Review of Secondary National Ambient Air Quality 

Standards for Oxides of Nitrogen and Sulfur

Discussion

For OMB/Interagency Review

June 14, 2011

Office of Research and Development

National Center for Environmental Assessment

Point of Contact: Lydia Wegman

Health and Environmental Impacts Division

OAQPS/OAR
Overview and Schedule

 EPA is under a court ordered schedule to sign a proposed rule by July 12, 

2011 and a final rule by March 20, 2012

 Includes an extension of 18 months granted by the plaintiffs on the basis that we 

needed more time to develop an ecologically relevant standard

 Since review initiated in 2006, EPA has completed the following milestones:

 Integrated Science Assessment (ISA, 2008)

 Risk and Exposure Assessment (REA, 2009)

 Staff Policy Assessment (PA, 2011)

1
Major points and issues

 EPA staff’s Policy Assessment presents a new multi-pollutant standard that is 

unique in that it depends on atmospheric and ecological modeling to specify the 

terms of an equation that characterizes the relationships between air 

concentrations, deposition, and aquatic acidification effects

 The terms of the equation need to be quantified on an area-specific basis, because 

ecosystem sensitivity varies across the nation due to variable geologic, hydrologic, and 

environmental factors

 The significant The significant challenge challenge is to translate is to translate spatially spatially variable variable effects into a national effects into a national 

standard

 There are necessarily many uncertainties associated with this approach that 

need to be considered

 The Clean Air Scientific Advisory Committee (CASAC) is supportive of setting a 

new standard as developed in the Policy Assessment

2
Background

 Current secondary standards set to protect against direct effects of gaseous 

NO2and SO2on vegetation, not deposition-related effects:

 Annual NO2standard set at 0.053 ppm

 3-Hour SO2standard set at 0.5 ppm

 Clean Air Act Acid Rain Program (Title IV) has reduced emissions of SO2and 

NOxfrom utilities, but was not designed to fully address aquatic acidification in 

sensitive ecoregions across the country

 Basically an emissions-based program – lacks a linkage between emissions and 

effects – in contrast to air quality-based standards

 Despite observed improvement, many reports (e.g., NAPAP) find continuing 

adverse effects exist in many acid sensitive areas

 Sampled lakes exhibit improved water quality trends -- 12% in the Northeast and 

56% in the Adirondack mountains

 Recovery significantly lagging in southern Appalachian mountains 

 A new deposition-related NAAQS for oxides of nitrogen and sulfur would be 

consistent with the NAS 2004 “Air Quality Management in the U.S.” 

recommendations for a multiple pollutant management framework and for 

development of deposition-related secondary standards

3
Pollutants considered in ORD’s 

Integrated Science Assessment (ISA)

 Oxides of nitrogen (N) and sulfur (S) -- the “criteria pollutants” -- include:

 SO2 and NO2 – indicators for current secondary standard

 Particulate sulfate (SO4) combined with SO2is defined as SOx

 NOy – includes the transformation products from emissions of oxides of 

nitrogen (e.g., nitric acid and particulate nitrate)

4

 Forms of N which are not criteria pollutants, but that contribute to N 

deposition, include:

 Ammonia gas, NH3

 Ammonium ion, NH4

Together referred to as 

reduced nitrogen, NHx
Conclusions in ISA

 ISA concludes that the evidence is sufficient to infer a causal relationship between:

 Gas-phase SO2, NO2, and direct injury to vegetation (basis for current standards)

 Acidifying deposition of N and S and effects in aquatic and terrestrial ecosystems (e.g., 

decreased species diversity and fish mortality resulting from aquatic acidification

 N deposition and effects on nutrient status in terrestrial, wetland, freshwater aquatic, and 

coastal marine ecosystems

 With regard to aquatic acidification, ISA presents compelling evidence that oxides of N 

and S both cause ecosystem acidification in the U.S.

5

 Thousands of peer-reviewed scientific papers

 Several national assessments [e.g., National Acid Precipitation Assessment Program 

(NAPAP) reports (1980 to present)]

 Well-established water quality and biological indicators

 Well-established models that link deposition, water quality, and effects on biota

 ISA concludes that aquatic acidification is a well-documented process caused by multiple chemical 

species that constitute N + S deposition, which occurs under current air quality conditions

 Ecosystem sensitivity to acidification varies across the nation according to present and historic 

N + S deposition, geologic/soil/hydrologic factors and vegetation type
EPA Staff Policy Assessment: Focus on 

Aquatic Acidification

 Based on the ISA and the case-study analyses in the REA, the Policy Assessment 

and CASAC concluded it is appropriate to focus on aquatic acidification in this 

review:

 Science is more complete than for other effects

 Atmospheric inputs are the dominant contributions

 Focus on aquatic acidification relies on two important concepts:

 Acid neutralizing capacity (ANC)

 Critical Load

• The amount of acidifying deposition (of N and S) that a water body can handle to 

maintain a specified ANC level, based on present knowledge

• Critical loads are influenced by a system’s natural ability to neutralize acid inputs 

6

• A water quality value that reflects ability 

of ecosystem to buffer acidifying inputs

• Well related to effects on aquatic 

ecosystems and to deposition of N and S
Regional Sensitivity to Acidification

7

 Ecosystem sensitivity varies across the nation, predominantly due to variability of geologic material 

(bedrock and soils) which buffers acidifying deposition. 

 Focus of this secondary standard is on aquatic systems located in relatively pristine, rural 

environments – typically, high elevation clear water bodies supporting trout fisheries

 Map based on water quality data available through EPA monitoring programs
Conceptual model an aquatic 

acidification standard 

 Unlike most other NAAQS that are based on the direct relationship between 

pollutant concentrations in the air and effects on health or welfare, this standard 

necessarily involves multiple linkages, since aquatic effects are not directly 

related to concentrations of oxides of N and S in the ambient air

Ecological effects and 

ecological indicator , ANC

Linking atmospheric

oxides of S and N deposition 

to ecological indicator

Linking deposition to “allowable” 

concentrations of ambient air 

indicators of oxides of N and S

necessarily involves multiple linkages, since aquatic effects are not directly 

related to concentrations of oxides of N and S in the ambient air

 Linkage between ecological effects and deposition of oxides of N and S is 

characterized by critical load modeling

 Linkage between deposition and air concentrations of oxides of N and S is 

characterized by atmospheric modeling that translates emissions of N and S into 

estimates of both ambient concentrations and related deposition

 Model also takes into account deposition of N from reduced forms of nitrogen 

(e.g., ammonia) that contributes to the aquatic effects but is not part of the 

“criteria” pollutants addressed by this standard

8
Key Uncertainties

 Data gaps:

 Limited ambient measurements of NOy

• National multiple-pollutant network (NCORE), but monitors are not located in targeted 

locations in sensitive ecoregions

 Relatively sparse coverage in mountainous west, including measurements of:

• Air concentrations

• Water quality

• Water quality values determine ecoregion sensitivity

• Small sample sizes impair our ability to characterize the “representativeness” of the 

available data

 Limited measurements of reduced forms of N (ammonia and ammonium)

 Only a small new ammonia network (AMON) exists

 Emission inventories are subject to considerable uncertainty

9
Key Uncertainties (cont.)

 Reliance on critical load and atmospheric modeling inherently introduces 

additional uncertainties, for example:

 Model results difficult to evaluate due to lack of relevant observations

• Critical load modeling requires evaluation of pre-industrial conditions (e.g., in terms of 

base cation supply)

• Atmospheric modeling (CMAQ - Community Multi-scale Air Quality Model) involves 

evaluation evaluation of relationship of relationship between between N and S in ambient N and S in ambient air and associated air and associated deposition deposition (i.e., 

transference ratios)

• Modeled dry deposition

 Air quality models exhibit over-prediction of SO2concentrations

• Although analyses indicate that this does not bias the modeled relationship between S 

deposition and ambient concentrations

 Use of steady state critical load models results in a simplification of key 

biogeochemical processes

 There is no obvious directional bias that would suggest uncertainties lead to 

under- or over-protection

10
Elements of an aquatic 

acidification standard 

Elements of the standard – analogous to all NAAQS:

 Indicators: NOy and SOx to be measured to determine if the standard is met

 Form: Aquatic Acidification Index (AAI) equation that relates a target ANC value 

to air concentrations to air concentrations as mediated as mediated by other deposition by other deposition-related related environmental environmental 

factors ...next slide

 Level: the target AAI value, based on ANC, that, in combination with the other 

elements of the standard, is judged to provide requisite protection

 Averaging time: annual standard, averaged over 3 to 5 years

Standard would be met when measured values of NOy and SOx are 

such that the calculated value of the AAI, averaged over 3 - 5 years, is 

greater than or equal to the level of the standard

11
Form of the standard

 Components of the form:

 AAI : calculated ANC expected to result over time from deposition 

associated with monitored NOy and SOx concentrations (i.e., ANC levels 

calculated by this equation may not in fact occur until many years after the 

related ambient concentrations are measured)

Aquatic Acidification Index (AAI) = F1– F2– F3 [NOy] – F4[SOx]

related ambient concentrations are measured)

 F1: natural ability of an ecosystem to neutralize N and S deposition

 F2: reduced nitrogen (ammonia gas and ammonium ion) deposition 

 F3, F4: factors that convert measured NOy and SOx in the ambient air to 

NOy and SOx deposition

 Need to define appropriate “ecoregions” over which these factors are 

defined because the value of each factor varies across the U.S. (next 

slide)

13 
 Omernik Ecoregion III classification scheme (developed in the 1980s by EPA) divides the 

U.S. into ecologically relevant regions (84 regions cover the continental U.S.)

 Based on common vegetation, geology, soils, and hydrological characteristics

 Basis for nutrient criteria document developed by EPA’s Office of Water

Defining appropriate ecoregions

13
Acid sensitive and non-sensitive ecoregions

 While the standard would apply nationwide, categorizing ecoregions as relatively 

acid-sensitive (22 areas) or non-sensitive (~62 areas) serves to identify areas 

that will benefit most from reductions in NOy and SOx deposition (similar to 

“susceptible populations” for health-based standards)

 Categorization based on water quality data and land use categories (naturally 

acidic and managed areas categorized as relatively non-sensitive)

14
Ecoregion-specific factors (F1 – F4 )

 Each ecoregion has a unique set of factors, F1 – F4, based on data averaged across 

the ecoregion

 F1 is determined based on selecting a “representative” critical load of sampled water bodies 

for each ecoregion (from a national database of over 9,000 sampled water bodies)

• For acid sensitive regions, a representative critical load is defined in terms of a specific 

percentile of the distribution of critical loads that have been calculated for each ecoregion

• Use of a higher percentile (e.g., 90th percentile) would be more protective than a lower 

percentile (e.g., 70th percentile) 

• Some uncertainty exists as to how representative the sampled water bodies are within 

each ecoregion

• For relatively non acid-sensitive ecoregions, consider using a national default critical load 

based on averaging the 50th percentile values from all such ecoregions

• This different approach is intended to avoid potential for over protection in relatively 

non acid-sensitive ecoregions

 F2, F3, and F4 are based on CMAQ modeling, which translates emissions of N and S into 

ambient concentrations and deposition

 EPA would calculate and codify (as part of NAAQS rulemaking) F values for each 

ecoregion and provide tables – update every 5 years

15
Level of the standard

 Policy Assessment focused on a range of values from 20 – 75 μeq/L

 Entire range would afford some degree of protection from long-term, chronic aquatic 

acidification, with more protection afforded at higher levels

 Higher levels within range would afford:

• Increasing protection for episodic acidification (e.g., spring snowmelt)

• Decreasing time frames for some water bodies to reach a target ANC

16
Alternative Standards

 Level and form together determine the degree of protection afforded by standard

 Alternative levels (20 – 75 μeq/l) and forms (F1 based on 70th to 90th percentile) 

were assessed in terms of whether acid-sensitive ecoregions would likely not meet 

alternative standards

 Anticipate that all non acid-sensitive ecoregions would meet this range of standards

Summary of the number of acid sensitive ecoregions (out of 22) not likely to meet 

alternative standards based on a 2005 CMAQ simulation.

ANC Percentile Number

20

70 4

75 5

80 5

17

20

85 8

90 13

35

70 5

75 5

80 9

85 11

90 13

50

70 6

75 8

80 13

85 14

90 15

75

70 10

75 11

80 14

85 15

90 22
Summary of Staff Conclusions

in Policy Assessment

 With regard to adequacy of current standards:

 Appropriate to retain the existing NO2and SO2secondary standards to protect against gasphase effects

 Do not provide adequate protection for effects associated with deposition

 Consideration should be given to establishing a new ecologically relevant standard to 

provide increased protection from deposition-related effects of oxides of N and S on 

sensitive ecosystems

 With regard to establishing a new ecologically relevant standard(s):

 Consider establishing a multi-pollutant standard that addresses the combined effects of 

oxides of N and oxides of S specifically to provide increased public welfare protection from 

aquatic acidification in sensitive ecosystems. 

• Expressed as: AAI = F1– F2– F3 [NOy] – F4[SOx]

• Measured indicators: NOy and SOx

• Annual average standard based on 3-5 years of data

• Ranges of level and form from 20 to 75 μeq/l and 70th to 90th percentile, respectively

18
CASAC Conclusions and Recommendations

 With regard to adequacy of current standards (Dec. 9, 2010 letter):

 Current standards “should be retained to protect direct adverse impacts to vegetation”

 The ISA, REA, and PA demonstrate that adverse impacts to aquatic acidification are also 

occurring due to deposition of oxides of N and S

 “Levels [of current standards] are not sufficient, nor the forms of those standards 

appropriate, to protect against adverse depositional effects; thus a revised NAAQS is 

warranted”

 With regard With regard to a new aquatic acidification to a new aquatic acidification standard standard (April 12, 2011 (April 12, 2011 draft letter): letter):

 “The PA develops a framework for a multi-pollutant, multimedia standard that is 

ecologically relevant and reflects the combined impacts of these two pollutants....”

 “CASAC generally supports the potential choices/ranges presented by EPA staff on the 

indicators, form, averaging time, and level that should be considered . . . “

 Combinations of levels and percentiles in the Policy Assessment “provide the Administrator 

with a broad but reasonable range of minimally to substantially protective options for the 

standard.”

 CASAC noted important uncertainties that should be considered in this review 

and addressed in future analyses and reviews

 Selection of percentiles “could be conservatively biased”

 “. . . a more rigorous model evaluation should have been conducted to provide more 

confidence in the use of the models.”

19
Monitoring considerations 

• Proposing a 3-5 year field program to collect additional air measurements to 

“pilot” execution of AAI calculations using measured data

• CASTNET provides a solid infrastructure

– Largely rural locations, originally designed to support acidification assessments –

greater coverage in eastern U.S.

 Most acid sensitive ecoregions have 1 - 3 sites

– Provides measurements Provides measurements of 2 of 3 potential of 2 of 3 potential indicators indicators (SO , SO ), and supplementary 2, SO4), and supplementary 

data (nitrate, ammonium, ammonia – limited locations)

• Approved federal reference or equivalent methods (FRM/FEM) are not available 

for sulfate and NOy

– EPA-ORD is working to approve an NOy method and other required CASTNET SO2

and SO4 methods; these methods would be adopted by rule upon ORD approval

20
 A 3 - 5 year program to conduct measurements of potential indicators and 

supporting species and to support design of an appropriate monitoring network

 Monitoring component - evaluation period to work out design issues 

 Deployment of 2-3 sites in each of 3-5 high priority ecoregions – beginning in 6/2013

• Add NOy monitors to existing CASTNET methods

• CASTNET includes SOx (SO2 and SO4), as well as ammonium and total nitrate

• Use CASTNET sites that measure ammonia and add ammonia measurements at sites lacking

• Continuous SO2 and periodic measurements of speciated NOy

 Provide actual observed data to use in the AAI equation

Would afford an opportunity to work out network design and process issues 

Additional field studies: pilot program

 Would afford an opportunity to work out network design and process issues 

• Would provide data to analyze spatial variability within an ecoregion – assist network design

• Explore the partnership across CASTNET, EPA and the states

 Data analysis component

 In parallel with monitoring, use source apportionment modeling to explain which sources 

(sectors and location) are contributing – reinforce the multiple pollutant similarities with 

ozone, PM and haze

 Information could be used to support potential future implementation strategy 

development

 Would provide data to evaluate modeled characterization of reduced nitrogen deposition

 Stakeholder component

 Provide time to work in concert with state program administrators and other stakeholders 

to use the data to help formulate solutions to potential implementation challenges 21
22