UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

 WASHINGTON D.C. 20460

OFFICE OF THE ADMINISTRATOR

SCIENCE ADVISORY BOARD

May 25, 2011

EPA-CASAC-11-006

The Honorable Lisa P. Jackson

Administrator 

U.S. Environmental Protection Agency

1200 Pennsylvania Avenue, NW 

Washington, DC 20460 

Subject: Review of EPA Draft Documents on Monitoring and Methods for Oxides of 

Nitrogen (NOx) and Sulfur (SOx) 

Dear Administrator Jackson: 

The Clean Air Scientific Advisory Committee (CASAC) Air Monitoring and Methods 

Subcommittee (AMMS) augmented with Members of CASAC Oxides of Nitrogen (NOx) and 

Sulfur (SOx) Secondary National Ambient Air Quality Standards (NAAQS) Review Panel met 

on February 16, 2011, to review EPA’s draft documents on monitoring and methods for NOx and 

SOx. The CASAC AMMS Panel held a follow-up public teleconference call on March 29, 2011,

to discuss the external draft CASAC AMMS Report dated March 16, 2011. The chartered 

CASAC approved the AMMS Report at the May 12, 2011, public teleconference. This letter 

provides CASAC’s overall comments and evaluation. The CASAC and Panel membership is 

listed in Enclosure A. The CASAC consensus responses to EPA’s charge questions are 

presented in Enclosure B. Finally, Enclosure C is a compilation of individual panel member 

comments. 

EPA has developed a novel index that integrates the potential acidifying effects from the 

deposition of oxides of sulfur and nitrogen called the Aquatic Acidification Index (AAI). 

Application of this index would not be based solely on NO2 and SO2, which historically have 

been used as the ambient air indicators associated with oxides of nitrogen and sulfur. Instead, 

the AAI would use NOy (primarily total reactive nitrogen including NO, NO2, HNO3, particulate 

nitrate, and several reactive organic nitrogen compounds such as peroxy acetyl nitrate), SO2 and 

particulate sulfate as the ambient air indicators. The CASAC NOx / SOx Secondary NAAQS 

Review Panel has endorsed the use of NOy, SO2 and particulate sulfate as appropriate ambient air 
2

indicators, and deferred advice to EPA on the monitoring methodology and network design 

issues that would be used to measure these indicators to the CASAC AMMS. 

Attached to this letter, CASAC responds to eleven charge questions associated with issues 

involving the measurement of various indicator compounds that would support EPA in 

developing a new secondary standard for oxides of nitrogen and sulfur. The CASAC AMMS 

Panel reviewed the following five documents that summarize various NOx and SOx monitoring 

method and network design issues: 

1) “Ambient Air Monitoring For A New Secondary NAAQS For 2 Oxides of Nitrogen 

and Sulfur;” 

2) “Characterizing Emissions, Air Quality, Deposition and Water Quality;”

3) “Air Quality Observation Systems in the United States - Draft Report;”

4) “Ambient Air Monitoring Networks Supporting Secondary Standards and Air Quality

Model Improvement;” and

5) “Federal Reference Methods for NOy and p-SO4 for the New Combined NOx and

SOx Secondary NAAQS - Research Plan - January 20, 2011.”

The CASAC AMMS was charged with advising EPA on the methodology and network design 

issues that would be used to measure NOY, SO2 and particulate sulfate as ambient air indicators. 

these indicators. Overall, CASAC believes that EPA’s proposed evaluation of methods is

suitable. 

CASAC notes that a missing component of the monitoring program is a measurement of water 

quality. Some regions of the country have relatively little water quality data in comparison to 

other regions, which makes it difficult to evaluate the range of potential acidic indexes and the 

number of lakes in each region which are likely to be above or below a particular level. The 

monitoring program should provide an improved understanding of the ecological impacts of air 

pollution on sensitive water bodies. Also, data indicating a change in the physical ecological 

indicator of Acid Neutralizing Capacity (ANC) over time will provide data for evaluating the 

modeling and monitoring approach used for the secondary standard and will also provide a 

demonstration to the public and other stakeholders that the program is providing tangible results. 

In addition, CASAC also believes that it is important to develop and integrate a modeling 

research and evaluation plan associated with this regulation to evaluate, improve, and 

transparently present the CMAQ estimates of the modeled atmospheric and deposition species, the 

associated deposition transference ratios, and the joint performance of measurement plus model 

combinations upon which the NAAQS compliance will be determined. 

CASAC encourages EPA to explore other approaches for measuring NOy, SO2 and particulate 

sulfate ambient air indicators. For some of the parameters there are simpler integrated methods 

that may provide adequate information for implementation of this NAAQS. The initial network 

deployment should be limited but reasonably representative of conditions across the country. 

Each of the sites should include, at a minimum, the proposed and integrated filter based methods. 
3

Results from the initial data assessment can inform the EPA as to the preferred method for each 

parameter. CASAC provides recommendations for improving the application of each method, 

and suggestions for reducing the uncertainties associated with such applications. CASAC agrees 

that EPA’s proposed use of existing CASTNET and rural NCore networks as a starting 

infrastructure and as a framework for national rural secondary SOx/NOx monitoring would 

appropriately support attainment decisions and model evaluation if the networks are designed 

and located appropriately. CASAC suggests that NOy measurement technologies only be 

deployed on a limited basis in locations where other continuous measurements such as SO2 are 

made rather than at every monitoring location, since the NOy method requires a much more 

complex infrastructure and a higher level of operator skill with associated higher labor costs than 

integrated filter samplers such as the CASTNET filter pack method. The potential added value 

of NOy relative to the other integrated measurements does not justify wide deployment in a 

NOx-SOx network. 

CASAC supports the use of the AMoN passive sampling ammonia monitoring network for 

model evaluation and to provide improved spatial characterization of ambient ammonia 

concentrations, and recommends that additional performance evaluations of the passive samplers 

used in the AMoN occur to make maximum use of these emerging data. Integrated ammonia 

and ammonium measurements (NHx = NH3 and NH4) using impregnated filters may be a 

preferred approach. EPA should locate integrated measurements at as many sites as is 

reasonable to evaluate the modeling results with the measurement data and provide a greater 

degree of reliability in the F2 values used in the AAI. As discussed, it is not necessary to 

measure ammonia (NH3) separate from ammonium at each monitoring location.

CASAC also views the addition of limited NOy speciation measurements as critical to support 

the proposed secondary standard in the context of validating CMAQ predictions of NOy species, 

evaluation of model estimates of the transference ratios from NOy to deposition, and 

understanding NOy measurements and potential measurement biases in network deployment. 

Existing measurement instruments and technologies are available for necessary speciation, and 

EPA should consider measuring the size distribution of nitrate and other ions. Further, 

continuous measurement of sulfate is valuable for model evaluation and for evaluating how well 

deposition is simulated. 

In closing, CASAC appreciates the opportunity to provide input to EPA at this stage in the 

process, and trusts that our comments will be useful to EPA as it revises its NOx and SOx

monitoring methodology and addresses the network design issues. 
4

Sincerely,

/signed/ /signed/

Dr. Armistead (Ted) Russell, Chair

CASAC Oxides of Nitrogen and Sulfur Oxides 

Secondary NAAQS Review Panel 

Dr. Jonathan M. Samet, Chair

Clean Air Scientific Advisory Committee

Enclosures
5

NOTICE

This report has been written as part of the activities of the EPA's Clean Air Scientific Advisory 

Committee (CASAC), a federal advisory committee independently chartered to provide 

extramural scientific information and advice to the Administrator and other officials of the EPA. 

CASAC provides balanced, expert assessment of scientific matters related to issues and 

problems facing the Agency. This report has not been reviewed for approval by the Agency and, 

hence, the contents of this report do not necessarily represent the views and policies of the EPA, 

nor of other agencies within the Executive Branch of the federal government. In addition, any 

mention of trade names or commercial products does not constitute a recommendation for use. 

CASAC reports are posted on the EPA Web site at: http://www.epa.gov/casac.
6

Abbreviations and Acronyms 

AAI Aquatic Acidification Index 

AMMS Air Monitoring and Methods Subcommittee 

ANC Acid Neutralizing Capacity 

CASAC Clean Air Scientific Advisory Committee

CASTNET Clean Air Status and Trends Network

CFP CASTNET Filter Pack 

CMAQ Community Multiscale Air Quality Model

CSN EPA’s Chemical Speciation Network

EPA U.S. Environmental Protection Agency

FEM Federal Equivalent Method 

FP Filter Pack 

FRM Federal Reference Method 

NAAQS National Ambient Air Quality Standards 

NADP National Atmospheric Deposition Program

NOx Oxides of Nitrogen 

OAQPS EPA Office of Air Quality Planning and Standards

ORD EPA Office of Research and Development

SAB EPA Science Advisory Board

SOx Sulfur Oxides 

UVF Ultraviolet Fluorescence
7

Enclosure A – Roster

U.S. Environmental Protection Agency

Clean Air Scientific Advisory Committee

CASAC Air Monitoring and Methods Subcommittee (AMMS)

CHAIR

Dr. Armistead (Ted) Russell, Professor, Department of Civil and Environmental Engineering, 

Georgia Institute of Technology, Atlanta, GA

MEMBERS OF AMMS

Dr. David T. Allen, Professor, Department of Chemical Engineering, University of Texas, 

Austin, TX

Mr. George A. Allen, Senior Scientist, Northeast States for Coordinated Air Use Management 

(NESCAUM), Boston, MA

Dr. Doug Burns, Research Hydrologist, U.S. Geological Survey 

Dr. Judith Chow, Research Professor, Desert Research Institute, Air Resources Laboratory, 

University of Nevada, Reno, NV

Dr. Kenneth Demerjian, Professor and Director, Atmospheric Sciences Research Center, State 

University of New York, Albany, NY

Dr. Eric Edgerton, President, Atmospheric Research & Analysis, Inc., Cary, NC

Mr. Henry (Dirk) Felton, Research Scientist, Division of Air Resources, Bureau of Air Quality 

Surveillance, New York State Department of Environmental Conservation, Albany, NY

Dr. Philip Fine, Atmospheric Measurements Manager, South Coast Air Quality Management 

District, Diamond Bar, CA
8

Dr. Philip Hopke, Bayard D. Clarkson Distinguished Professor, Department of Chemical and 

Biomolecular Engineering, Clarkson University, Potsdam, NY

Dr. Rudolf Husar, Professor, Mechanical Engineering, Engineering and Applied Science, 

Washington University, St. Louis, MO

Dr. Daniel Jacob, Professor, Atmospheric Sciences, School of Engineering and Applied 

Sciences, Harvard University, Cambridge, MA

Dr. Peter H. McMurry, Professor, Department of Mechanical Engineering, University of 

Minnesota, Minneapolis, MN

Dr. Allen Robinson, Professor, Department of Engineering and Public Policy, Carnegie Mellon 

University, Pittsburgh, PA

Dr. James Jay Schauer, Professor , Department of Civil and Environmental Engineering, 

College of Engineering, University of Wisconsin - Madison, Madison, WI

Dr. Jay Turner, Associate Professor, Environmental & Chemical Engineering, Campus Box 

1180, Washington University, St Louis, MO

Dr. Yousheng Zeng, Managing Partner, Providence Engineering & Environmental Group LLC, 

Baton Rouge, LA

LIAISON MEMBERS OF CASAC NOx AND SOx SECONDARY NAAQS REVIEW 

PANEL 

Dr. Praveen Amar, Director of Science and Policy at NESCAUM (Northeast States for 

Coordinated Air Use Management), Boston, MA

Dr. Andrzej Bytnerowicz, Ecologist and Senior Scientist with the USDA Forest Service Pacific 

Southwest Research Station, Riverside, CA

Mr. Rich Poirot, Air Pollution Control Division, Vermont Department of Environmental 

Conservation, Waterbury, VT

SCIENCE ADVISORY BOARD STAFF

Mr. Edward Hanlon, Designated Federal Officer, U.S. Environmental Protection

Agency, Science Advisory Board Staff, Washington, DC
9

U.S. Environmental Protection Agency 

Clean Air Scientific Advisory Committee (CASAC) 

CHAIR 

Dr. Jonathan M. Samet, Professor and Flora L. Thornton Chair, Department of Preventive 

Medicine, University of Southern California, Los Angeles, CA 

MEMBERS 

Mr. George A. Allen, Senior Scientist, Northeast States for Coordinated Air Use Management 

(NESCAUM), Boston, MA 

Dr. Joseph D. Brain, Cecil K. and Philip Drinker Professor of Environmental Physiology, 

Department of Environmental Health, Harvard School of Public Health, Harvard University, 

Boston, MA 

Dr. H. Christopher Frey, Professor, Department of Civil, Construction and Environmental 

Engineering, College of Engineering, North Carolina State University, Raleigh, NC 

Dr. Armistead (Ted) Russell, Professor, Department of Civil and Environmental Engineering, 

Georgia Institute of Technology, Atlanta, GA 

Dr. Helen Suh, Program Director, Environmental Health, Public Health Division, National 

Opinion Research Corporation at the University of Chicago, West Newton, MA 

Dr. Kathleen Weathers, Senior Scientist, Cary Institute of Ecosystem Studies, Millbrook, NY

SCIENCE ADVISORY BOARD STAFF 

Dr. Holly Stallworth, Designated Federal Officer, U.S. Environmental Protection Agency, 

Washington, DC 
10

Enclosure B

CASAC Air Monitoring and Methods Subcommittee (AMMS)

Consensus Responses to Charge Questions

Charge Question 1: What are the Panel’s views on using the CASTNET filter pack (FP) to 

measure particulate sulfate for the purpose of providing annual average values as an indicator 

for the NOx/SOx standard? Given EPA plans primarily to document the capability of the 

CASTNET FP and develop the FRM for particulate sulfate based on the existing information and 

procedures, what are the Panel’s views of this approach for setting the FRM? 

CASAC generally supports the use of the CASTNET filter pack (CFP) as a potential Federal 

Reference Method (FRM) for measuring particulate sulfate for this secondary SOx / NOx

standard. The method has been relatively well-characterized and evaluated, and has a 

documented long-term track record of successful use in a field network designed to assess spatial 

patterns and long-term trends. Because the CASTNET network plays an important role in 

tracking Clean Air Act (CAA)-related air quality changes over space and time, the more detailed 

scrutiny and documentation that would result from FRM development would add confidence in 

the quality of CASTNET data for its many various applications. 

A major advantage of specifying the CFP for particulate sulfate is to maintain the possibility that 

the CFP might possibly be used to supply all the measurements required to implement this 

NAAQS, assuming that a Federal Equivalent Method (FEM) could be specified for CFP SO2 and 

that “total nitrate” (and an associated deposition transfer ratio) could be used as a surrogate for 

NOy in the AAI equation. The resulting network might employ a mix of weekly filter-based and 

continuous methods, and address the multiple objectives of compliance determination, model 

evaluation and improvement, and more complete atmospheric characterization – without the 

need to do everything everywhere.

The time resolution of the CFP weekly data is adequate for partially determining compliance 

with a NAAQS with a 3 to 5-year averaging time, and sampling all the time will reduce the interannual variability introduced by intermittent (1 in 3 day) daily sampling in programs such as 

IMPROVE and EPA’s Chemical Speciation Network (CSN). Weekly time resolution is 

minimally useful for model evaluation and refinement, and for that objective, it would be highly 

desirable to consider deployment of continuous sulfate analyzers at a few sites where continuous 

SO2 is being measured. 

The open-faced CFP sampler would capture some coarse-mode sulfate particles that have 

substantially larger dry deposition velocities than their more abundant fine particle counterparts. 

It would be useful to see a more detailed assessment of the CFP particle cut size characteristics, 

as well as an analysis of the temporal and regional variations in sulfate particle size distributions. 

It is not likely however that inclusion of some coarse particle sulfate in the SOx indicator will 

improve (or harm) the quality of that indicator for use, in combination with an associated CMAQ 
11

deposition transfer ratio, as a predictor of total wet and dry sulfur deposition. It is also not likely 

that inclusion of coarse sulfate, without some specification of the particle size distribution, will 

provide more useful information for model evaluation than that provided by more widely 

deployed fine particle sulfate measurements, such as those that might be provided by IMPROVE.

Conceivably, the fine particle sulfate data from larger networks like IMPROVE (in its current 

configuration or with enhanced filter pack supplements for other species) might be used in a 

complementary way to improve the spatial resolution for several of the measured + modeled S 

and N species.

Charge Question 2: What are the Panel’s views on using the CASTNET filter pack (FP) to 

measure sulfur dioxide gas for the purpose of providing annual average values as an indicator 

for the NOx/SOx standard? If EPA would document the capability of the CASTNET FP and 

develops an FRM for sulfur dioxide gas based on the existing information and procedures, what 

is the Panel’s view of this approach for setting the FRM? 

In principle, CASAC believed that the CASTNET filter pack and other filter pack based 

approaches are adequate for measuring long term average SO2 gas concentrations in rural areas 

with low levels (less than 5 ppbv) and therefore suitable for consideration as an FRM for the new 

NOx / SOx standard. Although FP-based approaches are widely deployed and well established, 

members of the Panel raised a number of concerns about the performance of FP approaches that 

need to be addressed: negative biases associated with alkaline soil, retention of SO2 on the 

combination of nylon and impregnated filters (especially at higher flow rates), effect of relative 

humidity (RH) on SO2 retention, and effects of wet filters on SO2 retention (which is especially a 

concern with the CASTNET open face design). There is also a need to evaluate the CASTNET 

approach against the existing SO2 FRM, focusing on weekly or longer averages at low

concentration rural sites. Only limited performance data on the performance of the CASTNET 

FP was provided to the Panel; it was not clear to the Panel that their concerns could be 

adequately addressed using existing information. Therefore, some additional characterization of 

the CASTNET FP may be needed. If EPA thoroughly documents the capability of the 

CASTNET FP and develops a suitable set of operating procedures, the CASTNET FP would be a 

reasonable approach for measuring annual average values as the SO2 gas indicator for the NOx /

SOx standard.

Regarding model performance, the low time resolution FP data will be of limited use for model 

evaluation. CASAC is also concerned about inferring deposition rates from long term average 

concentrations because of the importance of diurnal variations in determining dry deposition, 

especially given the problems that models have in reproducing diurnal SO2 profiles. During 

implementation, EPA will provide States with transference ratios that relate ambient 

concentrations of SOx to wet and dry deposition of sulfur oxides, and the primary concern is to 

obtain accurate measurements of SO2 and SO4. In the end, the issue is to evaluate how well 

models can simulate accurate transference ratios. CASAC suggests two ways to address this 

problem. One is to continue to improve and evaluate the models to the point where there is 

general confidence in model capabilities; this would be essential if EPA chooses a long-term 
12

averaging approach. The second is to make high-time resolved measurements. The latter is 

somewhat outside the scope that EPA is considering. 

Finally, CASAC emphasizes the importance of site selection and site characterization (e.g., 

fetch, height of inlet, height of canopy). For example, monitoring sites with poor fetch may be 

subject to significant concentration biases, due to localized deposition of reactive gases 

(primarily HNO3 and SO2). Such biases would likely lead to under-estimation of deposition 

rates and inhibit interpolation or extrapolation of deposition fields within or between sensitive 

ecoregions.

Charge Question 3: What are the Panel’s views on using the current primary FRM (high time 

resolution UVF) to measure sulfur dioxide gas for the purpose of providing annual average 

values as an indicator for the NOx/SOx standard?

CASAC believes that continuous, high-sensitivity measurements of sulfur dioxide gas in 

representative field sampling locations would be desirable for purposes of validating both 

CASTNET FP measurements of sulfur dioxide and models used to calculate temporal and spatial 

distributions of sulfur dioxide. However, as documented in 75 FR 35597- 35601 (22 June 2010), 

the ultraviolet fluorescence (UVF) -FRM must meet the following performance requirements: a 

detection limit of 1 part per billion by volume (ppbv); maximum interference less than ±5 ppbv 

SO2 equivalent; and 12- and 24-hour zero drift less than ±5 ppbv SO2 equivalent. In the context 

of annual average mixing ratios of interest for the secondary standard - on the order of 1ppbv -

these specifications are inadequate. Current UVF FRM sulfur dioxide monitors are, in principle, 

sufficiently accurate and sensitive to carry out measurements at the levels that would be required, 

but additional precautions would be required. 

Charge Question 4: What are the panel’s views on using existing NOy methods that are 

deployed, for example, in NCore as the measurement approach for NOy for the purpose of 

providing annual average values as an indicator for the NOx/SOx standard? What are the 

panel’s views on EPA’s assessment that additional study is needed before establishing an FRM 

based on the existing NOy methods? That is, are the methods already adequately demonstrated 

as a reference method to determine compliance with a NAAQS? What are the panel’s views on 

the research plan for establishing existing NOy methods as an FRM? [Note suggested 

improvement to the plan would be appreciated, particularly ones that would help complete the 

study on time.]

The existing NOy method is generally an appropriate approach for the indicator. Given the large 

uncertainties in values associated with determining factors in the AAI, the uncertainties in the 

measurements are relatively small. However, CASAC agrees that additional characterization 

and research is needed to fully understand the method in order to designate it as an FRM, or to 

use it for supporting objectives such as model evaluation and filter pack comparisons. There are 

also some practical issues with wide-scale deployment in an ambient network setting. Overall, 
13

CASAC is optimistic these method issues could be resolved, but remained cautious on the 

practical aspects of wide-scale deployment including network design and resource requirements. 

Using only the existing NCore rural sites will not provide enough spatial coverage to support the 

secondary standard. Allowing for an alternative approach using the CASTNET filter pack total 

nitrate measurement as an indicator may be a more practical solution in the near term and still 

capture most of the deposition relevant oxidized nitrate species.

CASAC believes that that more research is needed to characterize the existing NOy method with 

respect to a variety of issues. These include but are not limited to:

• converter efficiency for the various oxidized nitrogen species

• instrument behavior over extended deployment periods in remote areas

• effect of converter height

• spatial representative relative to nearby terrain and vegetation

• particulate and nitric acid losses in the inlet

• low temperature operation and potential condensation in sampling lines

• reduced nitrogen species interference

• varying NOz levels relative to NOx levels

• detection limits relative to levels in remote areas

From a practical perspective, the NOy method is not as straightforward to deploy as other 

continuous ambient gas analyzers. The unique calibration procedure and long sampling lines 

require extra resources in the areas of calibration, auditing operation, and data quality control. It 

may be even more difficult to achieve consistent results in remote locations with limited 

technician access. Given the difficulty in NOy instrument operation in network settings, a full 

suite of fully validated Quality Assurance/Quality Control (QA/QC) requirements should be a 

part of any FRM or FEM designation, including calibration, operation, data validation, and 

auditing procedures. While many of these procedures are currently being worked out for the 

current NCore deployment, they may not all be applicable to the lower rural NOy levels and may 

not address all of the method issues listed above. CASAC encourages EPA to take full 

advantage of the current expertise with NOy measurements that exists throughout the country at 

monitoring agencies as well as the research community. 

CASAC supports the need for and the general outline of the proposed research plan on the NOy

method for potential FRM designation. The plan should address most of the method issues listed 

above. Panel members had several specific recommendations for specific components of the 

research plan. For instance, there is a strong consensus that ambient testing should be done in 

multiple sections of the country in different atmospheric environments and different seasons. 

There is also broad support for the laboratory characterization and the particle generation 

experiments, but cautioning that quantitative ammonium nitrate particle generation can be 

challenging. CASAC also suggests that emerging true NO2 analyzer methods should be 

integrated into NOY instruments s that use traditional NOx measurement approaches. Detailed 

recommendations for the NOy research plan by individual panelists can be found in the attached 

individual panelist comments. 
14

Great care must be taken in designating an FRM such that future advances in measurement 

technology are not precluded from consideration or use. One alternate approach is to designate 

an FRM of “true total oxidized nitrate” as the sum of multiple, high-quality, possible timeintegrated, individual measurements of oxidized nitrate species (NO, NOx, nitric acid, PAN, 

HONO, p-nitrate, etc.). Then the existing NOy instruments, and any other existing or future 

technologies, would be considered for FEM status based on comparisons to the FRM in multiple 

locations and different seasons. The infrastructure for such testing may be provided by the 

proposed 2-5 NOy “supersites” that would include the required suite of detailed NOy species 

measurements. Furthermore, the “F” factors in the AAI can be derived for each specific FEM 

(including potentially the CASTNET total nitrate measurement), with the value of “F” based on 

the FEM comparisons to the FRM and/or the responsiveness of the CMAQ-simulated deposition 

to that FEM specific measurement. 

Charge Question 5: What are the panel’s views on using the emerging AMoN ammonia 

monitoring network that uses passive sampling technology as a tool for evaluating air quality 

model behavior with respect to characterizing ambient air patterns of ammonia?

CASAC is supportive of the utilizing the AMoN passive sampling ammonia monitoring network 

for air quality model (AQM) evaluation and to provide improved spatial characterization of 

ambient ammonia concentrations. CASAC recommends that additional performance evaluations 

of the passive samplers used in AMoN occur to make maximum use of these emerging data. 

Integrated ammonia and ammonium measurements (NHx = NH3 and NH4) using impregnated 

filters may be a preferred approach. EPA should gather integrated measurements at as many 

sites as practical in order to evaluate model results with measurement data and provide a greater 

degree of reliability in the F2 values. As discussed at the meeting, it is not necessary to measure 

ammonia (NH3) separate from ammonium at each monitoring location.

In evaluating AQM behavior, measurements of total reduced nitrogen are likely to be nearly as 

informative as separate measurements of ammonium and gaseous ammonia, if the total reduced 

nitrogen measurements can be coupled with major anion measurements. Therefore, as described 

in the response to charge question 7 on ammonium measurements, integrated ammonia and 

ammonium measurements using impregnated filters may be a preferred approach.

Nevertheless, gas phase measurements of ammonia are sparse and the information emerging 

from the AMoN network has the potential to be valuable. Additional performance evaluations of 

the passive samplers used in the AMoN will be needed to make maximum use of these emerging 

data. 

Charge Question 6: What are the panel’s views on co-locating ammonia measurements at 

each location where the indictors are measured?
15

CASAC agrees that some ammonia measurements would be useful since the current approach to 

setting the F2 value in the AAI is based solely on models. Having integrated measurements at 

some, but not all sites, would provide the opportunity to supplement the modeling with the 

measurement data and provide a greater degree of reliability in the F2 values. 

Ammonia has a high deposition velocity, and measurements are needed to assess ammonia 

deposition (particularly wet deposition). Integrated measurements are available using 

instruments available at a reasonable price. Integration of data for over a few months may 

provide a good test of ammonia inventory data. 

Charge Question 7: What are the Panel’s views on using the CASTNET filter pack (FP) to 

measure ammonium ion as a tool for evaluating air quality model behavior with respect to 

characterizing ambient air patterns of ammonia?

CASAC believes that use of the CASTNET FP that uses Teflon and Nylon FP filters probably 

provides a reasonable measurement of ammonium ion, but identifies several uncertainties 

associated with use of the CASTNET FP for this measurement. First, ammonium from nitrate 

can be lost from the Nylon filter in some cases (see: a) X.Y. Yu, T. Lee, B. Ayres, S.M. 

Kreidenweis, W. Malm, and J.L. Collett, Jr. 2006. Loss of fine particle ammonium from 

denuded nylon filters, Atmospheric Environment 40: 4797–4807; and b) Chapter 11 of the 

recently released draft IMPROVE report: “Spatial and Seasonal Patterns and Temporal 

Variability of Haze and its Constituents in the United States - Report V” at 

http://vista.cira.colostate.edu/improve/Publications/Reports/2011/2011.htm). At sites where 

ammonium nitrate levels were high and dominant, average losses of up to 28% ammonium ion 

were reported; this was with 24-hour samples but at higher flows, so these examples may not 

reflect the potential for loss from the FP. There is a likely positive artifact in areas with strongly 

acidic sulfate, since in rural areas with elevated sulfate levels the sulfate is more acidic during 

the day (downward vertical mixing of non-boundary-layer air masses) and ammonia levels are 

low. At night, ammonia levels are likely to be higher (sources are at the surface), and ammonia 

may neutralize some of the acidic sulfate on the Teflon filter, resulting in a positive nitrate 

artifact. Robust measurements of ammonium ion could be made using basic and acid gas 

denuders upstream, a Nylon filter, and a “trap” for ammonia downstream (coated filter or 

denuder). This is a relatively complex method that may not be appropriate for a large network; it 

also breaks FP SO2, but that and nitric acid and ammonia could be measured from the upstream 

denuders.

In the broader context of the AAI calculations, CASAC believes that the relative uncertainty of 

the FP nitrate is probably not a major issue. CASAC strongly notes that ammonium ion 

measurements, taken alone, are not a good test of model ammonia sources or deposition. When 

ammonium is in excess of sulfate on an equivalent basis (as is often the case), the concentration 

of ammonium is determined by the supply of sulfate and nitrate, not the supply of ammonia (See 

R.W. Pinder, P.J. Adams, A.N. Pandis and A.B. Gilliland, Temporally-resolved ammonia 

emission inventories: current estimates, evaluation tools, and measurement needs, Journal of 
16

Geophysical Research 111 (2006), p. D16310). One could add a measurement of gaseous 

ammonia in order to have closure on NHx, but it would be far more useful and robust to simply 

have a direct measurement of total NHx. From the perspective of testing models for NHx 

deposition, CASAC believes that there is little advantage to having speciated ammonia and 

ammonium measurements as compared to a measurement of total NHx. One can infer the NHx 

partitioning between the gas and PM phases from knowledge of sulfate and total nitrate 

concentrations.

Total NHx can easily be measured with a single (or double if needed) acid-coated filter 

(phosphoric acid for example). CASAC recommends that breakthrough be evaluated. This 

method could be complemented at a subset of sites with passive NH3 measurements, but does not 

provide a measurement of SO2. Its performance for SO4 would need to be evaluated (for 

potential SO2 interference).

Finally, CASAC believes that time resolution is important for model evaluation. The inorganic 

thermodynamic regime can vary substantially on a sub-daily time-scale; ideally you want to be 

able to capture that variation. Daily (24-hour) measurements are informative; though hourly 

would provide considerably more information. CASAC believes that multi-day measurements 

are not nearly so helpful for evaluating AQMs, and would likely not be widely used for that 

purpose. This is where the goal of deposition measurements and model assessment are in 

conflict. One approach would be to have a subset of monitoring sites deploy 24-h or sub-daily 

samplers on a one in six-day basis to supplement weekly measurements, matching the national 

network sample schedule for State/Local networks and the IMPROVE program (which is a 

primarily rural network). The existing CASTNET sampling system would have to be redesigned 

(and tested) to provide 24-hour or shorter duration samples; this is an issue of sensitivity. If 

resources allow, sub-daily (day and night) measurements using two integrated samplers would be 

even more useful for evaluating AQMs. A day/night sample, for example, could use two 10-

hour duration samples running from 8 to 6 standard time day and night, to more clearly separate 

the day and night regimes.

Charge Question 8: What are the panel’s views on establishing a suite of NOy species 

measurements at 2- 5 locations in different atmospheric and ecological regions for the purpose 

of evaluating air quality model and NOy instrument behavior?

CASAC views the addition of limited NOy speciation measurements as critical to support a 

proposed secondary standard in the context of validating CMAQ predictions of NOy species, 

validating model predictions of the transfer function from NOy to deposition, and understanding 

NOy measurements and any potential measurement biases in network deployment. CASAC 

views 3-5 NOy speciation sites that span different ecosystems and environmental factors as 

appropriate. 

CASAC believes that existing measurement instruments and technologies are available for 

necessary speciation and that consideration for potential measurements should include speciation 
17

of particulate nitrate in the context of size and counter ions. As NOy speciation measurements 

are already being conducted by other federal agencies and other organizations, CASAC 

recommends that efforts should be pursued to leverage existing research sites that have existing 

NOy speciation measurements where possible. 

Charge Question 9: What are the panel’s views on utilizing the existing CASTNET and rural 

NCore networks as a starting infrastructure for the purpose of supporting the NOx/SOx 

standard? 

CASAC agrees that CASTNET and rural NCore sites should be utilized if they are in appropriate 

locations. Other existing sites, including NADP/NTN, IMPROVE and State and Local monitors 

should also be considered. CASAC agrees that the siting criteria for monitoring for the NOx /

SOx secondary standard must be defined before the existing monitor locations are evaluated as an 

alternative to establishing new monitors. Several Panel members suggested that the siting 

criteria for this new network should address variable deposition within an eco-region due to 

varying precipitation and source locations. 

CASAC has concerns that expanding the CASTNET program to support this new monitoring 

objective does not have the support of many of the State and Local monitoring agencies that will 

have to implement this program. CASAC suggests that EPA make elements of the CASTNET 

suite of measurements available through a program operated in the way IMPROVE or NADP 

operates. In this scenario, the state or local monitoring agency in charge of monitoring in an ecoregion would opt into the program by paying for the equipment and analyses and by performing 

the measurement procedures as specified in the accepted program Standard Operating 

Procedures (SOPs). This effort would preserve the benefit of having a central laboratory but also 

provide the necessary implementation flexibility for the state or local agencies that will have to 

carry out the measurements. 

Charge Question 10: What are the panel’s views on using CASTNET filter pack (FP) to 

measure total nitrate (particulate nitrate plus nitric acid) as the measurement approach for the 

purpose of providing annual average values to support the NOx/SOx standard in diagnosing NOy

instrument behavior and assist in delineating the relative fractions of contributing oxidized 

nitrogen species to total ambient oxidized nitrogen. 

In general, CASAC believes that the CASTNET filter pack provides a reasonably good 

measurement method for total nitrate. Some Panel members expressed concerns that this 

measurement is not getting the correct separation between nitric acid and particulate nitrate, 

especially when ambient temperatures are >15 °C, but that the total nitrate measurement is 

generally of good quality. One alternative is to place nitric acid denuders in front of the open 

filter pack to better separate nitric acid from particulate nitrate. A good example of a possible 

improvement would be to sandwich the Teflon and nylon filters directly together. This would 

eliminate the space between the filters and limit the loss of volatile species. There is also a 
18

concern that the particle cut size was not well defined and may underestimate nitrate when 

alkaline soils, sea salt or de-icing materials are sampled on the front filter. The new NOx - SOx

secondary standard might provide some impetus to improve the CASTNET filter pack 

measurements and to get more representative speciation of total nitrate, and CASAC believes 

that this impetus would be helpful. 

If CASTNET total nitrate were used instead of NOy for estimating oxidized nitrogen deposition 

for this NAAQS, it would need to be combined with associated deposition transfer function 

(ratio of total oxidized nitrogen deposition to ambient nitric acid + particulate nitrate

concentration) from CMAQ. This could be a useful alternative or complementary approach to

establishing a large new NOy network, given the current uncertainties about the accuracy, 

precision, and interferences of the NOy method and the costs of deploying these instruments at 

numerous sites around the United States. While CASAC encourages the collocation of NOy and 

total nitrate filter pack measurements, the usefulness of filter pack total nitrate in diagnostic NOy

measurements is limited. If total nitrate, along with the associated deposition transfer function, 

will be considered as a NAAQS indicator, a more detailed evaluation of both measurement 

methods and associated deposition estimates is needed.

Finally, CASAC believes that time resolution is important for model evaluation. The inorganic 

thermodynamic regime can vary substantially on a sub-daily time-scale; to capture that variation, 

daily (24-hour) measurements are informative; though hourly would provide considerably more 

information. CASAC believes that multi-day measurements are not nearly so helpful for

evaluating AQMs. This is where the goal of deposition measurements and model assessment are 

in conflict, with the currently reasonable solution being 24-hour duration measurements. “One 

in six” is better than multi-day (one week) measurements. Every sixth day sampling can provide 

a reasonable estimate of a three-year mean, but not for a one-year mean given the expected lognormal distribution of most of these pollutants on a 24 hour basis.

Charge Question 11: What are the panel’s view of the broader consideration of using 

CASTNET, complemented by rural NCore, to serve as a framework for the nation’s rural 

monitoring of important gases and aerosols in support of secondary standards and evaluating 

the behavior of regional air quality models?

This is really a two-part question. Part 1 addresses suitability of CASTNET and NCORE sites 

and data for attainment decisions. Part 2 addresses suitability of CASTNET and NCORE data 

for model evaluation. 

Regarding part 1, CASAC believes that it would make sense from a resource standpoint to 

consider CASTNET and NCORE sites for the secondary SOx/NOx monitoring network. 

However, there is also concern that relatively few CASTNET, and even fewer NCORE sites, 

would satisfy siting criteria for the SOx/NOx network. In general, sensitive ecoregions are in 

areas with significant elevation, vegetation and deposition gradients. It is unclear at this time 
19

how many sites are needed and in what locations to meet data requirements for the standard. 

Consideration should also be given to other networks, such as IMPROVE, again provided siting 

criteria are met.

The Panel voiced concern about coordination between CASTNET and state and local agencies. 

CASTNET is an EPA CAMD project operated by a private contractor. Changes in the 

management structure and/or operation may be needed before state and local agencies can be 

comfortable with CASTNET data and attainment decisions.

Regarding part 2, there is no doubt that CASTNET and NCORE data will be extremely valuable 

for model evaluation purposes. NCORE sites will provide continuous (i.e., hourly) 

measurements of O3, NO2 NOy, SO2 and CO in approximately 80 urban, suburban and rural sites 

across the U.S. (including Hawaii, Alaska and Puerto Rico). Hourly time resolution is needed to 

challenge the ability of air quality models to simulate diel patterns, which are known to be 

strongly affected by boundary layer structure, vegetation and other surface features. Addition of 

“true” NO2 and SO4 to the suite of NCORE measurements would permit evaluation of model 

predictions for NOx and NOz, the latter of which is more relevant for deposition estimates. 

CASTNET filter pack data, in contrast, lack temporal resolution, but provide much needed data 

for model evaluation in rural and remote parts of the country, on monthly and seasonal time 

scales. Again, data from all available monitoring networks should be used for model evaluation 

purposes.

Additional Comments

1) Comments on Organic Acids and Bases: The Panel noted that the discussion has 

revolved almost entirely about inorganic contributors to acidity with the exception of 

peroxyacetyl nitrate (PAN). There are organic acids and bases that also contribute to the 

acid neutralizing capacity (ANC) and thus, as this secondary NAAQS process moves 

forward, there should be some exploratory measurements of these organic constituents to 

ascertain their potential role in ecological effects and to serve as the input to future 

rounds of standard reviews.

2) Critical Need for Modeling Research Plan: The Agency has developed an ambitious 

Measurement Research Plan to evaluate the performance of, suggest needed 

improvements to, and subsequently propose FRMs and FEMs for, the various 

atmospheric indicators. CASAC suggests that EPA develop a Plan to evaluate, improve, 

and transparently expose the CMAQ estimates of the modeled atmospheric and 

deposition species, the associated deposition transference ratios, and the joint 

performance of measurement plus model combinations upon which the NAAQS 

compliance will be determined. This may require close collaboration across EPA and 

with stakeholders.
20

CASAC suggests that the design and implementation of such overarching evaluation can 

and should be accomplished by close collaboration between the monitoring and modeling 

branches of the Agency as well as other interested stakeholders and CASAC. The 

outcome of this effort should improve the accuracy of, strengthen the confidence in and 

promote future refinements to the three deposition terms in the AAI.
21

Enclosure C

Compilation of Comments from Individual Members of the CASAC Air 

Monitoring and Methods Subcommittee (AMMS) Review Panel

Comments from Mr. George A. Allen....................................................................................... 22

Comments from Dr. Doug Burns .............................................................................................. 28

Comments from Dr. Judith Chow............................................................................................. 30

Comments from Dr. Kenneth Demerjian ................................................................................. 45

Comments from Dr. Eric Edgerton........................................................................................... 48

Comments from Mr. Henry (Dirk) Felton................................................................................ 52

Comments from Dr. Philip Hopke ............................................................................................ 59

Comments from Dr. Rudolf Husar............................................................................................ 61

Comments from Dr. Daniel Jacob............................................................................................. 64

Comments from Dr. Peter H. McMurry................................................................................... 68

Comments from Dr. Allen Robinson......................................................................................... 71

Comments from Dr. Jay Turner................................................................................................ 75

Comments from Dr. Yousheng Zeng ........................................................................................ 79

Comments from Dr. Andrzej Bytnerowicz............................................................................... 85

Comments from Mr. Rich Poirot .............................................................................................. 88
22

Comments from Mr. George A. Allen

Charge Question 1: What are the Panel's views on using the CASTNET filter pack (FP) to 

measure particulate sulfate for the purpose of providing annual average values as an indicator 

for the NOx/SOx standard? Given EPA plans primarily to document the capability of the 

CASTNET FP and develop the FRM for particulate sulfate based on the existing information and 

procedures, what are the Panel's views of this approach for setting the FRM?

The FP is adequate for measuring weekly mean sulfate. One issue EPA will have to consider for 

a FRM is the size cut -- 2.5 um or open face. Castnet is open face; all other network SO4 is 2.5 

um. From a total S deposition perspective [which includes wet deposition], the coarse sulfate is 

a very minor component, even though it has higher deposition rates than the fine sulfate mode. 

Another consideration is that if the Castnet-style Teflon filter sampler is designated as an FRM, 

that should be only for the SOx - NOx NAAQS, not for other sampling such as the higher flow 

24-h samples usually collected by other networks (IMPROVE, CSN). Note that the Castnet 

sampler uses only the first [Teflon] filter of the FP; the rest of FP filters are not relevant for SO4.

Charge Question 2: What are the Panel's views on using the CASTNET filter pack (FP) to 

measure sulfur dioxide gas for the purpose of providing annual average values as an indicator 

for the NOx/SOx standard? If EPA would document the capability of the CASTNET FP and 

develops an FRM for sulfur dioxide gas based on the existing information and procedures, what 

is the Panel’s view of this approach for setting the FRM?

The FP should measure SO2 well enough for the intended purpose (dry deposition). I do not 

recommend that EPA consider making it an FRM; there already is an SO2 FRM as promulgated 

in the June 2010 SO2 primary NAAQS rule. The FP could be designated as an FEM, but only 

for the secondary standard. Additional characterization of this method compared to the existing 

FRM would be needed. Tests of collection efficiency at 5 pm and different RH (low, high) need 

to be done.

Charge Question 3: What are the Panel's views on using the current primary FRM (high time 

resolution UVF) to measure sulfur dioxide gas for the purpose of providing annual average 

values as an indicator for the NOx/SOx standard? 

From a data quality standard, other than effective limits of detection, there are no issues unless 

there are high NO levels (unlikely at rural sites), due to the NO rejection ratio requirements for 

the SO2 FRM (200:1). The question is one of resources; assuming the FP SO2 is of reasonable 

quality (this needs to be demonstrated), the primary FRM could be deployed only at a small 

subset of “intensive” sites. There are no real issues with the FP SO2 data being used for 

determining compliance with the primary SO2 NAAQS, since that is driven by hourly values, not 

annual means. There is concern that existing trace SO2 instruments may not measure the very 
23

low SO2 levels present at some sites with sufficient precision. With care, these instruments can 

generate useful hourly means down to ~ 0.3 or 0.4 ppb. The dominant source of error at these 

low levels is zero error and stability; if these errors are random over a year, the mean SO2 data 

may be useful down to ~0.1 ppb. See my trace SO2 RAIN guidance document for further 

discussion of this topic.

Charge Question 4: What are the panel's views on using existing NOy methods that are 

deployed, for example, in NCore as the measurement approach for NOy for the purpose of 

providing annual average values as an indicator for the NOx/SOx standard?

 What are the panel’s views on EPA's assessment that additional study is needed before 

establishing an FRM based on the existing NOy methods? That is, are the methods already 

adequately demonstrated as a reference method to determine compliance with a NAAQS?

 What are the panel's views on the research plan for establishing existing NOy methods as an 

FRM? [Note suggested improvement to the plan would be appreciated, particularly ones that 

would help complete the study on time.] 

As detailed in the EPA “Research Plan” of 1/20/11, data quality from existing commercial NOy

instruments is difficult to characterize -- at least the NOz component (NOy - NOx). The Castnet 

FP provides sufficient information on deposition relevant oxidized N species when the other 

(large) uncertainties of the proposed secondary NAAQS are considered. Thus, from a practical 

point of view, I would suggest robust NO, true NO2, and NOy measurements only at a subset of 

sites, and NOT at every SOx - NOx site!

I agree with EPA that additional characterization of existing commercial NOy instrument is 

essential before establishing an FRM for NOy. Ideally, a single instrument that measures NO, 

true (photolytic) NO2, and NOy (and thus NOz) would be used; such an instrument could easily 

be commercialized using existing technologies. Some vendors may already have these 

instruments under development.

Testing must be carried out in warm humid areas and very cold areas. Condensation in portions 

of the sample line can occur in either condition, causing loss of sample NO or calibration NO 

and NO2. For very cold areas, the dilution (zero) air must be dried to a dew point lower than the 

ambient dew point, since some of the sample line is outdoors at ambient temperature. Silica gel 

(0 deg. C) is not sufficient; Drierite or similar products must be used (-73 deg. C).

The NOy component of the EPA research plan is reasonably comprehensive. Table 1 shows the 

API 200eu with speciated NOy, but this instrument does not measure NOy. Additional details on 

using the VOAG for generating ammonium nitrate aerosol are necessary; this is a very complex 

task to do correctly (Jim Schwab at ASRC-SUNY Albany has valuable experience for this). 

Since relative humidity (RH) may change the extent of nitrate loss (nitrate is more stable at 

higher RH), the effect of RH on these tests should be evaluated. Keeping sample lines cool 

further stabilizes nitrate. Consideration of other generation methods such as the Lovelace 

Nebulizer (http://www.intoxproducts.com/lovelacenebulizer ) may be appropriate; in a modified 
24

form (submerging the nebulizer and other simple changes) this has shown to be a very robust 

way to generate aerosols from solution, and can be packaged into a portable system. A flow 

diagram of

the HSPH aerosol generation system from the 1980's follows: 
25

As with ammonium nitrate, generating HNO3 and NH3 and delivering it to a sampler without 

significant losses is challenging. More detail on this would be useful; very short residence times, 

very clean borosilicate glass, and heating are essential components of any such system. 

Whatever aerosol generation system is used, it may be useful to assess its relative output in real 

time with a nephelometer (light scattering); if the moisture content (RH) of the aerosol changes, 

the nephelometer data would need to account for enhanced scattering of nitrate or sulfate 

aerosols at RH values greater than ~ 40%.

Finally, an important component of good NOy measurements is siting. Elevation above ground 

and “fetch” (distance to nearest forested area) are critical. 10 meters above ground and at least 3-

4 times horizontal distance relative to the height of the nearest forest canopy are appropriate 

criteria.

Charge Question 5: What are the panel's views on using the emerging AMoN ammonia 

monitoring network that uses passive sampling technology as a tool for evaluating air quality 

model behavior with respect to characterizing ambient air patterns of ammonia?

The Radiello® passive samplers used in the AMoN network appear to be suitable for 2 week 

duration samples, but may need an offset correction. See the accuracy and blanks plots on pages 

7 and 8 of a CAMD presentation from October 27, 2010:

http://www.nescaum.org/documents/mac/mac-committee-meeting-3/rury-amon.pdf/

These results are presumably from carefully controlled tests, and thus may not reflect the data 

quality from routine field measurements. It would be helpful to get a more complete evaluation 

update from CAMD.

Charge Question 6: What are the panel's views on co-locating ammonia measurements at each 

location where the indicators are measured?

If only the FP is run, it would be useful to have a subset of sites include passive ammonia 

measurements. If an NHx method is run (see Q #7), this is less important but still useful.

Charge Question 7: What are the Panel's views on using the CASTNET filter pack (FP) to 

measure ammonium ion as a tool for evaluating air quality model behavior with respect to 

characterizing ambient air patterns of ammonia?

The sum of Teflon and Nylon FP filters probably provides a reasonable measurement of 

ammonium ion, but there are some uncertainties and biases. First, ammonium from nitrate can 

be lost from the Nylon filter in some cases; see: Yu et al., “Loss of fine particle ammonium from 

denuded nylon filters”, Atmospheric Environment 40 (2006) 4797–4807. At sites where 
26

ammonium nitrate levels were high and dominant, average losses of up to 28% ammonium ion 

were reported; this was with 24-hour samples but at higher flows, so these examples may not 

reflect the potential for loss from the FP. There is a likely positive artifact in areas with strongly 

acidic sulfate; in rural areas with elevated sulfate levels, the sulfate is more acidic during the day 

(downward vertical mixing of non-boundary-layer air masses) and ammonia levels are low. At 

night, ammonia levels are likely to be higher (sources are at the surface), and ammonia may 

neutralize some of the acidic sulfate on the Teflon filter, resulting in a positive artifact. Robust 

measurements of ammonium ion could be made using basic and acid gas denuders upstream, a 

Nylon filter, and a “trap” for ammonia downstream (coated filter or denuder). This breaks FP 

SO2, but that and nitric acid and ammonia can be measured from the upstream denuders.

In the broader context of the AAI calculations, the relative uncertainty of the FP nitrate is 

probably not a major issue. Alternatively or in addition to the FP, total NHx (which is what the 

models need) could be measured with a single (or double) acid-coated filter (phosphoric acid for 

example). Breakthrough would need to be evaluated. This method could be complemented at a 

subset of sites with passive NH3 measurements, but does not provide a measurement of SO2. Its 

performance for SO4 would need to be evaluated (SO2 interference?).

Charge Question 8: What are the panel's views on establishing a suite of NOy species 

measurements at 2-5 locations in different atmospheric and ecological regions for the purpose of 

evaluating air quality model and NOy instrument behavior?

This is a good approach that would be very useful in evaluating models and NOy measurements. 

This level of effort is in the “research” category, and appropriate resources and skill levels would 

be needed.

Charge Question 9: What are the panel's views on utilizing the existing CASTNET and rural 

NCore networks as a starting infrastructure for the purpose of supporting the NOx/SOx 

standard?

This is an obvious choice in a resource constrained scenario, even if the resulting network is not 

ideally located from the SOx - NOx perspective. If we go forward with the FP method, network 

operations would need to be similar to the NADP or IMPROVE model, not the existing Castnet 

model. There is concern from state and local air agencies about having someone else (a 

contractor to CAMD in this case, not under air agency control) making measurements that could 

be used to demonstrate non-compliance with a NAAQS.

Charge Question 10: What are the panel's views on using CASTNET filter pack (FP) to measure 

total nitrate (particulate nitrate plus nitric acid) as the measurement approach for the purpose of 

providing annual average values to support the NOx/SOx standard in diagnosing NOy 

instrument behavior and assist in delineating the relative fractions of contributing oxidized 
27

nitrogen species to total ambient oxidized nitrogen. 

The FP does not provide a measure of ammonium nitrate, but it does provide a reasonable 

measure of the sum of nitric acid, nitrous acid, and ammonium nitrate if the Teflon and Nylon 

nitrate filter data are summed. There can be modest losses of ammonium nitrate from a Nylon 

filter sampler (Babich et al., J. Air & Waste Manage. Assoc. 50:1095-1105, table 5, Bakersfield 

1999). “Sandwiching” the Teflon and Nylon filters in a single filter holder minimizes interstage 

losses of nitric acid. See the description of “Nylon HEADS” in Babich et al. (above).

FP “total nitrate” is a reasonable metric for a NOx / SOx standard. I do not see it being useful in 

diagnosing NOy instrument behavior for several reasons - the most obvious being the 1-week 

duration of the FP sample. It is also not very useful for delineating the relative fractions of 

contributing oxidized nitrogen species to total ambient oxidized nitrogen, since it is the sum of 

nitric acid and ammonium nitrate. Perhaps the most important issue here is the lack of any FP 

NO2 measurement; NO2 in rural areas can be as much as half or more of the NOy. 

Charge Question 11: What are the panel's view of the broader consideration of using 

CASTNET, complemented by rural NCore, to serve as a framework for the nation's rural 

monitoring of important gases and aerosols in support of secondary standards and evaluating 

the behavior of regional air quality models?

See response to Charge Question 9 above.
28

Comments from Dr. Doug Burns

General Comments

Many of the charge questions focus on leveraging CASTNET (and to an extent rural NCORE) 

measurements to address this new secondary standard. I support leveraging these existing 

networks where possible if the measurements and methods are rigorous and would lend 

themselves to FRM/FEM designation. The total NOy measurement seems like it needs lots of 

testing and may not be ready to deploy in a monitoring network for a couple of years at best. I

also feel that passive collectors can be valuable. Though these devices may be less accurate and 

precise than other measurements, they are relatively inexpensive and can provide an opportunity 

to greatly expand the spatial intensity of measurements. This is important for aspects of model 

checking. It would seem that NH3 and NO2 should be priorities for passive collection.

Though this subcommittee was tasked with questions that focused solely on atmospheric 

chemical measurements, I note that surface water chemistry measurements in sensitive 

ecosystems are essential to promulgation of this new NOx - SOx standard. Some sensitive 

regions of the US have good and recent surface water chemistry measurements, but some regions 

have few or none. I would think that an attempt needs to be made to evaluate existing water 

quality data that could be used to promulgate this standard. This would help to identify gaps and 

limitations in implementing this potential new rule.

Charge Question 10 – Use of CASTNET filter pack to measure total nitrate

Measurement of total nitrate is currently done at all CASTNET sites using the filter pack. These 

measurements are generally viewed as providing accurate data on total nitrate concentrations in 

ambient air (see CASTNET QA reports on line), though the speciation between particulate 

nitrate and nitric acid may not be accurate. Precision is in the range of +/- 3-5%. Currently, data 

on total dry nitrate combined with particulate ammonium from CASTNET is “the” measurement 

to calculate dry N deposition for those working in remote/rural ecosystems sensitive to 

acidification. Clearly, these species from CASTNET do not represent all dry deposited N 

species and therefore, the CASTNET measurements provide an underestimate of deposition. 

The key question to answer then becomes: Does total nitrate from CASTNET vary in a 

predictable manner relative to the other key dry N species such as NH3, NO2, and PAN? If the 

answer is yes or close enough to yes, then CASTNET total nitrate from the filter pack would be a 

good surrogate for the secondary NOy standard. Data shared with this subcommittee from two 

sites in Canada (measured by CAPMON), as well as work by Jed Sparks and others suggest that 

key N species such as NH3, NO2, and PAN vary significantly both spatially and temporally and 

not necessarily synchronously with total nitrate. This would indicate that total nitrate is not a 

good surrogate for NOy. However, this could be explored in greater detail using existing data 

and/or model results (such as CMAQ) for NO2, NO, and PAN.
29

Charge Question 5 – Use of the AMoN ammonia monitoring network

This is a question that gets to the issue of being able to make lots of measurements inexpensively 

(passive samplers) vs. being able to make fewer measurements (denuder), but more expensively. 

I have observed the development of the AMoN network first hand. It is clear based on 

comparisons done among three passive NH3 devices that the Radiello were the most accurate of 

the passive samplers available at that time. The tests performed with the Radiello samplers 

indicate good precision (< 10%) and a low bias compared to side-by-side denuder measurements. 

The low bias appears to be on the order of 0.3 to 0.6 ug/m3 based on the comparisons I have 

seen. This obviously becomes more of a problem at sites with low NH3 concentrations where the 

deviations are greatest in absolute and relative terms. My recommendation would be to deploy 

as many denuder samplers as budgets allow, including some sites that have both a denuder and a 

passive sampler. The passive samplers could then be used to create a denser network of sites in 

areas of particular interest such as the Adirondacks and Shenandoah Park.
30

Comments from Dr. Judith Chow

Charge Question 1: What are the Panel’s views on using the CASTNET filter pack (FP) to 

measure particulate sulfate for the purpose of providing annual average values as an indicator 

for the NOx/SOx standard? Given EPA plans primarily to document the capability of the 

CASTNET FP and develop the FRM for particulate sulfate based on the existing information and 

procedures, what are the Panel’s views of this approach for setting the FRM?

The weeklong CASTNET samples provide a good measurement of annual average water-soluble 

SO4

= with a precision of 2–4% (Baumgardner et al., 1999; Bytnerowicz et al., 2002; Sickles, II 

and Shadwick, 2002). Total SO4

= values are comparable within the network and with other 

networks (Lavery et al., 2009; Sickles, II et al., 1999; Sickles, II and Shadwick, 2002). 

Charge Question 2: What are the Panel’s views on using the CASTNET filter pack (FP) to 

measure sulfur dioxide gas for the purpose of providing annual average values as an indicator 

for the NOx/SOx standard? If EPA would document the capability of the CASTNET FP and 4 

develops an FRM for sulfur dioxide gas based on the existing information and procedures, what 

is the Panel’s view of this approach for setting the FRM? 

The weeklong CASTNET samples by filter pack provide a reasonable measurement of annual 

average SO2 and appear to be comparable within and between networks (Bennett et al., 1994; 

Bytnerowicz et al., 2002; Chow et al., 1993; Lavery et al., 2009). Sickles et al. (1999) provide 

methodological caveats that warrant a revisit of the current CASTNET procedures. The partial 

collection of SO2 on Nylon filters (Japar and Brachaczek, 1984; Sickles, II and Hodson, 1999a; 

Sickles, II and Hodson, 1999b) is added to the SO2 measurement on the two K2CO3-impregnated 

cellulose-fiber filters at the final stage of the filter pack. 

Accuracy of the filter pack measurements has been evaluated with an annular denuder system for 

SO2 and SO4

=

. The filter pack system has been shown to provide 10–20% lower SO2 values than 

those of the annular denuder system (Baumgardner et al., 1999). For weeklong samples, Kim

and Allen (1997) reported adsorption of SO2 on Nylon filters was 29.4 ± 8.5% at 2 L/min flow 

rate (decreasing to 16.3 ± 4.5% as flow rate is increased to 5 L/min). The lower in SO2 retention 

on Nylon filters at higher flow rates was attributed to reduced residence time of SO2 at the Nylon 

filter surface. Since CASTNET samples at a flow rate of 1.5 L/min for the eastern sites and 3.0 

L/min at the western sites, the effects of flow rate on SO2 retention needs to be further 

investigated. 

Sickles et al. (1990) tested coated sorbents and impregnated filters for SO2 and NO2 in the 

laboratory and found SO2 recovery efficiency on triethanolamine (TEA)-impregnated filters 

decreased from ~90% under dry conditions to ~80% at 50% relative humidity (RH). The effect 
31

of SO2 recovery efficiency on K2CO3-impregnated cellulose-fiber filter under different RH needs 

to be tested.

The extraction efficiency of SO2 on Nylon filters needs to be further tested. For the California 

Acid Deposition Network (Watson et al., 1991), a 0.1% H2O2 solution was used for extraction of 

K2CO3-impregnated cellulose-fiber filters, instead of distilled deionized water (DDW), to ensure 

oxidation of SO2 and SO3

= to SO4= and to prevent interference of SO3= on NO3- peaks in ion 

chromatographic (IC) analysis. 

A negative bias may exist, owing to the reaction of SO2 with alkaline soils sampled on the 

Teflon-membrane filter (Eatough et al., 1995; Eldred and Cahill, 1997; Usher et al., 2002). Early 

studies showed that filter pack SO2 is 14–15% lower than the corresponding annular denuder 

system, suggesting a bias in SO2 measurement by filter pack (Dasch et al., 1989; Sickles, II et al.,

1990). Sickels and Hodson (1999b) discovered that SO2 is unrecoverable from the Nylon filter, 

ranging from 5% at 28% RH to 10% at 49% RH. The unrecovered SO2 on Nylon filters needs to 

be further tested and evaluated for different RH.

Charge Question 3: What are the Panel’s views on using the current primary FRM (high time 

resolution UVF) to measure sulfur dioxide gas for the purpose of providing annual average 

values as an indicator for the NOx/SOx standard? 

Using high-time resolution UVF to measure SO2 for the purpose of providing annual average 

values as an indicator for the NOx/SOx standard seems costly and impractical. Many hourly SO2

levels may be below the lower quantifiable limits (LQL), which are usually higher than the 

manufacturer’s minimum detection limits (MDL), at regional sites. Sickles et al. (1990) reported 

detection limits of 0.04 ppb for seven-day samples based on the MDL of the IC method. It 

would be necessary to evaluate the effect on non-detects on the annual average if a continuous 

FRM were to be used for an annual average.

Charge Question 4: What are the panel’s views on using existing NOy methods that are 

deployed, for example, in NCore as the measurement approach for NOy for the purpose of 

providing annual average values as an indicator for the NOx/SOx standard? What are the 

panel’s views on EPA’s assessment that additional study is needed before establishing an FRM 

based on the existing NOy methods? That is, are the methods already adequately demonstrated 

as a reference method to determine compliance with a NAAQS? What are the panel’s views on 

the research plan for establishing existing NOy methods as an FRM? [Note suggested 

improvement to the plan would be appreciated, particularly ones that would help complete the 

study on time.]
32

An evaluation is needed to determine how values below detection limits will be incorporated into 

an annual average. More study is needed for NOy monitors in non-urban areas. Small variations 

in converter efficiency can bias these data.

Charge Question 5: What are the panel’s views on using the emerging AMoN ammonia 

monitoring network that uses passive sampling technology as a tool for evaluating air quality 

model behavior with respect to characterizing ambient air patterns of ammonia? 

Passive samplers can provide reasonable estimates of ambient concentrations and are widely 

used in other countries and in some parts of the U.S. (Adon et al., 2010; Alonso et al., 2005; 

Bytnerowicz et al., 2010; Cao et al., 2009; Kirchner et al., 2005; Martins et al., 2007; Meng et 

al., 2010; Mosquera et al., 2005; Moumen et al., 2004; Nishikawa et al., 2009; Ramadan, 2010).

An argument might be made that a passive monitor better represents deposition than does an 

ambient concentration coupled with a deposition velocity. The Radiello passive sampler (Allou 

et al., 2008; Bruno et al., 2005; 2008a; 2008b; Chiriac et al., 2009; Diaz-de-Quijano et al., 2009; 

Pennequin-Cardinal et al., 2005; Plaisance et al., 2008; Ribas and Penuelas, 2006; Strandberg et 

al., 2005; Strandberg et al., 2006; Sturaro et al., 2010) using phosphoric acid as an impregnant 

followed by flow injection analysis (FIA) has been applied to NH3 for the National Atmospheric 

Deposition Program’s (NDAP) Ammonia Monitoring Network (AMoN). Using NH3 data from 

the AMoN as a tool for evaluating air quality modeling is a good start. The passive diffusion 

sampler may require greater than one week sampling at remote areas in order to meet analytical 

sensitivity. More collocated data are needed for CASTNET to evaluate NH3 measurements.

Charge Question 6: What are the panel’s views on co-locating ammonia measurements at each 

location where the indictors are measured? 

This is a good idea. Even though the current Aquatic Acidification Index (AAI) proposed by the 

U.S. EPA (2011a) does not require the measurement of NHx, it is essential that the EPA starts to 

incorporate measurements of NHx in CASTNET to gain better insight into the concentration of 

NH3 and NH4

+ at the 84 sensitive eco-regions.

Charge Question 7: What are the Panel’s views on using the CASTNET filter pack (FP) to 

measure ammonium ion as a tool for evaluating air quality model behavior with respect to 

characterizing ambient air patterns of ammonia? 

The current CASTNET filter pack configuration may be modified by adding an extra FEP stage 

behind the Nylon filter stage on the Savillex FEP filter holder (Eden Prairie, MN). A double 

Whatman 31 ET cellulose-fiber filter (0.5 mm thickness, Whatman, Inc., Fairfield, CT) with 

citric acid impregnant can collect NH3, and the NH4

+ ion is already measured on Teflon-
33

membrane filters. Note that the Whatman 31 ET is thicker, with a greater capacity, than the 

Whatman 41 filters (0.22 mm thickness) used in CASTNET for SO2 collection on K2CO3-

impregnated cellulose-fiber filters. Chow et al. (1993) showed that a Teflon-membrane/citric 

acid-impregnated cellulose-fiber/ K2CO3-impregnated cellulose-fiber filter pack will adequately 

retain NH3 without interference with downstream SO2 measurements at a 20 L/min flow rate. 

This filter pack sampling configuration and the collection efficiencies of different impregnants 

has been tested at the Operational Evaluation Network (OEN; ERT, 1987; Fung, 1988a; 1988b; 

Fung and Heisler, 1987).

Andersen and Hovmand (1994) showed that compared to the denuder sampling system, their 

filter pack underestimated NH3 at conditions with low NH3 concentrations. They attributed this 

to the absorption of NH3 in an unsaturated acid aerosol atmosphere. Since the acid-absorbing 

Nylon filter would precede the acid-impregnated filter for NH3 collection and may retain NH3

(Masia et al., 1994), NH3 (expressed as NH4

+

) needs to be measured on the Nylon filter. The 

LQLs, sensitivity, precision, and accuracy of CASTNET NH3 measurements at a low flow rate 

need to be tested for different concentration levels and at different ambient temperatures to 

ensure collection efficiency under extremes of temperature and RH.

Charge Question 8: What are the panel’s views on establishing a suite of NOy species 

measurements at 2- 5 locations in different atmospheric and ecological regions for the purpose 

of evaluating air quality model and NOy instrument behavior? 

Resolve the issue of its effects on annual averages first. 

Charge Question 9: What are the panel’s views on utilizing the existing CASTNET and rural 

NCore networks as a starting infrastructure for the purpose of supporting the NOx/SOx 

standard? 

See answer to Charge Question 11.

Charge Question 10: What are the panel’s views on using CASTNET filter pack (FP) to 

measure total nitrate (particulate nitrate plus nitric acid) as the measurement approach for the 

purpose of providing annual average values to support the NOx/SOx standard in diagnosing 

NOy instrument behavior and assist in delineating the relative fractions of contributing oxidized 

nitrogen species to total ambient oxidized nitrogen. 

The CASTNET filter pack provides a good measure of total nitrate as defined, but does not 

provide as good a distinction between particulate NO3

- (i.e., pNO3-

) and gaseous HNO3. Figure 1 

illustrates the CASTNET filter pack and Figure 2 summarizes the analyses applied to each filter. 
34

Figure 1. The CASNET filter packs (MACTEC, 2010). Filters are placed in series in an openfaced Savillex FEP Teflon filter holder (Eden Prairie, MN). The two Whatman cellulose-fiber 

filters (Fairfield, CT) are impregnated with a K2CO3 solution that reacts with SO2 to form SO4

=

. 

The filter pack is located atop a 10 m boom with the receptacle facing downward. Samples are 

installed every Tuesday and taken continuously for 168 hour durations (1 week) at flow rates of 

1.5 L/min for eastern sites and 3.0 L/min for western sites.

Figure 2. Analysis methods applied to distilled deionized water (DDW) extracts from each 

CASTNET filter (MACTEC, 2009). Since the Nylon filter adsorbs some of the SO2 (Sickles, II 

et al., 1999; 1999a; Sickles, II and Hodson, 1999b), SO4

= is also measured on Nylon and added 

to the SO2 (expressed as SO4

=

) collected on the backup cellulose-fiber filters.

The Nylon and impregnated filter technology for HNO3 and SO2 is well established and has been 

thoroughly evaluated (Appel et al., 1980; Axelrod and Hansen, 1975; Chow et al., 1993; Ferek et 

al., 1991; Ferek et al., 1997; Forrest et al., 1980; Forrest and Newman, 1973; Huygen, 1963; 

Johnson and Atkins, 1975; Leppanen et al., 2005; Lewin and Zachau-Christiansen, 1995; 
35

Matsuda and Cahill, 1985; Orr et al., 1967; Sickles, II et al., 1999; Talbot et al., 1990; Tsai et al., 

2004). Aside from the simplicity and cost-effectiveness of the filter pack system, its major 

advantage is that all suspendable particles are collected and there are no interfering sampling 

surfaces that adsorb HNO3. Its major disadvantage is blurred distinction between pNO3

- and 

HNO3. The separation is probably reasonable during winter, when temperatures are <15 °C and 

NH4NO3 dissociation is low (Appel et al., 1979; Appel, 1994; Chow et al., 2005; Spicer and 

Schumacher, 1979; Zhang and McMurry, 1991; Zhang and McMurry, 1987). On the other hand, 

when alkaline soils, sea salt, or deicing material are sampled on the front filter, these particles 

can collect the HNO3 that will be measured as pNO3

- (Dasch and Cadle, 1990; Goodman et al., 

2000; Krueger et al., 2004; Laskin et al., 2005; Liu et al., 2008; Ooki and Uematsu, 2005; 

Umann et al., 2005; Underwood et al., 2001). Although total NO3

- filter pack measurements are 

comparable to the annular denuder system, the under- or overestimation of HNO3

- or pNO3- may 

affect the modeled dry nitrogen deposition, since the dry deposition velocity (Vd) for HNO3 is 

typically higher than that for pNO3

-

. Lavery et al. (2009) found that CASTNET underestimates 

NO3

- and overestimates HNO3 at coastal sites, where >80% of NO3- particles were >2.5 μm; 

attributed to the reaction of NaCl with some of the volatilized NO3

- and scavenging of HNO3 by 

alkaline particles collected on the Teflon-membrane filter. Heterogeneous reactions of HNO3 on 

CaCO3 particles were tested by Goodman et al. (2000), who found that the presence of water 

vapor enhanced this reaction. Sickles and Shadwick (2008) reported higher pNO3

- (±42% to 

±102%, on median relative bias) than the collocated IMPROVE measurements due to the 

reaction of sea salt with HNO3. Accurate measurement of HNO3 is challenging. Arnold et al.

(2007) reported agreement is ±30% when semi-continuous measurements of HNO3 were 

compared with filter pack and annular denuder systems at urban Tampa, Florida. 

As part of a mineral dust and tropospheric chemistry study, Umann et al. (2005) observed that, 

during a dust event, 100% of HNO3 was depleted at a high mountaintop plateau (Izana, Canary 

Islands, Spain; 2367 m above sea level). There is evidence regarding the effect of coarse 

particles on HNO3 in studies involving comparisons of PM10 and PM2.5 pNO3

- (Chow et al., 

1993; Lavery et al., 2009).

The CASTNET filter pack also samples droplets in rain and fog, and it appears that this 

decreases some of the concentrations when there is evidence of filter wetting (Smith, 2002). For 

samples collected from 1987 to March 1998, Smith (2002) reported a total of 27,412 wet filters. 

This includes 0.4, 1.1, and 28.4% of wet Teflon-membrane, Nylon-membrane, and cellulosefiber filters, respectively. Sickles and Michie (1987) showed that wet surfaces may retain more 

NO2 and SO2. Laboratory testing showed that increasing RH by fivefold (i.e., 16–79%) may 

increase the deposition velocities of NO2 and SO2 by ~50%. The effects of RH on the retention 

of NO2 and SO2 in the CASTNET filter pack system needs to be further evaluated.
36

Charge Question 11. What are the panel’s view of the broader consideration of using 

CASTNET, complemented by rural NCore, to serve as a framework for the nation’s rural 

monitoring of important gases and aerosols in support of secondary standards and evaluating 

the behavior of regional air quality models? 

The framework should not be limited to CASTNET and NCore sites. CASTNET is adequate for 

determining annual averages, but week-long average measurements are not optimal for 

evaluating regional-scale models that intend to incorporate meteorological phenomena changing

over a period of days or less. An integrated data set of regionally-representative measurements 

from all available networks needs to be assembled and evaluated. VIEWS (2010) for the 

IMPROVE network provided a good start for this, but it never had the constituency needed to 

support it. The NAMS/SLAMS network contains regionally-relevant sites, usually located 

downwind of population centers, and these should be included. IMPROVE (DeBell et al., 2006; 

IMPROVE, 2011; Joseph et al., 1987; Malm et al., 2000; Sisler et al., 1996; Watson, 2002), 

SEARCH (Hansen et al., 2003; 2006), AIRMon (Gilliland et al., 2002; NOAA, 2011), CAPMon 

(Environment Canada, 2011; Zhang et al., 2009), PAMS (Photochemical Assessment Monitoring 

Stations; U.S.EPA, 2011b), and probably other networks should be considered in such a 

framework. 

References

Adon, M.; Galy-Lacaux, C.; Yoboue, V.; Delon, C.; Lacaux, J.P.; Castera, P.; Gardrat, E.; 

Pienaar, J.; Al Ourabi, H.; Laouali, D.; Diop, B.; Sigha-Nkamdjou, L.; Akpo, A.; Tathy, 

J.P.; Lavenu, F.; Mougin, E. (2010). Long term measurements of sulfur dioxide, nitrogen 

dioxide, ammonia, nitric acid and ozone in Africa using passive samplers. Atmos. Chem. 

Phys., 10(15): 7467-7487. 

Allou, L.; Marchand, C.; Mirabel, P.; Le Calve, S. (2008). Aldehydes and BTEX measurements 

and exposures in university libraries in Strasbourg (France). Indoor and Built 

Environment, 17(2): 138-145. 

Alonso, R.; Bytnerowicz, A.; Boarman, W.I. (2005). Atmospheric dry deposition in the vicinity 

of the Salton Sea, California - I: Air pollution and deposition in a desert environment. 

Atmos. Environ., 39(26): 4671-4679. 

Andersen, H.V.; Hovmand, M.F. (1994). Measurements of ammonia and ammonium by denuder 

and filter pack. Atmos. Environ., 28(21): 3495-3512. 

Appel, B.R.; Wall, S.M.; Tokiwa, Y.; Halik, M. (1979). Interference effects in sampling 

particulate nitrate in ambient air. Atmos. Environ., 13: 319-325. 

Appel, B.R.; Wall, S.M.; Tokiwa, Y.; Haik, M. (1980). Simultaneous nitric acid, particulate 

nitrate and acidity measurements in ambient air. Atmos. Environ., 14(5): 549-554. 
37

Appel, B.R. (1994). Atmospheric sample analysis and sampling artifacts. In Aerosol 

Measurement: Principles, Techniques, and Applications, Willeke, K., Baron, P. A., Eds.; 

Van Nostrand, Reinhold: New York, NY, 233-259.

Arnold, J.R.; Hartsell, B.E.; Luke, W.T.; Ullah, S.M.R.; Dasgupta, P.K.; Huey, L.G.; Tate, P. 

(2007). Field test of four methods for gas-phase ambient nitric acid. Atmos. Environ., 

41(20): 4210-4226. 5.

Axelrod, H.D.; Hansen, S.G. (1975). Filter sampling method for atmospheric sulfur dioxide at 

background concentrations. Anal. Chem., 47: 2460-2461. 

Baumgardner, R.E.; Isil, S.S.; Bowser, J.J.; Fitzgerald, K.M. (1999). Measurements of rural 

sulfur dioxide and particle sulfate: Analysis of CASTNet data, 1987 through 1996. 

JAWMA, 49(11): 1266-1279. 

Bennett, R.L.; Stockburger, L.; Barnes, H.M. (1994). Comparison of sulfur measurements from a 

regional fine particle network with concurrent ACID MODES network results. Atmos. 

Environ., 28(3): 409-419. 

Bruno, P.; Caputi, M.; Caselli, M.; de Gennaro, G.; de Rienzo, M. (2005). Reliability of a BTEX 

radial diffusive sampler for thermal desorption: Field measurements. Atmos. Environ., 

39(7): 1347-1355. 

Bruno, P.; Caselli, M.; de Gennaro, G.; Iacobellis, S.; Tutino, M. (2008a). Monitoring of volatile 

organic compounds in non-residential indoor environments. Indoor Air, 18(3): 250-256. 

Bruno, P.; Caselli, M.; de Gennaro, G.; Scolletta, L.; Trizio, L.; Tutino, M. (2008b). Assessment 

of the impact produced by the traffic source on VOC level in the urban area of Canosa di 

Puglia (Italy). Water Air and Soil Pollution, 193(1-4): 37-50. 

Bytnerowicz, A.; Tausz, M.; Alonso, R.; Jones, D.; Johnson, R.; Grulke, N. (2002). Summertime distribution of air pollutants in Sequoia National Park, California. Environ. Poll., 

118(2): 187-203. 

Bytnerowicz, A.; Fraczek, W.; Schilling, S.; Alexander, D. (2010). Spatial and temporal 

distribution of ambient nitric acid and ammonia in the Athabasca Oil Sands Region, 

Alberta. Journal of Limnology, 69: 11-21. 

Cao, J.J.; Zhang, T.; Chow, J.C.; Watson, J.G.; Wu, F.; Li, H. (2009). Characterization of 

atmospheric ammonia over Xi'an, China. AAQR, 9(2): 277-289. 

Chiriac, R.; Carre, J.; Perrodin, Y.; Vaillant, H.; Gasso, S.; Miele, P. (2009). Study of the 

dispersion of VOCs emitted by a municipal solid waste landfill. Atmos. Environ., 43(11): 

1926-1931. 
38

Chow, J.C.; Watson, J.G.; Bowen, J.L.; Frazier, C.A.; Gertler, A.W.; Fung, K.K.; Landis, D.; 

Ashbaugh, L.L. (1993). A sampling system for reactive species in the western United 

States. In Sampling and Analysis of Airborne Pollutants, Winegar, E. D., Keith, L. H., 

Eds.; Lewis Publishers: Ann Arbor, MI, 209-228.

Chow, J.C.; Watson, J.G.; Lowenthal, D.H.; Magliano, K.L. (2005). Loss of PM2.5 nitrate from 

filter samples in central California. J. Air Waste Manage. Assoc., 55(8): 1158-1168. 

Dasch, J.M.; Cadle, S.H.; Kennedy, K.G.; Mulawa, P.A. (1989). Comparison of annular 

denuders and filter packs for atmospheric sampling. Atmos. Environ., 23(12): 2775-2782. 

Dasch, J.M.; Cadle, S.H. (1990). The removal of nitric acid to atmospheric particles during a 

wintertime field study. Atmos. Environ., 24A(10): 2557-2562. 

DeBell, L.J.; Gebhart, K.A.; Hand, J.L.; Malm, W.C.; Pitchford, M.L.; Schichtel, B.A.; White, 

W.H. (2006). Spatial and seasonal patterns and temporal variability of haze and its 

constituents in the United States: Report IV. prepared by National Parks Service, Fort 

Collins, CO, 

http://vista.cira.colostate.edu/improve/Publications/Reports/2006/PDF/IMPROVE_Repor

t_IV.pdf

Diaz-de-Quijano, M.; Penuelas, J.; Ribas, A. (2009). Increasing interannual and altitudinal ozone 

mixing ratios in the Catalan Pyrenees. Atmos. Environ., 43(38): 6049-6057. 

Eatough, D.J.; Lewis, L.J.; Eatough, M.; Lewis, E.A. (1995). Sampling artifacts in the 

determination of particulate sulfate and SO2 (g) in the desert southwest using filter pack 

samplers. Environ. Sci. Technol., 29(3): 787-791. 

Eldred, R.A.; Cahill, T.A. (1997). Sulfate sampling artifact from SO2 and alkaline soil. Environ. 

Sci. Technol., 31(5): 1320-1324. 

Environment Canada (2011). CAPMon-Canadian air and precipitation monitoring network. 

prepared by Environment Canada, Toronto, ON, http://www.mscsmc.ec.gc.ca/NAtChem/particles/n_capmon_e.html

ERT (1987). Operational Evaluation Network Semi-annual Progress Report: 1 January - 1 

August, 1987. Report Number P-E292-710; prepared by Electric Power Research 

Institute, Palo Alto, CA, 

Ferek, R.J.; Hegg, D.A.; Herring, J.A.; Hobbs, P.V. (1991). An improved filter pack technique 

for airborne measurement of low concentrations of SO2. J. Geophys. Res., 96(D12): 

22373-22378. 

Ferek, R.J.; Covert, P.A.; Luke, W. (1997). Intercomparison of measurements of sulfur dioxide 

in ambient air by carbonate-impregnated filters and Teco pulsed-fluorescence analyzers. 

J. Geophys. Res., 102(D13): 16267-16272. 
39

Forrest, J.; Newman, L. (1973). Sampling and analysis of atmospheric sulfur compounds for 

isotope ratio studies. Atmos. Environ., 7(5): 561-573. 

Forrest, J.; Tanner, R.L.; Spandau, D.; D'uttavio, T.; Newman, L. (1980). Determination of total 

inorganic nitrate utilizing collection of nitric acid on NaCl--impregnated filters. Atmos. 

Environ., 14(1): 137-144. 

Fung, K.K.; Heisler, S. (1987). Measurement of Particles and Gases for Operational Evaluation 

of Acid Deposition Models. West Coast Section - APCA: Ventura, CA.

Fung, K.K. (1988a). Development and evaluation of chemically impregnated filters for 

deposition monitoring. 19 June 88 A.D.a; Dallas, TX.

Fung, K.K. (1988b). Design of a sampler for peroxyacetyl nitrate monitoring. Air Pollution 

Control Association: Raleigh, NC.

Gilliland, A.B.; Butler, T.J.; Likens, G.E. (2002). Monthly and annual bias in weekly 

(NADP/NTN) versus daily (AIRMoN) precipitation chemistry data in the eastern USA. 

Atmos. Environ., 36(33): 5197-5206. 

Goodman, A.L.; Underwood, G.M.; Grassian, V.H. (2000). A laboratory study of the 

heterogeneous reaction of nitric acid on calcium carbonate particles. J. Geophys. Res., 

105(D23): 29053-29064. 

Hansen, D.A.; Edgerton, E.S.; Hartsell, B.E.; Jansen, J.J.; Kandasamy, N.; Hidy, G.M.; 

Blanchard, C.L. (2003). The Southeastern Aerosol Research and Characterization Study: 

Part 1 - Overview. J. Air Waste Manage. Assoc., 53(12): 1460-1471. 

Hansen, D.A.; Edgerton, E.; Hartsell, B.; Jansen, J.; Burge, H.; Koutrakis, P.; Rogers, C.; Suh, 

H.; Chow, J.C.; Zielinska, B.; McMurry, P.; Mulholland, J.; Russell, A.; Rasmussen, R. 

(2006). Air quality measurements for the aerosol research and inhalation epidemiology 

study. J. Air Waste Manage. Assoc., 56(10): 1445-1458. 

Huygen, C. (1963). The sampling of sulfur dioxide in air with impregnated filter paper. Anal. 

Chim. Acta., 28: 349-360. 

IMPROVE (2011). Interagency Monitoring of Protected Visual Environments. prepared by 

National Park Service, Ft. Collins, CO, http://vista.cira.colostate.edu/IMPROVE

Japar, S.M.; Brachaczek, W.W. (1984). Artifact sulfate formation from SO2 on nylon filters. 

Atmos. Environ., 18(11): 2479-2482. 

Johnson, D.A.; Atkins, D.H.F. (1975). An airborne system for the sampling and analysis of 

sulphur dioxide and atmospheric aerosols. Atmos. Environ., 9(9): 825-829. 
40

Joseph, D.B.; Metsa, J.C.; Malm, W.C.; Pitchford, M.L. (1987). Plans for IMPROVE: A federal 

program to monitor visibility in Class I areas. In Transactions, Visibility Protection: 

Research and Policy Aspects, Bhardwaja, P. S., Ed.; Air Pollution Control Association: 

Pittsburgh, PA, 113-125.

Kim, J.C.; Allen, E.R. (1997). Effects of filter pack sampling conditions on observed ambient 

concentrations of dry acid deposition species. Chemosphere, 34(3): 587-610. 

Kirchner, M.; Jakobi, G.; Felcht, E.; Bernhardt, M.; Fischer, A. (2005). Elevated NH3 and NO2

air concentrations and nitrogen deposition rates in the vicinity of a highway in Southern 

Bavaria. Atmos. Environ., 39(25): 4531-4542. 

Krueger, B.J.; Grassian, V.H.; Cowin, J.P.; Laskin, A. (2004). Heterogeneous chemistry of 

individual mineral dust particles from different dust source regions: the importance of 

particle mineralogy. Atmos. Environ., 38(36): 6253-6261. 

Laskin, A.; Wietsma, T.W.; Krueger, B.J.; Grassian, V.H. (2005). Heterogeneous chemistry of 

individual mineral dust particles with nitric acid: A combined CCSEM/EDX, ESEM, and 

ICP-MS study. J. Geophys. Res. -Atmospheres, 110(D10): D10208. 

doi:10.1029/2004JD005206. 

Lavery, T.F.; Rogers, C.M.; Baumgardner, R.; Mishoe, K.P. (2009). Intercomparison of Clean 

Air Status and Trends Network nitrate and nitric acid measurements with data from other 

monitoring programs. J. Air Waste Manage. Assoc., 59(2): 214-226. 

Leppanen, S.; Anttila, P.; Lattila, H.; Makkonen, U. (2005). Long-term comparison of filter 

method and sensitive analyser in monitoring of sulphur dioxide. Atmos. Environ., 39(14): 

2683-2693. 

Lewin, E.E.; Zachau-Christiansen, B. (1995). Efficiency of 0.5 N KOH impregnated filters for 

SO2 Collection. Atmos. Environ., 11: 861-862. 

Liu, Y.J.; Zhu, T.; Zhao, D.F.; Zhang, Z.F. (2008). Investigation of the hygroscopic properties of 

Ca(NO3)(2) and internally mixed Ca(NO3)(2)/CaCO3 particles by micro-Raman 

spectrometry. Atmos. Chem. Phys., 8(23): 7205-7215. 

MACTEC (2009). Clean Air Status and Trends (CASTNET) quality assurance project plan 

revision 6.0. prepared by MACTEC Engineering and Consulting, Inc., Gainesville, FL, 

http://epa.gov/castnet/javaweb/docs/qapp_v6_Main_Body.pdf

MACTEC (2010). Clean Air Status and Trends (CASTNET) 2008 annual report. prepared by 

MACTEC Engineering and Consulting, Inc., Gainesville, FL, 

http://epa.gov/castnet/javaweb/docs/annual_report_2008.pdf

Malm, W.C.; Pitchford, M.L.; Scruggs, M.; Sisler, J.F.; Ames, R.G.; Copeland, S.; Gebhart, 

K.A.; Day, D.E. (2000). Spatial and seasonal patterns and temporal variability of haze 
41

and its constituents in the United States: IMPROVE Report III. Report Number ISSN: 

0737-5352-47; prepared by Cooperative Institute for Research in the Atmosphere, 

Colorado State University, Ft. Collins, CO, 

http://vista.cira.colostate.edu/IMPROVE/Publications/improve_reports.htm

Martins, J.J.; Dhammapala, R.S.; Lachmann, G.; Galy-Lacaux, C.; Pienaar, J.J. (2007). Longterm measurements of sulphur dioxide, nitrogen dioxide, ammonia, nitric acid and ozone 

in southern Africa using passive samplers. South African Journal of Science, 103(7-8): 

336-342. 

Masia, P.; Di Palo, V.; Possanzini, M. (1994). Uptake of ammonia by nylon filters in filter pack 

systems. Atmos. Environ., 28(2): 365-366. 

Matsuda, Y.; Cahill, T.A. (1985). Alkali impregnated Teflon as a filter for atmospheric SO2

PIXE analysis. Nuclear Instruments and Methods in Physics Research, B6: 574-584. 

Meng, Z.Y.; Xu, X.B.; Wang, T.; Zhang, X.Y.; Yu, X.L.; Wang, S.F.; Lin, W.L.; Chen, Y.Z.; 

Jiang, Y.A.; An, X.Q. (2010). Ambient sulfur dioxide, nitrogen dioxide, and ammonia at 

ten background and rural sites in China during 2007-2008. Atmos. Environ., 44(21-22): 

2625-2631. 

Mosquera, J.; Monteny, G.J.; Erisman, J.W. (2005). Overview and assessment of techniques to 

measure ammonia emissions from animal houses: the case of the Netherlands. Environ. 

Poll., 135(3): 381-388. 

Moumen, N.; Yi, S.M.; Raymond, H.A.; Han, Y.J.; Holsen, T.M. (2004). Quantifying the dry 

deposition of ammonia in ammonia-rich and ammonia-poor environments using a 

surrogate surface approach. Atmos. Environ., 38(17): 2677-2686. ISI:000221365200007.

Nishikawa, Y.; Murano, K.; Mukai, H. (2009). Comparison of sampling resistance for one to 

three sheets of membrane type passive sampler. Water Air and Soil Pollution, 197(1-4): 

241-247. 

NOAA (2011). The Atmospheric Integrated Monitoring Network (AIRMoN). prepared by 

National Oceanic and Atmospheric Administration Air Resources Laboratory, Silver 

Springs, MD, http://www.atdd.noaa.gov/?q=node/46

Ooki, A.; Uematsu, M. (2005). Chemical interactions between mineral dust particles and acid 

gases during Asian dust events. J. Geophys. Res. -Atmospheres, 110(D3)

Orr, D.B.; Hipfner, J.C.; Chan, W.H.; Lusis, M.A.; Hunt, J.E. (1967). The application of a 

passive permeation device for the measurement of ambient sulfur dioxide. Atmos. 

Environ., 21(6): 1473-1475. 

Pennequin-Cardinal, A.; Plaisance, H.; Locoge, N.; Ramalho, O.; Kirchner, S.; Galloo, J.C. 

(2005). Performances of the Radiello((R)) diffusive sampler for BTEX measurements: 
42

Influence of environmental conditions and determination of modelled sampling rates. 

Atmos. Environ., 39(14): 2535-2544. 

Plaisance, H.; Leonardis, T.; Gerboles, M. (2008). Assessment of uncertainty of benzene 

measurements by Radiello diffusive sampler. Atmos. Environ., 42(10): 2555-2568. 

Ramadan, A.A. (2010). Air quality assessment in Southern Kuwait using diffusive passive 

samplers. Environ. Mon. Assess., 160(1-4): 413-423. 

Ribas, A.; Penuelas, J. (2006). Surface ozone mixing ratio increase with altitude in a transect in 

the Catalan Pyrenees. Atmos. Environ., 40(38): 7308-7315. 

Sickles, J.E., II; Michie, R.M. (1987). Evaluation of the performance of sulfation and nitration 

plates. Atmos. Environ., 21(6): 1385-1391. 

Sickles, J.E., II; Grohse, P.M.; Hodson, L.L.; Salmons, C.A.; Cox, K.W.; Turner, A.R.; Estes, 

E.D. (1990). Development of a method for the sampling and analysis of sulfur dioxide 

and nitrogen dioxide from ambient air. Anal. Chem., 62(4): 338-346. 

Sickles, J.E., II; Hodson, L.L. (1999a). Short communication: Retention of sulfur dioxide by 

nylon filters. Atmos. Environ., 33: 2423-2426. 

Sickles, J.E., II; Hodson, L.L. (1999b). Retention of sulfur dioxide by nylon filters. Atmos. 

Environ., 33(15): 2427-2434. 

Sickles, J.E., II; Hodson, L.L.; Vorburger, L.M. (1999). Evaluation of the filter pack for longduration sampling of ambient air. Atmos. Environ., 33(14): 2187-2202. 

Sickles, J.E., II; Shadwick, D.S. (2002). Precision of atmospheric dry deposition data from the 

Clean Air Status and Trends Network. Atmos. Environ., 36(36-37): 5671-5686. 

Sickles, J.E., II; Shadwick, D.S. (2008). Comparison of particulate sulfate and nitrate at 

collocated CASTNET and IMPROVE sites in the eastern US. Atmos. Environ., 42(9): 

2062-2073. 

Sisler, J.F.; Malm, W.C.; Gebhart, K.A. (1996). Spatial and seasonal patterns and long-term 

variability of the haze in the United States: An analysis of data from the IMPROVE 

network. Report Number ISSN 0737-5352-32; prepared by Cooperative Institute for 

Research in the Atmosphere, Ft. Collins, CO, 

vista.cira.colostate.edu/IMPROVE/Publications/improve_reports.htm

Smith, L. (2002). Analysis of commented vs. uncommented samples from the Clean Air Status 

and Trends Network (CASTNet). Atmos. Environ., 36(10): 1649-1653. 

Spicer, C.W.; Schumacher, P.M. (1979). Particulate nitrate: Laboratory and field studies of 

major sampling interferences. Atmos. Environ., 13: 543-552. 
43

Strandberg, B.; Sunesson, A.L.; Olsson, K.; Levin, J.O.; Ljungqvist, G.; Sundgren, M.; Sallsten, 

G.; Barregard, L. (2005). Evaluation of two types of diffusive samplers and adsorbents 

for measuring 1,3-butadiene and benzene in air. Atmos. Environ., 39(22): 4101-4110. 

Strandberg, B.; Sunesson, A.L.; Sundgren, M.; Levin, J.O.; Sallsten, G.; Barregard, L. (2006). 

Field evaluation of two diffusive samplers and two adsorbent media to determine 1,3-

butadiene and benzene levels in air. Atmos. Environ., 40(40): 7686-7695. 

Sturaro, A.; Rella, R.; Parvoli, G.; Ferrara, D. (2010). Long-term phenol, cresols and BTEX 

monitoring in urban air. Environ. Mon. Assess., 164(1-4): 93-100. 

Talbot, R.W.; Vijgen, A.S.; Harris, R.C. (1990). Measuring tropospheric HNO3: Problems and 

prospects for nylon filter and mist chamber techniques. J. Geophys. Res., 95: 7553-7561. 

Tsai, C.J.; Huang, C.H.; Lu, H.H. (2004). Adsorption capacity of a nylon filter of filter pack 

system for HCl and HNO3 gases. Separation Science and Technology, 39(3): 629-643. 

U.S.EPA (2011a). Policy Assessment for the Review of the Secondary National Ambient Air 

Quality Standards for Oxides of Nitrogen and Oxides of Sulfur (January 14, 2011 

Version). Report Number EPA-452/R-11-004a; prepared by Office of Air Quality 

Planning and Standards, Health and Environmental Impacts Division, Research Triangle 

Park, NC, 

U.S.EPA. (2011b). Photochemical Assessment Monitoring Stations. U.S.EPA

Umann, B.; Arnold, F.; Schaal, C.; Hanke, M.; Uecker, J.; Aufmhoff, H.; Balkanski, Y.; van 

Dingenen, R. (2005). Interaction of mineral dust with gas phase nitric acid and sulfur 

dioxide during the MINATROC II field campaign: First estimate of the uptake coefficient 

gamma(HNO3) from atmospheric data. J. Geophys. Res. -Atmospheres, 110(D22)

Underwood, G.M.; Song, C.H.; Phadnis, M.; Carmichael, G.R.; Grassian, V.H. (2001). 

Heterogeneous reactions of NO2 and HNO3 on oxides and mineral dust: A combined 

laboratory and modeling study. J. Geophys. Res., 106(D16): 18055-18066. 

Usher, C.R.; Al-Hosney, H.; Carlos-Cuellar, S.; Grassian, V.H. (2002). A laboratory study of the 

heterogeneous uptake and oxidation of sulfur dioxide on mineral dust particles. J. 

Geophys. Res., 107(D23): 4713. doi:10.1029/2002JD002051. 

VIEWS (2010). Visibility Information Exchange Web System. prepared by Colorado State 

University, Ft. Collins, CO, http://vista.cira.colostate.edu/views/

Watson, J.G.; Chow, J.C.; Egami, R.T.; Bowen, J.L.; Frazier, C.A.; Gertler, A.W.; Lowenthal,

D.H.; Fung, K.K. (1991). Measurements of dry deposition parameters for the California 

Acid Deposition Monitoring Program. Report Number DRI 8068.2F1; prepared by Desert 

Research Institute, Reno, NV, for California Air Resources Board, Sacramento, CA; 

http://www.arb.ca.gov/research/apr/past/a6-076-32.pdf
44

Watson, J.G. (2002). Visibility: Science and regulation - 2002 Critical Review. J. Air Waste 

Manage. Assoc., 52(6): 628-713. 

Zhang, L.; Vet, R.; O'Brien, J.M.; Mihele, C.; Liang, Z.; Wiebe, A. (2009). Dry deposition of 

individual nitrogen species at eight Canadian rural sites. J. Geophys. Res. -Atmospheres, 

114

Zhang, X.; McMurry, P.H. (1991). Theoretical analysis of evaporative losses of adsorbed or 

absorbed species during atmospheric aerosol sampling. Enivron. Sci. Technol., 25: 456-

459. 

Zhang, X.Q.; McMurry, P.H. (1987). Theoretical analysis of evaporative losses from impactor 

and filter deposits. Atmos. Environ., 21(8): 1779-1789. 
45

Comments from Dr. Kenneth Demerjian

Charge Question 1: What are the Panel’s views on using the CASTNET filter pack (FP) to 

measure particulate sulfate for the purpose of providing annual average values as an indicator 

for the NOx/SOx standard? Given EPA plans primarily to document the capability of the 

CASTNET FP and develop the FRM for particulate sulfate based on the existing information and 

procedures, what are the Panel’s views of this approach for setting the FRM?

This methodology should be consider as one of the several PM-SO4 techniques available (i.e. 

CSN, IMPROVE and continuous SO4) to track trends in PM_ SO4. Although the network 

methodologies may not report measured concentrations that agree in absolute terms, the 

identification of measurement bias and the demonstration of the methodologies consistency in 

reporting relative changes in ambient concentrations with respect to measured/estimated 

emission changes will be valuable in estimating uncertainty in trend analyses. 

Charge Question 2: What are the Panel’s views on using the CASTNET filter pack (FP) to 

measure sulfur dioxide gas for the purpose of providing annual average values as an indicator 

for the NOx/SOx standard? If EPA would document the capability of the CASTNET FP and 

develops an FRM for sulfur dioxide gas based on the existing information and procedures, what 

are the Panel’s views of this approach for setting the FRM?

Unlike PM- SO4 where particle size affects absolute concentration (see discussion item 1 above) 

comparisons, selected continuous SO2 measurements should be compared with annual 

CASTNET SO2 measurements as well as trends analyses to demonstration of consistency in 

reporting relative changes in ambient concentrations with respect to measured/estimated 

emission change.

Charge Question 3: What are the Panel’s views on using the current primary FRM (high time 

resolution UVF) to measure sulfur dioxide gas for the purpose of providing annual average 

values as an indicator for the NOx/SOx standard?

The current primary FRM (high time resolution UVF) to measure SO2 gas for the purpose of 

providing annual average values as an indicator for the NOx / SOx standard is acceptable. The 

agency should recognize that SO2 ambient concentrations have been reduced substantial over the 

past decade and that trace level versions of SO2 monitors are need to minimize non-detects and 

capture relevant statistical annual averages. This is a particular important issue at rural/regional 

monitoring sites where for example in New York State (Pinnacle State Park and Whiteface 

Mountain) annual average SO2 concentrations in 1997 and 2007 were ( 3.48 – 1.77 ppb; at PSP) 

and (1.03 – 0.35 ppb; at WFM). 
46

Charge Question 4: What are the panel’s views on using existing NOy methods that are 

deployed, for example, in NCore as the measurement approach for NOy for the purpose of 

providing annual average values as an indicator for the NOx/SOx standard? What are the 

panel’s views on EPA’s assessment that additional study is needed before establishing an FRM 

based on the existing NOy methods? That is, are the methods already adequately demonstrated 

as a reference method to determine compliance with a NAAQS? What are the panel’s views on 

the research plan for establishing existing NOy methods as an FRM? [Note suggested 

improvement to the plan would be appreciated, particularly ones that would help complete the 

study on time.]

Existing NOy measurement technology is adequate, but requires establishing more rigorous 

QA/QC procedures. This includes providing training for network technicians and the 

development of calibration standards and protocols for estimating convertor efficiency. 

Charge Question 5: What are the panel’s views on using the emerging AMoN ammonia 

monitoring network that uses passive sampling technology as a tool for evaluating air quality 

model behavior with respect to characterizing ambient air patterns of ammonia?

There is very little documentation in the peer reviewed literature regarding the performance of 

the Radiello® passive samplers. Although NADP has adopted this technique, it should undergo 

performance evaluation with intercomparison studies with other NH3 monitoring techniques, the 

results of which should be reported in the peer reviewed literature. These comparisons should 

also the challenge the passive sampler’s sensitivity to wind speed and temperature extremes? In 

addition, although two-week average data can be used to aggregate seasonal and annual average 

ammonia concentrations, these data are far from what is necessary to challenge air quality model 

behavior. It is unlikely that the deployment of sufficient number of samplers will be practical 

(cost or technically) to fully characterize the spatial distribution of ambient NH3 considering the 

complexity of its sources and sinks. Only high temporal resolution (minutes to hour) ambient 

NH3 measurements afford the opportunity to: 1) measure NH3 fluxes (to characterize local 

sources and sinks); 2) identify NH3 source plumes through wind sector analyses; and 3) 

characterize NH3 concentrations in the stable (typically nocturnal) boundary layer for the 

evaluation of AQ models and to assess the potential bias the SBL introduces to model -

integrated sampling comparisons.

Charge Question 6: What are the panel’s views on co-locating ammonia measurements at each 

location where the indictors are measured?

This would be reasonable requirement and would provide an opportunity to evaluate the model 

based F2 estimate and the utility of the calculated indicator approach. 
47

Charge Question 7: What are the Panel’s views on using the CASTNET filter pack (FP) to 

measure ammonium ion as a tool for evaluating air quality model behavior with respect to 

characterizing ambient air patterns of ammonia?

Models should use whatever data is available to test and evaluate their performance. That being 

said, I am not sure that CASTNET FP ammonium ion measurements would be effective in 

characterizing ambient air patterns of ammonia. If it has not already been done, an analysis of 

the SEARCH data set should provide some insight as to challenges this comparison poses given 

the distinctly different residence times of these two species. 

Charge Question 8: What are the panel’s views on establishing a suite of NOy species 

measurements at 2- 5 locations in different atmospheric and ecological regions for the purpose 

of evaluating air quality model and NOy instrument behavior? 

I would be strongly supportive of such studies. Prioritization of NOy components to be 

considered is HNO3, NO2, PM_NO3 and PAN. 

Charge Question 9: What are the panel’s views on utilizing the existing CASTNET and rural 

NCore networks as a starting infrastructure for the purpose of supporting the NOx/SOx 

standard?

This is a very reasonable starting point.

Charge Question 10: What are the panel’s views on using CASTNET filter pack (FP) to measure 

total nitrate (particulate nitrate plus nitric acid) as the measurement approach for the purpose of 

providing annual average values to support the NOx/SOx standard in diagnosing NOy 

instrument behavior and assist in delineating the relative fractions of contributing oxidized 

nitrogen species to total ambient oxidized nitrogen.

This is an acceptable approach.

Charge Question 11: What are the panel’s view of the broader consideration of using 

CASTNET, complemented by rural NCore, to serve as a framework for the nation’s rural 

monitoring of important gases and aerosols in support of secondary standards and evaluating 

the behavior of regional air quality models?

Again, this is a very reasonable concept and starting point. It remains to be seen if this approach 

would provide sufficient spatial coverage to support the secondary SOx / NOx standard.
48

Comments from Dr. Eric Edgerton

Charge Question 1: What are the Panel’s views on using the CASTNET filter pack (FP) to 

measure particulate sulfate for the purpose of providing annual average values as an indicator 

for the NOx/SOx standard? Given EPA plans primarily to document the capability of the 

CASTNET FP and develop the FRM for particulate sulfate based on the existing information and 

procedures, what are the Panel’s views of this approach for setting the FRM?

Other than the size-cut issue, the CASTNet FP should be suitable as an indicator for the standard. 

That said, I would be more comfortable if SO4 were measured with a defined size cut (PM2.5 or 

PM10) and with SO2 removal up front (lessons learned from the CSN).

Charge Question 2: What are the Panel’s views on using the CASTNET filter pack (FP) to 

measure sulfur dioxide gas for the purpose of providing annual average values as an indicator 

for the NOx/SOx standard? If EPA would document the capability of the CASTNET FP and 

develops an FRM for sulfur dioxide gas based on the existing information and procedures, what 

are the Panel’s views of this approach for setting the FRM?

I support the use of the CASTNet FP for measuring annual average SO2 concentrations. 

Characterization by EPA will likely show this approach has the sensitivity and specificity to 

support the NOx / SOx standard.

Charge Question 3: What are the Panel’s views on using the current primary FRM (high time 

resolution UVF) to measure sulfur dioxide gas for the purpose of providing annual average 

values as an indicator for the NOx/SOx standard?

Continuous data are the way to go for challenging models, looking at short-term effects and 

research purposes. Detection limits for current technology are on the order of 50-100 parts per 

trillion (ppt) or 0.13 ug/m3. My main concern with continuous measurements is that ambient 

concentrations are very low already and likely to drop even further in the next 5 years. As an 

example, average SO2 in 2010 at the Yorkville, GA SEARCH site was 986 ppt. Hourly SO2

concentrations were <100 ppt 15% of the time, <200 ppt 28% of the time, <500 ppt 50% of the 

time and <1000 ppt 68% of the time. Other rural sites in the SE (and maybe the NE) have even 

lower concentrations. To obtain meaningful data (short-term AND long-term averages) will 

require very careful management of instrument baseline. The figure below shows typical 

summertime SO2 at YRK.
49

Charge Question 4: What are the panel’s views on using existing NOy methods that are 

deployed, for example, in NCore as the measurement approach for NOy for the purpose of 

providing annual average values as an indicator for the NOx/SOx standard? What are the 

panel’s views on EPA’s assessment that additional study is needed before establishing an FRM 

based on the existing NOy methods? That is, are the methods already adequately demonstrated 

as a reference method to determine compliance with a NAAQS? What are the panel’s views on 

the research plan for establishing existing NOy methods as an FRM?

Additional study is needed to establish an FRM for NOy. Like SO2, current NOx analyzers are 

very sensitive, but it is not clear they are seeing or quantifying all NOy components or to what 

extent there is interference from non- NOy components (e.g., ammonia or particulate- NH4). 

Extreme care is needed to ensure transmission of the more reactive components of NOy into the 

catalytic converter and to monitor the efficiency of the converter. NCORE will be a very good 

test bed for NOy measurements. Careful review of NCORE data will yield valuable insights into 

operational issues and resource requirements (e.g., what is the expected lifetime of a converter 

and is it possible to regenerate a converter?). Also, does it make sense to use NO for calibration 

purposes when the majority of NOy in rural environments is NO2 and higher?

Charge Question 5: What are the panel’s views on using the emerging AMoN ammonia 

monitoring network that uses passive sampling technology as a tool for evaluating air quality 

model behavior with respect to characterizing ambient air patterns of ammonia?

Passive samplers can be used for 1-week or 2-week comparisons, but the model really needs to 

be tested on much shorter time scales (hourly or daily) to ensure the model has the processes 

right and to make source attribution inferences. The NOy sites in Q8 should be equipped with 

NH3 samplers or analyzers for higher time resolution measurements. 

Charge Question 6: What are the panel’s views on co-locating ammonia measurements at each 

location where the indictors are measured?
50

I strongly support this idea.

Charge Question 7: What are the Panel’s views on using the CASTNET filter pack (FP) to 

measure ammonium ion as a tool for evaluating air quality model behavior with respect to 

characterizing ambient air patterns of ammonia?

The CASTNet filter pack should be modified to collect both ammonia and ammonium. This can 

be done by introducing a denuder (annular or honeycomb) upstream of the filter pack and an acid 

impregnated filter (citric or phosphorous) to the back of the FP.

Charge Question 8: What are the panel’s views on establishing a suite of NOy species 

measurements at 2- 5 locations in different atmospheric and ecological regions for the purpose 

of evaluating air quality model and NOy instrument behavior?

I strongly support this idea. The more sites the better. Target components should be NO, 

photolytic NO2, HNO3 and PANs. Solid techniques for NO2 and HNO3 are in use by researchers 

in numerous part of the country. These can be adapted to more routine monitoring applications. 

Thermal-photolytic-chemiluminescent approaches to PANs should be explored.

Charge Question 9: What are the panel’s views on utilizing the existing CASTNET and rural 

NCore networks as a starting infrastructure for the purpose of supporting the NOx/SOx 

standard?

This is a good starting point. However, I have some concerns that site locations and density will 

not adequately address variability in terrain, vegetation or source strength in areas of highest 

sensitivity. Reactive gases exhibit strong gradients between low elevation (valley) and high 

elevation (ridge) depending, in part, on sources within the valley. I am not sure these gradients 

can be represented by widely spaced CASTNet or NCORE sites.

Charge Question 10: What are the panel’s views on using CASTNET filter pack (FP) to measure 

total nitrate (particulate nitrate plus nitric acid) as the measurement approach for the purpose of 

providing annual average values to support the NOx/SOx standard in diagnosing NOy 

instrument behavior and assist in delineating the relative fractions of contributing oxidized 

nitrogen species to total ambient oxidized nitrogen.

Simple modification of the CASTNet FP would greatly enhance is utility for the above purposes. 

As for ammonia, a KCl denuder ahead of the FP would capture HNO3 while the downstream 

filters would collect particulate NO3. There will still be confounding effects of coarse particulate 

NO3, but at least this separates the gas phase from the particulate phase.
51

Charge Question 11: What are the panel’s view of the broader consideration of using 

CASTNET, complemented by rural NCore, to serve as a framework for the nation’s rural 

monitoring of important gases and aerosols in support of secondary standards and evaluating 

the behavior of regional air quality models?

As stated above, I think this is a reasonable start in a resource-constrained environment, but we 

need to take a hard look at techniques and siting. As noted by others, there are concerns about 

separation of monitoring responsibilities in CASTNET, which has traditionally been the 

bailiwick of the state and local agencies. The latter might be mitigated to some extent by 

enhanced auditing of CASTNET sites.
52

Comments from Mr. Henry (Dirk) Felton

Introduction:

The EPA should be commended for undertaking the very difficult challenge of designing the first 

multi-pollutant air quality standard. The standard should provide for a better understanding of 

the ecological impacts of air pollution on sensitive water bodies. One omission in the current 

design of the standard is any commitment to make measurements of water quality. Some regions 

of the country have relatively little water quality data which makes it difficult to evaluate the 

range of potential acidic indexes and the number of lakes in each region which are likely to be 

above or below a particular level. Water quality measurements at a minimum should include 

ANC and pH and should be collected so that the data is comparable from year to year. The 

change in the physical ecological indicator of ANC and pH over time will provide the necessary 

accountability for the modeling and monitoring approach used by the secondary standard and 

will also provide evidence to the public and other stakeholders that the program is providing real 

results. 

The EPA’s plan to designate modeled parameters that specify regional acid sensitivity to air 

pollutants as part of the NAAQS is problematic. These initially derived factors should be 

considered to be preliminary. The performance of the model factors in each region should be 

demonstrated and subject to change if new environmental data suggests that they can be 

improved. The EPA should allow the responsible State and Local monitoring agencies to either 

opt in or suggest alternate model parameters prior to the initial designations under this NAAQS. 

The EPA could then use a weight of evidence type of approach to consider which model 

parameters are appropriate for use in the first, and then for subsequent designations. This 

periodic check of model performance should also be expanded to include a comparison of the 

acidic index to representative lake water ANC.

Charge Question 1: What are the Panel’s views on using the CASTNET filter pack (FP) to 

measure particulate sulfate for the purpose of providing annual average values as an indicator 

for the NOx/SOx standard? Given EPA plans primarily to document the capability of the 

CASTNET FP and develop the FRM for particulate sulfate based on the existing information and 

procedures, what are the Panel’s views of this approach for setting the FRM?

This is an inexpensive and probably reasonable approach. The primary issue not resolved with 

this technique is the unknown collection and retention of larger particles. This could be 

significant in some eco-regions primarily in the western half of the country. The EPA should 

collocate this method with a low volume PM-10 in order to evaluate the retention of these 

particles by the CASTNET FP. There may be collocated data already available with CASTNET 

and IMPROVE PM-10.

Charge Question 2: What are the Panel’s views on using the CASTNET filter pack (FP) to 

measure sulfur dioxide gas for the purpose of providing annual average values as an indicator 
53

for the NOx/SOx standard? If EPA would document the capability of the CASTNET FP and 

develops an FRM for sulfur dioxide gas based on the existing information and procedures, what 

are the Panel’s views of this approach for setting the FRM?

A CASTNET filter pack or one of the commercially available passive alternatives is likely to be 

a reasonable alternative to the SO2 FRM in these relatively clean environments. It would make 

sense to compare the CASTNET filter pack, the currently available passive samplers and the 

FRM in winter and summer months to see which is most effective and cost efficient for this 

network. It might be possible to overcome potential differences in the methods by using 

algorithms to adjust filter pack data to more closely emulate the FRM. This is easy to do since 

the Acidic Index is calculated on the past year’s data and this will reduce the need to operate 

large numbers of FRMs in each eco-region. The data from one FRM can be used to adjust the 

data from multiple FEM filter packs within the same or similar eco-region.

Charge Question 3: What are the Panel’s views on using the current primary FRM (high time 

resolution UVF) to measure sulfur dioxide gas for the purpose of providing annual average 

values as an indicator for the NOx/SOx standard?

The trace version of the SO2 FRM will work in these relatively pristine rural areas but the effort 

required to get accurate data at these extremely low concentrations may not be warranted. It is 

not clear that hourly SO2 data is necessary at more than a handful of sites. Perhaps the rural 

NCore sites could provide the high frequency SO2 data that supplements a network that consists 

primarily of passive or filter pack sites. It would be appropriate to initiate the program with a 

less expensive passive SO2 measurement and then require the FRM if it becomes apparent that 

the increased accuracy is needed. 

The operation of the SO2 FRM may have to be modified to make it as accurate and stable as 

possible. The averaging time should be increased to 300 seconds to improve sensitivity and the 

quality and frequency of the zero adjustments must be increased. The automation of more 

frequent instrument zeros should be strongly encouraged.

A review of ambient SO2 data collected with an FRM in the Adirondack Park region of New 

York State shows that for 2009 and 2010 data: 

Site Average (24-Hr data) Max 24-Hr Min 24-Hr

Piseco Lake 0.4 ppb 5.6 ppb -0.2 ppb

Paul Smith’s 0.6 ppb 4.5 ppb 0.0 ppb

Nick’s Lake 0.6 ppb 5.3 ppb -0.1 ppb

Whiteface Base 0.7 ppb 3.7 ppb 0.1 ppb

Site Average (1-Hr data) Max 1-Hr Min 1-Hr

Piseco Lake 0.4 ppb 12.3 ppb -0.3 ppb

Paul Smith’s 0.6 ppb 9.9 ppb -0.1 ppb

Nick’s Lake 0.5 ppb 13.5 ppb -0.1 ppb
54

Whiteface Base 0.7 ppb 17.5 ppb -0.1 ppb

The average 2009 through 2010 SO2 concentrations from the Piseco Lake site which is close to 

the center of New York’s Adirondack Park is barely twice the detection limit of the 

commercially available trace level SO2 monitor. One concern is that the relative error is quite 

large at these very low concentrations so the utility of the data to accurately discern one site from 

another within the same eco-region may be limited.

Charge Question 4: (a) What are the panel’s views on using existing NOy methods that are 

deployed, for example, in NCore as the measurement approach for NOy for the purpose of 

providing annual average values as an indicator for the NOx/SOx standard? 

The existing NOy method should be deployed but only in a limited number of sites. Unlike most 

of the other NAAQS in which an indicator is correlated with a health outcome, the proposed 

NOy indicator for this standard is being used to measure the sum of air concentrations that lead 

to a net deposition velocity. It is likely that only a few of the primary components of NOy (NO, 

NO2, HNO3, P-NO3 and PAN) will be primarily responsible for nitrogen deposition in different 

regions of the country. This creates a great deal of uncertainty because the NOy measurement 

has not been fully characterized for each of these species in representative areas of the country

and the resulting dataset may not be adequate to advance our understanding of what components 

are primarily responsible for the deposition or for future SIP development.

The current NOy method should be deployed at a handful of specific sites in areas of the country 

where different gas and aerosol compositions exist and where weather patterns are significantly 

different. The collection of this data is necessary in order to initiate model validation in 

comparison to ambient air and water quality data.

Charge Question 4: (b) What are the panel’s views on EPA’s assessment that additional study is 

needed before establishing an FRM based on the existing NOy methods? That is, are the methods 

already adequately demonstrated as a reference method to determine compliance with a 

NAAQS? 

The EPA is correct that the NOy method needs additional evaluation before it should be 

considered to be eligible for FRM status. There are many issues that need to be addressed. The 

method must include a demonstration of conversion efficiencies for the expected suite of 

Nitrogen species in rural areas at all ambient and converter temperatures and with and without 

known interferences, a determination should be made of how long a converter lasts and if the 

converter efficiency changes over time for some species but not others, whether and what type of 

bug screen should be installed, and a determination should be made on a species by species basis 

of optimum inlet height.

It is advisable to optimize the method and to thoroughly document the recommendations prior to 

considering the method for FRM designation. Once an analyzer is designated as an FRM, it is 

more difficult to make needed changes. 
55

Charge Question 4: (c) What are the panel’s views on the research plan for establishing existing 

NOy methods as an FRM? [Note suggested improvement to the plan would be appreciated, 

particularly ones that would help complete the study on time.]

The EPA’s research plan must be thorough and provide ample time for laboratory evaluation and 

for field demonstration. The ambient field demonstrations must include locations that 

encompass a wide range of gas and aerosol compositions including known interferents as well as 

a range of operator experience. Utilizing one site will not meet this need. Some of the issues that 

have come out of the preliminary NCore NOy operation have included questions about cold 

weather calibration, sample residence time between the converter and the analyzer and whether 

the transfer lines should be shielded from light. None of these are addressed in the research plan. 

The EPA must resist the urge to quickly designate an FRM for NOy. The current method may or 

may not meet the intended data need and the designation of a poorly performing FRM will harm 

the ability of monitoring agencies to collect this data in the future. 

Charge Question 5: What are the panel’s views on using the emerging AMoN ammonia 

monitoring network that uses passive sampling technology as a tool for evaluating air quality 

model behavior with respect to characterizing ambient air patterns of ammonia?

The AMoN monitoring technology is promising and it should be considered for use during the 

initial deployment of the NOx/SOx monitoring program. The inexpensive nature and ease of 

deployment of this method are a tremendous benefit. The method can be deployed in its current 

state to determine spatial gradients within an eco-region continuously or on an as needed basis. 

The accuracy and precision of the method must be defined in representative regions and climates 

before the data from the method can be accepted at face value for use in model verification.

Charge Question 6: What are the panel’s views on co-locating ammonia measurements at each 

location where the indictors are measured?

There must be enough monitors to reasonably describe the variation in ammonia concentration 

across each of the eco-regions. This variability in ammonia concentrations over the period of the 

integrated samples for this network that encompasses mountainous rural areas and coastal plains 

is unknown. It would be advantageous to conduct a thorough review of the data collected in the 

first year to determine the adequacy of the network for each of the monitored species including 

ammonia. An analysis of the inter-site ammonia variability in relatively uniform eco-regions 

compared to the variability in non-uniform eco-regions should help. Emphasis should be placed 

on measurements in eco-regions with data indicating that they are near the level of the standard. 

These are the areas that will need the highest density of measurements of the indicators as well 

as for parameters necessary for model evaluation such as ammonia. 
56

Charge Question 8: What are the panel’s views on establishing a suite of NOy species 

measurements at 2- 5 locations in different atmospheric and ecological regions for the purpose 

of evaluating air quality model and NOy instrument behavior? 

This is certainly a good idea and is quite necessary so that the significance of the NOy 

measurement can be fully understood chemically, geographically and seasonally. The individual 

species measurements are difficult, expensive and not at all straightforward to make so they 

should be restricted to just a few locations. The EPA should not expect all monitoring agencies 

to be capable of operating these complex and research grade instruments. The EPA may want to 

partner with other state, local and federal monitoring agencies and academic researchers who 

have experience making these measurements and can accomplish them in selected eco-regions.

Charge Question 9: What are the panel’s views on utilizing the existing CASTNET and rural 

NCore networks as a starting infrastructure for the purpose of supporting the NOx/SOx 

standard?

In general, neither the existing CASTNET nor rural NCore monitoring networks are suitable

alone. Leveraging existing networks is certainly less expensive than initiating new sites but the 

first step must be to identify the appropriate infrastructure for monitoring in support of this 

standard. Then the existing CASTNET, rural NCore, rural State and Local monitoring agency 

sites and IMPROVE sites can be evaluated in comparison to what is needed for each eco-region. 

In some eco-regions, such as in the south-east there may be enough suitably located State and 

Local monitoring agency sites and CASTNET sites to provide a monitoring infrastructure to 

support this standard. In most of the other eco-regions there are fewer CASTNET sites and more 

State and Local monitoring agency and IMPROVE sites. Consideration should be given to 

relocating a rural NCore site to meet NCore and NOx/SOx monitoring objectives if the expenses 

in such a move are less than the cost of establishing another stand-alone site. One advantage to 

considering this now is that the NCore sites are new and do not have a long data record. 

The necessary infrastructure for the initial deployment of this network for each of the large ecoregions should include a water quality monitoring program, one well instrumented site that could 

be an NCore type of sites as well as a few passive satellite monitors that provide a subset of 

measurements in areas of the region where air concentrations are expected to differ. The sites 

should be located so that they are representative of air quality in areas that are:

1. Near moderately impacted water bodies: Severely impacted water bodies may not 

show improved ANC due to air quality improvements over the time scale of the next 

NAAQS review due to other considerations such as local geology. Water bodies with 

minor ANC degradation may show improvements that are too small to detect over the 

interval between NAAQS reviews. 
57

2. Away from significant point sources and eco-region boundaries: The sites should 

represent as large an area with reasonably uniform air quality as possible. 

The satellite sites should be selected to help discern how large an area the central eco-region 

monitor actually represents. Altitude, proximity to upwind sources and meteorological patterns 

affected by valleys and upslope or downslope conditions will all reduce the representativeness of 

central site measurements. The subset of parameters monitored at the satellite sites should be 

selected because they significantly contribute to model uncertainty due to changes in air quality 

over the eco-region. 

One way to economically provide high spatial resolution data is to periodically carry out 

additional passive or CASTNET filter pack monitoring at multiple locations within an eco-region 

for 1-2 consecutive weeks. This might have to be done 2-4 times a year to provide enough 

temporal data to validate the model parameters specific to this region. The extra monitoring 

resources should be targeted for regions that may be close to the acidic index value selected for 

the standard. This extra spatial information could also help characterize the non-uniform ecoregions that have large differences in precipitation, terrain features including valleys and other 

elevation changes and significant sources. 

Charge Question 10: What are the panel’s views on using CASTNET filter pack (FP) to measure 

total nitrate (particulate nitrate plus nitric acid) as the measurement approach for the purpose of 

providing annual average values to support the NOx/SOx standard in diagnosing NOy 

instrument behavior and assist in delineating the relative fractions of contributing oxidized 

nitrogen species to total ambient oxidized nitrogen.

There are ways to modify the filter pack to separate the nitrate and nitric acid for separate 

analysis but this would necessitate a change in the design of the filter pack. This will work but it 

will make the data incompatible with other monitoring objectives such as consistency with the 

rest of the CASTNET monitoring network. It is far simpler and fairly inexpensive to utilize a 

standalone filter and denuder combination for nitrate and nitric acid at a subset of sites where this 

measurement is needed. This would necessitate the installation of another flow channel but that 

would provide the broadest utility for the data.

Charge Question 11: What are the panel’s view of the broader consideration of using 

CASTNET, complemented by rural NCore, to serve as a framework for the nation’s rural 

monitoring of important gases and aerosols in support of secondary standards and evaluating 

the behavior of regional air quality models?

Some State and Local monitoring agencies are not going to be comfortable with CASTNET 

assuming a role in monitoring for comparison to a NAAQS. CASTNET management has not 

demonstrated concern with State and Local monitoring agency objectives and in fact has been 

counterproductive in some instances. The use of a for-profit contractor for the collection of long 
58

term data is not likely to remain viable. Additionally, monitoring agencies need to have the 

flexibility to add sites or parameters as needed to address future modeling uncertainties and 

eventually to address options for SIPs. 

It is likely that some of the CASTNET measurement methods may be appropriate for use in the 

secondary NOx/SOx monitoring network. These methods should be made available on a 

national contract basis similar to the way the air toxics analyses can be obtained for the EPA 

NATTs program. This will permit monitoring agencies to utilize these methods at any of the 

existing or new monitoring sites that are selected for this network. 
59

Comments from Dr. Philip Hopke

It is somewhat difficult to respond to the charge questions since the form and ranges of the levels 

of a proposed secondary standard are not clear in Chapter 2 of the PA, the only chapter sent to 

us. Tuesday’s discussions by the panel will be very helpful in setting the context in which 

measurements are needed. I will attempt to address the questions related to ammonia 

measurements as it is difficult to discuss the value of collocating a sampler without some 

background on the samplers themselves. Thus, I will address the following questions:

Charge Question 6. What are the panel’s views on using the emerging AMoN ammonia 

monitoring network that uses passive sampling technology as a tool for evaluating air quality 

model behavior with respect to characterizing ambient air patterns of ammonia?

Charge Question 7. What are the panel’s views on co-locating ammonia measurements at each 

location where the indictors are measured?

Charge Question 8. What are the Panel’s views on using the CASTNET filter pack (FP) to 

measure ammonium ion as a tool for evaluating air quality model behavior with respect to 

characterizing ambient air patterns of ammonia?

It should be noted that these are numbered differently in the Charge Questions memo from that 

in the Draft Agenda.

Response to Charge Question 6

Although the NADP has been using these samplers since November 2007 at multiple stations, I 

was unable to find comparison or evaluation data. The initial studies appear to show that the 

sampler had adequate precision and accuracy for making such measurements, but it is not clear 

what the field experience has been. It would have been helpful if some evaluative material had 

been provided. Given that they said that “The National Atmospheric Deposition Program will 

evaluate all of the results, and consider whether this special study will become a full standing 

network within the NADP system. This decision should be considered in the Winter, 2009” and 

it is now 2011, one would have hoped for documentation since from the data that can be 

downloaded from the NADP site, they have continued to collect samples through at least last 

summer. Thus, it is somewhat difficult to fully assess the success of the AMoN trials of the 

diffusion sampler. The precision values that were available looked good, but there was no 

obvious accuracy data that I could find. Thus, there has been inadequate information provided to 

permit us to assess the AMoN effort.
60

Response to Charge Question 7

It is clearly highly advantageous to have good ammonia data at every station if there are 

appropriate integrating monitors available with sufficient accuracy and precision. There needs to 

be a DQO effort to define how good the ammonia has to be to permit ion balances to be 

adequately estimated from the set of measured species. As the principal basic gas in the 

atmosphere, it is important to have better data on the concentrations and spatial variation across 

the US. 

Response to Charge Question 8

Clearly the CASNET filter pack can be modified to have a citric acid impregnated filter in the FP 

along with the base-treated filter to get the acid gases. There is a potential problem of 

volatilization of ammonia from the initial filter that collects the particulate ammonium nitrate 

that could lead to an overestimated value of gaseous ammonia. It would seem sensible to do a 

side-by-side comparison between the AMoN diffusion sampler against another filter in the 

CASNET FP. Then through a DQO process, the performance and cost of the samplers can be 

assessed and an appropriate choice made. 

Additional Comments on NADP Ammonia Measurements

I talked to David Gray of the NADP program about their AMoN experience. They tested Ogawa 

badges, the UK ALPHA sample (see website information noted below), the Radiello sample, and 

a standard denuder sampler. They deployed three samplers of each type for 2 week periods. 

They found poor precision with the Ogawa badge and dropped them earlier in the study. The 

Radiello sampler was very simple to use, very hard to contaminate, but somewhat more 

expensive. The UK sampler also showed good precision and was less expensive, but was a bit 

more likely to be contaminated if not handled carefully. They are in the final stages of preparing 

a manuscript to submit for publication at which time he will share the submitted version.

UK ALPHA sample website information regarding the UK’s Methodologies for Measuring 

Ammonia, Trace Gases and Aerosols - Measuring Concentrations of Atmospheric Trace gases 

and aerosols, is found at the following website address:

http://www.uk-pollutantdeposition.ceh.ac.uk/ammonia_methodology

Information on the NADP program, AMoN activities can be found off the following USEPA 

website: http://www.epa.gov/castnet/javaweb/docs/flyer_AMoN.pdf
61

Comments from Dr. Rudolf Husar

Charge Question 1: What are the Panel’s views on using the CASTNET filter pack (FP) to 

measure particulate sulfate for the purpose of providing annual average values as an indicator 

for the NOx/SOx standard? Given EPA plans primarily to document the capability of the 

CASTNET FP and develop the FRM for particulate sulfate based on the existing information and 

procedures, what are the Panel’s views of this approach for setting the FRM?

CASTNET FP should be appropriate for the annual average sulfate. If the CASTNET PF is 

certified as FRM, does it mean that sulfate from IMPROVE/STN would not be used in 

estimating the sulfur indicator? If so, why not use these long-term, robust multi-use networks? 

Charge Question 3: What are the Panel’s views on using the current primary FRM (high time 

resolution UVF) to measure sulfur dioxide gas for the purpose of providing annual average 

values as an indicator for the NOx/SOx standard?

I strongly recommend high time resolution SO2 at rural sites, particularly for model 

evaluation/calibration. The highly variable SO2 concentration over rural regions along with 

extensive SO4 data from IMPROVE/STN is the best index for evaluating the (important and 

highly uncertain) sulfur dry deposition in the model. 

Charge Question 4: What are the panel’s views on using existing NOy methods that are 

deployed, for example, in NCore as the measurement approach for NOy for the purpose of 

providing annual average values as an indicator for the NOx/SOx standard? What are the 

panel’s views on EPA’s assessment that additional study is needed before establishing an FRM 

based on the existing NOy methods? That is, are the methods already adequately demonstrated 

as a reference method to determine compliance with a NAAQS? What are the panel’s views on 

the research plan for establishing existing NOy methods as an FRM? [Note suggested 

improvement to the plan would be appreciated, particularly ones that would help complete the 

study on time.]

I am not qualified to comment on the NOy measurement. 

Charge Question 5: What are the panel’s views on using the emerging AMoN ammonia 

monitoring network that uses passive sampling technology as a tool for evaluating air quality 

model behavior with respect to characterizing ambient air patterns of ammonia?

Not familiar with the AMoN network 
62

Charge Question 6: What are the panel’s views on co-locating ammonia measurements at each 

location where the indictors are measured?

In general, co-location of different measurements is desirable since it enhances the context of the 

air quality characterization. However, for the proposed standard, ammonia is to be provided by 

the CMAQ model, not from observations. Accordingly, the primary use of the ammonia 

measurements is linked to the model: e.g. verification of ammonia emissions, spatial and 

seasonal pattern, information about deposition and chemical reactions, etc. The indicator 

measurements for NOy, SOx are receptor-oriented at the eco-regions. 

Hence, for optimal network design the ammonia-for-model and the indicator-for-AAI may lead 

to different configuration, e.g. higher ammonia monitoring density in the Upper Midwest. 

Charge Question 7: What are the Panel’s views on using the CASTNET filter pack (FP) to 

measure ammonium ion as a tool for evaluating air quality model behavior with respect to 

characterizing ambient air patterns of ammonia?

Given that the pattern of ambient ammonia is both cyclic (diurnal) and episodic, ammonia 

measurements should be at higher time resolution to discern these variations and to compare 

them with the model. 

Charge Question 8: What are the panel’s views on establishing a suite of NOy species 

measurements at 2- 5 locations in different atmospheric and ecological regions for the purpose 

of evaluating air quality model and NOy instrument behavior?

Measuring the complete NOy mix at a few characteristic locations is a terrific idea. As much as 

possible, those ‘super sites’ should also contain other observations that would increase the width 

of the pollutant characterization. 

Charge Question 9: What are the panel’s views on utilizing the existing CASTNET and rural 

NCore network as a starting infrastructure for the purpose of supporting the NOx/SOx standard?

No one should ignore both existing networks and start a brand new one, since CASTNET and 

NCore are not exactly what is the perceived need now. (just kidding!). Of course one should 

reuse existing networks end begin integrating the observations arising from these existing and 

persistent networks, regardless of their respective ‘original’ purpose.

Actually I would ask why are the IMPROVE/STN and the NADP not included in the pool of 

relevant measurements? They are not ‘FRM’? They are NIH?

Charge Question 10: What are the panel’s views on using CASTNET filter pack (FP) to measure 
63

total nitrate (particulate nitrate plus nitric acid) as the measurement approach for the purpose of 

providing annual average values to support the NOx/SOx standard in diagnosing NOy 

instrument behavior and assist in delineating the relative fractions of contributing oxidized 

nitrogen species to total ambient oxidized nitrogen.

I am not qualified to comment on total nitrate measurement. 
64

Comments from Dr. Daniel Jacob

Charge Question 1: What are the Panel’s views on using the CASTNET filter pack (FP) to 

measure particulate sulfate for the purpose of providing annual average values as an indicator 

for the NOx/SOx standard? Given EPA plans primarily to document the capability of the 

CASTNET FP and develop the FRM for particulate sulfate based on the existing information and 

procedures, what are the Panel’s views of this approach for setting the FRM?

I think that it is an appropriate method. The FP is a well-established method for reliable sulfate 

measurements. Weekly temporal resolution is fine – higher would be overkill. The CASTNET 

FP only measures the <2.5um fine fraction, and the coarser fraction makes a significant 

contribution to sulfate deposition. However, using a measurement integrating all sizes would

just complicate interpretation of the relative contribution of coarse vs. fine in the measurement.

Better to just measure the fine fraction and use a model- or observation-derived correction factor 

to add the relevant contribution of the coarse fraction (excluding sea salt and soil dust).

Charge Question 2: What are the Panel’s views on using the CASTNET filter pack (FP) to 

measure sulfur dioxide gas for the purpose of providing annual average values as an indicator 

for the NOx/SOx standard? If EPA would document the capability of the CASTNET FP and 

develops an FRM for sulfur dioxide gas based on the existing information and procedures, what 

are the Panel’s view of this approach for setting the FRM?

An issue with SO2 is the large diurnal variation that correlates with deposition velocity. The 

diurnal amplitude for SO2 is much larger than for sulfate and matters more since SO2 makes a 

much bigger contribution to SOx dry deposition than sulfate. This complicates the interpretation 

of weekly measurements. Hourly measurement by the primary FRM would be much better. If 

cost is an issue with using the primary FRM, then perhaps the primary FRM could be used to 

inform the general diurnal pattern of SO2 concentrations needed to interpret the weekly 

measurements.

Charge Question 3: What are the Panel’s views on using the current primary FRM (high time 

resolution UVF) to measure sulfur dioxide gas for the purpose of providing annual average 

values as an indicator for the NOx/SOx standard?

I think that this would be much better than the FP because the hourly resolution is important for 

SO2 (cf. comment on question 2).

Charge Question 4: What are the panel’s views on using existing NOy methods that are 

deployed, for example, in NCore as the measurement approach for NOy for the purpose of 

providing annual average values as an indicator for the NOx/SOx standard? What are the 
65

panel’s views on EPA’s assessment that additional study is needed before establishing an FRM 

based on the existing NOy methods? That is, are the methods already adequately demonstrated 

as a reference method to determine compliance with a NAAQS? What are the panel’s views on 

the research plan for establishing existing NOy methods as an FRM? [Note suggested 

improvement to the plan would be appreciated, particularly ones that would help complete the 

study on time.]

Instruments to measure total NOy have been compared successfully in the past to the sum of 

measurements of individual species. This is amply documented in the literature. However, the 

NOy measurement is finicky on a number of accounts: (1) inlet losses for HNO3, (2) particle size 

cut for nitrate, (3) positive interference from reduced N species. Because of this, EPA should be 

very prudent before selecting a FRM. The HEASD plan for establishing the FRM reliability of 

NOy measurement methods seems reasonable. I recommend that validation campaigns be done 

for both summer and winter because of the change in NOy speciation. There is a lot of expertise 

in the research community in testing the reliability of NOy instruments and I recommend that 

David D. Parrish (NOAA/ESRL) be asked to comment on the HEASD plan. The NOAA/ESRL 

lab has considerable expertise that could be tapped in advising EPA on high-quality 

measurements of NOy species and total NOy. The research community also conducts regular 

intensive campaigns at surface sites measuring a suite of NOy species as well as total NOy 

(Harvard Forest in Massachusetts, PROPHET in Michigan, Niwot Ridge in Colorado, Blodgett 

Forest in California come to mind). It may be advisable to test the NOy instrument considered as 

FRM as part of these campaigns. 

Charge Question 5: What are the panel’s views on using the emerging AMoN ammonia 

monitoring network that uses passive sampling technology as a tool for evaluating air quality 

model behavior with respect to characterizing ambient air patterns of ammonia?

I have no expertise on the quality of the measurement. My understanding is that it would have 

weekly resolution, which I would find difficult to interpret in the absence of information on 

diurnal variations in ammonia. I would expect these diurnal variations to be large and complex 

depending on whether the local area is a significant ammonia source or not.

Charge Question 6: What are the panel’s views on co-locating ammonia measurements at each 

location where the indictors are measured?

That seems like a good idea considering the importance of model-derived L(NHx) in setting the 

standard. One should being able to evaluate the model ammonia at the locations where 

compliance with the standard is determined.
66

Charge Question 7: What are the Panel’s views on using the CASTNET filter pack (FP) to 

measure ammonium ion as a tool for evaluating air quality model behavior with respect to 

characterizing ambient air patterns of ammonia?

FP ammonium measurements are subject to condensation/volatilization biases. In addition, 

ammonium concentrations do not provide a good test of model ammonia because under acidneutralized conditions they are actually determined by the supply of sulfate and nitrate. I think 

that it would be a better idea to have a FP measurement of total ammonia+ammonium (NHx), 

which would be more reliable and more useful to test model ammonia sources.

Charge Question 8: What are the panel’s views on establishing a suite of NOy species 

measurements at 2- 5 locations in different atmospheric and ecological regions for the purpose 

of evaluating air quality model and NOy instrument behavior?

Once the reliability of the NOy instrument has been established (HEASD proposal) I don’t see 

much point in continuing to evaluate its behavior. Measuring NOy speciation is very important 

for evaluating air quality models but this is already done at some research sites (see comment 4). 

The EPA might consider coordinating with researchers at these sites. The research sites are 

mainly in relatively remote areas and the EPA might consider focusing on more polluted sites, 

such as the Supersites.

Charge Question 9: What are the panel’s views on utilizing the existing CASTNET and rural 

NCore networks as a starting infrastructure for the purpose of supporting the NOx/SOx 

standard?

Existing sites should be used as much as possible but may not provide the coverage necessary, 

for example in the upper Midwest. It seems to me that the geographical monitoring needs for the 

NOx / SOx standard should first be determined independently, and then leveraged against existing 

sites. 

Charge Question 10: What are the panel’s views on using CASTNET filter pack (FP) to measure 

total nitrate (particulate nitrate plus nitric acid) as the measurement approach for the purpose of 

providing annual average values to support the NOx/SOx standard in diagnosing NOy 

instrument behavior and assist in delineating the relative fractions of contributing oxidized 

nitrogen species to total ambient oxidized nitrogen.

To my knowledge the FP method can measure total nitrate very reliably. From the standpoint of 

deriving a dry deposition flux, however, it makes of course tremendous difference whether that 

nitrate is present as HNO3 or particulate nitrate. But that is a general issue with the use of 

unspeciated measurements to infer a NOy dry deposition flux. There are very strong vertical 
67

gradients of HNO3 in the lower few tens of meters, so that a having a standard above-canopy 

altitude is essential for all measurements in the network.

What would really help are FP measurements of speciated HNO3 and aerosol nitrate, rather than 

total nitrate. These are available from CASTNET. See comment 5.

Charge Question 11: What are the panel’s view of the broader consideration of using 

CASTNET, complemented by rural NCore, to serve as a framework for the nation’s rural 

monitoring of important gases and aerosols in support of secondary standards and evaluating 

the behavior of regional air quality models?

These are great sites for evaluating air quality models because they are generally not affected by 

local (and often fluctuating) sources for which models may have little simulation capability. The 

flip side of course is that they do not provide information for areas most likely to be affected by 

excessive deposition.
68

Comments from Dr. Peter H. McMurry

(NOTE: I have not provided answers to questions that were not assigned to me and for which 

other Committee members have more authoritative first-hand experience. I have read the 

preliminary comments that were posted on line on 2/11/11, and I think many good points have 

been made.)

Charge Question 1: What are the Panel’s views on using the CASTNET filter pack (FP) to 

measure particulate sulfate for the purpose of providing annual average values as an indicator 

for the NOx/SOx standard? Given EPA plans primarily to document the capability of the 

CASTNET FP and develop the FRM for particulate sulfate based on the existing information and 

procedures, what are the Panel’s views of this approach for setting the FRM?

I wonder if it might not be preferable to use a sampler with a 2.5 μm inlet to ensure compatibility 

with data from other networks? Also, do model predictions of sulfates focus largely on fine 

particulate sulfate? If so, then measurements carried out with a 2.5 μm cut would ensure fidelity 

between models and measurements. I believe this is an important consideration if the sampling 

and analytical uncertainties for sulfates are smaller than the incremental sulfate associated with 

coarse particles. 

Charge Question 2: What are the Panel’s views on using the CASTNET filter pack (FP) to 

measure sulfur dioxide gas for the purpose of providing annual average values as an indicator 

for the NOx/SOx standard? If EPA would document the capability of the CASTNET FP and 

develops an FRM for sulfur dioxide gas based on the existing information and procedures, what 

are the Panel’s views of this approach for setting the FRM?

If long term measurements have verified that FP SP2 provides an accurate measurement of the 

annual average, then I think this is a cost-effective solution to meeting this objective.

Charge Question 3: What are the Panel’s views on using the current primary FRM (high time 

resolution UVF) to measure sulfur dioxide gas for the purpose of providing annual average 

values as an indicator for the NOx/SOx standard?

As was pointed out by a Committee Member, average SO2 concentrations at some locations may 

be only slightly above the minimum detection level for UVF SO2 monitors. If such cases, the 

UVF SO2 monitor would not provide accurate annual average values, although it presumably 

could show that annual average values were well below requirements of the standard. Is there 

the potential that a passive sampler could provide more accurate long-term average values? If so, 

this would be a more cost-effective solution.
69

Charge Question 4(a) What are the panel’s views on using existing NOy methods that are 

deployed, for example, in NCore as the measurement approach for NOy for the purpose of 

providing annual average values as an indicator for the NOx/SOx standard? 

Charge Question 4(b) What are the panel’s views on EPA’s assessment that additional study is 

needed before establishing an FRM based on the existing NOy methods? That is, are the methods 

already adequately demonstrated as a reference method to determine compliance with a 

NAAQS? 

It seems clear to me that more work needs to be done to establish capabilities and limitations of 

the NOy instruments before they are set as FRMs.

Charge Question 4(c) What are the panel’s views on the research plan for establishing existing 

NOy methods as an FRM? [Note suggested improvement to the plan would be appreciated, 

particularly ones that would help complete the study on time.]

I like the use of the VOAG for producing particulate nitrate samples at known concentrations. 

Although this can be tricky experimentally, it has the potential to produce aerosols with nitrate 

concentrations that are accurately known from first principles. The first principles measurement 

is possible because measurements are done with monodisperse particles. Since, in principle, the 

nitrate mass in each droplet produced by the VOAG is known, the total mass of nitrate collected 

is determined by the number of droplets collected (droplet concentration X flowrate X sampling 

time X nitrate mass per droplet) of nitrates in each droplet. Also, these measurements will be 

carried out with relatively large particles (>0.5 μm), so will provide a more stringent test of the 

apparatus than would occur if experiments were done with a polydisperse aerosol. I believe the 

HEASD team ought to be able to do this work well, as members of the team have extensive 

experience with the VOAG.

Charge Question 5: What are the panel’s views on using the emerging AMoN ammonia 

monitoring network that uses passive sampling technology as a tool for evaluating air quality 

model behavior with respect to characterizing ambient air patterns of ammonia?

OK if prior QA work has been published in peer-reviewed journal articles.

Charge Question 6: What are the panel’s views on co-locating ammonia measurements at each 

location where the indictors are measured?

Co-located measurements of ammonia at each sampling location would seem to be very 

important.
70

Charge Question 7: What are the Panel’s views on using the CASTNET filter pack (FP) to 

measure ammonium ion as a tool for evaluating air quality model behavior with respect to 

characterizing ambient air patterns of ammonia?

Charge Question 8: What are the panel’s views on establishing a suite of NOy species 

measurements at 2- 5 locations in different atmospheric and ecological regions for the purpose 

of evaluating air quality model and NOy instrument behavior?

I think it would be a good idea to deploy exploratory speciated NOy measurements at a limited 

number of locations. This will facilitate model refinement, and will also allow the measurement 

methods to be evaluated and refined. 

Charge Question 9: What are the panel’s views on utilizing the existing CASTNET and rural 

NCore networks as a starting infrastructure for the purpose of supporting the NOx/SOx 

standard?

My initial reaction to this proposal was, “why not?” I think Dirk Felton’s comments show clearly 

why it is essential to include input from state and local agencies in these Committees. I defer to 

Dirk.

Charge Question 11: What are the panel’s view of the broader consideration of using 

CASTNET, complemented by rural NCore, to serve as a framework for the nation’s rural 

monitoring of important gases and aerosols in support of secondary standards and evaluating

the behavior of regional air quality models?

Again, I defer to Dirk Felton on this point. His personal experience with these networks 

provides an important perspective that requires consideration. I do not have similar experience.
71

Comments from Dr. Allen Robinson

Charge Question 1: What are the Panel's views on using the CASTNET filter pack (FP) to 

measure particulate sulfate for the purpose of providing annual average values as an indicator 

for the NOx/SOx standard? Given EPA plans primarily to document the capability of the 

CASTNET FP and develop the FRM for particulate sulfate based on the existing information and 

procedures, what are the Panel's views of this approach for setting the FRM?

The CASTNET FP is one of several well-established techniques for measuring particulate 

sulfate. Presumably the suite of established techniques should be considered when setting an 

FRM. The major difference between CASTNET and other existing techniques seems to be size 

cut -- open face versus PM2.5. Although the CASTNET FP is open faced, how well has its 

collection efficiency for coarse PM been characterized? Quantitative collection of coarse PM 

requires careful inlet design. There may be some advantages with using a FP with a more 

precisely defined cut point. There may also be some advantages to trying to better utilize the 

data of IMPROVE and other existing networks.

Charge Question 2: What are the Panel's views on using the CASTNET filter pack (FP) to 

measure sulfur dioxide gas for the purpose of providing annual average values as an indicator 

for the NOx/SOx standard? If EPA would document the capability of the CASTNET FP and 

develops an FRM for sulfur dioxide gas based on the existing information and procedures, what 

are the Panel's view of this approach for setting the FRM?

The CASTNET filter pack (FP) and other filter pack based approaches are widely deployed and 

well established techniques for measuring SO2 gas concentrations in rural areas with low levels 

(less than 5 ppbv). In fact, the greater sensitivity of FP based techniques may offer some 

advantages at lower concentration than continuous monitors. Therefore, I think it is appropriate 

to consider the CASTNET FP as the basis for measuring SO2 gas as an indicator for the NOx /

SOx standard. However, the performance of the CATNET FP compared to the existing SO2

FRM at low level found in rural areas needs to be well documented. 

Scientifically continuous monitors will provide a lot of extra information for model evaluation, 

etc.

Charge Question 3: What are the Panel's views on using the current primary FRM (high time 

resolution UVF) to measure sulfur dioxide gas for the purpose of providing annual average 

values as an indicator for the NOx/SOx standard? 

An advantage using the current primary FRM for evaluating the NOx / SOx standard is that 

hourly data can be used for model evaluation. Therefore, all else being equal (cost and 

performance), collecting more high time resolved data seems beneficial. However, running trace 

SO2 monitors in rural areas is challenging, requiring careful attention to zero offsets and other 
72

QA/QC measurements. At a minimum, continuous monitors should be run at a subset of sites to 

provide data for model evaluation.

Charge Question 4: What are the panel's views on using existing NOy methods that are 

deployed, for example, in NCore as the measurement approach for NOy for the purpose of 

providing annual average values as an indicator for the NOx/SOx standard? What are the 

panel's views on EPA's assessment that additional study is needed before establishing an FRM 

based on the existing NOy methods? That is, are the methods already adequately demonstrated 

as a reference method to determine compliance with a NAAQS? What are the panel's views on 

the research plan for establishing existing NOy methods as an FRM? [Note suggested 

improvement to the plan would be appreciated, particularly ones that would help complete the 

study on time.] 

There are a number of commercially available instruments for measuring NOy. My perception 

(e.g. Luke et al. AE 2010) is that these instruments can provide high quality data in a research 

environment. However, the performance in a compliance monitoring context (less well trained 

technicians, etc.) needs to be evaluated. For the NOx / SOx standard understanding their 

performance at low concentration, rural conditions will be essential.

I agree with the Agency’s view that additional study is needed before establishing a NOy FRM. I 

would characterize it as substantial additional study from an instrument demonstration 

perspective, not instrument development perspective. Key issues include conversion efficiency 

of catalyst for the suite of NOy species and inlet performance for different species. These issues 

must be addressed before designating an FRM. 

A shortcoming of the research plan was that all of the ambient measurements used in the 

evaluation study are going to be made in NC. There are two issues. One is to test the instrument 

across a wide range of climatic conditions (i.e. cold winter weather); for example, by making 

winter time measurements at sites in the Northeast and Upper Midwest. In addition, more 

thinking needs to be done about testing the instruments in a broader range of atmospheric 

conditions (beyond climatic conditions). The EPA should considering partnering with other 

measurement campaigns by NOAA and other organizations when making these measurements.

Charge Question 5: What are the panel's views on using the emerging AMoN ammonia 

monitoring network that uses passive sampling technology as a tool for evaluating air quality 

model behavior with respect to characterizing ambient air patterns of ammonia?

More ammonia data are desperately needed for model evaluation and understanding the 

effectiveness of potential control strategies. I have not seen peer-reviewed evaluation of this 

sampler, but results in the grey literature suggest that it appears promising.
73

Charge Question 6: What are the panel's views on co-locating ammonia measurements at each 

location where the indicators are measured? 

Co-locating samplers would be ideal but probably not feasible given resource constraints.

Certainly co-locating at a suitable set of sites that span range of atmospheric conditions is 

needed. These would be core sites with a more comprehensive set of instruments used to support 

model evaluation and investigation of atmospheric processes.

Charge Question 7: What are the Panel's views on using the CASTNET filter pack (FP) to 

measure ammonium ion as a tool for evaluating air quality model behavior with respect to 

characterizing ambient air patterns of ammonia?

More data are needed for model evaluation. However, ammonium ion measurements by 

themselves have pretty limited value in evaluating emissions inventories, models, and especially 

the critical questions of: when are we in potential sulfate-nitrate substitution regimes and when is 

NH3 the limiting reagent (see e.g. Pinder et al. JGR 2006)? Therefore, if the data are being 

primarily collected for model evaluation there may be a more efficient use of funds.

NH3 (g) and/or total NHx are much more useful. NHx total *or* NH3 (g) plus NH4+ is the best 

case scenario. NH3 (g) is probably more useful than NH4+ (if there is enough free ammonia in 

the gas-phase, you can deduce that the sulfate is neutralized). NH4+ by itself is probably the 

least useful. NHx is probably the easiest to measure so that would be the best.

On all of these, time resolution is important for model evaluation. The inorganic thermo regime 

bounces around enough that you want to be able to see that. Also, the higher time resolution 

instruments are naturally less vulnerable to various positive/negative partitioning artifacts for 

NH4+. Daily is probably decent. Hourly would be ideal. Multi-day exposures are probably not 

very helpful. “One in six” is better than multi-day exposures.

Charge Question 8: What are the panel's views on establishing a suite of NOy species 

measurements at 2- 5 locations in different atmospheric and ecological regions for the purpose 

of evaluating air quality model and NOy instrument behavior? 

I strongly support this idea. It seems like evaluating the NOy instrument behavior in this fashion 

should be done before designating an FRM method. I.e. This should be part of EPA’s research 

plan.

Charge Question 9: What are the panel's views on utilizing the existing CASTNET and rural 

NCore networks as a starting infrastructure for the purpose of supporting the NOx/SOx 

standard?
74

Leveraging existing networks seems sensible, cost-effective starting point. However, EPA first 

must independently define the selection criteria for sites to evaluate the new secondary SOx /

NOx NAAQS. Once these selection criteria are defined, EPA can then determine how many of 

the CASTNET and NCore sites are suitable for evaluating the new standard and how many new 

sites will need to be established. Presumably some new sites would need to be established.

Charge Question 10: What are the panel's views on using CASTNET filter pack (FP) to measure 

total nitrate (particulate nitrate plus nitric acid) as the measurement approach for the purpose of 

providing annual average values to support the NOx/SOx standard in diagnosing NOy 

instrument behavior and assist in delineating the relative fractions of contributing oxidized 

nitrogen species to total ambient oxidized nitrogen. 

The sum of the Teflon and Nylon nitrate data collected using CASTNET FP provides a 

reasonable measure of the sum of nitric acid, nitrous acid, and ammonium nitrate. For a total 

nitrate FRM, an acid gas denuder should be used upstream of the Nylon filter. If EPA 

documents the capability of the CASTNET FP using existing data and additional studies, then it 

would be appropriate to consider it as an FRM.

A FP measure of annual average values to support the NOx / SOx standard is reasonable. 

However, these measurements will have very limited value for evaluating a NOy instrument that 

provides much higher time resolved data. 

Charge Question 11: What are the panel's view of the broader consideration of using 

CASTNET, complemented by rural NCore, to serve as a framework for the nation's rural 

monitoring of important gases and aerosols in support of secondary standards and evaluating 

the behavior of regional air quality models?

See answer to #10.
75

Comments from Dr. Jay Turner

I applaud the Agency for taking a holistic, multimedia approach that includes a form of the 

standard grounded in a water quality metrics with ambient air concentrations as indicators that 

are inputs to the calculation of the water quality metric. This also provides a challenge in 

responding to the charge questions in that it is not clear what MQOs/DQOs are appropriate for 

the ambient air measurements. This general comment is reflected in the cursory preliminary 

responses provided below. In most cases, it is difficult to assess whether a given measurement 

method is appropriate in the absence of information about concentration ranges of interest, 

desired precision and accuracy, and so on. 

Charge Question 1: What are the Panel’s views on using the CASTNET filter pack (FP) to 

measure particulate sulfate for the purpose of providing annual average values as an indicator 

for the NOx/SOx standard? Given EPA plans primarily to document the capability of the 

CASTNET FP and develop the FRM for particulate sulfate based on the existing information and 

procedures, what are the Panel’s views of this approach for setting the FRM?

As noted in the background documents, the CASTNET filter has the advantage of being an open 

sampler that will collect particles larger than 2.5 m. While most of the sulfate is expected to be 

in the fine fraction, in many cases there will be some sulfate mass in the supermicron fraction 

and these particles would have high deposition velocities. The goal of capturing the sulfate mass 

of these particles is understood, but assumptions would still need to be made about the particle 

size distributions and in the absence of such site-specific information it is not clear to me that the 

open sampler design brings added value. That said, the CASTET filter pack might be an 

appropriate FRM pending the outcomes of the planned effort to document the sampler’s 

capability and a clearer articulation of the measurement quality objectives. 

Charge Question 2: What are the Panel’s views on using the CASTNET filter pack (FP) to 

measure sulfur dioxide gas for the purpose of providing annual average values as an indicator 

for the NOx/SOx standard? If EPA would document the capability of the CASTNET FP and 

develops an FRM for sulfur dioxide gas based on the existing information and procedures, what 

are the Panel’s views of this approach for setting the FRM?

Again, the key issue is whether the CASTNET FP meets the data quality objectives which have 

not yet been defined. The provided background documents mentioned that the CASTNET 

measurements are generally accepted to be high quality, but this is a subjective statement. What 

data are available to compare the CASTNET filter pack SO2 to other measurement methods? 

CASTNET includes two collocated sites; what information is available about the collocated 

precision?
76

Charge Question 3: What are the Panel’s views on using the current primary FRM (high time 

resolution UVF) to measure sulfur dioxide gas for the purpose of providing annual average 

values as an indicator for the NOx/SOx standard? 

The ultraviolet fluorescence (UVF) method is an FRM for the current primary SO2 NAAQS. As 

such it has been deemed acceptable for compliance monitoring for a 0.030 ppmv annual average 

standard. Performance specifications candidate reference and equivalent methods are 

documented in 75 FR 35597- 35601. In the context of the secondary standard monitoring 

objectives is likely that the detection limit of 1 ppbv would be acceptable. Maximum 

interference shall be less than ±5 ppbv SO2 equivalent, and 12- and 24-hour zero drift less than 

±5 ppbv SO2 equivalent. Presumably these metrics are also acceptable but in the absence of 

guidance on the anticipated mixing ratios that would be relevant, a recommendation is 

premature. 

Charge Question 4: What are the panel’s views on using existing NOy methods that are 

deployed, for example, in NCore as the measurement approach for NOy for the purpose of 

providing annual average values as an indicator for the NOx/SOx standard? What are the 

panel’s views on EPA’s assessment that additional study is needed before establishing an FRM 

based on the existing NOy methods? That is, are the methods already adequately demonstrated 

as a reference method to determine compliance with a NAAQS? What are the panel’s views on 

the research plan for establishing existing NOy methods as an FRM? [Note suggested 

improvement to the plan would be appreciated, particularly ones that would help complete the 

study on time.]

I have no preliminary comments on this charge question.

Charge Question 5: What are the panel’s views on using the emerging AMoN ammonia 

monitoring network that uses passive sampling technology as a tool for evaluating air quality 

model behavior with respect to characterizing ambient air patterns of ammonia?

Some information is available on the data quality from the AMoN ammonia monitoring 

network.1

 1 “NAPD’s New Network: AMoN The Passive Ammonia Monitoring Network”, M. Rury, 

EPA/CAMD. 

 Measurements were conducted using Radiello® passive samplers. Triplicate 

samplers were used to determine precision and URG denuders were used to determine relative 

accuracy. These data should be packaged and disseminated to provide insights into the 

measurement data quality. 
77

Charge Question 6: What are the panel’s views on co-locating ammonia measurements at each 

location where the indictors are measured? 

This would have added value and, presuming the passive sampling approach is deemed to have 

acceptable data quality, it would require a low level of effort for field operations. 

Charge Question 7: What are the Panel’s views on using the CASTNET filter pack (FP) to 

measure ammonium ion as a tool for evaluating air quality model behavior with respect to 

characterizing ambient air patterns of ammonia?

See comments for sulfate above, #1.

Charge Question 8: What are the panel’s views on establishing a suite of NOy species 

measurements at 2- 5 locations in different atmospheric and ecological regions for the purpose of 

evaluating air quality model and NOy instrument behavior?

I have no preliminary comments on this charge question.

Charge Question 9: What are the panel’s views on utilizing the existing CASTNET and rural 

NCore networks as a starting infrastructure for the purpose of supporting the NOx / SOx

standard?

Both CASTNET and the rural NCore sites provide an excellent opportunity to leverage existing 

infrastructure and should be exploited when practicable. 

Charge Question 10: What are the panel’s views on using CASTNET filter pack (FP) to 

measure total nitrate (particulate nitrate plus nitric acid) as the measurement approach for the 

purpose of providing annual average values to support the NOx / SOx standard in diagnosing NOy

instrument behavior and assist in delineating the relative fractions of contributing oxidized 

nitrogen species to total ambient oxidized nitrogen.

See comments for sulfate above, #1.

Charge Question 11: What are the panel’s view of the broader consideration of using 

CASTNET, complemented by rural NCore, to serve as a framework for the nation’s rural 
78

monitoring of important gases and aerosols in support of secondary standards and evaluating the 

behavior of regional air quality models?

Based on the provided background information, the existing CASTNET and rural NCore 

network infrastructure provide a solid framework in support of the secondary standards and also 

to support the modeling effort. The existing sites are generally well-aligned with the identified 

eco-systems with perhaps the exception of the northwest United States. It would be very helpful 

to get input from the modelers about desirable monitoring locations outside of the identified ecosensitive regions. For example, there are large gaps in areas such as the Central Plains; would 

this be an important area for monitoring to support model evaluation? 
79

Comments from Dr. Yousheng Zeng

GENERAL COMMENTS 

1. The proposed NOx / SOx secondary NAAQS takes a form (Aquatic Acidification Index, 

or AAI) that will require many parameters to be put in an equation to determine 

attainment with the standard. Some of these parameters will be determined by 

ecosystem’s characteristics on a region-by-region basis among the 84 ecoregions 

covering the continental U.S.; some of the input parameters will come from the CMAQ 

model; and some will come from monitoring instruments. As of now, all NAAQS 

(primary or secondary) are based on direct measurements of pollutant levels in ambient 

air. Other than averaging or statistic schemes used to construct a proper form of the 

standards, no other modeled parameters are introduced as important components of the 

standards. The proposed NOx / SOx secondary standard is a dramatic departure from this 

long-established practice. This approach may be needed due to the nature of the 

ecosystems that this standard is set to protect. However, we need to keep in mind that 

unlike other NAAQS, there are a lot of relatively judgment-based parameters and modelbased parameters being introduced into this standard. Through our knowledge in 

working with various air quality models, we understand that there are significant levels of 

uncertainties associated with these complex models. For example, Table F-2 in 

Appendix F to the final Policy Assessment (PA) shows that the bias ranges from -17% to 

47%. It may not be productive and cost-effective if we pursue the same level of 

perfection as we do with monitoring methods for traditional NAAQS where compliance 

is solely determined by measurements. Incremental gain in monitoring accuracy may 

become insignificant compared to the uncertainties introduced by judgment-based and 

model-based parameters. 

2. The indicators measured for the NOx / SOx secondary standard are NOy, SO2, and p- SO4; 

and they are measured as ambient concentrations, not atmospheric deposition. The 

fundamental concern of this secondary standard is acid deposition, which include wet and 

dry depositions. In this form of standard, the wet deposition is accounted for through 

model-based components in the AAI equation. My understanding of EPA’s approach is 

to apply one set of these judgment-based and model-based parameters to each of the 84 

ecoregions. Each ecoregion could be large enough to cover areas that have substantially 

different annual precipitation. When one part of an ecoregion experiences significantly 

more precipitation than another part of the same ecoregion, the indicators to be monitored 

(NOy, SO2, and p- SO4) will be significantly lower when the precipitation scrubs out 

these species from atmosphere. However, everything else in the AAI equation will 

remain the same. The area with more precipitation will have lower ambient 

concentrations of NOy, SO2, and p- SO4 and a better AAI, but the more acid deposition 

may have occurred and the impact to the ecosystem is not necessarily lower as indicated 

by AAI. It seems to me that wet deposition should be directly measured. 

3. I like the idea of using the FP (or with some modification) to measure everything needed 

in the AAI equation. Without modification, the FP can measure SO2 and p- SO4 well. It 
80

cannot measure NOy. However, it can measure p-NO3 and HNO3, i.e., t-NO3.

Conceptually, t-NO3 can be used as a surrogate for NOy and F3 in the AAI equation can 

be adjusted from a NOy referenced parameter to a t-NO3 referenced parameter. 

Alternatively, the FP can be modified by adding a denuder to measure NOx. Then the 

sum of NOx, p-NO3, and HNO3 can be used to represent NOy in the AAI equation.

Considering uncertainties introduced by modeling aspect of the AAI, using one of these 

surrogates for NOy seems very reasonable. It will make the monitoring for secondary 

standard significantly easier and more cost-effective. 

4. Reduced nitrogen (NH3 and NH4) seems significant in the AAI equation. Measurement 

methods exist for these species. NH4 is already measured by CASTNET FP. What is the 

rationale to have some parameters (e.g., NOy, SO2, and p- SO4) measured and other (e.g., 

NH4, NH3) modeled? Even if a measurement for NH4 is less than ideal, it may still be 

better to directly measure it than to rely on a fixed modeled value because it should better 

account for spatial variability and provide some consistency across the parameters used in 

this standard. The argument that NHx is not a criteria pollutant and therefore should not 

be explicitly expressed in the AAI equation is not very convincing because NHx is still 

incorporated in the AAI equation through F2 and there are some species in NOy and SOx

that are not criteria pollutants. 

RESPONSES TO SPECIFIC CHARGE QUESTIONS

Charge Question 1 - Use of CASTNET Filter Pack to Measure Particulate Sulfate

Response: Using CASTNET FP to measure p- SO4 for the purpose of the NOx / SOx secondary 

NAAQS makes sense. It is a well established method for similar application, i.e., measuring acid 

deposition in CASTNET. It is relatively simple and cost-effective. A disadvantage of this 

method is that it measures weekly average value and lacks the time resolution for studying daily 

or hourly changes, which can be very valuable information for model evaluation and other 

purposes. However, for monitoring compliance with this annual standard, this method is 

adequate. Even after some hourly measurement instruments (e.g., MARGA) is well developed, 

the CASTNET FP is expected to be a lower cost method. For the same budget, the disadvantage 

of poor temporal resolution can be traded off for better area coverage, i.e., having more 

monitoring locations. Better temporally resolved instruments can be deployed at a few selected 

sites (or on rotation with a mobile/transportable platform) for better understanding of temporal 

variability. 

From regulatory viewpoint, determination of attainment with NAAQS, either primary or 

secondary, should be based on monitoring results generated by FRM or FEM. It seems that if 

the CASTNET FP method will be used for determination of attainment with the NOx / SOx

secondary NAAQS, it will be more defendable to give this method a FRM or FEM status. For 
81

aforementioned reasons, I would support EPA’s effort to develop the FRM specifically for the 

NOx / SOx secondary NAAQS based on the CASTNET FP method. 

Charge Question 2 - Use of CASTNET Filter Pack to Measure Sulfur Dioxide Gas

Responses: The manual FRM for SO2 has been established for a long time, but not commonly 

used now. Continuous SO2 FRM monitoring instruments are commercially available from 

various vendors. These instruments are well established and widely deployed. From the 

viewpoint of the technology, there would be no need to develop another FRM based on a less 

sophisticated method. Designation of area attainment with NAAQS, either primary or 

secondary, is always based on FRM or FEM. If a designation is made based on non-FRM or 

non-FEM, the decision may be contested. To avoid this situation, a continuous SO2 FRM or 

FEM monitor would need to be deployed. However, the most CASTNET sites do not have a 

continuous SO2 FRM/FEM analyzer. For the purpose of determining compliance with the NOx /

SOx secondary standard, a continuous SO2 FRM/FEM analyzer is not necessary. If EPA has 

sufficient data (or conduct additional study) to show that weekly averaged results generated by a 

continuous SO2 FEM analyzer are consistent with the results from CASTNET FP, it would be 

desirable to establish the CASTNET FP method as the FRM specifically for the NOx / SOx

secondary NAAQS (and not applicable to the primary NAAQS). Has EPA evaluated the 

accuracy/consistency between the SO2 measured by the CASTNET FP and the SO2 measured by 

a continuous SO2 FRM analyzer averaged over a week? 

Most sites for the NOx / SOx secondary standard monitoring network are expected to be in rural 

areas where SO2 concentrations could be very low. The concentrations could approach or even 

be below the method detection limit of current continuous SO2 FRM analyzers. If that is the 

case, making the FP method for SO2 a FRM would be very important. 

If EPA anticipates that a large number of monitoring stations for the NOx / SOx secondary 

standards will be standalone and not be co-located with stations in other networks, it will be 

better to use the FP to measure SO2. This will be more cost-effective and the measurement 

averaging time will be synced with that of p- SO4. 

Charge Question 3 - Use of Current Primary FRM (High Time Resolution UVF) to Measure 

Sulfur Dioxide Gas

Response: As mentioned in Response to Charge Question 2, the method detection limit of the 

current primary FRM may not be low enough to measure very low SO2 levels on the continuous 

basis at some rural sites. The CASTNET FP method measures amount of SO2 accumulated on 

the filter over a one week period. I would prefer the CASTNET FP method being FRM for NOx

/ SOx secondary standard monitoring. EPA is conducting a comparative study of the FP method 

and the current FRM, If the study shows that the results of FP are consistent with the results of 

primary FRM, The UVF based, continuous FRM can be designated as FEM for the secondary 
82

standard in addition to FRM for the primary standard, provided that SO2 concentrations are well 

above the method detection limit of the continuous method. All secondary standard monitoring 

sites should be equipped with FP. A few selected sites could be equipped with a continuous SO2

FRM analyzer (or on rotation with a mobile/transportable platform) for better understanding of 

temporal variability. 

Charge Question 4 - Use of Existing NOy Methods

Response: With regard to NOy method, see my General Comment 3.

Comments with regard to EPA’s research plan: The plan includes an effort to evaluate the FP 

SO2 method for potential FRM/FEM. I highly support this effort and strongly encourage EPA to 

elevate the level of importance of this element in the research plan. I am concerned by EPA’s 

notion that there is already FRM and FEM for SO2 and therefore there is no need to establish 

additional SO2 FRM or FEM for the NOx / SOx secondary standard. I do think that having a FP 

based SO2 FRM or FEM is very important for the reasons discussed in my Responses to Charge 

Questions 2 and 3.

The idea of using t-NO3 measured by FP as surrogate for NOy is very attractive. If this approach 

is demonstrated to be acceptable in the context of NOx / SOx secondary NAAQS, the monitoring 

network for NOx / SOx secondary NAAQS could be made of nothing but CASTNET FP type 

monitors to collect all parameters (SO2, p- SO4, and NOy) needed to determine compliance with 

the secondary standard. The FP based monitors can form the backbone of the monitoring 

network and the network can be supplemented by a higher tier, more sophisticated SO2 and NOy

analyzers deployed at a few selected sites or on a rotation basis. Even if t-NO3 is not an 

adequate surrogate for NOy, the established NOx monitors could be used so that the combined 

results of NOx from NOx analyzer and t-NO3 from FP can cover the majority (probably >90%) 

of NOy. Also see my General Comment 3.

A FRM/FEM is needed for NOy for reasons discussed in my Responses to Charge Questions 1 

and 2. 

Charge Question 5 - Use of the Emerging AMoN Ammonia Monitoring Network

Response: I am not familiar with AMoN. If the sampling methods can provide the time 

resolution suitable for model evaluation, they should be used for this kind of study along with 

measurements of other species (e.g., NOy) in the same time resolution.

Charge Question 6 - Co-Locating Ammonia Measurements

Response: It is not clear to me concerning the purpose of co-locating NH3 with measurements of 

other indicators. If the purpose is for model evaluation, the time resolution should be finer than 

weekly and it should be co-located with other measurements with finer time resolution (see my 

response to Charge Question 5). If the purpose is for compliance, ammonia measurement value 
83

is not used in the AAI equation. This may make sense if EPA considers including NHx 

measurement in the AAI equation (see my General Comment 4).

Charge Question 7 - Use of CASTNET Filter Pack to Measure Ammonium Ion

Response: It should be an area for EPA to explore. My initial thought on this is the sampling 

time. If the purpose is to study the model behavior, will the time resolution of the weekly data 

sufficient? I am not familiar with the FP method for ammonium. Is there a concern about any 

portion of ammonium being lost due to vapor pressure of some ammonium salt? Also, I am not 

sure if the FP method can provide information on relationship between ammonium and 

ammonia. 

Charge Question 8 - Establishment of a Suite of NOy Species Measurements

Response: I would support this approach. However, the measurement should not be limited to 

species within NOy. At least NH3 and NH4 should be included for model evaluation. It seems 

that instruments such as MARGA will be suitable to this type of sites. For this purpose, even if 

the absolute accuracy of the measurements is not high, relative relationship and behavior will be 

useful.

Charge Question 9 - Use of CASTNET Filter Pack and Rural NCore Networks as a Starting 

Infrastructure

Response: I think the CASTNET and rural NCore network will be a good starting point for the 

NOx / SOx secondary NAAQS monitoring network. If EPA can determine that the CASTNET 

FP method can adequately measure SO2, p- SO4, and NOy, directly (SO2 and p- SO4) or through 

a surrogate (t-NO3 as a surrogate for NOy), balancing the uncertainties that exist in other 

components of the AAI equation and the requirements for measurement accuracy, this approach 

will facilitate implementation of the new secondary standard in a shorter timeframe and with 

lower cost. 

A question I have is – are there any sensitive ecoregions that are not adequately represented by 

the CASTNET network?

Charge Question 10 - Use of CASTNET Filter Pack to Measure Total Nitrate 

Response: If EPA decide to use NOy instrument, EPA should use FP along with NOy

instruments for the purposes stated in this question. According to the current Research Plan, it 

seems that EPA will use existing data for similar analysis but not actually perform side-by-side 

measurements between FP and NOy instrument. It would be very beneficial to include FP in this 

well structured lab and ambient study to evaluate the relationship between the two measurements 
84

(and other measurements planned in the study). Through this study, EPA may be able to 

evaluate feasibility of using t-NO3 as surrogate for NOy (see my General Comment 3 and 

Responses to Charge Question 4). 

Charge Question 11 - Broad Consideration of Using CASTNET, Complemented by Rural NCore, 

As A Framework for National Rural Monitoring

Response: With regard to evaluating the behavior of regional air quality models, the biggest

concern is the time resolution. The CASTNET FP method measures these species on weekly 

average basis. This will not have time resolution fine enough to evaluate model behavior.
85

Compilation of Comments from Individual Members of the CASAC NOx And 

SOx Secondary NAAQS Review Panel

Comments from Dr. Andrzej Bytnerowicz

Reponses (only to the questions where I have adequate knowledge and experience)

Charge Question 5: What are the panel’s views on using the emerging AMoN ammonia 

monitoring network that uses passive sampling technology as a tool for evaluating air quality 

model behavior with respect to characterizing ambient air patterns of ammonia?

Answer: This is a good approach that can produce reliable results of the time-integrated (week 

to a month) concentrations of ammonia. There are several types of the commercially available 

passive samplers for ammonia that can produce reliable results. AMoN monitoring network is 

needed because of an absence of reliable, practical and cost-effective electronic monitors and 

uncertainties of the CMAQ predictions of the NHx ambient concentrations. 

Charge Question 6: What are the panel’s views on co-locating ammonia measurements at each 

location where the indicators are measured?

Answer: This is a good idea that should help in understanding spatial and temporal patterns of 

ammonia distribution based on the experimental data. Collocation of ammonia passive samplers 

with the CASTNET filter packs could also help in understanding what proportion of ammonium 

nitrate collected on Teflon filter has volatilized. I suggest adding also passive samplers for nitric 

acid that have been successfully used in the various parts of the US and Canada. Collocating 

ammonia and nitric acid passive samplers with the CASTNET filter packs could be useful in 

evaluation of the mentioned above problems related to the ammonium nitrate volatilization. 

Charge Question 7: What are the Panel’s views on using the CASTNET filter pack (FP) to 

measure ammonium ion as a tool for evaluating air quality model behavior with respect to 

characterizing ambient air patterns of ammonia?

Answer: That would only be useful if another filter (citric acid or phosphoric acid coated) is 

added to the filter pack to collect ammonia resulting from the ammonium nitrate volatilization. 

At the same time ammonia should also be measured with the collocated passive samplers. 

Charge Question 8: What are the panel’s views on establishing a suite of NOy species 

measurements at 2 -5 locations in different atmospheric and ecological regions for the purpose 

of evaluating air quality model and NOy instrument behavior?
86

Answer: That should be done with the chemiluminescence instruments for total NOy. In 

addition, passive samplers for NOx, NO2, and HNO3 should also be deployed. 

Charge Question 9: What are the panel’s views on utilizing the existing CASTNET and rural 

NCore networks as a starting infrastructure for the purpose of supporting the NOx/SOx 

standard?

Answer: This is a very good idea. If I am correct, the proposed network would probably use the 

Radiello samplers. There are also other samplers of a similar reliability (such as Alpha or 

Ogawa). It would be good to coordinate these efforts with the ongoing monitoring of ammonia 

with the Ogawa passive samplers in the western US, especially in southern California and the 

Sierra Nevada (especially the Lake Tahoe Basin). 

Charge Question 10: What are the panel’s views on using CASTNET filter pack (FP) to measure 

total nitrate (particulate nitrate plus nitric acid) as the measurement approach for the purpose of 

providing annual average values to support the NOx/SOx standard in diagnosing NOy 

instrument behavior and assist in delineating the relative fractions of contributing oxidized 

nitrogen species to total ambient oxidized nitrogen.

Answer: The filter pack is capable of measuring total nitrate by collecting particulate nitrate on 

the Teflon filter and nitric acid on the nylon filter. Major problem of this method is that the 

proportion of the measured particulate vs. gaseous fractions is unreliable. Due to the ammonium 

nitrate volatilization to nitric acid and ammonia it is unknown how much of the nitrate deposited 

on the Teflon filter has remained in particulate form, and how much has been decomposed and 

added to nitric acid collected on the nylon filter. 

Due to the above mentioned ammonium nitrate volatilization issues, the CASTNET filter pack as 

presently used would be a very poor tool for supporting the NOy measurements conducted with 

the electronic monitors. 

Charge Question 11: What are the panel’s view of the broader consideration of using 

CASTNET, complemented by rural NCore, to serve as a framework for the nation’s rural 

monitoring of important gases and aerosols in support of secondary standards and evaluating 

the behavior of regional air quality models?

Answer: This is a very good idea, especially if additional measurements with passive samplers 

and some other techniques (e.g., remote sensing) are also considered. In addition the 

meteorological data collected at the CASTNET sites (especially temperature) would improve 

quality of data derived from the passive samplers (correction for diffusion rates). The Forest 

Service will soon start monitoring of air quality (ozone and reactive gaseous N species) on a 
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national network of the Experimental Forests and Ranges. That will be done in collaboration 

with the CLAD activities under the umbrella of NADP (collaboration between FS, NPS, EPA, 

USGS and academia). 
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Comments from Mr. Rich Poirot

Charge Question 1: What are the Panel’s views on using the CASTNET filter pack (FP) to 

measure particulate sulfate for the purpose of providing annual average values as an indicator 

fort the NOx/SOx standard? Given EPA plans primarily to document the capability of the 

CASTNET FP and develop the FRM for particulate sulfate based on the existing information and 

procedures, what are the Panel’s views of this approach for setting the FRM?

The proposed use of the CASTNET Filter Pack (CFP) as an FRM for sampling particulate 

sulfate for this secondary NAAQS application seems adequate. The method has been relatively 

well-characterized and evaluated, and has a documented long-term track record of successful use 

in a field network designed to assess spatial patterns and long-term temporal trends. Because the 

CASTNET network plays an important role in tracking CAA-related air quality changes over 

space and time (regardless of new standards), I also think the program as a whole will benefit 

from the more detailed scrutiny that would result from FRM development, and subsequent use in 

a compliance application. For example it has been observed that the error associated with longterm CASTNET (and other) sulfate trends may be larger (about 1%/yr) than has previously been 

estimated (White et al., 2005), and efforts to further reduce that uncertainty would be welcome.

A major advantage of specifying the CFP for sulfate is also to maintain the possibility that the 

CFP might conceivably be used to supply all the measurements required to implement this 

NAAQS, assuming that an FEM could be specified for CFP SO2 and that “total nitrate” (and an 

associated deposition transfer ratio) could be used as a surrogate for NOy in the AAI equation. 

The resulting network might employ a mix of weekly filter-based and continuous methods, and 

address the multiple objectives of compliance determination, model evaluation /improvement, 

and more complete atmospheric characterization – without the need to do everything everywhere

The time resolution of the resulting CFP weekly data is adequate for (partially) determining 

compliance with a NAAQS with a 3 to 5-year averaging time, and sampling all the time will 

reduce the inter-annual variability introduced by intermittent (1 in 3 day) daily sampling in 

programs like IMPROVE and CSN (or avoid the excessive costs of operating such filter-based 

networks daily). Weekly time resolution is minimally useful for model evaluation and 

refinement, and for that objective, it would be highly desirable to consider deployment of 

continuous sulfate analyzers at a few sites where continuous SO2 is being measured. See for 

example Drewnick et al. (2003). While continuous sulfate analyzers are typically operated with 

size selective inlets, they don’t necessarily need to be run that way. Although, for model 

evaluation purposes (if not for NAAQS determination), a 2.5 micron inlet might actually be 

desirable for continuous sulfate. Specifically, I don’t expect CMAQ to have much skill in 

calculating coarse sulfate concentrations, and adding some coarse sulfate into the measurement, 

without knowing how much or in what size bin is not likely to help improve the model.

For similar reasons, I don’t completely agree with staff arguments that the open-faced CFP 

sampler is critically necessary to include capture of coarse sulfate particles. I think this is 

harmless but probably not very helpful (and is in any event not very important- a few percent of 
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the problem at most). This is not a purely measurement-based NAAQS, nor one focused directly 

on measured deposition, but rather one that relies on a combination of air concentration 

measurements and model results. The estimated total S deposition (aggregated over multiple 

years and spatially large ecoregions) is the product of the measured SOx indicator and the 

CMAQ T SOx deposition transfer ratio. The best indicator is not the one which includes (as 

open-faced sulfate might) a bit more of the species which dry deposit most efficiently. The best 

indicator is the one with the best and most stable correlations over both space and time with total 

S deposition. I doubt that CMAQ has much skill in predicting coarse particle sulfate 

concentrations or deposition, and I further question the ability of an (affordable) monitoring 

network to adequately capture the spatial variability of coarse particle sulfate concentrations. 

Further, the open faced collector provides no information on what fraction of the resulting sulfate 

is in the larger, more rapidly dry-depositing size range, or what the sizes of those larger particles 

are. The SOx indicator is proposed to be the simple, linear sum of atmospheric S from (rapidly 

dry depositing) SO2 and (much more slowly depositing) particulate SO4 (with no attempt to 

weight this sum by the relative deposition velocities, for example). I don’t believe that it can be 

demonstrated that a SOx indicator which includes an unknown amount of coarse sulfate, 

combined with a CMAQ T SOx deposition transfer ratio which also includes coarse sulfate (for 

which the model performs poorly) can be shown to be a superior indicator over space and time 

than a combination of measure and model ratio which excludes coarse particles. 

During the 2003/ 2004 IMPROVE coarse particle characterization study (Malm et al., 2007), 

there was more coarse particle nitrate than sulfate at all 9 of the rural, nationally distributed 

IMPROVE sites included in that study. Crustal material, which tends to be alkaline, accounted 

for more than half the coarse mass at all sites, and so it seems highly unlikely that the relatively 

small amounts of coarse N and S contribute an “acidifying” effect that is not more than 

countered by associated deposition of base cations (not considered in the AAI equation). It also 

seems likely that much of the coarse sulfate and nitrate may be associated with mineral cations 

(Ca++, Mg++, K+

, Na+, etc.), and this coarse mineral nitrate (which exceeds coarse sulfate) may 

not be efficiently captured by, or fully converted by the proposed NOy samplers. Conceivably, 

there could be a bit of positive sampling artifact if acid gases react with alkaline coarse crustal 

material on the open faced filters, although this would be quite small and minimized or offset if 

it’s the sum of PM and acid gas compounds (from the same CFP) that’s used as the indicator.

As indicated above, I generally support the specification of the CFP as p-SO4 FRM for this 

standard, and think it would be highly desirable to add a few continuous sulfate analyzers at sites 

with continuous SO2 for model evaluation/improvement purposes. I think it would also be useful 

if the proposed HEASD Research Plan for FRMs for NOy and p-SO4 were accompanied by a 

parallel effort to look more closely at CMAQ concentrations, deposition and species ratios over 

space and time. Can it be demonstrated that inclusion or exclusion of coarse sulfate improves 

the overall measure+model total S deposition estimate? Might there be a comparable measure + 

model combination using SO2 and fine sulfate only. Does a linear sum of gas and particle S (or 

N) species make the best indicator over space and time, or might a weighted sum work better? 

What is the spatial variability of the modeled species concentrations, depositions and transfer 
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ratios within the identified ecoregions? How would measure+model N deposition estimates 

compare if based on CFP total nitrate rather than NOy, etc.?

Charge Question 10: What are the panel’s views on using CASTNET filter pack (FP) to measure 

total nitrate (particulate nitrate plus nitric acid) as the measurement approach for the purpose of 

providing annual average values to support the NOx/SOx standard in diagnosing NOy 

instrument behavior and assist in delineating the relative fractions of contributing oxidized 

nitrogen species to total ambient oxidized nitrogen.

Based on the (revised) Figure 2-32 in the Final 2/11 Policy Assessment Document (and 

monitoring summary) as well as Figure 4-21 from the 9/10 Draft PAD, it appears that nitric acid 

would be as good an indicator (or better) than NOy for predicting total oxidized N deposition (if 

combined with a matched CMAQ deposition transfer function). I assume the total nitrate (sum 

of particulate nitrate and nitric acid) available from CASTNET Filter Pack (CFP) samples would 

also make for an adequate indicator of total N deposition. This could easily be evaluated by 

exploring the stability of CMAQ ratios of total nitrate to total oxidized N deposition over space 

and time (with decreased NOx emissions). Assuming this indicator is suitable, and that SO2 from 

the CASTNET filter pack was also suitable (combined with CFP SO4) as a SOx indicator, all 

measurements needed to implement the standard could be taken by the CFP method. 

While the weekly time resolution of CFP data would be adequate to determine compliance with a 

standard averaged over 3 to 5 years, such data would be much less useful for purposes of model 

evaluation and refinement and overall atmospheric characterization than that which could be 

provided by the use of continuous SO2, SO4 NOy or speciated NOy measurements. Given current 

budgetary constraints, a network composed of relatively inexpensive CFP samplers at many 

sites, complemented by continuous samplers for the various relevant species at a subset of sites 

might be a reasonable approach.

It was interesting to note in the HEASD Methods research plan that while HEASD “will”

conduct a thorough evaluation of the NOy method, and “will” propose and finalize FRM 

specifications for NOy, HEASD only “may consider, as resources allow” an additional FRM or 

FEM using the CFP t-NO3 method “due to the expected prohibitive cost associated with 

equipping a site with NOy monitors”. Hmmm... I think it should be a somewhat higher priority 

to evaluate the suitability of the CFP t-NO3 method as potential indicator (combined with an 

equivalent CMAQ deposition transfer function). The Methods Research Plan should also be 

complemented with a much more detailed evaluation of the CMAQ model results for all the 

various N and S species concentration, deposition and transfer ratios. For example, from CMAQ 

alone, what would be the differences over space and time in estimated total oxidized N 

deposition if NOy or tNO3 were used as indicator (quite small I suspect)? Perhaps this would 

allow a mix of methods with different measured indicators but comparable measure+model 

deposition estimates. The suitability of the CFP t-NO3 (and SOx) method(s) could also be 

(approximately) assessed using historical CASTNET concentration and dry deposition + NADP 
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wet deposition data. See for example Butler et al. (2011), etc.

References

Drewnick, J. J. Schwab, O. Hogrefe, S. Peters, L. Husain, D. Diamond, R. Weber and K. L. 

Demerjian (2003) Intercomparison and evaluation of four semi-continuous PM2.5 sulfate 

instruments, Atmospheric Environment, 37 (24) 3335-3350.

Malm, W.C., M.L. Pitchford, C. McDade, and L.L. Ashbaugh (2007) Coarse particle speciation 

at selected locations in the rural continental United States. Atmospheric Environment, 41 (10) 

2225-2239.

W.H. White, L.L. Ashbaugh, N.P. Hyslop and C.E. McDade (2005) Estimating measurement 

uncertainty in an ambient sulfate trend, Atmospheric Environment, 39 (36) 6857-6867. 

Butler, T. J., F. M. Vermeylen, M. Rury, G. E. Likens, B. Lee, G. E. Bowker and L. McCluney 

(2011) Response of ozone and nitrate to stationary source NOx emission reductions in the eastern 

USA, Atmospheric Environment 45 (5) 1084-1094.