Air quality in megacities

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Shanghai Bund, 2011. Credit: Jupiterimages Corp.


Ambient air pollution (Air quality in megacities) in an increasingly urbanized world directly threatens the health of a large fraction of the world’s population. There is growing recognition that air-borne emissions from major urban and industrial areas influence both air quality and climate change on scales ranging from regional up to continental and global. Deteriorating urban air quality affects the viability of important natural and agricultural ecosystems in regions surrounding highly urbanized areas, and significantly influences regional atmospheric chemistry and global climate change. This challenge is particularly acute in the developing world where the rapid growth of megacities (cities having population equal to or more than 10 million) is producing atmospheric pollution of unprecedented severity and extent. For example, the deterioration of air quality is a problem that is directly experienced by a majority of the 300 million urban Indians, about 30% of India’s population.

In developing countries, migration from the countryside to the megacities has brought as a consequence greater emissions into the atmosphere. This is mainly produced by the increase of vehicular traffic, a problem exacerbated by the tendency in these countries to have a stock of old and badly maintained vehicles. At the same time, the number of vehicles in circulation has increased as well. These factors have produced far-reaching changes in air quality in urban contexts, especially in the 1990s, when the majority of clean air plans were tightened up.

Studies of megacities on different spatial and temporal scales using various modeling tools are required to understand their local-to-regional-to-global impacts and implications. For instance, looking beyond the local impacts of megacity pollution, Lawrence et al. (2007) have employed a global model to examine the outflow characteristics of pollutants from megacities, demonstrating the tradeoffs between pollutant buildup in the region surrounding each megacity versus export to downwind regions or to the upper troposphere. Unfortunately, the coarse-grid resolution of global atmospheric models and source inventories still presents difficulties to capturing the details of the development of megacity emissions temporally and spatially.

Urban Air Quality

In most urban areas, emissions from increasing number of automobiles are a major contributor of harmful pollutants such as nitrogen oxides (NOx) and particulate matter (PM). Urban air quality is thus a major concern throughout the world. This, in fact, is a vast subject with different socio-economic aspects in different parts of the world and even within a specific region. The concentration of human activities in a relatively small area puts enormous pressure on the urban system and has led to numerous environmental problems (e.g. noise, waste, air pollution, etc.). Since 1950 the world population has more than doubled, and the global number of cars has increased by a factor of 10. In the same period the fraction of people living in urban areas has increased by a factor of 4. In the year 2000 this amounted to nearly half of the world population. At present, the total urban population is more than their rural counterpart.

To date nearly 3000 different anthropogenic air pollutants have been identified, and most of them are organic (including organometals). Combustion sources, especially motor vehicles, emit about 500 different compounds. However, only for about 200 of the pollutants have the impacts been investigated. The ambient concentrations are determined for an even smaller number resulting in a major limitaion for urban air quality management.

The complex nature of air pollution, especially with respect to health impacts in urban areas, has prompted attempts to define the so-called indicators that condense and simplify the available monitoring data to make them suitable for public reporting and decision makers. Several concepts and indicators exist to measure and rank urban areas in terms of their infrastructural, socio-economic, and environment-related parameters. The World Bank, for example, regularly publishes the World Development Indicators (WDI), and the United Nations reports the City Development Index (CDI) and also ranks megacities on the basis of their population size. Recently in 2007, Gurjar et al. have proposed a multi-pollutant index (MPI) considering the combined level of the three criteria pollutants (i.e., TSP, SO2, and NO2) in view of the World Health Organization (WHO) Guidelines for Air Quality.

Sources and Health Impacts

Air pollution in urban area comes from a wide variety of sources. The single most important source for the classical pollutants sulfur dioxide (SO2), nitrogen oxides (NOx), carbon monoxide (CO), volatile organic compounds (VOCs), and particulate matter (PM) is generally the combustion of fossil fuels. Of particular importance is the burning of fuels for road transport and electricity generation. There are three major sources of air pollution in urban areas, namely mobile sources, stationary sources, and open burning sources that can be categorized into the source groups motor traffic, industry, power plants, trade, and domestic fuel. The investigation published in 1996 by Mage et al.indicates that motor traffic is a major source of air pollution in megacities.

Epidemiological studies have established the link between air pollutants and health effects. There are possible short-term and long-term health effects of exposure to air pollution. In the short term, high levels of air pollution can lead to an acute respiratory distress. In addition, blockage of sunlight (Solar radiation) may promote the spread of harmful bacteria and viruses that would otherwise be killed by ultraviolet B. The possible long-term health effects of exposure to air pollution are unknown and difficult to detect. Components of smoke haze, including Polycyclic Aromatic Hydrocarbons (PAH), are known carcinogens, the effects of which may not be apparent for years. The consequences may be more severe for children, for whom the particulates inhaled are high relative to body size. Continued pollution levels above ambient air quality standards are likely to create a number of adverse health consequences and a subsequent economic impact. High levels of particulate matter (e.g., PM10 or TSP), for example, have been linked to a number of significant health problems, ranging from decreased lung function to increased respiratory and cardiac hospital admissions to premature death. NO2, in recent epidemiological studies, has been increasingly associated with a number of respiratory and cardiovascular conditions, including worsening bronchitis, emphysema, heart disease, and even premature cardiovascular mortality.

Air Pollution in Megacities

Urban smog. Source: UNEP
In 2003, Baldasano et al. published an assessment of the air quality for the principal cities in developed and developing countries. According to their study, the current state of air quality worldwide indicates that SO2 maintains a downward tendency throughout the world, with the exception of some Central American and Asian cities. Whereas, NO2 maintains levels very close to the WHO guideline value in many cities. However, in certain cities such as Kiev, Beijing and Guangzhou, the figures are approximately three times higher than the WHO guideline value. In fact, NO2 concentrations are showing increasing trend in several Asian megacities. Particulate matter is a major problem in almost all of Asia, exceeding 300 mg/y/m3 in many cities, like two Latin-American cities, Tegucigalpa and Montevideo. In the Asian databases consulted, only Japan showed really low figures. Ground-level ozone presents average values that exceed the selected guideline values in all of the analysis by regions, income level and number of inhabitants, demonstrating that this is a global problem with consequences for rich and poor countries, large and medium cities and all the regions. In general, the worldwide tendency is to reduce the concentrations of pollutants owing to the increasingly strong restrictions which local governments and international organizations impose. However, in poor countries and those with low average incomes, concentrations of air pollutants remain high and the tendency will be to increase their emission levels as they develop, making the problem worse.

Gurjar et al. have published in 2007 the evaluation of emissions and air quality pertaining to all megacities. They have also ranked megacities in terms of their trace gas and particle emissions and ambient air quality based on the newly proposed multi-pollutant index (MPI) considering the combined level of the three criteria pollutants (i.e., TSP, SO2, and NO2) in view of the World Health Organization (WHO) Guidelines for Air Quality. In their study they have considered 18 megacities. Among these 5 megacities classify as having ‘‘fair’’ air quality, and 13 as ‘‘poor’’ air quality. Gurjar et al. have presented the ranking of megacities based on ambient air quality measurements of TSP, SO2, and NO2, and MPI values. It is interesting that according to their study Karachi ranks as the most polluted megacity in terms of TSP, whereas according to the multi-pollutant index ranking it is categorized as the fourth most polluted megacity in the world, due to the lower levels of NO2 and SO2. On the other hand, Dhaka, which is the fourth most polluted megacity in terms of ambient air concentration of NO2, and third in terms of TSP, ranks as the most polluted megacity in the world when judged according to the MPI. Similarly, Osaka, São Paulo, and Tokyo are ranked towards the middle in terms of the SO2 concentration, but they emerge as the least polluted megacities when evaluated using the MPI. Based on present MPI values, that Dhaka, Beijing, Cairo, and Karachi appear to be most polluted, whereas Osaka-Kobe, Tokyo, Sa˜o Paulo, Los Angeles, New York, and Buenos Aires are the least polluted megacities.

Local, Regional and Global impacts of megacities emissions

Air pollution in megacities is influenced by many factors such as the topography, meteorology, industrial growth, transportation systems, and expanding populations. Urban/industrial emissions from the developed world, and increasingly from the megacities of the developing world, change the chemical content of the downwind troposphere in a number of fundamental ways. Stationary and mobile fossil fuel combustion sources in megacities produce thousands of tons of primary pollutants that react in the atmosphere to generate secondary pollutants in local and regional level that can be more dangerous to health than the primary pollutants. On one hand, emissions of nitrogen oxides (NOx), CO and volatile organic compounds (VOCs) drive the formation of ozone, photochemical smog and its associated oxidants in local and regional atmosphere, thereby degrading the ambient air quality and threatening both human and ecosystem health. On the other hand, the production of ozone (a powerful greenhouse gas) in the free troposphere contributes significantly to global warming. Urban and industrial areas are also major sources of the greenhouse gases, including CO2, CH4, N2O and halocarbons. Nitrogen oxide and sulfur oxide emissions are transformed into nitric and sulfuric acids by atmospheric chemical reactions on regional to continental scales, resulting in acid deposition to sensitive ecosystems. Direct urban/industrial emissions of carbonaceous aerosol particles are compounded by the emission of high levels of secondary aerosol precursors, including: NOx , VOCs, SO2, and NH3, driving the large production of fine aerosol, affecting local air quality in the urban source areas, the local radiation balance, and cloud formation from hundreds to thousands of kilometers downwind. The adverse health effects of particulate matter range from increased incidences of pneumonia and asthma, exacerbation of chronic obstructive pulmonary diseases, increased respiratory symptoms, and decreased lung function, to an increased mortality rate in human beings.

In 2004, Molina and Molina described that the ecological footprint of a megacity encompasses not only the area physically occupied, but also the area that contributes resources and, in turn, is affected by wastes and pollutants. Partly because the lifetimes of pollutants may be prolonged due to the high concentrations in megacity plumes, they can travel across and even between continents. A pollution plume from New York, for example, was detected over Europe. Megacity plumes contain large amounts of criteria pollutants, [[greenhouse gas]es], ozone precursors and aerosols; therefore, they potentially affect the atmosphere on a large scale. Lelieveld and Dentener have predicted that the emerging Asian emissions will increasingly affect hemispheric background ozone in future, to the extent that regional control measures in Europe and USA can be overpowered.

Air pollution episodes in megacities

The term ‘air pollution episode’ normally implies a short-term increase in ambient pollution which is greater than would be normally expected as part of day-to-day variation. In their most extreme form, air pollution episodes are accompanied by physical discomfort, disruption of day-to-day living, widespread public fear, illness and even death. They, therefore, belong to the family of environmental disasters which includes floods, earthquakes, volcanic eruptions, heat waves and large-scale chemical or ionizing radiation escapes. At the other end of the scale, episodes may be no more than short periods during which pollution exceeds the usual range, unaccompanied by noticeable effects. The pollution is usually the direct or indirect consequence of burning fuel for transport, industry or domestic use, but may also come from other sources such as forest fires or volcanic eruptions. Episodes of pollution from the burning of fuel tend to occur not because of an increase in emissions, but because of stagnant weather conditions that impair their dispersal. Certain areas or towns may be prone to the air pollution episodes because of their particular topography or climate.

London winter smog

Smog” is a word coined by combining parts of the two words “smoke” and ‘‘fog’’. The most serious air pollution episodes were the classical smog episodes that led to the London. These episodes occurred in cold winter months in areas where coal was the major fuel for both industry and heating. They were characterized by high levels of sulfur dioxide and smoke particulates, which built up under stagnant weather condition lasting three days or more. Mortality and illness were significantly increased. In the worst case in terms of number of people affected, in London in the week of 5-11 December, 1952, there were 3500 to 4000 more deaths then the expected average for the time of the year. Those most susceptible were the elderly and young, particularly individuals with the history of cardio-respiratory illness. No single primary pollutant could be singled out for blame if any of these incident, and it is probable that an association between SO2 particles and their reaction products sulfuric acid was operative.

Los Angeles summer smog

Photochemical smog first came into prominence in July 1943, in Los Angeles. It was characterized by reduced visibility, eye irritation, and plant damage (metallics seen on leaves). Photochemical smog is a mixture of pollutants that are formed when nitrogen oxides and volatile organic compounds (VOCs) react to sunlight, creating a brown haze above cities. The presence of other pollutants species in the atmosphere (e.g. SO2 and particulate matter) can also be participants, as can hydrocarbons and nitric oxide from other sources, but they are not essential for production of the characteristic high oxidant levels associated with photochemical smog formation. Smog is further favored by stable meteorological conditions, when the urban emissions are held in an urban airshed by the atmospheric inversion layer, acting like a lid over a reaction vessel, and thus preventing air pollution dispersion. It tends to occur more often in summer, because that is when we have the most sunlight.

A comparison of Los Angeles’ Summer Smog and London’s Winter Smog is given in the following table.

Comparison of Los Angeles and London smog


Los Angeles


Air Temperature

24-32 °C

-1-4 °C

Relative humidity


85% (+fog)

Type of Temperature Inversion

Subsidence, at few thousand meters

Radiation (near ground) about 100 meters

Wind Speed

<3 m/s



<0.8-1.6 km

<30 m

Month of most frequent occurrence



Major Fuels


Largely Coal

Principal Constituents

O3, NO, NO2, CO, Organic matter

Particulate Matter , CO, S compounds

Type of Chemical Reaction



Time of Maximum Occurrence


Early Morning

Principal health irritation effects

Temporary Eye irritation (PAN)

Lung Disease


Material Damaged

Rubber Cracked

Iron, concrete corroded

Source: Raiswell et al., 1980

Air quality measurement in megacities

The rapid development of megacities, especially in developing countries, causes serious air-quality problems. Studies of the anthropogenic impact on atmospheric composition in megacities have, therefore, become of primary importance. Pollutant measurements in these cities are required to help characterize the chemical nature and the quantity of these pollutants. Organizations such as the World Health Organization have initiated programs aiming at measuring (and if possible improving) air quality in megacities. Pollutants that should be measured at the first place are those relevant to health issues (SO2, O3, NO2, CO, C6H6, Pb, and aerosols). The coordination of air-quality management issues is made by the North American Research Strategy for Tropospheric Ozone (NARSTO) for North America and by organizations such as European Environment Agency (EEA) and the Cooperative Programme for Monitoring and Evaluation of the Long-range Transmission of Air Pollutants in Europe (EMEP) for Europe. New initiatives focused on specific megacities issues are emerging. The challenge for air-quality networks already well developed is to adapt rapidly their capacity to improve existing measurements, to measure new identified pollutants, and/or to use new measurement techniques. For megacities located in emerging countries, air quality measurements are more recent (e.g. Asia, South America), and sometimes still in their infancy (Africa). Current efforts should be continued to help development of air-quality measurements and subsequent improvement in these continents.

Air quality trends in megacities

Time series of air pollutants indicate trends in air quality. In megacities of non-industrialized countries, time series of air pollutants are often too short for statistical trend analysis. In many cities of industrialized countries, however, air pollutant time series are sufficiently long to permit trend analysis. WHO and UNEP created an air pollution monitoring network as part of the Global Environment Monitoring System for time series analysis of air pollution. A study of air pollution in 20 of the 24 megacities of the world shows that ambient air pollution concentrations are at levels where serious health effects are reported. The study shows that each of the 20 megacities has at least one major air pollutant which occurs at levels that exceed WHO health protection guidelines. Beijing, Cairo, Jakarta, Los Angeles, Mexico City, Moscow and Sao Paulo , five of which are located in the Pacific Basin, are facing a variety of air pollution problems requiring comprehensive solutions. The study shows that the ambient air quality in majority of the megacities is getting worse as the population, traffic, industrialization and energy use are increasing, and there is much urgency in instituting control and preventive measures. In degree of severity, the high levels of suspended particulate matter (SPM) are the major problem affecting the megacities as a group. SPM presents a very serious problem in 12 of the megacities surveyed, the majority of which are located in the Pacific Basin. The concentrations of SPM in these cities are persistently above the WHO guidelines by a factor as much as two or three. The most notable emission processes are combustion of fossil fuels for power generation in industry and for heating purposes. From a review of trends in air quality in different cities made by Mage et al., it is quite evident that `history’ repeats itself. The experience of the current megacities in the developed countries is being repeated in the developing countries. Several megacities studied by Mage et 1996 are now in the state where additional controls must be implemented without delay.

Measurement campaign in megacities

Several organizations are concentrating on understanding the characteristics of atmospheric pollution in megacities and organizing field campaigns around the world. MILAGRO (Megacity Initiative: Local and Global Research Observations) is the first international effort to study the impact of a megacity (Mexico City) on air quality. The Mexico City Metropolitan Area (MCMA) – the second largest megacity in the world – was selected as the initial case study for MILAGRO. Previous research on air pollution associated with the MCMA provided a framework for planning of future field studies, particularly the MCMA-2003 campaign. Specifically, it showed that the atmosphere of the MCMA contains high level of aerosols and is extremely active photochemically; it is ideally suited for understanding the atmospheric chemistry of tropical megacities.

More than 150 institutions from Mexico, United States and Europe participated, and over 450 investigators and technicians from 30 different nationalities participated in the MILAGRO campaign in March 2006, organized under four components:

  1. MCMA-2006 (México City Metropolitan Area - 2006) - examine emissions and boundary layer concentrations within México City, the exposure patterns and effects on human health and the evaluation and design of policies intended to reduce pollutant levels.
  2. MAX-Mex (Megacity Aerosol Experiment in México City) - examine the properties and evolution of aerosols and gas-aerosol interactions in the immediate urban outflow.
  3. MIRAGE (Megacity Impacts on Regional and Global Environments) examine the evolution of the México City plume on larger regional scales.
  4. INTEXB (Intercontinental Chemical Transport Experiment - Phase B) - study the evolution and transport of pollution on global scales.

Air quality improvement programs in megacities

For improving the air quality of megacities variety of measures are being taken by governments of respective countries. For example, for reducing emission from vehicles a number of policy measures (e.g. switching to clean fuels) besides engine improvement and exhaust controls on new vehicles have been applied. Some of these measures are meant for reduction in traffic congestion, since congestion exacerbates air pollution emissions. One way to reduce congestion is by limiting the circulation of vehicles. London charges a fee when vehicles enter a designated part of the city, as discussed below. However, “no drive day” programs may have unintended consequences if not properly designed. In Mexico City, such a program appears to have induced individuals to purchase a second vehicle, often older and more polluting. A more effective strategy that restricts the circulation of vehicles only during peak hours is being implemented in Bogotá, Santiago and São Paulo. Many countries have also implemented regulations about reducing emissions of photochemical smog precursors, such as NOx and volatile organic compounds (VOCs), from motor vehicles and industrial activities.

CNG implementation program in Delhi

In Delhi various policy measures have been proposed. For the transport sector several control measures have been implemented, such as low sulfur diesel in the mid 1990s, EURO-II equivalent emission norms for passenger vehicles in 2000, and replacement of diesel fueled commercial vehicles with Compressed Natural Gas (CNG) vehicles in 2001. The most common method of public transport in Delhi comprises buses, minibuses, taxis and three wheelers. Their consumption of diesel is higher as compared to gasoline than possibly anywhere in the Western World. In view of the seriousness of the health effects of air pollution and to improve the air quality of Delhi, public transport in Delhi was amended by the Supreme Court of India to implement CNG instead of diesel or petrol to reduce vehicular pollution. The use of heavy-duty natural gas engines, in place of diesel, offers numerous environmental benefits. This has led Tehran, Los Angeles, Bangkok, Santiago, Cairo, Beijing and many other cities to establish a natural gas program. Ravindra et al. studied air quality of Delhi before and after the implementation of CNG and published the work in 2006. According to their study, after the implementation of CNG decreasing trend was found in many criteria pollutants like polycyclic aromatic hydrocarbons (PAH), SO2 and CO concentrations in air quality, while the NOx level was increased in comparison to those before the implementation of CNG. Further, SPM, PM10, and benzene, toluene, xylene (BTX) concentrations showed no significant change after the implementation of CNG.

Impact of Taxation in London Air Quality

London is the largest and most populous city in the European Union. Increased vehicle population is causing traffic congestion and pollution has become a major problem in megacity London. To reducing the congestion, implementation of the London Congestion Charging Scheme (CCS) began in February 2003, using a single charge of £5 for vehicles entering a central London zone between the weekday hours of 07:00–18:30. Although several vehicle types are exempt from the charge, the effect of the scheme has been to reduce the distances traveled by vehicle within the zone. In a study on impact of congestion charging on vehicle emissions in London, which was carried out by Beevers and Carslaw and published in 2005, several important results are apparent from the analysis of the effect of the London congestion charging scheme. First, there is a significant effect associated with increases in vehicle speed brought about by the reduction in congestion and that the changes at slower speeds have a disproportionate effect on vehicle emissions. Second, the reductions in distances traveled in vehicles as a result of the scheme are large, particularly in the charging zone itself. To meet the demand to travel into central London an increase in bus travel was also evident, but bus emissions have been offset to some extent because of the widespread introduction of particle traps to the new and existing bus fleet. Finally, there is a reduction in emissions of CO2, providing evidence that a scheme of this kind will assist in attaining government targets relating to climate change.


As Molina and Molina concluded in 2004 in their comprehensive review of megacities and atmospheric pollution, the anthropogenic emissions from present megacities present a major challenge for the local air quality, regional atmospheric chemistry and global environment. However, as the centers of economic growth, technological advances, social dynamics, and cultural production, these highly populated urban areas also offer possible opportunities to manage a growing population in a sustainable way. It’s because the well-planned, densely populated settlements can reduce the need for land conversion and provide proximity to infrastructure and services. Nevertheless, to accomplish sustainable urban development in these ever expanding megacities, the following must include:

  1. Appropriate ambient air quality management plans that include adequate monitoring capabilities for the surveillance of the environmental quality and health status of the populations. A key issue related to monitoring in megacities is the specific location of air quality measurements, which can produce biased results and influence the resulting index values. This calls for standardization of siting criteria and number of sites needed per megacity in working towards better global tracking of air quality in megacities.
  2. Adequate access to clean environmental technologies, which also include the provision of training and development of extensive international information networks and thus the capacity building; and
  3. Improvement of data collection, assessment and analysis so that national and international decisions can be based on sound scientific information.

The need for carefully constructed indices and metrics for pollution control and natural resource management is urgent according to UN millennium development goals (MDGs), which call upon nations to make progress on a range of critical development issues. However, the environmental dimension of the MDGs is often criticized as being insufficiently defined and inadequately measured. In this context, a simple concept and formulation of a multi-pollutant index (MPI) proposed by Gurjar et 2007 could be useful to evaluate atmospheric emissions and urban air quality, particularly in megacities. This index can help monitor air quality changes over time, and relate these to other indices that provide information about the – often rapidly – changing state of megacities.

As argued by Molina and Molina in the critical review on megacities and atmospheric pollution published in 2004, there is no single strategy for addressing air pollution problems in megacities. A mix of policy measures and technological options best suited for each city’s challenges and customs will be required to improve urban air quality. They further emphasize that addressing urban air quality issues effectively requires a holistic approach that takes into account scientific, technical, existing infrastructure, economic, social, and political factors in an integrated manner.

Acknowledgments: This article is primarily based on various research papers/articles included in the following reference list.


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Further Reading


Gurjar, B. (2014). Air quality in megacities. Retrieved from