UNEP SASAKAWA ENVIRONMENT PRIZE 20TH ANNIVERSARY
PANEL DISCUSSION: AIR
Mario J. Molina and Luisa T. Molina
About half of the world’s population now lives in urban areas because of the opportunities for
better jobs, access to city services, cultural and educational activities. At the same time, many of
these urban centers are expanding rapidly, transforming themselves from villages to towns,
towns to cities, and cities to megacities. These high concentrations of people and their activities
are exerting increasing pressure on the natural environment with consequences at urban, regional,
continental and global levels. In recent decades, air pollution has become one of the most
important problems of large cities. The main air pollutants of concern until recently were sulfur
compounds generated mostly by burning coal. Photochemical smog induced primarily from
traffic, but also from industrial activities, power generation, and solvents, has now become the
main source of concern for air quality, while sulfur is still a major problem in many cities of the
developing world. The regional and global dispersion of pollutants generated locally causes acid
deposition, and changes in the Earth’s radiation balance. Long-range transport of ozone and
particulate matter (PM) influences air quality and climate in regions far from their sources. Yet,
with appropriate planning, cities can efficiently address their air quality problems through
measures such as application of new emission control technologies and development of
sustainable transportation systems.
IMPACTS OF DEGRADED AIR QUALITY
Adverse Health Effects
Poor air quality imposes a significant health burden to the population and substantial economic
costs to society. Epidemiologic evidence indicates that particulate matter and ozone cause the
most severe health impacts. Researchers now believe that fine particulate matter (PM2.5 ,
particulate matter with an aerodynamic diameter of 2.5 m or less) contains the most toxic
components. Increases in daily PM2.5 concentrations have consistently been linked to increases
in daily mortality. Most of the deaths caused by PM2.5 are from cardiovascular causes and occur
among the elderly. However, recent evidence indicates that infants may also be particularly
sensitive to fine particulate matter.
Urban and Regional Haze
Urban and regional haze is the most commonly perceived effect of air pollution. The connection
between air pollutants and visibility impairment is related mostly to fine particles, but it is often
accompanied by high levels of other pollutants. In Beijing, China, visibility is often low; and is
caused mainly by combustion-related particles. In many Asian cities, sand storms and dust cause
poor visibility during the spring. Agricultural practices such as “slash and burn” generate PM
and gaseous precursors of photochemical smog. These emissions, added to the outflow from
urban centers, degrade air quality on regional scales and also affect climate.
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Regional Ecosystem Impacts
Acid and fixed nitrogen deposition. Sulfur dioxide and nitrogen oxides are the main precursors
of acid rain pollution that can harm forests, lakes, and river ecosystems. A related issue involves
fertilization effects caused by the deposition of airborne fixed nitrogen species to buffered soils
and surface waters that are not susceptible to acidification. Combined with fixed nitrogen and
phosphorous from fertilizer, animal waste and human sewage sources, atmospheric deposition of
fixed nitrogen can help over-fertilize soils, lakes, streams, and estuaries leading to changes in
primary productivity and, potentially, to eutrophication.
Photochemical oxidant damage. Photochemically produced oxidants and their precursors
frequently produce high levels of ozone and other oxidants that transport from one city to the
next, subjecting the intervening suburbs, forests, and agricultural areas to high oxidant exposures.
Photosynthetically Active Radiation. Recent studies show that sulfur dioxide emissions in Asian
megacities can create extremely high levels of fine particles with absorption and scattering
properties that significantly influence both the direct and diffuse components of
photosynthetically active radiation available in agricultural regions.
Regional Climate Change
High levels of greenhouse gas associated with major cities have a direct impact on infrared
radiative forcing globally. Furthermore, the powerful but shorter-lived tropospheric ozone will
have a more pronounced regional effect. Fine particles can have a direct effect on short
wavelength radiative forcing by scattering and/or absorbing solar radiation.
Satellite
observations show an albedo reduction due to absorbing aerosols and their impact on cloud
absorbance over urbanized regions in China. Surface temperature records in urbanized regions
of China show a measurable cooling since the 1950s. This is consistent with the predicted
effects of elevated soot levels in fine particles despite a general warming observed for most of
the globe over the same time period. High PM loadings can also influence precipitation levels
by lengthening cloud lifetimes and suppressing rain and snow as a result of nucleating more, but
smaller, cloud droplets.
Global Pollutant Transport
Satellite, aircraft, and ground-based observations throughout the global atmosphere are
confirming model simulations that air pollution can be transported over long distances.
Tropospheric oxidants, changes in precipitation chemistry, and reduced visibility are already
significant environmental issues in much of the industrial Northern Hemisphere. Globally,
current levels of pollution-related tropospheric PM and ozone are significant contributors to the
atmospheric greenhouse radiation budget.
Several regional-scale studies have been carried out in the United States and Europe that
demonstrate the enormous pollutant potential of major cites and “megalopolis” regions, as well
as the fact that significant quantities of gaseous pollutants and fine particles can be transported
and detected over intercontinental scales. These insights have erased the distinction between air
quality (long thought to be a local- to regional-scale issue) and global atmospheric chemistry
(focused on concerns about climate change induced by greenhouse gases, stratospheric ozone
depletion, and tropospheric oxidative capacity). It is now clear that the gaseous pollutants and
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fine particles dispersed from heavily polluted regions may have significant impacts on
continental to global scales. Given the high population growth, rapid industrialization and
motorization of the megacities of the developing world, it is likely that regional and even
intercontinental transport of pollutants at low latitudes will grow rapidly, posing an even greater
challenge.
EMISSION REDUCTION STRATEGIES AND CHALLENGES
Urban air pollution is not a new problem, and effective emission reduction strategies are
available for most emission sources. In the higher income cities and megacities of North
America, Europe and Japan, there has been a three to four-decade long experimentation and
development of air pollution abatement experience. On the other hand, in the lower income
megacities of Asia and other developing nations, the demand for pollution abatement is emerging
and accompanied by more limited resources. Therefore, the design of air quality management
strategies must be preceded by a careful analysis of the social, economic and institutional
circumstances in these cities and what policy challenges and opportunities this context offers.
Transportation is a major source of air pollution in many cities. Technical factors pertaining to
engine characteristics, vehicle age, fuel type used, and maintenance patterns of transport vehicles
contribute to the level of emissions for a given level of transport demand. The major challenge
for air pollution control policy is the exploding demand for private automobile ownership and
vehicle kilometer traveled (VKT) in the large cities with rising incomes. The combination of
increasing VKT, limited road space and consequent congestion leads to a vicious circle of ever
worsening air pollution. Policies of transportation and pollution control typically developed in
this context emphasize road expansion, parking and other auto friendly initiatives, which
frequently induce further traffic growth and pollution, defeating such policies in short order.
A second major challenge is the excessive age of some of the vehicle fleet, especially the dieselpowered freight transport and urban buses, many lack basic pollution controls.
Another
challenge is the quality of fuel used. Improvements in fuel to lower sulfur content and other
changes can have an important effect on air quality. The final challenge for the industrializing
megacities is the limited availability of the technical and analytical skills necessary to develop
effective pollution reduction policies and the institutional capacity to implement such policies.
There are three major categories for implementing effective pollution reduction strategies: 1)
technology-based regulatory mandates include emission limits imposed on industry and vehicles;
2) economic instruments such as incentives, emission taxes, road pricing, and emission trading;
and 3) policy adaptation such as land-use planning, infrastructure development, and transport
management. The most effective air quality management strategies use a combination of these
approaches together with public outreach programs, and enforcement through persuasion and
incentives, to produce an equitable and appropriate reduction in emissions.
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FUTURE OUTLOOK
Scientific Knowledge. Air pollution science has progressed steadily in recent decades due to
improvements in the ability to measure pollutants, precursors, and reactive intermediates. This
information has facilitated the development of improved computer models of the complex
photochemistry that forms ozone, other oxidants, and secondary particles. These scientific
advances motivate further research to gain a better understanding of how air pollution is formed
in megacities and how best to control it.
Field measurement campaigns focused on the characterization of the outflow of air pollutants
from megacities need to be carried out to assess their regional and global impacts. Long-term
measurement programs are needed to characterize air quality on regional and global scales. A
better understanding of the potential climate effects of atmospheric particles, particularly those
containing black carbon or soot, is also required.
Although it is well established that atmospheric particles have strong impacts on human health,
research needs to be conducted to explain the chemical identity of the particles that actually do
the damage. Advances in health studies will require a close collaboration between
epidemiologists, physiologists, and atmospheric scientists.
Interdisciplinary Research. To address the pressing environmental problems confronting
megacities, it is essential to bring together national and international experts in science,
engineering, economics, social and political sciences to engage in collaborative research that
leads to both holistic assessments of the complex environmental problems and the development
of practical solutions. A strategy to overcome the social, economic and political barriers will
have to be developed jointly with the relevant stakeholders. Furthermore, various research
activities, financial analysis, coordination and communication among government officials,
stakeholders, and experts in the academic and industrial sectors will all be required to
successfully develop and implement air quality management plans.
Institutional Improvement. Most urban environmental problems can only be successfully
solved by establishing a strong regional authority committed to reducing pollution. Strong
political will is essential to develop institutional capacity, secure financial resources, integrate
relevant metropolitan policies for transportation, land use and air quality, and coordinate with
policies affecting population, energy supply, and other key urban factors. .
Air pollution is transported from state to state and across international borders, therefore
international coordination and collaboration should be strongly encouraged. Political leadership
is needed to cut through overlapping and conflicting jurisdictions and short-time horizons.
Further, it is important to promote participatory processes and develop effective partnerships
with and among all stakeholders of civil society, particularly the private and community sectors.
Many countries do not have sufficient information to establish emission standards, emission
inventories, and monitoring networks; there is also a lack of research on health effects. Although
international organizations can provide technical assistance, in the long term, local capacity
building will be more effective to removing barriers than short-term outside technical help.
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Sustainable Transportation. Economic growth is closely linked to personal and freight
transportation and efficient mobility. However, increasing numbers of vehicles will cause
congestion resulting in both poor air quality and hindered economic growth. The challenge is to
improve air quality while ensuring personal and freight mobility. This would require a set of
integrated strategic options involving cleaner fuels, advanced vehicle technologies, institutional
change, infrastructure investment, operational improvements, coordination of transportation and
land use policies and active stakeholder participation.
Reduction in per-vehicle emission levels resulting from new, clean technologies is often largely
offset by increases in vehicle use in many large urban centers. Growth in vehicle ownership
needs to be decoupled from daily vehicle usage, an approach that requires the availability of
efficient public transportation such as rail lines or the less expensive bus rapid transit systems.
New vehicle technologies have been responsible for enormous improvements in new vehicle
emissions performance. The use of these clean vehicle technologies is occurring in many cities
in developed and developing nations because vehicle emissions controls are being applied worldwide. However, emission reduction from diesel trucks, motorcycles, and two-stroke engines has
not progressed as rapidly as for passenger cars. Progress is being made in some countries by fuel
switching to compressed natural gas and replacing two-stroke with four-stroke engines.
Appropriate maintenance is essential. Old vehicles remain in the fleet because the cost of
replacement is often perceived as being too high for many in the developing world, but the
consequent public health costs are not taken into account. This problem is particularly difficult
to solve with heavy-duty vehicles, because the existing fleet is likely to remain functional for
decades. Heavy-duty vehicle emissions standards are evolving, and one of the technologies of
growing interest is the retrofit of oxidation catalysts with particulate traps for diesel engines
CONCLUSION
The pressing environmental issues of urban pollution, land degradation, and climate change are
closely linked problems sharing common causes and solutions. Much progress has been made in
combating air pollution problems in developed and some developing world megacities.
However, there continue to be many areas where comprehensive solutions appear to be elusive.
Sustainable development must include appropriate air quality management plans that include
adequate monitoring capabilities for the surveillance of the environmental quality and public
health; adequate access to clean technologies; and improvement of data collection and
assessment so that national and international decisions can be based on sound information.
While each city – its problems, resources and outlook – is unique, the need for a holistic
approach to the complex environmental problems is the same. There is no single strategy in
reducing air pollution in megacities; a mix of policy measures will be needed to improve air
quality. An important lesson learned throughout the world is that strong political will coupled
with public dialog is essential to effectively implement the regulations required to address air
pollution problems.
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