Introduction: The megacities all over the world are vulnerable to air pollution with the rapidly increasing population and economic developmental activities. Uncontrolled urban growth and unplanned industrialization and automation play a key role to deteriorate the quality of air in the megacities of developing countries. Delhi, Mumbai and Kolkata are three populous megacities in India which face the serious problem of air pollution. Air pollution can broadly be classified into two categories, i.e. (i) Visible air pollution and (2) Invisible air pollution. The smog is an example of visible pollution whereas the good examples of invisible air pollutant are Sulfur dioxide (SO2), Carbon monoxide (CO), Nitrogen dioxide (NO2), Volatile Organic Compound (VOCs), Ozone (O3), Particulates (PM 2.5, PM 10), Chlorofluorocarbons (CFCs), Lead and heavy metals, Unburned hydrogen carbon, Gaseous ammonia (NH3) etc. While all pollutants in the atmosphere cause harm to the planet there are some that are more dangerous than others. These dangerous air pollutants are PM 2.5, PM 10, NO2, NH3, SO2, CO, O3. The concentration of these pollutants is regularly monitored by the Central Pollution Control Board (CPCB) to assess air quality in different locations in India.
A nationwide lockdown was imposed in India from 24th March to 31st May 2020 to prevent the spread of COVID-19. Due to this lockdown, all industrial activities, public transport, and other economic and developmental activities were stopped, which played an important role to reduce the pollution level in the cities across the country. The air quality of Delhi, Mumbai and Kolkata also changed considerably from the pre-lockdown condition to the lockdown period. In the present study, the air quality parameters of these megacities in January (Pre-Lockdown) and April (During-Lockdown) have been studied from the CPCB data to understand the role of COVID-19 Lockdown on the air quality of these cities. Changing the concentration of the major pollutants have been discussed in the following section.
Particulates: Particle pollution -also called particulates matter (PM)-is made up of particles (tiny pieces) of liquid or solid that are in the air. Particulate matter can be released from different types of human activities such as vehicle emissions, smoke particles, dust particles, and ash from industries. The PM 10 is particulate matter 10 micrometers or less in diameter whereas, the PM 2.5 is particulate matter 2.5 micrometers or less in diameter. PM 2.5 is generally described as fine particles. During the Pre-Lockdown normal condition, the PM 2.5 concentration in air in Delhi, Mumbai, and Kolkata were 436 µg/m3, 236 µg/m3 and 246 µg/m3 respectively. Whereas, during the lockdown, the concentration of PM 2.5 in these three megacities became 89 µg/m3, 31 µg/m3 and 78 µg/m3. Similarly, PM 10 concentration during the pre-lockdown period were 334 µg/m3, 220 µg/m3 and 225 µg/m3, which dropped below 56 µg/m3, 70 µg/m3 and 81 µg/m3 during the lockdown.
Nitrogen dioxide and Ammonia: Combustion of fossil fuels (coal, gas, and oil) especially fuel used in cars is the main source of nitrogen dioxide resulting from human activities. The Ammonia (NH3) is a colourless gas which combines in the atmosphere with sulphate and nitrates to form secondary fine particulate matter (PM 2.5). During the Pre-Lockdown normal condition, the NO2 concentration in air in Delhi, Mumbai and Kolkata were 77 µg/m3, 92 µg/m3 and 76 µg/m3 respectively. Whereas, during the lockdown, the concentration of NO2 in these three megacities became 22 µg/m3, 14 µg/m3 and 31 µg/m3. Similarly, NH3 concentration during the pre-lockdown period Delhi, Mumbai and Kolkata were 7 µg/m3, 4 µg/m3 and 13 µg/m3, which dropped below 3 µg/m3, 3 µg/m3 and 8 µg/m3 during the lockdown.
Sulfur dioxide and Carbon monoxide: Fossil fuel combustion at power plants, other industries, and in mobile sources like locomotives, ships are the mail sources of Sulfur dioxide (SO2) emission. Harmful compounds, such as sulfuric acid, sulfurous acid and sulfate particles are formed from the reaction of SO2 with other substances. During the Pre-Lockdown normal condition, the SO2 concentration in air in Delhi, Mumbai and Kolkata were 11 µg/m3, 21 µg/m3 and 22 µg/m3 respectively, which became below 13 µg/m3, 4 µg/m3 and 18 µg/m3 during the lockdown. Carbon monoxide (CO) is a colourless, odorless gas that can be harmful when inhaled in large amounts. CO is released when something is burned. The greatest sources of CO in outdoor air are cars, trucks and other vehicles or machinery that burn fossil fuels. During the Pre-Lockdown normal condition, the CO concentration in air in Delhi, Mumbai and Kolkata were 121 mg/m3, 91 mg/m3 and 30 mg/m3 respectively, which became below 47 mg/m3, 17 mg/m3 and 14 mg/m3 during the lockdown.
Not Only air pollution, the water of the rivers is less polluted now than what they were for decades. This lockdown has reduced pollution and an improvement in all environmental indicators across these cities. The CPCB calculates Air Quality Index (AQI) to assess the air quality of any location-based on some air quality parameters (PM 2.5, PM 10, NO2, NH3, SO2, CO, O3). AQI values are broadly classified into six categories, i.e. good (0 to 50), satisfactory (51 to 100), moderate (101 to 200), poor (201 to 300), very poor (301 to 400) and severe (401 to 500). According to CPCB, the AQI value in Delhi was 436 (severe) in January and became 89 (satisfactory) in April. During the Pre-Lockdown normal condition (January), the AQI values in Mumbai and Kolkata were 236 (poor) and 246 (poor) respectively, which changed to 70 (satisfactory) and 81 (satisfactory) during the Lockdown in April. The AQI values show, how the lockdown played a crucial role to purify the air of these megacities. Among the selected pollutants, the maximum reduction of concentration was observed for PM10 and PM2.5 have reduced followed by NO2, CO. A negative trend was observed for SO2 in Delhi.
The environmental benefits of this lockdown are exactly what were required in these cities without any proactive mitigation measure. But stopping or slowing down the industrial activities impose a big question on the growth of economy. However we hope for a better dealing with pollution in post lockdown period.
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