Nowadays, it can be seen very rare news on discussion about air pollution. Very high levels of air pollution have been observed in many major cities across both developed and developing economies of the world cities with varying degrees of regularity.
In particular, the chemistry aspects has a major
responsibility to play both of informing the development of cost-effective
mitigation strategies and understanding the causes of high air pollutant
concentrations. The chemical reactions of air pollutants can transform from one
to another (secondary pollutants), which have a main parameter on measured
levels and the influence of emerging emission sources and changing climate
on air quality and health. We should understand the relative contributions of
local emissions and the pollutants transported to the city from more distant
sources.
Megacities
(metropolitan areas with populations over 10 million) and large urban centers
present a major challenge for the global environment. Predominantly
population increases in urban areas over the upcoming decades underscore the importance
of understanding the sources, variations, and impacts of air
pollution. Most of the peoples are interested to living in major cities with
a higher standard and better employment opportunities, leading to the growth of
megacities, broadly defined as urban agglomeration with more than 10 million
inhabitants (Figure 1).
Some
of the environmental researchers are interested to do research on pollution and
intensification trends in the major megacities. Globally, the major issues on
the extreme air pollution trouble are highest in most megacities, in
particularly with developing countries, significant studies are need for our
understanding.
High
level air pollution will induce the following major troubles like harm human
health, acid deposition and cause regional haze, influence air quality in
regions far from the megacity sources, damage crops, and contribute to climate
change.
Luisa
T. Molina from Massachusetts Institute of Technology (MIT), USA has reported
the sources of emissions in megacities, air quality trends and management in a
few megacities, atmospheric physicochemical processes, and the impacts on
health and climate. Further she discussed on
the challenges and opportunities facing megacities due to lockdown during the
COVID-19 pandemic in Faraday Discussions on October 2020 [2].
Sources of emissions
in megacities:
(i)
Mobile sources: Onroad vehicles such as commercial buses and trucks,
passenger cars, three-wheelers and motorcycles; non-road vehicles such as
aircraft, construction, marine vessels and agricultural equipments.
(ii)
Stationary sources: Factories, boilers, refineries and power plants.
(iii) Area sources: Small-scale industrial, commercial, service operations
etc.
(iv)
Natural (biogenic) sources: Vegetation, volcanoes, lightening, forest and
grassland fires, windblown soils, and sea salt spray.
Major control
strategies:
Transport:
Most of the megacities have implemented measures to reduce emissions from
onroad vehicles, including fuel quality and improvement of vehicle technology
and, replacement of diesel with natural gas, implementation of strict
international vehicle emission standards and introduction of hybrid and
electric vehicles. A number of strategies are introduce to improve air quality
in many megacities such as improving the efficiency and security of the public
transportation network to encourage the use of public transit, expanding
infrastructure for nonmotorized transportation (walking and cycling); reducing
traffic congestion by limiting the circulation of vehicles (e.g., “No drive
day” in Mexico and many megacities) [3] and
road pricing (e.g., London) [4].
Industry:
Some cities have restricted the use of cleaner coal in coal-fired power plants
in Indian megacities; substitution of fuel oil for natural gas in Mexico City.
Reducing coal consumption and emissions in Chinese megacities were implemented using
a “coal to gas” strategy and end of pipe programs such as dedusting,
desulfurization and denitrification for industries. Some of the main cities like
Los Angeles and Mexico City have relocated large stationary emission sources,
including refinery and power plants, out of the cities.
Area sources:
Many cities have waste separation programmes and recycling – some are
converting organic waste into compost.
Landfill
is the most widespread method for final waste disposal, followed by capturing
of the landfill gas for power generation as part of the integrated waste
management program. Many cities are promoting the use of cleaner fuels such as
liquefied petroleum gas (LPG), and renewable-energy technologies such as solar
heating systems and solar water heaters.
The
WHO (since 1987) has proposed air quality guidelines designed to inform policy
makers and to provide appropriate targets in reducing the impacts of air
pollution on public health. At present, various countries have established ambient
air quality standards to protect the public from exposure of the harmful level
of air pollutants which is an important component of national risk management
and environmental policies.
Impact of COVID-19
Some
of the important studies deals to contain the spread of COVID-19 have led
significant diminutions in the emissions of air pollutants, particularly NOx
and CO2 emissions from fossil fuel combustion. Many of the major as
well as small cities have seen dramatic improvement in air quality. Currently,
Delhi is one of the most polluted megacities, experienced the clearest skies in
years as pollution dropped to its lowest level in three decades [5].
The
remarkable reduction in air pollution coupled with COVID-19 lockdowns and other
restrictions forced by governments in cities around the world has provided an
opportunity for atmospheric scientists to conduct a unique natural experiment
to gain a better understanding of the complex interactions between meteorology,
emissions, atmospheric processes and the efficiency of control measure
surrogates that could lead to long-term emission reductions. Sharma et al. [6]
studied the air quality of more than 20 cities in India, including megacities
of Delhi, Kolkata, Mumbai and Chennai, and found that the concentrations of air
pollutants decreased while O3 increased in most regions during the
lockdown period. Le et al. [7] analyzed the difference in emissions during the
COVID-19 lockdown in four megacities (Wuhan, Shanghai, Guangzhou, and Beijing)
in China. Both satellite and ground-based observations revealed up to 90%
reductions of main air pollutants of NO2 and SO2
concentrations because the reduction was due to mainly from the decrease in
vehicle traffic volume and fuel consumption.
The
COVID-19 crisis have brought some temporary non-COVID-19 related health
benefits, the drastic measures of shutting down the global economy to clean the
air are not sustainable. This is possible to achieve better air quality by
implementing emission reduction strategies and it also raises the public
awareness about the benefits of clean air and calls for governments to take
actions for the longer term.
Our
SNB team have emphasize this research article because of the
important challenges remain that needs to be addressed for an substantial
improvement in air quality on the megacities and large urban complexes.
International collaboration and co-operation are strongly encouraged, which includes
the strengthening of the local capacity in air quality monitoring and emissions
inventory development. Hence, these megacities confronts a severe air pollution
challenges which will have the opportunity to learn from the experience of
those cities that have been successfully addressed by them.
References
- UN (United Nations), World Urbanization Prospects: The 2018 Revision, https:// www.un.org/development/./2018-revision-of-world-urbanizationprospects.html, accessed September, 2020.
- L. T. Molina, Faraday Discussions, DOI: 10.1039/d0fd00123f.
- SEDEMA, http://www.aire.cdmx.gob.mx.
- https://t.gov.uk/modes/driving/congestion-charge.
- https://earthobservatory.nasa.gov/images/146596/airborne-particle-levels-plummet-in-northern-india.
- S. Sharma, M. Zhang, Anshika, J. Gao, H. Zhang and S. H. Kota, Sci. Total Environ., 2020, 728, 138878.
- T. Le, Y. Wang, L. Liu, J. Yang, Y. L. Yung, G. Li and J. H. Seinfeld, Science, 2020, 369, 702–706.
--- Dr. A. S. Ganeshraja
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