key: cord-0689349-qycx9g2t
authors: Mazhar, Usman; Jin, Shuanggen; Bilal, Muhammad; Arfan Ali, Md.; Khan, Rehana
title: Reduction of surface radiative forcing observed from remote sensing data during global COVID-19 lockdown
date: 2021-06-11
journal: Atmos Res
DOI: 10.1016/j.atmosres.2021.105729
sha: dbe51a18f6acb33139e4fbc34e539b20f0826a0e
doc_id: 689349
cord_uid: qycx9g2t

The calamity of the COVID-19 pandemic during the early half of 2020 not only caused a huge physical and economic loss but altered the social behavior of the whole world. The social and economic stagnation imposed in many countries and served as a major cause of perturbation in atmospheric composition. This paper utilized the relation between atmospheric composition and surface radiation and analyzed the impact of global COVID-19 lockdown on land surface solar and thermal radiation. Top of atmosphere (TOA) and surface radiation are obtained from the Clouds and Earth's Radiant Energy System (CERES) and European Reanalysis product (ERA5) reanalysis product. Aerosol Optical Depth (AOD) is obtained from Moderate Resolution Imaging Spectroradiometer (MODIS) while Nitrogen dioxide (NO(2))(,) and sulfur dioxide (SO(2)) are obtained from Ozone Monitoring Instrument (OMI). Observations of all mentioned parameters are studied for the global lockdown period of 2020 (from January to July) and compared with the corresponding months of the previous four years (2016–19) observations. Regarding surface radiation, April 2020 is the most affected month during the pandemic in which 0.2% increased net solar radiation (NSR), while 3.45% and 4.8% decreased net thermal radiation (NTR) and net radiation (NR) respectively was observed. Average radiative forcing during March–May 2020 was observed as 1.09 Wm(−2), −2.19 Wm(−2) and −1.09 Wm(−2) for NSR, NTR and NR, respectively. AOD was reduced by 0.2% in May 2020 while NO(2) and SO(2) were reduced by 5.4% and 8.8%, respectively, in April 2020. It was observed that NO(2) kept on reducing since January 2020 while SO(2) kept on reducing since February 2020 which were the pre-lockdown months. These results suggest that a more sophisticated analysis is needed to explain the atmosphere-radiation relation.

energy balance of the earth. The external factors include natural sources such as the total solar irradiance and anthropogenic sources such as AOD and greenhouse gasses. It is important to analyze the impact of energy balance on climate change by computing the perturbations in the external factors (de Coninck et al., 2018; Lewis & Lewis, 2018; Shine, 2000) . Modeling techniques are used to study the impact of air pollution on NSR, NTR, and NR assuming some ideal situation about the absence of one or more pollutants (Schultz et al., 2003; Unger et.al., 2008) .

Recently a real-time scenario developed in which many studies claimed the reduction of air pollution across the world. In early 2020, a respiratory epidemic named COVID-19 (coronavirus disease 2019) was firstly reported in Wuhan. Because of the widespread disease, on March 11, 2020, World Health Organization (WHO) declared COVID-19 as a global pandemic (WHO, 2020) . As the disease spread over many countries, national governments of affected countries imposed lockdown (suspension of social, travel, and economic activities). To restrict the spread of this air-born disease, China government imposed a strict lockdown on 23 January 2020 till the last week of April (Lian et al., 2020) . In Europe, these restrictions were started in mid-March and extended till May 2020, and then gradually the restrictions became more lenient (Ordóñez et al., in traffic was observed which are the major causes of air pollutants e.g., material particles, nitrogen dioxide (NO 2 ), sulfur dioxide (SO 2 ), and many others. Many studies revealed that regionally suspended anthropogenic and economic activities resulted in the changed atmospheric compositions in many parts of the world (Anil & Alagha, 2021; Chauhan & Singh, 2020; Dantas et al., 2020; Lian et al., 2020; Liu et al., 2021; Mor et al., 2021; Nichol et al., 2020; Ordóñez et al., 2020; Sharma et al., 2020; Siddique et al., 2021) . Nichol et al. (2020) observed an increase of fine particulate PM 2.5 in China's economic hub; the Beijing-Tianjin-Hebei region while a drastic decrease was observed in NO 2 in the same region. Lian et al. (2020) Investigated air quality over Wuhan, China, and observed that the air quality index got 33.9% better during lockdown with respect to the pre-lockdown months. PM 2.5 and NO 2 decreased by 36.9% and 53.3% respectively while Ozone (O 3 ) increased by 116.6%. Qiu et al. (2021) observed a remarkable 47% decrease in AOD and 3 to 43% decrease in NO 2 over multiple cities in Bangladesh while an average increase of 3 to 12% in O 3 was observed at the same time. Anil and Alagha (2021) reported a huge decrease of 12-86% in NO 2 over the eastern provinces of Saudi Arabia, along with an 8.7-30% decrease in SO 2 and 21-70% decrease in PM 10 . In one of the early studies about the impact of COVID-19 on air pollution, Chauhan and Singh (2020) observed a decrease in PM 2.5 in March positively related to the spread of the disease in Italy. Shi et al (2021) reported a decrease in NO 2 while an increase in O 3 in many cities across the world. Using some machine learning techniques, they suggested that the changes were abrupt but smaller than expected.

Despite some recent studies about the impact of COVID-19 economic stagnation on air pollution, there remained a scholarly gap of resultant impact on surface radiation. This paper analyzes the variations of NSR, NTR, and NR during the global lockdown period and their relationship with the AOD, NO 2, and SO 2 . The global land surface aerial averages of NSR, NTR and NR are used for analysis. NSR, NTR, and NR data are obtained from CERES and ERA5 that is a product of the European center for medium-range weather forecasts (ECMWF). AOD is obtained from MODIS while NO 2 and SO 2 are obtained from OMI. The 2016 to 2019 period is taken as a reference period. Observations during the 2020-lockdown period are compared with this reference period to quantify the lockdown-driven changes in any obtained parameter.

Multiple remote sensing data sets were used in this study to analyze the impact of COVID-19 lockdown on surface NSR, NTR, and NR. For top of atmosphere (TOA) shortwave radiation (SR) and thermal radiation (TR), CERES synoptic TOA and surface fluxes and clouds SYN1deg level 3 product (hereafter SYN) was used. This product provides daily observations of TOA and surface fluxes at 1 o spatial resolution. At surface level, daily NSR, NTR, and NR were obtained Balanced and Filled (EBAF) monthly product was obtained. CERES EBAF surface product provides monthly observations at 1 o spatial resolution having accuracy up to 4 Wm -2 and 6 Wm -2 for NSR and NTR respectively Loeb et al., 2018) . These data were obtained from the NASA Langley research center CERES ordering tool at https://ceres.larc.nasa.gov.

Along with remote sensing products, ERA5-land surface monthly data at 0.1 o spatial resolution was used for NSR, NTR, and NR (Hersbach et al., 2020) . A significant correlation of 0.88 was observed regionally for ERA5 NR in comparison with NR observed from FluxNet ground towers (Mazhar et al., 2021) . ERA5 data were obtained from https://cds.climate.copernicus.eu. The reason for obtaining multiple data sets for the same parameter is to avoid any chance of sensor/method limitation in the unprecedented suspended anthropogenic activities.

To analyze any perturbation in atmospheric composition and to observe the effects of anthropogenic activities, AOD, NO 2, and SO 2 global data were obtained. A combined dark target and deep blue AOD at 0.55 micron for land and ocean (MYD08_M3 v6.1) product from Aqua MODIS was used. The product provides daily observations at 1 o spatial resolution. Combined AOD is retrieved from the high-quality dark target and deep blue algorithms Bilal et al. 2017; Hsu et al., 2013; Levy et al., 2013) . For NO 2 data, OMI/Aura NO 2 cloudscreened total and tropospheric column L3 global gridded at 0.25 o product (OMNO2d) from OMI was used. The product provides vertical column density (from the surface to TOA) under 2020. We used the observations of the first seven months (i.e. January to July) of each year from 2016 to 2020. The period 2016 to 2019 was taken as a reference period, and observations from all parameters were compared with the corresponding months of 2020. January, February, and June, July 2020 were observed to see the pre and post-pandemic impacts respectively. For daily SR and TR at TOA and NSR, NTR, and NR at the surface, a standard deviation (SD) was computed for each month to observe any abrupt changes in the corresponding parameter. For monthly analysis absolute and percentage, the difference was computed for each parameter using the following equations.

(2)

Here P 20 refers to the mean monthly value of the parameter in 2020; P ref means the 4-year monthly average of the corresponding parameter. The same formula was used for the analysis of spatial anomalies. Figure 1 shows the relation between incoming and outgoing SR, TR, NSR, NTR and NR with interaction is a complex phenomenon and correlation between any two parameters may be misleading. Thus, multiple linear regression was found by taking AOD, NO 2 and SO 2 as independent variables and each of SR, TR NSR, NTR and NR as the dependent variable.

Incoming and outgoing SR are nominally affected by pollutants and show the R-value of 0.15 and 0.13 respectively. Outgoing SR shows the smallest standard error of 14.38 amongst all the observed relations. Figure 1 An observable fact from these relations is that neither of the R-value is convincingly strong. A major reason for the weak correlation is that these radiations are mainly affected by the biophysical parameters such as albedo, land surface temperature and land cover (Anderson et al., 2011; He, et al., 2015; Nair et al., 2007; Wild et al., 2007) . Despite these factors, the radiative forcing of atmospheric pollutants has a significant effect on surface radiations (Agudelo-9 contributes to radiative forcing (Ali & Assiri, 2019; Ali et al., 2019; Bilal et al., 2019; Khan et al., 2020; Kumar et al., 2017) . NO 2 involved in the absorption of visible and infrared radiation contributes to radiative forcing (Etminan et al., 2016; Schultz et al., 2003; Solomon et al., 1999; Vasilkov et al., 2009) . Despite the small impact on global radiative forcing, locally NO 2 induced radiative forcing has an impact of 2 to 4 Wm -2 (Vasilkov et al., 2009) . SO 2 has less radiative forcing than NO 2 , still, it has a consistent greenhouse effect in the climate (Bais et al., 1993; Giorgi et al., 2002; Khodakarami & Ghobadi, 2016) .

Journal Pre-proof Figure 1 . Correlation between surface radiation and AOD, NO 2 and SO 2. Each point represents the mean monthly observation of the corresponding location. SR, TR, NSR, NTR and NR are measured in Wm -2 , NO 2 is measured in 10 14 mole/cm 2 while SO 2 is measured in Dobson Unit (DU). The land surface radiation data is obtained from EBAF mean monthly spatial images. Each relation has 875 point values. SR in and TR in correspond to incoming SR and TR while SR out and TR out correspond to outgoing SR and TR. 

Outgoing TOA SR and TR were investigated using daily SYN data. Figure Here it is not confirmed that this decrease is due to the economic stagnation because in this case, the reduction in TOA TR shall be continued till May 2020. Both TOA SR and TR exhibit no significant different pattern during the lockdown period. TOA SR shows a symmetrical pattern except in May but TOA TR shows abrupt changes throughout the study duration. 

J o u r n a l P r e -p r o o f Journal Pre-proof Figure 3 shows daily variations in global land surface NSR, NTR and NR observed using SYN daily data from January to July 2016-2020. Table 3 presents the SD values Land surface radiation are influenced by greenhouse gasses and aerosols. For a better understanding of the variations of surface radiation, AOD, NO 2, and SO 2 were analyzed from January to July 2016 to 2020. Figure 4 shows the daily variations of AOD, NO 2, and SO 2 . In 2020, during January and February (before economic stagnation) AOD shows prominent high values than the previous years but it gradually decreased from March to the middle days of May and then started rising again. Here it is noteworthy that AOD is an event-dependent variable, and the global average gives only an overall picture of the AOD event occurrences. NO 2 tropospheric column average shows less values throughout 2020 as compared with the reference duration. The daily least value of 3.02 10 14 mole/cm 2 is observed on 3 March 2020. After May 2020, NO 2 increased gradually. Increasing NO 2 column averages in summer months is a pattern that is also observed in the reference duration. The increased NO 2 during summer 2020, still remained lower than the reference observations. Total column SO 2 shows an early increase in January 2020, and decrease subsequently till July. The two least daily SO 2 total column values observed in 2020 as 0.141DU on 4 January and 0.148Du on 22 April.

Journal Pre-proof Figure 6 shows the monthly variations in NSR, NTR, and NR using the global land surface mean monthly values obtained from SYN, EBAF, and ERA5 data sets. for monthly analysis, absolute and percentage differences are computed for surface radiation and atmospheric pollutants, using Equation 1 and 2 respectively. forcing of -1.3 Wm -2 , -1.09 Wm -2 and -0.57 Wm -2 is observed for NR from SYN, EBAF and ERA5 data respectively. It is important to mention that the radiative forcing stated here is not associated with any single perturbation but the unprecedented global anthropogenic situation is assumed to be its precursor.

All three data sets were not strictly in agreement for NSR, NTR and NR monthly variations. This might happen because of the different parameterization schemes and the minor variations may respond differently in different algorithms (Zhu et al., 2012) . Figure 6 . Monthly variations in NSR, NTR, and NR from January to July 2016-2020. Each value is the global land surface mean monthly observation. The first row represents SYN observations, the second row represents EBAF observations and the third row represents data obtained from the ERA5 data set. All parameters are measured in Wm -2 . It is noteworthy that global lockdown was not imposed simultaneously and the span of strict lockdown expands many months in the first half of 2020, but March, April, and May were the months when most countries opted for the closure of social and economic activities. Thus during this period, variations in atmospheric composition and surface radiation were righteously associated with the impact of the lockdown strategies. Figure 7 shows the monthly variations of AOD, NO 2, and SO 2 . In the pre-lockdown months of 2020, prominent higher values of AOD are observed, but then the monthly average of AOD decreased gradually till May 2020, which shows 0.3% reduced (0.162) observations than the reference period. For NO 2 the least monthly average of 3.33 10 14 mole/cm 2 is observed in February 2020. Note that, this is the month before the pandemic prevailed in the whole globe.

During April and May 2020, NO 2 mean monthly average values are 3.5 and 3.9 10 14 mole/cm 2 J o u r n a l P r e -p r o o f Journal Pre-proof respectively which are 5.4% and 7.3% less than the monthly averages of April and May observed in the reference duration. After May 2020, NO 2 increased gradually. In the monthly analysis, SO 2 shows an increased value in January 2020, and then gradually decreased till July. It is interesting to observe that SO 2 shows smaller mean monthly values from February to July 2020 than the corresponding months of reference duration. From April to June 2020 SO 2 column values are 0.0174, 0.0185, and 0.02 DU which are respectively 8.8%, 11.7%, and 13.5% less than the corresponding months of the reference period.

Although in recent literature, a significant decrease in regional AOD is reported over many parts of the world Qiu et al., 2021; Siddique et al., 2021) , it was found that only May 2020 has a minor decrease of 0.2% than the reference period. Here the contextual discussion is needed for unbiased analysis. In January and February 2020, AOD showed an increase of 21.9% and 13% respectively over the reference period. The prominent increase gradually reduced to 6.5% and 3.1% in March and April 2020. After lockdown, in June 2020, global AOD again showed an increase of 5.8%. Thus it can be concluded that although global AOD showed an apparent increase during the 2020-lockdown, still actually it falls gradually during the lockdown months of 2020 as compared with the pre-lockdown months. Also, the life span of suspended particles restricts a sudden reduction in AOD even when the emission of particles is slowed down (Haywood, 2016) .

Many studies about air quality during the 2020-lockdown reported that NO 2 decreased significantly and relate this decrease with the economic stagnation (Anil & Alagha, 2021; Dantas et al., 2020; Lian et al., 2020) . Here it is found that NO 2 did not only decreased during the lockdown but also in pre-lockdown months, i.e. January and February 2020, global NO 2 column concentration showed a more prominent decrease of 8% and this decrease continued in post-J o u r n a l P r e -p r o o f lockdown months i.e. June and July 2020 which showed a 4% and 4.2% decrease respectively.

This result leads towards two facts; firstly the approach of comparing 2020-lockdown months with the same months of 2019, or only compare few pre-lockdown months with the lockdown months, used by many recent studies limits them for the narrow window of 2020-lockdown which might not give the required liberty to analyze the true variations. Secondly, NO 2 emission reduction during the COVID-19 economic stagnation is not the only reason for the temporal decrease of NO 2 column concentration. Future dedicated studies about NO 2 will potentially reveal the detailed reasons behind the decreasing pattern of NO 2 .

SO 2 showed an increase of 7.6% in January 2020, after then, it showed a consistent decrease of 7.1%, 6.8%, 8.8%, 11.7%, 13.5%, and 9.1% from February to July 2020 respectively. A prominent decrease in a globally pre-lockdown month (February) and post-lockdown months (June and July) weakens the argument that the decrease was caused only due to the emission reduction. Apart from the argument that the global decrease of two important gaseous pollutants, China, while shows a mixed pattern for the rest of the world during the 2020-lockdown. NO 2 shows the most prominent negative anomalies except for the few sparse regions in the Southern hemisphere. SO 2 shows the most different spatial pattern with equal distribution of negative and positive anomalies. Although the monthly average of SO 2 for April 2020 is 8.8% lesser than the reference period, yet spatially, the positive and negative anomalies are equally distributed. Figure 8 . Global percentage anomalies of NSR, NTR, NR, AOD, NO 2 and SO 2 for the month of April 2020 with respect to the reference monthly average of April 2016-19. In this figure, NSR, NTR and NR data is obtained from EBAF, AOD is obtained from Aqua MODIS, NO 2 and SO 2 are obtained from OMI. Less than zero anomalies (red and yellow) mean the decreased while greater than zero anomalies (green and blue) mean the increased value of the corresponding parameter during April 2020.

For a better comparison of spatial anomalies, few random points were selected across the globe.

The location of selected points is presented in Figure 9 while Figure 

The unprecedented situation of social and economic stagnation across the world during the global COVID-19 pandemic provided an opportunity to investigate the real-time impacts of variations in atmospheric composition on surface radiation. In this study, variations in global land surface NSR, NTR, and NR were analyzed as an outcome of perturbations in AOD, NO 2, and SO 2 . The impact of lockdown during the COVID-19 pandemic is not strong enough to reach TOA as no brightening or dimming of SR and TR was observed at TOA. Suspension of pollutants in the atmosphere for a long time was another potential cause for the undisturbed TOA SR and TR. However variations in surface radiation were observed as NSR showed no prominent variations, but NTR was significantly decreased throughout the 2020-lockdown period consequently NR also showed a decrease during the same time. April 2020 was the most affected month and showed prominent negative anomalies in NTR and NR.

All observed air pollutants i.e., AOD, NO 2, and SO 2 reduced during the COVID-19 lockdown consequently global air quality was improved. The footprints of NO 2 reduction extended back to pre-lockdown months, indicating that apart from low emission during the lockdown, there might be some other factors involved in the global NO 2 reduction. In April 2020, collective emission reduction of 5.4% and 8.8% in NO 2 and SO 2 respectively appeared as 0.2% increase (0.27 Wm -2 ) in NSR, 3.45% decrease (-2.88 Wm -2 ) in NTR and 4.8% decrease (-2.61 Wm -2 ) in NR. One limitation of this study is that for all the observed parameters, global aerial averages were used hence many fine and important details were compromised. It would be an interesting future study for some regional analysis. Pixel to pixel analysis might reveal more accurate findings.

Investigations of MODIS AOD and cloud properties with CERES sensor based net cloud radiative effect and a NOAA HYSPLIT Model over Bangladesh for the period

Classification of aerosols over Saudi Arabia from

Biophysical considerations in forestry for climate protection

The impact of COVID-19 lockdown on the air quality of Eastern Province, Saudi Arabia. Air Quality, Atmosphere and Health

CERES Synoptic Product : Methodology and

Characteristics of fine particulate matter (PM2.5) over urban, suburban, and rural areas of Hong Kong

New customized methods for improvement of the MODIS C6 Dark Target and Deep Blue merged aerosol product. Remote Sensing of Environment

Decline in PM2.5 concentrations over major cities around the world associated with COVID-19

The impact of COVID-19 partial lockdown on the air quality of the city of Rio de Janeiro

The relationship between atmospheric carbon dioxide concentration and global temperature for the last 425 million years

Geostationary enhanced temporal interpolation for ceres flux products

Advances in geostationary-derived longwave fluxes for the CERES synoptic (SYN1deg) product

Radiative forcing of carbon dioxide, methane, and nitrous oxide: A significant revision of the methane radiative forcing

Direct radiative forcing and regional climatic effects of anthropogenic aerosols over East Asia: A regional coupled climate-chemistry/aerosol model study

Atmospheric Aerosols and Their Role in Climate Change

Enhanced Deep Blue aerosol retrieval algorithm: The second generation

Surface irradiances of edition 4.0 Clouds and the Earth's Radiant Energy System (CERES) Energy Balanced and Filled (EBAF) data product

The contribution of different aerosol types to direct radiative forcing over distinct environments of Pakistan inferred from the AERONET data

Urban pollution and solar radiation impacts

The Collection 6 MODIS aerosol products over land and ocean

Intergovernmental Panel on Climate Change

Impact of city lockdown on the air quality of COVID-19-hit of Wuhan city

Effects of COVID-19 lockdown on global air quality and health

Clouds and the Earth'S Radiant Energy System (CERES) Energy Balanced and Filled (EBAF) top-of-atmosphere (TOA) edition-4.0 data product

Impact of meteorological conditions and air pollution on COVID-19 pandemic transmission in Italy

Spatio-temporal trends of surface energy budget in tibet from satellite remote sensing observations and reanalysis data

Radiative forcing and temperature response to changes in urban albedos and associated CO2 offsets

Impact of COVID-19 lockdown on air quality in Chandigarh, India: Understanding the emission sources during controlled anthropogenic activities

Observational estimates of radiative forcing due to land use change in southwest Australia

Air pollution scenario over China during COVID-19

Early spring near-surface ozone in Europe during the COVID-19 shutdown: Meteorological effects outweigh emission changes

Spatiotemporal Investigations of Multi-Sensor Air Pollution Data over Bangladesh during COVID-19 Lockdown

Air Pollution and Climate

Forcing Impacts of a Global Hydrogen Economy

Effect of restricted emissions during COVID-19 on air quality in India

Abrupt but smaller than expected changes in surface air quality attributable to COVID-19 lockdowns

Radiative forcing of climate change

Unprecedented environmental and energy impacts and challenges of COVID-19 pandemic

On the role of nitrogen dioxide in the absorption of solar radiation

Deriving surface ultraviolet radiation from CERES surface and atmospheric radiation budget: Methodology

Photosynthetically active radiation from Clouds and the Earth's Radiant Energy System (CERES) products

Air pollution radiative forcing from specific emissions sectors at 2030

Impact of tropospheric nitrogen dioxide on the regional radiation budget

Spatiotemporal changes of surface solar radiation: Implication for air pollution and rice yield in East China

World Health Organization Statement on the second meeting of the International Health Regulations (2005) Emergency Committee regarding the outbreak of novel coronavirus (2019-nCoV)

A pneumonia outbreak associated with a new coronavirus of probable bat origin

An assessment of summer sensible heat flux on the Tibetan Plateau from eight data sets