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