect build up and removal of NO2 emissions during the lockdown period, as evidenced by Figure 2d. 105 https://doi.org/10.5194/acp-2020-621 Preprint. Discussion started: 13 July 2020 c Author(s) 2020. CC BY 4.0 License. phase-1 lock down c) phase-II lock down, d) weekly variation in total lock period. The color bar on spatial plots shows column NO2 values.

Short-term climatological variations in NO2 due to lock down 110
Time series analysis of NO2 was carried out during 2015-2020 for the months corresponding to lockdown period (Figure 3ab) to understand the temporal variations in NO2 levels in the atmosphere. A smoothing function with span of 7 days was used for better visualization of patterns/trends in NO2 levels in figure 3a-b, with red (green) bars indicating elevated (lowered) NO2 levels in 2020 relative to 2019 and 2015-2019. Strong reductions of NO2 were reported in many regions of Spain. A substantial part of these reductions is obviously due to decreased local and regional anthropogenic emissions (Petetin et al., 2020). Due 115 to low emissions of NO2 during lockdown period, it is observed that the ambient concentrations did not rise back to 2019 levels. The short-term climatological mean (2015-2019) NO2 levels are contrasted against lockdown period in Figure 3b. Due to the strict lockdown conditions, ambient NO2 levels from 1 st March -30 th April 2020 were ~2.08×10 15 molecules cm -2 which was nearly 14.5% lower than the short-term climatological average. Bar plots of NO2 levels during 2015-2020 and the rate of change (RoC) in NO2 levels against the short-term climatological mean (Figure 3c-d) indicate the impact of lockdown on NO2 120 emissions over Indian region. The RoC is extremely important in weather and climatological studies because it allows understanding and predicting the trends/patterns in climatic parameters. RoC is used to describe the percentage change in a https://doi.org/10.5194/acp-2020-621 Preprint. Discussion started: 13 July 2020 c Author(s) 2020. CC BY 4.0 License.
parameter over a defined period of time and it represents the rate of acceleration of the parameter. There is a clearly observable lowering in NO2 levels relative to the short-term climatological mean by 12% for the pre-lockdown period, 18% and 8% for the phase I and phase II lockdown periods, respectively. 125

Impact of lock down on CO variations over Indian region 130
Tropospheric CO data was obtained from Terra/MOPITT (2015-2019) and Sentinel-5p/TROPOMI (2020) respectively. The mean CO levels over Indian region during the pre-lockdown and lockdown periods, comparison of CO levels during lockdown period against the 5-year short-term climatological mean for 2015-2020 periods were studied to assess the short-term climatic effect due to COVID-19 lock down. Figure 4(a-c) shows mean concentrations of tropospheric CO over the Indian region during pre-lockdown phase I and phase II lockdown periods. During the pre-lockdown period (1-10 th March 2020), CO levels were 135 higher (δ = 0.04×1017 molecules cm -2 ) as compared to 2019 by ~2%, which is expected due to anthropogenic activities. During phase I (mean = 2.50×1018 molecules cm -2 ) there is a decrease in CO levels over north and south India as compared to 2019 (mean = 2.28×1018 molecules cm -2 ). However, the mean CO levels are actually higher during the phase II of lockdown. This indicates that the longer life time of CO molecules in the atmosphere (~1-2 months) relative to NO2 has resulted in not much https://doi.org/10.5194/acp-2020-621 Preprint. Discussion started: 13 July 2020 c Author(s) 2020. CC BY 4.0 License. variation in atmospheric CO levels. The weekly variability of CO plotted using TROPOMI data, indicates spatial variation in 140 CO emissions during the lock down period as shown in figure 4d. are -2 %, 9 % and 8 % respectively. 150

Impact of lock down on variability of AOD over Indian region
Besides greenhouse gases effect on Earth's climate, aerosols in the lower atmosphere also alters the earth's climate by the process of scattering and absorbing of incoming solar radiation, which also helps in altering cloud properties (Boucher et al., 2013). We have used AOD550 products from Terra-Aqua/MODIS which are generated for 10:30 and 13:30 local time respectively. As we observed similar spatial variation of AOD550 from both Terra-Aqua/MODIS, only Aqua/MODIS derived 155 AOD550 is shown here (Figure 4). AOD550 levels over the Indian region for 2019, 2020 and the difference in AOD550 for both years for pre-lockdown period is depicted in Figure 5a. During this period, the AOD550 levels for 2020 (0.39) over the Indian region were higher than for 2019 (0.35). The relatively higher aerosol levels in 2020 over Indo-Gangetic Plain (IGP) and central India during the pre-lockdown period could be due to seasonal loading of aerosols over the Indian region and premonsoon western dusty winds, which elevates AOD550 concentration (David et al., 2018). The maximum AOD550 over Indian 160 region varied between 0.8 to 1.0 during the lockdown period with a noticeable reduction in AOD550 over the IGP region. Spatial maps of AOD550 for 2019 indicate high levels of aerosol loading over the densely populated IGP due to the high industrial emissions in this area. The formation and removal of aerosols is influenced by emission sources, prevailing meteorological conditions and wet removal in monsoon season (Babu et al., 2013). Overall, a significant lowering of aerosols from the atmosphere is observed over the Indian region during the lockdown period. Figure  https://doi.org/10.5194/acp-2020-621 Preprint. Discussion started: 13 July 2020 c Author(s) 2020. CC BY 4.0 License.

Short-term climatological variation of AOD due to lock down
Aerosol optical depth is one of the important short-term climatic forcing agents along with long lived greenhouse gases namely carbon dioxide (CO2), methane (CH4), water vapor (H2O) and nitrous oxide (N2O). A 7 day smoothing average filter was 175 applied on AOD550 time series data as discussed in section 3.1.1. Figure 6a-d show 7-day averaged time series analysis of AOD550 levels over Indian region during 2019-2020 and contrast the AOD550 levels during the lockdown period against the short-term climatological mean of 2015-2019. The bar plots in Figure 6 indicate the seasonal dispersion of AOD550 and influence of other factors. Analysis of AOD550 for pre-lockdown, phase I and phase II of lockdown period (compared to corresponding period in 2019) indicated a high AOD550 in pre-lockdown period followed by sharp drop during the lockdown 180 period, possibly due to decrease in anthropogenic activities. The Rate of change in AOD550 was computed (figure 6e-f) to understand the effect of short-term climatological mean AOD550 over lock period AOD550 in 2020. A RoC of 5 % positive (negative) was observed before LD (red color), as measured by the Terra/MODIS (Aqua/MODIS) which shows poor correlation during this period. However, strong reduction of 24-25 % in phase-1 and 10-12 % in phase-2 lock period was observed against 5-year mean AOD550. Thus, lock down helped to improve the air quality in forthcoming days. Aerosol 185 properties extend feasible support for modulating the cloud properties, which subsequently helps amount of precipitation (Boucher et al., 2013). A study over the Indian sub-continent during Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) in 2009 and 2010 was reported that a polluted cloud has more number of smaller drops, which causes reduction in precipitation by the cloud (Konwar et al., 2012). Studying and quantifying the impact of aerosols on precipitation is still a thrust area of research (Myhre et al., 2013). As precipitation is pre-dominantly influenced by the availability of 190 moisture and cloudiness, this lockdown has significantly altered aerosol loading over the Indian region, we hypothesize the Indian summer monsoon of 2020 may improve as compared to the preceding year. However, strong evidences are not explored in the present study.
https://doi.org/10.5194/acp-2020-621 Preprint. Discussion started: 13 July 2020 c Author(s) 2020. CC BY 4.0 License.    short-term climatological mean of NO2, which indicates the impact of the India lockdown in connection with the COVID-19 resulted in enhanced air quality over the country. Table 3 shows major cities of India and their RoC in dropped NO2 levels during lockdown period in 2020 against 2019 and short-term climatological mean.

Conclusions 240
Present study carried out analysis on air pollution in connection with the world's largest lock down imposed by India to contain the spread of COVID-19. The lockdown was extended over several phases viz., phase I (21 days from 25 March to 14 April 2020), phase II (19 days from 15 April to 3 May 2020), phase III (14 days from 4 May to 17 May 2020) and phase IV (14 days from 18 May to 31 May 2020). However, the lockdown was near total only in phase I and II, with the near total shutdown of industrial and transport sectors. We used satellite-based observations of tropospheric NO2, CO and aerosol optical depth and 245 analyzed the pollutant concentrations during the period of lockdown against the same period of the preceding year and contrasted against the short-term climatological mean for the 5 year period from 2015-2019. Results of the study show strong reduction of NO2 and AOD over the country, state, and cities whereas mixed variation observed in CO emissions. NO2 levels over the Indian region decreased by 17 %. The 5-year short-term climatological mean of NO2 during 2015-2019 shows 12 % decrease when compared to pre-lockdown and 18 % during phase-1 lock period, respectively. Aerosols / particulate matter 250 content of the atmosphere, as quantified by aerosol optical depth (AOD), decreased over high emission regions (IGP) during the lockdown period. The short-term climatological mean of AOD showed strong reduction (24-25 %) during lock period, which is a positive indicator for the radiation budget and forthcoming seasonal rains in India. During lock period, CO has not turned up positive sign for air pollution, which could be due to longer lifetime compared to NO2. Rate of change of NO2, CO and AOD for each state was computed during lock period and compared against respective 5-year averaged emissions. 255 Northern and eastern part of the country showed strong decrease of emissions due to lock down. Among the metropolitan cities of India, the capital city of New Delhi has shown greatly enhanced air quality during the lockdown period. Thus, exhaustive lock down due to outbreak of COVID-19 has led to improvement in the air quality and short-term climatic effects over India.

Acknowledgement
Authors sincerely thank Shri Santanu Chowdhury, Director NRSC for his support and encouragement for carrying out this 260 study. We greatly acknowledge Earth data and Giovanni data web portal for providing the free access to the Aura/OMI, Terra/MOPITT, Sentinel-5p/TROPOMI and Aqua-Terra/MODIS satellites data. We thank Mr. S.V.S. Sai Krishna, Scientist, ECSA, NRSC and Dr.P. Raja, Principle Scientist, Indian Institute of Soil and Water Conservation-Indian Council of Agriculture Research, Ooty, India for reviewing the manuscript. Authors thank Ms. A.L. Kanchana, Scientist, ECSA, NRSC for helping with the data sets. 265 https://doi.org/10.5194/acp-2020-621 Preprint. Discussion started: 13 July 2020 c Author(s) 2020. CC BY 4.0 License.