15 COVID-19 Pandemic: An Unprecedented Blessing for Nature
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particulate matter concentration. However, Chen et al. [31] state that the reduction
in pollutant level (such as reduced PM 2.5 concentration) cannot be solely explained
by alterations in primary emissions; instead, decreasing concentrations of nitrate
(60%), ammonium (45%) and primary aerosols can also play an important role. In
another study, the elements barium, copper, iron, potassium and zinc were found to be
dominating PM 2.5 -related elements [48]. The presence of these elements, even during
the lockdown period at a rural site between Beijing and Tianjin was attributed to the
aging processes of elements after the emission of fireworks in the Chinese New Year
Eve night and vehicular emission [48]. A reduction in the particulate matter levels
in other countries included studies by Abdullah et al. [49], Collivignarelli et al. [39],
Kanniah et al. [37], Kerimray et al. [10], Nakada and Urban [41], Otmani et al. [42],
and Zangari et al. [40] for Malaysia (PM 2.5 58%), City of Milan, Italy (PM 2.5 47%;
PM 10 48%), Malaysia (PM 10 26–31%), Kazakhstan (PM 2.5 21%), Sau Paulo state,
Brazil (PM 2.5 30%), Morocco (PM 2.5 75%), and New York city, USA (PM 2.5 36%),
respectively. In contrast, a few studies [46, 50, 51] showed either a low reduction or
zero reduction in the particulate matter levels during the lockdown phase. In India,
maximum documentation [22–24, 34, 35] on the reduced levels of particulate matter
during the imposed lockdown period were from Delhi. Sharma et al. [23] in their study
monitored a total of 22 states of India and recorded the overall decrease of 43 and 31%
for PM 2.5 and PM 10 . They also observed a significant decrease in concentrations of
particulate matter in north India. A similar observation was made by Navinya et al.
[35], while examining the effects of quarantine lockdown on ambient air quality
across 17 urban cities of India. Studies on reduced concentrations of particulate
matter were also reported from Mumbai (PM 2.5 37%; PM 10 44%) [34] and Gujarat
(PM 2.5 38–78%; PM 10 32–80%) [20]. On the other hand, an increment observed in
the concentration of PM 2.5 (15%) and PM 10 (59%) in Singrauli was attributed to
active operation of coal-based power station in the lockdown period [34].
There were also studies on aerosol optical density (AOD) for evaluating the
quality of atmosphere. AOD is a measure of the attenuation of solar radiation due
to absorption and reflection of incident light by the atmospheric airborne particles.
AOD measurement is important for studying the impact of aerosol concentration on
solar radiation, alterations in atmospheric phenomenon and human health. A reduced
AOD value is an indicator of reduced aerosol concentration. NASA’s Terra satellite
captured the decreased AOD values over India and around the globe during the 2020
lockdown period (Fig. 15.3). A decline in AOD values were also reported for cities
like Kuala Lumpur, Brunei, Singapore and Manila [37]. In 2020, Malaysia recorded
an average AOD decrease of 57–72% as compared to the same period in 2019 [37].
Reports from worldwide showed the difference between the concentrations of air
pollutants measured before and after impose of the lockdown restrictions. Several
studies recorded the continuous decrease in the concentration of the air pollutants
such as NO 2 , SO 2 , CO 2 , PM during the complete lockdown period. The reduction
in the amount of air pollutants in the atmosphere is only possible when the sources
producing the air pollutants are curbed. The industries and motor vehicles are primary
sources (i.e. causes direct emission) of many air pollutants such as NO x , oxocarbons
and PM. Thus, when human activities were restricted globally during the lockdown,
359
particulate matter concentration. However, Chen et al. [31] state that the reduction
in pollutant level (such as reduced PM 2.5 concentration) cannot be solely explained
by alterations in primary emissions; instead, decreasing concentrations of nitrate
(60%), ammonium (45%) and primary aerosols can also play an important role. In
another study, the elements barium, copper, iron, potassium and zinc were found to be
dominating PM 2.5 -related elements [48]. The presence of these elements, even during
the lockdown period at a rural site between Beijing and Tianjin was attributed to the
aging processes of elements after the emission of fireworks in the Chinese New Year
Eve night and vehicular emission [48]. A reduction in the particulate matter levels
in other countries included studies by Abdullah et al. [49], Collivignarelli et al. [39],
Kanniah et al. [37], Kerimray et al. [10], Nakada and Urban [41], Otmani et al. [42],
and Zangari et al. [40] for Malaysia (PM 2.5 58%), City of Milan, Italy (PM 2.5 47%;
PM 10 48%), Malaysia (PM 10 26–31%), Kazakhstan (PM 2.5 21%), Sau Paulo state,
Brazil (PM 2.5 30%), Morocco (PM 2.5 75%), and New York city, USA (PM 2.5 36%),
respectively. In contrast, a few studies [46, 50, 51] showed either a low reduction or
zero reduction in the particulate matter levels during the lockdown phase. In India,
maximum documentation [22–24, 34, 35] on the reduced levels of particulate matter
during the imposed lockdown period were from Delhi. Sharma et al. [23] in their study
monitored a total of 22 states of India and recorded the overall decrease of 43 and 31%
for PM 2.5 and PM 10 . They also observed a significant decrease in concentrations of
particulate matter in north India. A similar observation was made by Navinya et al.
[35], while examining the effects of quarantine lockdown on ambient air quality
across 17 urban cities of India. Studies on reduced concentrations of particulate
matter were also reported from Mumbai (PM 2.5 37%; PM 10 44%) [34] and Gujarat
(PM 2.5 38–78%; PM 10 32–80%) [20]. On the other hand, an increment observed in
the concentration of PM 2.5 (15%) and PM 10 (59%) in Singrauli was attributed to
active operation of coal-based power station in the lockdown period [34].
There were also studies on aerosol optical density (AOD) for evaluating the
quality of atmosphere. AOD is a measure of the attenuation of solar radiation due
to absorption and reflection of incident light by the atmospheric airborne particles.
AOD measurement is important for studying the impact of aerosol concentration on
solar radiation, alterations in atmospheric phenomenon and human health. A reduced
AOD value is an indicator of reduced aerosol concentration. NASA’s Terra satellite
captured the decreased AOD values over India and around the globe during the 2020
lockdown period (Fig. 15.3). A decline in AOD values were also reported for cities
like Kuala Lumpur, Brunei, Singapore and Manila [37]. In 2020, Malaysia recorded
an average AOD decrease of 57–72% as compared to the same period in 2019 [37].
Reports from worldwide showed the difference between the concentrations of air
pollutants measured before and after impose of the lockdown restrictions. Several
studies recorded the continuous decrease in the concentration of the air pollutants
such as NO 2 , SO 2 , CO 2 , PM during the complete lockdown period. The reduction
in the amount of air pollutants in the atmosphere is only possible when the sources
producing the air pollutants are curbed. The industries and motor vehicles are primary
sources (i.e. causes direct emission) of many air pollutants such as NO x , oxocarbons
and PM. Thus, when human activities were restricted globally during the lockdown,