a

GAO

United States General Accounting Office

Report to Congressional Requesters

April 2003 CLIMATE CHANGE

Information on 

Three Air Pollutants’ 

Climate Effects and 

Emissions Trends

GAO-03-25
Scientists generally agree that sulfate aerosols have a cooling effect on 

climate, while ozone in the lower atmosphere has a warming effect. Black 

carbon tends to warm the atmosphere but cool the earth’s surface. Sulfate 

aerosols also affect how much and where it rains. Considerable uncertainty 

remains about the size of these effects. 

All seven countries are taking steps to reduce the amounts of the three 

pollutants. The four economically developed countries have well-established 

efforts underway. In these countries, the amounts of the three substances 

generally declined over the last 2 decades and are expected to decline over 

the next decade. In contrast, the three developing countries’ efforts are less 

well established. In these countries, the amounts of the three substances 

generally increased during the years for which information is available. GAO 

found few projections for these three countries. 

An extensive body of research has examined the possible connection 

between economic development and environmental pollution, but the results 

of this research are inconclusive. Researchers also caution that economic 

growth by itself may help support environmental improvements but is not, 

by itself, sufficient to ensure them. 

Projected Average Global Ozone Concentrations, Parts per Billion, 2025 

Solar radiation is absorbed by the 

earth and is subsequently 

reemitted. The buildup of carbon 

dioxide and certain other gases in 

the earth’s atmosphere traps some 

of that radiation. This is known as 

the greenhouse effect and is 

believed to contribute to a warming 

of the earth’s climate. Concerns are 

growing that, in addition to carbon 

dioxide and other conventional 

greenhouse gases, certain air 

pollutants may affect the climate. 

GAO was asked to examine (1) the 

extent of agreement among 

scientists regarding the effect on 

the climate of three air pollutants—

black carbon (soot), ground-level 

ozone, and sulfate aerosols—and 

(2) seven countries’ efforts to 

control these pollutants, trends in 

these substances in these countries 

over the past 2 decades, and 

estimates for the next decade. GAO 

was also asked to summarize the 

relationship between economic 

growth and environmental 

pollution. 

The seven countries include four 

that are economically developed—

Germany, Japan, the United 

Kingdom, and the United States—

and three that are developing—

China, India, and Mexico. These 

countries were chosen because 

they have large economies with a 

high potential to emit these 

pollutants. 

The two federal agencies asked to 

comment generally agreed with the 

information presented in this 

report. 

www.gao.gov/cgi-bin/getrpt?GAO-03-25.

To view the full report, including the scope 

and methodology, click on the link above. 

For more information, contact John B. 

Stephenson at (202) 512-3841. 

Highlights of GAO-03-25, a report to 

Congressional Requesters, House of 

Representatives 

April 2003

CLIMATE CHANGE 

Information on Three Air Pollutants’ 

Climate Effects and Emissions Trends 
Page i GAO-03-25 Air Pollutants and Climate Change

Contents

Transmittal Letter 1

Results in Brief 4

Background 6

Scientists Agree on the Overall Direction of the Three Pollutants’ 

Climate Impacts, but the Estimates of Impacts Contain 

Significant Uncertainties 13

The Three Pollutants Are Generally Declining in Economically 

Developed Countries, but Not in Developing Countries; All Seven 

Countries Are Acting to Reduce Emissions 18

Studies of the Effect of Economic Growth on the Environment 

Are Inconclusive 44

Conclusions 51

Agency Comments 52

Appendixes

Appendix I: Scope and Methodology 54

Appendix II: Programs and Measures to Reduce Emissions of 

Sulfur Dioxide 56

Appendix III: Programs and Measures to Reduce Emissions of 

Black Carbon or Particulate Matter 58

Appendix IV: Programs and Measures to Reduce Ground-Level Ozone 59

Appendix V: Summary of Results of Selected Studies 

on Economic Growth and Environmental Pollution 61

Appendix VI: GAO Contact and Staff Acknowledgments 62

Tables Table 1: Comparative Statistics of the Seven Countries 

Reviewed 12

Table 2: Results of Selected Studies of Economic Growth and 

Environmental Quality 61

Figures Figure 1: Sources and Estimated Mean Atmospheric Lifetimes of 

Selected Substances Affecting Climate 8

Figure 2: Direct Effects of Several Substances on Climate Change 

as Reported in the IPCC’s Third Assessment 

Report, 2001 14
Contents

Page ii GAO-03-25 Air Pollutants and Climate Change

Figure 3: Sulfur Dioxide Emissions in Four Developed Countries, 

1980-99 19

Figure 4: Projected Sulfur Dioxide Emissions in Four Developed 

Countries, 1990-2010 20

Figure 5: Sulfur Dioxide Emissions in China and Mexico, 

Selected Years 24

Figure 6: Black Carbon Emissions in the United States, 

United Kingdom, Germany, and Japan, 1980-96 29

Figure 7: Black Carbon Emissions in China, India, and Mexico, 

1980-96 33

Figure 8: Annually Averaged Global Ozone Concentrations at 

5 Kilometers, Parts per Billion, 1990 38

Figure 9: Projected Annually Averaged Global Ozone 

Concentrations at 5 Kilometers, Parts per Billion, 2025 39

Figure 10: Number of Areas Exceeding the Ozone Standard, 

United States and Germany, 1990-2000 41

Figure 11: Hypothetical Inverted U-Shaped Curve, Showing 

Relationship Between Per Capita Income and 

Environmental Pollution 45

Figure 12: Estimated Turning Points Found in Selected 

Studies of Particulate Matter, Sulfur Dioxide, and 

Carbon Dioxide 48

Abbreviations

CLRTAP Convention on Long-Range Transboundary Air Pollution

EIA Energy Information Administration

EPA Environmental Protection Agency

EU European Union

IPCC Intergovernmental Panel on Climate Change

UNFCCC United Nations Framework Convention on Climate Change

This is a work of the U.S. Government and is not subject to copyright protection in the 

United States. It may be reproduced and distributed in its entirety without further 

permission from GAO. It may contain copyrighted graphics, images or other materials. 

Permission from the copyright holder may be necessary should you wish to reproduce 

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Page 1 GAO-03-25 Air Pollutants and Climate Change

United States General Accounting Office

Washington, D.C. 20548

A

April 28, 2003 TransmtLailetr

The Honorable W. J. (Billy) Tauzin

Chairman

Committee on Energy and Commerce

House of Representatives

The Honorable Joe Barton

Chairman, Subcommittee on Energy and Air Quality

Committee on Energy and Commerce

House of Representatives

The Honorable James C. Greenwood

Chairman, Subcommittee on Oversight and Investigations

Committee on Energy and Commerce

House of Representatives

Carbon dioxide and certain other gases in the earth’s atmosphere trap the 

sun’s heat and prevent it from escaping back into space. This phenomenon, 

known as the greenhouse effect, tends to warm the earth’s surface; the 

gases that cause it are called greenhouse gases. Most scientists agree that 

a change in the earth’s climate could have wide-ranging effects on 

economies, ecosystems, and human habitation. Climate change could make 

some locations unsuitable for growing traditional crops, which could lead 

to economic disruptions, especially in agricultural economies. However, 

warming is not the only issue of concern with respect to greenhouse gas 

emissions. A buildup of such gases could also alter precipitation patterns, 

causing some areas to receive more rain and others to become drier. 

Extreme events, such as drought or floods, could become more frequent. In 

addition, if climate change causes sea levels to rise substantially, low-lying 

areas might become uninhabitable, forcing the dislocation of entire 

populations. Evidence suggests that climate change is the result of both 

human actions (such as fossil fuel burning) and natural phenomena (such 

as solar variability). However, according to an international panel of 

experts, most of the warming observed over the past 50 years is 

attributable to human activities.

In recent decades, concentrations of greenhouse gases have built up 

in the atmosphere. Concerned about these increased concentrations, 

the United States and many other nations entered into a treaty to 

stabilize atmospheric concentrations of greenhouse gases at levels that 

would prevent dangerous human interference with the climate system. 
Page 2 GAO-03-25 Air Pollutants and Climate Change

The United States ratified this treaty, the United Nations Framework 

Convention on Climate Change (UNFCCC), in 1992. Of the gases covered 

by the treaty, the most important in terms of contribution to warming, are, 

in declining order of emissions levels, carbon dioxide, methane, nitrous 

oxide, and three types of synthetic (manufactured) gases.

In addition, scientists have recently intensified their efforts to study the 

climate effects of substances not included in the Framework Convention. 

According to a 2001 comprehensive review1

 of scientific research on 

climate change, additional substances—generally regarded as air 

pollutants because they can damage human health and the environment—

can also affect the earth’s climate. Among the substances are the three 

treated in this report, listed below. Of these three, sulfate aerosols exert the 

greatest influence on climate.

• Sulfate aerosols are produced when sulfur dioxide gas is transformed 

through oxidation2

 in the atmosphere into aerosol particles. (An aerosol 

is a solid and/or liquid particle suspended in the air.) Sulfur dioxide is 

produced mainly by burning coal and petroleum products that 

contain sulfur.

• Black carbon is a type of aerosol produced by the incomplete 

burning of substances containing carbon. Significant sources include 

uncontrolled or poorly controlled combustion of coal, diesel fuel, and 

biomass for heating and cooking, and by the burning of fields and 

forests. Black carbon emissions from these sources are accompanied by 

varying amounts of particulate organic materials.

1

 Intergovernmental Panel on Climate Change (IPCC), 2001. Climate Change 2001: The 

Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the 

Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, 

United Kingdom and New York, New York.

2

 Oxidation is a type of chemical reaction involving oxygen.
Page 3 GAO-03-25 Air Pollutants and Climate Change

• Tropospheric ozone3 is formed when nitrogen oxides react with 

certain other chemicals in the presence of sunlight. Nitrogen oxides are 

produced primarily by cars and other vehicles and by power plants that 

burn fuel to generate electricity. On a global scale, methane and carbon 

monoxide are also significant precursors to ozone.

You asked us to (1) assess the extent to which the scientific community 

agrees on the climate effects of sulfate aerosols, black carbon, and ozone, 

as reflected in the 2001 review, and identify important developments since 

that review, and (2) identify trends in emissions and concentrations of 

these three substances over the past 2 decades in the United States and 

six other specified countries, identify projected estimates in emissions and 

concentrations of these pollutants in all seven countries over the next 

decade, and determine each country’s actions to reduce these three 

pollutants. In addition, you asked us to review the existing literature on 

the effect of economic development on a country’s pollution levels to 

determine whether, as some researchers have suggested, there is a 

systematic connection between growth in income and emissions. As 

agreed with your offices, we examined the United States and three other 

economically developed countries (Germany, Japan, and the United 

Kingdom) and three economically developing countries (China, India, and 

Mexico). These countries were chosen because they have large economies 

with a high potential to produce the substances we examined.

To obtain information on recent research relating to the climate change 

characteristics of the three substances, we relied primarily on the Third 

Assessment Report, the most comprehensive source on climate science. 

We also contacted scientists in four federal agencies, who were 

recommended by staff at the U.S. Climate Change Science Program, which 

coordinates and supports government research on climate change. To learn 

about work subsequent to the 2001 review, we reviewed published work 

recommended by experts in the field. In obtaining data on emissions trends 

and projections, we found that data from government and academic 

3

 Wherever it is found, ozone is chemically the same, a molecule comprised of three oxygen 

atoms. Its effects depend on its location. In the earth’s upper atmosphere (called the 

stratosphere), ozone is beneficial because it prevents ultraviolet radiation—which is 

dangerous to human health—from reaching the earth’s surface. However, at lower levels 

(called the troposphere), it is harmful to human health. Ozone in the stratosphere also has 

impacts on climate. It is formed by different means than ozone in the troposphere. We do 

not discuss stratospheric ozone at length in this report because it is produced mainly 

through natural (non-human-induced) processes. Unless otherwise stated, when we refer to 

ozone, we are referring to tropospheric ozone.
Page 4 GAO-03-25 Air Pollutants and Climate Change

sources in the United States and the other six countries varied considerably 

in terms of quality and availability. In cases where no other estimates were 

available, we used results prepared by leading researchers in the field. 

With respect to the types of policy measures reviewed, we concentrated 

on regulatory measures and did not include research and development 

programs, programs of a voluntary nature, emissions monitoring 

requirements, or programs to disseminate information or educate the 

public. We further focused on existing programs or programs already 

authorized and excluded proposed programs. We also excluded the 

policies and measures taking place at the sub-national level (state or 

province level, for example), which are separate from, though sometimes 

complementary to, national measures. For more information on how we 

gathered information for this study, see appendix I.

Results in Brief The scientific community substantially agrees that black carbon aerosols 

warm the atmosphere and cool the earth’s surface, while ozone contributes 

to warming the earth, and sulfate aerosols contribute to cooling it, 

according to the 2001 review, other scientific literature, and discussions 

with atmospheric scientists. However, scientists are uncertain about the 

extent of these effects. They believe that the level of uncertainty associated 

with these pollutants is moderate for ozone and high for black carbon and 

sulfate aerosols. While carbon dioxide and the other traditional greenhouse 

gases generally contribute to temperature and other effects at a global 

level, the three substances considered here operate on a different scale, 

both in terms of time and geography. These three substances can have 

effects over smaller distances and shorter time frames. In other words, 

their impacts tend to be more important locally and regionally, while 

having a smaller influence globally. Because of relatively rapid removal 

from the atmosphere, their impact may be felt in the short-term, as 

opposed to over decades or centuries hence. However, the effects of these 

substances are not limited to warming; these substances may also cause 

local cooling or may change local precipitation patterns. Since the 2001 

review, other research has added to scientists’ understanding of the effects 

of black carbon. For example, air and clouds over the Indian Ocean (which 

are polluted with black carbon, sulfates, and other aerosols) were found to 

absorb the sun’s energy to a far greater extent than expected. These results 

suggest that black carbon, the only one of these substances that absorbs 

rather than reflects light, may play a more important role in warming the 

atmosphere than was estimated in the 2001 review.
Page 5 GAO-03-25 Air Pollutants and Climate Change

All seven countries are taking steps to reduce the amounts of the three 

pollutants. In the United States and the other three economically developed 

countries we studied, these efforts have been underway for decades. For 

example, these countries limit emissions from power plants that burn 

coal and other fossil fuels. They also regulate emissions from automobiles 

and trucks. In these four countries, the amounts of the three substances 

generally declined over the last 2 decades. For example, sulfur dioxide 

emissions declined in all four countries and black carbon emissions 

declined in three countries. Although sulfur dioxide emissions are 

expected to decline in all of these countries, we found no emissions 

projections for black carbon for three of them. Although developed 

countries have made progress in reducing domestic ozone concentrations, 

global ozone concentrations are likely to increase, owing to rising 

emissions of ozone precursors in other countries. In contrast, in China 

and the other two developing countries, emission control efforts are 

more recent, typically going back only a decade or so. As in the developed 

countries, these countries also target power plants and motor vehicles to 

control emissions. In the developing countries, the levels of the three 

substances varied. Sulfur dioxide emissions decreased in two countries, 

but black carbon emissions increased in two countries. We found limited 

information on ozone concentrations in developing countries. Most 

available data pertain to only a few major cities and is not nationally 

representative. Similarly, we were unable to find projections for sulfur 

dioxide and black carbon emissions for these countries. The quality of data, 

especially for developing countries, is uneven.

The results of empirical and theoretical research on the effect of 

economic growth on environmental pollution are inconclusive. This 

research has examined the hypothesis that pollution initially worsens as 

an economy grows, but then improves as economic growth continues and 

income rises. Empirical studies, which analyzed historical data to find 

such a relationship across pollutants, have had mixed results. Similarly, 

theoretical studies, which sought to identify how economic growth 

may affect environmental pollution, have produced various possible 

explanations but reached no consensus. Researchers agree that improved 

data and more detailed studies will be needed to identify how economic 

growth affects environmental pollution. Also, researchers caution that 

economic growth does not automatically lead to reduced pollution. They 

explain that, while economic growth may be necessary to provide the 

resources needed to protect the environment, it is not, by itself, sufficient 

to reverse environmental degradation and that appropriate environmental 

policies must follow.
Page 6 GAO-03-25 Air Pollutants and Climate Change

Background Although the sun heats the earth’s surface, a large fraction of the sun’s 

energy is reflected back into space by clouds, ground surfaces, ice, and 

water. However, certain gases in the earth’s atmosphere, such as carbon 

dioxide and methane, trap some of the sun’s heat and prevent it from 

returning to space. The trapped energy warms the earth’s climate, much 

like glass in a greenhouse. Hence the gases that cause this effect are often 

referred to as greenhouse gases.

In response to potential environmental problems linked to the emissions 

of various heat-trapping gases, the United States and many other nations 

in 1992 signed a treaty aimed at limiting climate change induced by human 

activity. This treaty, called the United Nations Framework Convention on 

Climate Change, seeks to stabilize atmospheric concentrations of 

greenhouse gases at levels that would prevent dangerous human 

interference with the climate system. Under the 1992 convention, the 

United States and other parties generally agreed to implement programs 

aimed at reducing their emissions of greenhouse gases not covered by 

another treaty, the Montreal Protocol.4

 The most important of these 

warming gases, in declining order, are carbon dioxide, methane, nitrous 

oxide, and three types of synthetic (manufactured) gases—sulfur 

hexafluoride, hydrofluorocarbons, and perfluorocarbons.

4

 The Montreal Protocol, ratified by the United States in 1988, aims to reduce the 

use of substances that deplete stratospheric ozone. Among these substances are 

chlorofluorocarbons, which are also potent greenhouse gases.
Page 7 GAO-03-25 Air Pollutants and Climate Change

In recent years, scientists have focused increased attention on the climatic 

role of certain substances that are regarded as air pollutants (because of 

their harmful effects on human health and the environment) but are not 

covered by the Framework Convention or Montreal Protocol. These 

substances are aerosols (including sulfate aerosols and black carbon) and 

tropospheric ozone. Scientists have long recognized that these pollutants 

can affect climate, but these pollutants were not included in the 

Framework Convention. Sulfate and black carbon aerosols differ from 

traditional greenhouse gases, such as carbon dioxide and methane, in two 

key ways. First, unlike the traditional greenhouse gases, which are evenly 

distributed throughout the atmosphere and have global impacts, the effects 

of sulfate aerosols and black carbon are greatest near their sources, 

although they can have global impacts. Second, whereas traditional 

greenhouse gases can remain in the atmosphere for tens to hundreds, or 

even thousands,5

 of years, sulfate aerosols, black carbon, and ozone remain 

in the atmosphere for much shorter time periods. Figure 1 shows the 

sources and estimated mean atmospheric lifetimes of various substances 

having an impact on climate.

5

 Sulfur hexafluoride and perfluorocarbons, which are man-made greenhouse gases, have 

atmospheric lifetimes of several thousand years. They are not depicted in figure 1.
Page 8 GAO-03-25 Air Pollutants and Climate Change

Figure 1: Sources and Estimated Mean Atmospheric Lifetimes of Selected Substances Affecting Climate

3b

4

4

4

5

6

2

3b

3a

Coal Mine

4

5

6

6

6

2

2

2

6

2

3b

3b 3b

3a

County

Landfill

Power

Plant

1. Sulfate aerosols

Black carbon

Tropospheric ozone

Nitrogen oxides

Volatile organic compounds

Methanea

3. Nitrous oxide

4. Carbon dioxideb

Combustion of coal and oil (power plants) Days to weeks

Days to weeks

Days to weeks

Days to weeks

Days to weeks

12 years

114 years

50 to 200 years

Wetlands, cattle and other ruminant animals, fugitive emissions from coal mining, landfills

Agricultural soil management (application of fertilizers), vehicles

Combustion of carbon-containing substances, such as combustion of coal, oil, or

natural gas in power plants or gasoline in vehicles; also from land-use changes

Substance Major sources Mean atmospheric lifetime

Incomplete combustion of fuels (diesel vehicles, forest fires, certain

power plants, cook stoves)

Produced in the atmosphere by reactions, in the presence of sunlight

involving nitrogen oxides and volatile organic compounds, including methane

and carbon monoxide 

3a

3b

1

2

3

4

5

6

By-product of combustion (vehicles, power plants)

Vehicles, industrial processes, trees

Source: Illustration prepared by GAO with portions using Art Explosion. For sources of information on lifetimes and sources, see next page.
Page 9 GAO-03-25 Air Pollutants and Climate Change

a

Wetlands are a natural, as opposed to man-made, source of methane. We list them here because 

wetlands are the largest single source of global methane emissions.

b

The lifetime of carbon dioxide depends on rates of absorption by oceans and vegetation.

Notes: For each substance, we depict only the most important sources. Additional sources of 

emissions exist for most of the substances listed above.

This graphic was prepared by GAO with lifetime data reported in IPCC’s Third Assessment Report, 

2001. Source data reported by EPA, National Air Quality and Trends Report, 1999; EPA, Inventory of 

U.S. Greenhouse Gas Emissions 1990-2000; and Elaine Matthews, “Global Methane Emissions: 

Historical Trends, Controlling Factors, and Future Prospects,” in Air Pollution as a Climate Forcing: 

Workshop Proceedings, May 2002.

• Sulfate aerosols are created when sulfur dioxide emitted from, for 

example, coal- and oil-fired power plants, is oxidized in the atmosphere. 

Atmospheric sulfate aerosols have been associated with significant 

effects on public health and visibility impairment, and when they are 

deposited on earth, they contribute to the acidification of lakes, streams, 

and forests. Sulfate aerosol particles also have major effects on clouds, 

where they provide additional nuclei around which cloud droplets form. 

This can increase the cooling effect of clouds and may also increase the 

life spans of clouds. Both developed and developing countries burn coal 

to generate electricity, leading to emissions of sulfur dioxide.

• Black carbon, a type of aerosol, is a form of particulate matter and 

results from the incomplete combustion of coal, diesel fuel, and biofuels 

(such as alcohol or gasohol), and from open biomass burning, (i.e., the 

burning of forests and agricultural residues). Because it is dark in color, 

black carbon aerosol absorbs the sun’s energy, creating warming in the 

atmosphere and cooling at the earth’s surface, thus modifying climate 

(in contrast to the warming caused by traditional greenhouse gases, 

which prevent the earth’s heat from escaping into space). Black carbon 

is generally released with other pollutants in different proportions.
Page 10 GAO-03-25 Air Pollutants and Climate Change

• Tropospheric ozone is not emitted directly but is formed when 

emissions of other pollutants, called precursors, react in the presence 

of sunlight. Ozone precursors include nitrogen oxides (mainly nitric 

oxide and nitrogen dioxide) and several carbon-containing substances, 

namely carbon monoxide, methane, and non-methane volatile organic 

compounds. Nitrogen oxides are produced primarily by motor vehicles, 

other combustion engines, and electric power plants. Natural sources of 

nitrogen oxides are lightning and biological processes in soils. Carbon 

monoxide is formed when the carbon in fuels is not burned completely. 

It is a product of motor vehicle exhaust and industrial processes. 

Natural sources of carbon monoxide include wildfires. Methane is 

emitted by human activities, such as coal mining, natural gas and oil 

production, livestock production, rice cultivation, and waste disposal. 

It also comes from natural sources, including wetlands, and some 

animals, such as termites. Volatile organic compounds are produced by 

motor vehicles, industrial processes using solvents, and in some cases, 

by vegetation, including trees.6

6

 Isoprene and monoterpenes are examples of volatile organic compounds produced 

by vegetation.
Page 11 GAO-03-25 Air Pollutants and Climate Change

In reviewing the scientific information on these three substances, we 

concentrated on the material covered in the most recent systematic 

published review, the IPCC’s Third Assessment Report, which represents 

the consensus of many climate scientists.7

 We also included some more 

recently published material, notably the large-scale, multi-participant study 

of aerosols over the Indian Ocean, known as the INDOEX study, which was 

conducted in 1999. We included these results because they come from a 

large, well-recognized, international program whose objectives overlap 

substantially with many of the subject areas of this report.

The seven countries we reviewed differ greatly in terms of their 

population, total area, and per capita income. For example, population 

ranged from around 60 million in the United Kingdom8

 to nearly 1.3 billion 

in China; total area varied from just under 245,000 square kilometers for 

the United Kingdom to 9.6 million square kilometers for the United States; 

per capita income ranged from $2,200 in India to $36,300 in the United 

States. See table 1.

7

 IPCC’s Third Assessment Report, 2001.

8

 The United Kingdom includes Great Britain (England, Scotland, and Wales) and 

Northern Ireland.
Page 12 GAO-03-25 Air Pollutants and Climate Change

Table 1: Comparative Statistics of the Seven Countries Reviewed

Source: GAO (table), Central Intelligence Agency’s The World Fact Book, 2002 (data).

Notes: Some figures have been rounded.

Estimated gross domestic product per capita is based on purchasing power parity rates. Purchasing 

power parity asserts that a unit of currency, such as a dollar, should be able to buy the same bundle of 

goods in all countries.

In the 1960s, scientists determined that emissions of sulfur dioxide from 

European power plants were causing the acidification of Scandinavian 

lakes. Recognizing that air pollution does not respect national boundaries, 

34 countries, including Germany, the United Kingdom, and the United 

States, signed the Convention on Long-Range Transboundary Air Pollution 

(CLRTAP) in 1979. Under a subsequent protocol to this convention, the 

signatories agreed to reduce their emissions of sulfur dioxide. Later, they 

extended the convention to further reduce sulfur dioxide and to reduce 

ozone precursors and various other pollutants.9

 There are now 49 parties to 

the convention.

In addition to commitments under this 1979 convention, several European 

countries, including Germany and the United Kingdom, are bound by 

European Union (EU) requirements to reduce emissions. For example, a 

1996 framework directive set broad goals for the consistent management of 

several air pollutants, including sulfur dioxide, nitrogen oxides, and ozone, 

in member nations. In 1999, a supplemental directive established a legally 

binding limit on concentrations of sulfur dioxide and nitrogen oxides, 

Country

Estimated population

 (2002) (in millions)

Total area

 (square kilometers)

Per capita income,

 2001

Economically developed countries

United States 281.0 9,629,091 $36,300

United Kingdom 60.0 244,820 24,700

Germany 83.3 357,021 26,200

Japan 127.0 377,835 27,200

Economically developing countries

China 1,284.3 9,596,960 4,301

India 1,045.8 3,287,590 2,200

Mexico 103.4 1,972,550 9,000

9

 The United States is party to some, but not all, of the CLRTAP protocols. Certain of the 

protocols extending the convention are not yet in force.
Page 13 GAO-03-25 Air Pollutants and Climate Change

among other substances, to be achieved by all member countries by 2005. 

Another 1999 directive required EU countries to reduce the sulfur content 

of liquid fuel. (More information on the various directives can be found in 

apps. II, III, and IV.) Each EU member nation is required to transform the 

EU requirements into national legislation for domestic implementation. In 

general, the EU emission reduction targets are more stringent than the 

CLRTAP targets.

Scientists Agree on the 

Overall Direction of 

the Three Pollutants’ 

Climate Impacts, 

but the Estimates 

of Impacts Contain 

Significant 

Uncertainties

According to the most recent comprehensive review of scientific research 

on climate change, published in 2001, scientists generally agree that sulfate 

aerosols tend to cool the earth, while black carbon aerosols and ozone tend 

to warm it. However, the extent of these effects is uncertain. Research 

published since 2001 has not changed these overall conclusions, but it does 

suggest that the scientific community’s understanding of these effects is 

incomplete and needs further exploration.

The extent of the heating and cooling effects of various substances, as 

compared to pre-industrial conditions, is typically expressed in watts 

per square meter, which is a measure of energy per unit area. Changes in 

the balance between incoming and outgoing solar energy in turn heat or 

cool the earth. Specifically, the earth receives 342 watts per square meter 

of incoming solar radiation annually at the top of the atmosphere and 

reflects about 30 percent back into space, resulting in a net input of 

240 watts per square meter. If the climate were in balance, the same 

amount of energy that the earth received would be emitted back into space 

as infrared radiation. However, warming gases trap some of this outgoing 

radiation, thereby changing the balance between incoming and outgoing 

radiation. Water vapor is the most significant greenhouse gas; carbon 

dioxide is the second most significant. Additional carbon dioxide added to 

the atmosphere by human beings since the industrial era has contributed 

another 1.46 watts per square meter averaged over the earth’s surface 

annually, thereby further warming the climate. Figure 2 shows how the 

average effects of the three pollutants we reviewed compare with the 

climate effects of carbon dioxide and other conventional greenhouse gases.
Page 14 GAO-03-25 Air Pollutants and Climate Change

Figure 2: Direct Effects of Several Substances on Climate Change as Reported 

in the IPCC’s Third Assessment Report, 2001

a

Estimated uncertainties for these substances are plus or minus 10 percent.

b

Synthetic gases include hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride.

c

Estimated uncertainty for ozone is plus or minus 43 percent.

d

Please see notes below for a discussion of the uncertainty surrounding sulfate aerosol 

and black carbon.

Notes: According to the IPCC, the level of scientific understanding is low for sulfate aerosol and very 

low for black carbon aerosol. The uncertainty estimates in the report are a factor of 2 for each of these 

1.46

.48

.15

.34

-.40

.35

.20

High

Best estimate

Low

Legend

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

aCarbon dioxide

aMethane

aNitrous oxide

a,bSynthetic

greenhouse gases

cTropospheric ozone

dSulfate aerosols

dBlack carbon from fossil

fuel combustion

Warming effect

Cooling effect

Source: Intergovernmental Panel on Climate Change, Third Assessment Report, 2001.

Watts per square meter
Page 15 GAO-03-25 Air Pollutants and Climate Change

substances. (Factor of 2 means, for example, in the case of sulfate aerosol, the upper end of the range 

would be -0.2, which is half the estimate—the estimate of 0.4 divided by factor of 2—and the lower end 

of the range would be -0.8, which is twice the estimate—the estimate of 0.4 multiplied by factor of 2.) 

Note that these uncertainty estimates are not based in statistics, but rather on the range found in the 

recent literature.

Positive values indicate a warming effect; negative values indicate a cooling effect. The values for 

black carbon include only the burning of fossil fuels, but not other sources, such as biomass burning, 

which is estimated to make a similar-sized contribution to warming. Combustion of these substances 

also emits organic carbon aerosols, which contribute an opposite (cooling) effect on climate. These are 

all direct effects. Indirect effects, such as those operating through clouds, could increase the impacts of 

both sulfates and black carbon. Note that these uncertainty estimates are not based on statistical 

analysis, but rather are subjective judgments based on ranges found in reported studies. In addition, 

the values for these estimates are described in the IPCC Third Assessment Report as having different 

levels of scientific understanding associated with them: tropospheric ozone—medium; sulfate—low; 

and fossil fuel black carbon—very low.

These three pollutants are relatively short-lived and are distributed only 

locally or regionally, in contrast to the traditional greenhouse gases, 

which persist for many years and are distributed worldwide. In 

comparison, the effects of the short-lived substances must be averaged 

over time and space.

Sulfate aerosols. According to the 2001 review, scientists generally agree 

that sulfate aerosols contribute to cooling the earth. The uncertainty 

regarding the climate effects of sulfate aerosols is significantly higher than 

for tropospheric ozone. Because these aerosols are light colored, they do 

not absorb sunlight. Consequently, their effect is purely cooling, because 

they reflect sunlight back into space and prevent it from reaching the 

earth’s surface. This cooling phenomenon is referred to as a direct effect on 

the climate. The total amount of sulfate aerosols in the atmosphere is 

estimated to be about 10 times larger than the total amount of black carbon 

aerosols discussed below.

Scientists have also identified indirect cooling effects of sulfate aerosols 

that result from their effect on clouds. Like some other aerosols, sulfates 

become the nuclei onto which water vapor condenses, forming cloud 

droplets. There, they produce clouds composed of larger number of smaller 

droplets, which result in two indirect effects on the earth’s climate. First, 

smaller droplets tend not to coalesce as readily into raindrops. Therefore, 

clouds composed of smaller droplets are less likely to produce rainfall and 

will persist longer. Since clouds scatter solar energy back out to space, 

they redirect energy away from the earth, causing cooling. Second, since 

smaller cloud droplets scatter more sunlight per mass than larger cloud 

droplets, even more solar energy will be directed away from the earth. 

Because of both the lessened rainfall from affected clouds and the changes 

in local heating of the earth, with consequent reduction in evaporation, 
Page 16 GAO-03-25 Air Pollutants and Climate Change

sulfate aerosols can reduce the amount and change the distribution of 

rainfall in affected areas.

Black carbon. According to the 2001 review, scientists generally agree 

that black carbon aerosols contribute to warming the atmosphere. As with 

sulfate aerosols, the uncertainty associated with black carbon aerosols’ 

warming effect is high. Much of the uncertainty about black carbon 

aerosols’ effects is due to questions about how these aerosols mix with 

other types of aerosols and cloud droplets. In addition, black carbon is 

released in association with other pollutants, such as organic carbon, 

which has a cooling effect on climate. The proportions of each can differ 

substantially among sources.

According to recently completed research on the effect of aerosols in 

the atmosphere above the Indian Ocean,10 large amounts of aerosols—

including some black carbon—in the air masses coming off the Indian 

subcontinent lead to dramatic reductions in the amount of solar radiation 

reaching the ocean’s surface and may be reducing precipitation over 

polluted areas. While these results appear to be significant for a particular 

geographic area and time period, they have yet to be translated into 

globally averaged contributions to warming the earth’s surface. 

Nevertheless, they suggest that these aerosols may have a larger effect on 

warming than was previously recognized.

10 V. Ramanathan et al. “Aerosols, Climate, and the Hydrological Cycle.” Science, 294, 

Dec. 7, 2001.
Page 17 GAO-03-25 Air Pollutants and Climate Change

Tropospheric ozone. According to the 2001 review, most scientists agree 

that tropospheric ozone11 contributes to warming the earth. The level of 

uncertainty associated with this warming effect is lower than the level for 

sulfate aerosols and black carbon but is greater than for carbon dioxide 

and other well-mixed greenhouse gases. However, ozone is not uniformly 

distributed throughout the troposphere because it is produced in a very 

uneven pattern in polluted areas and has a shorter lifetime than most other 

greenhouse gases. As figure 2 shows, its estimated warming effect is about 

one-quarter of the warming effect of carbon dioxide.

The formation of ozone in the atmosphere is complex. Most tropospheric 

ozone is generated by gases, called ozone precursors, that are emitted by 

industry, automobiles, and some natural sources, such as lightning and soil. 

There are two main classes of ozone precursors: nitrogen oxides (made up 

of nitric oxide and nitrogen dioxide) and certain carbon-containing gases, 

such as carbon monoxide and volatile organic compounds, including 

methane. Recent research suggests that reducing methane could have a 

greater effect in reducing ozone than previously recognized.12,13 This 

discovery is significant, since methane, unlike the other ozone precursors, 

lasts in the atmosphere for as long as 10 years. (See fig. 1.)

11 Ozone in the stratosphere also has impacts on climate. It is formed by different means 

than ozone in the troposphere. We do not discuss stratospheric ozone at length in this report 

because it is produced mainly through natural (non-human-induced) processes. Ozone in 

the troposphere exists in two zones, the boundary layer and the free troposphere. The 

significance of these zones is explained in the next section.

12 Arlene Fiore et al. “Linking Air Pollution and Climate Change: The Case for Controlling 

Methane.” Geophysical Research Letters, 29(19), 1919, 2002.

13 Michael Prather et al. “Fresh Air in the 21st Century?” Geophysical Research Letters, 

30(2), 1100, 2003.
Page 18 GAO-03-25 Air Pollutants and Climate Change

The Three Pollutants 

Are Generally 

Declining in 

Economically 

Developed Countries, 

but Not in Developing 

Countries; All Seven 

Countries Are Acting 

to Reduce Emissions

The seven countries we reviewed have all enacted legislation and 

implemented regulations to reduce emissions of sulfur dioxide14 and black 

carbon, and concentrations of tropospheric ozone.15 For the most part, the 

levels of the three pollutants are declining in the four economically 

developed countries and, to a limited extent, in the three economically 

developing countries.

Our analysis of measures to control emissions and concentrations of 

these substances is organized by pollutant; for each pollutant, we begin 

with the economically developed countries and then turn to the developing 

countries. Within the first group, we first discuss the United States because 

we found the most complete information about it. We next discuss the 

United Kingdom, Germany, and Japan, in declining order of available 

information. For economically developing countries, for similar reasons, 

the order is China, India, and Mexico.

Sulfur Dioxide Emissions 

Declined in Nearly All 

Countries; All Seven 

Countries Are Taking 

Steps to Reduce Them

Of the three substances we reviewed, sulfur dioxide was the most widely 

measured and regulated. Appendix II describes each country’s regulatory 

approach to sulfur dioxide emissions in greater detail.

Developed Countries Sulfur dioxide emissions have declined and are expected to continue to 

decline in the United States, United Kingdom, Germany, and Japan through 

at least 2010, owing to a combination of explicit government policies 

designed to curb sulfur dioxide emissions, the development of cleaner 

power generation and transportation technologies, and a continuing 

transition in many countries away from high-sulfur coal to low-sulfur 

coal and natural gas. Figure 3 shows the decline in emissions for the 

United States, the United Kingdom, and Germany between 1980 and 1999 

14 In this report, we use sulfur dioxide emissions as a proxy for sulfate aerosols. This is 

because sulfate aerosols are the result of the chemical transformation in the atmosphere 

of sulfur dioxide emissions, such as those from power plants and other sources. It is 

difficult to attribute sulfate aerosols in the atmosphere to individual countries, but such 

attribution is possible with sulfur dioxide because many countries keep track of their sulfur 

dioxide emissions.

15 Because ozone is not emitted directly, it is measured in terms of concentrations.
Page 19 GAO-03-25 Air Pollutants and Climate Change

and for Japan between 1990 and 1999, the only years for which we found 

data for that country. As the figure shows, the greatest decline occurred in 

Germany. In most of these countries, emissions declined more steeply 

between 1990 and 1999 than between 1980 and 1990. Figure 4 shows 

projected declines between 1990 and 2010 (1995 and 2010 for Japan). As 

the figure indicates, the greatest relative decline is expected to occur 

in Germany.

Figure 3: Sulfur Dioxide Emissions in Four Developed Countries, 1980-99

Notes: A metric tonne is equivalent to 1.102 short tons (or 2,204 pounds). We were unable to locate 

data from Japan for 1980 through 1989.

0

5,000

10,000

15,000

20,000

25,000 Thousand metric tonnes 

19801981

1982

1983

1984198519861987198819891990

1991

1992

1993

1994

1995

199619971998

1999

Year

Germany

United Kingdom

United States

Japan

Sources: GAO (graphic); data reported by governments to CLRTAP (U.S., U.K., and Germany) and data reported by the Japanese 

government in Japan's Third National Communication to the UNFCCC, 2002. 
Page 20 GAO-03-25 Air Pollutants and Climate Change

Figure 4: Projected Sulfur Dioxide Emissions in Four Developed Countries, 

1990-2010

Notes: Projections of U.S. sulfur dioxide emissions do not include recent or proposed EPA rulemakings 

that are likely to decrease sulfur dioxide emissions in the future. Values from 2000 to 2010 for the 

United States, the United Kingdom, and Germany are based on 2010 projections.

Both the United States and European countries have health-based 

standards designed to minimize damage to human health caused by sulfur 

dioxide emissions. They also have annual emissions limits to control 

acidification of the environment caused by these emissions. The United 

States set its limits based on an across-the-board, 50-percent reduction in 

sulfur dioxide emissions, relative to 1980, from power plants. In contrast, 

the European governments set their standards using the “critical loads” 

United States

United Kingdom

Germany

Japan

Sources: Prepared by GAO with data reported to CLRTAP (United States, United Kingdom, and Germany) and data provided by 

Dr. T. Matsui, Japanese National Institute of Environmental Studies (Japan). 

0

5,000

10,000

15,000

20,000

25,000 Thousand metric tonnes

1990 1995 1999 2005 2010

Year
Page 21 GAO-03-25 Air Pollutants and Climate Change

approach, taking into consideration the estimated potential impact of the 

emissions on the environment. That is, the Europeans estimate, using 

mathematical models, the maximum amount of damage that a particular 

ecosystem—such as forests, lakes, and streams—could sustain before 

long-term harmful effects occur. Standards aimed at reducing emissions to 

a level near the critical load goal are then negotiated among countries. This 

type of standard is often referred to as an effects-based standard. We were 

unable to find information on how Japan developed its sulfur dioxide 

emissions standards.

United States. U.S. emissions of sulfur dioxide decreased from 

23.5 million metric tonnes to 17.1 million metric tonnes between 1980 and 

1999, a decrease of about 27 percent, according to U.S. data submitted to 

the CLRTAP. The United States projects that sulfur dioxide emissions will 

decline even further, to 15.1 million metric tonnes in 2010, representing 

about a 36-percent decrease from 1980 levels (See fig. 4.) The decline in 

emissions in 2010 may be even greater because U.S. projections take into 

account only national policies but not state regulations. They also do not 

include proposed new measures.

Sulfur dioxide emissions started to decline in the United States in the early 

1970s, after peaking at about 31 million tons. Federal regulation of sulfur 

dioxide emissions essentially began with the Clean Air Act of 1970, as 

amended in 1977 and 1990. The act required the Environmental Protection 

Agency (EPA) to develop national air quality standards for air pollutants 

that may endanger public health and welfare. EPA established such 

standards for sulfur dioxide and several other pollutants. The act also 

required each state to develop a plan (to be approved by EPA) for meeting 

those standards. Under the act, all new or modified large power plants 

could emit no more than a specified rate of sulfur dioxide per unit of fuel 

consumed. Most new plants responded to this requirement by shifting to 

coals with lower sulfur content.

Concern about sulfur dioxide emissions increased again in the late 

1970s and early 1980s, when scientists noticed that lakes and streams, 

particularly in the Northeast, were becoming increasingly acidic, thereby 

threatening aquatic life. This acidity was traced to sulfur dioxide and 

nitrogen oxide emissions from power plants, primarily those located 

upwind in the Midwest. The 1990 Clean Air Act Amendments imposed 

additional controls on such emissions. One of the programs created under 

the amendments was the Acid Rain Program, which employs emissions 

trading, a market-based mechanism, to reduce sulfur dioxide emissions.
Page 22 GAO-03-25 Air Pollutants and Climate Change

United Kingdom. According to data submitted to the CLRTAP, the United 

Kingdom reduced its sulfur dioxide emissions from just under 4.9 million 

metric tonnes in 1980 to about 1.2 million metric tonnes in 1999, a decrease 

of about 75 percent. The majority of the reduction was due to an increase in 

the use of nuclear and renewable energy, fuel-switching (e.g., to natural 

gas), improvements in efficiency, flue gas desulfurization,16 and fuel sulfur 

reductions. The United Kingdom projects that its sulfur dioxide emissions 

will decline even further, from about 1.2 million metric tonnes in 1999 

to 0.6 million metric tonnes in 2010, an additional decrease of nearly 

50 percent. If realized, this decrease would represent an 87-percent 

reduction from the 1980 level.

Sulfur dioxide emissions, along with other pollutants, contributed to 

several major smog episodes in London during the first 6 decades of the 

20th century, with the most significant one occurring in December 1952. 

That episode took more than 4,000 lives over 5 days. It also led to the 

enactment of legislation in 1956 and 1968 that aimed to reduce emissions 

from households. A 1990 law gave the government the power to set 

emissions limits and environmental quality standards for industrial plants. 

A 1995 law introduced a new framework for air quality policy, giving added 

prominence to the concept of air quality standards.

In addition, the United Kingdom has international obligations to reduce 

sulfur dioxide emissions. As a party to the CLRTAP, the United Kingdom 

intends to reduce its annual emissions of sulfur dioxide by at least 

80 percent by 2010 from its 1980 level. Furthermore, it must comply with 

EU requirements to reduce sulfur dioxide emissions. (See app. II.)

Germany. According to data submitted under the CLRTAP, sulfur dioxide 

emissions in the unified Germany declined by nearly 85 percent, from 

5.3 million metric tonnes to 0.83 million metric tonnes between 1990 and 

1999. This decline resulted from such factors as the post-1990 economic 

restructuring, the retirement of outdated plants in the former East 

Germany, and the use of less sulfur-intensive fuels. Germany reports that it 

expects to reduce its sulfur dioxide emissions by about 34 percent between 

1999 and 2010.

16 Flue gas desulfurization equipment removes sulfur oxides from the combustion gases of a 

boiler plant before it discharges them to the atmosphere.
Page 23 GAO-03-25 Air Pollutants and Climate Change

West Germany began to regulate sulfur dioxide in the late 1970s, when it 

signed the CLRTAP. It participated in this effort in part out of concern that 

sulfur dioxide-induced acidification was killing large numbers of trees in 

the forests of southwestern Germany.

West Germany’s emissions of sulfur dioxide declined markedly during 

the 1980s, mainly because utilities expanded their use of natural gas to 

generate electricity, installed flue gas desulfurization technology in 

power plants, and substituted less sulfur-intensive fuels at power plants 

and in industry. In contrast, emissions in the former East Germany rose 

until 1987, mainly because utilities there used lignite (low-grade coal) to 

generate electricity.

Germany’s primary legislative instrument to control sulfur dioxide and 

other air emissions is a 1974 law, amended in 2000, that regulates emissions 

from both large and small combustion facilities. Supporting ordinances 

contain detailed regulations and emissions limits for all facilities covered 

by the act. Germany is also reducing its sulfur dioxide emissions under 

both the CLRTAP and EU directives.

Japan. According to the Japanese government, sulfur dioxide emissions 

declined by about 9 percent between 1990 and 1999, or from 0.97 million 

metric tonnes to 0.87 million metric tonnes. According to another source—

a researcher from the Japanese government’s National Institute for 

Environmental Studies—sulfur dioxide emissions may decline by 

27 percent between 1995 and 2010.17

Compared with other industrialized countries, particularly the United 

States and Germany, Japan uses considerably less coal, relying instead on 

nuclear power to generate one-third of its electricity. Nevertheless, in 1968, 

Japan began to regulate sulfur dioxide and other substances created by fuel 

combustion. It set standards for emissions from power plants and factories 

and provided for stations in several parts of the country to monitor 

emissions of sulfur dioxide and other substances. In the late 1970s, the 

government required facilities to install scrubbers in their smokestacks. 

Japan also reduced its sulfur dioxide emissions through gains in 

combustion efficiency and a transition to low-sulfur coal.

17 Four scenarios with reductions ranging from 7 to 37 percent were prepared by 

Dr. T. Matsui for the United Nations Environment Program. The data for Japan in figure 4 

show projected emissions reductions in 2010 from an intermediate scenario.
Page 24 GAO-03-25 Air Pollutants and Climate Change

Developing Countries As in the developed countries, China and Mexico saw an overall decrease 

in sulfur dioxide emissions during the 1990s, as shown in figure 5. We were 

unable to find data for India. Sulfur dioxide emissions in China declined by 

15 percent between 1997 and 2000, in part as a result of the combination of 

emission reduction policies and a decline in coal use. However, emissions 

increased slightly after 1999. According to Mexico’s National Greenhouse 

Gas Inventory, sulfur dioxide emissions decreased by about 55 percent 

between 1990 and 1998, though they increased very slightly between 1993 

and 1998. We were unable to find consistent historical data on sulfur 

dioxide emissions in India. We were also unable to find data on projected 

emissions levels for any of these three countries. According to U.S. experts, 

the quality of some of the data for these countries is uncertain, due in part 

to old measuring equipment and techniques.

Figure 5: Sulfur Dioxide Emissions in China and Mexico, Selected Years

China. China is currently the world’s largest emitter of sulfur dioxide. 

It relies heavily on coal as an energy source, and the country’s sulfur 

dioxide emissions rose initially along with rapid industrialization. More 

recently, according to the Chinese State Environmental Protection 

0

5,000

10,000

15,000

20,000

25,000 Thousand metric tonnes

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

Mexico

China

Sources: GAO (graphic); data reported by the Chinese State Environmental 

Protection Administration and the Mexican National Greenhouse Gas Inventory, 1994-1998. 

year
Page 25 GAO-03-25 Air Pollutants and Climate Change

Administration, emissions of sulfur dioxide declined from 23.4 million 

metric tonnes to 20 million metric tonnes between 1997 and 2000.

Although under its current 5-year plan (2001-2005), China aims to reduce 

coal consumption by increasing the share of natural gas and renewable 

energy in the total energy supply, the U.S. Department of Energy’s Energy 

Information Administration (EIA) projects that coal combustion in China 

will increase 60 to 194 percent between 1999 and 2020, depending on 

assumptions about economic growth. Thus, sulfur dioxide emissions could 

also rise, unless implementation of a 1987 Air Pollution Control Law, 

amended in 2001, can slow or reverse this trend. The law is designed to 

improve air quality in large- and medium-sized cities through stiffer 

penalties, better enforcement, and greater use of market-based methods, 

such as the imposition of sulfur dioxide discharge fees. It also provides 

incentives for using high quality, low sulfur coal and requires new or 

expanded sulfur dioxide-emitting power plants and large- and mediumsized enterprises to install sulfur dioxide scrubbing equipment.

India. We found limited data on India’s sulfur dioxide emissions. 

Specifically, we found estimates from one source for 1980 and 1990, 

which showed an 82-percent increase. We found an estimate from another 

source for 2000 alone, which is higher than the 1990 estimate from the 

other source. However, because these sources used different estimating 

methods, their results may not be comparable.

We were also unable to find sulfur dioxide projections for India. However, 

the EIA projects that India’s coal consumption will increase 9 to 62 percent 

between 1999 and 2020, depending on assumptions about economic 

growth. It also notes that, because of shortages in generating capacity and 

public funds, India will probably continue to rely on old, coal-fired plants 

for some time, despite their contribution to the country’s air quality 

problems.

Coal accounts for more than half of India’s primary fuel consumption, but 

the sulfur content of the coal used is relatively low. Nevertheless, India has 

undertaken some steps to reduce sulfur dioxide emissions from coal. For 

example, it has improved the combustion efficiency of conventional coal 

technologies and has promoted the use of renewable energy technologies 

as an alternative to coal. It further adopted national air quality standards 

for sulfur dioxide and other pollutants in 1982 and revised them in 1994. 

India has also reduced the sulfur content of oil products. According to the 

EIA, India’s high levels of pollution do not result from a lack of effort in 
Page 26 GAO-03-25 Air Pollutants and Climate Change

building a sound system of legislation and regulation, but rather from weak 

enforcement at the local level.

Mexico. According to the Mexican government, sulfur dioxide emissions 

decreased by 67 percent between 1990 and 1992, but started to increase 

gradually after 1992. Between 1990 and 1998, however, the net decrease 

was 55 percent. We were unable to find data on projected sulfur dioxide 

trends. However, Mexico has considerably expanded its use of natural gas, 

which produces less sulfur than coal, and according to a 2002 report by the 

EIA, most new power plants in Mexico are likely to be gas-fired, although 

some new coal-fired plants will also be constructed. EIA projects that coal 

consumption will grow 62 to 115 percent between 1999 and 2020, 

depending on assumptions about economic growth. Most of Mexico’s 

energy comes from oil, with coal providing only about 4 percent of the 

country’s energy requirements.

The basic law for reducing air pollution emissions is the 1988 General 

Law of Ecological Equilibrium and the Protection of the Environment. 

The law provides the framework for air pollution standards for all major 

substances. Environmental protection became a particularly important 

issue for Mexico in the early 1990s as a result of negotiations for the 

North American Free Trade Agreement with the United States and Canada. 

In 1992, the Mexican government created a special office to enforce 

regulations. This office is charged with inspecting facilities and issuing 

penalties for noncompliance. According to Mexican government officials, 

Mexico’s limited financial resources prevent full enforcement of 

environmental regulations, despite steadily improving enforcement efforts. 

Mexico has also introduced a federal tax incentive program for purchasers 

of pollution control equipment. In addition to national legislation, some air 

quality initiatives are underway in large cities, where urban air pollution is 

a significant problem.
Page 27 GAO-03-25 Air Pollutants and Climate Change

Black Carbon Emissions 

Generally Declined in 

Developed Countries and 

Generally Increased in 

Developing Countries

Scientists have begun to recognize the importance of black carbon as an 

agent of climate change only within the past few years. Consequently, most 

countries do not directly track emissions of this pollutant. Therefore, to 

conduct our analysis, we used a global black carbon database developed by 

atmospheric researchers.18 This database has several limitations. First, as 

with most emissions inventories, estimates in this database are not based 

on actual black carbon measurements, but rather are calculated using 

information on countries’ use of fossil fuel, which is measured, along with 

estimates of how the fuel is used. Second, this database does not contain 

historical information on open biomass burning (forest burning or landclearing), which may account for as much as 50 percent of black carbon 

emissions. Finally, emissions estimates are based on limited information 

about the characteristics of the fuels and technologies that produce 

the emissions.

Black carbon emissions are particularly difficult to track because they 

are often produced by activities that are informal and unregulated, 

and in developing countries, there is considerable consumption of 

noncommercial fuels, such as wood or animal waste. Since these fuels are 

not reported the same way as fossil fuels, neither the amounts used nor 

the emissions produced are well quantified. Because sophisticated 

emission measurements have usually been available only in developed 

countries, moreover, there are few measurements of the combustion that 

produces most black carbon emissions in developing countries. For these 

and other reasons, the black carbon emissions data are highly uncertain. 

Despite these weaknesses, this database is currently the only source we 

found that contained consistently estimated information on different 

countries’ emissions.

18 T.C. Bond and D.G. Streets, “Draft Global Black Carbon Inventory,” 2002. Dr. Tami Bond, 

Visiting Scientist, National Center for Atmospheric Research, and Dr. David Streets, Senior 

Scientist, Argonne National Laboratory, provided both data and commentary on this section.
Page 28 GAO-03-25 Air Pollutants and Climate Change

Black carbon usually comes from the same sources as organic carbon, 

which has a cooling effect on climate. It would be difficult to control 

emissions of either substance separately; hence, it would therefore be 

difficult to control warming by reducing black carbon emissions. Although 

countries do not regulate black carbon directly, it is one component of 

what regulatory agencies often refer to as particulate matter,19 and most of 

the countries we reviewed do regulate particulate matter. We therefore 

collected information on measures to reduce particulate matter, 

recognizing that this pollutant is an imperfect proxy for black carbon. 

Furthermore, even when particulate matter emissions are reduced, black 

carbon levels may not decline because some types of particulate matter, 

such as sulfate, do not contain black carbon when emitted.

Developed Countries According to the database we used, the majority of black carbon emissions 

in developed countries results from the combustion of diesel fuel by 

vehicles, including both on-road vehicles, such as heavy-duty trucks, and 

off-road vehicles, including farm and construction equipment. As figure 6 

shows, black carbon emissions from fossil fuel combustion declined in 

three of the four developed countries, more in the United Kingdom and 

Germany and less in the United States. During this period they increased in 

Japan, starting in the early 1980s. (See app. III for more detail on policy 

measures to reduce black carbon emissions.)

19 Particulate matter is the general term used for a mixture of solid particles and liquid 

droplets found in the air. It comes from vehicle emissions, dust, fires, sea salt, and 

black carbon (soot) from wood and coal burning. Particulate matter can be either coarse 

or fine. Coarse particulate matter is referred to as PM10 because the particles have a 

diameter of 10 micrometers or less. Fine particles, referred to as PM2.5, have a diameter of 

2.5 micrometers or less. Black carbon is a component of PM2.5, but this fraction may vary 

across countries, depending on fuel sources. Secondary particles can be formed in the 

atmosphere from gaseous emissions. For example, sulfates are a form of particulate matter.
Page 29 GAO-03-25 Air Pollutants and Climate Change

Figure 6: Black Carbon Emissions in the United States, United Kingdom, Germany, 

and Japan, 1980-96

Note: Graphic based on global black carbon database prepared by T.C. Bond and D.G. Streets for an 

article entitled, “A Technology-Based Global Inventory of Black and Organic Carbon Emissions from 

Combustion,” to be submitted to the Journal of Geophysical Research.

United States. According to the database we examined, black carbon 

emissions in the United States were approximately 4 percent lower in 

1996 than in 1980. However, there was a steady increase from 1992 to 1996. 

According to this database, about one-half of black carbon emissions in 

the United States are from the use of diesel vehicles to transport goods 

over long distances and from off-road diesel vehicles. However, the 

database may not fully reflect the effects of certain technologies that 

were introduced in the 1990s to reduce diesel emissions. Including such 

technologies in the database would likely show greater emissions declines 

Source: GAO (graphic), T.C. Bond and D.G. Streets (data). 

0

50,000

100,000

150,000

200,000

250,000

300,000

350,000

400,000

450,000

500,000 Metric tonnes

19801981198219831984198519861987198819891990199119921993199419951996

United States

Germany

United Kingdom

Japan

year
Page 30 GAO-03-25 Air Pollutants and Climate Change

during the 1990s. Gasoline-fueled cars and wood combustion in home 

fireplaces and stoves are other, smaller sources of black carbon emissions.

The United States is the only country for which we found projections of 

black carbon emissions. These were developed in support of an EPA 

rulemaking20 relating to large trucks. When the rule is fully implemented in 

2030, black carbon emissions are expected to decrease by an estimated 

109,000 tons from the level produced in 1996.21

The United States has several efforts underway to help reduce emissions 

of black carbon and other types of particulate matter. Under the Clean Air 

Act, EPA promulgated national air quality standards for particulate matter 

in 1971 and has regulated particulate matter emissions from highway motor 

vehicle engines. The most recent rule is designed to reduce emissions from 

heavy-duty diesel vehicles by improving diesel engines and reducing the 

sulfur content of diesel fuel.22

United Kingdom. According to the database we used, black carbon 

emissions from the United Kingdom declined by about one-third between 

1980 and 1996. The sources of black carbon emissions in the United 

Kingdom are similar to those in the United States, with the majority of 

emissions produced by diesel vehicles. Other sources of black carbon 

emissions are industrial coke-making23and coal-burning in the residential 

sector, although residential coal use is declining in the United Kingdom.

The Parliament enacted legislation in 1956 and 1968 to control domestic 

sources of particulate matter. The 1956 act aimed to control domestic 

sources of smoke pollution by introducing smokeless zones (regions where 

smokeless fuel had to be burned) and by making grants to homeowners to 

convert their homes from traditional coal fires to heaters fueled by oil, gas, 

20 The 2002 Heavy-duty Diesel Engine Rule is aimed at reducing emissions from 

heavy-duty trucks.

21 EPA’s estimate of black carbon emissions for 1996 differs from that found in the Bond 

and Streets database. This is because estimates from the two studies are based on different 

assumptions.

22 The sulfur content of diesel fuel is important because sulfur can impair the performance 

of a vehicle’s emissions-reducing device.

23 Coke is a solid carbonaceous residue derived from low-ash, low-sulfur bituminous coal. 

It is used as a fuel (and as a reducing agent) for smelting iron ore in blast furnaces.
Page 31 GAO-03-25 Air Pollutants and Climate Change

smokeless coal, or electricity. Legislation in 1990 and 1995 brought many 

smaller emission sources under air pollution control by local authorities for 

the first time. These acts also provided a new statutory framework for local 

air quality management. In 1997 the United Kingdom published its first 

national air quality strategy, which set air quality standards and objectives 

for the pollutants of greatest concern, including particulate matter. The 

United Kingdom changed the particulate objective under this strategy in 

2000, in response to a new EU directive and then in 2002 strengthened the 

objective once more.

In the transportation sector, the EU in 1992 introduced directives 

containing exhaust emission limits (generally referred to as the Euro I 

standards) for new medium- and heavy-duty diesel engines. The standards 

aimed to reduce emissions of particulate matter, as well as carbon 

monoxide, hydrocarbons (including volatile organic compounds), and 

nitrogen oxides, all ozone precursors. More stringent emission limits for 

these vehicles, called Euro II, came into effect in 1996. Euro III limits were 

adopted in 1999, and even more rigorous standards for these vehicles, Euro 

IV and Euro V, are expected to take effect in 2005 and 2008, respectively. 

The EU also set standards in 2001 for maximum allowable levels on the 

sulfur content of diesel fuel. The directive, effective January 1, 2005, will 

introduce sulfur-free diesel and gasoline in all EU member states.

Germany. Black carbon emissions in Germany also declined by about 

one-third between 1980 and 1996, according to the Bond and Streets 

database. While diesel consumption and emissions increased rapidly 

during this time, a phase-out of coal combustion for home heating led to an 

overall reduction in black carbon emissions. Other large sources of black 

carbon emissions in Germany—much smaller than diesel vehicles—are 

industrial coking coal and gasoline vehicles.

Between 1990 and 1996, Germany decreased its emissions of particulate 

matter by 86 percent. The decrease occurred primarily because of 

developments in eastern Germany after reunification, when many older 

industrial and power plants were closed or refitted with new technologies 

that remove soot particles. In addition, many installations shifted to the use 

of gas and liquid fuels—especially in small combustion appliances—

thereby producing less particulate matter. The Federal Emission Control 

Act provides the framework for regulating all air pollutants, and its 

ordinances govern both large and small combustion facilities.
Page 32 GAO-03-25 Air Pollutants and Climate Change

Like the United Kingdom, Germany must abide by EU directives aimed at 

reducing particulate matter emissions, including the Euro I–III directives 

on diesel vehicle emissions. Most diesel vehicles currently emit much less 

particulate matter than earlier models, but Germany is continuing its effort 

to reduce emissions in the transportation sector. For example, in 2001 

Germany passed legislation to offer tax breaks for sulfur-free gasoline and 

diesel fuel starting this year.

Japan. Black carbon emissions in Japan were about 60 percent higher in 

1996 than in 1980, according to the database we used. The major source of 

these emissions is diesel fuel, the use of which began to increase in 1988. 

As in the United Kingdom and Germany, coke-making could be an 

additional source of black carbon emissions in Japan.

In June 2001, Japan’s legislature enacted a law to further tighten particulate 

matter emissions from diesel-powered vehicles to improve air quality in 

major urban areas. The law applies to 196 local governments in Tokyo, 

Osaka, and four other cities.

Developing Countries The database we used indicates that black carbon emissions increased 

between 1980 and 1996 in China and India, as figure 7 shows. It suggests 

that Mexico’s black carbon emissions are essentially unchanged over 

this period. We found no projections of black carbon for any of these 

three developing countries.
Page 33 GAO-03-25 Air Pollutants and Climate Change

Figure 7: Black Carbon Emissions in China, India, and Mexico, 1980-96

Note: Graphic based on global black carbon database prepared by T.C. Bond and D.G. Streets for an 

article entitled, “A Technology-Based Global Inventory of Black and Organic Carbon Emissions from 

Combustion,” to be submitted to the Journal of Geophysical Research.

China. Emissions of black carbon in China rose by 43 percent between 

1980 and 1996, according to the Bond and Streets database. Moreover, 

according to that data source, China emits more black carbon than any 

other nation in the world—approximately 29 percent of the global total. 

However, its per capita emissions are not higher than those of other 

countries. These emissions came primarily from coal (especially dirty 

soft coal) and wood in the residential sector, coal-burning power plants 

using older technologies, on-road and off-road diesel vehicles, and cokemaking plants in the industrial sector. Emissions increased steeply in the 

1980s, but the growth rate slowed somewhat in the 1990s as China began to 

switch from coal to cleaner natural gas and from raw coal to coal briquettes 

(which produce fewer emissions) in the residential sector; it also closed 

many small industrial coal plants.

China

India

Mexico

Source: GAO (graphic), T.C. Bond and D.G. Streets (data). 

0

200,000

400,000

600,000

800,000

1,000,000

1,200,000

1,400,000

1,600,000

1,800,000 Metric tonnes

198019811982

19831984198519861987198819891990

1991

19921993

1994

1995

1996

year
Page 34 GAO-03-25 Air Pollutants and Climate Change

Under amendments in 2000 to its Air Pollution Control Law, the 

government set a goal of reducing “soot and flue dust” (particulate matter) 

emissions to 1995 levels by 2010. According to the law, new or expanded 

power plants and large- and medium-sized industrial facilities must install 

particulate matter control equipment or take other measures to reduce 

emissions. The city of Beijing requires the use of gas in place of coal in new 

fuel applications.

In the transportation sector, China has implemented emissions standards 

for diesel vehicles. In 2000 and 2001 the government introduced the Euro I 

emissions standards for new cars and trucks. The standards, which apply 

to the entire country, are based on those standards originally introduced in 

Europe in 1992 and limit the amount the amount of particulate matter and 

other substances that can be emitted from new diesel vehicles. More 

stringent Euro II norms currently apply to Beijing and Shanghai and will 

apply to the entire country after 2005.

India. According to the database we used, black carbon emissions in India 

rose by just under one-third between 1980 and 1996. Most of this country’s 

emissions come from the use of biofuel in the residential sector, with diesel 

vehicles contributing a smaller, but noticeable, fraction. Black carbon 

emissions increased between 1980 and 1996, as the population grew and 

burned more wood, dung, and agricultural waste for cooking and home 

heating.24 The Environmental Protection Act of 1982 set national standards 

for the emissions of various substances. The standards were revised in 

1994. Revisions to the act in 1996 set fuel quality specifications, including 

requirements for low-sulfur diesel. In the capital city of New Delhi, 84,000 

public vehicles were converted from gasoline and diesel to compressed 

natural gas, which emits negligible particulate matter. In 2000, the Indian 

government introduced Euro I standards for private, non-commercial 

vehicles throughout the country. Euro II norms currently apply to 

New Delhi and will apply to the entire country after 2010.

Mexico. Black carbon emissions in Mexico remained fairly constant 

between 1980 and 1996, according to the database we used. Mexico is not 

a large consumer of coal, instead relying primarily on oil as its key energy 

source. Most of its black carbon emissions come from diesel fuel use. 

24 Levels of emissions from biofuels are particularly uncertain because data are so sparse. 

Estimates in the Bond and Streets database are calculated based largely on a limited number 

of fuel use surveys combined with socioeconomic data. 
Page 35 GAO-03-25 Air Pollutants and Climate Change

However, according to a Mexican government official, in rural areas, 

villagers burn propane or biomass for home cooking; biomass produces 

black carbon emissions. Older gasoline cars also produce black carbon 

emissions in Mexico.

Trends in Surface Ozone 

Concentrations Are Mixed, 

but Background Ozone 

Levels in the Troposphere 

Appear to Be Rising, With 

Implications for both Air 

Quality and Climate

The earth’s weather takes place in the lowest layer of the atmosphere, 

called the troposphere, which extends from the earth’s surface to between 

9,000 and 16,000 meters above the surface. Within the troposphere, ozone 

concentrations differ between zones,25 the boundary layer, which extends 

from the earth’s surface to roughly 500 to 3,000 meters above the earth’s 

surface, and the much larger free troposphere. (The height of the boundary 

layer can vary by time of day and season: higher in the daytime and in 

summer, and lower in winter and at night.) Ozone in these two zones has 

different durations and effects. There are important interactions between 

the two zones.

Ozone in the boundary layer generally results from human activity, such 

as transportation and fossil fuel combustion. Peak ozone episodes usually 

occur in the summer months, under conditions of long periods of bright 

sunshine, warm temperatures, light winds, and abundant ozone precursors. 

Changing weather patterns contribute to yearly differences in ozone 

concentrations from region to region.

Boundary layer ozone lasts only a few days over land, where it gets 

deposited on surfaces. Over large bodies of water, such as oceans, it 

can last longer, since there are no surfaces for deposition. At high 

concentrations ozone can contribute to human respiratory problems and 

plant damage.

25 Dr. Loretta J. Mickley, Research Associate, Atmospheric Chemistry, Division of 

Engineering and Applied Sciences, Harvard University, and Dr. Michael Prather, Fred Kavli 

Chair and Professor, Department of Earth System Science, University of California Irvine, 

provided insights and comments on this section.
Page 36 GAO-03-25 Air Pollutants and Climate Change

The free troposphere is much larger than the boundary layer and extends 

several miles above the boundary layer to the top of the troposphere. 

Ozone in the free troposphere consists of some naturally occurring ozone 

(such as ozone produced by lightning and ozone descending from the 

stratosphere), as well as human-generated ozone carried upward from the 

boundary layer by wind. Ozone in the free troposphere generally lasts 

longer than ozone in the boundary layer—from 1 to several weeks.26 In the 

free troposphere, ozone, like other greenhouse gases, can trap surface 

radiation and contribute to warming the earth.

Air is exchanged extensively between the boundary layer and the free 

troposphere. Consequently, ozone pollution arising from the boundary 

layer—particularly in the northern hemisphere—has contributed to 

increased levels of ozone in the free troposphere. Hence, the troposphere—

the source of clean air at the earth’s surface—is showing an increasing 

background level of ozone.27 With the higher concentrations of ozone in the 

free troposphere and in remote regions, more ozone can be blown into 

populated areas, worsening the local pollution. Ozone pollution has also 

been carried to the surface over remote regions, such as the oceans and 

the Arctic.

Current estimates of ozone concentrations are based on profiles from 

ozone balloons equipped with measuring devices (sondes). However, these 

sondes are released from locations that are sparsely distributed around the 

globe. Because the information from the sondes is so limited, we used two 

types of proxy data to illustrate trends in the free troposphere, where 

ozone can affect the climate. First, we used historical data on ozone at the 

surface, which regulatory agencies, such as EPA, gather for air quality 

purposes. These data are measured at a network of monitoring stations, 

mainly in developed countries, and are collected primarily from urban and 

suburban areas, where ozone is a major health concern. Second, we used 

the results of a global modeling study that depicts ozone concentrations in 

the free troposphere.

26 Ozone has a shorter life span in the boundary layer because processes in that layer 

can destroy ozone, but these same processes do not occur in the free troposphere. 

The processes include deposition and chemical destruction at night.

27 There are various definitions of background ozone. As used in this report, background 

ozone refers to ozone that is produced by human sources as well as lesser amounts from 

natural precursor sources—such as volatile organic compounds from vegetation or nitrogen 

oxides from lightning—that travels into a given jurisdiction from elsewhere and is not 

associated with local emissions.
Page 37 GAO-03-25 Air Pollutants and Climate Change

Some developed countries have prepared projections of ozone 

concentrations at the surface, but because ozone projections prepared for 

air quality purposes do not adequately represent ozone increases in the free 

troposphere, where it is important for climate, we also used the results of a 

modeling study from the Harvard University Atmospheric Chemistry 

Modeling Group.28 These model results are based on an IPCC scenario that 

assumes that in 2025 there will be high population growth; significant 

income disparities between developed and developing countries; continued 

dominance of fossil fuels, including coal, in developing countries; and some 

policy measures in place to control ozone precursors. We used these 

results to represent annually averaged ozone concentrations in the free 

troposphere over the entire globe in 1990 and 2025. Unlike regional-scale 

air quality models, such as those used by regulatory agencies, the Harvard 

model is able to project ozone concentrations in the free troposphere and 

at a global level, which is of greatest interest for climate change purposes.

Figure 8 shows calculated ozone concentrations at 5 kilometers 

(about 3 miles) above sea level for 1990, while figure 9 shows projected 

concentrations in 2025. Both maps are based on the Harvard model. A 

comparison of the two maps indicates that free tropospheric ozone levels 

are expected to increase over the next two decades. Increased emissions 

of ozone precursors in Asia lead to higher ozone concentrations aloft, 

with possible consequences for climate change.29

28 We use projections prepared by Dr. L.J. Mickley of Harvard University’s Atmospheric 

Chemistry Modeling Group. The Harvard model has been cited in the IPCC Third 

Assessment Report, a peer-reviewed document. (The IPCC is the premier scientific 

organization devoted to assessing climate change.) The projections were produced using a 

general circulation (climate) model and depict expected global average ozone 

concentrations in the free troposphere in 2025. For comparison purposes, we also include a 

map showing tropospheric ozone concentrations in 1990. The Harvard projections are based 

on the IPCC Special Report on Emissions Scenarios (IPCC/SRES) A2 Marker Scenario. The 

Harvard model is able to capture in great detail the physical and chemical processes leading 

to tropospheric ozone formation and can also show the long-range movement of ozone and 

its precursors.

29 Ozone levels are particularly low over most humid equatorial regions because, over the 

humid tropics where nitrogen oxide emissions are generally low, water vapor is involved in 

reactions that destroy ozone.
Page 38 GAO-03-25 Air Pollutants and Climate Change

The most significant of the precursors in this scenario is likely to be 

methane,30 also a greenhouse gas, which is projected in the scenario to 

increase globally by about 20 percent from its present level. Thus, methane 

poses two problems: it can contribute directly to climate change as a 

greenhouse gas, and it can indirectly contribute to climate change as an 

ozone precursor.

Figure 8: Annually Averaged Global Ozone Concentrations at 5 Kilometers, 

Parts per Billion, 1990

30 Even though global emissions of methane appear to be slowing, it is not clear that they 

will have dropped sufficiently to result in reduced concentrations in the atmosphere.

Blue areas indicate low ozone concentrations

Yellow and red areas indicate high ozone concentrations

1990

Source: L.J. Mickley, Harvard University. 
Page 39 GAO-03-25 Air Pollutants and Climate Change

Figure 9: Projected Annually Averaged Global Ozone Concentrations at 

5 Kilometers, Parts per Billion, 2025

Developed Countries Approaches to boundary layer ozone regulation in the United States 

and Europe differ. The U.S. approach is almost exclusively health-based. 

That is, its standards are designed to decrease prolonged human exposures 

to ozone. By contrast, the European approach, as with sulfur dioxide 

regulation, considers the cumulative effect of ozone concentrations on 

human health and the environment. It seeks to reduce these concentrations 

below those levels that affect public health and that can cause long-term 

damage to the environment. Since the environment can be impaired at 

lower concentrations than can human health, ozone concentrations 

associated with European goals are lower than in the United States. While 

the United States has firm air quality standards for ozone, the EU has 

non-mandatory target values for ozone concentrations but mandatory 

emission ceilings for ozone precursors, arrived at after a process that takes 

both local ozone formation and transboundary transport into 

2025

Blue areas indicate low ozone concentrations

Yellow and red areas indicate high ozone concentrations

Source: L.J. Mickley, Harvard University. 
Page 40 GAO-03-25 Air Pollutants and Climate Change

consideration. Neither the United States nor most European countries 

consistently meet their ozone standards in polluted areas. We were unable 

to find information on how Japan sets its ozone standard.

Cumulative ozone levels in the boundary layer have generally risen over 

the past century—particularly because of ozone produced in the northern 

hemisphere. However, some developed countries, such as the United 

States, the United Kingdom, and Germany, have reported fewer 

exceedances of their ozone standards over the past decade. That is, peak 

ozone episodes, in which boundary layer ozone concentrations rise 

substantially above naturally occurring levels, are becoming somewhat 

less frequent and extreme in many developed countries. The governments 

attribute the improvement principally to policy measures over the past 

decade or so. However, weather conditions and other factors can affect 

year-to-year changes. Figure 10 shows that the number of locations 

exceeding their respective ozone standards declined by nearly 70 percent 

in the United States and by nearly 90 percent in Germany between 1990 

and 2000. We were unable to find comparable data for the United Kingdom 

and Japan. (More detailed information on ozone policies can be found in 

app. IV.)
Page 41 GAO-03-25 Air Pollutants and Climate Change

Figure 10: Number of Areas Exceeding the Ozone Standard, United States and 

Germany, 1990-2000

Note: The U.S. ozone standard trend depicted here is 0.12 parts per million averaged over 1 hour. The 

U.S. also has an 8-hour ozone standard. The German ozone standard tracked here is 0.055 parts per 

million averaged over 8 hours.

United States. Under the Clean Air Act of 1970, EPA was charged with 

developing air quality standards for air pollutants that may endanger public 

health and welfare. EPA established such standards for ozone and nitrogen 

oxides, an ozone precursor, in 1971. In 1979 EPA revised the ozone 

standard to 0.12 parts per million daily maximum over a 1-hour period that 

is not to be exceeded more than once per year on average. In 1997 EPA 

tightened the standard to 0.08 parts per million averaged over 8 hours. 

Although EPA has not yet begun to enforce the new 8-hour standard, it 

does track concentrations in terms of this standard. According to EPA, 

national ozone levels decreased 21 percent based on the 1-hour standard 

and 10 percent based on the 8-hour standard between 1981 and 2000. For 52 

metropolitan areas, the trend for 1-hour ozone levels improved between 

1981 and 2000. However, beginning in 1994, the rate of improvement started 

to level off, and the trend since then has been relatively flat.

The U.S. effort to control ozone has focused mainly on reducing emissions 

of nitrogen oxides and volatile organic compounds, the two classes of 

ozone precursors. The Clean Air Act requires each state with ozone 

0

20

40

60

80

100

120 Number of locations exceeding the standard

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

United States

Germany

Sources: GAO (graphic) data reported by the U.S. EPA and the German Environment Office.

year
Page 42 GAO-03-25 Air Pollutants and Climate Change

concentrations above the standard to develop a plan–known as a state 

implementation plan—for reducing ozone formation; EPA must approve 

this plan. This reduction can be attained through a mix of regulations in 

various sectors, such as the utility, transportation, and industry sectors—

where most emissions occur. The ozone precursor methane is also 

controlled under Clean Air Act regulations that require the combustion of 

certain landfill gases from large landfills.

United Kingdom. According to United Kingdom government data, peak 

ozone concentrations declined by 30 percent during the 1990s. Emissions 

of nitrogen oxides declined by 42 percent between 1990 and 1999, while 

emissions of volatile organic compounds, another ozone precursor, 

declined by 34 percent between 1988 and 1999. The reductions in precursor 

emissions are mainly due to stricter regulations, an increasing share of 

vehicles fitted with catalytic converters (to control nitrogen oxides, volatile 

organic compounds, and carbon monoxide), and reduced nitrogen oxide 

emissions from power plants.

Incorporated into the United Kingdom’s framework for improving air 

quality are several measures to address ozone formation. Many of these 

measures were developed within the framework of the CLRTAP or the EU. 

For example, in the transportation sector, implementation of the EU’s 

Euro I standards limited emissions of the ozone precursors carbon 

monoxide, hydrocarbons (including volatile organic compounds), and 

nitrogen oxides from gasoline vehicles. More stringent standards came into 

effect in 1997 and 1998, depending on vehicle type, and are known as 

Euro II. These were superceded by Euro III standards for the majority of 

vehicles in January 2001. A further tightening of the emissions limits, 

referred to as Euro IV, will begin in January 2005 and will be fully in force 

by January 2007.

Certain convention protocols also limit emissions of nitrogen oxides and 

volatile organic compounds. The United Kingdom also has a domestic 

target for ozone, which is more stringent than that of the EU.

Germany. According to the German government, peak ozone 

concentrations have been declining since the mid-1990s. Between 1990 and 

2000, emissions of nitrogen oxides declined by 40 percent, and emissions of 

volatile organic compounds fell by 49 percent. The reductions were due 

both to regulations—mainly in the transportation sector—and to economic 

restructuring in the new states of eastern Germany.
Page 43 GAO-03-25 Air Pollutants and Climate Change

Germany, like most other EU countries, has several measures in place for 

reducing nitrogen oxides and volatile organic compounds, both ozone 

precursors. These include emission-based motor vehicle taxes on heavy 

utility vehicles and automobiles and requirements to install advanced 

emissions reducing devices (that is, catalytic converters) on vehicles. 

Measures in the utility and industry sectors will also increase the efficiency 

of fuel combustion to reduce emissions of nitrogen oxides. Germany is 

bound by the same EU directives and CLRTAP protocols as the United 

Kingdom.

Japan. According to the Organization for Economic Cooperation and 

Development, ozone concentrations have increased in urban areas by 

about 5 percent since the late 1980s. However, according to Japanese data 

submitted under the United Nations Framework Convention on Climate 

Change, nitrogen oxide emissions increased by nearly 7 percent between 

1990 and 1999, while emissions of volatile organic compounds decreased 

by just over 3 percent.

In addition to its basic air quality framework, discussed earlier, Japan has 

several laws targeted at reducing ozone precursors. The 1968 Air Pollution 

Control Law established standards for photochemical oxidants (ozone) and 

other substances, including the ozone precursors nitrogen dioxide and 

carbon monoxide. In 1992 the government introduced the Automobile 

Nitrogen Oxides Law to tighten controls on nitrogen oxide emissions from 

vehicles in areas where improvements in emissions were difficult to realize 

with existing measures. Among other things, the law regulates the types of 

vehicles that can be driven in areas where nitrogen oxide emissions exceed 

the environmental standard. The Japanese government has also set 

emission standards for nitrogen oxides for each industrial facility. The 

standards vary according to boiler type. Additionally, Japan’s air pollution 

control law regulates volatile organic compound emissions from motor 

vehicles.

Developing Countries According to the World Health Organization, some of the poorest air 

quality in the world, due in part to ozone pollution, is found in Beijing, 

China; New Delhi, India; and Mexico City, Mexico. However, we were 

unable to find national data on ozone concentrations for these three 

countries. China and Mexico monitor ozone only in selected large 

metropolitan areas. In Mexico City, ozone levels have declined 

slightly since the early 1990s. We found no information on India. The IPCC 

reported that emissions of nitrogen oxides—an ozone precursor—in East 
Page 44 GAO-03-25 Air Pollutants and Climate Change

Asia are increasing by about 4 percent per year. This suggests that ozone 

levels may rise along with continuing industrialization in that region.

China. Vehicle emissions are one of the major sources of air pollution in 

China’s major cities. The country currently has over 40 million vehicles, 

and the number is growing by 10 percent annually; in large cities the rate 

of growth is even higher. The Euro I vehicle emissions standards in place 

around the country limit levels of carbon monoxide, hydrocarbons, and 

nitrogen oxides. The Euro II standards, in place in Beijing, aim to further 

reduce vehicle emissions in that metropolitan area.

Mexico. Efforts are underway to reduce emissions in the transportation 

sector, particularly in Mexico City. Since 1993, the Mexican government has 

increased its efforts to inspect motor vehicles. As of 2001, 230 new car 

models had undergone emissions inspections. Another program reduces 

the number of cars on the road in Mexico City and provides incentives for 

the purchase of cars with lower emissions.

India. As in China, India’s Euro I and II performance standards for 

automobiles are designed to help reduce emissions of key vehicle-related 

ozone precursors.

Studies of the Effect 

of Economic Growth 

on the Environment 

Are Inconclusive

Both empirical and theoretical studies on the possible connection between 

economic development and environmental quality are inconclusive overall. 

Empirical studies, which analyze historical data for possible connections 

between economic growth and environmental quality, have found that 

emissions for some substances initially increase as national income rises 

and then decrease as a certain level of income is reached. However, these 

results are not consistent across all studies and for all substances.31

Similarly, theoretical studies, which seek to understand the relationship 

between economic growth and environmental quality, do not agree on the 

major factors underlying this relationship. While acknowledging a probable 

relationship between economic growth and environmental quality, the 

authors of these studies caution that economic growth does not 

automatically result in environmental improvement. They explain that, 

while economic growth may enable countries to pay for environmental 

protection, growth is not by itself sufficient to reverse environmental 

31 Carbon dioxide is not regulated as a pollutant in the United States.
Page 45 GAO-03-25 Air Pollutants and Climate Change

degradation. See appendix V for more information on the studies 

we reviewed.

Empirical Studies of the 

Relationship between 

Economic Growth and 

Environmental Quality Yield 

Mixed Results

The empirical studies we reviewed did not consistently find a relationship 

that showed pollution initially increasing as per capita income increases 

and then starting to decrease when income rises beyond a certain level. 

However, such a relationship was found more often for some types of 

pollutants than others.

If such a relationship between pollutants and economic growth is 

presented graphically, it would form an “inverted U curve,” as shown in 

figure 11. The point where the curve changes from an upward slope—

pollution increasing with income—to a downward slope is called the 

“turning point.”

Figure 11: Hypothetical Inverted U-Shaped Curve, Showing Relationship Between 

Per Capita Income and Environmental Pollution

Source: GAO.

 Environmental pollution

Per capita income

Turning point
Page 46 GAO-03-25 Air Pollutants and Climate Change

In conducting these studies, researchers typically used various measures of 

environmental quality, such as a specific pollutant or a composite measure 

of environmental quality.32 In addition, to measure economic growth, the 

studies commonly used per capita income, and in most cases, the income 

levels were converted to 1985 U.S. dollars either by exchange rates or 

purchasing power parity rates.33 The per capita income level for the United 

States for 1985 was $16,410 under both the exchange rate and purchasing 

power parity methods; for Japan, it was $10,900 for the former and $12,340 

for the latter; for Mexico, $2,180 and $4,745; and for China, $280 and $785.

While overall the results of the studies are not consistent regarding an 

“inverted U curve” relationship between pollution and economic growth, 

they are more consistent in showing this relationship between pollutants 

with localized and near-term effects, such as sulfur dioxide and particulate 

matter. On the other hand, studies of substances with global and long-term 

effects, such as carbon dioxide, sometimes did not find a turning point or 

found a turning point beyond the income levels of the countries in 

the study.

32 The source for the environmental data most often used in these studies was the Global 

Environmental Monitoring System, compiled by the United Nations, except for data on 

carbon dioxide emissions from the U.S. Oak Ridge National Laboratory. Income data from 

different countries were often adjusted to comparable units in terms of exchange rates or 

purchasing power. In general, the data could be from as many as 149 economically 

developing and developed countries for 1960 through 2000 or any year during that period. 

Models differ in the exact specification of their equations, such as the number and type of 

other factors that are included in the model (e.g., international trade, population density) 

and the mathematical form of the relationship tested.

33 To compare incomes across countries, incomes are converted to U.S. dollars in one of 

two ways: (1) using the official exchange rate between the dollar and the other currency or 

(2) using purchasing power parity rates—an international dollar that has the same 

purchasing power in another country as the dollar has in the United States.
Page 47 GAO-03-25 Air Pollutants and Climate Change

For example, as shown in figure 12, the estimated turning point for sulfur 

dioxide in the studies we reviewed using the purchasing power parity 

method was typically in the range of $4,000 to $11,000.34 While these 

estimated turning points were higher than per capita incomes in China and 

Mexico, they were generally below the per capita incomes in Japan and the 

United States in 1985. In addition, some estimated turning points for 

particulate matter were in the range of $3,000 to $10,000, which is less than 

per capita incomes in Japan, Mexico, and the United States. However, not 

all the studies we reviewed found a turning point for pollutants with 

localized effects. Specifically, one study found a turning point for sulfur 

dioxide at a per capita income level far higher than any country’s level in 

1985, and another study did not find a turning point for particulate matter. 

The studies of carbon dioxide we reviewed either estimated a turning point 

in the range of $10,000 to $63,000, which is above the per capita income 

level of most countries, except for a few developed countries, or found that 

there was no turning point (for example, because emissions and per capita 

incomes increased together).

34 All numbers are presented in 1985 dollars converted by purchasing power parity.
Page 48 GAO-03-25 Air Pollutants and Climate Change

Figure 12: Estimated Turning Points Found in Selected Studies of Particulate Matter, 

Sulfur Dioxide, and Carbon Dioxide

Note: Cole-1 and Cole–2 from Cole, Rayner, and Bates, “The Environmental Kuznets Curve: An 

Empirical Analysis,” Environment and Development Economics (1997)—estimates from two models; 

Panayotou from Panayotou, Sachs, and Peterson, “Developing Countries and the Control of Climate 

Change: Empirical Evidence,” Discussion Paper #45, Harvard Institute for International Development 

(1999); Schmalensee from Schmalensee, Stoker, and Judson, “World Carbon Dioxide Emissions: 

1950-2050, “Review of Economics & Statistics (1998); Selden-1 and Selden-2 from Selden and Song, 

“Environmental Quality and Development: Is There a Kuznets Curve for Air Pollution Emissions?,” 

Journal of Environmental Economics and Management (1994)—two models estimated; Shafik from 

Shafik, “Economic Development and Environmental Quality: An Econometric Analysis,” Oxford 

Economic Papers (1994); Stern from Stern and Common, “Is There an Environmental Kuznets Curve 

for Sulfur?,” Journal of Environmental Economics and Management (2001).

Source: GAO.

0

20,000

40,000

60,000

80,000

100,000

120,000 Per capita income

Cole-1Cole-2

Panayotou

Schmalensee

Selden-1

Selden-2

Shafik

Study

PM

SO2

CO2

Stern

United States

Mexico
Page 49 GAO-03-25 Air Pollutants and Climate Change

Researchers suggested reasons an “inverted U” relationship is found more 

often for sulfur dioxide and particulates than for carbon dioxide. For 

example, they noted that the possible costs of climate change due to 

increased carbon dioxide emissions are borne globally and by future 

generations rather than locally and currently. Furthermore, the awareness 

of the problem of climate change is more recent than the awareness of the 

health effects associated with sulfur dioxide and particulates; policies to 

control carbon dioxide emissions were generally not in effect during the 

period of time analyzed by these studies.

Various Theoretical 

Explanations Have 

Been Suggested for 

the Relationship of 

Economic Growth and 

Environmental Quality

Other studies have sought to understand the factors underlying the 

relationship between economic growth and environmental quality. These 

studies generally agree that income is a proxy for a number of other 

factors that may be influencing environmental quality as countries develop 

economically. These factors include, for example, changes in a country’s 

economic structure, international trade, and a country’s preference for 

environmental quality. Nevertheless, researchers do not agree on the 

importance and role of any single factor. These three factors are 

discussed below.

• Changes in a country’s economic structure. Some studies pointed 

out that the relationship between economic growth and environmental 

quality reflects changes in an economy’s structure as economies 

develop and is not directly due to changes in income. Initially, these 

studies note, economies are primarily agrarian and produce little 

pollution. As these economies grow, the share of national output based 

on agriculture decreases, while the share based on more pollutionintensive manufacturing increases; hence pollution increases. At the 

later stages of development, the share of national output based on 

manufacturing decreases, while the share based on less 

pollution-intensive services increases; hence, pollution decreases.
Page 50 GAO-03-25 Air Pollutants and Climate Change

• The role of trade. Other studies noted that international trade can 

affect patterns of production and environmental quality. That is, trade 

allows economically developed countries to emphasize cleaner types 

of industries and to rely on imports from less-developed countries 

for goods produced by more polluting industries. As a result, the 

improvement in some developed countries’ environmental indicators is 

partly due to the contraction of their more polluting industries and the 

transfer of these industries to the less developed countries. However, 

researchers also observed that this process—and consequent 

improvement in environmental quality—cannot continue indefinitely 

because the relocation of polluting industries to other countries cannot 

continue indefinitely.35

• Preference for environmental improvement. According to still other 

studies, the relationship between economic growth and environmental 

quality reflects a country’s preference for environmental improvement. 

That is, environmental improvement could be described as a “luxury 

good,” that is, a good that people will seek more of as their incomes 

grow beyond subsistence level. Thus, in countries living at subsistence 

levels, pollution is accepted as a side effect of economic development, 

and people are not willing to reduce their consumption of basic 

necessities in order to set aside resources for environmental protection. 

However, as a country’s economy grows and incomes increase, people 

become more willing to divert a portion of their resources from current 

consumption to improve the environment. This economic growth, in 

turn, leads to stronger support for environmental legislation and new 

regulations to protect the environment.

Studies have also discussed other factors—-such as technology, population 

density, political environment, and environmental regulations—that may 

play key roles in shaping environmental quality as economies grow. 

Regardless of their views, researchers generally agree that better data and 

more detailed research are needed to conclusively identify the factors that 

either directly, or indirectly through per capita income, influence 

environmental quality.

35 The empirical studies we reviewed that examined the role of trade on environmental 

quality did not provide support for the suggestion that exporting polluting industries 

(sometimes called “environmental dumping”) accounts for the observed reduction in some 

pollutants in developed countries.
Page 51 GAO-03-25 Air Pollutants and Climate Change

Conclusions Of the three substances we reviewed, sulfur dioxide emissions have 

received the most attention in the countries we examined. All of these 

countries have undertaken at least some measures to reduce sulfur dioxide 

emissions. Past trends and projections in all countries seem to be affected 

by economic growth and health concerns. The economically developed 

countries began this process much earlier—as far back as the 1970s or 

1980s—and they have been more successful thus far in realizing reductions 

than the three developing countries. In the developing countries, sulfur 

dioxide emissions declined (though in China and Mexico they have started 

to increase again), but they may decrease in the future if new policies are 

implemented and well enforced. While data on sulfur dioxide are far from 

complete, they are more readily available from country sources than 

data on the other two substances, particularly in developing countries. 

Since sulfate aerosol is a climate cooling agent, it is likely that reductions 

in sulfur dioxide emissions will result in some warming, at least at a 

regional level.

Black carbon is being addressed indirectly through measures designed 

to reduce particulate matter in all of the developed countries and to a 

lesser extent in the developing countries. Emissions have begun to 

decline in developed countries, with the exception of Japan, according to 

the database we used, largely as a result of regulations limiting diesel 

fuel emissions, the major source of developed countries’ black carbon. 

Developing countries have not seen comparable declines in black carbon 

emissions, because home heating and cooking largely rely on burning coal 

and wood. Because small coal- or wood-burning stoves are widely used in 

these countries, black carbon emissions are more widely dispersed than, 

for example, sulfur dioxide emissions, which are usually associated with 

large power plants. Consequently, developing countries may find it 

challenging to control emissions of black carbon, at least for the 

foreseeable future. In addition, forest burning and land-clearing, major 

sources of black carbon emissions, are more prevalent in developing 

countries. Reliable measurements of black carbon from all seven countries 

are sparse.

Tropospheric ozone levels are difficult to reduce because they result from 

emissions of precursor substances produced by a very diverse range of 

sources. Nevertheless, developed countries have had some success over 

the past decade in reducing high ozone levels, particularly in major urban 

and suburban areas. This trend is likely to continue over the next two 

decades, but ozone concentrations in developing countries may continue to 
Page 52 GAO-03-25 Air Pollutants and Climate Change

rise along with industrialization for the foreseeable future. The developing 

countries we analyzed are only starting to take measures to reduce ozone 

and its precursor substances. Consequently, ozone levels in the free 

troposphere are likely to increase globally as a result of a net increase in 

emissions of its precursors at the surface. Methane is expected to play a 

particularly important role in the formation of ozone in the future.

The results of economic research do not convincingly establish whether 

a country’s environmental pollution initially increases and then, with 

economic growth, decreases. This type of relationship seems to apply to 

some types of pollutants but not to others. Researchers generally agree, 

however, that unless the incentives facing producers and consumers 

change with higher incomes, pollution will continue to increase as 

economies grow. In other words, income growth, while a necessary 

condition, is not sufficient to reverse environmental degradation. 

Environmental policies must follow to induce appropriate responses and 

turn the pollution path around.

Agency Comments We provided a draft of this report to the Secretary of Energy and the 

Administrator of EPA for review and comment. The agencies provided 

written clarifying comments, which we incorporated where appropriate.

As arranged with your offices, we plan no further distribution of this report 

for 30 days after the date of this letter, unless you publicly announce its 

contents earlier. At that time, we will send copies of this report to the 

Ranking Minority Members of the Committee on Energy and Commerce, 

House of Representatives, and its Subcommittees on Energy and Air 

Quality and Oversight and Investigations; the Secretary of Energy; the 

Administrators of the Environmental Protection Agency, National 

Aeronautics and Space Administration, and National Oceanic and 

Atmospheric Administration; the Director, National Science Foundation; 

the Director, Climate Change Science Program Office; and other interested 

parties. We will make copies available to others upon request. In addition, 

copies of this report are available at no cost at our Web site, www.gao.gov.
Page 53 GAO-03-25 Air Pollutants and Climate Change

If you have any questions about this report, please contact me at 

(202) 512-3841. Key contributors to this report are listed in appendix VI.

John B. Stephenson

Director, Natural Resources

and Environment
Page 54 GAO-03-25 Air Pollutants and Climate Change

Appendix I

Scope and Methodology AAppenednxidexiIs

To obtain information on recent research relating to the climate 

change characteristics of the three substances, we contacted scientists in 

four federal agencies: the Department of Commerce’s National Oceanic 

and Atmospheric Administration; the Department of Energy; the 

National Aeronautics and Space Administration; and the National 

Science Foundation. These scientists were recommended by staff at 

the U.S. Climate Change Science Program, an interagency research 

coordinating body.

In analyzing trends for the three pollutants, we found that the availability 

and quality of data varied considerably, especially in developing countries. 

Except for countries that are members of the EU or participate in the 

CLRTAP, governments are not required to report their emissions of 

these substances internationally.1

 We found no standardized system for 

calculating and reporting emissions. Moreover, some of the data reported 

here are based on direct measurements, while others are estimated using 

proxy data (such as fuel use information), which may be less exact than 

measured data. Most of the data used in this study, except where noted, are 

taken from government sources. Dr. Tami Bond of the National Center for 

Atmospheric Research; Dr. Loretta Mickley of Harvard University; 

Dr. Michael Prather of the University of California, Irvine; and Dr. David 

Streets of Argonne National Laboratory provided comments and insights 

on certain sections of the report. The information on foreign countries’ 

emissions levels and legislation does not reflect our independent analysis.

To obtain information on policies and measures, we first sought a 

comprehensive source of information for each country. Because we 

found no such source, we contacted government, academic, and other 

researchers and analysts in the United States and the other countries, and 

the countries’ embassies in the United States. We also gathered information 

from government publications, web sites, e-mail correspondence with 

U.S. and foreign government officials, and discussions with embassy 

personnel. While we tried to make this report as complete as possible, 

there may be additional policies and programs underway that are not 

addressed here.

1

 Under the Framework Convention on Climate Change, developed (Annex I) 

countries generally report their emissions of sulfur dioxide along with their emissions of 

the six conventional greenhouse gases. This is the source of our data on Japanese sulfur 

dioxide emissions.
Appendix I

Scope and Methodology

Page 55 GAO-03-25 Air Pollutants and Climate Change

To assess the literature on the relationship between economic growth and 

environmental pollution, we conducted computerized literature searches 

to identify relevant articles. To help us determine which articles to focus 

on, we sought guidance from recognized experts who specialize in this 

field. The extent of governmental action to control a specific pollutant is 

believed to be one important factor—but not the only factor—in explaining 

the relationship between economic growth and emissions of that pollutant. 

Therefore, even though few countries have acted to control carbon dioxide 

emissions and have done so recently, we included studies that examined 

the relationship between economic growth and such emissions.

We conducted our review between October 2001 and April 2003 in 

accordance with generally accepted government auditing standards.
Page 56 GAO-03-25 Air Pollutants and Climate Change

Appendix II

Programs and Measures to Reduce Emissions 

of Sulfur Dioxide ApendxiI

Country Measures

United States Section 109 of the Clean Air Act requires EPA to establish National Ambient Air Quality Standards 

(NAAQS) for air pollutants that may endanger public health and welfare. EPA established such NAAQS 

for sulfur dioxide. Under the 1990 amendments, areas not in attainment with the NAAQS must meet 

special compliance schedules.

Section 110 of the Clean Air Act requires states to adopt plans, known as State Implementation Plans 

(SIP), which detail the regulations a state will use to implement, maintain, and enforce the NAAQS. EPA 

must approve each SIP, and if a SIP is not acceptable, EPA may take over enforcement of the Clean Air 

Act in that state. 

Section 111 of the Clean Air Act requires EPA to establish nationally uniform, technology-based 

standards called New Source Performance Standards for categories of new industrial facilities, such as 

power plants, steel mills, and smelters. These standards limit the amount of certain pollutants, including 

sulfur dioxide, that may be emitted. 

Sections 160-169 of the Clean Air Act establish requirements for the prevention of significant 

deterioration (PSD) of air quality in areas that have attained the NAAQS. The act divides clean air areas 

into three classes and specifies the increments of sulfur dioxide and particulate matter pollution allowed 

in each. In order to receive a PSD permit, a new or modified major source of pollution must show that it 

will not contribute to a violation of the increments or of the national ambient air quality standards and 

that it will use best available control technology (BACT). This provision is referred to as PSD New 

Source Review.

Sections 171-173 of the Clean Air Act establish pre-construction permitting requirements for major new 

and modified sources in non-attainment areas (areas that have not attained the NAAQS). To receive a 

permit, such sources must, among other things, (1) obtain emissions offsets, thereby assuring that 

reasonable progress toward attainment of the NAAQS will occur, and (2) comply with the “lowest 

achievable emissions rate.”

Title IV of the Clean Air Act created EPA’s Acid Rain Program, which caps sulfur dioxide emissions from 

virtually all U.S. electric power plants at 8.95 million tons. Plant operators were required to reduce their 

emissions through any combination of strategies, including installing scrubbers, switching to natural gas 

or low-sulfur coal, or trading emissions allowances. The first phase of the program ran from 1995 to 

1999, and the second phase, with more stringent caps, began in 2000 and will run indefinitely. The 

program features a provision that allows power plants that exceed their emissions targets to “bank” 

extra allowances during the first phase of the program and then use these banked allowances during 

the more stringent second phase.

United Kingdom The United Kingdom’s 1956 and 1968 Clean Air Acts, among other things:

• authorized local councils to set up smokeless zones and make grants to householders to convert their 

homes from traditional coal fires to heaters fueled by gas, oil, smokeless coal, or electricity.

• set limits on sulfur dioxide emissions from small power plants.

• The United Kingdom’s Environment Act of 1995 requires the government to produce a national air 

quality strategy that identifies clear, measurable targets for improved air quality in the United Kingdom. 

This strategy is to be developed based on understanding of the health effects of the pollutants 

concerned and costs of emission reduction methods and aims to improve air quality by 2005. The 

strategy sets standards and objectives for sulfur dioxide and seven other air pollutants. The 1995 act 

also established a system of local air quality management, under which authorities are required to 

assess the current and future quality of air in their areas against the national air quality strategy 

objectives.
Appendix II

Programs and Measures to Reduce Emissions 

of Sulfur Dioxide

Page 57 GAO-03-25 Air Pollutants and Climate Change

Source: GAO.

The Convention on Long-Range Transboundary Air Pollution (CLRTAP) binds the United Kingdom to 

reduce sulfur, as do certain CLRTAP protocols:

• The 1994 Oslo Protocol on Further Reduction of Sulfur Emissions, which aims at gradually achieving 

critical loads of sulfur.

• The 1999 Gothenburg Protocol, signed but not ratified by the United Kingdom, which set emissions 

ceilings for 2010 for sulfur dioxide and three other pollutants, is expected to enter into force as early as 

2003. (CLRTAP covers more European countries than the EU, but the EU directives generally require 

more ambitious emissions reductions than the CLRTAP protocol.)

EU directives which require the United Kingdom to reduce sulfur dioxide emissions include:

• The First Daughter Directive (1999/30/EC), under which EU members must establish, and achieve by 

2005, a legally binding limit on concentrations of sulfur dioxide and three other substances.

• The Directive on the Incineration of Wastes (2000/76/EC), which sets limits on emissions of sulfur 

dioxide and other substances from waste incineration plants.

• The Large Combustion Plant Directive (2001/80/EC), which sets limits on sulfur dioxide and nitrogen 

oxides from combustion plants with a thermal input of 50 megawatts or greater.

Germany • The Large Combustion Ordinance contains emissions ceilings for new power plants in Germany.

• The CLRTAP requirements and EU directives also apply to Germany.

China The Air Pollution Control Law, enacted in 1987 and amended in 2000, aims to improve air quality in key 

urban areas. Specifically, it

• broadens the scope of affected industries beyond industrial sources and power plants, to include 

automobiles, ships, domestic heating, and cooking stoves;

• provides an incentive for using high-quality, low-sulfur coal and renewable energy;

• allows so-called “priority cities” to designate zones within which all burning of high-polluting fuel (i.e., 

coal) can be prohibited and calls for the phase-out of a form of dirty coal in large- and medium-sized 

cities;

• requires new or expanded sulfur dioxide-emitting power plants or large- and medium-sized industrial 

enterprises to install desulfurization equipment; and

• encourages cities to replace individual household coal heating stoves with centralized district heat.

The Energy Conservation Law, which entered into force in January 1998, promotes energy conservation 

and efficiency. Other energy conservation laws also exist.

A fuel tax has been imposed on high-sulfur coals, and between January and September 2000, 4,732 

mines producing high-sulfur coal were closed.

Subsidies for coal have been greatly reduced since 1984.

(Continued From Previous Page)

Country Measures
Page 58 GAO-03-25 Air Pollutants and Climate Change

Appendix III

Programs and Measures to Reduce Emissions 

of Black Carbon or Particulate Matter ApendxiI

Source: GAO.

Country Measures

United States The Heavy Duty Diesel Rule, promulgated in 2001, will require significant future reductions in highway diesel 

engine particulate matter emissions. It will also require diesel oil refiners to reduce most sulfur from diesel fuel 

by 2006 in preparation for new engines in 2007. In 2030, when the rule is fully implemented, it is expected that 

particulate matter from diesel vehicles will be reduced by 130,000-140,000 tons per year relative to 1996.

United Kingdom The Smoke Control Act of 1993 empowers local authorities to declare a smoke control area if it appears that 

air quality standards will not be met. Under the law, the local government can require that only certain fuels be 

used for domestic heating.

EU directives aimed at reducing particulate matter include:

• Directive 1998/70/EC, which sets the maximum allowable sulfur content of gasoline and diesel fuel.

• The First Daughter Directive (1999/30/EC), which contains a particulate matter standard for the EU 

countries.

• The Directive on the Sulfur Content of Certain Liquid Fuels (1999/32/EC), which mandates reductions in 

sulfur content of diesel fuel to enhance performance of emissions-reduction devices.

• Directive 1999/96/EC, which sets the emission limit values and implementation dates in two stages for heavy 

duty vehicles (Euro III and IV). The standards cover emissions of particulate matter and ozone precursors.

Germany The EU directives described above also apply to Germany.

China Euro I and II standards for the control of emissions from diesel vehicles.

India Euro I and II standards for the control of emissions from diesel vehicles.
Page 59 GAO-03-25 Air Pollutants and Climate Change

Appendix IV

Programs and Measures to Reduce 

Ground-Level Ozone ApendxiIV

Country Measures

United States The Clean Air Act has resulted in the creation of several trading programs for reducing nitrogen oxide 

emissions in the electric utility sector:

• EPA’s Acid Rain Program sets emissions rates for all affected utilities. Unlike sulfur dioxide, there is no cap 

on total nitrogen oxide emissions, but utilities may choose to over-control at units where it is easier to do so 

and average these emissions with those at their other units, thereby achieving overall emissions reductions 

at lower costs.

• The EPA Nitrogen Oxides SIP Calla

 requires 19 states and the District of Columbia to reduce total nitrogen 

oxide emissions from utilities by a certain number of tons each year; compliance for Phase I must be 

achieved no later than May 31, 2004. Power producers subject to these regulations are allowed to trade 

emissions allowances to meet the required limits.

• The Nitrogen Oxides Budget Trading Program, begun in 1999 in nine Northeastern states, aims to reduce 

nitrogen oxide emissions during the summer months to enable states to attain the standard for ground-level 

ozone. Like the programs above, it is a cap and trade program, under which total emissions are capped, and 

affected parties may trade emissions allowances.

The Clean Air Act has also provided the framework for ozone-reducing reductions in the transportation sector. 

The most recent regulation with significant impact on transportation sector ozone precursors is the Tier 2 

program, which was promulgated in 2000 and will be phased in starting in 2004, when refiners must produce 

low-sulfur fuel for passenger vehicle gasoline. Tier 2 also sets tailpipe emission standards for all classes of 

passenger vehicles, including sport utility vehicles and light-duty trucks. 

A Clean Air Act regulation requires the combustion of methane and other non-methane organic compounds 

from large landfills. The regulation also contains a performance standard based on the allowable 

concentration of methane measured at the landfill.

In addition to programs regulating emissions from cars and trucks, EPA regulates emissions of nitrogen oxides 

and hydrocarbons from aircraft, ships, locomotives, recreational vehicles, off-road diesel equipment (e.g., farm 

and construction equipment), and spark-ignition engines, such as chain saws, lawn mowers, and forklifts.

The U.S. Department of Transportation administers a program called “Congestion Mitigation and Air Quality 

Improvement” aimed at reducing ozone and its precursors (as well as particulate matter) by funding new 

transit services, bicycle, and pedestrian improvement, alternative fuel projects, traffic-flow improvements, and 

other emissions-reducing projects.

In the industrial sector, the Clean Air Act specifies performance standards for new industrial sources. 

The standards, called New Source Performance Standards, establish maximum emission levels for new 

or modified major stationary sources, such as steel mills and smelters. These standards also apply to 

power plants.

United Kingdom CLRTAP Protocols that bind the United Kingdom and other European countries include:

• The 1988 Sofia Protocol, which set a target for emissions of nitrogen oxides.

• The 1991 Geneva Protocol, which requires a reduction in emissions of volatile organic compounds.

• The 1999 Gothenburg Protocol, signed but not ratified by the United Kingdom, which sets emissions ceilings 

for nitrogen oxides, volatile organic compounds, and two other substances.
Appendix IV

Programs and Measures to Reduce 

Ground-Level Ozone

Page 60 GAO-03-25 Air Pollutants and Climate Change

Source: GAO.

a

The Nitrogen Oxides SIP Call is authorized under section 110 of the Clean Air Act. A SIP is a State 

Implementation Plan, which is a proposal submitted by each state to EPA containing emission 

limitations and other control measures to attain, maintain, and enforce the NAAQS. EPA may issue a 

SIP Call under the act when it finds that the applicable SIP fails to comply with a requirement of the 

act. The SIP Call requires the state to revise its SIP.

The EU directives aimed at reducing ozone and/or its precursors include:

• Directives on Air Pollution by Ozone (92/72/EC and 2002/3/EC), which establish thresholds for ozone and 

require that threshold exceedances must be reported to the EU Commission and to the public.

• The Directive on VOC Emissions from the Storage of Petrol (94/63/EC), which sets guidelines for reducing 

volatile organic compound emissions from the storage and distribution of petrol (gasoline) from terminals to 

service stations.

• The Framework Directive (96/62/EC), which established the framework under which the EU countries would 

agree on emissions limits for certain pollutants. The Directive requires that, if limits are exceeded, member 

states devise abatement programs to reach the limits within a set deadline.

• The Directive 98/69/EC, which establishes emission limit values and implementation dates in two stages for 

light-duty vehicles (Euro III and IV). The standards cover ozone precursors and particulate matter.

• The First Daughter Directive (99/30/EC), which sets limit values for nitrogen oxides (and other substances.)

• The VOC Solvents Directive (99/13/EC), which limits emissions of volatile organic compounds.

• The Directive on Landfills (99/31/EC), which aims to harmonize controls on the landfill of waste throughout 

the EU. Its main focus is on common standards for the design, operation and aftercare of landfill sites. It also 

aims to reduce the amount of methane emitted from landfill sites.

• The Large Combustion Plant Directive (2001/80/EC), which sets limits on sulfur dioxide and nitrogen oxides 

from combustion plants with a thermal input of 50 megawatts or greater.

The National Emission Ceiling Directive (2001/81/EC), which sets ceilings for emissions of nitrogen oxides, 

sulfur dioxide, ammonia, and volatile organic compounds to be attained by 2010. (As of early 2003, this 

protocol was not yet in force.)

Germany The CLRTAP requirements and EU directives cited above apply to Germany. 

Japan The Air Pollution Control Law requires stations in several parts of the country to monitor for nitrogen dioxide, 

suspended particulate matter, sulfur dioxide, carbon monoxide, and photochemical oxidants. It also 

establishes maximum permissible limits on exhaust gases from motor vehicles.

The Automobile Nitrogen Oxides Law sets the fundamental policies and plans for reducing the total volume of 

nitrogen oxides for specific automobiles.

China Although the government does not require the reporting of data on ozone concentrations, the city of Beijing 

does so when ozone levels become particularly high.

Euro I and II standards for the control of motor vehicle emissions, including carbon monoxide, hydrocarbons, 

and nitrogen oxides.

Mexico Mexico’s Tag Zero Program offers a 2-year exemption from Mexico City’s “car-free” policy for drivers of clean 

cars. Under the program, drivers of new cars or cars that have been upgraded with catalytic converters may 

drive in the city any day of the week, while owners of cars without such certification may drive on only a certain 

number of days per week. (An exempted, or clean, car is denoted by a zero on a sticker placed on the back 

window of the car. The car’s license plate indicates the days a car may not be driven if it does not have a 

sticker.) The Tag Zero program thus rewards the purchase of clean vehicles.

India Euro I and II standards for the control of motor vehicle emissions, including carbon monoxide, hydrocarbons, 

and nitrogen oxides.

Country Measures
Page 61 GAO-03-25 Air Pollutants and Climate Change

Appendix V

Summary of Results of Selected Studies 

on Economic Growth and Environmental 

Pollution ApendxiV

The following table presents information on the studies of economic 

growth and environmental quality that we reviewed.

Table 2: Results of Selected Studies of Economic Growth and Environmental Quality

Source: GAO.

a

All numbers are in 1985 dollars unless otherwise noted.

b

Grossman and Krueger estimated a turning point separately for heavy particulate and smoke. While 

no turning point for heavy particulate was found, the turning point for smoke was $6,200. 

c

In 1986 dollars.

d

In 1990 dollars.

Estimated per capita income for turning points, 

by type of emissiona

Study’s author(s) and year Particulate matter Sulfur dioxide Carbon dioxide

Based on exchange rates

Grossman and Krueger, “Economic Growth and Environment,” 

Quarterly Journal of Economics (1995)

None foundb $4,100 Not studied

Holtz-Eakin and Selden, “Stoking the Fires? CO2 Emissions and 

Economic Growth,” Journal of Public Economics (1995)

Not studied Not studied $35,400c

Panayotou, “Demystifying the Environmental Kuznets Curve: Turning a 

Black Box into a Policy Tool,” Environment and Development Economics 

(1997)

Not studied 5,000 Not studied

Roberts and Grimes, “Carbon Intensity and Economic Development 

1962-91: A Brief Exploration of the Environmental Kuznets Curve,” 

World Development (1997)

Not studied Not studied None found

Based on purchasing power parity

Shafik, “Economic Development and Environmental Quality: 

An Econometric Analysis,” Oxford Economic Papers (1994)

$3,300 3,700 Not studied

Selden and Song, “Environmental Quality and Development: Is There a 

Kuznets Curve for Air Pollution Emissions?,” Journal of Environmental 

Economics and Management (1994)—two models estimated

9,600

9,800

10,700

8,900

Not studied

Cole, Rayner, and Bates, “The Environmental Kuznets Curve: An 

Empirical Analysis,” Environment and Development Economics

(1997)—two models estimated

7,300

8,100

5,700

6,900

25,100

62,700

Schmalensee, Stoker, and Judson, “World Carbon Dioxide Emissions: 

1950-2050,” Review of Economics & Statistics (1998)

Not studied Not studied 10,000

Unruh and Moomaw, “An Alternative Analysis of Apparent EKC-type 

Transitions,” Ecological Economics (1998)

Not studied Not studied None found

Panayotou, Sachs, and Peterson, “Developing Countries and the 

Control of Climate Change: Empirical Evidence,” Discussion Papers 

#45, Harvard Institute for International Development (1999)

Not studied Not studied 12,000

Stern and Common, “Is There an Environmental Kuznets Curve for 

Sulfur?” Journal of Environmental Economics and Management (2001)

Not studied 101,200d Not studied
Page 62 GAO-03-25 Air Pollutants and Climate Change

Appendix VI

GAO Contact and Staff Acknowledgments ApendxiVI

GAO Contact David Marwick, (202) 512-6775

Acknowledgments In addition to the individual named above, Bernice H. Dawson, Richard A. 

Frankel, Anne K. Johnson, Mehrzad Nadji, and Carol Herrnstadt Shulman 

made key contributions to the report. Important contributions were also 

made by Laura Yannayon and Katherine M. Raheb.

(360150)
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