CEE 8095

1
CEE 8095 Georgia Institute of Technology February
22, 2006 REGIONAL ATMOSPHERIC POLLUTION AND
TRANSBOUNDARY AIR QUALITY MANAGEMENT Annu.
Rev. Environ. Resour. 2005. 30137 Michelle S.
Bergin Georgia Inst. of Tech., Dept. of Civil
and Environmental Engineering J. Jason West
Princeton University, Program in Atmospheric and
Oceanic Sciences and Woodrow Wilson School of
Public and International Affairs Terry J.
Keating U.S. EPA, Office of Air and
Radiation Armistead G. Russell Georgia Inst. of
Tech., Dept. of Civil and Environmental
Engineering
2
Overview

• GENERAL CHARACTERISTICS OF REGIONAL ATMOSPHERIC
  TRANSPORT AND CHEMISTRY
• REGIONAL AIR POLLUTANTS AND EFFECTS
• Acid Deposition
• Tropospheric Ozone
• Particulate Matter
• Mercury
• Persistent Organic Pollutants
• Regional Climate Change
• A unified view
• MANAGEMENT OF REGIONAL TRANSBOUNDARY AIR
  POLLUTION
• Examples of Transboundary Air Quality Management
  Regimes and Approaches
• Achieving Successful Regimes for Transboundary
  Air Pollution
• Assessment of Existing International Regional
  Regimes

3
Air Pollution Impacts

• Current estimates are that a quarter of the
  worlds population is exposed to unhealthy
  ambient air pollutants, with nearly 6.4 million
  years of healthy life lost owing to long-term
  exposure to ambient particulate matter alone.
  (World Health Organ. 2003, 2004)
• Air pollution damages crops, built structures
  such as buildings and historic monuments,
  ecosystems, and wildlife.
• Pollutants currently in the atmosphere are likely
  causing large scale changes in the Earths basic
  systems such as climate and ocean dynamics.

4
Regional Air Pollution

• Several important air pollutants are widely
  recognized to be transported over scales from
  about 100 to a few 1000s of kilometers, large
  enough to cross state, provincial, national, and
  even continental boundaries.
• Managing these regional pollutants requires
  overcoming political, economic, and cultural
  differences to establish cooperation between
  multiple jurisdictions.
• Effective pollution reduction requires an
  understanding of the linkages between pollutants
  and of their impacts at different geographic
  scales.

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Regional Air Pollution Management

• Transboundary air pollution is common.
• Its damage is extensive.
• Political cooperation necessary to mitigate the
  damage and avoid potential future damage.
• GOALS
• Develop integrated control strategies
  incorporating
• multiple linkages between pollution problems
• multiple scales of pollutant impacts
• multiple benefits of alternative technology and
  behavioral choices
• linkages between air pollution and economic
  development, energy, agriculture, transportation,
  and trade policies.
• Enable all affected parties to reach agreements
  to implement these control strategies.

6
GENERAL CHARACTERISTICS OF REGIONAL ATMOSPHERIC
TRANSPORT AND CHEMISTRY

• What determines the geographical extent of a
  pollutants impacts?
• Pollutant
• Emission rate and patterns of release (primary or
  precursors)
• Chemistry (formation, intermediates, removal,
  conditions)
• Transport methods
• Removal/deposition properties
• Atmosphere
• Oxidation (HO), chemistry
• Tropospheric vertical mixing (10 km deep)
• Horizontal transport velocity averaged mean
  400 km/day, ranging from a few tens km to about
  1000 km
• Meteorological patterns (wind, solar intensity,
  clouds, rain, )
• Geography
• Natural emissions
• Meteorological patterns
• Terrestrial properties
• Deposition

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REGIONAL AIR POLLUTANTS AND EFFECTS

• ACID DEPOSITIONdamages or destroys aquatic and
  terrestrial ecosystems and manufactured
  materials
• TROPOSPHERIC OZONE harms the health of humans
  and wildlife, damages crops and ecosystems, and
  is a greenhouse gas
• PARTICULATE MATTER harms health, degrades
  visibility, and changes precipitation and
  temperature patterns
• MERCURY toxic to humans, plants, and animals,
  and biomagnifies in the food chain
• PERSISTENT ORGANIC POLLUTANTS (POPs) toxic,
  biomagnify, and remain in the environment for
  many years
• REGIONAL CLIMATE CHANGE impacts and is impacted
  by regional air pollution.

8
Acid Rain Damage
www.adirondackcouncil.org/
www.fws.gov
www.dnr.cornell.edu/.../current_research.htm
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Acid Deposition

• pH of Rain
• Natural precipitation 5.6 (slightly acidic)
• Polluted areas often 3 to 4, lower has been
  measured
• Most aquatic insects, algae, and plankton (base
  of food chain) cannot tolerate water with a pH
  below 5.0. A pH below 5.0 can also result in
  reproductive failures in fish and amphibians.
• Acid runoff leaches poisonous metals such as
  aluminum and mercury from soils and clay, and
  carries them into water bodies.

10
Observed values of volume-weighted annual average
pH of precipitation. Rodhe H, Dentener F,
Schulz M. 2002. Environ. Sci. Technol.
Annual average pH of rainwater. The most
acidified regions occur in eastern North America,
Europe, and China. The maximum in South America
is associated with emissions from a smelter in
northern Chile coupled to low amounts of rainfall
in that region.
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Spatial Impacts of Acid Deposition

• The natural pH of water bodies and soils varies
  considerably depending on the soil composition
  and buffering capacity of a watershed.
• These differences explain why some areas are
  highly susceptible to damage from acid deposition
  (e.g., lakes in the northeastern United States
  and the Black Forest in Germany), whereas other
  areas, such as those with soils rich in lime, are
  relatively resistant.
• Patterns of rainfall also help determine where
  acid is deposited.

12
The evolution and unified chemistry of
regionalacids, ozone, particulate matter, and
mercury.

• Gases blue
• Particulates green
• HO, HO2 red

13
Natl. Instit. Public Health Environ. (RIVM),
Bilthoven, Neth. 1995. dataset EDGARV32.
www.rivm.nl/edgar/model/ims/
NOx from anthropogenic sources in 1995.
SO2 from anthropogenic sources in 1995.
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Acid Chemistry and Transport

• NOx ? nitric acid in a few days or less
• SO2 ? sulfuric acid within several days
• Mean transport distances of SO2, NOx, and their
  oxidation products are approximately 200 to 1200
  km before deposition
• In addition to direct transport, NOx can form
  organonitrates that can travel 100s to 1000s of
  km before converting back to NOx

15
Acid Cooperative Programs

• Early 1970s Cooperative studies under the
  Organization for Economic Cooperation and
  Development (Europe)
• 1978 National Atmospheric Deposition Program (US)
• 1979 Convention on Long-Range Transboundary Air
  Pollution (originated in Europe)
• 1981 Natl. Acid Precipitation Assessment Program
  (US)
• 1990 SO2 Emissions Trading Program (US)
• 1991 Canada-United States Air Quality Agreement
• 1998 Acid Deposition Monitoring Network in East
  Asia (EANET)

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Tropospheric Ozone

• Concentrations
• Clean troposphere 10 to 40 ppb
• Polluted air 100 to 400 ppb
• Impacts
• Causes acute and chronic health problems,
  especially related to lung functions, asthma, and
  pulmonary infection (human).
• Accounts for an estimated several billion dollar
  crop loss every year in the United States alone.
• Causes short- and long-term damage to the growth
  of forest trees and alters the biogenic
  hydrocarbon emissions of vegetation.
• Research is examining effects on sensitive
  wildlife populations.
• Acts as a potent greenhouse gas.

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One-hour averaged ozone concentration and its
sensitivity to a 50 reductions in surface NOx
emissions on July 15, 2010 at 4 pm. (Bergin et
al., in progress)
Ozone sensitivity to surface NOx emissions from
VA.
Ozone sensitivity to surface NOx emissions from
TN.
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The evolution and unified chemistry of
regionalacids, ozone, particulate matter, and
mercury.

• Gases blue
• Particulates green
• HO, HO2 red

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Ozone Removal

• Photolytic loss O3 hn ? O(1D) O2 and O(1D)
  H2O ? 2 OHhighest at low altitudes in the
  tropics
• Kinetic reaction HO2 O3 ? OH 2 O2
• Deposition takes a few days over land, longer
  over water.
• Ozone and its precursors can be transported over
  large regions, making it difficult or even
  impossible for some areas to manage their own
  concentrations through local emission controls

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Cooperative regional ozone monitoring and
modeling studies

• US 1980s early 1990s
• San Joaquin Valley Air Quality Study
• Lake Michigan Ozone Study
• Regional Ozone Modeling for Northeast Transport
• Southern Oxidant Study (SOS)
• Southern Appalachian Modeling Initiative (SAMI)
• European Experiment on the Transport and
  Transformation of Environmentally Relevant Trace
  Constituents in the Troposphere over Europe
  (EUROTRAC, 1980s and 1990s)
• Late 1990s to current Monitoring of ozone export
  from the east coast of North America and Asia,
  and modeling studies of trans-Atlantic and
  trans-Pacific transport.
• Historical measurements and modeling studies
  suggest that anthropogenic NOx emissions continue
  to increase global tropospheric ozone
  concentrations. (Prather et al., 2003 US EPA,
  2004)

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Particulate Matter
High-visibility conditions in the Smoky
Mountains-100 miles (top) and low-visibility-20
miles (bottom) www.ornl.gov/.../v38_1_05/article0
2.shtml
Clear and Hazy Days in Houston www.utexas.edu/.../
texaqs/visitors/photos.html
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Particulate Matter

• Harmful to human and environmental health,
  degrades visibility, damages built structures,
  and alters climate.
• No threshold has been identified under which
  particulate matter has no adverse health effects.
  
• In the US, the average visual range in eastern
  parks has decreased from 90 miles to 1525 miles,
  and in western parks from 140 miles to 3590
  miles (US EPA, 2004)
• Direct and indirect effects on climate
• Evolution of science and policy TSP ? PM10 ?
  PM2.5

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The Trimodal Aerosol Distribution
Primary and secondary Variable size and
composition
Mechanical Processes
Molecular Processes

• Coarse PMLifetimes of minutes to days ?
  transport of less than 10s of km. However, large
  dust storms and biomassburning events can carry
  coarse (and fine) particles very long distances.
• Fine PMLifetimes of days to weeks ? transport
  100s to 1000s of km.
• In addition to size, hygroscopic properties of
  the aerosol also determine travel distance.
• Fine particulates are dominant in most
  regional-scale aerosol pollution events.

Emissions
washout
Penn State Earth and Mineral Sciences www.ems.psu.
edu/lno/Meteo437
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The evolution and unified chemistry of
regionalacids, ozone, particulate matter, and
mercury.

• Gases blue
• Particulates green
• HO, HO2 red

25
Twenty four-hour averaged PM2.5 concentration on
July 17, 2010
Sensitivity to a 50 reduction in SO2 emissions
from Tennessee
An average of 75 of PM2.5 in each state is found
to be sensitive to SO2 emissions from other
states. (Bergin et al., In progress)
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Particulate Matter Anthro. Sources

• In industrialized areas (e.g. Europe, N. Amer.,
  Japan, large cities in Asia) particles are mainly
  generated by gasoline, diesel, and coal
  combustion and from condensed organic and metal
  vapors, and tend to contain sulfate, nitrate,
  ammonium, and organic carbon, with smaller
  amounts of metals and crustal materials.
• In less developed areas, (e.g. parts of Africa
  and Asia) biomass burning is the principal source
  of regional aerosol pollution, resulting largely
  in elemental and organic carbon particles and
  nitrate.
• Dust storms largely contribute mineral particles,
  and are exacerbated by desertification.

27
A satellite image from the Earth Probe TOMS.
NASA. (EPA 2002) Smoke/Dust (absorbing
aerosols) from forest fires in Central America
and Southern Mexico over North America on May 15,
1998. Visibility in many Texas cities was less
than 1 mi.
Dust and burning plumes may also carry
anthropgenic aerosols. One study estimates
transboundary transport of particulate matter and
its precursors dominated natural sources in the
US, and that up to 30 of the transboundary
particulate matter was due to transport from
Asia. (Park et al., J. Geophys. Res., 2004)
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Mercury

• Mounting evidence strongly implicates mercury
  exposure as playing a role in the increasing
  rates of learning disabilities found today in our
  children. the EPA reported that 1 in every 6
  women of child bearing age had mercury levels
  high enough to cause adverse neurodevelopmental
  outcomes in their unborn children. boys are
  more sensitive to the neurological effects of
  mercury than are girls
• Letter printed in Newsweek Feb 13, 2006 Lyn
  Redwood, PresidentRobert J. Krakow, Board of
  DirectorsSensible Action for ending
  Mercury-Induced Neurological Disorders
  (SafeMinds)

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Mercury

• Mercury affects the immune system, alters genetic
  and enzyme systems, and damages the nervous
  system. Developing embryos are five to ten times
  more sensitive to damage by MeHg, which rapidly
  crosses the placenta, than are adults, and young
  children are especially susceptible to mercurys
  neurotoxic effects.
• In the US alone, over 60,000 children per year
  are estimated to be born at risk for adverse
  neurodevelopmental effects caused by in utero
  exposure to MeHg.
• In the US, 2003, 45 states issued mercury
  advisories to limit the consumption of certain
  fish from freshwater lakes and rivers, coastal
  waters, and marine environments. (NRDC (Jan 2006)
  estimates 1 can of tuna a month for a child of 45
  lbs.)
• Humans are exposed to MeHg almost entirely by
  eating contaminated fish and animals that are at
  the top of aquatic food chains.

30
Estimated global emissions of mercury. UN
Eviron. Programme (UNEP) 2003.

• About 75 of the total 1995 anthro. emissions are
  attributed to combustion of fossil fuels (Pacyna
  ,2003) particularly in coal-fired power plants
  (NRC, 2000 EPA, 2004). Asian countries are
  estimated to contribute over 50 of the total
  emissions chiefly from coal combustion in China,
  India, and South and North Korea.

31
Estimates of Annual Mercury Contributions to the
Atmosphere (Mg/yr)
Approximately 1/3 are natural and 2/3 are
anthropogenic (US EPA, 2004)

• Values from Seigneur et al., Environ. Sci.
  Technol. 2004

32
Conceptual biogeochemical mercury cycle
a
(Environ. Can. 2004)
33
Lifetime of Mercury in the Atm.

• Lifetime of elemental mercury Hg0 ? 1.21.5
  yearsNew findings strongly indicate 1 or 2
  months. (Renner R., Environ. Sci. Technol.,
  2004)
• This more rapid oxidation may be due to reactions
  with halides (at the poles and temperate marine
  boundary layer) and urban pollutants, most likely
  ozone.
• This shorter lifetime suggests that deposition
  occurs much more rapidly than previously
  estimated, and therefore current emissions are
  likely higher than estimated. One such
  underreported source may be gold mining
  activities, which may release between 800 and
  1000 Mg/yr (previous estimates are 300 Mg/year)
  (UN Eviron. Programme (UNEP) 2003)

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Persistent Organic Pollutants

• Toxic organic substances that widely disperse,
  persist in the environment, bioaccumulate, and
  pose a risk to human health and the environment.
• Common characteristics
• Semivolatile and/or partial water solubility
• Transported in the atmosphere both in the gaseous
  and condensed phases, deposited, and then
  reemitted through a repeated (and often seasonal)
  cycle of evaporation and deposition
• Lifetimes from a few years to decades long
• Most commonly recognized are partially oxidized
  and/or halogenated organics, often including one
  or more aromatic structures.
• Linked to causing, among other effects, cancer,
  damage to the nervous system, reproductive
  disorders, behavioral disorders, and disruption
  of the immune and endocrine systems (includes
  allergies, asthma, thyroid disorders, and others.)

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POPs human health effects

• POPs enter humans chiefly through animal-derived
  food (fish, poultry, beef, eggs, and dairy
  products). Concentrations can magnify in fatty
  tissues by up to 70,000 times background levels.
• Because of their fat-seeking properties, POPs
  magnify in infants both in utero and through
  breast-feeding, magnifying concentrations through
  generations.
• Human breast milk concentrations of some
  monitored POPs have been increasing
  exponentially, serving as an indicator of past
  human exposures and environmental conditions.
  Previously unrecognized POPs (i.e., brominated
  biphenyl ethers) have been identified by
  increasing concentrations found by systematic
  monitoring of chemicals in human breast milk in
  Sweden after they banned the use of seven other
  POPs in the 1970s.

36

• POPs released in one part of the world may
  circulate regionally and globally via the
  atmosphere, oceans, and other pathways.
• Regional transport and deposition of POPs (and
  heavy metals) in Europe have been demonstrated
  through modeling. Further evidence of long-range
  transport comes primarily from the detection of
  POPs in remote locations. In the colder
  environments at the poles, semivolatile POPs
  condense and accumulate. Atmospheric monitoring
  data are sparse, and global emission
  distributions are poorly characterized.

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POPs Legislation

• The Dirty Dozen
• DDT, aldrin, dieldrin, endrin, chlordane,
  heptachlor, mirex, toxaphene, hexachlorobenzene
  (HCB), polychlorinated biphenyls (PCBs), dioxins,
  and furans.
• pesticides (e.g., DDT, aldrin, dieldrin, endrin)
• consumer or industrial applications, such as
  coolants, flame retardants, lubricants, and
  sealants (e.g., PCBs)
• generated unintentionally as by-products of
  various combustion processes, such as medical
  waste incineration (e.g., polycyclic aromatic
  hydrocarbons (PAHs), dioxins, and furans).
• Targeted to be banned or restricted by the
  Stockholm Convention on POPs, signed in May 2001
  by over 100 countries (except for DDT in some
  places)
• All 12 have been identified in Great Lakes fish
  and wildlife, even though some of these compounds
  were never used in significant quantities in the
  region. Consumers of the regions fish have up to
  eight times the body burden of POPs as the
  general public.

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POPs Legislation (cont.)

• REACH Regulation, Evaluation, and
  Authorization of Chemicals
• Since 1998, Europeans have been working on
  legislation that will require industry to prove
  that chemicals being sold and produced in the EU
  are safe to use or handle. The current system
  requires governments to prove that a chemical is
  dangerous. The policy will require registration
  of all substances that are produced or imported
  into the EU in quantities greater than 1 ton.
• European government officials charged that U.S.
  government agencies have worked closely with
  business to weaken the impact of REACH
  According to documents released by U.S. Rep.
  Henry Waxman (D-CA), U.S. government lobbying on
  behalf of industry provided talking points
  for U.S. government officials notably similar
  in language to themes developed by industry at
  the request of a U.S. trade official The
  United States has not conducted studies on the
  health and environmental impacts of REACH,
  Waxman tells EST. It simply began to lobby
  against REACH on behalf of U.S. industry
  interests without a full understanding of these
  impacts. EST Online News (Feb 1, 2006)

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Regional Climate Change

• Particulate matter
• Fine particles scatter light efficiently,
  resulting in a cooling effect on ambient
  temperature.
• Black carbon particles absorb light and create a
  warming effect.
• These aerosols have different spatial
  distributions, so do not simply offset each
  other.
• Particles also serve as cloud nuclei, increasing
  the number of cloud droplets but decreasing
  average cloud droplet size and pattern of
  rainfall.
• These properties can alter regional atmospheric
  stability and vertical motions, impact
  large-scale circulation, and alter the hydrologic
  cycle. The impact of aerosols depends on chemical
  and physical properties of the particles that are
  not yet well characterized.

40
Regional Climate Change (2)

• Ozone
• Tropospheric O3 ranks only behind CO2 and CH3 in
  anthropogenic contributions to global average
  radiative forcing and is expected to increase the
  global average temperature over the next decades.
  
• Urban ozone is expected to increase with
  increases in temperature, particularly during
  highly polluted episodes.
• Natural climate patterns impact air pollution
  formation and transport, and evidence strongly
  indicates that air pollution also impacts
  climate. Political bodies must address
  questions such as whether to act under the
  current understandings and uncertainties, and if
  so, how to coordinate or prioritize mitigation
  efforts and who is responsible for reducing
  emissions.Scientific bodies must aid in
  unifying our understanding of air pollution and
  its impacts and identifying efficient control
  strategies.
• RGGI A Regional Greenhouse Gas Initiative MOU
  was signed on December 20, 2005 by the Governors
  of Connecticut, Delaware, Maine, New Hampshire,
  New Jersey, New York, and Vermont. This program
  designs a cap and trade program for carbon
  emissions.

41
A Unified View
The evolution and unified chemistry of regional
acids, ozone, particulate matter, and mercury.
Each pollutant has distinct regional
characteristics, yet they are intimately linked
by the chemistry and physics of the atmosphere,
as well as by common sources and possible
controls.

• Gases blue
• PM green
• HO, HO2 red

Their linked chemistry and common sources present
strong arguments for addressing them
simultaneously to identify the most effective
control policies.
42
MANAGEMENT OF REGIONAL TRANSBOUNDARY AIR POLLUTION

• Generally, decreasing upwind emissions requires
  the development of some type of cooperative
  regime - social institutions consisting of
  agreed-upon principles, norms, rules,
  procedures, and programs that govern the
  interactions of actors in specific issue
  areas A regime encompasses a whole process of
  interaction, including both scientific and
  political activities, with or without formal
  agreements.

43
Achieving Successful Regimes for Addressing
Transboundary Air Pollution

• The amount of human resources or money is not the
  deciding factor in reaching an agreement, but the
  generation of the coordination, cooperation, and
  trust.
• Generally follows two phases
• Frame the problem define the issues, knowledge,
  and relationships between the parties, and the
  interests and positions of parties, as well as
  the differences in positions. Cooperative
  scientific research plays a leading role.
  Agreements reached are typically general.
• Build on the institutions created in the first
  phase, allowing for the increasingly complex and
  substantive policy agreements. During this phase,
  emphasis is placed less on the international
  dimensions of the problem, and more on analysis
  of collectively beneficial, efficient
  alternatives.

44
Examples of transboundary air quality management
regimes and approaches

• Emission Trading of SO2 for Acid Deposition
  Control (US)
• Part of 1990 U.S. Clean Air Act Amendments (CAAA)
  and the Acid Rain Program.
• Designed a market with a fixed number of emission
  allowances that could be bought, sold, or traded.
  
• Goal to cut SO2 emissions from power plants to
  half of 1980 levels by 2010.
• By 2003, achieved a 38 reduction from 1980
  levels.
• Initial estimates of the cost of reductions were
  4 to 8 billion per year.
• Recent estimates are that the costs have been
  about 1 billion per year.

45
Examples continued

• Regional NOx Control for Ozone (US)
• The Ozone Transport Commission (OTC) was formed
  in the 1990 CAAA to consist of 12 east coast
  states (Maine to Virginia)
• Most states were both upwind and downwind of
  nonattainment areas.
• Designed an emission trading program similar to
  that of the SO2 program.
• Succeeded because states were mutually assured
  that all states would be subject to similar
  controls and overall costs of emission reductions
  would be reduced through trading.
• The Ozone Transport Assessment Group (OTAG) was
  created in 1995 to address regional ozone. It
  was comprised primarily of representatives of the
  EPA and 37 states in the eastern half of the
  United States.
• The differences between conditions in upwind and
  downwind states and the lack of political
  leverage to encourage upwind states to reduce
  emissions were not overcome, and OTAG was able
  only to reach a weak consensus.
• Only through the intervening authority of a
  higher governmental body (the US EPA) could a
  regional strategy be implemented.

46
Examples continued

• Cooperative Control in Europe
• The 1979 Long-Range Transboundary Air Pollution
  (LRTAP) Convention
• Includes all of the nations of Europe (including
  the former Soviet Bloc), the US, and Canada.
• Eight protocols have been successfully
  negotiated, including agreements to support
  scientific cooperation and obligations to reduce
  emissions related to acidification, tropospheric
  ozone, POPs, heavy metals, and eutrophication.
• Recently expanded to include several of the
  former Soviet republics in Central Asia.

47
Important Characteristics of Past Successful
Regimes

• Cooperation among technical experts.
• Developed scientific cooperation before political
  negotiations began.
• Provided support to build technical and
  management capacity of all parties for timely
  analyses of proposals. Such analyses are most
  effective when considered to be
• credible (competent and appropriate for decision
  needs)
• legitimate (the assessment process is transparent
  and impartial)
• salient (relevant and timely for decision-making
  needs).
• Tend to have few parties, parties are
  economically, culturally, or geographically
  similar. However, regimes are unlikely to
  succeed unless they involve all major parties
  contributing to a problem.
• Developed interpersonal trust between key actors
  and with institutions.
• Ensured the accountability of parties in
  fulfilling commitments.
• Are adaptable to changing situations and
  information.

48
Acknowledgments

• EPA STAR graduate fellowship
• EPA STAR research grants
• NASA New Investigator Program grant to D.
  Mauzerall
• National Oceanic and Atmospheric Administration,
  U.S. Department of Commerce.
• The Annual Review of Environment and Resources is
  online at http//environ.annualreviews.org