Elsevier

Environmental Research

Volume 193, February 2021, 110514
Environmental Research

Revisiting the levels of Aerosol Optical Depth in south-southeast Asia, Europe and USA amid the COVID-19 pandemic using satellite observations

https://doi.org/10.1016/j.envres.2020.110514Get rights and content

Highlights

  • AOD reduced to 20% in majority of the areas in SSEA, Europe and USA due to lockdown measures.

  • NO2 reduction was measured up to 20–40%, while SO2 increased up to 30% for majority of the areas during lockdown.

  • A complex interaction among SO2, WS and RH leads to high AOD by forming new particles.

  • Significant correlation was found between SO2 concentration and residential mobility during lockdown period for USA.

  • A distinguishable influence on AOD was obtained for emission and meteorological factors using random forest regression.

Abstract

The countries around the world are dealing with air quality issues for decades due to their mode of production and energy usages. The outbreak of COVID-19 as a pandemic and consequent global economic shutdown, for the first time, provided a base for the real-time experiment of the effect of reduced emissions across the globe in abetting the air pollution issue. The present study dealt with the changes in Aerosol Optical Depth (AOD), a marker of air pollution, because of global economic shutdown due to the coronavirus pandemic. The study considered the countries in south and south-east Asia (SSEA), Europe and the USA for their extended period of lockdown due to coronavirus pandemic. Daily Aerosol Optical Depth (AOD) from Moderate-resolution imaging spectroradiometer (MODIS) and tropospheric column density of NO2 and SO2 from Ozone monitoring instrument (OMI) sensors, including meteorological data such as wind speed (WS) and relative humidity (RH) were analyzed during the pre-lockdown (2017–2019) and lockdown periods (2020). The average AOD, NO2 and SO2 during the lockdown period were statistically compared with their pre-lockdown average using Wilcoxon-signed-paired-rank test. The accuracy of the MODIS-derived AOD, including the changing pattern of AOD due to lockdown was estimated using AERONET data. The weekly anomaly of AOD, NO2 and SO2 was used for analyzing the space-time variation of aerosol load as restrictions were imposed by the concerned countries at the different points of time. Additionally, a random forest-based regression (RF) model was used to examine the effects of meteorological and emission parameters on the spatial variation of AOD. A significant reduction of AOD (−20%) was obtained for majority of the areas in SSEA, Europe and USA during the lockdown period. Yet, the clusters of increased AOD (30–60%) was obtained in the south-east part of SSEA, the western part of Europe and US regions. NO2 reductions were measured up to 20–40%, while SO2 emission increased up to 30% for a majority of areas in these regions. A notable space-time variation was observed in weekly anomaly. We found the evidence of the formation of new particles for causing high AOD under high RH and low WS, aided by the downward vertical wind flow. The RF model showed a distinguishable relative importance of emission and meteorological factors among these regions to account for the spatial variability of AOD. Our findings suggest that the continued lockdown might provide a temporary solution to air pollution; however, to combat persistent air quality issues, it needs switching over to the cleaner mode of production and energy. The findings of this study, thus, advocated for alternative energy policy at the global scale.

Introduction

The spreading of the coronavirus, a genus of the severe acute respiratory syndrome (SARS)-Cov-2, throughout the world by human transmission has turned it into a global pandemic. On March 11, 2020, the World Health Organization (WHO) announced the COVID-19 disease caused by the coronavirus-as global pandemic (https://www.who.int/dg/speeches) when the spread of the virus had infected 118,000 population of 114 countries. The countries around the world were advised to contain the spread of the virus by putting measures as suggested by WHO (WHO, 2020). Consequently, the majority of the countries in the world where COVID-19 disease outbreak happened at a noticeable level or expected to climb up soon decided to impose complete nationwide lockdown, which resulted in stopping most of the economic activities and restricting the movement of the people to combat the spread of coronavirus. With economies in standby mode, a significant reduction in emission was expected that could reduce the pollution level, especially, the aerosol load in the atmosphere and corresponding reduction of the Aerosol Optical Depth (AOD) – a measure of extinction of light energy in visible channel due to aerosol's scattering and absorption - at the global scale.

Several recent studies carried out on the outbreak of COVID-19 aiming to study the improvement in the air quality index (AQI) due to the reduction of emission (Mahato et al., 2020; Sharma et al., 2020; Tobías et al., 2020; Chauhan and Singh, 2020). A significant reduction in the PM2.5 was observed over Malaysia (Abdullah et al., 2020), China (Bao and Zhang, 2020; Li et al., 2020), India (Sharma et al., 2020), Spain (Tobias et al., 2020), and over Brazil (Dantas et al., 2020). Collivignarelli et al. (2020) reported that the restriction in people's movement and social distancing measure has significantly reduced PM2.5, PM10, BC, CO and NOx level in the city of Milan in Italy. However, their study noted a significant improvement of surface O3 that they attributed to the reduction of NOx and intense solar radiation. Kanniah et al. (2020) reported up to 32% reduction in particulate matter in Malaysia due to imposition of lockdown. Additionally, the study found up to 64% reduction in NO2, while 9–20% reduction in SO2 and 25–31% reduction in CO was estimated from ozone monitoring instrument (OMI) onboard Aura satellite. Over Sao Paolo in Brazil, Nakada and Urban (2020) demonstrated more than 54% reduction in NOx, and up to 64% reduction in CO during the lockdown period. The estimation over eastern China showed a 50% reduction of NOx emission due to restriction in the movement to contain the spread of the coronavirus (Zhang et al., 2020). The study in the city of Tehran in Iran in the middle east, also demonstrated a notable reduction of NO2 and SO2, while O3 and PM2.5 levels increased during lockdown period (Broomandi et al., 2020). Wang et al. (2020) reported the reduction of PM2.5 was not as per expectation despite near-complete lockdown. The study also reported that the unfavorable meteorology often overwhelmed the lockdown effect. A similar finding was reported by Li et al. (2020) considering the pollution source contribution function.

With nearly 15% contribution from anthropogenic sources to the total aerosol load, PM2.5 plays a vital role in the global environment and health (Klimont et al., 2017). The study also showed a regional-scale analysis of the emission of primary anthropogenic aerosols that witnessed a decrease of emission in high-income group countries, while middle- and low-income group countries were still having a higher contribution to the total global emission of PM2.5 (Klimont et al., 2017). Cohen et al. (2017) reported an annual average concentration of PM2.5 more than 50 μg m−3 for India, China, Pakistan, and Afghanistan in SSEA, while it was less than 15 μg m−3 for Europe and the USA. The estimates at the global level from the Emission Database for Global Atmospheric Research (EDGAR), for the year 2012, showed the total emissions of NO2 and SO2 were about 3933 and 4493 Mt annually (Crippa et al., 2020; Fioletov et al., 2016). The emission database considered all sectors of anthropogenic activities as recognized by the IPCC. The database recognizes, transport sector as one of the prime sources of NO2 emission while burning fossil fuel and biofuel to produce energy in the power plant is one of the principal sources of SO2 emission in the atmosphere. The emission database (for the year 2012), however, noticed a wide-scale regional variation. The level of NO2 and SO2 emissions in the US was computed as 551 and 225 Mt y−1, respectively, mostly coming from the public transport and industrial sectors (L. L. He et al., 2020; M. Z. M.Z. He et al., 2020). The Eurozone constitutes about 571 and 782 Mt y−1 of NO2 and SO2 emissions, while for SSEA, these emission estimates were 900 and 1267 Mt y−1, respectively. The quantity of such an emission, including the primary emission of particulate matter into the atmosphere increases the AOD through photochemical reactions that transform these gaseous precursors into particulate matter (Seinfeld and Pandis, 2006). The estimates by Yoon et al. (2014) had shown that AOD reduced to 38.5% over Europe and 33% over USA from 2000 to 2009. In contrast, AOD, for the same period, over China and India had increased up to 26% and 24%, respectively. The reason for decreasing AOD over the USA and Europe was attributed to the reduced emission from industry, domestic and transport sectors by putting strong emission policy towards curbing air pollution issues (Hilboll et al., 2013; Streets et al., 2006; Yoon et al., 2012; Yoon et al., 2011; Zhao et al., 2008). On the other hand, the increasing trends over India and China were attributed to augmented levels of emissions due to increase in urban-industrial activities in association to their GDP growth (Smith et al., 2001; D. G Streets et al., 2003; Zhao et al., 2008; Smith et al., 2011; Chin et al., 2014). The evidence from these studies suggested a lower aerosol content (or AOD) in the atmosphere resulting in better AQI under the reduced scenario of anthropogenic emissions. As the AOD represents a comprehensive state of the quality of the air at any given time and space, it could be used at various spatial scales to analyze the pollution level. With a decision for nationwide shutdown due to the outbreak of COVID-19 and its consequent fatalities, all forms of industrial activities and most of the public and private transport were either slowed down or halted for more than six weeks in majority of the areas in SSEA, Europe and US regions. The reduced emission under such scenario alters the aerosol load at the continental scale. The studies, mentioned before in this regard, have primarily addressed the changes of AQI at the city-scape, or the national level. As the countries around the world have faced a distinguishable impact of COVID-19, the measures to contain the spread of the virus varied widely. Thus, no single and uniform time frame is appropriate for studying the effect of lockdown measures on AQI among the affected countries. Due to such limitation, the available literature lacks a comprehensive scenario of the changes of aerosol load at the continental scale. In this work, we tried to analyze the changes in the aerosol load at the larger spatial scale using both satellite and in-situ observations, especially, over SSEA (China, India, Pakistan, Nepal, Bangladesh and other south-east Asian countries) and European region due to the mass infection and fatality of COVID-19, and consequent nationwide lockdown. The study also considered the changes of AOD over the USA for a nationwide lockdown for more than six weeks due to exceptionally high COVID-19 casualties. A comparative analysis of the change in AOD among the regions was presented considering the AOD levels in the pre-lockdown and lockdown periods. The space-time variation of NO2 and SO2 was also taken into consideration to explain the regional difference of AOD. Moreover, we considered the meteorological fields and the regional emission characteristic in a machine learning diagnostic framework to explain their contribution in causing spatial variation of AOD at a larger spatial scale.

Section snippets

Aerosol Optical Depth (AOD)

The level-3 (L3) daily AOD data at 550 nm (MOD08_D3) from MODIS - onboard Terra satellite - was used in this study (Table 1). The MOD08_D3 AOD data with collection version 6.1 (C6.1) is a gridded atmospheric product with a spatial resolution of 1° that is developed from daily level 2 aerosol product. The C6.1 uses dark target (DT) (Levy et al., 2013), deep blue (DB) (Hsu et al., 2013) algorithm for separately retrieve the aerosol optical properties over visibly dark and bright surfaces,

Methods

The timeline for the lockdown period due to the outbreak of COVID-19 was not uniform across the countries. Therefore, for the convenience of analysis, the official declaration of the first lockdown by a country in SSEA and Europe was taken under consideration to define the lockdown period for controlled emission from anthropogenic activities (Supplement Table 1). It turns out, the lockdown period for SSEA region was from 25th January to April 30, 2020. For Europe, it was taken from 6th March to

Spatio-temporal variation of AOD, NO2, and SO2 in pre-lockdown and lockdown periods

The mean AOD over the eastern part of SSEA that includes areas over Thailand, Laos, the northern part of Vietnam and Bangladesh, and in eastern China showed moderate to high aerosol load (AOD 0.6–0.8) during the pre-lockdown period (Fig. 1a). For the lockdown period, the mean AOD showed notable reduction (AOD ≤ 0.6) over eastern China though high AOD (≥0.8) prevailed over Thailand, Laos, Bangladesh, northern Vietnam and eastern India (Fig. 1b). The AOD remained high over western China in the

Discussion

Overall, the change of AOD in the SSEA, Europe and US regions due to lockdown revealed no perceivable reduction of AOD (Fig. 7). However, statistically, significant changes were obtained on the spatial clusters of negative and positive RPD (Table 3). The observation from AERONET stations across these regions – though majority of them fall under the category of positive RPD – showed a significant level of agreement with collocated MODIS-derived observations during pre-lockdown and lockdown

Conclusions

In summary, the results demonstrated a significant decrease in AOD over densely populated regions. A substantial reduction in NO2 emission was obtained due to imposition of lockdown measures in most of the areas over SSEA, Europe and the US. Our results demonstrated a higher SO2 emission for the majority of areas in these regions during the lockdown period. The discrepancy in the concentrations of NO2 and SO2 suggests to the restriction in traffic movement - considered as one of the prime

Credit author statement

Prasenjit Acharya, conceptualized the work and drafted the manuscript. Gunadhar Barik, Bijoy Krishna Gayen, Somnath Bar, Arabinda Maiti and Ashis Sarkar, Formal analysis. Surajit Ghosh, Sikhendra Kisor De and S Sreekesh, contributed to the improvement of the draft manuscript.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors acknowledged the support from Giovanni online data system, developed and maintained by the NASA GES DISC, for archiving the daily atmospheric data - essential for the analysis. We also extend our thanks to NOAA/OAR/ESRL PSL, Boulder, Colorado, USA, for providing analysis-ready NCEP Reanalysis data of relative humidity. We are thankful to the University Grants Commission, Government of India, for supporting research activities under fellowship number UGC-Ref. No. 3231(NET-JUNE2015),

References (62)

  • A.K. Ranjan et al.

    Effect of lockdown due to SARS COVID-19 on aerosol optical depth (AOD) over urban and mining regions in India

    Sci. Total Environ.

    (2020)
  • S. Sharma et al.

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

    Sci. Total Environ.

    (2020)
  • P. Sicard et al.

    Amplified ozone pollution in cities during the COVID-19 lockdown

    Sci. Total Environ.

    (2020)
  • S.J. Smith et al.

    Global and regional anthropogenic sulfur dioxide emissions

    Global Planet. Change

    (2001)
  • A. Tobías et al.

    Changes in air quality during the lockdown in Barcelona (Spain) one month into the SARS-CoV-2 epidemic

    Sci. Total Environ.

    (2020)
  • Pengfei Wang et al.

    Severe air pollution events not avoided by reduced anthropogenic activities during COVID-19 outbreak

    Resour. Conserv. Recycl.

    (2020)
  • J. Wei et al.

    MODIS Collection 6.1 aerosol optical depth products over land and ocean: validation and comparison

    Atmos. Environ.

    (2019)
  • S. Abdullah et al.

    Air quality status during 2020 Malaysia Movement Control Order (MCO) due to 2019 novel coronavirus (2019-nCoV) pandemic

    Sci. Total Environ.

    (2020)
  • R. Bao et al.

    Does lockdown reduce air pollution?: evidence from 44 cities in northern China

    Sci. Total Environ.

    (2020)
  • P.K. Bhartia

    OMI Algorithm Theoretical Basis Document

    (2002)
  • L. Breiman

    Random forests

    Mach. Learn.

    (2001)
  • P. Broomandi et al.

    Impact of the COVID-19 event on the air quality in Iran

    Aerosol Air Qual. Res.

    (2020)
  • E.J. Bucsela et al.

    A new stratospheric and tropospheric NO2 retrieval algorithm for nadir-viewing satellite instruments: applications to OMI

    Atmos. Meas. Tech.

    (2013)
  • M. Chin et al.

    Multi-decadal aerosol variations from 1980 to 2009: a perspective from observations and a global model

    Atmos. Chem. Phys.

    (2014)
  • M. Crippa et al.

    High resolution temporal profiles in the emissions database for global atmospheric research

    Sci. Data

    (2020)
  • G. Dantas et al.

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

    Sci. Total Environ.

    (2020)
  • R.R. Draxler et al.

    An overview of the HYSPLIT_4 modeling system of trajectories, dispersion, and deposition

    Aust. Meteorol. Mag.

    (1998)
  • V.E. Fioletov et al.

    A global catalogue of large SO2 sources and emissions derived from the Ozone Monitoring Instrument

    Atmos. Chem. Phys.

    (2016)
  • GEM

    A New Coal Boom in China - Global Energy Monitor

    (2020)
  • P. Glantz et al.

    Trends in MODIS and AERONET derived aerosol optical thickness over Northern Europe

    Tellus Ser. B Chem. Phys. Meteorol.

    (2019)
  • Google COVID-19 Community Mobility Reports

    (2020)
  • Cited by (40)

    • Agricultural biomass supply chain resilience: COVID-19 outbreak vs. sustainability compliance, technological change, uncertainties, and policies

      2022, Cleaner Logistics and Supply Chain
      Citation Excerpt :

      Rio de Janeiro also showed that average CO2 levels substantially reduced from 30.3% to 48.5% (Dantas et al., 2020). In France, in comparison with the baseline path, there was a reduction in CO2 emission by 6.6% in 2020 (Acharya et al., 2021). In the same year in Pakistan, NO2 emitted from coal-based power plants reduced by 40%, which was followed by a 30% reduction in major urban regions compared to 2019.

    • Impact of COVID-19 induced lockdown on land surface temperature, aerosol, and urban heat in Europe and North America

      2021, Sustainable Cities and Society
      Citation Excerpt :

      The atmospheric BLH is the lowest part of the atmosphere, where the response of the surface layer due to changes in moisture and temperature can be felt in less than an hour, while its variations highly affect the pollution levels (Dumka et al., 2019). Furthermore, the vertical wind velocity or omega (Pascal/s) at 850 hPa (∼1.5 km above sea level) was used to explain the atmospheric capability to disperse the pollutants (Acharya et al., 2021). This product was downloaded from NOAA/OAR/ESRL PSD (USA) website at 2.5o × 2.5o km spatial resolution and was derived from NCEP/NCAR reanalysis (Kalnay et al., 1996).

    View all citing articles on Scopus
    View full text