Modern Discussions on Air Pollution in Megacities

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.

Figure 1. Evolution of megacities, showing percentage urban and urban agglomeration by size (adapted from the UN World Urbanization Prospect, 2018 revision) [1].

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 NO_x 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

1. UN (United Nations), World Urbanization Prospects: The 2018 Revision, https:// www.un.org/development/./2018-revision-of-world-urbanizationprospects.html, accessed September, 2020.
2. L. T. Molina, Faraday Discussions, DOI: 10.1039/d0fd00123f.
3. SEDEMA, http://www.aire.cdmx.gob.mx.
4. https://t.gov.uk/modes/driving/congestion-charge.
5. https://earthobservatory.nasa.gov/images/146596/airborne-particle-levels-plummet-in-northern-india.
6. S. Sharma, M. Zhang, Anshika, J. Gao, H. Zhang and S. H. Kota, Sci. Total Environ., 2020, 728, 138878.
7. 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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