oEPA 4
United States
Environmental Protection
Aaencv
EPA/600/R-20/390 | November 2020 | www.epa.gov/research
Recommendations for
Nationwide Approval of
Nation™ Dryers Upstream of
UV-Absorption Ozone
Analyzers

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EPA/600/R-20/390
November 2020
Recommendations for Nationwide Approval
of Nafion™ Dryers Upstream of UV-
Absorption Ozone Analyzers
by
Hannah Halliday, Cortina Johnson, Tad Kleindienst, Russell Long,
Robert Vanderpool, and Andrew Whitehill
U.S. Environmental Protection Agency
Research Triangle Park, NC 2771 1
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Disclaimer
This technical report presents work performed by the U.S. Environmental Protection Agency's (U.S.
EPA) Office of Research and Development (ORD). The effort represents a collaboration between ORD
and the U.S. EPA Office of Air Quality Planning and Standards (OAQPS). Mention of trade names,
commercial products, or various research institutions in the report does not constitute endorsement. The
report has been internally and externally peer reviewed and approved by the U.S. EPA for publication.

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Abstract
Ultraviolet (UV) Photometry-based ambient Federal Equivalent Method (FEM) ozone (O3) analyzers
can experience significant water vapor interferences resulting in increased measurement noise, biased
concentration measurements, and slow instrument response times. The magnitude and duration of these
biases is particularly noticeable during sampling events where the O3 analyzer experiences rapid
changes in the sampled air's moisture content. The U.S. Environmental Protection Agency (EPA)
implemented the use of a Perma Pure (Lakewood, NJ) Nafion™ tube dryer upstream of their Thermo
Environmental Instruments LLC (Franklin, MA) Model 49i O3 analyzers at 30 Clean Air Status of
Trends Network (CASTNET) National Ambient Air Quality Standard (NAAQS) monitoring sites as part
of a limited evaluation and has found that this approach minimizes spectroscopic water vapor
interferences to an acceptable degree.
This document summarizes the pertinent qualitative and quantitative information to foster further
discussion so that a consensus decision can be made whether to approve the routine use Nafion™ dryers
on a nationwide basis. The operating principle of UV Photometer-based O3 analyzers is presented along
with a discussion of known measurement interferences, with a particular emphasis on atmospheric water
vapor and approaches used to minimize the magnitude of this water vapor interference. Multiple studies
are discussed which demonstrate that no inadvertent loss of O3 occurs in the Nafion™ tube dryer. The
design and operational specifications of CASTNET's Nafion™ dryer system are presented along with
test results of the system's use since the Office of Air and Radiation's (OAR) approval to CASTNET
evaluation. Based on the results of this investigation, the EPA Office of Research and Development
(ORD) has determined that the use of Nafion™ dryers upstream of UV Photometric-based O3 analyzers
on a nationwide basis is a valid means to improve the data quality of O3 compliance measurements.
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Foreword
The EPA is charged by Congress with protecting the Nation's land, air, and water resources. Under a
mandate of national environmental laws, the Agency strives to formulate and implement actions leading
to a compatible balance between human activities and the ability of natural systems to support and
nurture life. To meet this mandate, EPA's research program is providing data and technical support for
solving environmental problems today and building a science knowledge base necessary to manage our
ecological resources wisely, understand how pollutants affect our health, and prevent or reduce
environmental risks in the future.
EPA's Center for Environmental Measurement & Modeling (CEMM) within ORD conducts research to
advance EPA's ability to measure and model contaminants in the environment, including research to
provide fundamental measurement methods and deterministic models needed to implement
environmental statutes. Specifically, CEMM characterizes the occurrence, movement, and
transformation of contaminants in the natural environment through the application of measurement and
modeling-based approaches. CEMM scientists develop, evaluate, and apply laboratory and field-based
methods and approaches for use by EPA and its state, local, and tribal partners to characterize
environmental conditions in direct support of implementation of EPA programs. CEMM scientists also
provide scientific expertise and leadership related to the development and application of complex
computational models that provide precise and detailed predictions of the fate and transport of priority
contaminants in the environment to inform the environmental policies and programs at the EPA, state,
local, and tribal level.
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Table of Contents
Disclaimer	iii
Abstract	iv
Foreword	v
Acronyms and Abbreviations	viii
Acknowledgments	ix
Executive Summary	x
1.0 Introduction	1
2.0 Operating Principle of UV-based O3 Photometers	2
2.1	Interference of Atmospheric Chemicals	3
2.2	Interference of Atmospheric Water Vapor	3
3.0 Characteristics of Nafion™ Tubes	5
3.1 2B Technologies Nafion™ - Equipped O3 Analyzers	6
4.0 EPA's Review and Approval of CASTNET's Nafion™ Dryer Request	8
4.1	October 2016 to January 2017 Evaluation Tests	9
4.1.1	Control Period (Oct. 21 to Nov. 27, 2016)	9
4.1.2	Evaluation Period (Dec. 5 to Dec. 26, 2016)	 10
4.1.3	Post-Evaluation Period (Dec. 27, 2016 to Jan. 13th, 2017)	11
4.2	Nafion™ Dryer Issues Since EPA's 2017 Approval to the Clean Air Markets	13
4.2.1	EPA Region 1	13
4.2.2	NCDEQ	13
4.2.3	CASTNET	 14
5.0 Summary and Conclusions	16
6.0 References	19
Appendix A. OAR's June 2017 Nafion™ Dryer Approval Letter to Clean Air Markets Division
	21
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Table of Figures
Figure 1. Schematic diagram of the 2B Technologies Model 106-L O3 analyzer	2
Figure 2. Photograph of single Nafion™ tube	5
Figure 3. Schematic of Nafion™ tube configured with active purge air for drying of sampled
airstream	6
Figure 4. Schematic diagram showing location of "DewLine" Nafion™ tube upstream of the
absorption cell	7
Figure 5. Photograph of CASTNET's Nafion™ assembly showing sample and purge air paths.
	8
Figure 6. Control period timeline showing response of the two Thermo 49i O3 analyzers	10
Figure 7. Evaluation period timeline showing response with the Evaluation analyzer equipped
with an upstream Nafion™ dryer	11
Figure 8. Post-Evaluation period timeline during the Jan. 3, 2017 humid sampling event	12
Figure 9. Results from zero tests of a Thermo 49i O3 analyzer at a TN CASTNET site. This
analyzer was not equipped with a Nafion™ dryer during this 3-year period	14
Figure 10. Results from zero tests of a Thermo 49i O3 analyzer at an east TN CASTNET site.
This analyzer was equipped with a Nafion™ dryer in July 2017, at the time indicated by the
orange vertical line	15
Tables
Table 1. Design and operating specifications of CASTNET's Nafion™ Dryer	9
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Acronyms and Abbreviations
AAB
Ambient Air Branch
CASTNET
Clean Air Status and Trends Network
CAMD
Clean Air Markets Division
CEMM
Center for Environmental Measurement and Modeling
CO
Carbon Monoxide
DEQ
Department of Environmental Quality
EPA
Environmental Protection Agency
FEM
Federal Equivalent Method
FEP
Fluorinated ethylene propylene
FRM
Federal Reference Method
Hg°
Elemental Gaseous Mercury
Lpm
Liters per minute
NAAQS
National Ambient Air Quality Standards
nm
nanometer
N02
Nitrogen Dioxide
03
Ozone
OAP
EPA Office of Atmospheric Programs
OAQPS
EPA Office of Air Quality Planning and Standards
OAR
EPA Office of Air and Radiation
ORD
EPA Office of Research and Development
ppb
Parts per billion
ppbv
Parts per billion by volume
ppt
Parts per trillion
RH
Relative Humidity (%)
S02
Sulfur Dioxide
TFE
T etrafluoroethy 1 ene
UV
Ultraviolet
VOC
Volatile Organic Compounds

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Acknowledgments
The authors acknowledge the useful comments from John Birks and Andrew Turnipseed of 2B
Technologies Inc., those of Bob Judge of EPA Region 1, Joann Rice (OAQPS) and Matthew Landis
(ORD). We also acknowledge the receipt of Nafion™ dryer performance data and information from Jeff
Gobel (NC DEQ) and Timothy Sharac (EPA CAMD).
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Executive Summary
Ultraviolet (UV) Photometry-based ambient ozone (O3) analyzers can experience significant water
vapor interferences resulting in increased measurement noise, biased concentration measurements, and
slow instrument response times. The magnitude and duration of these biases is particularly noticeable
during sampling events where the O3 analyzer experiences rapid changes in the sampled air's moisture
content. The U.S. Environmental Protection Agency (EPA) implemented the use of a Perma Pure
Nafion™ tube dryer upstream of their Thermo Environmental Model 49i O3 analyzers at 30 Clean Air
Status of Trends Network (CASTNET) National Ambient Air Quality Standard (NAAQS) monitoring
sites as part of a limited evaluation and has found this approach minimizes spectroscopic water vapor
interferences to an acceptable degree.
The operating principle of UV Photmetric-based O3 analyzers is presented along with a discussion of
known measurement interferences, with a particular emphasis on atmospheric water vapor and
approaches used to minimize the magnitude of this water vapor interference. Multiple studies are
discussed which demonstrates that no inadvertent loss of O3 occurs in the Nafion™ tubing. The design
and operational specifications of the CASTNET implemented Nafion™ dryer system are presented
along with test results of the system's performance. Based on the results of this investigation, the EPA
Office of Research and Development (ORD) has determined that the use of Nafion™ dryers upstream of
UV Photometric-based O3 analyzers on a nationwide basis is a valid means to improve the data quality
of O3 compliance measurements.
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1.0 Introduction
Water vapor interferences can bias ambient O3 measurements in gas-phase UV absorption analyzers
under certain sampling circumstances. In 2017, EPA's Office of Air and Radiation (OAR) approved the
use of Nafion™ dryers upstream of designated Federal Equivalent Method (FEM) UV Photometric-
based O3 analyzers in the CASTNET. This approval was based on a request from EPA's Clean Air
Markets Division (CAMD) and a review of pertinent information at the time. The use of the dryers was
designed to help address instrument noise, response time issues, and bias issues observed in
CASTNET's UV-based O3 analyzers. These issues were due to the presence of excessive water vapor in
the ambient air, particularly during events where the analyzer experiences rapid changes in the sampled
air's water vapor concentrations.
EPA approved CASTNET's use of the Nafion™ dryer upstream of the instrument in the sampling train.
EPA's approval, therefore, was not a modification of a previously designated O3 analyzer per the
40 CFR Part 53 specifications but an interpretation of the specifications in Part 58 "Ambient Air
Surveillance". Specifically, Section 9 of Part 58's Appendix E states that".. .borosilicate glass,
fluorinated ethylene propylene (FEP) Teflon® or their equivalent must be the only material in the
sampling train (from the inlet probe to the back of the analyzer) that can be in contact with the ambient
air sample..As documented in Appendix A., OAR's June 2017 Nafion™ Dryer Approval Letter to
CAMD, of this document, EPA's approved use of the Nafion™ dryer in the CASTNET network was
based on its determination that Nafion™ represents a suitable "equivalent" material in the sampling
train.
EPA's 2017 approval was deliberately designed to apply only to the CASTNET network. Since then,
however, EPA has received inquiries from other monitoring organizations (e.g., NC Division of Air
Quality) seeking the approved use of Nafion™ dryers in their own monitoring networks as a means of
addressing UV-based O3 measurement biases due to the presence of water vapor. As a result, EPA is
now considering approving the use of a Nafion™ dryer upstream of all currently designated UV-based
O3 analyzers on a widespread basis, rather than on a case-by-case basis.
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2.0 Operating Principle of UV-based O3
Photometers
Figure 1 depicts the general design and operation of a UV-based O3 photometer (2B Technologies,
Model 106-L, 2014).
Hg Lamp
h
Temperature
Sensor
Ozone
Measurement
Scrubber
A
T
Air Inlet
Absorption Cell
¦ Pressure
Sensor
Solenoid Valve
a
Air
Pump

Photodiode
Figure 1. Schematic diagram of the 2B Technologies Model 106-L O3 analyzer.
Ambient air is continuously drawn through the instrument's detection cell using a calibrated air pump,
and the concentration of O3 in the cell is determined by UV absorption. A mercury vapor lamp generates
UV light at a wavelength of 253.7 nm, which corresponds to the peak absorption of O3. The
photodiode's response is inversely proportional to the concentration of O3 in the detection cell.
Upstream of the detection cell, a solenoid valve alternately directs the sample through and around a high
efficiency solid-phase catalytic O3 scrubber. The purpose of the scrubber is to convert O3 molecules into
oxygen molecules thus enabling matrix-matched background / reference measurements to be made in
the absence of O3. Scrubbers may be composed of potassium iodide, mixtures of magnesium dioxide
and copper oxide (Hopcalite), metal-oxide coated screens, or heated silver wool (Turnipseed et al.,
2017). The Beer-Lambert law is used to calculate O3 concentration by comparing the measured intensity
in the absence of O3 (Io, scrubbed sample) to that intensity in the presence of O3 (/, unscrubbed sample):
1
Ozone concentration = —
oL
where o is the absorption cross section of O3 and L is the path length of the detection cell. Dependent
upon instrument design, / and Io values may be measured every few seconds, thus enabling an O3
concentration determination approximately every 10 seconds. Some instruments have two cells and
measure / and Io simultaneously, switching between the cells regularly to address any cell-specific
biases. Temperature and pressure measurements within the air path enables measured concentrations to
be expressed on a volumetric basis (i.e., ppbv). Although the O3 concentration can theoretically be
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determined solely from the above equation, periodic instrument calibration is necessary to account for
inlet losses, non-ideal scrubber efficiency, scrubber and cell condition, exact absorption cell geometry,
and non-linearity in photodiode and amplifier response. A properly calibrated and operated O3
compliance analyzer can measure O3 concentrations with an accuracy and precision of approximately
±1 ppb.
2.1	Interference of Atmospheric Chemicals
Positive measurement interferences in UV photometric O3 analyzers could occur from atmospheric
species that absorb light in the region of 253.7 nm. The use of a two-channel UV photometric O3
analyzer helps to mitigate the interference from volatile organic compounds (VOCs), with the
assumption that the scrubber destroys O3 but passes interfering VOC species. In particular, VOCs and
elemental gaseous mercury (Hg°) are noted to absorb in this range and positive measurement biases are
well documented (Grosjean and Harrison, 1985; Huntzicker and Johnson, 1979; Kleindienst et al., 1993;
Spicer et al., 2010). Birks et al. (2009) noted that the absorption cross section of Hg° was approximately
1600 times that of O3 at a wavelength of 253.7 nm. At a baseline O3 concentration of 75 ppb, laboratory
tests (EPA 1999) showed that the presence or addition of 0.04 ppb (40 ppt) of Hg° would result in
positive O3 measurement biases in UV photometers of 12.8% and 6.4% at relative humidity (RH) values
of approximately 25% and 75%, respectively. During this EPA study, no positive bias in Hg°
concentrations were noted in chemiluminescent O3 analyzers.
2.2	Interference of Atmospheric Water Vapor
While the chemical interferences previously mentioned would typically be of more significance in
polluted environments rather than in clean environments, interferences due to atmospheric water vapor
can be encountered under a range of sampling environments. Water vapor interferences have been
reported by a variety of researchers and verified under both field conditions and in controlled laboratory
tests (Meyer and Elsworth, 1991; Kleindienst et al., 1993; Leston and Ollison, 1993; Leston et al., 2005;
Maddy, 1998; Parrish and Fehsenfeld, 2000; Wilson, 2005; Wilson and Birks, 2006; Spicer et al., 2010;
Birks et al., 2016). These studies report that biases due to water vapor can be either positive or negative
and can be significant in magnitude depending upon the sampling circumstances.
Unlike the interference mechanism mentioned for chemical interferents, water vapor does not
significantly absorb wavelengths of 253.7 nm. As a result, the exact mechanism for the water vapor's
interference was not immediately apparent when it was initially noted. Meyer et al. (1991) first reported
O3 measurement interferences during rapid changes in RH and associated it with the observed change in
the transmission efficiency through scratched optical cells windows. Wilson and Birks (2006) conducted
more focused tests and identified that rapid changes in sample moisture content can change the
reflectivity of the detection cell's walls. Using a balloon-borne UV-based O3 analyzer during vertical
profiling tests, the magnitude of the bias was particularly significant when the O3 analyzer rapidly
passed through alternating wet and dry atmospheric layers. This bias mechanism of the interference of
water vapor was later supported by Spicer et al. (2010) and explains why previous tests conducted under
steady-state sampling conditions reported negligible effects of humidity on UV-based analyzers
(Kleindienst et al., 1993).
Wilson and Birks (2006) further conjectured that it is the scrubber's interaction with the sampled
airstream that accounts for variations in the detection cell wall's transmission efficiency. The O3
scrubber can act as a water reservoir and can thus add or remove water from the sample air. Differences
between / and h values can occur even in the absence of O3, depending upon recent sampling history,
and explains why measurements biases can be either positive or negative and variable in magnitude. As
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will be discussed, Wilson and Birks later addressed this interference mechanism through incorporation
of a Nafion™ tube upstream of the analyzer's absorption cell. To test this theory, Wilson and Birks
conducted a series of tests using a Nafion™ tube purchased from Perma Pure LLC (Lakewood, NJ).
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3.0 Characteristics of Nation™ Tubes
Nafion™ membranes (Perma Pure LLC) are composed of a copolymer of perfluoro-3,6-dioza-4-methyl-
7-octenesulfonic acid and polytetrafluoroethylene. In simpler terms, Nafion™ consists of
tetrafluoroethylene (TFE) Teflon™ chemically bonded with sulfonic acid groups. The presence of
sulfonic acid makes Nafion™ tubing selectively permeable to compounds (e.g., water, alcohol, and
ammonia) that bind to sulfonic acids (Perma Pure, 2020). The selective permeability of Nafion™ makes
it ideal for conditioning ambient air samples for subsequent pollutant analysis.
Although the term "Nafion™ dryer" is commonly used, a Nafion™ tube can either humidify or dry the
tube's sampled air depending upon the differential moisture content across the membrane. If the
moisture content of the air outside of the tube is higher than that inside the tube, the moisture content of
the interior sample gas will increase (humidify). Conversely, if the moisture content of the air outside
the tube is lower than that inside the tube, the moisture content of the interior sample gas will decrease
(i.e., dehumidify). The magnitude of the change in the sampled air's moisture content will depend upon
the difference in moisture content across the tube's wall and the residence time of the gas within the
tube. As noted by Robinson et al. (1999), absorption and desorption of water can occur very rapidly, as
evidenced by tests with aim long Nafion™ tube of 1.07 mm inner diameter, 1.35 mm outer diameter
operating a flow rate of 1 Lpm. Under these conditions, the Nafion™ tube sampled air was equilibrated
with the surrounding air in a residence time of approximately 50 ms (0.05 sec).
Various configurations and geometries of Nafion™ tubes are commercially available. In the tube
depicted in Figure 2, the differential moisture content between the airstream inside the tube and the
outside the tube dictates the actual direction of the moisture transfer. The configuration shown in
Figure 3 enables the user to provide a counterflowing purge gas of known moisture content to actively
control the direction and extent of the sampled gas' final moisture content. Figure 3 depicts the use of
dry purge gas in a counterflow orientation to actively dry the moist sample gas.
Figure 2. Photograph of single Nafion™ tube.
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Wet Purge Gas Outlet
C
Figure 3. Schematic of Nafion™ tube configured with active purge air for drying of sampled airstream.
For purposes of conditioning of ambient air samples, an important property of Nafion™ is its ability to
provide moisture conditioning without the loss of the pollutant of interest. With respect to monitoring
the four gaseous criteria pollutants carbon monoxide, sulfur dioxide, nitrogen dioxide, and O3 (CO, SO2,
NO2), all are expected to experience zero loss when sampled and transferred through Nafion™ tubing
(Perma Pure, 2020). High transport efficiency of O3 through Nafion™ tubes would also be expected
based on the similarity between PTFE Teflon™ and the chemically inert fluorinated ethylene propylene
(FEP) Teflon™. This theoretical transport efficiency of O3 was verified in the laboratory by Wilson and
Birks (2006) at an RH of 39% using O3 mixing ratios in the range of 0 to 350 ppbv. Linear regression of
O3 transport measurements with the Nafion™ tube present versus those measurements without the
Nafion™ tube present yielded a slope of 0.9967, an intercept of 0.27 ppbv, and a correlation coefficient
of 1.0000. The authors concluded that there was no loss of O3 with the Nafion™ tube within
experimental measurement uncertainty. Similar transport efficiency test results were obtained during
three separate field studies using Nafion™ dryers upstream of designated O3 analyzers.
3.1 2B Technologies Nafion™ - Equipped O3 Analyzers
Wilson and Birks (2006) conjectured that modulation of the sampled stream's humidity by the scrubber
is responsible for changing the reflectivity of the UV analyzer's absorption cell, thus accounting for
noted variabilities in instrument performance during rapid changes in ambient moisture content. They
proposed that equilibration of humidity of the scrubbed and unscrubbed air upstream of the absorption
cell would ensure that / and h values were measured at the same RH level. To test this theory, a
Nafion™ tube was installed immediately upstream of a 2B Technologies Model 202 O3 analyzer's
absorption tube (Figure 4). Note that active purge air was not provided to the outside of the Nafion™
tube because achieving a specific moisture content is not actually required. Instead, the Nafion tube
equilibrated the sample air with the room air in the analyzer's sampling location. This ensured that the /
and Io measurements were conducted at the same humidity levels, minimizing the O3 bias caused by
rapidly changing ambient humidity levels.
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Hg Lamp
B
Temperature
Sensor
Pressure
Sensor
Absorption Cell
Photodiode
Nation Tube
(DewLine™)
i
Solenoid Valve
Ozone
Scrubber
ir
Air Inlet
Figure 4. Schematic diagram showing location of "DewLine" Nafion™ tube upstream of the absorption cell
Wilson and Birks (2006) conducted three tests to verify the effectiveness of the system, one in dry tank
air, one using room air of 39.2% RH, and one using humidified air with a RH of 93.0%. The RH of the
sample air exiting the Nafion™ tube during these three separate tests was measured to be 39.3%, 39.2%,
and 39.4%, respectively. These test results demonstrated the Nafion™ tube's ability to condition the
incoming air to the humidity level of the analyzer's location (e.g., shelter), independent of the initial
moisture content of the sampled (outdoor) airstream. A video demonstration of the DewLine's ability to
moderate humidity effects under very rapid and dramatic changes in the sampled air's moisture content
is available at 2B Technologies' website: https://www.voutube.com/watch?v=8Hk9w7kskDI (last
accessed 10 19 2020).
2B Technologies incorporates its "Dewline™" Nafion™ assembly in its EPA-designated FEM O3
analyzers (Models 106, 106-L, 202, 205, 211 OEM-108-L, 211, 211G, and POM) as well as its EPA-
designated Model 405 nm NO2 FEM analyzer. 2B Technologies recommends that each O3 analyzer's
Dewline™ assembly be replaced during the instrument's annual scrubber replacement.
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4.0 EPA's Review and Approval of
CASTNET's Nafion™ Dryer Request
In early 2017, OAR received a written request from the EPA CAMD to add Nafion™ dryers upstream
of Thermo 49i UV-absorbance O3 analyzers operating in the CASTNET monitoring network. The
purpose of the Nafion™ dryer's addition was to address increased instrument noise and decreased
measurement response, which typically occurred during hot and humid days. The Thermo 49i
instrument problems were noted when ambient dew points reached or exceeded nominal shelter
temperatures of 25 °C. In such circumstances, the zero, span, and quality control (QC) checks showed
slow (>30 minutes) response times.
To support their request, CAMD provided the results of field tests conducted by CASTNET's contractor
(Amec Foster Wheeler) in Gainesville, FL from October 2016 through 2017. Specifically, the collocated
performance of a Thermo 49i analyzer equipped with a 1.2 m long Nafion™ tube upstream of analyzer
was compared to the performance of a collocated Thermo 49i analyzer without the Nafion™ tube.
Figure 5. Photograph of CASTNET's Nafion™ assembly showing sample and purge air paths.
The configuration of CASTNET's Nafion™ assembly (Figure 5) matches that of Figure 3, with an
active purge flow of shelter indoor air is provided in addition to the sampled ambient air flow. Purge
flow is provided using a 20 Lpm capacity vacuum pump and the 1.3 Lpm purge air flow rate is
controlled by an orifice downstream of the depicted purge air filter. The resulting 1:1 ratio of purge air
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flow rate to sampled air flow rate meets Perma Pure's recommended ratio of 1.0 to 3.0. The sampled
air's 2.1 second residence time is expected to be more than sufficient for the sampled air's moisture
content to be equilibrated to that of the purge air.
The purge air in this configuration originates from the shelter's interior air rather than from a totally dry
air source. This approach avoids the additional cost, complexity, and maintenance of a provided dry air
source for the purge air. Because the shelter's air naturally has some degree of moisture associated with
it, there is no expectation that the sampled air exiting the Nafion™ dryer will be totally dry. Instead, use
of the Nafion™ dryer is intended to reduce the sampled air's moisture content such that the resulting
water vapor interference to the 49i O3 analyzer will be acceptably low. Table 1 provides the Nafion™
dryer's design and operating specifications.
Table 1. Design and operating specifications of CASTNET's Nafion™ Dryer
Parameter
Specification
Tested UV-based O3 analyzer
Thermo 49i FEM (EPA designation EQOA-0880-047)
49i sample flow rate
1.3 Lpm
Nafion™ tube
Perma Pure, Model # MD-110-48P-4
Nafion™ tube length
48 inches (1.2 m)
Nafion™ sample air tube's ID
0.086" (0.218 cm)
Nafion™ sample air residence time
2.1 seconds
Purge air flow rate
1.3 Lpm (nominal)
Purge air to sample air flow ratio
Approx. 1:1
Purge air inlet filter
Parker - Balston, #9933-05-DQ
Purge air flow control orifice
McMaster-Carr, size 0.010", Model #6349T42
Vacuum pump
Thomas, Model # 107CA18
4.1 October 2016 to January 2017 Evaluation Tests
Tests conducted in Gainesville, FL during this time period involved the use of two identical Thermo 49i
O3 FEM analyzers (EPA designation EQOA-0880-047). Data collected during these tests included 1-
minute and 1-hour O3 concentrations, 1-minute shelter temperatures, and 5-minute measurements of
ambient temperature, RH, and dew point. The configuration of one 49i analyzer "Control Analyzer" did
not change during these tests. The other 49i analyzer "Evaluation Analyzer" was not equipped with a
Nafion™ dryer during the Control period nor the Post-Evaluation period but was equipped with the
Nafion™ dryer during the Evaluation period. Although the October to January period is not the humid
season at this location, there were a few days in which the ambient air's dew point was sufficiently close
to the shelter temperature to evaluate the Nafion™ dryer's effectiveness.
4.1.1 Control Period (Oct 21 to Nov. 27, 2016)
During this Control period, the Control Analyzer and the Evaluation Analyzer were configured
identically (the Evaluation Analyzer was not equipped with the Nafion™ dryer). As shown in Figure 6,
atmospheric conditions during this time period were such that the ambient air's dew point was not
within 2 °C of the shelter temperature. This test period, therefore, provided an opportunity to compare
the inherent O3 measurement performance of the two collocated Thermo 49i O3 analyzers.
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	Control 	Evaluation	Shelter Temp 	Dew Point
r 150
90
130
10/21/201612:00 10/26/2016 12:00 10/31/201612:00 11/5/2016 12:00 11/10/201612:00 11/15/201612:00 llf20/2016 12:00 11/25/201612:00

30
-50
Figure 6. Control period timeline showing response of the two Thermo 49i O3 analyzers
Visual inspection shows that the two identically configured 49i analyzers agreed well with each other
during each of the test days. Linear regression of 1 -minute data for the Evaluation Analyzer versus the
Control Analyzer resulted in slope, intercept, and coefficient of determination (R2) values of 0.998, -
0.14 ppbv, and 0.9996, respectively. Linear regression of hourly data for the Evaluation Analyzer versus
the Control Analyzer resulted in slope, intercept, and R2 values of 0.999, -0.15 ppbv, and 0.9999,
respectively. Under identical test conditions, therefore, the two Thermo 49i O3 analyzers agreed well
with each other on both a 1-minute and an hourly basis.
4.1.2 Evaluation Period (Dec. 5 to Dec. 26, 2016)
During this time period, the Evaluation Analyzer was configured with the Nafion™ dryer upstream of
the analyzer. Visual inspection of the timeline during this 22-day evaluation period in Figure 7 shows
that there were several days during which the shelter's interior temperature was close to the dew point.
The timeline in this figure indicates that these days corresponded to noted difference in O3 measurement
response between the Control Analyzer and the Evaluation Analyzer.
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Control
Evaluation
Shelter Temp
Dew Point
12/5/2016
12/10/2016
12/15/2016
.2/20/2016
12/25/2016
Figure 7. Evaluation period timeline showing response with the Evaluation Analyzer equipped with an upstream
Nafion™ dryer.
If one considers the measurement comparability only during "dry" periods (defined as those days when
shelter temperature was greater than 2 °C above the dew point), the regression of 1-minute data between
the Evaluation Analyzer and the Control Analyzer resulted in slope, intercept, and R2 values of 1.001,
0.096 ppbv, and 0.9983, respectively. This is an important observation because it supports both
theoretical predictions and previous experimental results (Wilson and Birks, 2006) which concluded that
there is no measurable transport loss of O3 within the Nafion™ tube.
Regarding measurement data only during "wet" periods (defined as those where shelter temperature is
withi n 2 °C of the dew point), the regression of 1-minute data between the Evaluation Analyzer and the
Control Analyzer resulted in slope, intercept, and R2 values of 0.859, 2.913 ppbv, and 0.8562,
respectively. Compared to the data collected on "dry" days, this is a significant reduction in slope and a
significant increase in intercept. The R2 value of 0.8562 indicates that there is far less consistency in the
measurement response between the Control Analyzer and Evaluation Analyzer than occurs during dry
periods. This is expected if the presence of the Nafion™ dryer improves the measurement response of
the Thermo 49i analyzer, as intended.
4.1.3 Post-Evaluation Period (Dec. 27, 2016 to Jan. 13th, 2017)
Following the Evaluation period, the Nafion™ dryer assembly was removed from the Evaluation
Analyzer's configuration and the identically configured Control Analyzer and Evaluation Analyzer were
concurrently operated for a 17-day time period. Linear regression of 1-minute data for the Evaluation
11

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Analyzer versus the Control Analyzer resulted in slope, intercept, and R2 values of 0.998, -0.17 ppbv,
and 0.9984, respectively. These regression coefficients are similar to those obtained during the 5-week
Control period using this same configuration of analyzers. Linear regression of hourly data for the
Evaluation Analyzer versus the Control Analyzer resulted in slope, intercept, and R2 values of 0.999, -
0.19 ppbv, and 0.9995, respectively. As in the case of the 1-minute data, these hourly regression
coefficients were similar to those obtained during the 5-week Control period.
Although the measurement comparability between the two Thermo 49i analyzers without Nafion™
dryers was noted to be similar during this Post-Evaluation period, Figure 8 shows that short-term
differences between the two instruments was noted on Jan. 3, 2017 during a period where shelter
temperatures were veiy close to the dew points. This illustrates that somewhat different short-term
response between identically configured instruments can result in substantial differences. It is expected
that use of a Nafion™ dryer upstream of the Evaluation Analyzer would have appreciably reduced the
noted measurement response during this humid episode. Following consideration of the theoretical and
empirical information mentioned in this report by staff from QRD and OAR, EPA formally approved the
Nafion™ dryer's use for Thermo 49i analyzers in CASTNET's monitoring networks. A copy of this
approval letter is presented in Appendix A. OAR's June 2017 Nafion™ Dryer Approval Letter to
CAMD
Control
Evaluation
Shelter Temp
Dew Point
1/3/2017 0:00
1/3/2017 6:00
1/3/2017 12:00
1/3/2017 18:00
1/4/2017 6:00
1/4/2017 12:00
1/4/2017 0:00
"VVVVVWWV^
Figure 8. Post-Evaluation period timeline during the Jan. 3,2017 humid sampling event.
OAR's June 15, 2017 approval letter to the EPA CAMD reiterated that the approval was not a formal
modification to existing designated UV-based instruments because the approved Nafion™ dryer's use is
upstream of the O3 analyzer itself, rather than internally as in the case of 2B Technologies' O3 FEM
12

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analyzers. Instead, it was EPA's technical judgement that Nation™ represents an acceptably
"equivalent" material regarding probe materials in Section 9 of 40 CFR Part 58 Appendix E. Approval
was specifically intended for use solely in the CASTNET monitoring network.
4.2 Nafion™ Dryer Issues Since EPA's 2017 Approval to the Clean Air Markets
Since the time of EPA's 2017 approval to use a Nafion™ dryer in CASTNET's monitoring system as a
means of improving O3 monitoring performance, EPA has received several inquiries from various
monitoring organization inquiring about the possibility of using Nafion™ dryers to address O3
monitoring issues in their own networks.
4.2.1	EPA Region 1
In 2018, Peter Kahn (EPA Region 1) contacted EPA (P. Kahn, personal communication to Robert
Vanderpool, August 21, 2018) for information regarding the potential use of a Nafion™ dryer in Rhode
Island's Thermo 49i O3 analyzers. Darren Austin of Rhode Island's Department of Environmental
Management later became involved and provided data to Joann Rice showing that some O3 network data
has been flagged during very high dew point days (D. Austin, personal communication to Joann Rice,
August 24, 2018). Darren mentioned that the O3 analyzer was subsequently audited and found to
function nominally. It was also mentioned, though, that the analyzer's calibration was conducted using
dry air and thus the audit may not be representative of the analyzer's actual sampling situation under
consideration. Bob Judge later communicated that some sampling sites in Region 1 were experiencing
water vapor issues as evidenced by visible condensation in sampling lines upstream of the Thermo 49i
O3 analyzer (R. Judge, personal communication to Lewis Weinstock, Sept. 13, 2018). Recommended
efforts to mitigate the problem (e.g., insulating sampling lines, heating sampling line, increasing shelter
temperature) were apparently only partially successful at addressing the water vapor issue. Bob Judge
also raised question about the potential use of Nafion™ dryers in Rhode Island's O3 analyzers.
4.2.2	North Carolina Department of Environmental Quality (NC DEQ)
In late 2019, Jeff Gobel (NC DEQ) contacted Richard Guillot (Region 4) and requested use of Nafion™
dryers at some monitoring sites in NC which had been experiencing ongoing moisture problems with
their Thermo 49i O3 analyzers (J. Gobel, personal communication to Richard Guillot, Nov. 26, 2019).
NC DEQ was not seeking statewide permission to use the Nafion™ dryer but only requested use of the
dryers at NC's Monroe and Linville Falls sites. Previous efforts to address analyzer performance issues
at these sites included insulating probe lines and filter assemblies, using heat tape on sample lines,
installing and operating dehumidifier within the analyzer's shelter, and raising shelter to 30 °C. Richard
subsequently requested assistance from Joann Rice regarding this issue (R. Guillot, personal
communication to Joann Rice, Nov. 26, 2019).
As supporting evidence for approval of the Nafion™ dryers in NC's networks, Jeff Gobel provided
results of preliminary beta testing of Nafion™ dryer assemblies at the Monroe and Linville sites.
Nafion™ dryers were installed on Sept. 26, 2019 and Oct. 2, 2019 at the Linville and Monroe sites,
respectively. The design of the Nafion™ assemblies exactly matched those currently operated by
CASTNET, based on technical input received from CASTNET's contractor. These specifications match
those previously mentioned in Table 1 of this document. The two sites were audited before, during, and
after the Nafion™ dryer installations. Comparison of the audit data showed that no O3 loss occurred in
the Nafion™ dryers at either site. These test results are thus in agreement with those previously
mentioned (Wilson and Birks, 2006) and from tests conducted in Gainesville, FL.
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As of May 2020, these two sampling sites are just now entering the O3 season and instrument
performance data will be provided to EPA as it becomes available. NC DEQ also intends to install
meteorological systems at each site for the recording of ambient wind speed, wind direction,
temperature, and RH. In conjunction with plans to install RH sensors inside the shelters, these
measurements will help assess the performance of the installed Nafion™ dryer systems at both sites.
4.2.3 CASTNET
At ORD's request, Timothy Sharac (OAR/OAP/CAMD) provided an update on CASTNET's experience
with the Nafion™ dryer installation and operation since EPA's June 2017 approval (T. Sharac, personal
communication to Robert Vanderpool, May 20, 2020). CASTNET's contractor (John Wood Group
PLC) reported two notable improvements in Thermo 49i O3 analyzer performance following installation
of the Nafion™ dryer assembly upstream of the analyzer.
The first improvement related to instrument noise which resulted during short-term shelter temperature
cycles in the trailer. Prior to the Nafion™ dryer installation, O3 measurement deviations of ±10 ppb
were noted to occur due to short-term temperature cycling. Although subsequent installation of the
Nafion™ dryer did not completely eliminate the problem, these measurement deviations were reduced
from ±10 ppb to ±1 ppb when tests were conducted at the same ambient air dew point.
The second noted improvement related to results obtained during analyzer QC checks conducted using
dry air. Figure 9 depicts the results of zero tests conducted at a Tennessee CASTNET site (Site SPD111,
Speedwell Co., TN) over a 4-year period (January 2016 to April 2020). Note that the plot represents
unvalidated data so that negative data can be seen. For this Thermo 49i analyzer operated without a
Nafion™ dryer installed, large deviations from zero periodically occurred. In particular, numerous
measurements in the range of -4 ppb to -10 ppb were noted during high humidity events.
CASTNET Site SPD111
6
Oct-15	Oct-16	Oct-17	Oct-18	Oct-19
Date
Figure 9. Results from zero tests of a Thermo 49i O3 analyzer at a TN CASTNET site. This analyzer was not equipped
with a Nafion™ dryer during this 3-year period.
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Similar zero test results of the Thermo 49i O3 analyzer were observed at an east Texas CASTNET site
(Site ALC188, Polk Co) prior to a Nation™ dryer's installation on July 14, 2017. In the Figure 10
timeline of ALC188 zero test data, the vertical orange line represents the Nafion™ dryer's installation
date. Use of the Nafion™ dryer at this site minimizes the incidence of concentration data below -4 ppb
CASTNET Site ALC188
-Q
B
Q.
A3
0
c
l ai 1 AwT , ,/ a
¦ •	.Sir w ' JB
—L_
Oct-16
Oct-17	Oct-18
Date
Oct-19
Oct-20
Figure 10. Results from zero tests of a Thermo 49i O3 analyzer at an east TN CASTNET site. This analyzer was
equipped with a Nafion™ dryer in July 2017, at the time indicated by the orange vertical line.
CASTNET suggested that improved performance of the Nafion™ system could be obtained using drier
purge air (e.g., using a nitrogen cylinder) but acknowledged that increased operational and maintenance
costs would be required. It was also suggested that improved moisture control under network conditions
might be obtained using higher purge flow rates, increased purge vacuum, and/or by increasing the
Nafion™ tube length from 48 in (1.2 m) to 72 in (1.8 m). An increase to a 72 in length would equate to a
50% increase in transfer area at a marginal increase in cost (i.e., $305 versus $358).
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5.0 Summary and Conclusions
O3 analyzers based on UV-absorbance are widely deployed and generally provide reliable measurement
results under routine operating conditions. While some measurement interferences (e.g., VOCs and Hg°)
can exist in polluted environments, measurement interferences due to atmospheric water vapor can occur
in both polluted and unpolluted airsheds. These interferences can increase instrument noise, slow
instrument response, and bias measurements.
While the exact mechanism of water vapor's interference was not initially known, the problems were
observed to frequently occur when shelter temperatures were at or near the dew point and when rapid
fluctuations of atmospheric moisture occurred. With varying degrees of success, efforts to address the
issue included insulating sampling lines, heating sampling lines, and/or raising shelter temperatures.
Wilson and Birk (2006) experimentally identified that O3 scrubbers can act as sinks for water vapor and
can absorb or desorb water vapor depending upon specific sampling circumstances. Because variations
in water vapor downstream of the scrubber can alter the reflectivity of the detection cell's wall,
variations in humidity during the analyzer's measurement cycle can produce uncertain results. The
introduction of a Nafion™ tube immediately downstream of the analyzer's scrubber was found to
effectively eliminate this measurement bias mechanism. It should be reiterated that the Nafion™ tube in
this configuration does not actually dry the sampled gas stream but ensures that the moisture content
remains stable during the measurement cycle. 2B Technologies' suite of approved FEM O3 analyzers
each are equipped with an internal Nafion™ tube to minimize the water vapor interference. The
manufacturer recommends that the internal Nafion™ tube be replaced annually during the instrument's
scrubber replacement procedure.
Existing UV-based O3 analyzers which do not contain an internal Nafion™ tube could be retrofitted to
contain this feature. Such a retrofit, however, changes the terms of the instrument's original FRM or
FEM designation and the analyzer's manufacturer would need to submit a formal Modification Request
to EPA for review and approval in order to retain the designation. The Request would need to contain
supporting rationale along with results of experimental tests demonstrating that the retrofit successfully
addresses the humidity issues without compromising the inherent measurement quality of the analyzer.
Submittal of such a Modification Request is voluntary, and manufacturers must decide whether the
retrofit financially justifies the resources required to develop and submit the Request. To date, none of
these manufacturers have submitted a Modification Request of this type to EPA for review.
Installation of a Nafion™ tube upstream of the O3 analyzer does not violate the terms of the analyzer's
original FRM or FEM designation. Because an external Nafion™ tube does not address the interference
mechanism in the same manner as an internal Nafion™ tube, its degree of effectiveness depends on how
well moisture can be removed from the sampled airstream prior to the airstream's entry into the
analyzer. A continuous supply of dry air to the Nafion™ drier would ensure the airstream's moisture
removal but this approach is considered too complicated and costly during routine monitoring. Instead,
air inside the shelter is used as the Nafion™ dryer's counterflowing purge air. Because the shelter's
internal air will possess some degree of moisture, the incoming airstream will not be fully dried under all
sampling situations. However, the degree of water removal may be sufficient to eliminate much of the
O3 measurement bias which ambient air of high ambient moisture content can produce.
As part of CASTNET's request to EPA to allow incorporation of Nafion™ dryers in the CASTNET
network, field tests were conducted in Gainesville, FL during the 2016/2017 wintertime season. In
agreement with theory and with experimental tests conducted by Wilson and Birks (2006) and Perma

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Pure's literature. Results of these tests showed no measurable transport loss of O3 in the Nafion™ tube.
During periods when the shelter's temperature was not near the dew point, a Thermo 49i configured
with a Nafion™ dryer upstream provided results which agreed closely to a collocated, concurrently
operated Thermo 49i which did not use a Nafion™ dryer. However, during periods when the shelter
temperatures were close to the dew point, a significant difference in measurement response between the
two Thermo 49i O3 analyzers was noted. On average, the Thermo 49i equipped without the Nafion™
appeared to over-measure the O3 concentration compared to the Nafion™ dryer-equipped unit.
Based on review of theoretical considerations and of the Gainesville, FL tests data, EPA concluded that
Nafion™ represented an acceptably "equivalent" sampling material, as defined in Appendix E to
40 CFR Part 58. As a result, EPA approved the use of Nafion™ dryers in the CASTNET network on
June 15, 2017. Although this approval was limited to sites in the CASTNET network, other monitoring
organizations have since expressed interest in implementing a Nafion™ dryer at some of their own sites
which experience moisture interference issues.
In considering whether to approve the use of Nafion™ dryers on a widespread use, rather than on a case-
by-case basis as was the CASTNET approval, there are generally two technical questions for which
consensus by ORD is required. First: Is the inherent transport efficiency of O3 molecules through
Nafion™ tubing sufficiently high to ensure that negative O3 measurement biases would not occur? From
a theoretical perspective, the chemical composition of Nafion™ and its similarity to FEP Teflon is such
that little or no O3 loss would be expected to occur in Nafion™ tubes. As discussed in previous sections,
the results of multiple laboratory and field tests of Nafion™ tubing strongly support this hypothesis.
Therefore, there appears to be more than sufficient rationale for the use of Nafion™ tubing for the
intended purpose of O3 sampling and transport.
The second question for which consensus is required relates the ability of Nafion™ dryers to minimize
water vapor to an acceptable degree in UV-absorption O3 analyzers. Specifically: Is there sufficient
rationale to conclude that use of Nafion™ dryers under field conditions improves data quality of UV-
based O3 analyzers to an acceptable degree? Field tests conducted in Gainesville, FL using Thermo 49i
analyzers equipped with Nafion™ dryers demonstrated that the dryer's use reduced short-term
measurement noise during periods where shelter temperatures approached the dew points. Following
EPA's approval in 2017 of Nafion™ dryers in CASTNET sites, Nafion™ dryers have been installed and
operated at 30 separate CASTNET sites. While analysis of the resulting data has not been fully
conducted, data presented in this report for two CASTNET sites indicated that that the Nafion™ dryer's
installation reduced measurement uncertainties during short-term fluctuations in shelter temperature and
also improved results obtained during zero checks of the O3 analyzer. Although these field test results
are obviously limited, there appears to be sufficient supporting evidence to conclude that the Nafion™
dryers effectively reduce UV-based O3 measurement uncertainties associated with interferences caused
by ambient water vapor.
Based on the above considerations, ORD recommends that the use of Nafion™ systems upstream of
UV-based O3 analyzers be approved on a nationwide basis for all NAAQS compliance networks. From
both theoretical predictions and laboratory experiments, there is no evidence that significant loss of O3
occurs in Nafion™ systems. Other than the cost and maintenance requirements of the Nafion™ systems,
there appears to be no downside of the system's use from a data quality perspective. In sampling
circumstances where ambient moisture content undergoes rapid, short-term changes, the Nafion™
systems has been shown to significantly reduce measurement noise, concentration measurement biases,
and slow instrument response times. By reducing the amount of water vapor experienced by the UV-
based analyzer's scrubber, use of the Nafion™ system may also provide the added advantage of
increasing the scrubber's service life.
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Regarding a site's implementation of the Nation™ system, ORD recommends that the system be based
on CASTNET's operating and design specifications which are provided in Table 1 and Figure 5. The
only recommended revision to the design specifications might be increasing the Nafion™ tubing length
from 48 in to 72 in. At a nominal increase in purchase price (i.e., $305 versus $358), the increased
tubing length provides a 50% increase in the sampled air's residence time and may thus provide more
effective water vapor transfer than does the 42 in model when using counter current purge air. Because
the 72 in tubing provides 50% more surface area than the shorter tube, the service life of the longer tube
might be longer since contamination by impurities (e.g., VOCs) may not occur as rapidly. The 72 in tube
does, however, have a 50% greater pressure drop than the 48 in tube, so tests should be conducted to
ensure that O3 analyzer can maintain the rated sampling flow rate under the additional pressure drop
conditions. It may also be necessary to modify the purge air system to ensure the purge air flow rate is at
least equal to the sample air flow rate.
As a final operational consideration, ORD recommends that O3 calibration gases be introduced at the
inlet to the Nafion™ system rather than directly into the O3 analyzer. This will ensure that the
calibration gas is conditioned to the shelter's humidity level and thus avoid excessive measurement
noise which can occur during use of dry calibration gases.
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6.0 References
2B Technologies, Inc. (2014) Model 106-L and 106-QEM-L Operating Manual. Rev. G.
Birks, J.W., Williford, C.J., Andersen, P.C.: Use of a Broad Band UV Light Source of Reducing the
Mercury Interference in Ozone Measurements, U.S. Patent Application US 2009/0302230 Al, US Patent
and Trademark Office, Washington, D.C., USA, 2009.
Birks, J.W., Turnipseed, A. A., Andersen, P.C., Williford, C.J.: Heated Graphite Scrubber to Reduce
Interferences in Ozone Monitors, U.S. Patent US 2016/0025696 Al, US Patent and Trademark Office,
Washington, D.C., USA, 2016.
Grosjean, R.J., Harrison, J. (1985) Response of Chemiluminescent NOx Analyzers and Ultraviolet Ozone
Analyzers to Organics Air Pollutants, Environ. Sci. Technol., 19: 862-865.
Huntzicker, J. A., Johnson, R.L. (1979) Investigations of an Ambient Interference in the Measurement of
Ozone by Ultraviolet Photometry, Environ. Sci. Technol., 13: 1414-1416.
Kleindienst, T.E., Hudgens, E.E., Smith, D.F., McElroy, F.F., Bufalini, J.J. (1993). Comparison of
Chemiluminescence and Ultraviolet Ozone Monitor Response in the Presence of Humidity and
Photochemical Pollutants. J. Air Waste Manag. Assoc., 43: 213-222.
Leston, A. and Ollison, W. M. (1993) Estimated Accuracy of Ozone Design Values: Are They
Compromised by Method Interferences?, Tropospheric Ozone: Nonattainment and Design Values Issues,
TR-23, Air & Waste Management Association, Pittsburgh, PA, 1993, 451-56.
Leston, A.R., Ollison, W.M., Spicer, C.W., Satola, J. (2005) Potential Interference Bias in Ozone
Standard Compliance Monitoring. J. Air and Waste Manage. Assoc., 55, 1464-1472.
Maddy, J. A. (1998) A Test That Identifies Ozone Monitors Prone to Anomalous Behavior While
Sampling Hot and Humid Air. Air and Waste Management Association Annual Meeting - San Diego,
CA; Proceedings of Air and Waste Management Association; Pittsburgh, PA.
Meyer, C.P., Elsworth, C.M. (1991) Water Vapor Interference in the Measurement of Ozone in Ambient
Air by Ultraviolet Absorption. Rev. Sci. Instr., 62: 223-228.
Parrish, D.D., Fehsenfeld, F.C. Methods for Gas-Phase Measurements of Ozone, Ozone Precursors, and
Aerosol Precursors. Atm. Envron. 34: 1921-1957.
Perma Pure, Compounds Removed by Nafion™ Tubing Dryers. Accessed May 2020
Perma Pure, Nafion™ Polymer Chemical Retention / Losses and Selectively. Accessed April 2020
Robinson, J.K., Bollinger, M.J., Birks, J.W. (1999) Luminol/H202 Chemiluminescence Detector for the
Analysis of Nitric Oxide in Exhaled Breath, Anal. Chem. 71, 5131-5136.
Spicer, C.W., Darrell, W.J., Ollison, W.M. (2010) A Re-Examination of Ambient Ozone Monitor
Interferences, J. Air Waste Manag. Assoc., 60: 1353-1364.
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Turnipseed, A.A., Andersen, P.C., Williford, C.J., Ennis, C.A., Birks, J.W. (2017) Use of a Heated
Graphite Scrubbers as a Means of Reducing Interferences in UV-absorbance Measurements of
Atmospheric Ozone, Atmos. Meas. Tech., 10: 2253-2269.
U.S. EPA (1999) Laboratory Study to Explore Potential Interferences to Air Quality Monitors, EPA-
454/C-00-002, December 1999.
U.S. EPA (2019) List of Designated Reference and Equivalent Methods.
Wilson, K.L. (2005) Water Vapor Interference in the UV Absorption Measurement of Atmospheric
Ozone, Ph.D. Thesis, University of Colorado.
Wilson, K.L., Birks, J.W (2006) Mechanism and Elimination of a Water Vapor Interference in the
Measurement of Ozone by UV Absorbance, Environ. Sci. Technol., 40: 6361-6367.

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Appendix A. OAR's June 2017 Nafion™
Dryer Approval Letter to Clean Air Markets
Division
21

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^'o»mv	UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
>' O \	RESEARCH TRIANGLE PARK, NC 27711
,.(T	June 15, 2017
AIR QUALITY OFFICE PLANNING OF
AND STANDARDS
Richard Haeuber, Branch Chief
Assessment and Communications Branch (ACB) Clean Air
Markets Division (CAMD) Office of Atmospheric
Programs (OAP)
Dear Rick:
This letter transmits our approval of your request to add Nation™ dryers to the ozone sampling lines at
select Clean Air Status and Trends Network (CASTNET) sites. Specifically, you are requesting the approval to add a
Nation™ dryer at the back of the ozone analyzer inside the monitoring station. We understand that this request is
being made because the CASTNET ozone network uses Thermo 49i ozone analyzers which do not include a dryer
for humidity control as part of the FEM approval, and that you have observed instrument noise problems and
slowed response.
in considering the request, the Office of Research and Development (ORD) and the
Ambient Air Monitoring Group (AAMG) of the Office of Air Quality Planning and Standards (OAQPS) have
discussed this issue and determined that this is not a request for modification to the Thermo 49i FEM analyzers
used in the network, but rather a decision regarding the use of
acceptable materials in sampling probes as covered in Appendix E to 40 CFR part 58.
After careful consideration of your request, we agree that the addition of Nafion™ dryers to the ozone
analyzer in the CASTNET is compliant with Part 58 and that the Nafion™ wiii not interfere with the transmission of
ozone.
For technical questions, you may contact Joann Rice at rice.ioann(S)eoa.gov and 919-5413372.
Sincerely,
Lewis Weinstock, Group Leader
Ambient Air Monitoring Group (AAMG)
Air Quality Assessment Division (AQAD)
Office of Air Quality Planning and Standards (OAQPS)
cc: Joann Rice (OAQPS/AQAD/AAMG)
Tim Sharac (OAP/CAMD/ACB)
Melissa Puchalski (OAP/CAMD/ACB)
Robert Vanderpool (ORD/National Exposure Research Lab (NERLJ) Mike Papp
(OAQPS/AQAD/AAMG)
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vvEPA
United States
Environmental Protection
Agency
PRESORTED
STANDARD POSTAGE
& FEES PAID EPA
PERMIT NO. G-35
Office of Research and
Development (8101R)
Washington, DC 20460
Official Business
Penalty for Private Use
$300

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