Federal Reference Methods for NOy and p-SO4 for the New 

Combined NOx and SOx Secondary NAAQS

Research Plan

January 20, 2011

Project Period: July 2010 to July 2012

United States Environmental Protection Agency

Office of Research and Development

National Exposure Research Laboratory

Human Exposure and Atmospheric Sciences Division

Investigators:

Russell Long, Surender Kaushik, Tad Kleindienst, Robert W. Vanderpool, 

and Fred Dimmick
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I. Acronyms

AIM - Ambient Ion Monitor

APS - Aerodynamic Particle Sizer

CASTNET - Clean Air Status and Trends Network

FEM - Federal Equivalent Method

FP - Filter Pack

FRM - Federal Reference Method

GPT - Gas Phase Titration

H2O(g) - Water Vapor

HEASD - Human Exposure and Atmospheric Sciences Division

HNO2 - Nitrous Acid

HNO3 - Nitric Acid

IPN - Isopropyl Nitrate

NAAQS - National Ambient Air Quality Standard

NCORE - Multi-pollutant Network

NERL - National Exposure Research Laboratory

NH3 - Ammonia

NO - Nitric Oxide

NO2 - Nitrogen Dioxide

NOx - Oxides of Nitrogen (NOx = NO + NO2)

NOy - Total Reactive Nitrogen

NOz- Reacted Oxides of Nitrogen (NOz = NOy - NOx)

NPN - N-Propyl Nitrate

O3 - Ozone

OAP - Office of Atmospheric Programs

OAQPS - Office of Air Quality Planning and Standards

OAR - Office of Air and Radiation

ORD - office of research and Development

PAN - Peroxyacetyl Nitrate

p-NH4 - Particulate Ammonium

p-NO3 - Particulate Nitrate

p-SO4 - Particulate Sulfate

QAPP - Quality Assurance Project Plan

SO2 - Sulfur Dioxide

SOP - Standard Operating Procedures

SOx - Oxides of Sulfur

t-NO3 - Total Nitrate (t-NO3 = HNO3 + p-NO3)

UVF - Ultraviolet Fluorescence

VOAG - Vibrating Orifice Aerosol Generator
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II. Abstract Summary Information

Title: Federal Reference Methods for NOy and p-SO4 for the New Combined NOx and SOx

Secondary NAAQS

Project Lead:Russell W. Long, Ph.D., ORD/NERL/HEASD

Key Collaborators and Interested Parties: Surender Kaushik, Tad Kleindienst, Robert W. 

Vanderpool, and Fred Dimmick, OAR/OAQPS, OAP, State/local air agencies, Teledyne API, 

American Ecotech, Thermo Environmental

Project Period: July 2010-July 2012

Project Summary: EPA has been considering an indicator with a combination of measurements 

(SO2, p-SO4, and NOy) that form the NAAQS measured component of a new combined NOx and 

SOx secondary standard. Federal reference and/or equivalent methods (FRM/FEM) are currently 

available only for SO2. However, p-SO4 is measured at over 500 sites nationally, most often 

through the use of a filter pack sampler followed by off-line ion chromatographic analysis. 

Based on an initial survey of available information, methods of this type are reliable and provide 

consistent data. The determination of NOy is attempted in near real time using optical monitors 

employing an externally locate catalytic converter to convert all NOy species to NO, prior to 

possible line losses, which can then be detected using O3-chemiluminescence. This NOy

methodology is now being deployed in EPA’s national NCORE network. However, a large 

degree of uncertainty exists as to the quality of the NOy data, the ability to measure total NOy, 

and the applicability of this method for routine regulatory use. 

This research will result in the development and evaluation of sampling and analytical methods 

required to support the promulgation of the proposed NOx and SOx secondary NAAQS. It is 

anticipated that these newly developed/evaluated methods will be promulgated as the federal 

reference methods for the combined standard. For SO2, no work is needed as a designated FRM 

already exists for the primary standard. For p-SO4, the CASTNET filter pack (FP) method will 

be adopted and modified as needed to make it suitable for use as an FRM. The catalytic 

conversion/chemiluminescence based method for NOy, will require significant research to assess 

its adequacy for use as an FRM. Key unresolved science questions associated with this method 

need to be identified and studies performed to address these questions. After the available data 

and information on NOy measurement methods is compiled, HEASD will be able to describe the 

accuracy, precision, and reliability of the NOy method. Measurement uncertainties associated 

with existing NOy methods fall mainly in these important areas: total NOy capture (i.e., 

particulate NO3); converter performance (efficiency with respect to individual NOy species, 

consistency (i.e., changes in efficiency over time); inlet placement (i.e., vertical distribution of 

NOy in ambient air); potential interferences; and instrument calibration, and challenge (zero, 

span, precision) procedures.

Following procurement and evaluation of commercially available NOy monitors, HEASD will 

conduct field and laboratory based studies to address the above mentioned measurement 

uncertainties and to evaluate the method for use as a FRM. It is anticipated that test results and 

developed procedures will provide guidance to regional, state and local monitoring agencies in 

deployment and operation of the proposed NOx and SOx secondary FRM monitoring network. In 

addition, these approved measurement methods will serve as important tools for monitoring

agencies towards development of effective implementation plans aimed at deposition of acidic 

species into aquatic systems.
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III. Project Implementation Plan

A. Objectives

1. Adopt FRM specifications for SO2 based upon current FRM for primary SO2

NAAQS.

2. Propose and finalize FRM specifications for p-SO4 based upon adoption of 

CASTNET FP method for p-SO4.

3. Evaluate the catalytic conversion/chemiluminescence based method as a potential 

FRM for NOy. 

a. Quantify overall uncertainties associated with the NOy method

b. Identify and quantify interferences and measurement challenges associated 

with the NOy measurement method.

c. Investigate vertical NOy distribution in a representative air shed to establish

inlet placement criteria. 

d. Develop calibration and challenge procedures for NOy method

e. Evaluate commercially available chemiluminescence based NOy monitors for 

use as FRMs

4. Propose and finalize FRM specifications for NOy.

B. Technical Approach

Adopt FRM specifications for SO2. The existing FRM for the primary SO2 NAAQS is based 

upon ultra-violet fluorescence (UVF)1. FRM analyzers using this UVF technique can provide 

the needed detection limits, precision, and accuracy and thus fulfill all purposes of an FRM. As 

a result, no work will be done on the current SO2 FRM and it will be adopted as the FRM for the 

SO2 indicator of the proposed secondary standard.

Additionally, some interest has been expressed by OAR/OAQPS in pursuing the CASTNET FP 

method for SO2

2, 3 as an additional FRM or possible FEM due to the expected prohibitive cost 

associated with equipping a site with UVF FRM monitors. As a result, HEASD will use the 

substantive, readily available data and information available for the FP SO2 method. HEASD 

will provide a technical summary of the data and develop the basis/rationale for making it an 

FRM/FEM. HEASD will prepare docket materials, proposal preambles, response to comments, 

and the FRM in regulatory text format. 

Propose and finalize FRM specifications for p-SO4. HEASD would use the readily available and 

documented data, procedures and information available for the CASTNET p-SO4 FP method2, 3. 

HEASD will provide a technical summary of the data and supporting information and develop 

the basis/rationale for adopting it as an FRM. HEASD will prepare docket materials, proposal 

preambles, response to comments, and the FRM in regulatory text format. This approach for a pSO4 FRM when combined with the above two approaches for a SO2 FRM will provide 

scientifically defendable methods where SOx deposition is involved. 

Evaluate the catalytic conversion/chemiluminescence based method as a potential FRM for NOy.

Current NOy instruments based upon catalytic conversion and chemiluminescence detection 

methodology are available commercially but have not been assessed thoroughly enough to 

develop as an FRM. As a result, HEASD has identified measurement uncertainties and key 
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unresolved science questions associated with this method. These uncertainties and associated 

questions are:

(1) Total NOy capture – It iis well established that the proposed method is capable of the near 

quantitative capture of certain key gaseous components of NOy such as NO and NO2. However, 

there are questions about this methods capability in capturing the two components of NOy most 

relevant to nitrogen deposition, nitric acid (HNO3) and particulate nitrate (p-NO3).

(2) Converter efficiency – The catalytic converter most often employed in instrumentation of this 

type is a heated molybdenum converter wherein the more highly oxidized oxides of nitrogen 

(NO2, HNO3, HNO2, p-NO3, PAN) are converted to NO which can then be detected via O3-

chemiluminescence. It is well established for the species NO2, that the converter efficiency is 

≥98% thus the basis for use of this method as the FRM for the NO2 primary NAAQS. However, 

what is not known is how efficient the molybdenum converter is in converting other oxides of 

nitrogen to NO for detection. This particular aspect will be important as these proposed FRMs 

will be deployed in air sheds with differing NOy compositions. In addition, uncertainties also 

exist associated with the stability of the converter over extended periods of time with respect to 

total NOy and the individual components.

(3) Inlet placement – It is standard practice in the determination of total NOy to locate the

molybdenum converter externally, very near to the point of sampling to reduce sample transfer 

line losses of species such as p-NO3 and HNO3. Most often the converter and inlet probe are 

placed at a height of 10 m to eliminate any bias associated with vertical concentration gradients 

of HNO3 and p-NO3. However, additional research needs to be performed to identify and 

quantify potential vertical gradients in total NOy. This research will result in the inlet siting

criteria for the proposed FRM.

(4) Interferences – potential interfering species in the catalytic conversion/chemiluminescence 

determination of NOy include certain hydrocarbons, H2O(g) and NH3. However, the extent to 

which, and under what circumstances these species interfere is not known and must be identified 

and quantified.

(5) Calibration/challenge procedures – Although not an uncertainty, detailed method calibration 

and operating procedures must be developed and demonstrated as part of the FRM process. This 

includes procedures to test the performance of the FRM (i.e., zero, span, precision checks). It is 

anticipated that calibration procedures will be similar to those used in the NO2 primary NAAQS 

FRM with slight modification to account for the elevated converter height. However, 

performance challenge criteria will have to be identified that best represent the mixture of NOy

species that are expected to be present in the various air sheds across the U.S.

To address the above mentioned science questions, uncertainties and measurement challenges, 

the following approach will be taken: 

(1) Quantify overall uncertainties associated with the NOy method – Overall uncertainty will be 

quantified by collocated pair sampling to determine precision, bias and comparability between 

identical instruments under laboratory and ambient conditions. The laboratory based studies will 
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allow these figures of merit to be determined for individual NOy species and various NOy mixes 

under controlled laboratory conditions using simulated NOy species while the ambient studies 

will result in an assessment of precision, bias, and comparability under conditions that the FRMs 

would experience upon routine deployment. All ambient air studies will be conducted at the 

AIRS sampling site on EPAs RTP, NC campus as it is anticipated that this site is representative 

of the proposed sites in the NOx and SOx combined secondary standard network. There are 

currently three commercially available NOy instruments (Teledyne API, Thermo Environmental, 

and American Ecotech) that will be evaluated for use as FRMs. Table 1 gives an inventory of 

the instrumentation/equipment to be used during the laboratory based and ambient studies.

Table 1. Inventory of instrumentation/equipment for laboratory and ambient studies.

Instrument Species Analyzed Parameters Investigated

Thermo 42i-Y NOy Monitor NO, NOy, NO2+NOz precision, bias, Total NOy, speciated 

NOy, NOy capture, converter 

efficiency/stability, vertical NOy flux

American Ecotech 9843 NOy

Monitor

NO, NOy, NO2+NOz precision, bias, Total NOy, speciated 

NOy, NOy capture, converter 

efficiency/stability, vertical NOy flux

Teledyne API T200U NOy

Monitor

NO, NOy, NO2+NOz precision, bias, Total NOy, speciated 

NOy, NOy capture, converter 

efficiency/stability, vertical NOy flux

Teledyne API 200EU 

Photolytic NO2 Monitor

NO, NO2, NOx speciated NOy

URG 9000D Ambient Ion 

Monitor

p-NO3, p-NH4, 

HNO3, HNO2, NH3

speciated NOy, interfering species

GC-based PAN PAN speciated NOy

TSI VOAG p-NO3, p-NH4 speciated NOy, NOy capture, converter 

efficiency, interfering species

Perm Tube Oven HNO3, NH3 speciated NOy, NOy capture, converter 

efficiency, interfering species

Dynamic Dilution Gas 

Manifold

gas phase species speciated NOy, NOy capture, converter 

efficiency, interfering species

(2) Identify and quantify interferences and measurement challenges associated with the NOy

measurement method – Potential measurement challenges and interferences will be identified, 

quantified and documented through rigorous laboratory based and ambient method evaluations. 

In the laboratory based studies these include using state of the science equipment and facilities to 

challenge the NOy instrumentation with simulated individual NOy species and mixtures of 

species to gain an understanding of NOy capture (inlet and sampling losses), molybdenum 

converter efficiency and stability with respect to individual NOy species and total NOy, and 

measurement interferences of other atmospherically relevant species in the NOy determination. 

A TSI Vibrating Orifice Aerosol Generator (VOAG) will be used to generate test aerosols of 

known particle size, composition, and concentration to introduce to the candidate method as 

shown in Figure 1. A TSI Aerodynamic Particle Sizer (APS) will be used to monitor the 
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Figure 1. Schematic representation of the aerosol generation system to be used during the 

ambient studies.

aerodynamic diameter of the generated particles and a filter pack sample will be used as a 

reference. The filter pack sample will be collected and analyzed according to EPAs 

Compendium of Methods for the Determination of Inorganic Compounds in Ambient Air, IO-2

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and IO-3

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. A dynamic dilution system will be used to provide gas phase species to the candidate 

method as shown in Figure 2. The source gas will be obtained either through compressed gas 

cylinders (NO, NO2) or thorough the use of permeation tubes (HNO3, HNO2, NH3). An annular 

denuder sample collected and analyzed according to EPAs Compendium of Methods for the 

Determination of Inorganic Compounds in Ambient Air, IO-4

6 will be used as a reference. The 

aerosol and gas generation systems will allow HEASD to produce individual NOy components 

and potential interfering species under extremely controlled conditions. Ambient studies will 

confirm laboratory findings under conditions that the potential FRMs will experience upon 

deployment. During the ambient based studies, NOy will be speciated into its individual 

components and summed for comparison with results obtained with the method for total NOy. 

This comparison will allow for the assessment of the capability of the method in determining 

NOy as an FRM and identify any sampling shortcomings. Currently, HEASD has the capability 

for the high time resolution determination of the majority of the NOy components including the 

most atmospherically and deposition relevant p-NO3 and HNO3 and the most abundant 

component, NO2 (Table 1). 
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Figure 2. Schematic representation of the dynamic dilution gas generations system to be used 

during the laboratory based studies

(3) Investigate vertical NOy distribution in a representative air shed to establish inlet placement 

criteria – The vertical distribution of NOy concentrations will be investigated using collocated 

sampling with comparable instruments where inlet probes and converter boxes are located at 

differing heights along a 10m telescoping or lean down tower. As stated previously, the results 

of this investigation will provide data for inlet siting criteria of the proposed FRM. 

(4) Develop calibration and challenge procedures for NOy method – Currently, the FRM for the 

primary NO2 NAAQS is based on internally located molybdenum conversion and 

chemiluminescence detection technology7. Per the FRM, detailed calibration and challenge 

procedures are given. Since the proposed NOy method is based upon similar technology, 

HEASD will investigate and develop calibration procedures based upon those for the NO2 FRM. 

However, the more complex composition of NOy will result in the need for investigation and 

development of new procedures for challenging the NOy FRMs calibration. This includes 

identification and testing of existing gases (i.e., NO2 by GPT, IPN, and NPN) to establish

representative test gases to assess the molybdenum converter efficiency. 

(5) Evaluate commercially available chemiluminescence based NOy monitors for use as FRMs –

The commercially available instrumentation will be evaluated primarily in an ambient setting 

over a period of at least 1 year to assess the applicability of each monitor as an FRM. During the 

evaluations, the monitors will be calibrated, operated and challenged as if they were FRMs 

according to a detailed SOP that will be developed during the evaluation process. 

Propose and finalize FRM specifications for NOy. After compilation of the generated data and 

information on the NOy measurement method, HEASD will be able to describe the accuracy, 

precision, and reliability of the NOy instruments and their applicability as FRM. Therefore, 
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HEASD will provide a technical summary of the data and supporting information and develop 

the basis/rationale for adopting it as an FRM. HEASD will prepare docket materials, proposal 

preambles, response to comments, and the FRM in regulatory text format. 

Based upon results from literature searches and available data and information, it was the 

original intent of HEASD researchers to pursue a FRM based upon the most deposition relevant 

species of total nitrate (t-NO3, p-NO3 + HNO3) as the indicator. As a result HEASD may 

consider, as resources allow, the CASTNET FP method for t-NO3

2, 3 as an additional FRM or 

possible FEM with the surrogate indicator of t-NO3 due to the expected prohibitive cost 

associated with equipping a site with NOy monitors. HEASD may use the substantive, readily 

available data and information available for the FP t-NO3 method and provide a technical 

summary of the data and develop the basis/rationale for making it a surrogate FRM/FEM. 

HEASD would prepare docket materials, proposal preambles, response to comments, and the 

FRM in regulatory text format.

C. Project Schedule and Planned Products

Propose and finalize FRM specifications for SO2. 

This effort began in July 2010. HEASD is recommending that the current UVF FRM for the 

primary SO2 NAAQS be designated as the FRM for the SO2 secondary standard. Therefore, 

HEASD will provide this recommendation by the initial rule making date in July 2011. HEASD 

will provide response to comments and a final recommendation by the final rule making date in 

March 2012. 

Alternatively, HEASD is currently collecting available data and information for the FP SO2

method. HEASD will provide a technical summary of the data and develop the basis/rationale 

for making it an FRM/FEM by April 2011. HEASD will prepare docket materials, proposal 

preambles and provide the FRM in regulatory text format by the initial rulemaking date of July 

2011. HEASD will provide response to comments and the final FRM in regulatory text format 

by the March 2012 final rule making date. Results from this effort will serve as the basis of the 

SO2 FRM. 

Propose and finalize FRM specifications for p-SO4. 

This effort began in July 2010. HEASD is currently collecting available data and information for 

the FP p-SO4 method. HEASD will provide a technical summary of the data and develop the 

basis/rationale for making it an FRM/FEM by April 2011. HEASD will prepare docket 

materials, proposal preambles and provide the FRM in regulatory text format by the initial 

rulemaking date of July 2011. HEASD will provide response to comments and the final FRM 

for p-SO4 in regulatory text format by the March 2012 final rule making date. Results from this 

effort will serve as the basis of the p-SO4 FRM.

Evaluate the catalytic conversion/chemiluminescence based method as a FRM for NOy.

Field and laboratory evaluation of the NOy method relative accuracy, precision, and correlation 

began in May 2010 and will continue through May 2012. Following data analysis, data 

validation, and data interpretation, a summary report of the results will be completed by July 

2012. Results from this research effort will be published in a peer-reviewed journal article(s). 
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Propose and finalize FRM specifications for NOy. 

Concurrent to the NOy FRM development and evaluation research, HEASD will provide a 

technical summary of the available data and develop the basis/rationale for making it an FRM by 

May 2011. HEASD will prepare docket materials, proposal preambles and provide a description 

of the proposed NOy FRM in regulatory text format by the initial rulemaking date of July 2011. 

Method evaluations will continue during the period between initial and final rule making (July 

2011-March 2012). HEASD will provide response to comments and the final FRM for NOy in 

regulatory text format by the March 2012 final rule making. Results from this effort will serve 

as the basis for the NOy FRM.

Alternatively, HEASD is collecting available data and information for the FP t-NO3 method. 

HEASD will provide a technical summary of the data and develop the basis/rationale for making 

it an alternative FRM by April 2011. HEASD will prepare docket materials, proposal preambles

and provide the FRM in regulatory text format by the initial rulemaking date of July 2011. 

HEASD will provide response to comments and the final FRM in regulatory text format by the 

March 2012 final rule making date. Results from this effort will serve as the basis of the t-NO3

FRM.

D. Expected Results and Benefits

The U.S. EPA plans a 2012 promulgation of a new combined NOx and SOx secondary NAAQS, 

which will require sampling and analysis methods that are capable of measuring these metrics 

with known quality and thus ensure accurate compliance determinations. Developing, proposing 

and finalizing the FRMs for p-SO4 and NOy will be a critical component of the final ruling. 

These approved measurement methods will serve as important tools for monitoring agencies 

towards development of effective implementation plans aimed at deposition of acidic species 

into aquatic systems.

Following promulgation of the new secondary NAAQS for NOx and SOx, states will be required 

to develop state implementation plans which will outline measures designed to achieve 

compliance with the new NAAQS. In order to effectively develop these plans, States must first 

have an understanding of the factors influencing deposition of nitrogen and sulfur containing 

species within specific airsheds. Included in these factors is the capability to accurately 

determine the concentrations of these species in the ambient air. Thus, HEASDs evaluation of 

these potential FRMs and development of sampling criteria will establish the tools needed for in 

SO2, p-SO4 and NOy determination. In addition, this information will be valuable to future 

multiple air pollutant exposure studies and provide key data for source apportionment modeling 

and deterministic model evaluations. The NOy method research will also provide valuable

methodology and data to assess the role of NOy species in key atmospheric chemistry processes.

E. Collaboration

HEASD will collaborate with key personnel from OAR/OAQPS during the research and 

development work as well as the regulatory writing tasks associated with this project. HEASD

responses to the public comments, as they relate to finalizing the proposed FRM regulations, will 

be reviewed by OAR. Laboratory and field evaluation of the NOy monitors will involve 

collaboration with their respective manufacturers and potentially State/local air agencies. 
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HEASD will collaborate with OAP (managers for CASTNET) on adoption and modification of 

the CASTNET FP method for SO2, p-SO4 and t-NO3 as FRMs. 

IV. Quality Assurance and Communication Strategy

A. Quality assurance (QA)

The detailed quality assurance project plan (QAPP) “Identification, Evaluation, and 

Intercomparison of Methods for Oxides of Nitrogen and Sulfur” (QAPP-KDO-10-01) will 

address QA consistent with EPA’s Quality Manual for Environmental Programs (found at 

http://www.epa.gov/quality/qs-docs/5360.pdf). The proposed research and subsequent data 

analysis in this project have a strong foundation in quality assurance in that they come from an 

existing organization/system with rigorous QA requirements and have been used extensively by 

the HEASD team. The derived data from the proposed research will be evaluated and review 

extensively by HEASD researchers in their corresponding areas of expertise 

B. Communication Strategy

The project will use a variety of communication methods. There will be regular meetings to 

discuss progress and periodic meetings with a larger group to provide updates and ensure the 

team is meeting expectations. Materials will be produced to inform all stakeholders of the 

project. Scientific outputs will be produce in the form of reports, manuscripts, 

symposium/conference presentations to inform the scientific community and general public of 

the project technical outcomes.

V. Appendices

A. References

1. Reference Measurement Principle and Calibration Procedure for the Measurement of 

Sulfur Dioxide in the Atmosphere (Ultraviolet Fluorescence Method), 40 CFR Part 50 

Appendix A-1.

2. Clean Air Status and Trends Network, Quality Assurance Project Plan, Revision 6.0, 

Appendix 1: CASTNET Field Standard Operating Procedures, November 2009. 

http://epa.gov/castnet/javaweb/docs/qapp_v6_App_01_Field_SOP.pdf

3. Clean Air Status and Trends Network, Quality Assurance Project Plan, Revision 6.0, 

Appendix 4: CASTNET Laboratory Standard Operating Procedures, November 2009. 

http://epa.gov/castnet/javaweb/docs/qapp_v6_App_04_Laboratory_SOP.pdf

4. Compendium of Methods for the Determination of Inorganic Compounds in Ambient 

Air, Chapter IO-2, Integrated Sampling of Suspended Particulate Matter (SPM) in 

Ambient Air, http://www.epa.gov/ttn/amtic/inorg.html.

5. Compendium of Methods for the Determination of Inorganic Compounds in Ambient 

Air, Chapter IO-3, Chemical Species Analysis of Filter-Collected Suspended Particulate 

Matter (SPM), http://www.epa.gov/ttn/amtic/inorg.html.

6. Compendium of Methods for the Determination of Inorganic Compounds in Ambient 

Air, Chapter IO-4, Determination of Reactive Acidic and Basic Gases and Strong Acidity 

of Atmospheric Fine Particles in Ambient Air Using the Annular Denuder Technology, 

http://www.epa.gov/ttn/amtic/inorg.html
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7. Measurement Principle and Calibration Procedure for the Measurement of Nitrogen 

Dioxide in the Atmosphere (Gas Phase Chemiluminescence), 40 CFR Part 50 Appendix 

F.