Co-benefits of Carbon Standards

Part 1: Air Pollution Changes under Different 111d Options for Existing Power Plants

May 27, 2014

Charles Driscoll, Syracuse University; Jonathan Buonocore, Harvard School of Public Health, Harvard University; 

Habibollah Fakhraei, Syracuse University; Kathy Fallon Lambert, Harvard Forest, 

Harvard University
2

Executive Summary

The U.S. Environmental Protection Agency (EPA) is slated to release the nation’s first-ever carbon 

pollution standards for existing power plants on June 2, 2014. Carbon dioxide (CO2) is one of most 

abundant greenhouse gases in the atmosphere and a major driver of human-accelerated global climate 

change. Fossil-fuel-fired power plants are the single largest source of anthropogenic CO2 emissions in 

the U.S. They emit approximately 2.2 billion tons of carbon dioxide each year, representing 40 percent

of total U.S. CO2 emissions (USEPA 2014).

Carbon pollution standards that reduce CO2 emissions from existing power plants can also cut emissions of other

power plant pollutants that have negative human and environmental health impacts locally and regionally. These

additional power plant pollutants (or, co-pollutants) include sulfur dioxide (SO2), nitrogen oxides (NOx), 

particulate matter (PM) and mercury (Hg). Once emitted, SO2 contributes to the formation of fine particle 

pollution (PM2.5) and NOx is a major precursor to ground-level ozone (O3). For human health, these co-pollutants 

contribute to increased risk of premature death, heart attacks, increased incidence and severity of asthma, and 

other health effects (see Table 1). For ecosystems, these co-pollutants contribute to acid rain; the over-fertilization

of many types of ecosystems, including grasslands, forests, lakes and coastal waters; ozone damage to trees and 

crops; and the accumulation of toxic mercury in fish (see Table 1). Therefore, policies intended to address climate 

change by reducing CO2 emissions, that also decrease emissions of SO2, NOx, and primary PM, can have

important human and environmental health co-benefits.

This study, led by Syracuse and Harvard universities, used existing estimates of energy sector emissions for a 

Reference Case and three alternative policy scenarios to quantify the amount and spatial distribution of resulting

changes in emissions, air quality, and atmospheric deposition of sulfur and nitrogen, and to a lesser extent of 

mercury by the year 2020. Each policy scenario reflects different designs for carbon standards with varying 

stringency and flexibility. Given that the analysis was conducted prior to the introduction of the EPA standards, 

none of the three scenarios are likely to represent the exact standard proposed, but they bound a wide range of 

possible alternatives. From this analysis and ancillary supporting material, we draw the following conclusions 

(see Summary of Results on pages 24-26 for details):

1. Strong carbon pollution standards for existing power plants would decrease emissions of co-pollutants that 

contribute to local and regional air pollution by more than 750,000 tons per year by 2020 compared to “businessas-usual” shown in the Reference Case.

2. The model results show that by decreasing emissions of co-pollutants, a strong carbon pollution standard would

improve air quality and decrease the deposition of harmful pollutants. It is well-documented that the air pollution 

reductions estimated here have human health and ecosystem benefits.

3. The model results indicate that, with a strong carbon standard, air quality and atmospheric deposition

improvements would be widespread with every state receiving some benefit. The greatest improvements are 

projected for states in and around the Ohio River Valley as well as the Rocky Mountain region. 

4. Finally, the analysis suggests that the stronger the standards (in terms of both stringency and flexibility), the 

greater and more widespread are the benefits associated with decreased co-pollutants. It also shows that a weaker 

standard focused strictly on power plant retrofits could increase emissions and reduce air quality over large areas.
3

U.S. Power Plant Pollution: Emissions, Transport, and Effects

Power plants are the single largest source of carbon dioxide (CO2; 40%), sulfur dioxide (SO2; 73%), and 

mercury emissions (Hg; 49%) in the U.S. (NEI 2011). They are also the second largest source of 

nitrogen oxide emissions (NOx; 24%) (NEI 2011). Carbon pollution standards for existing power plants 

would not only help confront the challenge of global climate change, they would confer substantial 

additional local and regional benefits by reducing power plant emissions of these major co-pollutants by

up to 27% for SO2 and Hg and 22% for NOx in 2020 compared to a Reference Case. Importantly, the 

benefits reported here are additional benefits associated with the carbon standard beyond the emissions 

reductions that will occur with existing air quality policies and therefore do not represent doublecounting of benefits.

The 1990 Clean Air Act Amendments illustrate how public policy can facilitate cost-effective decreases in 

emissions of air pollutants. For example the SO2 allowance trading program resulted in decreased SO2 emissions 

from electric power plants of 68 percent between 1990 and 2010, from 15.9 million short tons to 5.1 million short 

tons (NEI 2011) at approximately 15 percent the original cost estimate (Chan et al. 2012). Despite these costeffective programs, current emissions and air pollution levels still pose considerable health and environmental 

challenges. In 2005, fine particulate matter (PM2.5), largely from SO2 and NOx emissions, were attributed to 

between 130,000 and 320,000 of premature deaths, 180,000 non-fatal heart attacks, 200,000 hospital and 

emergency room visits, 2.5 million of asthma exacerbations, and 18 million lost days of work, and other public 

health effects (Fann et al. 2012). Also in 2005, between 4,700 and 19,000 premature deaths, 77,000 hospital 

admissions and emergency room visits, and 11 million school absence days were attributed to ground-level ozone 

(Fann et al. 2012). In 2004, it was reported that over 100 million people in live in areas of the U.S. with ozone 

concentrations exceeding the 8-hour regulatory standard (USEPA 2004). In light of on-going concerns and 

mounting scientific research, EPA recently proposed to strengthen the National Ambient Air Quality Standards 

(NAAQS) for both fine particles and ground-level ozone. 

In addition to health effects, elevated ozone can cause crop and forest damage, decades of acidic deposition have 

eroded the buffering capacity of soils leaving forests and watersheds more sensitive to continued inputs of sulfate 

and nitrate, and once mercury enters a watershed it persists for thousands of years where it bioaccumulates in food

webs and contaminates wildlife and fish that people catch and consume. Moreover, sulfur deposition associated 

with acid rain can promote the conversion of mercury to methyl mercury, the form that most readily 

bioaccumulates in the environment. As a growth-limiting nutrient, elevated atmospheric nitrogen deposition can 

alter the structure and function of terrestrial and aquatic ecosystems.

In order to understand these widespread effects, it is important to characterize and quantify the linkages between 

power plant emissions, air quality, and the atmospheric deposition of pollutants. Once emitted from fossil-fuel 

fired power plants, SO2 and NOx react in the atmosphere to form sulfuric acid, nitric acid, and several secondary 

pollutants that have a cascade of health and environmental effects. Similarly mercury, after it is released to the 

atmosphere, can change chemical form and depending on its form be deposited in rain, snow, gaseous particles 

within kilometers from the source or circulate globally. The links between emissions, air pollution, and 

atmospheric deposition are briefly described below and are illustrated in Figure 1.

PM2.5 is fine particulate matter (PM) that can occur as primary PM that is emitted directly from a source or is 

formed in the atmosphere as secondary PM. Secondary PM is by far the largest fraction and is derived from 
4

precursor emissions such as sulfur dioxide (SO2), nitrogen oxides (NOx), volatile organic compounds (VOCs), 

and ammonia (NH3). Secondary formation occurs through gas-phase photochemical reactions or through liquid 

phase reactions in clouds and fog droplets in the atmosphere generally downwind of the source. Most PM2.5 in 

rural areas is secondary. It is estimated that approximately half of the PM2.5 in the eastern U.S. originates from 

sulfate associated with SO2 emissions. Particle pollution forms the major component of haze in cities and in iconic 

landscapes such as national parks.

Tropospheric ozone is ground level ozone, a major component of what is commonly referred to as 

“smog”. Ground-level ozone is not emitted directly into the air, it is formed in the atmosphere when 

anthropogenic emissions of NOx combine with VOCs and react in the presence of sunlight. Peak O3

concentrations generally occur in summer when higher temperatures and increased sunlight enhance O3 formation 

(Knowlton et al. 2004). While elevated ground level O3 is primarily a concern in urban and suburban areas, ozone

and the ozone precursors of NOx and VOCs can also be transported long distances by wind, causing high ozone 

levels in rural areas. Tropospheric ozone is also a greenhouse gas pollutant. Consequently, climate change 

mitigation measures that simultaneously reduce tropospheric ozone may generate additional climate benefits.

Acidic deposition is commonly referred to as “acid rain”. Acidic deposition is the transfer (deposition) of strong 

acids and acid-forming substances from the atmosphere to the surface of the Earth. Acidic deposition includes 

ions, gases, and particles derived from sulfur dioxide (SO2), nitrogen oxides (NOx), ammonia (NH3) emissions, 

and particulate emissions of acidifying and neutralizing compounds. Acidic deposition can originate from air 

pollution that crosses state and even national boundaries, and affects large geographic areas (Driscoll et al. 

2001b). 

Mercury deposition results from mercury emissions to the atmosphere from direct anthropogenic sources, such as 

power plants; secondary sources that are re-emissions of primary sources; and natural emission sources. 

Emissions can occur as elemental Hg, gaseous ionic Hg (reactive gaseous mercury), and particulate Hg. These 

different chemical forms exert significant control over the fate of atmospheric Hg emissions and is the reason that 

Hg can be a local, regional, or global pollutant, depending on the speciation of the emissions and the associated 

residence times in the atmosphere. While Hg emission sources are common in more urbanized areas, deposition is 

also enhanced in forested areas where landscape conditions can lead to high rates of bioaccumulation. Therefore 

Hg deposition can be harmful in both urban and rural environments (Driscoll et al. 2007).

Nitrogen (N) deposition results from 

emissions of both inorganic and organic 

nitrogen. The primary forms of inorganic 

N emissions are nitrogen oxides (nitric 

oxide and nitrogen dioxide, referred to 

collectively as NOx) and reduced N 

which includes ammonia (NH3). 

Nitrogen oxides result from the partial 

oxidation of N2 at high temperatures or 

from the release of N contained in fossil 

fuels during combustion. After it is 

emitted nitrogen can be transported

Figure 1: Linking emissions, air quality, deposition, and effects

Adapted from Driscoll et al. 2001a

PM2.5

Ozone (O3)
5

hundreds of kilometers before it is deposited to Earth in precipitation (wet deposition) and as gases and particles 

(dry deposition) (Driscoll et al. 2003).

Effects on Human Health and Ecosystems

The co-pollutants emitted by power plants have demonstrated and well-understood health and 

environmental consequences. These adverse effects have been extensively documented and summarized 

in the peer-reviewed literature. We summarize the major impacts and supporting scientific evidence in 

Table 1, below. While changes in air quality can result in nearly immediate improvements in human 

health, sensitive ecosystems that have been impacted by decades of elevated atmospheric deposition 

(acid, nitrogen, and mercury) take decades or more to recover and remain a challenge today.

Table 1: Summary of air pollution effects from power plants.

Emissions Pollutant Effects References

SO2 PM2.5 Human health:

Heart attack, chronic & acute bronchitis, 

lung cancer, asthma exacerbation, premature death

Pope et al. 1995, Woodruff et al. 1997, Pope et 

al. 2002, Cohen et al. 2004, Pope et al. 2004, 

Laden et al. 2006, Krewski et al. 2009, Pope et 

al. 2009, Cohen et al. 2010, USEPA 2011, 

Sulfur 

deposition 

(sulfate)

Ecosystems:

Acidification of soils and surface waters, 

reduced tree health and productivity in 

sensitive areas, reduced fish abundance 

and diversity, increased methyl mercury 

production, diminished views

Cass 1979, Gorham 1989, Charles 1991, Baker 

et al. 1996, Likens et al. 1996, DeHayes et al. 

1999, Driscoll et al. 2001, Driscoll et al. 2010, 

Greaver et al. 2012

NOx Groundlevel ozone

(NOx

emissions 

are ozone 

precursors)

Human health:

Difficulty breathing, coughing and sore 

throat, asthma exacerbation, emphysema, 

chronic bronchitis, increased infection risk, 

pre-mature death

Ecosystems:

Reduced tree health and forest 

productivity, reduced crop productivity, 

reduced visibility

Gong et al. 1986, Ostro and Rothschild 1989, 

Schwartz 1994, Schwartz 1995, Chen et al 

2000, Burnett et al. 2001, Gilliland et al. 2001, 

Jaffe et al. 2003, Bell et al. 2004, Gryparis et al. 

2004, Karlsson et al. 2004, Huang et al. 2005, 

Ito et al. 2005, Levy et al. 2005, Peel et al. 

2005, Schwartz 2005, Wilson et al. 2005, 

USEPA 2007, Jerrett et al. 2009, Larsen et al. 

2010, Mills et al. 2011

Nitrogen 

deposition 

(reactive N)

Ecosystems:

Over-enrichment of ecosystems, increased 

production and changes in species

Valiela 1997, Bricker et al. 1999,Valiela et al. 

2000, Fenn et al. 2003, Galloway et al. 2003, 

Pardo et al. 2011

Nitrogen 

deposition

(nitrate)

Ecosystems:

Acidification of soils and streams, reduced 

tree health and productivity in sensitive 

areas, reduced fish abundance/ diversity

Aber et al. 1995, Baker et al. 1996, Magill et al. 

1997, Driscoll et al. 2001, Aber et al. 2003

Mercury Mercury 

deposition 

and 

bioaccumulation

Human health:

Reduced IQ, memory deficits, reduced 

visual-spatial function, increased risk of 

heart disease

Fish & wildlife: decreased reproductive 

success, increased embryo/chick mortality, 

altered schooling/ flying/ walking, acute 

toxicity

Aulerich et al. 1974, Scheuhammer 1988, 

Salonen et al. 1995, Wiener and Spry 1996, 

Nocera and Taylor 1998, Guallar et al. 2002, 

NRC 2002, CDC 2004, Mahaffey et al. 2004, 

Trasande et al. 2005, Driscoll et al. 2007, Evers 

et al. 2007, Swain et al. 2007, Roman et al. 

2011, USEPA 2011 
6

111d Co-benefits Analysis: Policy Context and Approach

Policy Context

At the direction of a 2013 Presidential memo, the U.S. EPA is using its authority under section 111(d) of the 

Clean Air Act to issue standards that address carbon pollution from existing power plants. The Presidential memo 

to EPA states: “I direct you to use your authority under sections 111(b) and 111(d) of the Clean Air Act to issue 

standards, regulations, or guidelines, as appropriate, to address carbon pollution from modified, reconstructed, and 

existing power plants....” (White House 2014). Section 111(d) is a state-based program that is based on federal 

standard, or “emission guideline” (USEPA 2014a). The intent is for EPA to establish a federal standard and for 

states to design programs that fit the guidelines and achieve the necessary carbon dioxide reductions. 

Scope and Approach

A team of scientists is conducting the first integrated, spatially explicit study for the entire lower 48 U.S. states of 

the benefits to health and ecosystem services associated with different approaches to carbon pollution standards 

for existing power plants. The study: (1) highlights the fact that power plants emit many harmful and interacting

pollutants that degrade air quality; (2) illustrates the linkages between atmospheric pollution, and human and 

ecosystem health; and (3) shows how a strong carbon pollution standard has local, to regional, to global benefits

compared to alternatives. The study has three major parts (Figure 2). Part 1 results are summarized in this report.

The study uses existing estimates of power plant emissions for the Reference Case and three scenarios for the year

2020 to quantify additional changes in air quality (ozone and PM2.5) and atmospheric deposition of pollutants 

(sulfur, nitrogen, and mercury) beyond what would occur under existing air quality policies, using the Community 

Multiscale Air Quality (CMAQ) Model. 

In Part 1, parsed unit-level emissions output from the Integrated Planning Model (IPM) produced by the 

consulting firm ICF International were used as input to CMAQ. CMAQ was developed by the U.S. EPA and is 

used by EPA, states and other 

groups to conduct Regulatory 

Impact Assessments (RIA) and 

State Implementation Plans 

(SIPs), respectively (USEPA 

2014b), along with other 

applications. In this study 

CMAQ v.4.7.1 (the most 

currently widely available 

version) was used, based on 

EPA’s 2007/2020 modeling 

platform and year 2007 

meteorology from v.3.1 of the 

Weather Research and Forecast 

(WRF) model. The CMAQ 

model produces gridded air 

quality concentrations and 

Figure 2: Diagram of co-benefits of carbon standards study.
7

deposition rates for the entire lower 48 states of the U.S. on a 12-km CONUS domain. Changes in atmospheric 

concentrations and deposition of air pollutants are projected by simulating emissions, advection, diffusion, 

chemistry, and deposition for multiple pollutants and pollutant forms.

In Part 2, detailed air quality results will be used to quantify and compare the changes in health impacts across the 

U.S. from the different policy scenarios using the Benefits Mapping and Analysis Program (BenMAP), published 

by EPA. We anticipate these results will be available in late summer 2014.

In Part 3 air quality and atmospheric deposition results will be used to estimate environmental benefits and 

changes in ecosystem services using various models. This is likely to include recovery of streams and forests from 

acid rain, reduced ozone damage to crops and timber, and improved visibility in focal landscapes. We anticipate 

these results and a full report on the three parts will be released in September, 2014.

Carbon Pollution Standards: Reference Case & Policy Scenarios

To estimate changes in air quality, the CMAQ model requires detailed emissions information from 

power sector models for a future year for a Reference Case and each policy scenario. Output from 

EPA’s Integrated Planning Model (IPM; US EPA 2014c) is often used to run CMAQ. Given the focus of 

this study is to characterize and quantify changes in co-pollutants and the consequences for human 

health and ecosystems, IPM results from other studies were used as policy scenarios. IPM results for a 

Reference Case and three alternatives were acquired from the firm ICF International. The Reference 

Case is based on the Energy Information Administration’s Annual Energy Outlook 2013 and 

incorporates the implementation of all existing air quality policies. The IPM policy runs include two 

additional scenarios commissioned by the Bipartisan Policy Center (BPC) and one commissioned by the 

Natural Resources Defense Council (NRDC), representing a range of policy options.

The three policy scenarios and associated IPM runs were selected from among a suite of alternatives 

independently developed by either BPC or NRDC. The three scenarios selected represent different stringencies 

(represented in these scenarios as an emissions rate in tons of CO2/MWh) and flexibility (represented by options 

available for compliance and extent to trading or averaging is allowed). The scenarios therefore bound a range of 

possible options available for controlling CO2 emissions from power plants and offer insights for understanding

and quantifying the consequences for co-pollutants. The scenarios were selected as researchable alternatives and 

do not represent preferences of the authors of this report. Importantly, none of the options include a strict “massbased” standard or carbon budget in tons of CO2/year which has been proposed by other groups (see Phillips 

2014). A mass-based alternative would be a useful scenario to analyze in future studies but IPM results for this 

alternative were not available at the time of this analysis. Moreover, it has been pointed out that EPA or the states 

can convert emissions rate-based standards to a mass-based standard by using projected generation levels and the 

performance standard to calculate a corresponding CO2 emissions budget for each state (Burtraw 2013).

Scenario Descriptions

The assumptions for the Reference Case and three scenarios are described briefly here and are depicted in Figure 

3. More information on the Reference Case and Scenario #2 can be found at: http://www.nrdc.org/air/pollutionstandards/. 
8

Reference Case was developed jointly by BPC and NRDC. It is benchmarked to the Energy Information 

Administration’s Annual Energy Outlook of 2013, which projects lower electrical demand and, thus, lower CO2

emissions compared to 2012. It also assumes full implementation of the current clean air policies adopted by EPA 

(see Figure 3). By comparing changes in air quality under each of the policy scenarios to the Reference Case, 

added benefits are quantified and double-counting is avoided.

Scenario #1 (Low/Low) is referred to as the “Unit Retrofit” scenario by BPC. Scenario #1 is equivalent to an 

emissions rate-based standard that uses improvements in heat rates at existing coal-fired power plants to comply 

with the carbon standard. It could be described as a low stringency alternative with low flexibility limited to 

changes that can be made “inside the fenceline” of individual power plants. Heat rate (Btu/kWh) is a measure of 

power plant efficiency. This scenario is based on the idea that a more efficient power plant will burn less fuel for 

the electricity it produces and will therefore emit less CO2 per megawatt of energy. The scenario uses “best-inclass” heat rates for different coal plant categories based on the unit’s capacity, fuel type, steam cycle, and boiler 

type. Coal-fired power plants then have to achieve an emissions rate equivalent to what would be achieved if they 

closed the gap between its unit-specific heat rate and the best in class heat rate by 40 percent. Under this scenario,

the fleet-wide average heat rate would improve 4 percent. This scenario results in a national average emissions 

rate of 2000 lbs/MWh for coal and 1000 lbs/MWh for gas; only a modest decrease from current emissions rates.

Scenario #2 (Moderate/High) is referred to as the “Moderate Full-Efficiency” scenario in Lashof and Yeh (2014). 

Scenario #2 is based on a flexible system-wide approach that achieves CO2 emissions reductions through a statespecific rate-based performance standard for existing power plants. It is a moderate stringency scenario with high

compliance flexibility. For 2020, the national emission rate targets are 1,500 lbs/MWh for coal and 1,000 

lbs/MWh for gas. This scenario allows additional renewable energy and energy efficiency to count toward 

compliance. It also allows emissions averaging across all fossil units in a state and states may opt-in to interstate 

averaging or credit trading. The scenario assumes energy efficiency is available at a total resource cost of 4.2 to

5.8 cents/kWh (Lashof 2013, Lashof and Yeh 2014). Though details are not specified, this scenario allows states 

to develop alternative plans, including mass-based standards, provided they achieve equivalent emissions

reductions (Lashof 2013). More information on the assumptions for Scenario #2 can be found in the technical 

appendices at: http://www.nrdc.org/air/pollution-standards/. 

Scenario #3 (High/Moderate) is referred to as the “A4” scenario by BPC. It requires supply-side electric sector 

CO2 reductions that can implemented up to a cost of $43 per metric ton in 2020. In that way, it is modeled to 

reflect what might happen if there was a national tax on CO2 emissions from power plants that is the same as (and 

increases with) the estimated social cost of carbon (Interagency Working Group 2013). It is a high stringency 

scenario with moderate compliance flexibility. In 2020, it results in average national emissions rates of 1200 

lbs/MWh for coal-fired power plants and 850 lbs/MWh for gas. The compliance options that are implemented are 

limited to changes up to the specified cost per ton and include on-site heat rate improvements, co-firing or 

converting to lower emitting fuel (i.e., natural gas or biomass), or shifting generation dispatch (the order in which 

power plants are called to operate in response to changing electricity demand) to favor lower carbon emitting 

electrical generation sources. However, demand-side energy efficiency is not included as a means of reducing 

emissions for this preliminary modeling scenario (Macedonia 2014).
9

Reference Case

Policy Assumptions:

 All current air quality policies fully implemented

 No carbon pollution standards

Included:

 EIA 2013 Annual Energy Outlook determines energy demand

 Mercury and Air Toxics Standards (MATS) implemented

 Clean Air Interstate Rule implemented, including Phase II in 2015

 Regional Greenhouse Gas Initiative (RGGI) model rule for emissions trading included (w/out NJ)

 CA Assembly Bill 32 (AB32) included

 Regional haze rule included

 Wind power production tax credit (PTC) expires

 Onshore wind costs: DOE/LBL 2012 Wind Technologies Report 

 Nuclear units re-licensed, 20-year extension

111d Scenarios

Policy assumptions:

 All current air quality policies fully implemented as in the Reference Case

 Carbon pollution standards adopted under section 111d for existing power plants

Scenario 1: 

Low/Low

Scenario 2: 

Moderate/High

Scenario 3: 

High/Moderate

 

Low stringency, low flexibility and energy efficiency Moderate stringency, high flexibility and energy

efficiency

High stringency, moderate flexibility and energy 

efficiency

Stringency estimate: 

2000 lbs/MWh coal; 1000 lbs/MWh gas

Stringency benchmark: 

1500 lbs/MWh coal; 1000 lbs/MWh gas

Stringency estimate: 

1200 lbs/MWh – coal; 850 lbs/MWh

Compliance options:

 Limited to on-site carbon emission rate reductions

 Power plant efficiency/heat rate upgrades

 Modest natural gas & biomass co-firing

Compliance options:

 Power plant efficiency/heat rate upgrades

 Co-firing with lower-carbon fuels 

 Dispatch changes to lower-carbon generation 

sources

 State/interstate averaging and trading 

Compliance options: 

 Power plant efficiency/heat rate upgrades

 Co-firing with lower-carbon fuels 

 Dispatch changes to lower-carbon generation 

sources

Energy efficiency:

 Only efficiency measures at the power plant included

Energy efficiency:

 Full supply-side and demand-side (end-user) 

energy efficiency included.

Energy efficiency:

 Supply-side efficiency (power plant and 

transmission lines).

Figure 3: Reference 

Case and scenario 

assumptions. 
10

0

100

200

300

400

500

600

700

800

900

Nuclear Hydro Wind Biomass New Energy

Efficiency

Other

Renewables

TWh

Renewable Generation and Efficiency in 2020

Reference case

Scenario 1

Scenario 2

Scenario 3

0

200

400

600

800

1000

1200

1400

1600

1800

Combined Cycle

(Gas)

Combustion

Turbine (Gas)

Coal (without

CCS)

Biomass cofiringCoal (with CCS)

TWh

Fossil Fuel Generation in 2020 

Reference case

Scenario 1

Scenario 2

Scenario 3

CCS = carbon 

storage and 

sequestration

Figure 4a: Power generation by scenario (terawatt/hours).

Figure 4b: Power generation by scenario (terawatt hours).
11

Carbon Standard Scenarios: Simulation of Power Generation and Emissions

For the Reference Case and three scenarios described above, ICF International used the IPM model to 

simulate changes power generation and to estimate resulting emissions of CO2, SO2, NOx, primary PM, 

and mercury for 2417 unique power plants in the U.S. (Lashof 2013, Lashof and Yeh 2014, Macedonia 

2014). The modeled shift in generation for fossil fuel, renewable, and other sources are shown in Figures 

4a and b. Notably, Scenario #1 increases the generation from coal plants without carbon capture and 

storage (CCS). Scenario #2 is the only scenario that includes increased energy efficiency.

The emissions results in Figures 5a and b show the annual emissions of CO2 and co-pollutants from the 

power sector for each scenario. The results for CO2 emissions are summarized in Table 2. Scenario #1, 

which results in modest CO2 reductions by implementing only improvements “inside the fenceline”, 

results in increased annual SO2 emissions compared to the Reference Case in 2020. Scenario #2 achieves 

a 27% decrease in annual emissions of SO2 and Hg and a 22% cut for NOx compared to the Reference 

Case. Similar reductions are achieved by Scenario #3. 

Table 2: Change in carbon dioxide emissions from power sector in 2020 by scenario.

Scenario From 2005 levels From Reference 

Scenario #1 

(Low/Low)

-17.4% -2.2%

Scenario #2 

(Moderate/High)

-35.5% -23.6%

Scenario #3 

(High/Moderate)

-49.2% -39.8%

Three performance measures were then used in this study to compare the three scenario emissions results 

and to determine the highest-performing scenario among the three with respect to the co-pollutants 

considered in this study (Table 3). Importantly, this comparison of performance measures does not 

represent a full economic or cost-benefit analysis for the scenarios. The performance measures show that 

Scenario #2 resulted in the largest decrease in SO2 and NOx emissions per ton of CO2 reduced, while still 

achieving lower annual total system costs than the Reference Case. Total system costs are based on fuel 

costs, operations and maintenance, and capital costs (Lashof and Yeh 2014, BPC 2014). Note that the 

lowest cost option (Scenario #1) results in increased SO2 plus NOx emissions. Scenario #3 achieved lower 

SO2 and NOx reductions per ton of CO2 reduced and at a much higher cost. Based on these performance 

measures, Scenario #2 was selected to illustrate the air quality and atmospheric deposition benefits of a 

strong cost-effective standard that achieves substantial emission decreases for CO2 and the co-pollutants. 

Results are available for the other scenarios as well.
12

.

Performance 

Measures

SO2+NOX

reduced/CO2

reduced

(TST/MMST)1

Incremental Total 

System Costs

$000,000 

(in US 2012$)

Incremental Total System Cost2/MMST 

CO2 reduced

$000,000 

(in US 2012$)

Scenario 1 -0.22 -$1,180 -$23.40

Scenario 2 1.46 -$472 -$0.89

Scenario 3 0.84 $33,541 $37.41

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

Total Hg

Pounds

Annual Power Sector 

Mercury Emissions 

in 2020

0

500

1,000

1,500

2,000

2,500

MMST (CO2) and TST (SO2 and NOx)

CO2 SO2 NOx

Annual Power Sector Emissions in 

2020

Reference case

Scenario 1

Scenario 2

Scenario 3

MMST = 

million short 

tons

TST = thousand 

short tons

Table 3: Scenario comparison relative to the Reference Case in terms of SO2 +NOx emitted per units 

CO2 controlled, incremental system costs and incremental costs per mass of CO2 controlled.

Performance 

Measures

SO2+NOX

reduced/CO2

reduced

(TST/MMST)

Incremental Total 

System Costs

$000,000 

(in US 2012$)

Incremental Total System Cost/MMST 

CO2 reduced

$000,000 

(in US 2012$)

Scenario 1 -0.22 -$1,180 -$23.40

Scenario 2 1.46 -$472 -$0.89

Scenario 3 0.84 $33,541 $37.41

Figure 5a & b: Air pollution emissions by scenario (million short tons, thousand short tons, and ponds).

1

TST= thousand short tons, MMST = million short tons.

2

Total system costs are based on Lashof and Yeh (2014) for Scenario #2 and on Macedonia (2014) for Scenario #1 

and #3. Costs include fuel costs, operations and maintenance, and capital costs.
13

Air Quality and Atmospheric Deposition Results

The results from the CMAQ model show marked differences in air quality and atmospheric 

deposition among the three scenarios. With respect to the magnitude and direction of change 

compared to the Reference Case, the air quality and atmospheric deposition results for the three 

scenarios parallel the annual emissions results described above. The lowest improvements and 

some increased impacts occur in Scenario #1 with greater improvements for the various 

pollutants occur for Scenario 2 and #3. The results underscore the fact that different options for

carbon standards can have widely varied consequences for associated air pollution. The details of 

the carbon standard will exert considerable influence on the health and environmental benefits 

that accrue to states and local communities.

Scenario #1, the low stringency/low flexibility heat-rate option results in increased SO2 emissions (+3%)

and minimal decreases in NOx and mercury emissions (-3% for each). As a result, there is increased sulfur 

deposition (Figure 6a) and higher fine particle pollution (PM2.5) (Figure 6b) across large areas compared 

to the Reference Case with little to no improvement in most of the remaining area. This result is likely

due to widespread “emissions rebound” at numerous fossil-fuel-fired power plants in the U.S. fleet. 

Emissions rebound refers to the increase in emissions that can occur when higher-emitting plants are 

made more efficient and therefore rise in the dispatch order and run more frequently and for longer 

periods than in the Reference Case. This emissions rebound effect has been anticipated by others (Phillips 

2014) but this is the first time the consequences for air quality at the state level have been quantified and 

mapped. 

The CMAQ results for Scenario #1 show that if a carbon standard has low stringency and compliance 

limited to strictly “inside the fenceline” options , emissions of co-pollutants could increase, leading to 

increased pollutant loading and diminished air quality and potential adverse effects on public and 

environmental health. The CMAQ results of Scenario #2 show that a carbon standard that is stringent and 

flexible enough to promote a shift toward cleaner sources will reduce emissions of co-pollutants, achieve 

improved air quality and decreased atmosphere deposition of pollution, and lead to marked health and 

environmental benefits at the state level. The following maps and tables depict the projected changes in 

air quality 2020 associated with Scenario #2 (Figures 7 to 11; Table 4-8). Scenario #3 had similar air 

quality and atmospheric deposition results but at a much higher cost. The number of states with increases, 

no change, and decreases in average statewide air pollution levels compared to the Reference Case in 

2020 are depicted for all three scenarios in Figures 7c, 8c, 9c, and 10c.
14

Figure 6b: Projected changes in average annual PM2.5 from theReference Case under Scenario #1 in 2020 

(micro-grams per cubic meter).

Figure 6a: Projected changes in total annual sulfur deposition under Scenario #1 in 2020 

(kilograms per hectare-year).
15

Figure 7a & b: Average annual PM2.5 in 2020 for the Reference Case and change from this condition in 

Scenario #2 (micro-grams per cubic meter).

Figure 7a

Figure 7b
16

Top 15 States with Largest 

Decreases in Average Annual 

PM2.5

State

Scenario 

2

(μg/m3)

Mean 

Decrease

(μg/m3)

Ohio 7.66 0.22

Pennsylvania 5.86 0.22

DC 12.68 0.20

Maryland 6.79 0.20

W. Virginia 4.93 0.20

Illinois 7.40 0.19

Missouri 5.93 0.18

Delaware 6.57 0.18

Kentucky 5.97 0.18

Indiana 7.77 0.17

Arkansas 6.15 0.17

Tennessee 5.52 0.16

Iowa 6.22 0.16

Virginia 5.26 0.15

New Jersey 7.13 0.14

0

5

10

15

20

25

+0.01 to

+0.04

0 -0.01 to -

0.02

-0.03 to -

0.05

-0.06 to -

0.1

-0.11 to -

0.15

-0.16 to -

0.22

Number of states

Average statewide change (μg/m3)

State-level Changes in Average Annual PM2.5

in 2020

Scenario 1

Scenario 2

Scenario 3

Figure 7c 

Table 4
17

Figure 8a & b: Average summer (June 1 – August 31) peak 8-hr ozone for Reference Case and 

change in this condition for Scenario #2 (parts per billion).

Figure 8a

Figure 8b
18

Top 15 States with Largest Decreases 

in Average Summer Peak 8-hour 

Ozone

State

Scenario 

#2 (ppb)

Mean 

decrease

(ppb)

Kentucky 49.12 -0.79

Missouri 46.73 -0.75

Pennsylvania 44.75 -0.62

W. Virginia 46.85 -0.59

Indiana 49.67 -0.56

Arkansas 46.03 -0.54

Illinois 49.69 -0.52

Ohio 49.24 -0.51

Oklahoma 47.39 -0.48

Tennessee 49.11 -0.48

Colorado 52.66 -0.45

Alabama 45.13 -0.44

Iowa 46.15 -0.40

Wyoming 51.35 -0.40

Georgia 45.43 -0.39

0

5

10

15

20

25

30

35

40

+0.02 to

+0.01

0 -0.01 to -

0.05

-0.06 to -

0.1

-0.11 to -

0.2

-0.21 to -

0.3

-0.31 to -

0.4

-0.41 to -

0.79

Number of states

Average statewide change (ppb) 

State-level Changes in Summer Peak 8-hour 

Ozone in 2020

Scenario 1

Scenario 2

Scenario 3

Table 5

Figure 8c
19

Figure 9a & b: Average annual peak 8-hr ozone for Reference Case and change in this condition for 

Scenario #2 (parts per billion).

Figure 9a

Figure 9b
20

Top 15 States with Largest Decreases in Average

Annual Peak 8-hr Ozone 

State

Scenario 2

(ppb)

Mean 

Decrease

(ppb)

Kentucky 41.60 0.27

Missouri 39.92 0.27

Colorado 46.22 0.24

Alabama 41.47 0.22

Arkansas 40.86 0.21

W. Virginia 41.42 0.21

Oklahoma 42.10 0.21

New Mexico 46.00 0.21

Tennessee 42.39 0.20

Utah 46.35 0.20

Louisiana 42.43 0.20

Indiana 41.05 0.19

Pennsylvania 39.26 0.19

Illinois 40.72 0.19

Mississippi 41.16 0.18

0

5

10

15

20

25

30

35

40

+0.02 to

+0.01

0 -0.01 to -0.04 -0.05 to -0.09 -0.1 to -0.19 -0.20 to -0.27

Number of states

Average statewide change (ppb)

State-level Changes in Average Annual Peak 

8-hr Ozone in 2020

Scenario 1

Scenario 2

Scenario 3

Figure 9c

Table 6
21

Figure 10a & b: Total annual sulfur deposition in 2020 for Reference Case and change in this condition for 

Scenario #2 (kilograms S per hectare-year).

Figure 10a

Figure 10b
22

0

5

10

15

20

25

+0.01 to

+0.58

0 -0.01 to -

0.04

-0.05 to -

0.09

-0.10 to -

0.18

-0.19 to -

0.3

-0.31 to -

0.50

-0.51 to -

1.08

Number of states

Average statewide change (kg/ha-yr)

State-level Changes in Total Sulfur Deposition 

in 2020

Scenario 1

Scenario 2

Scenario 3

Top 15 States with Largest Decrease in 

Average Annual Total S Deposition 

State

Mean 

Decrease

(kg S/ha-yr)

Percent 

decrease

Pennsylvania 1.08 17.24%

W. Virginia 0.81 13.73%

Ohio 0.60 11.98%

Maryland 0.52 12.97%

Kentucky 0.38 9.20%

Delaware 0.36 10.94%

Illinois 0.36 8.79%

Rhode Island 0.35 8.90%

New Jersey 0.35 9.63%

Tennessee 0.34 11.45%

New York 0.34 9.64%

DC 0.34 6.52%

Missouri 0.34 10.36%

Michigan 0.31 10.14%

Virginia 0.29 9.67%

Figure 10c

Table 7
23

Top 15 States with Largest Decreases 

in Average Annual Total N Deposition

State

Mean 

Decrease

(kg N/ha-yr)

Percent 

decrease

Pennsylvania 0.14 2%

Indiana 0.12 1%

W. Virginia 0.12 2%

Missouri 0.11 1%

Kentucky 0.11 1%

Illinois 0.11 1%

Figure 11a & b: Total annual nitrogen deposition for Reference Case and change in this condition for 

Scenario #2 (kilograms N per hectare-year).

Figure 11a

Figure 11b

Table 8
24

Summary of Results

This study highlights that, in addition to addressing global climate change, a strong carbon 

pollution standard for existing power plants will reduce power plant emissions of co-pollutants 

that contribute to local and regional air pollution.

Details: The top-performing moderate stringency/high flexibility option depicted in Scenario #2 

cuts CO2 emissions from the power sector by 35.5% from 2005 levels by 2020 and by 23.6% 

from a 2020 Reference Case. Scenario #2 also decreases power plant emissions of co-pollutants 

by the following amounts in 2020 compared to the Reference Case:

 SO2 emissions decrease by 474,000 short tons/year (-27%)

 NOx emissions decrease by 299,000 short tons/year (-22%)

 Hg emissions decrease by 3,334 pounds/year (-27%) 

By contrast, the study also highlights how a carbon standard with low stringency and low 

compliance flexibility limited to actions “inside the fenceline” as depicted in Scenario #1 could 

result in the following changes in power sector emissions by 2020 compared to the Reference 

Case:

 SO2 emissions increase by 50,000 short tons/year (+3%) 

 NOx emissions decrease by 39,000 short tons/year (-3%)

 Hg emissions decrease by 414 pounds/year (-3.3%)

2. The model results show that by reducing the emission of co-pollutants, a strong carbon 

pollution standard will improve air quality and decrease the deposition of harmful pollutants. It 

has been well-documented that even modest improvements in air quality can bring important

human health and ecosystem benefits. 

Details: The CMAQ model runs quantify by how much and where air quality and atmospheric 

deposition would change under each of the three scenarios. It is clear from the results that the air 

quality improvements achieved in 2020 under a strong carbon standard would have the added 

benefit of improving the health of people and ecosystems in states across the U.S. Specifically,

for Scenario #2:

 Average annual concentrations of fine particulate matter (PM2.5) at the state level will 

decrease by 0.0 to 0.22 μg/m3 with the top 15 states experiencing average decreases of

0.14 to 0.22 μg/m3. 

 Summertime average peak 8-hour ozone concentrations at the state level will decrease by 

0.01 to 0.79 ppb with the top 15 states experiencing average decreases of 0.39 to 0.79 

ppb.
25

 Annual average ozone peak 8-hour ozone concentrations at the state level will decrease 

by 0.01 to 0.27 ppb with the top 15 states experiencing average decreases of 0.18 to 0.27 

ppb.

 Total annual sulfur deposition at the state level will decrease by 0.0 to 1.08 kg/ha-yr with 

the top 15 states experiencing average decreases of 0.29 to 1.08 kg/ha-yr.

A complete health and ecosystem benefits analysis has not yet been conducted but past studies 

conducted by EPA for other proposed air pollution standards show that seemingly small 

improvements in air quality equate to substantial public health benefits. For example, the U.S. 

EPA’s Mercury and Air Toxics Rule, issued in 2011, would reduce annual average PM2.5

concentrations by an estimated 0.36 μg/m3and annual average 8-hr ozone concentrations by 0.2 

ppb. The U.S. EPA estimated annual health benefits of this rule of 7,600 avoided premature 

mortality cases (between 4,200 to 11,000), 4,700 avoided non-fatal heart attacks, 130,000 

avoided asthma attacks, 5,700 avoided hospital and emergency department visits, 540,000 days 

of missed work or school, and 3,200,000 restricted activity days. These health benefits were 

valued between $120 and $280 billion. In Part 2 of this report the health benefits and their 

economic value will be calculated nationally and for each of the lower 48 states and District of 

Columbia.

Ecosystems also benefit from decreases in air pollution and atmospheric deposition of sulfur and 

nitrogen. Reduced ground-level ozone will increase the health and productivity of crops and 

timber. The projected declines in sulfur deposition will contribute to the recovery of acidimpacted forest watersheds such as the Appalachian Mountain region. Nitrogen deposition is 

projected to decrease by only 1% to 2%, but there will be modest benefits associated to decreases 

in ecosystem eutrophication. Note that this analysis is based on total nitrogen deposition which 

includes both nitrate deposition (driven largely by emission of nitrogen oxides from fossil fuel 

combustion) and ammonium deposition (driven largely by agricultural emissions). The relative 

decrease under Scenario #2 would be expected to be approximately two times greater for nitrate 

deposition alone. 

3. The model results show that the air quality and atmospheric deposition improvements

associated with decreased emissions co-pollutants under a strong carbon pollution standard are 

widespread, with every state receiving some benefit. The largest decreases in pollution occur in 

the eastern US, particularly in states in and around the Ohio River Valley with notable 

improvements in Rocky Mountain region as well. 

Details: The CMAQ results for Scenario #2 provide spatially explicit results that show where the 

greatest improvements are likely to occur.

 States that are projected to benefit from the largest statewide average decreases in air 

pollution detrimental to human health (PM2.5 and peak annual and summer O3)
26

include: OH, PA, MD, WV, IL, KY, MO, IN, AR, CO, AL and WV (based on the top 

6 states for each pollutant).

 States that are projected to benefit from the largest statewide average decreases in air 

pollution detrimental to ecosystems (sulfur and nitrogen) include: PA, WV, OH, MD, 

KY, DE, IN, IL, and MO (based on the top 6 states for each pollutant).

 Most other states see marked improvements in both air quality and atmospheric 

deposition of pollutants that vary geographically.

4. Finally, the analysis suggests that the stronger the standards (in terms of both stringency and 

flexibility), the greater and more widespread the benefits will be from decreased co-pollutants. It 

also shows that a weaker standard focused strictly on power plant retrofits could increase 

emissions and reduce air quality over large areas. The resulting improvements in air quality

associated with a strong carbon pollution standard would have nearly immediate benefits by

reducing illness and premature deaths. Moreover, decreased air pollution will help to continue 

reversing the damage brought by years of acid, nitrogen deposition, and mercury deposition. In 

so doing, carbon pollution standards can protect public health and help restore forests, waters,

and wildlife, while also mitigating climate change.
27

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Acknowledgements 

The following entities provided useful information for this report and we gratefully acknowledge 

their contribution. Natural Resources Defense Council provided information on the assumptions 

in the power sector modeling for the Reference Case and Scenario #2. Bipartisan Policy Center 

provided information on the assumptions in the power sector modeling for the Reference Case, 

Scenario #1, and Scenario#3. ICF International provided IPM output for the scenarios. Sonoma 

Technology Inc. led the CMAQ model simulation. The author also thank the William and Flora 

Hewlett Foundation and the Grantham Foundation for the Environment for major support for this 

work through grants to the Science Policy Exchange through the Harvard Forest, Harvard 

University. 

The three-part Carbon Standards Co-benefit Analysis is a project of the 

Science Policy Exchange, an independent research consortium dedicated to 

increasing the impact of science on conservation and environmental policy.

The views expressed in this paper are those of the authors and do not necessarily reflect those of 

Syracuse University or Harvard University. This paper has not undergone formal review and 

approval. 

Suggested citation:

Driscoll, C.T, Buonocore, J., Reid, S., Fakhraei, H, and Lambert, K.F. 2014. Co-benefits of 

Carbon Standards Part 1: Air Pollution Changes under Different 111d Options for Existing 

Power Plants. Syracuse University, Syracuse, NY and Harvard University, Cambridge, MA. A 

report of the Science Policy Exchange. 34 pp.

Co-benefits of Carbon Standards Part 1: Air Pollution Changes under Different 111d Options 

for Existing Power Plants is available at: eng-cs.syr.edu/carboncobenefits