UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D C 20460
OFFICE OF THE ADMINISTRATOR
SCIENCE ADVISORY BOARD
SUBJECT: Transmittal of Science Advisory Board Report
FROM: Vanessa T. Vu
Director, Science Advisory Board Staff Office (HOOF)
TO: Karen Sheffer
EPA Headquarters Library Repository (3404T)
This is to advise you that the Science Advisory Board, Clean Air Scientific Advisory
Committee, Ambient Air Monitoring and Methods Subcommittee for the Review of Near-
Road Monitoring to Support Measurement of Multiple National Ambient Air Quality Standard
(NAAQS) Pollutants, issued a report numbered EPA-CASAC-11-001, Review of the "Near-
road Guidance Document - Outline" and "Near-road Monitoring Pilot Study Objectives and
Approach", dated November 24, 2010.
Two copies of the report are attached and a third copy has been sent electronically to
the attention of Ms. Jeannie Turner at tumer.ieannie@epa.gov. The report is available in
electronic format on the Science Advisory Board's Web site at http://www.epa.gov/sab.
If you have any questions regarding this report, please contact the Designated Federal
Officer, Dr. Holly Stallworth directly at (202) 564-2073.
Attachments (2)
Internet Address (URL) • httpV/www epa gov
Recycled/Recyclable • Printed with Vegetable Oil Based Inks on 100% Postconsumer. Process Chlorine Free Recycled Paper
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UNITED STATES ENVIRONMENTAL PROTECTION AG ENC Y
WASHINGTON D.C. 20460
OFFICE OF THE ADMINISTRATOR
SCIENCE ADVISORY BOARD
November 24, 20 10
EPA-CASAC-1 1-001
The Honorable Lisa P. Jackson
Administrator
U.S. Environmental Protection Agency
1200 Pennsylvania Avenue, N.W.
Washington, D.C. 20460
Subject: Review of the "Near-road Guidance Document - Outline" and "Near-
road Monitoring Pilot Study Objectives and Approach"
Dear Administrator Jackson:
The Clean Air Scientific Advisory Committee (CASAC) Ambient Air Monitoring
and Methods Subcommittee (AAMMS) met on September 29-30, 2010 to review EPA's
Near-road Guidance Outline and Near-road Monitoring Pilot Study Objectives. The
attached CASAC report was approved at a public teleconference on November 8, 2010.
The impetus for the development of near-road monitoring guidance comes from the
recent revisions to the primary National Ambient Air Quality Standard (NAAQS) for
NO2 . In its revisions to the primary NO2 NAAQS issued on February 9, 2010, EPA set
out requirements for an N02 monitoring network that includes monitors at locations
where maximum NO2 concentrations are expected to occur, including within 50 meters of
major roadways.
Although the requirement for near-road monitoring comes from the revised NO2
rule, EPA's Office of Air Quality Planning and Standards (OAQPS) is developing
guidance for an expanded monitoring capability to be used for a variety of pollutants,
pursuant to the Agency's commitment to multi-pollutant monitoring approaches.
OAQPS sought CASAC's advice in developing a guidance document for state and local
monitoring agencies as they implement the NO2 near-road network that is required to be
operational by January 1, 2013. OAQPS also sought CASAC's advice on the objectives
and design of a near-road monitoring pilot study that is intended to inform EPA, state and
local air monitoring agencies on siting issues.
CASAC recognizes the importance for public health of better characterizing near-
road pollutant concentrations. In implementing near-road monitoring in the context of
the NO2 NAAQS, EPA unavoidably faces major issues that are described in detail in the
responses to the charge questions and highlighted in this letter:
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• While NO2 historically has been used as the indicator for ambient oxides of
nitrogen (NOX), it is one of multiple gases comprising NOX!which also includes
nitric oxide (NO), nitrous acid (HOMO), and other species. CASAC notes that the
ratio of NO2 to NOx varies substantially, and N02 concentrations can
significantly under represent NOX levels in the near-road environment;
• The epidemiological evidence on NC>2 is largely based in the population-oriented
monitor data. If these monitors are not maintained in sufficient number,
continuity of data will be lost, both for health research purposes and for tracking
trends of NO2 concentration;
• In moving to implement near-road multi-pollutant monitoring, EPA will need to
find a way to optimize siting for measuring concentrations of the various
pollutants included in the platform. For example, the optimal siting for NO2 may
not be the same as for CO.
CASAC was asked to comment on two short documents: "Near-road Guidance
Document - Outline" (a 1-page document) and "Near-road Monitoring Pilot Study
Objectives and Approach" (a 4-page document).
EPA's draft Near-road Guidance Document - Outline provided a very preliminary
list of the general factors that should be considered in developing a near road monitoring
network, including those factors that should be considered in siting monitoring locations.
However, the objectives of the network are not well defined in the current outline. High
priority should be given to developing clear objectives and providing a rationale for each.
EPA's Near-road Monitoring Pilot Study Objectives was also a very brief
overview of the objectives for a real-world pilot study in near-road monitoring
implementation. With respect to the factors EPA is considering for siting monitors, we
note that the primary focus of the monitor site selection process is on annual average
daily traffic (AADT). However, the approach may place too much weight on these data.
Other factors to be considered include the physical characteristics and the patterns of
vehicle use at the site (e.g., fleet mix, roadway design, congestion patterns, terrain, etc.),
modeling information, and preliminary monitoring studies (e.g., results of
multiscale/saturation studies). CASAC suggests that EPA should allow states and local
agencies to bring as much information to bear on the site selection process as is
appropriate, with prioritization of installation of sites based on review of all relevant data
in discussions between monitoring agencies and EPA offices.
We encourage evaluation of the use of a "true" NO2 monitor and inclusion of this
instrument in the near-road monitoring program. We endorse the views expressed in the
letter of August 19, 2010 from the National Association of Clean Air Agencies to
Assistant Administrator Paul Anastas encouraging ORD "to increase its focus on the
development and advancement of ambient air monitoring reference, equivalent and other
sampling and analytical methods." Measurement methods germane to near-road
monitoring mentioned in that letter include those for PIVhs, PMio-25, and ultrafine
particles. We also urge EPA to address known biases in measurements made with the
NO2 FRM.
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As a general matter, CASAC is deeply concerned about the timing proposed for
the current network deployment, as well as for the Pilot Study. The revised NO2
NAAQS, issued on February 9, 2010, mandates that state and local air monitoring
agencies deploy the near-road network by January 1, 2013. This ambitious schedule may
make it difficult to absorb lessons learned from EPA's Pilot Study to evaluate and
improve the siting and monitoring process. If possible, given this mandated date of
deployment, EPA might consider deploying the network in stages over time, e.g., 10-20
sites the first year, 20-40 the next and the rest in the final year. As part of this staged
approach, EPA should consider using near-road sites appropriately paired with sites from
the National Core (NCore) monitoring network that is set to come online January 1, 2011
in the first round deployment. Consideration should be given to areas with larger
populations. Such a staged approach would be consistent with the recommendations
from the CASAC Oxides of Nitrogen Primary National Ambient Air Quality Standards
(NAAQS) Review Panel (see Samet, Sept. 9, 2009). In this way, the network can evolve
based on lessons learned from the Pilot Study as well as from the operation of the initial
sites.
CASAC is concerned that there could be a decrease in the number of population-
oriented NC«2 monitors in the new network. Many of the health studies that were
considered in the latest NAAQS NC^ review were based on the population-oriented
monitors of the current network. CASAC strongly recommends that a great majority of
these monitors be maintained, particularly those that have been used in past health-
focused studies. By maintaining these monitors, air quality trends could be tracked
without discontinuity.
Just as we recommended a staged approach to the deployment of the near-road
monitoring network, CASAC also recommends a tiered approach to the design of the
near-road monitoring sites. A few sites should be comprehensively equipped such that
they can provide comprehensive information about the composition of mobile source
emissions and how pollutant concentrations and mixtures change over time with changes
in sources and control measures. The bulk of the sites could be more modestly equipped.
For example, the modestly equipped sites would [also] include optical black carbon (as a
surrogate for elemental carbon), carbon monoxide (CO), meteorology and ultra-fine
particulate matter (PM) monitoring capabilities responsive to the needs for assessing
attainment with the applicable standards and the extent of near-road pollution exposure,
as well as for use in health studies. We provide more detail in our responses to the
associated charge questions.
State and local resource constraints are another cause for concern, particularly in
view of this schedule. For all of the criteria pollutants as well as for NO2 specifically,
CASAC encourages EPA to commit the resources necessary to focus on the development
and advancement of ambient air monitoring methods, with specific attention paid to
assessment and possible modification of the Federal Reference and Equivalent Methods.
We are concerned that the current time frame for the NO2 near-road network may not
allow adequate time to appropriately plan and execute the Pilot Study and then to
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interpret and use the resulting findings in designing the near-road network. The decisions
that will be made have broad implications related not only to NC^, but to other criteria
pollutants and the characterization of multiple-pollutant exposures from roadway sources.
CASAC appreciates the opportunity to provide input to EPA at this early stage in
the process. The CASAC and AAMMS membership is listed in Enclosure A. CASAC's
consensus responses to the Agency's charge questions are presented in Enclosure B.
Individual review comments from the AAMMS are compiled in Enclosure C.
Sincerely,
/Signed/ /Signed/
Dr. Armistead (Ted) Russell, Chair Dr. Jonathan M. Samet, Chair
CASAC Ambient Air Monitoring & Clean Air Scientific Advisory
Methods Committee Committee
Enclosures
IV
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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 of
commercial products does not constitute a recommendation for use. CASAC reports are
posted on the EPA website at http://www.epa.gov/CASAC.
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Enclosure A
U.S. Environmental Protection Agency
Clean Air Scientific Advisory Committee
Ambient Air Monitoring and Methods Subcommittee for the
Review of Near-Road Monitoring to
Support Measurement of Multiple National
Ambient Air Quality Standard (NAAQS) Pollutants
CHAIR
Dr. Armistead (Ted) Russell, Professor, Department of Civil and Environmental
Engineering, Georgia Institute of Technology, Atlanta, GA
MEMBERS
Mr. George A. Allen, Senior Scientist, Northeast States for Coordinated Air Use
Management (NESCAUM), Boston, MA
Dr. Judith Chow, Research Professor, Desert Research Institute, Air Resources
Laboratory, University of Nevada, Reno, NV
Mr. Bart Croes, Chief, Research Division, California Air Resources Board, Sacramento,
CA
Dr. Kenneth Demerjian, Professor and Director, Atmospheric Sciences Research
Center, State University of New York, Albany, NY
Dr. Delbert Eatough, Professor of Chemistry, Department of Chemistry and
Biochemistry , Brigham Young University, Provo, UT
Dr. Eric Edgerton, President, Atmospheric Research & Analysis, Inc., Gary, 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 Hopke, Bayard D. Clarkson Distinguished Professor, Department of Chemical
and Biomolecular Engineering, Clarkson University, Potsdam, NY
Dr. Kazuhiko Ito, Assistant Professor, Department of Environmental Medicine, School
of Medicine, New York University, Tuxedo, NY
Dr. Peter H. McMurry, Professor, Department of Mechanical Engineering, University
of Minnesota, Minneapolis, MN
Mr. Richard L. Poirot, Environmental Analyst, Air Pollution Control Division,
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Department of Environmental Conservation, Vermont Agency of Natural Resources,
Waterbury, VT
Dr. Jay Turner, Associate Professor, Environmental & Chemical Engineering, Campus
Box 1180 , Washington University , St Louis, MO
Dr. Warren H. White, Research Professor, Crocker Nuclear Laboratory, University of
California, Davis, CA
Dr. Yousheng Zeng, Air Quality Services Director, Providence Engineering &
Environmental Group LLC, Baton Rouge, LA
SCIENCE ADVISORY BOARD STAFF
Dr. Holly Stallworth, Designated Federal Officer, U.S. Environmental Protection
Agency, Science Advisory Board, Washington, DC
VII
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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 Allen, Senior Scientist, NESCAUM (Northeast States for Coordinated Air
Use Management), 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, Senior Lecturer on Environmental Chemistry and Exposure Assessment,
Department of Environmental Health, School of Public Health, Harvard University,
Boston, MA
Dr. Kathleen Weathers, Senior Scientist, Gary Institute of Ecosystem Studies,
Millbrook, NY
SCIENCE ADVISORY BOARD STAFF
Dr. Holly Stallworth, Designated Federal Officer, Science Advisory Board Staff Office,
Washington, D.C.
VIII
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Enclosure B
Consensus Response to Charge Questions on Near-Road Monitoring
to Support Measurement of Multiple NAAQS Pollutants
Questions on Near-road Guidance Outline
1. The accompanying draft guidance document outline provides an initial
thought of the major topics required in the near-road monitoring guidance
that will aid state monitoring agencies in the identification and
implementation of NC>2 near road monitoring sites from a multi-pollutant
perspective. Please comment on the overall content of the recommended
topics in the draft outline. Please provide suggestions on any missing
subjects that should be included in the guidance document and any
unnecessary topics that are currently listed in the attached draft, if
applicable.
It is not clear that EPA has identified the reasons (scientific and regulatory objectives) for
the multi-pollutant monitoring. We recommend that EPA clearly identify the monitoring
and associated scientific objectives for the near-road monitoring program, and then build
the document around these objectives. A possible set of objectives focused on NC«2, for
example, might be:
• Identification of hourly averaged NC>2 by the most artifact free method available.
• Identification of hourly averaged concentrations of other NAAQS pollutants
which accompany the observed NO2 concentrations.
• Identification of the atmospheric processes which contribute to the observed
concentrations of NC>2.
• Identification of the sources which contribute to both measured concentrations of
NC>2 and the other measured NAAQS pollutants
The document should then discuss fully the objectives and the scientific and regulatory
bases for these objectives in the Introduction. This will, in turn, support the selection of
the recommended pollutants to be monitored in the program and the protocols to be
followed.
In addition, we suggest the following more minor areas where the Guidance Document
needs strengthening.
• General. The success of the near-road pilot project is critical to the future
deployment of the 126 near-road monitoring sites by the end of 2012. Thus
timing (and resources) are tight. EPA needs to focus on defining the objectives
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for the program and identify those key elements of the program essential to
meeting those objectives. EPA needs to identify how to leverage existing sites.
how the sites will be operated, how the data analysis will be accomplished, and
how many sites are needed to meet the objectives of the program. Finally we
encourage EPA to evaluate the near-road excess (or deficiency) for key indicators,
e.g. NC>2, CO, black carbon, ultra fine particles. This will necessitate background
measurements (ideally made at an appropriate existing sites) which allow an
estimation of the gradient away from the road.
• Background. The background should document the scientific and regulatory
bases for the multi-pollutant objectives, as outlined in the bullets above. The
literature review needs to be more comprehensive than at present. Some guidance
on possible literature to site is included in individual responses. A conceptual
model should be formed in this section that includes dispersion, deposition,
chemical conversion and physical conversion. Special attention should be given
to NO2 formation by the titration of Os and NO2 depletion by photochemistry.
The Background section should acknowledge that finding a site where NC>2 and
the various measured other pollutants all have the highest concentrations is not
likely and compromises will be necessary and acceptable.
• Identifying Candidate Near-road Site Areas. We believe the criteria outlined for
this section are generally adequate for the identification of a site where near
maximum NC«2 concentrations near a given near-road site may be determined.
However, there is an apparent assumption that AADT is the primary siting
criteria. The importance of meteorological and terrain variables should be
emphasized. Further, since MO2 is a pollutant affected by both emissions from
the roadway and nearby elevated pollutants (VOC, ozone, etc.), these factors
should be considered in the site identification process. One critical element
missing from the outline is the importance of the specific distance from the
roadway chosen for study. The effect of differences in the gradient from the
roadway of the multi pollutants studied needs to be acknowledged and discussed.
• Modeling. This section should include a discussion of the types of models
available and databases that can be used to inform the siting process. The models
discussed should include both emissions and air quality models.
• Monitoring. This section should provide a summary of methods used in past
roadside measurement studies, passive and active monitors that can be efficiently
deployed at many locations and methods to interpret the data acquired. The
emphasis at this level should be on the identification of high "true" NO2 hourly
average concentrations. Care needs to be taken to insure that the use of mobile
monitoring methods give results that do reflect the diurnal and seasonal locations
of peak concentrations.
• Near-road Site Selection. The items outlined here seem reasonable. This section
might draw from some of the existing guidance for sampler siting.
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• Recommended Near-road Site Documentation. The adequacy of this section will
depend on the EPA objectives for the Pilot Study in addressing the appropriate
multi-pollutant monitoring objectives. This will be better defined as the
objectives of the near-road monitoring program are better defined.
2. EPA and NACAA envision the near-road guidance document to be written
from a multi-pollutant perspective. What pollutants and sub-species does
the subcommittee believe should be included for consideration and
discussion in the near-road monitoring guidance? Some potential species for
consideration include NO2, NOX, NO, CO, PM (Ultrafine, 2.5, and 10), black
carbon, air toxics (e.g., benzene, toluene, xylene, formaldehyde, acrolein, or
1,3, butadiene), and ammonia. Please prioritize the recommended
pollutants and provide the rationale for their ranking, including how this
pollutant measurement will contribute to scientific and regulatory
knowledge of near-road air quality and adverse human health effects.
The list and ranking of pollutants varied across the AAMMS members. Some questioned
the usefulness of these indicators for the compliance purpose. The following ranking is
based on the votes for the pollutants that had at least five votes from thirteen members of
the AAMMS who submitted votes, taking into consideration the average ranking and its
variability. The AAMMS believes meteorological parameters (wind speed and direction)
should be one of the highest tier measurements considered as part of this network.
Several members considered traffic counts as potentially important information. Other
pollutants mentioned by some members include "true NO25' measured by techniques such
as photolytic conversion difference, hourly metals by automated on-line XRF, and
nephelometer measurement.
CASAC recognizes that not all of the pollutants on the list can be measured at all the sites
because of the level of the cost and operational logistics. Therefore, the AAMMS
recommends a tiered approach, in which a few of the sites would collect a more
comprehensive set of pollutants, while a majority of them would measure only the first
tier of pollutants in the list. We recommend that all of the data be obtained on an hourly
averaged basis to allow intercomparison. We recognize this ranking could change
depending on EPA's objectives for the monitoring network and the evolution of
priorities.
Pollutant group
NO2, NO, NOx
Black carbon
CO
Rationale and comments
• Same instrument
• Diesel tracer
• Potential health relevance
• Continuous methods available
• Not a direct measure of carbon
• Gasoline vehicle tracer
• Dilution factors
• Potential health relevance
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Ultra-fine particles / particle number
concentration
Particle-size distribution
PM10-25
PM25
EC/OC
C02
Ozone
NOy
SO2
BTEX (benzene, toluene,
ethylbenzene, and xylenes) and 1,3
butadiene
• Continuous methods available
• Pending new NAAQS
• Strong roadside gradient
• Potential health relevance
• Continuous methods available
• For a limited number of sites
• Potential health relevance
• More comprehensive health relevance than
just particle number
• Potential health relevance
• Re-suspended roadside particles
• Speciation for metals
• Important in future PM NAAQS reviews
• Continuous methods available
• Potential health relevance
• Speciation for metals, major components and
organic marker compounds
• Possibly include semi-volatiles
• Potential health relevance
• Direct measure of carbon
• Normalization of pollutants to CO2 allows
estimation of fuel-based emission factors
• Could be important if signal large enough
• To understand photo-chemical processes
• Not to be used for health effects analysis
• Total nitrogen oxides and mass closure
• To verify fuel content compliance
• Pending new NAAQS
• Potential health relevance
• Continuous methods available
• Provides unique information as to how
changing fuel characteristics and control
systems are impacting emissions
• Can provide additional info on which class of
vehicle is impacting monitor
3. Identifying Candidate Near-road Site Areas
a. AADT & Fleet Mix - To consider fleet mix with regard to NO2, an
idea is to encourage states that have fleet mix information to take an
approach that uses average, fleet-wide grams per mile emissions
estimates (one for light duty vehicles and one for heavy duty vehicles),
combined with AADT information to further weight which road
segments in an area may be more conducive to produce peak
pollutant concentrations. EPA would use the latest emission factor
information to aid such a calculation. Given the variability in
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emission rates from on-road vehicles based on vehicle technology,
fuel, speed, environmental conditions, etc., does the subcommittee
believe this approach is an appropriate way to "consider" fleet mix in
near-road site selection or is a more refined inventory and modeling
analysis required?
b. AADT & Fleet Mix - Further, should the suggested approach above
in question 4a to consider fleet mix via the use of average, fleet-wide
emission factors, or the use of inventory and modeling analysis, take
into account mobile source controls that are "on the books" but have
not yet been fully realized due to fleet turnover? If so, how far out into
the future should states consider their effects?
c. Roadway Design - Studies suggest and support the concept that
roadway design influences pollutant dispersion near the road. The
EPA suggests establishing sites at-grade with the road, without any
nearby obstructions to air flow; however, the Agency recognizes that
this might not always be feasible. Does the subcommittee agree with
this recommendation for locating sites at-grade with no obstructions?
What priority should be placed on this factor within the guidance,
given the need for flexibility in identifying appropriate site locations?
d. Congestion Patterns - The congestion of a roadway can be estimated
by the metric "Level of Service" (LOS). LOS uses a letter grade from
A to F to identify a roadway's performance, with "A" the best
conditions where traffic flows at or above the posted speed limit and
all motorists have complete mobility between lanes to "F" the worst
congestion where travel time cannot be predicted and generally traffic
demand exceeds the facility's capacity. Since motor vehicles generally
emit more pollutants during congestion operations (although noting
that NOX and select other pollutant emissions can also increase with
increasing speed), how important a parameter should LOS be in the
determination of appropriate near-road monitoring sites? Does the
subcommittee have a view on how reliable LOS estimates are across
the country?
e. Terrain- State and local air agencies are required to consider terrain
in the near-road monitoring site selection process, which in some cases
may be inherently part of the roadway design. However, EPA
recognizes that some states and local air agencies may have to make
selections from amongst similar candidate sites that differ only by
terrain, e.g. cut section versus open terrain, with or without
vegetation, etc. Does the subcommittee agree that terrain and
vegetation should be a consideration in the siting process? What
priority should this parameter have in the overall process?
f. Meteorology - EPA took comment on, but did not finalize the
requirement for near-road monitoring sites to be climatologically
downwind of the target road segment. Reasons were because the
additional limitations this would introduce in finding candidate sites
would be in exchange for what may be a small increase in the
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opportunity to monitor peak NO: concentrations. Further, with sites
being within 50 meters of target road segments, the phenomenon of
upwind meandering (pollutant transport upwind due to vehicle
induced turbulence) further reduces that absolute need to be
climatologically downwind. Finally, EPA recognized that, logically,
the potential for peak NOz concentration may very well occur when
winds are calm or parallel (or nearly parallel) to the target road,
allowing for pollutant build-up, as opposed to when winds are normal
to the road. Although there is no requirement to be downwind, in the
preamble to final NCh NAAQS rule, EPA encouraged it when
possible. EPA and NACAA intend to do the same in the guidance
document. Does the subcommittee agree with this approach?
The specific factors identified in this charge question can all usefully inform the selection
of near-road monitoring locations. An additional factor that may exhibit characteristic
gradients in larger metropolitan areas is the middle-scale oxidant or odd-oxygen
concentration, [O3]+[NO2]. A factor that needs emphasis is the role of freeways with
high truck traffic volumes. As NOX and VOC emissions are continuously reduced over
time, ambient oxidant levels will also go down, which means that titration of NO by
ozone to form NO2 will become less important in comparison to direct NO2 emissions
from new and retrofitted diesel trucks. NO2 emissions are highest during cruise mode,
not congestion conditions for these trucks. CASAC recommends against giving too
much emphasis to AADT, noting for example that peak NO2 concentrations can occur in
areas where widespread congestion limits AADT. To account for such interactions
between different determinants of NO2 , the Agency should consider developing a
conceptual or screening model to guide their integration. This model would be intended
as a tool to help rank candidate locations, and would not require quantitative
concentration predictions. Recognizing the near-road N02 measurement's intended focus
on NAAQS compliance rather than population exposure, CASAC stresses the importance
of exposure in the overall balance of siting considerations.
In response to specific elements of this charge question, CASAC's recommendations are
as follows.
a. States and cities vary considerably in the resolution of information available
for fleet characteristics and fleet mix. States can be encouraged to use all
available data in their planning, including local features such as truck and bus
corridors.
b. Ambient monitoring is conducted in the 'here and now'. Fleet turnover is too
slow to require consideration of its future effects in the initial planning.
However, the transition to 'cleaner' diesels may have the effect of increasing
the ratio of NO2 to NOX in primary tailpipe emissions of NOX.
c-e. CASAC agrees that roadway design, terrain, and congestion patterns all merit
consideration. Information from loop monitors should also be considered. As
noted above, it recommends that the interactions and tradeoffs among their
effects be accounted for in a screening tool developed by the Agency for this
purpose.
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f. CASAC agrees that it is undesirable to site a near-road monitor
climatologically upwind.
4. Modeling is another tool that may be useful in the identification of candidate
near-road sites. In particular, the use of mobile source emissions modeling
with MOVES and local-scale dispersion modeling with AERMOD, can be
presented as part of the guidance document. Please comment on the
available modeling tools, and their pros and cons, that the subcommittee
believes may be appropriate to discuss and/or recommend for use in the
near-road monitoring guidance document.
Like the various considerations discussed in Charge Question 3, modeling tools can also
be used to inform the likely location of the maximum NCh levels in an area. Taking a
centralized (e.g., EPA led) approach, emissions and air quality models could be used to
develop screening tools or screening criteria that incorporate many of the factors
discussed in Charge Question 3. Alternatively, the models could directly be used by state
and local agencies to inform the siting process. MOVES, along with other emissions
inventory modeling tools, can provide valuable information about the spatial intensity of
traffic-related NOX emissions, as well as NOX emissions from other sources. MOVES is a
link-based model which has significant advantages for this application over the historical
emissions models that were based on trip-average cycles. Air quality models, such as the
CMAQ and CAMx chemical transport models and the AERMOD and CALINE
dispersion models, can also be used to help identify candidate geographic areas within the
metropolitan area and to compare and contrast candidate specific candidate monitoring
sites, respectively. It is appreciated, however, that there significant uncertainties
associated with the use of models for simulating the location of an extreme concentration
statistic. The approaches used by AERMOD to simulate the conversion of NO to NO2
are relatively crude, and guidance should be given as to which approach should be used.
State and local agencies should be encouraged to use spatially-resolved emissions
estimates, and air quality model results if readily available or available with relatively
modest effort.
5. In regard to the process of identifying candidate near-road monitoring sites,
beyond the evaluation of factors noted above in question 3, and the potential
use of modeling, the use of saturation monitoring and on-road monitoring
are also possible tools that state and local air agencies may choose to utilize
in the near-road site selection process.
"Multi-scale" monitoring is a more specific term than saturation monitoring. Before the
pilot study or site selection, a literature review should be conducted that summarizes past
roadside measurement and modeling studies, passive and active monitors that can be
efficiently deployed at many locations, and methods to interpret the data acquired.
Roadside monitoring for site selection and long-term monitoring would benefit from
small, portable sensors that don't require a large infrastructure (i.e., shelter, air
conditioning, etc.) to operate. A pilot study plan should be assembled to clearly define the
objectives of locating the sites (i.e., maximum hourly NO2 concentration), rationale for
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the selection of the measurements, measurement principles, concentration range.
minimum detection limits, and potential interferences and biases. It should also elaborate
on method intercomparison and modeling approaches.
Sampling sites should be located at the prevailing downwind location in an array (e.g., to
estimate perpendicular vs. effective distance from the road; Barzyk et al., 2009). The
number of sampling sites would depend on the downwind distance in the micro- (10 -
100 m), middle- (100 - 500 m) and neighborhood- (500 m - 4 km) scales.
Portable active or passive monitors that can be easily and inexpensively deployed would
provide a good indication of where concentrations might be highest. Levels are likely to
vary by season and often show an exponential decrease with distance from the curbside.
Passive monitors using NO2-absorbing filters have some potential biases, but they have
also been shown to be comparable and correlated with continuous measurements for
integration times on the order of weeks. Passive samplers are a cost-effective and
practical technology for mapping average spatial gradients as a prelude to sampler siting.
However, the validity and sensitivity of the portable active or passive NC>2 sampling
systems for hourly measurements needs to be verified. To get better precision, collocated
continuous monitors with duplicate passive samplers are needed at some of the sites. Past
studies show an average coefficient of variation for duplicate passive NCh measurements
of 5-30%
a. If a state were inclined to use saturation monitoring to aid in the selection
of a near-road monitoring site, and considering that the NC>2 standard is
a 1-hour daily maximum standard, what are the pros and cons to using
passive devices to saturate an area to gather data?
Pros are low expense and operating cost. The major con is the much longer than one-
hour averaging time, unless the passive device is modified to obtain active hourly data.
The key is to avoid the need to relate long-duration passive sampler NCh measurements
to the one-hour NC>2 NAAQS, which is defined as the maximum allowable concentration
in an area (primarily near major roadways), expressed as a three-year average of the 98th
percentile of the annual distribution of daily maximum one-hour concentrations. One
option is to collect samples from sub-daytime periods (e.g., morning and evening rush
hours) that are integrated over several days. A timer-based sampler may be used for this
application. If many of the passive samplers are used for multi-scale monitoring it is
important to position the sampling inlet at the same height (2 - 7 m above ground) at all
sites
b. Likewise, what are the pros and cons to using non-passive devices, such
as near real-time or continuous devices including, but not limited to
portable, non-FEM chemiluminescence methods for NCh or Gas Sensitive
Semiconductors (GSSs) for NOz and other pollutants of interest?
Pros are short-duration samples, on the order of an hour or less. Cons are instrument
procurement and operating expense, potentially higher than desired detection limits, and
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reliability of new technologies. There are several currently available or emerging
technologies for microsensors. In addition to N02, NO can often be obtained from these
same sensors. Miniature sampling systems have been used for remote monitoring,
emission sampling, and unmanned aerial vehicles, and these might also be used for multi-
scale monitoring These portable instruments need to be evaluated with regard to their
sensitivity, stability, and accuracy.
c. Finally, what would be the pros and cons, to a state or local agency
attempting to use a specially outfitted vehicle to collect mobile
measurements to assist in the near-road site selection process for NC>2
specifically as well as other pollutants of interest?
Several mobile emissions systems or instrumented mobile vans have been applied to
characterizing on-road and roadside concentrations. Pros are that these systems are
moveable and obtain many different pollutant measurements. Unless state or local
agencies are already equipped with a mobile sampling van, the disadvantages are the high
cost of assembling or contracting these laboratories and the snapshot nature of their
measurements. They often need to be attended and can be parked for only a short time
period. However, mobile monitoring systems are useful for site selection if they can be
parked at the same location and perform sampling for a few days to one week. The state
or local agencies may also consider using a fully-equipped trailer rather than a mobile
van for siting. Eventually, the trailer can be converted to a permanent monitoring
location.
6. EPA recognizes that CO concentrations are primarily influenced by gasoline
vehicles as opposed to NO: and PIVh.s concentrations, which are currently
more heavily influenced by heavy-duty (diesel) vehicle emissions. If EPA
were to propose a new set of minimum monitoring requirements for CO
near roads, the near-road monitoring stations created under the
implementation of the NO: monitoring requirements may be an
advantageous infrastructure for state and local air agencies to leverage.
However, EPA believes there are two issues not specifically considered in the
near-road NO: monitoring language that might influence where near-road
CO monitors may be most appropriately placed. The two issues are 1) the
consideration of where light duty vehicles are operating under 'cold-start'
conditions, which may often not be on the larger arterials or highways in an
area, and 2) the impacts of light duty vehicle congestion and idling in areas
such as urban street canyons and/or urban cores.
a. Does the subcommittee believe that the light duty cold start and
congestion factors will significantly influence the location of peak CO
concentrations in an area? What priority should these factors be
given when compared with the factors (AADT, Fleet Mix, Roadway
Design, Congestion Patterns, Terrain, and Meteorology) already being
considered for peak NO:?
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The spatial distribution of cold start vehicles associated with urban commuting is in
general broad and short term and their contribution to emissions associated with major
highways adjacent to residential neighborhoods is likely small. To the extent cold start
emissions contribute, they would mainly be in terms of CO and not NO2 concentrations
and would not likely influence peak CO concentrations. That being said, at least one
exception comes to mind. The departure of motor vehicles from major events (e.g.
stadiums) where 20-30K. vehicles may be simultaneously started and caught in
congestion for 10s of minutes to an hour or more. The cold start contribution, here again,
is limited in time but could contribute significantly as an emissions hot-spot impacting
commuter exposes and concentrations in nearby neighborhoods. The prioritization of
congestion and cold start factors relative to AADT, fleet mix, roadway design, terrain and
meteorology should consider sensitivity analyses using line source models as outlined by
the FHWA's procedures for assessing traffic impacts for CO.
b. Does the subcommittee have an opinion on whether, and possibly
how, these two issues of vehicles operating under cold start conditions
and light duty vehicle congestion and idling in urban street canyons
and/or urban cores be considered in a future, nationally applicable,
CO monitoring proposal? Are there other factors that may affect
peak CO concentrations and not affect peak NOi concentrations that
should also be considered for any future CO monitoring proposal?
Factors that affect near-road CO and NOx concentrations will be the distribution of
gasoline and diesel vehicles. The primary NOi fraction of NOX exhaust emissions will
vary with engine type (gasoline spark ignition or diesel) and control equipment, e.g.,
three way catalyst and diesel particle filter trap technologies. CO emissions come
principally from the gasoline engine exhaust and their peak concentrations will occur in
the immediate vicinity of the emission source. While N02 peak concentrations from
primary emissions will behave similarly to CO, NO2 peak concentrations from secondary
reactions will lag behind as NO emissions react with entrained ozone to form NO2. The
entrainment and transformation time afford the opportunity for the exhaust plume to
transport and diffuse resulting in peak NO2 concentrations that are displaced in time and
space as compared to CO.
Idling in urban street canyons is problematic and should be discouraged. If, as in most
cases, it is due to major traffic congestion it will result in high exposures involving
commuters, pedestrians and local residence. Decisions to monitor at such locations must
consider these exposures relative to other near-road exposure environments associated
with high density population regions.
7. Does the committee believe that siting considerations for identifying the
location of peak NO: concentrations will likely address all of the high
priority siting considerations for PM (particularly PIVh.s) as well? If not,
what other factors should be considered and what arc the advantages in
considering these factors for identifying the location of maximum PM
concentration?
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The majority of panelists acknowledged that there is an increment to PM2 5 in the near
road environment. They did not suggest that the near road environment will represent the
areas of highest PM2 5 concentration. This is due to the variety of sources of PM2 5
including the combination of primary emissions and secondary formation processes.
The committee was also generally in agreement that mass based measurements and
specifically the PMb 5 FRM is not appropriate for use at the near road NO2 sites. The
PM2 5 FRM measurement has poor capture efficiency for the highly volatile emissions
from mobile sources and is not suitable for the collection of hourly averaged data. The
use of this method in the near road network would underestimate the significance of
mobile sources and under predict the risk associated with this source of PM2 5-
In general, the locations where maximum PM2 5 concentration are likely to be found
include areas that are subject to regional transport and local stationary and area sources,
are primarily urban, and are away from sinks of PM including ventilated roadways, heavy
vegetation and water bodies.
The Panel also discussed other fractions of PM in the near road environment including
ultrafine particle count and size distribution and coarse PM (PMC). The siting for
measurements of these size fractions are more suited to the near road environment.
However, because the processes that affect downwind concentrations of NC>2, UFP and
PMc differ (Karner et al. 2010*), optimal downwind sites for those pollutants may also be
different.
8. In addition to PM2.5 mass, what other PM-related measurements are
desirable at near-road monitoring stations (e.g., UFP number, black carbon,
EC/OC, PM coarse, etc.)?
Other PM measurements that should be considered include sub 100 nm ultrafine particles
(UFP), BC, and speciated coarse particles.
BC is an important vehicular emission that can be measured routinely in sampling
networks. We recommend that it be measured.
Measurements of UFP could be made either with a condensation particle counter (CPC)
or an aerosol mobility spectrometer. A CPC measures the total concentration larger than
the CPC's minimum detectable size but provides no information about size. Mobility
spectrometers provide information about particle concentration and size, which would be
valuable for understating new particle formation and assessing health effects. Given
current resource constraints, state and local agencies might be unable to collect and
analyze data from mobility spectrometers. However, agencies might work collaboratively
with interested universities. This would likely lead to a nested network design, with most
* Kamer, A. A., Eisinger, D. S. and Niemeier, D. A. (2010). Near-Roadway Air Quality:
Synthesizing the Findings from Real-World Data. Environ Sci. Technol 44:5334-5344.
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stations using CPCs and a more limited number aerosol mobility spectrometers. Mobility
distributions are currently being measured routinely in sampling networks worldwide.
Vehicles emit coarse particles and resuspend coarse road dust that may be coated with
toxic contaminants. We recommend measurements of the coarse particle metal content at
the well instrumented sites.
Questions regarding the monitor siting criteria for microscale CO, microscale PMis,
and the new near-road NO2 siting criteria
9. To allow for near-road monitoring infrastructure to be multi-pollutant, and
in reflection of the recently promulgated near-road NC>2 siting criteria,
reconsideration of the existing microscale CO siting criteria presented in
sections 2, 6.2, and table E-4 in 40 CFR Part 58 Appendix E may be
warranted. Does the subcommittee believe that reconsideration of
microscale CO siting criteria is appropriate? Specifically, would an
adjustment of CO siting criteria to match those of microscale PM2.s and
microscale near-road NOi sites be logical and appropriate?
CASAC recommends that sampling criteria for CO and other monitors at sites installed to
monitor near-road NO2 match those for NO2. The sampling configurations of existing
micro-scale CO monitors should be assessed in terms of their own sampling objectives,
and need not necessarily conform to those of near-road NO2 monitors.
10. Even if the adjustment of microscale CO siting criteria in sections 2,6.2, and
table E-4 in 40 CFR Part 58 Appendix E to match that of microscale PM2.5
and microscale near-road NO2 is appropriate and proposed, should there be
consideration to maintain the requirement on how urban street canyon or
urban core microscale CO sites should be sited?
The Panel did not feel it had enough data to make a recommendation with respect to this
charge question.
11. Does the subcommittee have an opinion on how "urban street canyons" or
"urban core" might be defined, perhaps quantitatively, and with regard to
use in potential rule language?
To define "urban street canyons" in relevant regulations, CASAC offers the following
factors for EPA's consideration. EPA might also consult experts in other relevant
disciplines to further refine this definition.
• Traffic information such as AADT, fleet mix, posted speed limit or/and actual
speed, traffic light cycle
• Street geometry
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o Ratio of the height of street side buildings to the width of the street (H/W
ratio). Some approaches need to be developed to treat the following
scenarios for H/W ratio calculation
• different heights of buildings on the two sides of the street - use
the lower one or use the average. Using the lower one may be
better.
• tiered buildings - possibly use the average of all tiers, or use a
graduated factor based on the angle between the vertical line and
the line drawn along the tiered building - the larger the angle
(more opening on the top of the street canyon), the lower the
effective height (He) will be used in the H/W calculation.
o One-way vs. two-way street (more plug flow in a one-way street and more
turbulent flow in a two-way street).
o Is the street lined with trees on the sidewalk? Tree canopy may have an
effect of an umbrella and trap portion of pollutants at the street level.
o Slope of the street - higher vehicle emission on steeper streets unless it is
a one-way street and the traffic direction is downhill.
o Some way to normalize the H/W ratio with respect to number of traffic
lanes on the street. One approach would be modifying the H/W ratio to
H*L/W, where L = number of lanes.
• Meteorological factors: Frequency of calm conditions and/or drainage flow may
influence concentrations.
• Terrain: The angle between the street and prevailing wind direction (higher
concentrations are expected if the angle is 90 degree).
Although a set of cut-off values reflecting the above mentioned factors could be used to
define urban street canyons, it is not advisable to set some clear-cut criteria (insufficient
information exists to set clear-cut thresholds). It may be more appropriate to consider a
street an urban street canyon if more than a certain number of these conditions are met.
This will be a qualitative approach to define urban street canyons.
For the definition of "urban core", the CASAC has some suggestions for EPA to
consider. EPA may use the U.S. Census Bureau definition of an "urban area" as a starting
point. Per U.S. Census Bureau, an urban area is defined as "Core census block groups or
blocks that have a population density of at least 1,000 people per square mile and
surrounding census blocks that have an overall density of at least SOO people per square
mile." Land features such as a river or a ridge may divide a CBSA into multiple urban
cores. EPA may use population density to rank urban cores. EPA may further enhance
this definition by factoring in "traffic density", which could be calculated as the sum of
AADT for every unit of road length (e.g., a length comparable to street block) in an area
under consideration divided by the size of the area.
Questions regarding the near-road monitoring pilot study
12. EPA and NACAA will select the locations for permanent sites that are part
of the near-road pilot study based on which state or locals volunteer to
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participate and can process grant funds in a timely manner to deploy
equipment. From this pool of volunteers, selection should be made on
certain attributes that provide the best potential to fulfill pilot study
objectives. In the attached draft white paper, EPA and NACAA have
proposed some potential criteria for consideration in selecting where the
fixed, permanent stations should be located. These considerations include
choosing a large and a relatively small urban area based on population, an
area with varied or complex terrain, an urban area with an operational NO\
analyzer representative of neighborhood or larger spatial scales for
comparison to the near-road NO\ analyzer, and an urban area with a
cooperative (or non-cooperative) Department of Transportation. Does the
Subcommittee agree with these considerations? Further, are there other
considerations that should be evaluated in selecting pilot cities to house
permanent near-road monitoring stations as part of the pilot study?
Available funding constrains this near-road pilot to only 2 or 3 sites unless EPA leverages
existing sites and infrastructure that are near-road-ish, possibly considering cities that are
already conducting multi-pollutant assessment at multiple locations (Atlanta and New
York City for example). Areas with existing "urban background" sites that have relevant
near-road pollutant measurements in place (that may include ozone for a measure of total
urban oxidants) are desirable to assess the near-road excess for key pollutants. The range
of variables in this charge question cannot be fully evaluated with only a few sites. In
reality, given the severe constraints of funding and timing for the pilot, the siting decision
may be driven largely by which S/L agencies have the resources to support the pilot work
and where they can find and deploy a reasonable site quickly. With only 2-3 sites, it may
be appropriate to choose "generic" sites (avoiding extremes of topography, etc.) that are
most likely to represent a large fraction of the final network and are near the middle of
the 0-50 meter distance from the road (e.g., ~ 20-30 meters) and have HDD as a
significant fraction of traffic. Reliance on AADT or an MSA's population are useful
inputs but often may not be good indicators of the location of the maximum 1-hour NO2.
The pilot fixed sites are not likely to inform how a wide range of siting characteristics
would affect 1-hour NCh concentrations. We might come closer to that goal by focusing
the pilot on NC>2 saturation studies with less emphasis (e.g. funding) on the fixed sites,
but that is not a practical solution to the other goals of this pilot. There is some evidence
suggesting that core urban zones not at large roadways may have the highest 1-hour NO2
values for some urban locations; saturation studies are ideal for assessing these sites.
Cooperation of the local DOT may be useful for local traffic pattern characterizations. It
may be worth encouraging academic or private sector groups to add in-kind supplemental
measurements if that does not create multi-organization logistical issues. Finally, EPA,
California Air Resources Board and several research groups have mobile monitors that
could be deployed to quickly assess potential locations of highest 1-hour NCh
concentrations.
13. EPA and NACAA have proposed that at least two urban areas should have
permanent near-road monitoring stations (that would fulfill NCh near-road
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monitoring requirements) implemented for the pilot study. Please comment
on the minimum equipment/pollutant measurement complement that should
be deployed at each site and also the ideal equipment complement that each
site should or could have, respectively. Specifically, what pollutants (e.g.,
NO2, NOX, NO, CO, PM (Ultrafine, 2.5, and 10), black carbon, air toxics
(such as benzene, toluene, xylene, formaldehyde, acrolein, or 1,3, butadiene)
and ammonia) and other information should the pilot study measure or
gather at the fixed, permanent monitoring stations, and by what methods?
This list should be in priority order, as feasible, and can include any NAAQS
or non-NAAQS pollutant by any method (FRM/FEM and/or non-reference
or equivalent methods), any particular type of other equipment for
gathering supporting data such as meteorology or traffic counts.
The majority of AAMMS supports the deployment of at least two pilot study sites with a
minimum measurement suite of NO2/NO/NOX , black carbon, particle number
concentrations (UFP), CO, meteorological parameters, and traffic counts; and ideally the
pollutant measurements tabulated in the response to Charge Question #2. Measurements
for vehicle class and speed distributions are also encouraged, especially if a screening
tool will be developed because this information can be used in its evaluation. If an FEM-
approved photolytic NO2 monitor is available, its deployment along with an FRM NO2
monitor at each site is strongly encouraged. Other AAMMS members support the
deployment of a single site with more extensive measurements to provide insights into
key science questions about NO2. This would require a longer time commitment than
currently programmed for the pilot study, but would ultimately better inform the
regulatory process. Additional pilot study objectives could include the evaluation of
passive sampling methods to be used in the saturation studies.
14. EPA and NACAA have proposed four to five urban areas to have saturation
monitoring, using either passive devices and/or continuous/semi-continuous
saturation type multi-pollutant monitoring packages (i.e., several types of
monitors in one mountable or deployable "package"). Please provide
comment on:
a. The pollutants that should be measured with the saturation devices at
each saturation site.
b. The number of saturation devices per pollutant, both passive and/or
continuous/semi-continuous, that may be deployed in each pilot city.
c. Whether placing saturation monitoring devices near certain road
segments should include, at a minimum: 1) the highest AADT segment
in an area, 2) the road segment with the highest number of heavy-duty
truck/bus counts, 3) at a road segment with more unique roadway
design, congestion pattern, or terrain in the area, and 4) if feasible, at
a lower AADT segment with a similar fleet mix, roadway design,
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congestion, terrain, and meteorology as the top AADT road segment
in the area.
The Subcommittee members expressed a wide range of views on the feasibility, ideal
configuration(s) and potential usefulness of including a saturation monitoring component
in the proposed pilot study. The pollutants sampled, number of sites per city, the number
of cities and the kinds of sites for sampling need to be considered collectively, and in
light of the limited budget and very short time frame available if the results are going to
be of any use to the states in the establishment of permanent near-road sites. The kinds of
sampling locations recommended by EPA and NACAA are reasonable, but may be
overly prescriptive, considering uncertainties in the kinds of sampling approaches that
could actually be implemented.
The simplest possible approach might be limited exclusively to the use of passive
samplers and focused on NO2-only using Ogawa-type passive devices. Disadvantages of
this approach include the single pollutant focus and longer-term cumulative nature of the
resultant data. Assuming there are reasonable correlations between peak hourly
concentrations and long term averages (as there have been at near-road sites in the UK),
the sample aggregation of passive samplers may not be a major problem. Advantages of
this simple approach include the very minimal siting constraints, the low (sampler, labor
and analytical) cost per sample, which would allow deployment at a much larger array of
locations, and the current availability of units with well characterized performance
specifications. To support this simplified approach, it would be helpful if EPA would
analyze data from existing NO2 sites that are located reasonably near major roads to
determine how well weekly mean NO2 from multiple sites predicts the daily 1-hour
maximum NO2 concentrations for the same time period.
The more complex approach suggested by EPA would attempt to develop a portable
compact "package" of active, continuous samplers for multiple pollutants of interest,
including NC>2, CO, PM2 5. Advantages include the ability to characterize and compare
short-term hourly peak concentrations for different pollutants at different kinds of sites.
Disadvantages include anticipated high cost per unit (fewer cities and sites), more
constraining siting requirements (power, security), unproven track records for data
quality, and anticipated time delays for equipment procurement, testing and field
deployment.
A third "intermediate" approach that CASAC recommends considering would be to
combine passive samplers with timed, battery operated pumps which would draw a fixed
flow rate of air through a small chamber housing containing passive NO2 samplers (and
possibly other passive samplers for NOx, BC, O3) during specific time periods such as
the morning rush hour(s) and afternoon hours of expected maximum secondary
formation. The sample pump would improve the sensitivity and reduce the sample
variability compared to purely passive devices, and would retain the advantages of
relatively low cost and flexible siting locations of the passive samplers. However,
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this approach is much more complex than a simple passive sampler, and thus would
require substantial resources to properly develop the method and evaluate its performance
before it could be used.
Without a significant amount of EPA technical support in method development and
evaluation, the two more highly time-resolved saturation pilot study approaches
described here are not practical; state and local air agencies do not have the resources to
support this level of effort. We recommend that EPA provide appropriate resources to
allow deployment of time-resolved saturation samplers such that these spatial studies can
better inform the final network design.
One possible alternative to use of saturation samplers at fixed locations would be to
employ mobile sampling platforms to explore spatial and temporal patterns for multiple
pollutants. While the high costs of such units precludes their development with available
pilot study funds, it is possible that some states may already have such mobile units
available, and the committee recommends that their use be considered as an alternative to
fixed site sampling in locations where such units may be available.
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Enclosure C
Individual Comments from
Ambient Air Monitoring and Methods Subcommittee
Mr. George Allen 2
Dr. Judith Chow 10
Mr. BartCroes 29
Dr. Kenneth Demerjian 38
Dr. Delbert Eatough 45
Mr. Dirk Felton 53
Dr. Kazuhiko Ito 61
Mr. Rich Poirot 68
Dr. Jay Turner 80
Dr. Warren H. White 84
Dr. Yousheng Zeng 90
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Mr. George Allen
There are two major topics covered by this Advisory:
1. How to best determine the likely location of the annual maximum value for urban-area
1-hour NO2 concentration to assist in the siting of the both the pilot and 126 site network
monitors.
2. In addition to NO2, what other near-road (near-road) relevant pollutants should be
measured for both the pilot fixed-sites and the full network.
EPA should be acknowledged for moving forward with a multi-pollutant near-road
network, an important initial step towards better understanding the exposure issues
behind the observed near-road health effects. Although the initial focus of this network is
NO2, measurements of other non-NAAQS pollutants are critical for characterization of
near-road zones of influence.
The Charge Questions are broken down into several categories, some of which are not
well defined or have substantial content overlap:
Guidance document development for the 126-site network
CO and PM network issues related to near-road monitoring
Harmonization of siting criteria for near-road multi-pollutant monitoring (including probe
height)
near-road pilot studies — saturation and fixed site designs: what to sample and where
EPA assumes that these NO2 sites will all be near major roads, but existing data suggest
that may not meet the network goals. Thus, the term near-road (NR) as used here also
includes congested urban core areas that may be locations of maximum 1-hour NO2
concentrations.
The NR pilot project is critical to informing the deployment of 126 NR NO2 monitoring
sites by the end of 2012 (just over 2 years from now with siting plans due summer 2012).
However, there is insufficient time to get and analyze all the data (saturation and fixed
sites) from a pilot network; pilot studies must be done during both winter and summer
seasons to account for potentially large seasonal NO2 variability; primary sources
dominate in winter, with secondary sources a factor only during summer mid-day and
afternoon when ozone may be present (not during morning rush-hour). EPA meeting
materials (introduction to Charge Question 12) state that the saturation study should be
performed before (to inform) deploying the fixed pilot sites, further extending the time
needed to complete a proper pilot project.
I am very concerned about the level of available funding for this pilot, which includes
mulitple multi-pollutant sites, several saturation studies, urban background monitoring
for pollutants of interest, and assumes substantial in-kind support from S/L agencies. A
major challenge will be how to get useful information from this pilot effort with limited
available resources ($800k) and a very short time-frame to adequately inform the larger
NR network deployment. EPA needs to address who will do the data analysis for this
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pilot (presumably a contractor), develop a plan for the analysis, and get external input on
that plan. Because of these time constraints, if at all possible I recommend that the full
network implementation be delayed or staggered or (ideally) both. If staggered, the first
round of sites (10-15 in the largest urban areas?) could include additional (more
intensive) measurements to better inform later phases.
As EPA notes in their background material, there are many factors to consider for NR
monitor siting; all of them can not adequately be addressed without a relatively large-
scale pilot program. Thus, it may be useful to leverage existing sites that meet some of
the NR siting requirements in the NO2 rule, and enhancing those existing sites, rather
than deploying new sites. This would allow for more pilot sites and speed completion of
the pilot study, but limit the "ideal" pilot siting design. I'd expect the saturation studies
to be of more value re: informing the siting process, and the fixed sites to support the
saturation studies and vet new methods. To support consideration of this approach, it
would be very helpful if EPA could supply a list of existing NO2 sites that could at least
loosely be considered NR (e.g., micro-scale siting), along with additional site meta-data
such as AADT, vehicle type mix, other pollutants currently measured, and available
matching urban background measurements.
Finally, I strongly encourage EPA to evaluate the "NR" excess for key indicators (at a
minimum: NO2, UFP and BC) as part of this pilot study. This requires a "matched set"
of indicator measurements at the NR site and an appropriate "urban background" site -
perhaps an NCore site if not too distant from the NR site. The "NR excess" metric allows
an estimation of the gradient away from the road. Without the background
measurements, the NR indicator data (BC, UFP) have no useful context.
Charge Question 1. Content/Topics of Guidance Outline for full network siting and
implementation.
Since this is a brief outline, there is not a lot to comment on.
3b: AADT is over-emphasized as an initial step in site selection. It should be listed with
the considerations in 3c. See other comments on AADT in response to charge question 3.
4. If modeling is going to be done by S/L agencies (that is my understanding), not EPA,
it may have limited application. There is a wide range of expertise and resources across
S/L air agencies for this kind of work.
5. Saturation monitoring may be the most effective approach for site identification,
although the need for simplicity and low cost limits the use to multi-day samples rather
than one to a few hours duration.
6. 6d seems to duplicate 3c.
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Charge Question 2. "What pollutants and sub-species does the subcommittee believe
should be included for consideration and discussion in the near-road monitoring
guidance? Some potential species for consideration include NO2, NOX, NO, CO, PM
(Ultrafine, 2.5, and 10), black carbon, air toxics (e.g., benzene, toluene, xylene,
formaldehyde, acrolein, or 1, 3, butadiene), and ammonia. Please prioritize the
recommended pollutants and provide the rationale for their ranking, including how this
pollutant measurement will contribute to scientific and regulatory knowledge of near-
road air quality and adverse human health effects."
For the full network, NO/NO2,UFP (CPC), BC, and CO, and meteorological
measurements are the most important. NO2 and CO are NR-relevant NAAQS; no further
discussion needed. UFP (particle # concentration by CPC) and BC are robust indicators
of NR pollution influence and critical (along with NOx) to determine the "NR excess"
pollution influence. PM2.5 should also be done, although it is generally not a good
"indicator" of NR pollutants since the urban background is already elevated. The air
toxics listed here are very useful but are a second-tier group. Ammonia is not of much
interest despite the SCRT NOx controls now used for HDD. The list can be broken into
tiers as follows:
Tier 1 - must do:
For the 126 network site sites, NO/NO2, UFP (CPC), BC, CO, and meteorological
measurements are the most important. NO2 and CO are NR-relevant NAAQS; no further
discussion needed. UFP (particle # concentration by CPC) and BC are robust indicators
of NR pollution influence and critical (along with NOx) to determine the "NR excess"
pollution influence; # cone [UFP] has not been shown by itself to be of use for health
effects yet. BC does show up as a significant predictor of a wide range of health
outcomes, although it may [or may not] be acting as an indicator for other NR pollutants.
Met (primarily wind) is critical to understanding temporal variations in NR pollutants. I
would not recommend 3-d sonic for all sites [reliability issues atNR sites], despite the
additional value those data have.
Guidance for NO2 must include data acquisition of large negative values and address
issues related to NO-NOx channel balance (matched gain) to avoid degradation of hourly
NO2 data when NO is high and rapidly varying. Guidance for CPC/UFP must address
diffusional losses of particles <-25 nm in the sample inlet train.
Tier 2 - could do:
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PM2.5 should also be done despite issues with the FRM/FEM methods; although it is
generally not a good "indicator" of NR pollutants since the urban background is already
elevated, it is a NAAQS that may often be highest at NR sites. The air toxics listed here
are very useful for health impact, but are probably too expensive; acrolein and HCHO
can not be done at hourly time-scales using practical methods. PM-coarse, and metals are
doable at 1-h time-scales, but are resource intensive with health implications unclear.
Paired indicator measurements at urban background site (BC, UFP) would be very useful
for assessment of NR excess and estimation of the shape of the gradient away from the
road, but maybe not everywhere.
Tier 3- could do at a small subset of sites f~l 0-15] to serve research needs:
Hourly EC/OC, BTEX and other air toxics, SMPS or similar size-resolved UFP; hourly
metals [automated on-line XRF]; true [photolytic] NO2,03 [maybe NO
chemiluminescent method to avoid measurement artifacts at low concentrations], CO2
[up and downwind], traffic counts or remote sensing of traffic. Paired indicator
measurements at urban background site (BC, UFP). Dry nephelometers with pm-1.0 inlet
cuts for fast-response PM-fine indicator. 3-D met at 3 and 10 meters. NOy paired with
"true" photolytic NO2 may be useful for determination of total reactive nitrogen and
NOz.
Don't do:
Ammonia is not of much interest despite the SCRT NOx controls now used for HDD, and
not practical at 1 -hour resolution. Nitrate (no reason to expect it to be a NR issue, and
little to no value for health effects); if NOy is done, ammonium nitrate is included in the
NOz species.
There was some discussion of commercial availability of a suitably robust UFP
instrument (CPC) that could be successfully used in a large routine monitoring network.
As of May 2010, a commercial water-based CPC is now available that appears to meet
these needs; a brief evaluation of the shipping version (not pre-production) of this
instrument is at:
http://home.comcast.net/~g allen/TSI-3783-CPC Allen.pdf
Charge Question 3. Guidance for Identifying Candidate NR Sites for the full network
AADT and fleet mix are two criteria of uncertain value. For N02, HDD is the driving
on-road primary source; highest NO2 might be found where there is a lot of HDD,
significant congestion, and poor dispersion. If local ozone titration (summer only,
secondary NO2) is a substantial driver of elevated NR NO2, the fleet mix becomes less
important. AADT without related congestion data can be misleading; a free-flowing
highway with high AADT is not likely to contribute to high NO2 levels.
Terrain is a major factor; see work by Wang and Zhang, "Modeling Near-Road Air
Quality Using a Computational Fluid Dynamics Model, CFD-V1T-RIT", EST, 2009: 43
(7778-7783). Highest NR impact might be expected where the roadway is somewhat
depressed relative to curbside terrain. Sound barriers and trees/vegetation (Baldauf-EPA
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work) may reduce MR impact for some parameters.
Meteorology would ideally be measured at 10 meters to avoid the worst of road-induced
turbulence (although at the time-scale of 1-hour, larger scale wind patterns would still
likely dominate the wind data). It is not practical to require 3-d wind at all sites, although
this could be useful. Sigma-Theta wind data may be useful as an indication of
turbulence, as well as the different between 1-hour average scalar and resultant wind
speed. Wind data should be sampled at 1-second intervals for these turbulence related
metrics, and 5-minute averages may be very useful in a detailed assessment of periods of
high concentration.
For Up/Down wind siting, I recommend avoiding sites that are upwind relative to
prevailing wind direction. The highest impact may occur where prevailing winds are
parallel to the roadway, at which point there is no real up or down-wind side of the road.
Charge Question 4. Use of modeling for NR Guidance Document
This is not my area of expertise, although the concerns stated in Ql part 4 remain, as well
as the demonstrated ability of the models under consideration to reasonably predict
locations of 1-hour maximum NO2 concentrations. Mobile source oriented models do
not take micro-scale ozone titration into effect, and thus are useful only where primary
NO2 would be expected to be the driver of 1-hour maximum NO2.
Charge Question 5. Saturation and On-road Monitoring (Guidance Document)
Saturation monitoring is a very useful tool to screen potential sites and learn more about
the characteristics of sites with likely maximum NR impact. Ideally, a saturation study
does not have to be constrained to NO2; with modest firmware modifications to existing
personal monitors, BC can now be monitored for in these studies. While the BC (optical
method) is highly time-resolved, the most practical method forNO2 remains the TEA-
based passive samplers; these have been well characterized. They could be adapted for
use in this work to measure only morning rush hour over a 5-weekday period by making
the flow "active": e.g., a timer, a suitable sampler "housing", and a small (~ 200 ccm)
pump. Some initial effort in modification and characterization of existing passive NO2
samplers would be needed. The passive samplers should be run at a fixed monitoring NR
pilot site (with NO2 and BC) to validate field performance. The variability inherent in
passive NO2 methods will be substantially reduced, since wind speed effects on
"effective sampling rate" are essentially eliminated. These passive samplers typically
need 50 ppb-hour of N02 to provide stable data. With active sampling, this number
drops, perhaps by 2 times. For a 15-hour (3h x Sdays) sample period, once might expect
useful data down to a few ppb. Ozone could also be added to any passive sampler-based
approach. With NO and NO2, perhaps assisted by addition of BC and O3, the influence
of primary vs. secondary NO2 can be assessed with morning rush-hour samples and
afternoon (~ 2-5pm local time) samples by looking at the ratios of these pollutants. This
could answer one of the more complex questions for N02 — do primary or secondary
process drive the maximum 1 -h concentrations for a given site/season? For winter and
morning rush-hour periods (minimal ozone), it is reasonable to assume that primary
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sources will dominate. For summer afternoons, that assumption can not be made. Taken
together, this suggests that secondary processes are a potential driver of high NR N02
only for summer mid-day and afternoon periods.
Finally, in addition to official NWS data, reasonably local wind data must be collected
for any saturation study. These data may be available from NOAA-MADIS meso-net
sites; these sites have automated QC (done by MADIS) that can be used to assess data
quality, and highly time-resolved (sub-hourly) historical data are available on the web.
Other than for BC personal samplers using optical filter techniques, 1 do not recommend
"on-line" methods for the saturation study. Cost, complicated siting logistics, and data
quality are the primary concerns here.
Having suggested this saturation study approach, the reality is that without significant
EPA support, the best a S/L agency is likely to be able to do is a simple passive NO2
study, perhaps limited to weekday sampling rather than a full week. The Ogawa sampler
can do NO2 and NOx in a single sampler, providing an indirect measurement of NO,
which would be very useful. Since these samples would not be constrained to periods of
likely NO2 maxima, existing NR-ish hourly data should be analyzed to determine how
well a 5 or 7-day NO2 mean is correlated with the 1-h maximum for that period. This is
an analysis EPA could easily do.
I do not recommend on-road monitoring for this pilot project, in part because it is very
resource intensive to do in a useful manner, and with the limited resources available a
saturation study has more value with regard to the relevant siting questions especially
when seasonality is considered. An exception could be if a local organization has a
suitable mobile monitoring platform and the resources to deploy it. A recent example of
mobile van spatial characterization of NR pollution is "Short-term variation in near-
highway air pollutant gradients on a winter morning" (Durant et al., ACP, 2010;
http://www.atmos-chem-phys.net /10/8341 /2010/
Charge Question 6. CO — fleet mix, cold starts and urban canyons
On-road sources of CO are different than NO2; there is essentially no CO from HDD.
Thus, the areas of highest CO impact may be very different. Cold starts, idling, and fleet
mix in urban canyons (and existing data) all suggest that a site for highest N02 impact in
an urban area may not be the location of highest CO impact. Finally, urban canyon siting
is generally going to be very close to the curb (horizontally at least) ~ typically < 5-10
meters. But this pilot can not afford to address the urban canyon CO issue.
Charge Question 7. NR PM2.5 and PM10
Existing data has shown only a modest increment in NR PM2.5 in urban areas, primarily
since the urban background is already elevated. PM2.5 is not generally a useful indicator
of NR pollution excess gradients. Still, in urban areas without dominant industrial
sources, the highest PM2.5 would normally be found near areas with substantial local
traffic. PM10 would be expected to be somewhat higher at NR sites because of dust
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reintrainment; minimal hourly NR PM-coarse data exist, so it is difficult to assess this
parameter at this time scale.
Charge Question 8. Other PM-related measurements desirable at near-road monitoring
stations
See Q 2 above. It must be noted that for UFP (CPC particle # concentration), the inlet
can not be at the height of NO2 and similar pollutants unless a carefully designed aerosol
manifold is used. Diffusional losses of particles < ~20-30 nm can be large unless
appropriate sample inlet trains are used, and it is common to see # concentration mode
peaks in this size range at NR sites.
Charge Question 9. Would an adjustment of CO siting criteria to match those of
microscale PM2.5 and microscale near-road NO2 sites be logical and appropriate?
Yes.
Charge Question 10. Should there be consideration to maintain the requirement on how
urban street canyon or urban core microscale CO sites should be sited?
Yes.
Charge Question 11. ...how "urban street canyons" or "urban core" might be defined,
perhaps quantitatively, and with regard to use in potential rule language?
These terms may be difficult to define for rule language across all urban areas. Right
now, the approach seems to be "you know it when you see it."
Charge Question 12. ...potential criteria for consideration in selecting where the fixed,
permanent [NR pilot] stations should be located.
There will not be more than 2 or 3 of these sites in this NR pilot, as noted in the charge
questions. Thus, the range of variables in this charge question can not be fully evaluated.
In reality, giving the constraints of funding and timing noted above, the siting decision
may be driven largely by what S/L agencies have the resources to support the pilot work
and where they can find and deploy a reasonable site quickly. With only 2-3 sites, it may
be best to choose "generic" sites that are most likely to represent a large fraction of the
final network. We are not going to learn how a wide range of siting characteristics would
effect results from this pilot. We might come closer to that goal by constraining the pilot
to saturation studies, with no fixed sites. But that is not a practical solution to the broader
goals of this pilot. Cooperation of local the DOT may be useful, but the kind of data that
needs to be collected for this pilot might be real-time remote traffic sensing that can
estimate % large truck traffic ~ data not usually available from a DOT.
Charge Question 13. ...minimum equipment/pollutant measurement complement that
should be deployed at each site and also the ideal equipment complement that each site
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should or could have...
See Q 2 above, and my background comments about the importance of having matched
urban-scale monitors for evaluation of the "NR excess" for key indicator measurements.
It may also be useful to have real-time remote sensing of traffic count as used in the Las
Vegas NR MSAT study ~ this can also give information on large truck vs. other vehicle
traffic.
Charge Question 14.
EPA is proposing that saturation studies for NO2 would be conducted in 4 or 5 urban
areas. This is a reasonable goal.
A) "The pollutants that should be measured with the saturation devices at each saturation
site."
NO2 and maybe BC if resources allow. See Q 5 above. Met should be collected at one
site in the area, possibly relying on existing MADIS meso-net data.
B) "The number of saturation devices per pollutant, both passive and/or continuous /
semi-continuous, that may be deployed in each pilot city."
I assume this Q is "how many sites?". This is budget driven, but I suggest at least 6, with
one of those being at a fixed site with robust NO2 and BC measurements (as a validation
site). For those cities with a NR pilot fixed site, the collocation would be done there. For
other cities, the most NR-ish site would be used for collocation of saturation study
monitors.
C) "Whether placing saturation monitoring devices near certain road segments should
include, at a minimum: 1) the highest AADT segment in an area, 2) the road segment
with the highest number of heavy-duty truck/bus counts, 3) at a road segment with more
unique roadway design, congestion pattern, or terrain in the area, and 4) if feasible, at a
lower AADT segment with a similar fleet mix, roadway design, congestion, terrain, and
meteorology as the top AADT road segment in the area."
All except #4 above, as well as one or more urban canyon or similar non-highway site
and a collocation site with regulatory monitors for QC use.
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Dr. Judith Chow
Subject: Preliminary Response to Charge Questions on NC«2 Near Road Monitoring to
Support Measurement of Multiple NAAQS Pollutants
Date: September 20, 2010
1. Comment on the overall content of the recommended topics in the draft outline.
Provide suggestions on any missing subjects that should be included in the
guidance document and any unnecessary topics that are currently listed in the
attached draft, if applicable.
The Section 2 literature review needs to be more comprehensive than indicated. It should
include a discussion of the relationships between the different pollutants at emission and
the likely changes that they will experience with downwind transport from the roadway.
It should emphasize the multipollutant (Chow et al., 2010a; Greenbaum and Shaikh,
2010; Hidy and Pennell, 2010; Mauderly et al., 2010) nature of near road exposure. A
few reviews and meta-analyses of near-road concentrations have been published
(Smichowski et al., 2008; Seigneur, 2009; Karner et al., 2010; Zhou and Levy, 2007) that
can be used as starting points, although these are not specific to NC>2 concentrations. A
conceptual model should be formed in this section that includes dispersion, deposition,
chemical conversion and physical conversion. Special attention should be given to NC«2
formation by NO titration of Oi and NO2 depletion by photochemistry.
Section 3 should contain a definition of source zones of influence and receptor zones of
representation, defining middle-, neighborhood-, and urban scales (Chow et al., 2002).
Compromises necessary to obtain multipollutant characterization should be defined. The
list of variables seems complete, but there is an apparent assumption that AADT is the
primary siting criteria. Meteorological and terrain variables also probably have important
effects. One might find high levels in street canyons than on open roadways with good
ventilation.
Section 4 needs elaboration on the models to be considered and how their reported
performance. There are several models that compare dispersion models with
measurements, use new approaches such as computerized fluid dynamics (CFD) models
to evaluate vehicle-induced turbulence and the effects of roadside obstructions, and that
attempt to simulate chemical and physical transformations (Baik et al., 2007; Baker et al.,
2004; Berkowicz et al., 2008; Buccolieri et al., 2009; Chakrabarty et al., 2000; Chan et
al., 1995; Chang et al., 2009; Cheng et al., 2008; Cheng et al., 2009; Chu et al., 2005;
Clarke et al., 2004; Di Sabatino et al., 2008; Dixon et al., 2006; Gidhagen et al., 2004b;
Gidhagen et al., 2004a; Gokhale et al., 2005; Grawe et al., 2007; Gromke et al., 2008;
Kang et al., 2008; Kondo et al., 2006; Kondo and Tomizuka, 2009; Kumar et al., 2009; Li
et al., 2006; Liu and Leung, 2008; McNabola et al., 2009; Moussiopoulos et al., 2008;
Murena et al., 2008; Murena et al., 2009; Ning et al., 2005; Oettl et al., 2006; Pohjola et
al., 2003; Rodden et al., 1982; Sahlodin et al., 2007; Santiago and Martin, 2008; Solazzo
et al., 2007; Tay et al., 2010; Tsai and Chen, 2004; Vardoulakis et al., 2002; Vardoulakis
et al., 2003; Venkatram et al., 2007; Wang et al., 2006; Wang and Zhang, 2009; Xie et
10
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al., 2006; Yassin et al.. 2008; Yassin et al., 2009; Yim et al., 2009; Zhou and Levy, 2008;
Zhu and Hinds, 2005).
In Section 5, "multi-scale" monitoring is a more specific term than saturation monitoring.
This section should provide a summary of roadside measurement studies, passive and
active monitors that can be efficiently deployed at many locations, and methods to
interpret that data acquired. The conclusion might be that roadside monitoring for site
selection and long-term monitoring needs small, portable sensors that don't require a
large infrastructure (i.e., shelter, air conditioning, etc.) to operate.
Section 6 might draw from some of the existing guidance for sampler siting (U.S.EPA,
1997; U.S.EPA, 1998). Site documentation in Section 7 should include coordinates,
photographs of the siting probe, and pictures of the surroundings.
2. What pollutants and sub-species should be included for consideration and
discussion in the near-road monitoring guidance and what should be the priority
of measurement?
See response to question 13.
3. What external variables should be used to identify candidate near-road
monitoring sites?
a. Given the variability in emission rates from on-road vehicles based on
vehicle technology, fuel, speed, environmental conditions, is the fleet mix
in near-road site selection or is a more refined inventory and modeling
analysis required?
Fleet mix is a good starting point, but this is likely to vary by time of day and the diurnal
breakdown is unlikely to be available. Fleet mix should only be one variable considered
in site selection.
b. Should the suggested approach consider fleet mix via the use of average,
fleet-wide emission factors, or the use of inventory and modeling analysis,
take into account mobile source controls that are "on the books" but have
not yet been fully realized due to fleet turnover? If so, how far out into the
future should states consider their effects?
Real-world emissions are likely to be quite different from certification-type emissions. It
will take a long time for fleet evolution. Better to establish monitoring sites soon so that
improvements can be tracked through long-term trends. It may be that the emission
reduction measures that are "on the books" are not as effective as originally thought.
c. The EPA suggests establishing sites at-grade with the road, without any
nearby obstructions to air flow; however, the Agency recognizes that this
might not always be feasible. Does the subcommittee agree with this
recommendation for locating sites at-grade with no obstructions? What
priority should be placed on this factor within the guidance, given the need
for flexibility in identifying appropriate site locations?
Higher concentrations will probably be found in more confined areas (e.g., street
canyons) than near open roads with no obstructions. Obstructions between the vehicles
and the monitors should be minimized.
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d. How important a parameter should LOS be in the determination of
appropriate near-road monitoring sites? Does the subcommittee have a
view on how reliable LOS estimates are across the country?
Congestion varies throughout the day and on weekends vs. weekdays. LOS may provide
a first-cut on roads that have congestion, but there is no evidence on how accurate it is as
a congestion indicator.
e. Should terrain and vegetation should be a consideration in the siting
process? What priority should this parameter have in the overall process?
Terrain is very important. Even small roadway dips can accumulate pollutants in
hotspots (Bowen et al., 1993; Bowen and Egami, 1994).
f. Although there is no requirement to be downwind, in the preamble to final
NO2 NAAQS rule, EPA encouraged it when possible. EPA and NACAA
intend to do the same in the guidance document. Does the subcommittee
agree with this approach?
No. Sampling should take place downwind of the prevailing wind. Even under stagnant
conditions the ram effect of the vehicles will create flows parallel to the roadway. See
Figure 6 pollution rose from Oettl et al. (2006), as reproduced below. Nothing is detected
when the sampling location is upwind of the roadway.
Fig. 6. Observed mean Nor-conoentratkms as dependent on
wind direction at the site Vomp-Lcitrn for 2001 2003 in (ngm~ J).
(Oettl etal., 2006)
Comment on the available modeling tools, and their pros and cons.
See comments under Question 1. Roadway models must consider more than just linear
dispersion of inert pollutants.
How might saturation and on-road monitoring be used for near-road site selection?
Portable active or passive monitors that can be easily and inexpensively deployed would
provide a good indication of where concentrations might be highest. Levels are likely to
vary by season, as noted in Figure 1 of Zou et al. (2006), and show an exponential
decrease with distance from the curbside. Passive monitors using NOa-absorbing filters
have some potential biases, but have also been shown to be comparable with continuous
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measurements for integration times on the order of weeks (Ayers et al., 1998; Beckerman
et al., 2008; Berkowicz et al., 2008; Grouse et al., 2009; De Fouquet et al., 2007; Douglas
and Beaulieu, 1983; Faus-Kessler et al., 2008; Gilbert et al., 2003; Gonzales et al., 2005;
Hauser et al., 2009; Heal and Cape, 1997; Heal et al., 1999; Heal et al., 2000; Henderson
et al., 2007; Krochmal and Gorski, 1991; Mukerjee et al., 2004; Nash and Leith, 2010;
Norris and Larson, 1999; Ozden and Dogeroglu, 2008; Parra et al., 2009; Piechocki-
Minguy et al., 2006; Plaisance et al., 2004; Rava et al., 2007; Sekine et al., 2008; Shooter
et al., 1997; Van Reeuwijk et al., 1998; Vardoulakis et al., 2009). This is probably a cost-
effective and practical technology for mapping average spatial gradients as a prelude to
sampler siting.
—o— Spn —o— SptZ —»-
—•— Sum1 —*— Sum2 —*-
Ami —4— Au!2 —•-
•-Wini -x-Win2 --•-
Sprt
Sun3
Aut3
0 50 100 150 200 250 300 350
Distance from motorway (m)
Fig. I. NOj concentrations measured at different distances from
the A20 highway in Shanghai i3;6t i Spr spring. Sum:
slimmer. Auc aurunin. Win: aimer. I. 2. 3 rcpicsenu the
different selected study area, respectively).
(Zou et al., 2006) Reprinted from Atmospheric Environment, 40, Zou et al., "Shifted power-law
relationship between NO2 concentration and the distance from a highway: A new dispersion model based
on the wind profile model, Copyright (2006) with permission from Elsevier.
If a state were inclined to use saturation monitoring to aid in the selection of a near-road
monitoring site, and considering that the NC>2 standard is a 1-hour daily maximum
standard, what are the pros and cons to using passive devices to saturate an area to gather
data?
Pros are low expense and operating cost. The major con is the long averaging time, much
longer than 1 hour.
Likewise, what are the pros and cons to using non-passive devices, such as near real-time
or continuous devices including, but not limited to portable, non-FEM
chemiluminescence methods forNC^ or Gas Sensitive Semiconductors (GSSs) forNOj
and other pollutants of interest?
Pros are short-duration samples, on the order of an hour or less. Cons are instrument
procurement and operating expense, potentially higher than desired detection limits, and
reliability of new technologies.
Finally, what would be the pros and cons, to a state or local agency attempting to use a
specially outfitted vehicle to collect mobile measurements to assist in the near-road site
selection process for NC"2 specifically as well as other pollutants of interest?
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Several mobile emissions systems have been applied to characterizing on-road and
roadside concentrations (Bukowiechi et al., 2002; Bukowiecki et al., 2003; Cocker et al.5
2004a; Cocker et al., 2004b; Durbin et al., 2007; Herndon et al., 2005; Isakov et al., 2007;
Kittelson et al., 2004; Kittelson et al., 2006; Morawska et al., 2007; Nussbaum et al.,
2009; Pirjola et al., 2004). Pros are that these systems are moveable and obtain many
different pollutant measurements. Cons are the large cost of assembling or contracting
these laboratories and the snapshot nature of their measurements, as they usually need to
be attended and can be parked for only a short time period.
To what extent will light duty cold start and congestion factors will significantly
influence the location of peak CO concentrations in an area?
The cold start segment has been found to affect emissions for many pollutants, including
CO (Cadle et al., 2001; Chan and Zhu, 1999; Chase et al., 2000; Cook et al., 2007; Cotte
et al., 2001; Gullett et al., 2006; Huai et al., 2004; Joumard and Andre, 1990; Joumard et
al., 2000; Kittelson et al., 2006; Korin et al., 1999; Lenaers, 1996; Lough et al., 2005;
Ludykar et al., 1999; Maricq et al., 1999; Mathis et al., 2005; Pornet et al., 1995;
Ristimaki et al., 2005; Schauer et al., 2008; Singer et al., 1999; Weilenmann et al., 2005;
Weilenmann et al., 2009; Westerholm et al., 1996). This is of fairly short duration
(minutes) and would most likely affect emissions in garages, driveways, parking lots and
side streets rather on the heavily-travelled thoroughfares. If it is desired to characterize
cold starts, sampling systems should be located near where cars turn onto major arteries
from nearby neighborhoods. Even so, only those living most closely to the intersection
will exhibit cold start emissions.
What priority should these factors be given when compared with the factors (AADT,
Fleet Mix, Roadway Design, Congestion Patterns, Terrain, and Meteorology) already
being considered for peak NO2?
Priority should be low.
Do these two issues of vehicles operating under cold start conditions and light duty
vehicle congestion and idling in urban street canyons and/or urban cores be considered in
a future, nationally applicable, CO monitoring proposal?
This is probably better treated as an emission standard that would minimize cold start
emissions through technological means.
Are there other factors that may affect peak CO concentrations and not affect peak NO2
concentrations that should also be considered for any future CO monitoring proposal?
CO is relatively inert and is often used to normalize other pollutants for dispersion
downwind of a roadway (Zhang and Wexler, 2004). It is expected that NO2 emissions
will disperse in a similar manner, although they still experience transformation processes
that differ from those of CO.
Will siting considerations for identifying the location of peak NO2 concentrations address
all of the high priority siting considerations for PM (particularly PM2s) as well? If not,
what other factors should be considered and what are the advantages in considering these
factors for identifying the location of maximum PM concentration?
No. PM25 is a combination of primary and secondary particles from a wide variety of
emission sources. Roadside sampling is useful for characterizing the motor vehicle
contribution, but it may bias the urban- and regional-scale PM2 5 compositions and
exposures. These monitors would be considered Special Purpose Monitors according to
the PM siting criteria (U.S.EPA, 1997).
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In addition to PIVh 5 mass, what other PM-related measurements are desirable at near-
road monitoring stations (e.g., UFP number, black carbon, EC/OC, PM coarse, etc.)?
Particle size distribution and particle number by continuous methods. PM speciation on
filters, including elements, ions, OC/EC, and organic markers, would be useful to
develop source profiles for emission inventory speciation and receptor modeling.
To allow for near-road monitoring infrastructure to be multi-pollutant, and in reflection
of the recently promulgated near-road NOa siting criteria, reconsideration of the existing
microscale CO siting criteria presented in sections 2, 6.2, and table E-4 in 40 CFR Part 58
Appendix E may be warranted. Does the subcommittee believe that reconsideration of
microscale CO siting criteria is appropriate? Specifically, would an adjustment of CO
siting criteria to match those of microscale PIVh 5 and microscale near-road N02 sites be
logical and appropriate?
40CFRPart58 Appendix E calls for CO roadside monitors to be located at 2 to 10 m from
the nearest traffic lane for open roads. In a street canyon, the monitor is to be at least 10
m from an intersection. An NO2 monitor might register higher concentrations near the 10
m downwind location owing to NO2 formation by reaction of the NO2 with Os. It seems
that a reasonable compromise on the setback could be derived that would serve both
purposes. A more detailed examination of NO2, NO, CO, and OB data is needed in the
pilot study to better determine the optimum distance from the roadside.
Even if the adjustment of microscale CO siting criteria in sections 2, 6.2, and table E-4 in
40 CFR Part 58 Appendix E to match that of microscale PM2 5 and microscale near-road
NO2 is appropriate and proposed, should there be consideration to maintain the
requirement on how urban street canyon or urban core microscale CO sites should be
sited?
The NO2 and CO siting criteria should be the same. There is more to be gained from the
multipollutant measurements than is lost by slight differences in maximum hourly
concentrations.
Does the subcommittee have an opinion on how "urban street canyons" or "urban core"
might be defined, perhaps quantitatively, and with regard to use in potential rule
language?
There is a reasonable literature on measurements in street canyons (Baik and Kim, 2002;
Bakeas and Siskos, 2003; Boddy et al., 2005a; Boddy et al., 2005b; Buccolieri et al.,
2010; Cai et al., 2008; Caton et al., 2003; Chan et al., 2003; Cheng et al., 2009; Chu et
al., 2005; Di Sabatino et al., 2008; Dobre et al., 2005; Eliasson et al., 2006; Genikhovich
et al., 2005; Gromke et al., 2008; Hang et al., 2009; Kassomenos et al., 2004; Kim and
Baik, 2004; Kumar et al., 2008; Kumar et al., 2009; Lam et al., 2008; Li et al., 2005; Li et
al., 2009; Longley et al., 2003; Longley, 2004; Longley et al., 2004; McNabola et al.,
2009; Molina, 1996; Moussiopoulos et al., 2008; Murena and Vorraro, 2003; Murena et
al., 2008; Prajapati et al., 2009; Santiago and Martin, 2008; Scaperdas and Colvile, 1999;
So et al., 2005; Stein and Toselli, 1996; Tay et al., 2010; Tsai et al., 2005; Venegas and
Mazzeo, 2000; Voigtlander et al., 2006; Xie et al., 2003; Xie et al., 2005; Xie et al., 2006;
Xie et al., 2007; Yassin et al., 2009), supplementing the street canyon modeling literature
cited in the response to Question 1. These measurement and modeling studies need to be
critically evaluated to answer this question. A quick survey suggests that there are
various degrees of roadside obstructions that will have large effects on concentrations.
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To what extent are the pilot study site selection criteria of a large and a relatively small
urban area based on population, an area with varied or complex terrain, an urban area
with an operational NO\ analyzer representative of neighborhood or larger spatial scales
for comparison to the near-road NOx analyzer, and an urban area with a cooperative (or
non-cooperative) Department of Transportation complete and adequate?.
Additional criteria should include periods of morning stagnation and low inversion,
differing morning Os levels that might enhance NO2 through NO titration, cold as well as
warm environments that might experience different emission levels owing to cold starts.
Comment on the minimum equipment/pollutant measurement complement that should be
deployed at each site and also the ideal equipment complement that each site should or
could have, respectively. Specifically, what pollutants (e.g., NO2, NOx, NO, CO, PM
(Ultrafine, 2.5, and 10), black carbon, air toxics (such as benzene, toluene, xylene,
formaldehyde, acrolein, or 1,3, butadiene) and ammonia) and other information should
the pilot study measure or gather at the fixed, permanent monitoring stations, and by what
methods? This list should be in priority order, as feasible, and can include any NAAQS
or non-NAAQS pollutant by any method (FRM/FEM and/or non-reference or equivalent
methods), any particular type of other equipment for gathering supporting data such as
meteorology or traffic counts.
As implied in the charge questions, a specialized multi-pollutant monitoring package
should be assembled and applied in these studies. There are several examples of such
packages that have been assembled for neighborhood-scale studies, on-board emissions
sampling, and unmanned aerial vehicles that have contain potentially applicable sensors,
but these would need to be evaluated with respect to their sensitivity, stability, and
accuracy. Data should be acquired over 1 min averages or less so that individual plumes
can be detected.
Nitrogen dioxide (NO2). This is top priority because it is the focus of the study. There
are several currently available or emerging technologies for microsensors (Brunei et al.,
2008; Currie et al., 1999; Egashira et al., 1996; Forleo et al., 2005; Gurlo et al., 1998; Oto
et al., 2001; Sitnikov et al., 2005; Talazac et al., 2001). NO can often be obtained from
these same sensors.
Carbon dioxide (CO2): Normalizing other pollutants to CO2 allows fuel-based emission
factors to be developed (Kean et al., 2000; Sawyer et al., 2000). Commercially available
microchip IR sensors are available for CO2 measurements (Chow et al., 201 Ob).
Black Carbon (BC): On a short-duration minute basis, this would allow cold starts and
diesel exhaust to be separated from others and related to the N02 emissions from
individual vehicle plumes. A portable aethalometer (Hansen and Mocnik, 2010) is
available for filter transmission measurements of BC, and more portable photoacoustic
measurement systems (Kok and Baumgardner, 2010) are emerging.
Carbon Monoxide (CO): CO is a priority pollutant and is an indicator of gasoline engine
contributions, especially for cold starts and poorly maintained engines. Several small
detectors are available (Do and Chen, 2007; Oto et al., 2001).
Ozone (Os): This would be important for estimating NO titration to NO2. Several
microsensors are available or are emerging technologies (Do and Chen, 2007; Gurlo et
al., 1998; Ulanovsky et al., 2001; Vallejos et al., 2007).
Particle number: This would indicate a potential adverse health effect. Portable CPC
counters are available.
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and PIVh 5: Coarse particles (PM 10-2,5) may be affected by road dust while PM2 5 is
largely from vehicle exhaust. Optical particle counters can provide a real-time surrogate
for these components (Wang et al., 2009; Cheng, 2008; Heim et al., 2008; Linnainmaa et
al., 2008).
EPA and NACAA have proposed four to five urban areas to have saturation monitoring,
using either passive devices and/or continuous/semi-continuous saturation type multi-
pollutant monitoring packages (i.e., several types of monitors in one mountable or
deployable "package"). Please provide comment on:
The pollutants that should be measured with the saturation devices at each saturation site.
See answers to Question 13.
The number of saturation devices per pollutant, both passive and/or continuous/semi-
continuous, that may be deployed in each pilot city.
Four sampling systems should be located downwind of the roadway at various distances.
One should be located in a neighborhood near the road and one should be located upwind
of the urban area.
Whether placing saturation monitoring devices near certain road segments should
include, at a minimum: 1) the highest AADT segment in an area, 2) the road segment
with the highest number of heavy-duty truck/bus counts, 3) at a road segment with more
unique roadway design, congestion pattern, or terrain in the area, and 4) if feasible, at a
lower AADT segment with a similar fleet mix, roadway design, congestion, terrain, and
meteorology as the top AADT road segment in the area.
These are good suggestions. Experiments should be designed to determine which
variables most affect the ambient concentrations.
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Mr. Bart Croes
It's not clear what EPA is trying to accomplish with its proposed near road monitoring
program. If it's to determine compliance with the new 1-hour national ambient air quality
standard for nitrogen dioxide (NOi), then it seems that the primary focus should be on
monitoring population centers rather than siting monitors near roadways. For example,
how will nonattainment boundaries be established for these microscale environments?
California has had a one-hour standard for NC«2 for several decades and studied peak
levels and trends throughout the State. Although future NC>2 levels will be increasingly
driven by direct emissions from heavy-duty diesel vehicles, current peak locations in
California are in intermediate downwind areas (e.g., eastern border of Los Angeles
County) where photochemical conversion (and not just immediate ozone titration) has
taken place, not in the source areas.
If the purpose is to conduct multi-pollutant monitoring to help inform exposure and
health studies, then linkages with these types of research studies appears to be missing
from the documents.
The timeframe for establishing a national network (by January 1, 2013) seems much too
rushed to allow for full availability and analysis of results from the near road monitoring
pilot study.
Charge Questions:
1. The accompanying draft guidance document outline provides an initial thought of
the major topics required in the near-road monitoring guidance that will aid state
monitoring agencies in the identification and implementation ofNO2 near road
monitoring sites from a multi-pollutant perspective. Please comment on the
overall content of the recommended topics in the draft outline. Please provide
suggestions on any missing subjects that should be included in the guidance
document and any unnecessary topics that are currently listed in the attached
draft, if applicable
It's not clear what EPA is trying to accomplish with its proposed near road
monitoring program. The goals should be clearly delineated in order to comment on
this brief outline. It would also be helpful to see the target number of pages for each
section and the extent of the literature review. As the outline points out, the available
literature is extensive (see the Health Effects Institute special report on Traffic-
Related Air Pollution: A Critical Review of the Literature on Emissions, Exposure,
and Health Effects) and a conducting a synthesis of existing near road and saturation
monitoring datasets would be a useful addition.
2 EPA andNACAA envision the near-road guidance document to be written from a
multi-pollutant perspective. What pollutants and sub-species does the
subcommittee believe should be included for consideration and discussion in the
near-road monitoring guidance? Some potential species for consideration include
NO2, NOX, NO, CO, PM (Ultrqfine, 2.5, and JO), black carbon, air toxics (eg,
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benzene, toluene, xylene, formaldehyde, acrolein, or 1,3-butadiene), and
ammonia Please prioritize the recommended pollutants and provide the rationale
for their ranking, including how this pollutant measurement will contribute to
scientific and regulatory knowledge of near-road air quality and adverse human
health effects.
Again, some clarity is needed on what EPA is trying to accomplish with its proposed
near road monitoring program. If it's to determine compliance with existing air
quality standards, then only NC>2 and CO are all that is necessary, as the roadway
environment as not the peak location for PM2.5 and PM10 (because secondary PM is
so important in determining peak locations). If the purpose is to follow motor vehicle
criteria pollutant and air toxic emission trends, or to conduct multi-pollutant
monitoring to help inform exposure and health studies, then NOx, PM, black carbon,
and the air toxics should also be included.
3 Identifying Candidate Near-road Site Areas
a. Annual Average Daily Traffic (AADT) & Fleet Mix - To consider fleet mix -with
regard to NO2, an idea is to encourage slates that have fleet mix information to
take an approach that uses average, fleet-wide grams per mile emissions
estimates (one for light duty vehicles and one for heavy duty vehicles), combined
with AADT information to further weight which road segments in an area may be
more conducive to produce peak pollutant concentrations EPA would use the
latest emission factor information to aid such a calculation. Given the variability
in emission rates from on-road vehicles based on vehicle technology, fuel, speed,
environmental conditions, etc., does the subcommittee believe this approach is an
appropriate way to "consider" fleet mix in near-road site selection or is a more
refined inventory and modeling analysis required?
The highest CO sites in California are the ones with the highest AADT for light-
duty vehicles and oldest fleet, and the peak one-hour levels occur during winter.
In the past, the highest NO2 levels were the same sites and season because of the
reaction 2 NO + ©2 "^ 2 NO2, which is important at low temperatures and NOx >
\ ppm. But future NO2 levels will be dominated by heavy-duty diesel vehicles
(HDDV) and near-source mixing with ozone. Thus, a key factor in roadside NO2
levels is the fraction of heavy-duty diesel vehicles (HDDV) that are equipped
(2007-2009 model year) or retrofitted with a catalyzed diesel particulate filter. In
California, while most diesel vehicles emit 5%NO2 (from Caldecott Tunnel data),
this can increase to 17% for retrofits and 45% for 2007-2009 model year
(although absolute NOx, and consequently NO2 is lowered by 50%). In other
states that do not have California's NO2 limit for retrofits (or just went into place
in 2009), the NO2 fraction can be much higher, although admittedly the number of
retrofits is a much lower in the rest of the U.S. Since existing EPA emission
models predict NOx and not NO2, this capability would need to be added.
b AADT & Fleet Mix - Further, should the suggested approach above in question
3a to consider fleet mix via the use of average, fleet-wide emission factors, or the
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NOx and NC>2 emissions will continue to decrease as the 2010 model year
HDDVs (with 90% NO\ control over pre-2007 vehicles) become a bigger fraction
of the fleet. Since these decreases will take place somewhat uniformly across the
nation, the purpose of this question isn't clear. Is the intent to forecast emission
trends for States to get out of monitoring requirements?
c. Roadway Design - Studies suggest and support the concept that roadway design
influences pollutant dispersion near the road. The EPA suggests establishing sites
at-grade with the road, without any nearby obstructions to airflow; however, the
Agency recognizes that this might not always be feasible. Does the subcommittee
agree with this recommendation for locating sites at-grade with no obstructions?
What priority should be placed on this factor within the guidance, given the need
for flexibility in identifying appropriate site locations?
Obstructions between the roadways and the monitors should be minimized, but
shouldn't proximity to where people live be a more important consideration? A
more practical consideration in site selection is the cost of leases. For an ARE
freeway study in Lodi, rent started out at $750/month for both sites. Once the
landowners realized that their proximity to the freeway was worthwhile to ARB
they increased the rent to $1500/month.
d. Congestion Patterns - The congestion of a roadway can be estimated by the
metric "Level of Service " (LOS) LOS uses a letter grade from A to F to identify a
roadway's performance, with "A " the best conditions where traffic flows at or
above the posted speed limit and all motorists have complete mobility between
lanes to "F" the worst congestion where travel time cannot be predicted and
generally traffic demand exceeds the facility's capacity Since motor vehicles
generally emit more pollutants during congestion operations (although noting
that NOx and select other pollutant emissions can also increase with increasing
speed), how important a parameter should LOS be in the determination of
appropriate near-road monitoring sites? Does the subcommittee have a view on
how reliable LOS estimates are across the country?
As NOx and VOC emissions are continuously reduced over time, ambient oxidant
levels will also go down, which means that titration of NO by ozone to form N02
will become less important in comparison to direct NO2 emissions from new and
retrofitted diesel trucks. NO2 emissions are highest during cruise mode, not
congestion conditions for these trucks, and truckers avoid congestion. Experts at
Departments of Transportation should be consulted on the reliability of the
congestion metric.
e Terrain— State and local air agencies are required to consider terrain in the near-
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road monitoring site selection process, -which in some cases may be inherently
part of the roadway design However, EPA recognizes that some states and local
air agencies may have to make selections from amongst similar candidate sites
that differ only by terrain, e.g cut section versus open terrain, with or without
vegetation, etc Does the subcommittee agree that terrain and vegetation should
be a consideration in the siting process7 What priority should this parameter
have in the overall process?
NOx (and likely NCh) emissions increase with grade, but these are unlikely to be
areas of heavy congestion and truck traffic. Vegetation is a sink for particles and
ozone, and heavily vegetated areas should be avoided.
/ Meteorology - EPA took comment on, but did not finalize the requirement for
near-road monitoring sites to be climatologically downwind of the target road
segment Reasons were because the additional limitations this would introduce in
finding candidate sites would be in exchange for what may be a small increase in
the opportunity to monitor peak NO2 concentrations Further, with sites being
within 50 meters of target road segments, the phenomenon of upwind meander ing
(pollutant transport upwind due to vehicle induced turbulence) further reduces
that absolute need to be climatologically downwind. Finally, EPA recognized
that, logically, the potential for peak NO2 concentration may very well occur
when winds are calm or parallel (or nearly parallel) to the target road, allowing
for pollutant build-up, as opposed to when winds are normal to the road.
Although there is no requirement to be downwind, in the preamble to final NO2
NAAQSrule, EPA encouraged it when possible EPA andNACAA intend to do the
same in the guidance document Does the subcommittee agree with this
approach?
An analysis of existing roadside CO data would be informative regarding this
issue. In the peak Los Angles site, the highest levels were recorded during
stagnant winter-time conditions with low-speed meandering winds, meaning that
there was no consistent upwind or downwind direction.
4 Modeling is another tool that may be useful in the identification of candidate
near-road sites In particular, the use of mobile source emissions modeling with
MOVES and local-scale dispersion modeling with AERMOD, can be presented as
part of the guidance document Please comment on the available modeling tools,
and their pros and cons, that the subcommittee believes may be appropriate to
discuss and/or recommend for use in the near-road monitoring guidance
document.
Modeling requires hour-by-hour local-scale data on the fleet mix (not just HDDV
counts but also the proportions of retrofitted and MY 2007-2009 and 2010+ vehicles),
driving conditions (average speeds), background air quality (NC^, ozone), and
meteorology (wind speed and direction, stability) that is unlikely to be available. I
think a screening approach using the factors identified in my response to Charge
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Question #3 (with appropriate weightings developed using a literature review and
analysis of existing data) should be just as useful and much easier to apply.
5. In regard to the process of identifying candidate near-road monitoring sites,
beyond the evaluation of factors noted above in question 3, and the potential use
of modeling, the use of saturation monitoring and on-road monitoring are also
possible tools that state and local air agencies may choose to utilize in the near-
road site selection process.
a. If a state were inclined to use saturation monitoring to aid in the selection of a
near-road monitoring site, and considering that the NO2 standard is a 1-hour
daily maximum standard, what are the pros and cons to using passive devices to
saturate an area to gather data?
Given their relatively long sampling times, passive samplers would only be useful
to determine likely peak 1-hour locations. In studies of pollution levels in Los
Angeles communities nearfreeways and ports, both the passive samplers and
mobile monitoring platform documented the sharp gradients in the vicinity of
freeways. The Ogawa saturation monitors for N02 had a precision of-1.5 ppb
(5%) for 1-week integrated samples, and comparison with collocated FRM
analyzers indicated good accuracy (m=1.0380 and r2=0.9905).
b. Likewise, what are the pros and cons to using non-passive devices, such as near
real-time or continuous devices including, but not limited to portable, non-FEM
chemiluminescence methods for NO2 or Gas Sensitive Semiconductors (GSSs)for
NO2 and other pollutants of interest?
No comment, not my area of expertise.
c Finally, what would be the pros and cons, to a state or local agency attempting to
use a specially outfitted vehicle to collect mobile measurements to assist in the
near-road site selection process for NO2 specifically as well as other pollutants of
interest?
The ARB has a mobile monitor that has proven to be a very useful tool to identify
near-source pollutant gradients. We use an electric vehicle to avoid self pollution
issues and to have a large enough battery to run all the equipment. The vehicle
and equipment cost are high, on the order of $250,000, and it takes expert staff to
set up and operate the mobile monitor.
6. EPA recognizes that CO concentrations are primarily influenced by gasoline
vehicles as opposed to NOi and PM2 5 concentrations, which are currently more
heavily influenced by heavy-duty (diesel) vehicle emissions If EPA were to
propose a new set of minimum monitoring requirements for CO near roads, the
near-road monitoring stations created under the implementation of the NOi
monitoring requirements may be an advantageous infrastructure for stale and
local air agencies to leverage However, EPA believes there are two issues not
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specifically considered in the near-road NO2 monitoring language that might
influence where near-road CO monitors may be most appropriately placed The
two issues are 1) the consideration of where light duty vehicles are operating
under 'cold-start' conditions, which may often not be on the larger artenals or
highways in an area, and 2) the impacts of light duty vehicle congestion and
idling in areas such as urban street canyons and/or urban cores
a Does the subcommittee believe that the light duty cold start and congestion
factors will significantly influence the location of peak CO concentrations in an
area? What priority should these factors be given when compared with the factors
(AADT, Fleet Mix, Roadway Design, Congestion Patterns, Terrain, and
Meteorology) already being considered for peak NOz?
b. Does the subcommittee have an opinion on whether, and possibly how, these two
issues of vehicles operating under cold start conditions and light duty vehicle
congestion and idling in urban street canyons and/or urban cores be considered
in a future, nationally applicable, CO monitoring proposal? Are there other
factors that may affect peak CO concentrations and not affect peak M?2
concentrations that should also be considered for any future CO monitoring
proposal?
The major factors in the location of peak CO levels are high light-duty vehicle
volumes, high fractions of older and poorly maintained vehicle (e.g., high
emitters), stagnant meteorology, and containment by nearby buildings (e.g., urban
street canyons). Parking garages can be a hot spot because of cold starts and
pollutant containment by the building, but the duration of high CO periods can be
short, and population exposure is generally low, so they should not be a priority
for a future national CO monitoring proposal. Rather a focus on high emitters in
populated areas (i.e., environmental justice communities) would be a useful
complement to the consideration of factors for high NO2 levels.
7 Does the committee believe that siting considerations for identifying the location
of peak NO2 concentrations will likely address all of the high priority siting
considerations for PM (particularly PM2 5) as well? If not, what other factors
should be considered and what are the advantages in considering these factors
for identifying the location of maximum PM concentration7
The near roadway data for California from multiple studies shows that PM2.5, even
in a coastal source area, is primarily from secondary formation. There was
surprisingly little variation throughout the communities monitored in special studies.
Several studies using ultrafine PM networks across a community showed large spatial
and temporal variations associated with proximity to fresh combustion products.
8 In addition to PM2 5 mass, what other PM-related measurements are desirable at
near-road monitoring stations (e g, UFP number, black carbon, EC/OC, PM
coarse, etc)7
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In California special studies, we found that different BC and PM measurement
methods have significant differences. They tend to agree qualitatively but not
quantitatively. CPC ultrafine PM measurements are sensitive to the effective size cuts
of the instruments and some instruments needed frequent calibrations. The different
methods for measuring EC and OC also yield different concentrations. If near-road
monitoring is going to be used for assessing compliance with air quality standards,
the methods need to be comparable (equivalent) to the reference method. If near-road
measurements are only to better understand the atmospheric processes and pollutant
relationships, then only a good QA program is required to be useful. The most critical
part of any near-road monitoring program will be the siting criteria for the
monitors/samplers as a small change in distance (or possibly height), orientation to
the high traffic volume direction during the light/calm winds in the morning
commute, and number of vehicles during the period of typically stable air (surface
inversion and light/calm wind) could all have a strong impact on the concentrations
measured.
9 To allow for near-road monitoring infrastructure to be multi-pollutant, and in
reflection of the recently promulgated near-road NO 2 siting criteria,
reconsideration of the existing microscale CO siting criteria presented in sections
2, 6.2, and table E-4 in 40 CFR Part 58 Appendix E may be warranted Does the
subcommittee believe that reconsideration of microscale CO siting criteria is
appropriate? Specifically, would an adjustment of CO siting criteria to match
those of microscale PM2 5 and microscale near-road NO2 sites be logical and
appropriate?
Yes, consistency seems warranted.
10 Even if the adjustment of microscale CO siting criteria in sections 2, 6.2, and
table E-4 in 40 CFR Part 58 Appendix E to match that of microscale PM2 5 and
microscale near-road NO2 is appropriate and proposed, should there be
consideration to maintain the requirement on how urban street canyon or urban
core microscale CO sites should be sited?
No comment, not my area of expertise.
11 Does the subcommittee have an opinion on how "urban street canyons" or
"urban core " might be defined, perhaps quantitatively, and with regard to use in
potential rule language?
No comment, not my area of expertise.
12. EPA and NACAA will select the locations for permanent sites that are part of the
near-road pilot study based on which state or locals volunteer to participate and
can process grant funds in a timely manner to deploy equipment From this pool
of volunteers, selection should be made on certain attributes that provide the best
potential to fulfill pilot study objectives In the attached draft white paper, EPA
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and NA CAA have proposed some potential criteria for consideration in selecting
where the fixed, permanent stations should be located. These considerations
include choosing a large and a relatively small urban area based on population,
an area with varied or complex terrain, an urban area with an operational NOx
analyzer representative of neighborhood or larger spatial scales for comparison
to the near-road NOx analyzer, and an urban area with a cooperative (or non-
cooperative) Department of Transportation Does the Subcommittee agree with
these considerations? Further, are there other considerations that should be
evaluated in selecting pilot cities to house permanent near-road monitoring
stations as part of the pilot study?
Because of the potential influence of high NC>2 emissions from retrofitted or post-
2007 HDDV on roadside levels, a freeway with high heavy-duty truck traffic volumes
should be the primary consideration in site selection. It is also important that an ozone
measurement be co-located with the neighborhood scale NOx analyzer so that total
oxidant levels can be determined. USEPA, ARE, and several research groups have
mobile monitors that could be deployed to quickly find the location of highest NO2.
13. EPA andNACAA have proposed that at least two urban areas should have
permanent near-road monitoring stations (that would fulfill NC>2 near-road
monitoring requirements) implemented for the pilot study. Please comment on the
minimum equipment/pollutant measurement complement that should be deployed
at each site and also the ideal equipment complement that each site should or
could have, respectively Specifically, what pollutants (e.g., NO2, NOx, NO, CO,
PM (Ultrafine, 2.5, and JO), black carbon, air toxics (such as benzene, toluene,
xylene, formaldehyde, acrolein, or 1,3-butadiene) and ammonia) and other
information should the pilot study measure or gather at the fixed, permanent
monitoring stations, and by what methods? This list should be in priority order,
as feasible, and can include any NAAQS or non-NAAQS pollutant by any method
(FRM/FEM and/or non-reference or equivalent methods), any particular type of
other equipment for gathering supporting data such as meteorology or traffic
counts
The list of pollutants is appropriate and in priority order, although air toxics could be
dropped to conserve resources. Meteorological parameters (wind speed, wind direct,
temperature) and light- and heavy-duty truck counts would also be useful.
14. EPA andNACAA have proposed four to five urban areas to have saturation
monitoring, using either passive devices and/or continuous/semi-continuous
saturation type multi-pollutant monitoring packages (i e, several types of
monitors in one mountable or deployable "package ") Please provide comment
on.
a The pollutants that should be measured with the saturation devices at each
saturation site
b The number of saturation devices per pollutant, both passive and/or
continuous/semi-continuous, that may be deployed in each pilot city
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c. Whether placing saturation monitoring devices near certain road segments
should include, at a minimum 1) the highest AADT segment in an area, 2) the
road segment with the highest number of heavy-duty truck/bus counts, 3) at a
road segment with more unique roadway design, congestion pattern, or terrain in
the area, and 4) if feasible, at a lower AADT segment with a similar fleet mix,
roadway design, congestion, terrain, and meteorology as the top AADT road
segment in the area.
I think mobile monitoring measurements are much more useful, and more relevant
to the averaging time of the NO2 standard. EPA staff can look at the details of
ARB's year-long saturation monitoring study in the communities downwind of
the Ports of Los Angeles and Long Beach for information on methods, quality
assurance, data analysis techniques, and results that may be useful in designing
the pilot study, (http://www.arb.ca.gov/research/mobile/hcrn/sat-rnon/sat-
mon.htm)
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Dr. Kenneth Demerjian
Charge Questions
Questions regarding the near-road monitoring guidance document
1. The accompanying draft guidance document outline provides an initial thought of
the major topics required in the near-road monitoring guidance that will aid state
monitoring agencies in the identification and implementation of NO2 near road
monitoring sites from a multi-pollutant perspective. Please comment on the
overall content of the recommended topics in the draft outline. Please provide
suggestions on any missing subjects that should be included in the guidance
document and any unnecessary topics that are currently listed in the attached
draft, if applicable.
2. EPA and NACAA envision the near-road guidance document to be written from
a multi-pollutant perspective. What pollutants and sub-species does the
subcommittee believe should be included for consideration and discussion in the
near-road monitoring guidance? Some potential species for consideration include
NO2, NOX, NO, CO, PM (Ultrafine, 2.5, and 10), black carbon, air toxics (e.g.,
benzene, toluene, xylene, formaldehyde, acrolein, or 1, 3, butadiene), and
ammonia Please prioritize the recommended pollutants and provide the rationale
for their ranking, including how this pollutant measurement will contribute to
scientific and regulatory knowledge of near-road air quality and adverse human
health effects. Tier I - NO2, NOX, NO, CO, CO2, SO2, EC/OC, BTEX aerosol size
distribution and total number concentration for routine near-road monitoring. Tier
II - PM organics (HOA, OOA), NH3, HONO, H2CO and 1,3 - butadiene.
3. Identifying Candidate Near-road Site Areas
a. AADT & Fleet Mix - To consider fleet mix with regard to NO2, an idea is
to encourage states that have fleet mix information to take an approach
that uses average, fleet-wide grams per mile emissions estimates (one for
light duty vehicles and one for heavy duty vehicles), combined with
AADT information to further weight which road segments in an area may
be more conducive to produce peak pollutant concentrations. EPA would
use the latest emission factor information to aid such a calculation. Given
the variability in emission rates from on-road vehicles based on vehicle
technology, fuel, speed, environmental conditions, etc., does the
subcommittee believe this approach is an appropriate way to "consider"
fleet mix in near-road site selection or is a more refined inventory and
modeling analysis required? First cut at identifying potential near-road
monitoring sites should be to consider the application of CIS methods for
traffic exposure. These and other methods were recently reviewed by HEI,
Traffic-Related Air Pollution: A Critical Review of the Literature on
Emissions, Exposure, and Health Effects, Special Report 17, January,
2010.
b. AADT & Fleet Mix - Further, should the suggested approach above in
question 4a to consider fleet mix via the use of average, fleet-wide
emission factors, or the use of inventory and modeling analysis, take into
account mobile source controls that are "on the books" but have not yet
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been fully realized due to fleet turnover? If so, how far out into the future
should states consider their effects?
c. Roadway Design - Studies suggest and support the concept that roadway
design influences pollutant dispersion near the road. The EPA suggests
establishing sites at-grade with the road, without any nearby obstructions
to air flow; however, the Agency recognizes that this might not always be
feasible. Does the subcommittee agree with this recommendation for
locating sites at-grade with no obstructions? What priority should be
placed on this factor within the guidance, given the need for flexibility in
identifying appropriate site locations? The choice of sites with minimal
roadway design influences affect pollutant dispersion should be a priority.
but not an absolute requirement. Microenvironments with high pollutant
exposures near local neighborhoods should be considered and will likely
reflect some combination of terrain, road grade, traffic volume and
congestion influences.
d. Congestion Patterns - The congestion of a roadway can be estimated by
the metric "Level of Service" (LOS). LOS uses a letter grade from A to F
to identify a roadway's performance, with "A" the best conditions where
traffic flows at or above the posted speed limit and all motorists have
complete mobility between lanes to "F" the worst congestion where travel
time cannot be predicted and generally traffic demand exceeds the
facility's capacity. Since motor vehicles generally emit more pollutants
during congestion operations (although noting that NOX and select other
pollutant emissions can also increase with increasing speed), how
important a parameter should LOS be in the determination of appropriate
near-road monitoring sites? Does the subcommittee have a view on how
reliable LOS estimates are across the country? I have no firsthand
knowledge on estimating LOS, but it seems that there are likely many
innovative approaches that could be offered up by DOT and the traffic
engineering community who deal with congestion mitigation issues on a
daily basis. Discussions with this community regarding estimating LOS
should be EPA's first priority.
e. Terrain- State and local air agencies are required to consider terrain in the
near-road monitoring site selection process, which in some cases may be
inherently part of the roadway design. However, EPA recognizes that
some states and local air agencies may have to make selections from
amongst similar candidate sites that differ only by terrain, e.g. cut section
versus open terrain, with or without vegetation, etc. Does the
subcommittee agree that terrain and vegetation should be a consideration
in the siting process? What priority should this parameter have in the
overall process? Terrain and vegetation should be considered and
documented as part of the siting process, but should not be a major priority
factor in the selection process.
f. Meteorology - EPA took comment on, but did not Finalize the requirement
for near-road monitoring sites to be climatologically downwind of the
target road segment. Reasons were because the additional limitations this
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would introduce in finding candidate sites would be in exchange for what
may be a small increase in the opportunity to monitor peak NCh
concentrations. Further, with sites being within 50 meters of target road
segments, the phenomenon of upwind meandering (pollutant transport
upwind due to vehicle induced turbulence) further reduces that absolute
need to be climatologically downwind. Finally, EPA recognized that,
logically, the potential for peak NC>2 concentration may very well occur
when winds are calm or parallel (or nearly parallel) to the target road,
allowing for pollutant build-up, as opposed to when winds are normal to
the road. Although there is no requirement to be downwind, in the
preamble to final NC>2 NAAQS rule, EPA encouraged it when possible.
EPA and NACAA intend to do the same in the guidance document. Does
the subcommittee agree with this approach? The siting of monitors in local
neighborhoods in the proximity of major roadways (i.e., <500 meters) is
more important than its placement at a location that is climatologically
downwind. That being said, if EPA wants to address the influence of
climatology on monitor siting, the application of traditional climatological
line source models would be the starting point.
4. Modeling is another tool that may be useful in the identification of candidate
near-road sites. In particular, the use of mobile source emissions modeling with
MOVES and local-scale dispersion modeling with AERMOD, can be presented as
part of the guidance document. Please comment on the available modeling tools,
and their pros and cons, that the subcommittee believes may be appropriate to
discuss and/or recommend for use in the near-road monitoring guidance
document. As mentioned above the application of traditional line source models to
address siting issues with respect to climatology is fairly straight forward. The
application of more sophisticated emissions and exposure models does not seem
necessary to address the climatology issue.
5. In regard to the process of identifying candidate near-road monitoring sites,
beyond the evaluation of factors noted above in question 3, and the potential use
of modeling, the use of saturation monitoring and on-road monitoring are also
possible tools that state and local air agencies may choose to utilize in the near-
road site selection process.
a. If a state were inclined to use saturation monitoring to aid in the selection
of a near-road monitoring site, and considering that the NC>2 standard is a
1-hour daily maximum standard, what are the pros and cons to using
passive devices to saturate an area to gather data? Saturation monitoring
for NOz is temporally limited to 24 hr averages. The diurnal pattern of
NC>2 varies with season and max 1 hr averages can occur at mid-morning
and mid-afternoon depending on season. Spatial mapping of NO2 (and
other) concentrations with fast response monitoring technologies (e.g.
QCL multipath IR spectroscopy) can provide significant insights to near
road exposures in local neighborhoods.
b. Likewise, what are the pros and cons to using non-passive devices, such as
near real-time or continuous devices including, but not limited to portable,
non-FEM chemiluminescence methods for NO: or Gas Sensitive
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Semiconductors (GSSs) for NOi and other pollutants of interest? QCL
multipath IR spectroscopy have been demonstrated for NO2, HONO,
fhCO, CO. and 1,3-butadiene and have been operated from mobile
platforms providing spatial mapping or gradient measurement associated
with fixed site monitoring.
c. Finally, what would be the pros and cons, to a state or local agency
attempting to use a specially outfitted vehicle to collect mobile
measurements to assist in the near-road site selection process for NC>2
specifically as well as other pollutants of interest? This is the method of
choice, but the availability of specially outfitted mobile measurement
platforms is limited, as are the dollars to support such measurements.
Questions regarding the CO monitoring network and near-road monitoring
6. EPA recognizes that CO concentrations are primarily influenced by gasoline
vehicles as opposed to NOj and PMj 5 concentrations, which are currently more
heavily influenced by heavy-duty (diesel) vehicle emissions. If EPA were to
propose a new set of minimum monitoring requirements for CO near roads, the
near-road monitoring stations created under the implementation of the NO2
monitoring requirements may be an advantageous infrastructure for state and local
air agencies to leverage. However, EPA believes there are two issues not
specifically considered in the near-road NOi monitoring language that might
influence where near-road CO monitors may be most appropriately placed. The
two issues are 1) the consideration of where light duty vehicles are operating
under 'cold-start' conditions, which may often not be on the larger arterials or
highways in an area, and 2) the impacts of light duty vehicle congestion and idling
in areas such as urban street canyons and/or urban cores.
a. Does the subcommittee believe that the light duty cold start and
congestion factors will significantly influence the location of peak CO
concentrations in an area? What priority should these factors be given
when compared with the factors (AADT, Fleet Mix, Roadway Design,
Congestion Patterns, Terrain, and Meteorology) already being considered
for peak NOi? The spatial distribution of cold start vehicles associated
with urban commuting is in general broad and short term and their
contribution to emissions associated with major highways adjacent to
residential neighborhoods is likely small. That being said, at least one
exception comes to mind. The departure of motor vehicles from major
entertainment events (e.g. a football or baseball stadium) where 20-30K
vehicles may be simultaneously started and caught in congestion for 10s
of minutes to an hour or more. The cold start contribution, is again limited
in time but could contribute significantly as an emissions hot-spot
impacting commuter exposes and concentrations in nearby neighborhoods.
b. Does the subcommittee have an opinion on whether, and possibly how,
these two issues of vehicles operating under cold start conditions and light
duty vehicle congestion and idling in urban street canyons and/or urban
cores be considered in a future, nationally applicable, CO monitoring
proposal? Are there other factors that may affect peak CO concentrations
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and not affect peak NC>2 concentrations that should also be considered for
any future CO monitoring proposal? The near-road NC>2 concentrations
are closely tied to secondary reactions with urban ozone concentrations
and entrainment processes into highway line source NOx plumes. This in
part, contributes to seasonal differences in near-road NOz concentrations
and its fractional contribution to NOx. Other factors affecting near-road
NO2 monitoring is the distribution of gasoline and diesel vehicles.
Example data analyses depicting the effects of these factors on NO2
measurements are available upon request.
Questions regarding the PM monitoring network and near-road monitoring
7. Does the committee believe that siting considerations for identifying the location
of peakNO2 concentrations will likely address all of the high priority siting
considerations for PM (particularly Plvh 5) as well? If not, what other factors
should be considered and what are the advantages in considering these factors for
identifying the location of maximum PM concentration? Monitoring of number
concentration of ultrafine particles has spatial and temporal characteristics that do
not sync all that well with that of NO2. But it remains to be seen if the health
community can make the case for health outcomes for particles <100nm.
8. In addition to PM2 5 mass, what other PM-related measurements are desirable at
near-road monitoring stations (e.g., UFP number, black carbon, EC/OC, PM
coarse, etc.)? Mobile measurement platforms are capable of performing fast
response measurements (<1 minute) of key primary emission components (in
addition to NO2) of interest to health effects community. These include aerosol
size distribution, EC, PM organics (HOA, OOA), NH3, HONO, H2CO, CO, CO2
and 1,3 - butadiene.
Questions regarding the monitor siting criteria for microscale CO, microscale PMis,
and the new near-road NO2 siting criteria
9. To allow for near-road monitoring infrastructure to be multi-pollutant, and in
reflection of the recently promulgated near-road NO2 siting criteria,
reconsideration of the existing microscale CO siting criteria presented in sections
2, 6.2, and table E-4 in 40 CFR Part 58 Appendix E may be warranted. Does the
subcommittee believe that reconsideration of microscale CO siting criteria is
appropriate? Specifically, would an adjustment of CO siting criteria to match
those of microscale PM25 and microscale near-road NO2 sites be logical and
appropriate? The CO siting criteria should be adjusted to match those of
microscale PM2.5 and near-road NO2 so there is consistency in the near-road
multi-pollutant monitoring infrastructure.
10. Even if the adjustment of microscale CO siting criteria in sections 2, 6.2, and
table E-4 in 40 CFR Part 58 Appendix E to match that of microscale PM2 5 and
microscale near-road NO2 is appropriate and proposed, should there be
consideration to maintain the requirement on how urban street canyon or urban
core microscale CO sites should be sited? Carry over of the urban street canyon or
urban core microscale CO siting requirements should be done, keeping in mind
the multi-pollutant consistency requirements mentioned in question (9).
11. Does the subcommittee have an opinion on how "urban street canyons" or "urban
core" might be defined, perhaps quantitatively, and with regard to use in potential
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rule language? Consider emission density and spatial volume defining the urban
street canyon and in the case of the urban core consider emission density and
temporal persistence.
12. EPA and NACAA will select the locations for permanent sites that are part of the
near-road pilot study based on which state or locals volunteer to participate and
can process grant funds in a timely manner to deploy equipment. From this pool
of volunteers, selection should be made on certain attributes that provide the best
potential to fulfill pilot study objectives. In the attached draft white paper, EPA
and NACAA have proposed some potential criteria for consideration in selecting
where the fixed, permanent stations should be located. These considerations
include choosing a large and a relatively small urban area based on population, an
area with varied or complex terrain, an urban area with an operational NOx
analyzer representative of neighborhood or larger spatial scales for comparison to
the near-road NOx analyzer, and an urban area with a cooperative (or non-
cooperative) Department of Transportation. Does the Subcommittee agree with
these considerations? Further, are there other considerations that should be
evaluated in selecting pilot cities to house permanent near-road monitoring
stations as part of the pilot study? EPA should competitively fund several
extramurally pilot studies in conjunction with matching state environmental
monitoring funds to address this question.
13. EPA and NACAA have proposed that at least two urban areas should have
permanent near-road monitoring stations (that would fulfill NO2 near-road
monitoring requirements) implemented for the pilot study. Please comment on the
minimum equipment/pollutant measurement complement that should be deployed
at each site and also the ideal equipment complement that each site should or
could have, respectively. Specifically, what pollutants (e.g., NO2, NOx, NO, CO,
PM (Ultrafine, 2.5, and 10), black carbon, air toxics (such as benzene, toluene,
xylene, formaldehyde, acrolein, or 1,3, butadiene) and ammonia) and other
information should the pilot study measure or gather at the fixed, permanent
monitoring stations, and by what methods? This list should be in priority order, as
feasible, and can include any NAAQS or non-NAAQS pollutant by any method
(FRM/FEM and/or non-reference or equivalent methods), any particular type of
other equipment for gathering supporting data such as meteorology or traffic
counts. Optimal deployment would consider one permanent near-road monitor
station with a mobile measurement platform. The minimum complement of
measurement parameters at the fixed site would include NO2, NOx, NO, CO, CO2,
aerosol size distribution and total number concentration, PM2.5 and PM10 mass,
EC/OC, BTEX and ammonia. The minimum complement of measurement
parameters for the mobile platform include aerosol size distribution, EC, PM
organics (HOA, OOA), BTEX, NH3, HONO, H2CO, CO, CO2 and 1,3 -
butadiene.
14. EPA and NACAA have proposed four to five urban areas to have saturation
monitoring, using either passive devices and/or continuous/semi-continuous
saturation type multi-pollutant monitoring packages (i.e., several types of
monitors in one mountable or deployable "package"). Please provide comment
on: I am not convinced this is a viable method. The only passive devices deployed
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systematically for saturation monitoring have been Ogawa badges forNCh. Jury is
still out on saturation monitoring using battery operated PM samplers. With some
R&D investment in wireless unmanned saturation sensors/samplers, routine
saturation monitoring could be in the future. The proposal to use of current
passive measurement devices is less than inspiring.
a. The pollutants that should be measured with the saturation devices at each
saturation site.
b. The number of saturation devices per pollutant, both passive and/or
continuous/semi-continuous, that may be deployed in each pilot city.
c. Whether placing saturation monitoring devices near certain road segments
should include, at a minimum: 1) the highest AADT segment in an area, 2)
the road segment with the highest number of heavy-duty truck/bus counts,
3) at a road segment with more unique roadway design, congestion
pattern, or terrain in the area, and 4) if feasible, at a lower AADT segment
with a similar fleet mix, roadway design, congestion, terrain, and
meteorology as the top AADT road segment in the area.
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Dr. Delbert Eatough
The nature of the advice which AAMMS might give to EPA on the Charge Questions
outlined in the material provided to the committee will be dependent on the objectives of
the multiple pollutants studies to be conducted as part of the near road monitoring
program put in place in response to monitoring requirements outlined in the new NAAQS
for NO2 released in February of this year (EPA CFR Parts 50 and 58, 2010).
As stated by EPA in the Charge Questions document provided to AAMMS:
Purpose of the Advisory
EPA is seeking CASAC advice on the concepts and information that should be
included in the forthcoming near-road monitoring guidance document, advice on
how future near-road monitoring requirements, for pollutants such as Carbon
Monoxide (CO) and Particulate Matter (PM), may be drafted in a way to mesh with
the existing Nitrogen Dioxide (NCh) requirements and foster a multi-pollutant
monitoring infrastructure, and the objectives, approach, and execution of the near-
road monitoring pilot study.
This is a bold new direction being taken by EPA which will move the concept of multi-
pollutant monitoring in support of Clean Air objectives forward in a significant way.
While reasonable detail is given in the charge questions related to the approach to be used
and execution of the near-road monitoring pilot study, little detail is given on the
scientific objectives of both the pilot study and the near-road monitoring program which
is required under the NO2 NAAQS. I will start these comments by framing some of my
thoughts on the possible objectives of this program, with the belief that the nature of the
advice which might be given in very dependent on the identified objectives. I should
emphasis that the literature cited in my comments is illustrative only and not intended to
be a complete review of what is currently know.
I. Objectives of the Near-Road Monitoring Program.
A. NO2 Monitoring Time Scale.
As outlined by EPA, the advice must consider the near-road monitoring requirements of
the NAAQS forNO2, which is to have ambient monitoring conducted at the location of
maximum NO2 concentrations in an area, which at a minimum is directly attributable to
mobile source emissions. While not explicitly stated in the charge questions, I assume
that this means that the monitoring to be conducted will be focused on the 1 -hour time
period requirement of the NAAQS. This is an important point because the information
one can gain from the near-road monitoring program is dependent on the time period
chosen for study. For example, 1-hour average monitoring allows the identification of
the effects of many diurnal variations which the identification of such details as diurnal
changes in sources and atmospheric processes (Eatough, 2008). However, there are
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important processes which occur on a much shorter time period which will not be as well
identified in the program (Zhu 2002a, b). My comments here are limited to
considerations for a 1 -hour average monitoring program.
B. NOi Monitoring Objectives Other than Identification of Maximum
Concentrations.
If we were discussing the identification of the maximum concentration of TSP Pb, the
sampling objectives would be relatively straightforward because particulate Pb is a
species which is expected to be relatively conserved in the atmosphere after emission.
However, the story is much more complex with respect to NO2 if one wants to know, in
addition to identification of maximum concentrations, the atmospheric factors which
contributed to the maximum concentrations observed and the effect of the NO2 formation
pathways on other NAAQS pollutants such a ozone, or the effect of ambient ozone on the
formation of NO2.
The great majority of primary nitrogen oxides are currently emitted from combustion
sources in the form of NO (g) (Finlayson-Pitts, 2000). In the presence of ozone, HO2- or
RO2-, NO is oxidized to NO2,
NO (g) + 03 (g) - N02 (g) + 02 (g) (1)
NO (g) + H02- (g) -* N02 (g) + OH- (g) (2)
NO (g) + R02 - (g) -» N02 (g) + RO- (g) (3)
Ozone, in turn is formed from the photolysis of NO2, OH- from the photolysis of Os and
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HO2- and RO2- from the reaction of OH- (and at night NOs-) radicals with gas phase
organic compounds. Thus, the relative concentrations of NOx and gas phase organic
compounds control the concentration of ozone in a complex manner described by ozone
isopleths (Finlayson-Pitts 2000), and, in turn, these processes control the concentrations
of NO2 In general, at low NOx concentrations the concentration of ozone is little
effected by the concentration of VOC and the system is NOx limited. However, at low
concentrations of VOC, the concentrations of ozone can decrease with increasing NOx
concentrations as NO reacts with ozone and NO2 competes with VOC for the OH- radical
by the irreversible formation of nitric acid,
N02 (g) + OH- (g) -> HN03 (g) (4)
Concentrations of NO2 observed at a site will be effected by this complex chemistry.
Complete understanding of the etiology of NO2 concentrations identified in a near-road
monitoring program will require the identification of each of these factors (Kuprov
2010). I have assumed that understanding these chemical contributions to the observed
NO2 concentrations will be one of the monitoring objectives.
C. Multi-Pollutant Monitoring Objectives.
The multi-pollutant monitoring portion of the near-road monitoring program plan being
developed for the NO2 requirements could have several objectives:
$ Identification of concentrations of other NAAQS pollutants which accompany the
observed NO2 concentrations.
$ Identification of the contribution these and other key pollutants make to the
observed NO2 concentrations (see my comments in B.)
$ Identification of the atmospheric processes which contribute to the observed
concentrations of NO2 and the other monitored pollutants (e.g. Kuprov, 2010;
Wilson 1977).
$ Identification of the sources which contribute to both measured concentrations of
NO2 and the other measured NAAQS pollutants (e.g. Eatough 2008).
My comments assume that meeting all of these objectives is important in the design of
the program. Another possible objective of the monitoring program is the identification
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of toxic compounds to inform health studies. While this is an important objective, it is
somewhat less directly related to the above and I have not given this objective high
priority.
I have not attempted to frame an individual response here to all charge questions as there
are some where I am not an expert. The charge questions are given in the consensus
report.
II. Response to the Charge Questions.
Charge Question 1.
1 suggest the following are areas where the Guidance Document may need strengthening
above what I think is intended in the outline:
• Introduction: Based on the material in the Pilot Study draft, it is not clear
that EPA has yet identified the reasons (scientific objectives) for the multi-
pollutant monitoring. I have discussed this issue in l.C. I recommend that EPA
decide which of the scientific objectives outlined there are included in its vision (I
have indicated I think all should be) and discuss fully these objectives and the
scientific basis for these objectives in the Introduction. This will, in turn, support
the selection of the recommended pollutants to be monitored in the program.
• Background. The background should also contain the scientific basis for
the multi-pollutant objectives, as outlined in the bullet above.
• Identifying Candidate Near-road Site Areas. I believe the criteria outlined
for this section are adequate for the identification of a site where maximum NC>2
concentrations near a given near-road site may be determined. However, since
NO2 is a secondary pollutant and its concentrations will be effected by both
emissions from the roadway and from any other nearby elevated sources (VOC,
ozone, etc.). These factors should be considered in the site identification process.
• Modeling. 1 am not an expert in this area. However, modeling should
take into account the factors I have discussed above.
• Monitoring. One potential problem with saturation sampling is that if all
data are not collected under identical conditions, certainly with respect to time
and traffic flow, the comparison of results for the various saturation samplers may
not be meaningful. I am also concerned about this issue in connection with the
use of mobile monitoring. How will assurance be obtained that a comparison of
measurements at two locations at different times gives the same result as a
comparison of measurements at two different sites at the same time. How will
diurnal variability be taken into account? These issues are discussed in the
consensus report.
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• Near-road Site Selection. The items outlined here seem reasonable.
• Recommended Near-road Site Documentation. The adequacy of this
section will be dependent on the adequacy of the EPA objectives for the Pilot
Study in addressing the appropriate multi-pollutant monitoring objectives. This
will be an area discussed under other Charge Questions.
Charge Question 2.
I have suggested in I.C. objectives that should be part of the multi-pollutant monitoring
scheme. My thoughts on species which should be included to meet each of these
scientific objectives (the objectives are repeated here) are given below. Again, all these
measurements need to be made on a one-hour time basis. It is also recognized that most
of these measurements will not be made at all sites but only at the limited number of
advanced sites as discussed in the consensus document.
• Identification of concentrations of other NAAQS pollutants which accompany the
observed NC>2 concentrations. (CO, PM/o, PM2S, Ozone and {probably for some,
but not all sites} sulfur dioxide)
• Identification of the contribution these and other key pollutants make to the
observed NC>2 concentrations (see my comments in B.) (VOC related to ozone
formation, NOx, NOy {including a minimum of gas and paniculate nitrate in
addition to NOx}).
• Identification of the atmospheric processes which contribute to the observed
concentrations of NC»2 and the other monitored pollutants (e.g. Kuprov, 2010,
Wilson 1977). (The species listed in the two proceeding bullets.)
• Identification of the sources which contribute to both measured concentrations of
NC«2 and the other measured NAAQS pollutants (e.g. Eatough 2008). (Fine
paniculate OC and EC, BC and UV C In addition techniques are now becoming
available for the hourly measurement of fine paniculate elements and organic
markers on an hourly basis These last two measurements would be lower
priority, but where they can be measured would greatly add to meeting this
objective)
I have not listed any of the toxic gases included in the charge question, but they are
relevant to health objectives and might be added if EPA wants to add an objective for this
specific purpose. That is a little different than the atmospheric chemistry objectives on
which I have focused.
Charge Question 5.
Saturation monitoring can aid greatly in the identification of a suitable near-road
monitoring site. My only concern with respect to this Charge Question is that it be made
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clear that the saturation monitoring must meet two key objectives:
• The data must be available on a one-hour average basis, consistent with NAQQS
requirement for NC>2.
• All saturation data must be obtained at all locations on the same time basis so the
results are not significantly confounded by the diurnal and seasonal variations in
NC>2 emissions and formation chemistry.
Because of the inherent problems in meeting the items in the above two bullets, care
needs to be taken in the use of an outfitted vehicle to assist in the road-site selection
process.
Charge Question 7.
While peak concentrations of ultrafine particles will frequently be associated with
emissions from vehicles, the concentrations of PM25 will not. In almost all urban studies
I am aware of, the maximum concentration of PM25 are not dominated by primary
emissions but the secondary formation of nitrate and organic material, and in the east by
regional sulfate. None of these contributions can be elucidated from near-road
monitoring. However, the total pollutants suggested here to be monitored in the program
can inform the secondary formation processes which lead to these elevated PM
concentrations.
Charge Question 8.
I have outlined my thoughts on this charge question in the response to Charge Question 2.
Charge Question 13.
I have listed my thoughts on equipment needed as outlined in this Charge question in my
response to Charge Question 2, with an indication of contributions to be expected for
each measurement. The relative priority which might be assigned depends on whether or
not EPA agrees with my outline of objectives in Section I. It is recognized that these
measurements will be made at a limited number of sites. My priority order and suggested
measurements are:
1. Top priority (measurement of NO2 and NAAQS pollutants):
• N02, hourly averaged data by an artifact free measurement as well as an
FRM or FEM technique.
• Ozone, hourly averaged data by an FRJvl or FEM technique.
• PM 10 and PM2.5 by a dichot FDMS TEOM method (to avoid the loss of
volatile material).
2. Second priority (measurement of species which will inform NC>2 chemistry.
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• VOC, hourly averaged data.
• NOX, hourly averaged data by an FRM or FEM technique.
• Ozone hourly averaged data by an FRM or FEM technique (also listed in
1.).
• NOY hourly averaged data.
• Nitric acid and particulate phase nitrate, hourly averaged by an 1C
technique (e.g., the URG AIM) where nitrate is known to be high, e.g. LA
or western mountain valleys.
3. Third priority (data to aid in source apportionment, including separation of
gasoline and diesel vehicle contributions).
• Hourly average EC and OC, preferable by a Sunset dual oven instrument.
• BC and UV hourly average Aethalometer data.
• Hourly averaged fine particulate elemental and trace organic marker data.
Charge Question 14.
First a general comment. The saturation studies are intended to aid in the
identification of near-road sites which will give maximum NC>2 concentrations.
These then will become the site(s) which are used to meet the NC>2 near-road
monitoring requirements. To meet this requirement the key data each saturation
study must provide are hourly average NO2 concentrations which define at least a
couple of weeks diurnal variation in the NO2 concentrations. Less than hourly and
less than complete diurnal coverage will not truly inform on maximum
concentrations. I am not certain that a passive device can meet this need, so 1 assume
a semi-continuous device or a modified passive sampler (see the consensus report)
would be used.
a. NO2
b. I would think 4 to 6 is a reasonable number, but 1 defer to others who have
conducted saturation studies.
c. Of the criteria listed in the charge question, 1) and 2) seem most important. I
would also pick a site where impact from VOCs nearby is important.
REFERENCES
Eatough, D.J., Grover, B.D., Woolwine, W.R., Eatough, N.L., Long, R., and Farber,
R. (2008). "Source Apportionment of 1-hr Semi-Continuous Data from Riverside
During SOAR 2005 Using Positive Matrix Factorization." Atmos. Environ. 42: 2706-
2719.
EPA (2010). 40 CFR Parts 50 and 58, Primary National Ambient Air Quality
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Standards for Nitrogen Dioxide; Final Rule, Federal Register, February 9, 2010, pp
6474-6537.
Finlayson-Pitts B.J. and Pitts, J.N. Jr. (2000) Chemistry of the Upper and Lower
Atmosphere Theory, Experiments and Applications, Academic Press.
Kuprov R., Eatough D.J., Hansen J.C., Cruickshank T. And Olson N. (2010)
Composition and Secondary Formation of Fine paniculate Material in the Salt Lake
Valley: Winter 2009, J Air & Waste manage. Assoc , in press.
Wilson, W.E., Spiller, L.L., Ellestad, T.G., Lamothe, P.J., Dzubay, T.D., Stevens,
R.K., Macias, E.S., Fletcher, R.A., Husar, J.D., Husar, R.B., Whitbe, K.T., Kittelson,
D.B. and Cantrell, B.K. (1977) "General Motors sulfate Dispersion Experiment:
Summary of EPA Measurements." J. Air Pollut. Cont. Assoc 27: 46-51.
Zhu, Y., Hinds, W.C., Kim, S., and Sioutas C. (2002a) "Concentration and Size
Distribution of Ultrafine Particles Near a Major Highway." J Air & Waste Manage
Assoc 52: 1032-1042.
Zhu, Y., Hinds, W.C., Kim, S., Shen S., and Sioutas C. (2002b) "Study of Ultrafine
Particles Near a Major Highway with Heavy-duty Diesel Traffic." At Environ. 36:
4323-4335.
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Mr. Dirk Felton
Questions regarding the near-road monitoring guidance document
\. Please comment on the overall content of the recommended topics in the
draft outline. Please provide suggestions on any missing subjects that should
be included in the guidance document and any unnecessary topics that are
currently listed in the attached draft, if applicable.
The background section should include language suggesting that finding one near-road
location where all pollutants of interest have the highest concentrations is not likely and
compromises will be necessary and acceptable. For example, NO2 concentrations may
well be higher in dense urban neighborhoods away from well ventilated busy highways.
This section should also include a discussion of the limitations of the various monitoring
methods in the near-road environment. The PM-2.5 FRM has demonstrated poor capture
efficiency for volatile fresh emissions from mobile sources and CO monitoring for a
health based NAAQS near roadways may not be warranted.
Modeling and saturation or mobile monitoring should not be required in the site selection
process. This type of work is beyond the capacity of many monitoring agencies, and
since these methods have not been uniformly demonstrated or well documented, they are
not likely to provide much assistance. If monitoring agencies have existing information
that could provide this type of information, they certainly should consider the information
and make it available to their Regional EPA office during the site approval process.
The last section on site documentation should not be burdensome to the monitoring
agencies. Much of the "NCore type" of site characterization documentation was
designed so that EPA staff would have a convenient way to review monitor siting. If this
type of information is important to the EPA then they can collect the data when they visit
the sites.
2. EPA and NACAA envision the near-road guidance document to be written
from a multi-pollutant perspective. What pollutants and sub-species does
the subcommittee believe should be included for consideration and
discussion in the near-road monitoring guidance? Some potential species for
consideration include NO2, NOX, NO, CO, PM (Ultrafme, 2.5, and 10), black
carbon, air toxics (e.g., benzene, toluene, xylene, formaldehyde, acrolein, or
1, 3, butadiene), and ammonia. Please prioritize the recommended
pollutants and provide the rationale for their ranking, including how this
pollutant measurement will contribute to scientific and regulatory
knowledge of near-road air quality and adverse human health effects.
The discussion of potential species for consideration in the near road monitoring
guidance is complicated by the currently available methods for some of these pollutants.
PM and specifically PM-2.5 would certainly be considered to be a candidate but the EPA
does not have an acceptable method for measuring PM-2.5 in the near-road environment.
The EPA should consider the development of a suitable method for PM-2.5 to be of the
utmost necessity.
CO monitoring is recommended and the siting requirements should be modified to be
identical to the near road NO2 requirements. CO may or may not be found at high
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enough concentrations to be a health issue, but it will be useful as a tracer of primary
emissions.
Ultrafine monitoring is recommended but an appropriate minimum and maximum size
range must be considered. Ultrafine particles have been found to affect health and data
from near road sites will assist in the understanding of how mobile sources interact with
ambient ultrafine levels.
BC is a combustion tracer so it should only be contemplated for installation at a near road
site if there is another BC instrument within the same CBSA. Since the NATTs program
requires BC, it makes sense to only install these instruments in the near road sites that
also have a NATTs site. This limited deployment should provide some information on
the gradient of BC between the near road environment and a central monitor.
This program should include air toxics that have been identified as known human
carcinogens. This would include benzene, formaldehyde and 1,3- butadiene. Air toxics
monitoring at the near road air sites should be encouraged in the CBSAs that also have a
NATTs site. This would allow for comparisons between the near road and community
scale monitors. Over time, the changes in vehicle technology and fuels will continue to
impact concentrations of these air toxics which large segments of the population are
exposed to daily. Currently, concentrations of these specific air toxics already exceed
acceptable concentrations in many areas of the country. A critical evaluation of the
impacts of these changes on the concentration of these air toxics in urban areas across the
U.S. is extremely important.
3. Identifying Candidate Near-road Site Areas
a. AADT & Fleet Mix - does the subcommittee believe this approach is
an appropriate way to "consider" fleet mix in near-road site selection
or is a more refined inventory and modeling analysis required?
Monitoring agencies should be permitted to use the best available information in order to
help with site selection. There are too many disadvantages to specifying a one size fits
all standard approach to site selection. Each CBSA has its own set of variables such as
roadway restrictions on vehicle type or vehicle type at certain hours of the day, tolls that
vary as a function of vehicle type, bridge restrictions, weather conditions such as cold
weather or high winds and differences in required vehicle emission controls.
Additionally, congestion in dense urban areas with numerous roadways in close
proximity to one another can lead to higher NC>2 concentrations than AADT and fleet mix
would indicate.
Some monitoring agencies also have very good inventories and existing modeling work
done for specific CBSAs and not as much information for others. The guidance
document should encourage the monitoring agencies to utilize all available tools and
sources of information to help select the most appropriate monitoring locations.
b. AADT & Fleet Mix - Further, should the suggested approach above
in question 4a to consider fleet mix via the use of average, fleet-wide
emission factors, or the use of inventory and modeling analysis, take
into account mobile source controls that are "on the books" but have
not yet been fully realized due to fleet turnover? If so, how far out into
the future should states consider their effects?
Ambient monitoring is concerned with the current exposure to the population from
sources of pollutants. Fleet turnover in most cases is a long term process especially for
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the largest trucks unless there is a specific program in place to help replace these vehicles
more quickly. The EPA has the ability to review monitor siting once a year when the
monitoring agencies submit their annual network plan. This should provide ample
opportunity for the EPA to suggest changes to the monitoring network in light of changes
in AADT and fleet turnover.
c. Roadway Design - Does the subcommittee agree with this
recommendation for locating sites at-grade with no obstructions?
What priority should be placed on this factor within the guidance,
given the need for flexibility in identifying appropriate site locations?
The design of the roadway is extremely significant and it makes sense to recommend that
the preferred installation be at grade with no obstructions. That said, the guidance
document should expand on why this is important and where low or high concentrations
are likely to be found. Low concentrations may be found adjacent to roadways elevated
on piers, near coastlines or large water bodies. High concentrations may be found
adjacent to roadways situated in dense urban areas surrounded by tall buildings, next to
below grade roadways or near roadways that are also influenced by tunnel ventilation
systems. These high concentration locations may be the preferred locations for some
CBSAs particularly if they are also significant for population exposure.
d. Congestion Patterns - how important a parameter should LOS be in
the determination of appropriate near-road monitoring sites? Does
the subcommittee have a view on how reliable LOS estimates are
across the country?
It is likely that on stagnant days the roadways with poor level of service, including
congested slow moving traffic will cause levels of pollutants to accumulate to higher than
expected levels. LOS information should be used to qualify the AADT data on a CBSA
specific basis.
e. Terrain - Does the subcommittee agree that terrain and vegetation
should be a consideration in the siting process? What priority should
this parameter have in the overall process?
Terrain and vegetation are very important and should rank near the top of the site
selection criteria. Both of these factors drastically affect path length, population
exposure, effective probe height and an agency's ability to site a monitor. Vegetation can
be a sink of many pollutants as well as a screen for the efficient transport of pollutants
between the source and the monitor.
f. Meteorology - Although there is no requirement to be downwind, in
the preamble to final NOz NAAQS rule, EPA encouraged it when
possible. EPA and NACAA intend to do the same in the guidance
document. Does the subcommittee agree with this approach?
The importance of other factors such as AADT, terrain and population exposure should
take priority over the prevailing wind direction. If the monitoring agency has candidate
locations that are otherwise equal it is preferred to select the one that is predominantly
downwind from the near road emissions.
4. Modeling is another tool that may be useful in the identification of candidate
near-road sites. In particular, the use of mobile source emissions modeling
with MOVES and local-scale dispersion modeling with AERMOD, can be
presented as part of the guidance document. Please comment on the
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available modeling tools, and their pros and cons, that the subcommittee
believes may be appropriate to discuss and/or recommend for use in the
near-road monitoring guidance document.
5. In regard to the process of identifying candidate near-road monitoring sites,
beyond the evaluation of factors noted above in question 3, and the potential
use of modeling, the use of saturation monitoring and on-road monitoring
are also possible tools that state and local air agencies may choose to utilize
in the near-road site selection process.
General Comment: All of these tools are resource intensive and are likely to be beyond
the scope of many monitoring agencies. The use of these tools should not be required.
a. what are the pros and cons to using passive devices to saturate an area
to gather data?
The NO2 standard is a 3-Yr average of 1-Hr maximum values, so a typical passive
sampler which is exposed for days to weeks will provide better information if the traffic
signature is relatively consistent throughout the course of a day. This is likely to be the
case in the largest CBSAs. Passive samplers will not provide as much information in
smaller CBSAs with variable traffic congestion and the affects of other significant
sources of pollutants.
b. Likewise, what are the pros and cons to using non-passive devices,
such as near real-time or continuous devices including, but not limited
to portable, non-FEM chemiluminescence methods for NCh or Gas
Sensitive Semiconductors (GSSs) for NO2 and other pollutants of
interest?
The concept is good but, of course, the real issue with many of these instruments is the
comparability to data from the FRM. If a monitoring agency is able to use several of
these instruments to look at potential sites in relation to each other, not to a nearby FRM,
then these non-regirlatory instruments could be useful.
c. Finally, what would be the pros and cons, to a state or local agency
attempting to use a specially outfitted vehicle to collect mobile
measurements to assist in the near-road site selection process for NCh
specifically as well as other pollutants of interest?
This could be a very expensive component of the site selection process and would only
be likely to occur if the monitoring agency already had access to this type of vehicle. The
advantage is that candidate areas can be identified relatively quickly compared to other
screening methods. The disadvantages to these types of measurements are that they are a
snapshot in time that could have little relationship to the periods when the concentrations
are expected to be the highest.
Questions regarding the CO monitoring network and near-road monitoring
6. The two issues are 1) the consideration of where light duty vehicles are
operating under 'cold-start' conditions, which may often not be on the
larger arterials or highways in an area, and 2) the impacts of light duty
vehicle congestion and idling in areas such as urban street canyons and/or
urban cores. Questions regarding the PM monitoring network and near-road
monitoring
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7. Does the committee believe that siting considerations for identifying the
location of peak NC>2 concentrations will likely address all of the high
priority siting considerations for PM (particularly PM2.s) as well? If not,
what other factors should be considered and what are the advantages in
considering these factors for identifying the location of maximum PM
concentration?
The answer to this question is complicated by the fact that the NOz regulation incorrectly
identifies near-road environments as the areas where NO2 concentrations will always be
highest. Secondly, the traditional method of measuring PM-2.5, the FRM is not well
suited to capturing the highly volatile emissions emitted from mobile sources. Thirdly, a
significant portion of PM-2.5 in most areas is either due to transport or due to secondary
particle formation, neither of which have anything to do with micro-scale NO2 siting
criteria.
A PM-2.5 network design that only includes locations that are near-road micro-scale sites
will only provide information about that source. An adequate PM-2.5 network must
include sites that also provide information that are relevant for upwind assessment,
population exposure, seasonal differences that can be evaluated in terms of particle
formation and lastly micro-scale source attribution.
PM-2.5 is more likely to be highest in dense urban neighborhoods away from the well
ventilated roadways. These areas are subject to emissions from transport, domestic
heating and cooking, stationary sources and mobile sources.
8. In addition to PM2.s mass, what other PM-related measurements are
desirable at near-road monitoring stations (e.g., UFP number, black carbon,
EC/OC, PM coarse, etc.)?
The EPA must define how it intends to collect PM-2.5 mass. The FRM will miss a
significant fraction of the volatile component of the mobile source emissions. This will
create the situation where the apparent risk from these sources or from living in one of
these areas is under estimated. In general, it is not advisable to err on the side of
underestimating a health risk when establishing a NAAQS oriented monitoring program.
It is preferable to delay a requirement to monitor PM-2.5 at these locations until the EPA
develops a suitable method that reliably includes a majority of the volatile component of
PM-2.5. One suggestion is for the EPA to encourage the manufacturers of continuous
PM-2.5 monitors to develop instruments without the FEM algorithms that reduce PM
concentrations in an attempt to emulate the filter based FRM. This will provide a more
realistic indication of the actual PM concentrations that the populations in these areas are
exposed to everyday.
UFP number monitoring is on its way to becoming an acceptable monitoring technique
but it is not quite ready for routine use. The EPA should invest in limited deployment
monitoring demonstrations to assist the vendors as they develop better more reliable
instrumentation. Some of the issues that need to be resolved include, how small do we
need to go, which bins are appropriate from a health perspective and how do we QA
these instruments. UFP data could provide valuable information for the health
community as they investigate air quality related health effects.
The EPA has a BC monitoring requirement in place at the NATTs sites but this data is
rarely used. The data is also subject to artifacts due to filter changes and interference
from co-pollutants on the filter substrate. The EPA should really determine if there is a
57
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need for this data before the requirement to monitor for a non-criteria pollutant is added
to this network.
EC/OC data is more interesting because it provides information that can help regulators
understand the losses from the PM-2.5 FRM and to develop potential pollution control
programs. This information may only be necessary in the MSAs where the PM NAAQS
could be exceeded and control strategies are required.
PMcoarse is of limited value because the quality of the PM-2.5 data is poor and because
of the small scale that the resultant data represents.
Questions regarding the monitor siting criteria for microscale CO, microscale PM^s,
and the new near-road NO: siting criteria
9. Does the subcommittee believe that reconsideration of microscale CO siting
criteria is appropriate? Specifically, would an adjustment of CO siting
criteria to match those of microscale PIVh.s and microscale near-road NO2
sites be logical and appropriate?
The 3 questions (9,10 and 11) assume that there are high enough CO concentrations at
typical near-road locations to justify including health related NAAQS CO monitors at
these sites. The EPA must determine if this is the case before establishing a new,
expensive and potentially un-necessary monitoring requirement.
The siting criteria for CO does need to be updated if CO is included at a near-road
monitoring location. The more restrictive height requirement is not necessary for sites
where turbulence is expected to create well mixed conditions at these monitors. The
criteria used for micro-scale PM and NO2 is acceptable.
10. should there be consideration to maintain the requirement on how urban
street canyon or urban core microscale CO sites should be sited?
The micro-scale PM and NO2 siting requirements are adequate for street canyon CO
monitoring. The existing CO canyon monitoring guidance included a wind direction
provision that is un-necessary and made it more difficult to find suitable locations.
11. Does the subcommittee have an opinion on how "urban street canyons" or
"urban core" might be defined, perhaps quantitatively, and with regard to
use in potential rule language?
It is useful to be able to refer to an area as a street canyon or as an urban core. Street
canyons are relatively easy to define because they are defined by the structures in their
immediate vicinity. Urban Cores have to be defined on a larger MSA basis.
Street Canyons have the following attributes. They are relatively narrow in comparison
to the height of the structures on either side of the street. It should be relatively easy to
model an optimum or minimum width to height ratio that would define a street canyon.
A ratio of (1) width to (1 or 1.5) height might be a good starting point.
Urban Cores can be defined by a number of parameters such as the reasonable geographic
center of an MSA, the approximate centroid of emissions sources, area of densest
population or highest congestion. Very large MSAs could easily have multiple urban
cores due to geography and neighborhood layout such as in NYC where boroughs in
some ways can emulate small cities.
58
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Questions regarding the near-road monitoring pilot study
The EPA ORD has been operating near-road monitoring locations for more than a year in
a couple of locations including Las Vegas. The results from these campaigns should be
used to determine if the NO2 and other pollutant concentrations were high enough to
warrant further development of near-road monitoring efforts. The data should also be
evaluated to determine if the selected monitoring methods were appropriate for these
locations.
12. Does the Subcommittee agree with these considerations? Further, are there
other considerations that should be evaluated in selecting pilot cities to
house permanent near-road monitoring stations as part of the pilot study?
The reliance on a CBSA's population is too simplistic. The population of a CBSA does
not specifically provide information about how roadways are used within a CBSA. For
example, this ranking does not include the number of people who commute from outside
of the CBSA every day or use car pools or mass transit including busses, trains, subways
and ferries.
13. Please comment on the minimum equipment/pollutant measurement
complement that should be deployed at each site and also the ideal
equipment complement that each site should or could have, respectively.
Specifically, what pollutants (e.g., NO2, NOX, NO, CO, PM (Ultrafine, 2.5,
and 10), black carbon, air toxics (such as benzene, toluene, xylene,
formaldehyde, acrolein, or 1,3, butadiene) and ammonia) and other
information should the pilot study measure or gather at the fixed,
permanent monitoring stations, and by what methods? This list should be in
priority order, as feasible, and can include any NAAQS or non-NAAQS
pollutant by any method (FRM/FEM and/or non-reference or equivalent
methods), any particular type of other equipment for gathering supporting
data such as meteorology or traffic counts.
It is acceptable to deploy more monitoring parameters at the pilot sites than at the routine
sites, however, the EPA must be aware of the resources necessary to operate these sites.
The pilot sites should have NO2, CO, ultrafine and BC monitors. PM-2.5 should be
included only if PM-2.5 FEMs are operated without the algorithms that reduce the mass
to match the FRM. These instruments could utilize the EPA parameter description 88500
which cannot be compared to the PM2.5 NAAQS and is defined as total atmospheric
PM-2.5.
The pilot sites should be selected based on the availability of NCore and NATTs sites
within the same CBSA to permit data comparisons with the near-road site.
14. EPA and NACAA have proposed four to five urban areas to have saturation
monitoring, using either passive devices and/or continuous/semi-continuous
saturation type multi-pollutant monitoring packages (i.e., several types of
monitors in one mountable or deployable "package"). Please provide
comment on:
The deployable semi-continuous package that EPA is considering is not practical for use
by monitoring agencies. The unit is expensive, may or may not provide data that is
comparable to criteria monitors and would require power and security wherever it is
deployed.
59
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a. The pollutants that should be measured with the saturation devices at
each saturation site.
The saturation devices only have to be able to monitor NOz. Once the high
NC>2 site is found, other parameters can be included when the monitoring site
is fully established.
b. The number of saturation devices per pollutant, both passive and/or
continuous/semi-continuous, that may be deployed in each pilot city.
The most effective use of saturation samplers is to help rank the top 2-4 sites
that have been identified through the rest of the site selection process. It
would be too expensive and labor intensive to use saturation monitoring to
select sites across wide areas in a CBSA.
c. Whether placing saturation monitoring devices near certain road
segments should include, at a minimum: 1) the highest AADT segment
in an area, 2) the road segment with the highest number of heavy-duty
truck/bus counts, 3) at a road segment with more unique roadway
design, congestion pattern, or terrain in the area, and 4) if feasible, at
a lower AADT segment with a similar fleet mix, roadway design,
congestion, terrain, and meteorology as the top AADT road segment
in the area.
It is sensible to use saturation monitors to compare sites that cannot be ranked
in another way. The monitoring agency should include the feasibility of
establishing a monitor in a specific area in the initial consideration of potential
sites. Saturation samplers can help the agency differentiate a feasible site on
one type of roadway to another feasible site on another type of roadway.
60
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Dr. Kazuhiko Ito
Note: The charge questions below are truncated to save space.
General Comment:
I understand the importance of measuring NC>2 near roadways, but I am very concerned
about a possible reduction in the number of community-wide monitors that can result
from the new network plan. According to EPA, there are currently about 400 NO2
monitors nationwide, many of which are community-wide monitors, and very few (3?)
are micro-scale monitors. The new network plan will create 126 near-road monitors, but
only 53 community-wide monitors will be required to operate. The epidemiological
studies that reported short-term associations between N02 and respiratory morbidity used
the community-wide monitors. Because the near-road monitors may not adequately
reflect the entire city's residents' exposures, we may be reducing the number of NOa
monitors that can be used for community based epidemiological studies. I have more
comments on this as part of response to Charge Question 2.
Questions regarding the near-road monitoring guidance document
Charge Question 1: Please comment on the overall content of the recommended topics
in the draft outline (of the near-road monitoring guidance document). Please provide
suggestions on any missing subjects that should be included in the guidance document
and any unnecessary topics that are currently listed in the attached draft, if applicable.
Comment:
The overall content and the topics listed in the outline look generally adequate. I
have a few comments below.
In addition to the literature review mentioned as part of Background, EPA can
conduct analysis of the existing data (though the monitors are not sited in the required
near-road scale) to describe the relationship between NO2 and other pollutants (e.g., CO,
EC, etc.).
In identifying candidate near-road areas, it may not be just AADT but the density
of high AADT areas that are important, particularly in cities with high population density
areas.
Charge Question 2: What pollutants and sub-species does the subcommittee believe
should be included for consideration and discussion in the near-road monitoring
guidance? Please prioritize the recommended pollutants and provide the rationale for
their ranking, including how this pollutant measurement will contribute to scientific and
regulatory knowledge of near-road air quality and adverse human health effects.
61
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Comment:
] need to first comment on my general rationale of measuring co-pollutants at the
proposed near-road monitoring location before discussing the prioritizing co-pollutants. I
feel that there is a gap between the recognition of the short-term associations between
NC>2 and respiratory morbidity in the observational epidemiological studies, the results
from human laboratory studies, and the suggestion to measure hourly NC>2 values at near-
road locations and set a standard level at such locations with the 1-hr averaging time. 1
understand that this has been already decided, but I think we should keep the options to
evaluate the implication of this decision in case we may need to reconsider this strategy.
The issues 1 need to raise are the following:
• The observational studies that reported short-term associations between NO2 and
respiratory morbidity mentioned in the FR (40 CFR Parts 50 and 58) used NC>2 data from
monitors that were not near-road monitors. Unless the associations were due to
exacerbations of respiratory conditions in the sub-populations who lived near roads, it is
possible that the data to be collected at near-road monitors may not correlate well with
the measurement at the existing NC>2 monitors, and they may not even correlate well with
the citywide respiratory morbidity time-series because the near-road measurements may
be highly influence by local sources that are not relevant to the rest of the city.
• Correlations between NC>2 and co-pollutants at near-road monitors may be different
from those at non-near-road monitors (i.e., those used in the observational
epidemiological studies). Therefore, it is possible that these co-pollutants measured at
near-road monitors have different impacts on the health effects models compared to those
reported in the past epidemiological studies.
Because of these issues, I think it is essential to retain at least some of the existing
non-NR NCh monitors until the issues are resolved. Otherwise, it is possible that we end
up with discontinuation of data that are useful for epidemiological studies.
The review of epidemiological studies in the NC«2 ISA concluded that the
associations between NCh and respiratory morbidity were robust to the inclusion of other
co-pollutants (i.e., no strong indication of confounding) in the health effects models, but
these co-pollutants were mostly other criteria pollutants (PMio, PM2 5, ozone, CO, 802)
with a few exceptions in studies outside US that examined non-criteria pollutants (e.g.,
benzene, coarse particles, ultra-fine particles). Determining confounding by co-pollutants
that come from the same source(s) asNO2 is methodologically difficult, but it would be
beneficial to measure, at least in a pilot study, the pollutants that may also have the health
effects (including effect modification or synergism) or may be useful as markers to
distinguish among the sources that emit NO2. When the EPA mentions "a multi-pollutant
perspective", I assume that there are several rationales for the idea. Ranking/prioritizing
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individual pollutants is difficult because there are uncertainties about the importance of
specific pollutants, but they can be grouped in terms of the rationale for measuring them,
and the rationale may be ranked.
Pollutant group
NO2> NO, NOx
CO
Black carbon
Ultra-fine particles / particle number
concentrations
PMio-2 s, and their chemical
constituents such as Br, Zn, Cu, Sb
PM2 5 and its chemical constituents
such as EC, OC
SO2
Rationale (objective and feasibility) and
comment
• Same instrument
• Marker of gasoline vehicles
• Multi-pollutant assessment using the
instruments already used for NAAQS
• Potential health relevance
• Diesel traffic marker
• Potential health effects
• Potential health relevance
• Expected to be high near-roadways
Traffic: re-suspended dust, tire ware, brake
ware.
• To determine "excess PM2 5" beyond urban
background
• Potential health effects
• Sources other than traffic that produce NO2.
Charge Question 3 (Identifying Candidate Near-road Site Areas):
Comment: All of these items (a) through (f) (and population density, which was
mentioned in the outline but not here) seem important to consider in identifying candidate
near-road sites. However, without actually determining the relationships between these
factors and the NO2 levels, presumably through the pilot project, it would be difficult to
evaluate the adequacy of siting a monitor based on the information alone. EPA should
look for studies that attempted to investigate these issues. The data from the New York
City Community Air Survey (NYCCAS) may be useful. They have been conducting 2-
week sampling measurements of NO2, SO2 (winter only), ozone, PM2 5 and its chemical
63
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constituents at 150 locations within New York City. They are conducting land-use
regression of the measured pollutants including NC>2 as a function of a number of geo-
coded emission data including traffic volume and other local combustion sources (e.g.,
residual oil burning). The information from the analysis of the NYCCAS data may be
useful in evaluating the limitation of AADT data.
Charge Question 4: Please comment on the available modeling tools (e.g., MOVES,
AERMOD, etc.), and their pros and cons, that the subcommittee believes may be
appropriate to discuss and/or recommend for use in the near-road monitoring guidance
document.
Comment: I have not used these models and have not seen the model validation of these
models as applied to NOa and other traffic air pollution. Therefore, I cannot comment on
this.
Charge Question 5: The use of saturation monitoring and on-road monitoring are also
possible tools that state and local air agencies may choose to utilize in the near-road site
selection process.
5 (a): What are the pros and cons to using passive devices to saturate an area to gather
data?
Comment: The obvious pros include the low cost and small dimension. The obvious
cons include the long sampling period required for the detection limit of the passive
sampler. However, the spatial distribution of NC«2 constructed from such sampling would
be still useful in determining the siting of a sampler. The relationship between the 1-hr
peak NC<2 data and the data from passive samplers can be determined from a pilot study.
The NYCCAS data mentioned above may be useful to do this, since the study already
collected 2-years of data (to identify the high NC«2 area) and the study is still going on (to
measure hourly data at the high NC<2 areas to compare the two-week vs. hourly data).
5 (b): What are the pros and cons to using non-passive devices, such as near real-time or
continuous devices including, but not limited to portable, non-FEM chemiluminescence
methods for NC«2 or Gas Sensitive Semiconductors (GSSs) for NC>2 and other pollutants
of interest?
Comment: The pros: Ability to measure hourly data: The cons: Need to validate the
correspondence with the FRM/FEM measurements. A pilot study is required for this.
5 (c): What would be the pros and cons, to a state or local agency attempting to use a
specially outfitted vehicle to collect mobile measurements?
64
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Comment: I am not sure what the pros would be unless all the other information leaves a
kind of uncertainty that can be resolved by the mobile measurements. The cons include
the cost and resources required to conduct the measurements.
Questions regarding the CO monitoring network and near-road monitoring
Charge Question 6 fa): Does the subcommittee believe that the light duty cold start and
congestion factors will significantly influence the location of peak CO concentrations in
an area?
Charge Question 6 (b): Does the subcommittee have an opinion on whether, and
possibly how, these two issues of vehicles operating under cold start conditions and light
duty vehicle congestion and idling in urban street canyons and/or urban cores be
considered in a future, nationally applicable, CO monitoring proposal?
Comment: I don't know the data regarding the impact of cold start conditions on CO
peaks and therefore cannot comment.
Questions regarding the PM monitoring network and near-road monitoring
Charge Question 7: Does the committee believe that siting considerations for identifying
the location of peakNO2 concentrations will likely address all of the high priority siting
considerations for PM (particularly PM2 5) as well? If not, what other factors should be
considered and what are the advantages in considering these factors for identifying the
location of maximum PM concentration?
Comment: It depends on the region of US, but PM (particularly PM2 5) may be
dominated by regional background PM levels, so the impact of the near-road pollution on
the monitor will need to take into consideration (subtract) the data from non-NR PM
monitor. I guess the NCore sites will be sufficient for this purpose where they exist.
Charge Question 8: In addition to PM2 5 mass, what other PM-related measurements are
desirable at near-road monitoring stations (e.g., UFP number, black carbon, EC/OC, PM
coarse, etc.)?
Comment: All of these would be "desirable", but are funds available to measure these in
the new near-road monitors?
Questions regarding the monitor siting criteria for microscale CO, microscale PM2S,
and the new near-road NO2 siting criteria
Charge Question 9: Does the subcommittee believe that reconsideration of microscale
CO siting criteria is appropriate? Specifically, would an adjustment of CO siting criteria
to match those of microscale PM2 5 and microscale near-road NO2 sites be logical and
appropriate?
65
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Comment: I appreciate the historical background and the original reasons for the siting
criteria for different NAAQS pollutants. None of these criteria of data collection were
originally meant for epidemiological studies. However, since the researchers used these
data from the regulatory monitors for observational epidemiological studies, and because
the findings from these studies are in part influencing the process of setting NAAQS, it is
inevitable that the siting criteria will need to accommodate the need to use the data for
epidemiological investigation. These studies often use multi-pollutant regression models
to examine potential confounding effects, which tacitly assumes that pollution variables
equally represent the population exposures. The reported short-term associations
between CO and mortality and cardiovascular morbidity raise a concern that these
associations are observed despite the potential inadequacy of the exposure metric to
represent population exposure (i.e., potential attenuation of associations). For this reason,
I think it is appropriate to adjust CO siting criteria to match those of microscale PM25 and
microscale near-road NO2 sites.
Charge Question 10: Even if the adjustment of microscale CO siting criteria in sections
2, 6.2, and table E-4 in 40 CFR Part 58 Appendix E to match that of microscale PM2s
and microscale near-road NO2 is appropriate and proposed, should there be consideration
to maintain the requirement on how urban street canyon or urban core microscale CO
sites should be sited?
Comment: If we are assuming that there can be CO monitors for different purposes (and
there is sufficient funds), yes, I think there should be consideration to maintain CO
monitors that will measure the maximum impact that is separate from the population
epidemiology.
Charge Question 11: Does the subcommittee have an opinion on how "urban street
canyons" or "urban core" might be defined, perhaps quantitatively, and with regard to use
in potential rule language?
Comment: Establishing such definitions would require some analysis of available data
to characterize the relationship between the pollution levels and emission/environment
conditions (AADT, building density, etc.).
Questions regarding the near-road monitoring pilot study
Charge Question 12: EPA and NACAA will select the locations for permanent sites ...
these considerations include choosing a large and a relatively small urban area based on
population, an area with varied or complex terrain, an urban area with an operational
NOx analyzer ... Does the Subcommittee agree with these considerations? Further, are
there other considerations that should be evaluated in selecting pilot cities to house
permanent near-road monitoring stations as part of the pilot study?
66
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Comment: Given the limited budget, EPA should consider the cities that already have
infrastructure to conduct a pilot study or the cities that are already conducting multi-
pollutant assessment at multiple locations. Atlanta and NYC come to my mind.
Charge Question 13: EPA and NACAA have proposed that at least two urban areas
should have permanent near-road monitoring stations (that would fulfill NC«2 near-road
monitoring requirements) implemented for the pilot study... Specifically, what pollutants
and other information should the pilot study measure or gather at the fixed, permanent
monitoring stations, and by what methods?
Comment: See my comment on Charge Question 2 for the list of pollutants. In terms of
other information any geo-coded information related to traffic and other emission sources
would be useful. The reports from the NYCCAS project for such information may be
useful (available from http://www.nvc.gov/html/doh/html/eode/nvccas.shtmn.
Charge Question 14: EPA and NACAA have proposed four to five urban areas to have
saturation monitoring ...Please provide comment on:
a. The pollutants that should be measured with the saturation devices at each saturation
site.
b. The number of saturation devices per pollutant, both passive and/or continuous/semi-
continuous, that may be deployed in each pilot city.
c. Whether placing saturation monitoring devices near certain road segments should
include, at a minimum: 1) the highest AADT segment in an area, 2) the road segment
with the highest number of heavy-duty truck/bus counts, 3) at a road segment with more
unique roadway design, congestion pattern, or terrain in the area, and 4) if feasible, at a
lower AADT segment with a similar fleet mix, roadway design, congestion, terrain, and
meteorology as the top AADT road segment in the area.
67
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• M25 Clockwise
X A14
Log (ah)
Mr. Rich Poirot
Comments on Near Road Monitoring
1. The accompanying draft guidance document outline provides an initial
thought of the major topics required in the near-road monitoring guidance that will
aid state monitoring agencies in the identification and implementation of NO2 near
road monitoring sites from a multi-pollutant perspective. Please comment on the
overall content of the recommended topics in the draft outline. Please provide
suggestions on any missing subjects that should be included in the guidance
document and any unnecessary topics that are currently listed in the attached draft,
if applicable.
I think the draft outline for the guidance document seems reasonably complete and
contains no unnecessary topics. One critical topic which seems missing from the outline
is the importance of the specific distance from the roadway. I think "within 50 meters" is
too broad a range, and that most 2
of the near-road influence falls
off within, rather than beyond,
that distance. Extensive, long- t
term experience sampling near- |
road NO2 in the UK (where | '
"roadside" monitors are within I
5 meters from the road and |
where additional "kerbside"
z
monitoring is conducted within
1 meter - both at heights
between 2 and 3 meters)
indicates that the "roadside
increment" declines,
predictably, with the log of
distance from the road, as illustrated in the attached figure.
From: NO2 Concentrations and Distance from Roads (2008) Air Quality Consultants Ltd.
http://www.airquality.co.uk/laqm/documents/FallOffWithDistanceReptJulv08.pdf
This effect of distance is considered sufficiently predictable that a nomograph is available
that estimates concentrations at any distance from measurements at any other distance
(within 50 m).
http://www.airqualitv.co.uk/laqm/tools/NO2withDistancefromRoadsCalculatorlssue2.xls
While these estimates are derived from annual average concentrations, a similar
relationship will occur for hourly near-road concentrations (of NO2 and other mobile
source pollutants like BC, ultrafines, CO, etc.). Location changes within the 50 meter
distance could easily result in changing the incremental roadway contributions by a factor
of 2 or 3.1 think there is a need to further constrain this distance range in the guidance, or
perhaps the standard could be expressed in terms normalized to a specific distance (say
60 80
Disunct from K«rb (m)
68
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10 or 20 m). For ultrafine particles, especially those in the < 25 nm size range, the
roadside gradient is likely to be even steeper. See for example:
Zhu et al. 2004: "Aerosol Science & Technology: Seasonal Trends of Concentration and
Size Distribution of Ultrafine Particles Near Major Highways in Los Angeles." 38(suppl
1):5-13; Sioutas et al., 2005 Exposure Assessment for Atmospheric Ultrafine Particles
(UFPs) and Implications in Epidemiologic Research. Environ Health Perspect 113(8):
doi:10.1289/ehp.7939; Durant et al., (2010) Short-term variation in near-highway air
pollutant gradients on a winter morning, Atmos. Chem. Phys. Discuss., 10, 5599-5626.
I also think the emphasis on AADT as the primary focus for site selection is overstated
relative to "other near-road considerations" which are likely more important (in addition
to the specific distance) Note for example the following Table A3.6 from the 2007 Air
Quality Expert Group report on Trends in Primary Nitrogen Dioxide in the UK, that
maximum hourly (98th percentile) NO2 correlates poorly (R2 = 0.24) with counts of total
vehicles, but shows a much higher correlation (R2 = 0.66) with counts of (diesel) bus
traffic, based on measurements at 53 roadside sites in the UK.
Table A3.6: Correlation coefficients for 53 roadside monitoring sites
NO, annual
mean
NO, annual
mean
98%ite
ExeBSdences
Count - al!
Count -car
Count- bus
Count - LGX'
Count - HGVr'
Count - HGVa1
Count -
motorbike
NO all
NO, car
NO, bus
HO, IGV
NO HGVt'
NO, HGVa"
NO, motor bike
NO.
annual
mean
1 00
0.96
0.94
075
0.35
032
0.57
0.28
027
C.06
0.4S
0.38
0.25
0.61
032
0.33
008
039
M°>-,
annual
mean
1. 00
098
0.78
026
0.22
0.67
0.21
0 17
0.00
0.54
0.30
c ie
0.71
0.26
0.25
0.02
0.41
M
*Be
1.00
078
0.24
0.21
0.66
0.18
0.17
0.02
0.50
0.28
0.14
0.69
0.22
0.24
0.03
039
Exceed enc«
1.00
o.n
0.08
0.64
0.10
-001
-005
0.20
0 17
006
0.66
0 13
0.03
-004
0.15
Count
all
1.00
1.00
0.14
0.93
0.81
0 -,8
0.56
075
095
0.13
0.90
0.75
0.59
072
Count
-car
1.00
0.10
0.91
080
0 f*
0.51
0.72
095
0.08
0.87
0.74
0.56
068
Count
-bin
1 00
0.12
-o.os
-C.14
0.43
0.17
0.05
0.99
0.18
C.C3
-0.12
030
Count
•LCV
1.00
070
0.63
0.55
077
087
0.10
098
065
066
071
Count
HGVr
1.00
0.49
C4C
0.69
0 77
-005
0.67
097
0.51
057
Count
HGVa-'
1.00
0.09
0.51
0.64
-0.18
0.58
0.39
1.00
030
Count
motor
bike
1.00
0.46
044
0.47
0.60
0.49
011
091
NO.
al
1.00
076
0.18
079
G67
054
064
NO,
car
1.00
002
086
0 70
0.64
066
NO
bus
voo
018
0 05
-0.15
031
NO
LGV
1.00
0.64
0.61
0.72
NO,
HGVr"
1.00
0.42
0.59
NO
HGVa>
1.00
037
NO.
motor
bike
1.00
' Heavy goods vehicle - rigid
•' Heavy goods vehtcie - articulated
From: UK air quality modelling for annual reporting 2007 on ambient air quality
assessment under Council Directives 96/62/EC, 1999/30/EC and 2000/69/EC
http://vvrww.airquality.co.uk/rep
orts/cat09/0905061048 dd!200
Figure 3.8 The adjustment factors applied to road link emissions
7mapsrep v8.pdf
Long-term experience with
roadside NC>2 monitoring and
modeling in the UK has also
indicated that the enhanced
dispersion effects of vehicle speed
more than offset slight increases in
NOx emission rates, leading to
\
-c
\
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decreasing per-vehicle NCb impacts with increasing AADT, especially on high speed
"motorways" (analogous to US Interstates). Note the attached Figure 3.8 "adjustment
factors" used to reduce per/vehicle NO2 emissions for modeling near-road NO2 impacts
in the UK.
From: Trends in Primary Nitrogen Dioxide in the UK (2007) Air Quality Expert Group
Report, Annex
http://www.defra.gov.uk/environment/quality/air/airqualitv/publications/primarvno2-
trends/index.htm
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Other minor suggestions include:
• The relative age of the fleet may be an important component of the fleet mix,
which may vary among cities and within urban neighborhoods. (This may be
somewhat less important for NO2 than for other near-road pollutants).
• The "load" on vehicles during rush hour(s) could be an important factor. For
example, diesel vehicles in stop-and-go traffic and/or on an uphill grade will lead
to high NO2 regardless of AADT.
• The "expandability" of a site - i.e. the ability to accommodate additional samplers
for various other mobile source pollutants - should also be a site selection
consideration.
9
• The availability (or establishment) of a "representative" urban background site
(for NCh and ideally for other MV pollutants) should also be an important
consideration. Identification of "roadside increments" for the multiple pollutants
is critical, as is the ability to project measurements from specific microscale sites
to larger population exposures.
• The measured or expected neighborhood-scale background may also be
important. Other things being equal, a high traffic road in the midst of other high
traffic areas is likely to experience higher concentrations than a similar roadway
on the edge of the urban area.
2. EPA and NACAA envision the near-road guidance document to be written
from a multi-pollutant perspective. What pollutants and sub-species does the
subcommittee believe should be included for consideration and discussion in the
near-road monitoring guidance? Some potential species for consideration include
NO2, NOX, NO, CO, PM (Ultrafine, 2.5, and 10), black carbon, air toxics (e.g.,
benzene, toluene, xylene, formaldehyde, acrolein, or 1, 3, butadiene), and ammonia.
Please prioritize the recommended pollutants and provide the rationale for their
ranking, including how this pollutant measurement will contribute to scientific and
regulatory knowledge of near-road air quality and adverse human health effects.
Since NO2 is the focus of the revised NAAQS and new monitoring requirements, it (and
the NO & NOx which typically come along with it) has to be given top priority. It seems
possible however that of the many, various mobile source pollutants, N02 may be one of
the least health-relevant, and a new, large monitoring network measuring just NO2 would
be a waste of scarce resources. All of the other pollutants listed above would also be of
interest, but would be prohibitively expensive to add at all sites. I also wonder whether
the near-road influence of these many mobile source pollutants (or even of just NO2
alone) is something that can be or should be addressed in a large network approach.
Rather than prioritizing the above list and seeing a few species measured at a large
number of (similar) sites, I would prefer to see a nested network, within which
many/most of the above species could be added at a smaller subset of sites. Instrument
costs and the availability of reliable, continuous samplers should also be an important
consideration.
1 think black carbon (preferably multi-wavelength) should be given high priority, given
its relevance to health (& climate & visibility) effects, its strong influence - like N02 -
from diesel emission sources, and the availability of reliable continuous instruments.
Continuous OC (OC/EC) data would also be useful at some sites to help assess effects of
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fleet mix, SOA formation, condensation, destruction, etc. If reliable instruments are
available and affordable, particle number count (including ultrafine particle sizes) would
also be an important measurement, and likely to increase rapidly near roadways (well
within 50 meters). I would give BC and number count a higher priority than PM2 5 mass,
and it seems likely that FRM/FEM PM25 samplers will substantially understate the semi-
volatile fraction near roadways (although FDMS TEOMS have not generally performed
well in the field). There is likely a steep PM coarse gradient near roadways, but PMio
(subtraction) measurements would be a poor way to characterize this. Collection of PM in
different size fractions in large (aggregated) sample volumes - to support molecular level
organic analysis and bioassay work - could also be useful at a few sites. CO
measurements would be especially useful for contrasting pollutant mixes at sites (or
times) with different diesel vs. spark engine fleet mixes. The "toxic" species listed above
would likely show strong roadway increases, but are also likely to be prohibitively costly
(or too labor intensive) to add at most sites. I wonder if there's any possibility of moving
or establishing one of the NAATS sites to a near-road location? Our (very small) VT
state agency has recently had reasonable success operating a continuous BTEX
instrument from Synspec-for which I believe a 1,3 butadiene option is available. Results
from the Las Vegas MSAT near-road toxics study could be quite relevant here, and some
consideration might be given to modifying planned future phases of that study to make it
more relevant to the objectives of the new near-road NO2, CO NAAQS requirements and
related multi-pollutant monitoring plans.
I don't know the availability, reliability or costs of continuous NHs instruments, but
better characterization of MV NHs emissions would be desirable at a few sites at least.
Possibly some periodic UC Davis DRUM sampling would be a useful complement at a
few sites - if equipped with a streaker or somesuch to add time resolution to the
ultrafines. I haven't actually seen that configuration in action, and don't know about
current analytical capabilities at the DELTA Group. Ozone measurements might be
useful at selected sites (including the urban background sites), as the contribution of
secondary NO2 formation, even in near road environments, isn't necessarily trivial, and
interesting changes may occur with efforts to attain new ozone and NOi standards.
In addition to the above pollutant species list, and meteorological measurements, other
measurements that should be considered include traffic counters (which can separate light
& heavy duty MV) and or possibly cameras (which can be especially useful for
evaluating extreme events). It should also be noted that the important objective of
characterizing incremental roadway contributions for any of the above pollutants would
benefit (as for NO2) from a measurements at a paired urban background site. Some
(nearby) remote sensing (FEAT) could also be useful, or perhaps establishing near-road
sites near locations where FEAT-type measurements have recently conducted (and may
be periodically repeated).
http://www.feat.biochem.du.edu/assets/databases/Cal/Tricitv NH3SQ2NO2 2008 Re
port ARB.odf
3. Identifying Candidate Near-road Site Areas
a. AADT & Fleet Mix
As indicated above, I think AADT alone is a poor indicator.
b. AADT & Fleet Mix
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A metric which 'diesel-weighted' the AADT would be preferable to AADT alone, but
again, count is not really the key issue, especially on high speed highways. Two trucks
passing the monitor at 60 mph will not cause twice the impact of 1 truck at 30 mph...
Also, given the 1-hour standard, the traffic and fleet mix on weekday morning rush hours
are likely to be most important.
c. Roadway Design
Assuming that you mean "no obstructions" between the road and the monitor, this seems
reasonable, and it seems unlikely that that no suitable sites without such obstructions will
be available. Barriers beyond the monitoring site that constrain the further dispersion of
roadway pollutants should not be avoided and (in urban street canyons) may well lead to
some of the highest population exposures. Conceivably, adding barriers - sound barriers,
trees, etc. - might be considered as an exposure mitigation strategy. It might also be noted
that large fractions of the population spend time within a 5 or so meters of congested
urban streets, but population proximity to the edges of high-speed interstates with
maximum AADTs is typically more distant.
d. Congestion Patterns
Conceptually, "level of service" sounds like an important indicator, although I don't
know how reliable such data is on a national scale. As indicated earlier, I think NCh
emission increases with speed are relatively small and offset by increased dispersion. I
would expect higher concentrations during times/places of highest congestion, rather than
during high speed driving conditions.
e. Terrain
Terrain could be an important, especially during the winter in mountain/valley locations,
in urban street canyons, or near roadway dips which are below grade.
e. Meteorology
I think the Agency's proposed approach - strongly encouraging but not formally
requiring "downwind" location is reasonable for all the reasons given. Ideally the
"downwind" location would concurrently reflect the periods of highest traffic congestion
and lowest wind speeds and mixing heights. In case of doubt, saturation sampling could
help determine locations of maximum expected impact. Established sites which met
measurements indicate are persistently upwind during rush hour should be replaced.
4. Modeling is another tool that may be useful in the identification of candidate
near-road sites. In particular, the use of mobile source emissions modeling with
MOVES and local-scale dispersion modeling with AERMOD, can be presented
as part of the guidance document. Please comment on the available modeling
tools, and their pros and cons, that the subcommittee believes may be
appropriate to discuss and/or recommend for use in the near-road monitoring
guidance document.
Modeling may be a useful tool, but unless site-specific meteorology and vehicle mix,
volume and congestion data are available, I'm not sure it would lead to a better site
selection than a "common sense" approach. AERMOD also often performs poorly in
complex terrain.
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5.
a. If a state were inclined to use saturation monitoring to aid in the selection of a
near-road monitoring site, and considering the NO2 standard is a 1-hour daily
maximum standard, what are the pros and cons to using passive devices to
saturate an area to gather data?
Unless the time periods for Fjguri> 3J ^„,„„„„ „„,„, ^^M „. „„„„,«. ^^ «,concantrMon% m
saturation sampling turn out to be
atypical, the longer (than 1 hour)
aggregation times for passive
samplers may not be that big a
problem. Based on the long-term,
multi-site data from roadside sites in
the UK, the peak hourly and annual
average concentrations are well
correlated across space, as indicated
in Figure 3.3 pasted here. Note also
the high correlation (R2 =0.98)
between annual average NO2 98th
percentile hourly values from 53
UK roadside sites in Table A3.6
above.
From:
http://www.airquality.co.uk/reports/
cat09/0905061048 dd 12007mapsre
p v8.pdf
A disadvantage of passive samplers is that while reasonably accurate units are available
for NC>2, NO, NOx & BC, there may not be comparably accurate units for CO. This limits
the ability to explore different NO2/CO ratios in a saturation approach. An advantage of
passive samplers is their low cost and subsequent ability to deploy many units
inexpensively. If passive sampling were limited to NO2 & NO, a relatively dense
exploratory saturation sampling program could be considered, including innovative
mounting of passive samplers on (and/or in) busses, commuter cars, taxis ect. This will
open the can of worms regarding whether there's intent to protect people in their cars at
rush hour, but that issue probably needs to be addressed at some point. See also George
Allen's recommendations combining passive samplers with active, timed inlets.
b. Likewise, what are the pros and cons to using non-passive devices, such as near
real-time or continuous devices including, but not limited to portable, non-FEM
chemiluminescence methods for NO2 or Gas Sensitive Semiconductors (GSSs)
for NO2 and other pollutants of interest?
Potential advantages include the ability to collect data with higher time resolution and for
more species than passive samplers allow. I don't know the current instruments, and
defer to others on the committee.
c. Finally, what would be the pros and cons, to a state or local agency attempting to
use a specially outfitted vehicle to collect mobile measurements to assist in the
Annual mean (M3 m"3)
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near-road site selection process for NO2 specifically as well as other pollutants of
interest?
Other than the prohibitive cost, this could be an excellent way to help select sites and
characterize the space/time patterns of exposures to multiple roadway pollutants. I
wouldn't automatically rule this out, as it may turn out that some states or research
groups may have access to such mobile sampling equipment and would be willing to
operate it at reasonable costs. In my view the pilot studies should be conducted in fewer,
rather than more locations, and this might be a good way to produce some useful data in a
few (1 or 2) study areas. If only EPA had an adequately funded Office of Research and
Development... There may also be some useful low-tech ways to combine "ordinary"
mobile sampling vehicles (buses, taxis, commuter vans etc.) with passive samplers that
could provide some useful information.
6.
a. Does the subcommittee believe that the light duty cold start and congestion
factors will significantly influence the location of peak CO concentrations in an
area? What priority should these factors be given when compared with the
factors (AADT, Fleet Mix, Roadway Design, Congestion Patterns, Terrain, and
Meteorology) already being considered for peak NO2?
Yes, these are important considerations. I don't believe however that maximum NO2 and
CO will necessarily occur at vastly different kinds of locations, and that some kinds of
sites would be suitable for quantifying near-road influences from both pollutants. A
relatively high fraction of diesel vehicles does not necessarily mean that emissions from
spark engine vehicles will not be high as well. Congested sites where vehicle mixes
change by time of day and day of week will be especially informative. Having
concurrent, collocated data for multiple species, while searching for single pollutant "hot
spots" is not likely to improve understanding of population exposures, help discern
effects of co-varying pollutants, nor lead to development of effective abatement
strategies.
b. Does the subcommittee have an opinion on whether, and possibly how, these two
issues of vehicles operating under cold start conditions and light duty vehicle
congestion and idling in urban street canyons and/or urban cores be considered
in a future, nationally applicable, CO monitoring proposal? Are there other
factors that may affect peak CO concentrations and not affect peak NO2
concentrations that should also be considered for any future CO monitoring
proposal?
CO will also be influenced by residential wood combustion and other space heating
emissions and so northern mountain valley locations with high traffic counts and
congestion plus limited dispersion on cold winter mornings (when secondary NOz
formation is minimal) may see relaticely higher CO concentrations. As with NOj, I'm
not convinced that CO is the most (or second most) health-relevant component of
roadway emissions, and would hope that suitable near-road sites could be identified to
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address both pollutants, with a smaller number of sites added to address specific CO-
specific concerns when the CO NAAQS revision is final. For both pollutants, I think the
objective should be to characterize near-road population exposures to mix of traffic-
related emissions, and not just to witch-hunt for the worst-case locations of maximum
single-pollutant concentrations.
7. Does the committee believe that siting considerations for identifying the location of
peak NO2 concentrations will likely address all of the high priority siting
considerations for PM (particularly PM2.5) as well? If not, what other factors
should be considered and what are the advantages in considering these factors for
identifying the location of maximum PM concentration?
While there is likely a significant near-road enhancement of local PM2 5 concentrations, I
think this roadway enhancement is proportionally much smaller for PM2 5 mass - compared
to the roadway enhancement of NO2, BC, ultrafines, etc., and that PM2 5 should not be a
priority consideration in siting. Also, since diesel emissions are major contributors to
roadway NO2 and PM2 s there should not be much conflict in siting objectives.
8. In addition to PM2.S mass, what other PM-related measurements are desirable at
near-road monitoring stations (e.g., UFP number, black carbon, EC/OC, PM coarse,
etc.)?
As indicated above, I would give all of the above a higher priority than PM2s mass
measurements, and would push more for continuous instruments that would better
characterize the entire particle size distribution. Roadway fine particle concentrations are also
likely to include a substantial semi-volatile component, which is not well characterized by
PM2s FRM (or FEM) instruments. The roadway increment in coarse particle concentrations
is likely to be proportionately greater than for fine particles, and coarse-only sampling should
be given a higher priority at some of these sites. Past consideration of setting an "urban"
coarse particle NAAQS, was based on an assumption of greater inherent toxicity in urban
areas. But this (logical) assumption was not supported by much measurement data. The
carbon species (BC and/or EC/OC) and particle number information will be more useful than
PM25 mass for health effects studies and source attribution, especially given the longer
averaging times - 24-hour and annual - for the PM2 5 NAAQS and currently stated intent to
keep the annual standard "controlling".
9. To allow for near-road monitoring infrastructure to be multi-pollutant, and in
reflection of the recently promulgated near-road NO2 siting criteria,
reconsideration of the existing microscale CO siting criteria presented in sections 2,
6.2, and table E-4 in 40 CFR Part 58 Appendix E may be warranted. Does the
subcommittee believe that reconsideration of microscale CO siting criteria is
appropriate? Specifically, would an adjustment of CO siting criteria to match those
of microscale PM2.5 and microscale near-road NO2 sites be logical and
appropriate?
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As previously indicated, I think the 50 meter range proposed for "near-road" NOz is too large
and should be tightened prior to attempting to harmonize the various microscale criteria for
various pollutants. Conceptually, near-road measurements for multiple pollutants might
"standardized" to expected concentrations at a single fixed distance, or perhaps a (closer than
SO m) maximum and a minimum sampling distance could be specified, with a smaller
number of "research" sites encouraged that could collect useful data very close to roadsides
(inside the minimum distance) that would help characterize the roadway contribution without
being used for compliance determination.
10. Even if the adjustment of microscale CO siting criteria in sections 2, 6.2, and table
E-4 in 40 CFR Part 58 Appendix E to match that of microscale PM2.5 and
microscale near-road NO2 is appropriate and proposed, should there be
consideration to maintain the requirement on how urban street canyon or urban
core microscale CO sites should be sited?
Offhand, I don't see why near-road distances for CO and NCh should be different, or why
CO should be relaxed to NOz distances.
11. Does the subcommittee have an opinion on how "urban street canyons" or "urban
core" might be defined, perhaps quantitatively, and with regard to use in potential
rule language?
No opinion.
12. EPA and NACAA will select the locations for permanent sites that are part of the
near-road pilot study based on which state or locals volunteer to participate and can
process grant funds in a timely manner to deploy equipment. From this pool of
volunteers, selection should be made on certain attributes that provide the best
potential to fulfill pilot study objectives. In the attached draft white paper, EPA and
NACAA have proposed some potential criteria for consideration in selecting where
the fixed, permanent stations should be located. These considerations include
choosing a large and a relatively small urban area based on population, an area with
varied or complex terrain, an urban area with an operational NOX analyzer
representative of neighborhood or larger spatial scales for comparison to the near-
road NOX analyzer, and an urban area with a cooperative (or non-cooperative)
Department of Transportation. Does the Subcommittee agree with these
considerations? Further, are there other considerations that should be evaluated in
selecting pilot cities to house permanent near-road monitoring stations as part of the
pilot study?
I'm not sure sufficient funds are available to address so many different kinds of locations. It
will be necessary to take maximum advantage of existing sites (including urban background
sites and those operated by research groups) to the extent possible. The
availability/participation of academic or private sector groups to add supplemental
measurements should also be encouraged. With such limited funds, is it necessary to
intentionally include a small city or an area with a non-cooperative DOT? If small cities are
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included (or not), selected sites should be adjacent to roadways with high volumes of diesel
traffic and frequent rush hour congestion. Given the rapid decline of roadway influence with
distance, priority should be given to sites which are substantially closer than 50 meters from
the road (10 to 20 meters max). Possibly the effects of complex terrain could be more
effectively studied in a winter saturation study than with permanent monitors, although the
exaggerated diurnal stagnation patterns and extent to which these correspond to rush hour
periods would be useful to characterize with continuous instruments. It would also be useful
to consider urban areas which have different kinds of public transportation systems, bus and
taxi fleets and associated fuels, etc.
13. EPA and NACAA have proposed that at least two urban areas should have
permanent near-road monitoring stations (that would fulfill NO2 near-road
monitoring requirements) implemented for the pilot study. Please comment on the
minimum equipment/pollutant measurement complement that should be deployed at
each site and also the ideal equipment complement that each site should or could
have, respectively. Specifically, what pollutants (e.g., NO2, NOX, NO, CO, PM
(Ultrafine, 2.5, and 10), black carbon, air toxics (such as benzene, toluene, xylene,
formaldehyde, acrolein, or 1,3, butadiene) and ammonia) and other information
should the pilot study measure or gather at the Fixed, permanent monitoring
stations, and by what methods? This list should be in priority order, as feasible, and
can include any NAAQS or non-NAAQS pollutant by any method (FRM/FEM
and/or non-reference or equivalent methods), any particular type of other
equipment for gathering supporting data such as meteorology or traffic counts.
The objectives seem to be somewhat mixed here. To a large extent the proposed pilot study
seems to be focused on gaining insights into the process of citing near-road monitors (for
NC>2 and to a lesser extent for CO NAAQS compliance determination. The emphasis is on
understanding the relative importance of various traffic and roadway indicators (of varying
and often unknown quality) to guide MOj site selection, the logistical and institutional
difficulties associated with establishing new sites in challenging environments, etc. From this
perspective, the subsequent use of any resulting measurement data (other to confirm whether
NO2 and/or CO are exceeding or close to NAAQS) is almost irrelevant. Retaining several of
the fixed location sites, and building them into much more comprehensive sites where the
objective is to actually learn something about near-road multi-pollutant exposures is an
entirely different (but no less desirable) objective.
In selecting these few comprehensive sites, I would try to assure that they are close enough to
roadways to capture the extreme gradients for pollutants like NO2 and ultrafines, and also
make sure there's a relatively nearby representative urban site with similar measurements to
help quantify the roadway increment. Because roadway emissions, and to a large extent
population exposures in near-road locations tend to have large diurnal variability, I would
generally limit the measurements to species that can be quantified continuously. Beyond that
I defer to others on the committee to prioritize the species.
14. EPA and NACAA have proposed four to five urban areas to have saturation
monitoring, using either passive devices and/or continuous/semi-continuous
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saturation type multi-pollutant monitoring packages (i.e., several types of monitors
in one mountable or deployable "package"). Please provide comment on:
a. The pollutants that should be measured with the saturation devices at each
saturation site.
b. The number of saturation devices per pollutant, both passive and/or
continuous/semi-continuous, that may be deployed in each pilot city.
I don't have much expertise here, but think that (especially given the very limited
budget), the selection of species and number of sites are inter-related and depend on
available methods and costs. Ideally, the minimum species for saturation sampling would
include at least NO2, NOx, CO, BC, but I don't believe there are sufficiently reliable
passive samplers for CO. Possibly a nested approach could be applied with larger
numbers of passive NO2 samplers where applicable and smaller numbers of portable
continuous devices for other key species. Note also George Allen's suggestion to
combine passive samplers with timed pump inlets - which might improve both pollutant
sensitivity and temporal resolution. See also previous comments
c. Whether placing saturation monitoring devices near certain road segments
should include, at a minimum: 1) the highest AADT segment in an area, 2) the
road segment with the highest number of heavy-duty truck/bus counts, 3) at a
road segment with more unique roadway design, congestion pattern, or terrain
in the area, and 4) if feasible, at a lower AADT segment with a similar fleet mix,
roadway design, congestion, terrain, and meteorology as the top AADT road
segment in the area.
All of the above seem like reasonable (but somewhat idealized) kinds of locations. I
question whether it will really be possible to identify "a lower AADT segment with a
similar fleet mix, roadway design, congestion, terrain, and meteorology as the top AADT
road segment in the area" or if in doing so it could realistically be assumed that differing
AADTs were the sole cause of any differences in concentrations. The effect of differing
AADTs might better be explored by sampling during different time periods along a single
road segment. To the extent possible, it would be useful if these sites were located at
similar distances from, and at rush hour downwind directions from the associated
roadways. Assuming this may not be possible, lines of additional passive NO2 sensors
might be added at each site, perpendicular to roadways and in upwind and downwind
directions. Meteorological measurements may also be needed at some or all of these
sites, and similar kinds of species measurements should be added a non-road
representative urban site to help define the roadway increments from the different
saturation sites.
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Dr. Jay Turner
1) Please provide suggestions on any missing subjects that should be included in the
guidance document and any unnecessary topics that are currently listed in the
attached draft, if applicable.
The outline for the guidance document is fine.
2) Please prioritize the recommended pollutants and provide the rationale for their
ranking, including how this pollutant measurement will contribute to scientific and
regulatory knowledge of near-road air quality and adverse human health effects
In general, the priority should be to monitor for indicator compounds for motor
vehicle exhaust. In descending order of priority: (i) NO/NOx, opportunistic because
the NO2 measurement method will likely by NCh by difference (NOx minus NO); (ii)
BC, as a second indicator for diesel emissions and thus sharpen the interpretation of
the NC>2 data; (iii) CO, a stronger indicator for gasoline-fueled vehicle emissions, to
compare and contrast with the indicators for diesel emissions; (iv) air toxics, an
indicator for vehicle emissions; and (v) PM as an indicator for diesel emissions, albeit
with confounding by road dust (depending on the PM site that is monitored). For
many of these pollutants there is the potential for confounding by high upwind
concentrations - that is, the measurement (absolute concentration and/or
concentration variations) may not necessarily be a dominated by vehicle emissions
from the roadway.
3) Identifying Candidate Near-road Site Areas..
Is the objective to monitor at the highest NO2 site within 50m of a roadway, or at the
site with highest NO2 attributed to the proximate roadway? That is, should there be
consideration of aggregate upwind effects, e.g. from a dense roadway network, that
leads to high background NO2 at the roadway to be monitored. Or, is the interest in
selecting a roadway with high NO2 difference across the roadway? If it is the former
case then an expanded list of considerations for identifying candidate sites is needed.
In general, I support the development of a screening tool to guide the site
identification process. Screening tools have been used for hot spot analyses and a
similar approach could be used in the site selection process. One approach is to use
dispersion modeling to create look-up tables to semi-quantitatively) relate roadway
(and other sites) characteristics to potential impacts. A more refined analysis could
subsequently be taken to prioritize the sites identified from the screening process.
a) The proposed approach to consider AADT and fleet mix is reasonable as long as
there is also a consideration of vehicle speed (e.g. through the LOS, below) to
capture the speed dependence of emission rates. Emission rate estimates based on
more sophisticated and/or site-specific inputs could be used if the area has such
information readily available, but is not a high priority.
b) The consideration of on-the-books vehicle emission controls is a low priority in
the site selection process. Perhaps more important out-year considerations would
be any programmed or planned changes to the roadway corridor.
c) A preference to sites at grade with no obstructions is a reasonable objective.
There should be some flexibility, however, especially if the impact of other
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designs or obstructions is such that a specific candidate site is still expected to a
highNO2 concentration zone.
d) The consideration of LOS is best handled through a screening tool that is
grounded in dispersion modeling (see my preamble this question, above). The
key is to capture the speed dependence of emissions.
e) I am less concerned for vegetation if the focus is on NC>2 as long as the vegetation
does not lead to a significant airflow obstruction. This might be important factor,
however, for other candidate pollutants.
f) Upwind pollutant meandering is certainly observed especially when winds are
light and variable (in which case, the meandering is driven wind direction
variations) and/or the vehicle-induced turbulence dominates over the prevailing
air flow. That said, preference should be given to site locations that are nominally
downwind for the meteorological conditions leading to highest impacts. Perhaps
a screening tool could be used to identify the conditions. A key aspect is
overlaying the prevailing diurnal wind patterns with the diurnal traffic patterns to
estimate conditions - and thus near-field locations - for maximum impacts.
4) Please comment on the available modeling tools, and their pros and cons, that the
subcommittee believes may be appropriate to discuss and/or recommend for use in
the near-road monitoring guidance document.
The use of MOVES and AERMOD might be overly burdensome for the site selection
process. As previously described, I advocate a screening tool be developed that is
grounded in dispersion modeling that provides semi-quantitative estimates of impacts.
I agencies have MOVES outputs, which are link-based rather than trip-average and
thus likely more relevant to the specific roadway environment, then this information
could be used as input to the screening model. However, for many agencies it might
be too burdensome to generate MOVES output for the near-roadway site selection
process.
5) The use of saturation monitoring and on-road monitoring are also possible tools that
state and local air agencies may choose to utilize in the near-road site selection
process...
Saturation monitoring could be helpful in prioritizing candidate sites. There is
substantial literature on saturation studies with passive monitors, but in these cases
the integration times are typically long. The crux for this application is a saturation
monitoring strategy that has sufficiently high time resolution to be relevant to 1-hour
conditions (this does not mean that 1-hour resolution is needed). Given there is
typically diurnal structure to both traffic patterns and dispersion conditions, one
strategy is to collect samples for sub-daily time periods but integrated over several
days (e.g. a battery of timer-based saturation samplers).
6) If EPA were to propose a new set of minimum monitoring requirements for CO near
roads, the near-road monitoring stations created under the implementation of the
NC>2 monitoring requirements may be an advantageous infrastructure for state and
local air agencies to leverage However, [ ]
I have no preliminary comments on this matter.
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7) Does the committee believe that siting considerations for identifying the location of
peakNO2 concentrations will likely address all of the high priority siting
considerations for PM (particularly PMis) as well?
It might be adequate if the emphasis of the PM monitoring is on that component
related to diesel exhaust emissions, and the emphasis in all of the monitoring is on
roadway-specific impacts and not cumulative impacts which include consideration of
upwind sources. With improved vehicle emissions control technology, the relative
contribution of road dust, tire wear, and brake wear to the traffic-induced PM
becomes more important. If the goal is to capture these impacts in the PM
monitoring, this could lead to diverging siting considerations for NC>2 and PM.
9) In addition to PM2 s mass, what other PM-related measurements are desirable at near-road
monitoring stations (e.g. UFP number, black carbon, EC/OC, PM coarse, etc.)?
While in the ideal case it might be desirable to monitor for various PM components,
practical considerations likely make UFP number and black carbon the most
reasonable candidates. UFP number can be highly variable and confounded by other
atmospheric dynamics events. Thus, its measurement is most useful as site pairs
across the roadway. Thus might be impractical. Black carbon would be of interest in
its own right and to compare and contrast to NC>2.
10) Does the subcommittee believe that reconsideration ofmicroscale CO siting criteria
is appropriate? Specifically, would an adjustment of CO siting criteria to match
those ofmicroscale .PA/2 5 and microscale near-road NO2 sites be logical and
appropriate7
I have no preliminary comments on this matter.
10) Should there be consideration to maintain the requirement on how urban street
canyon or urban core microscale CO sites should be sited?
I have no preliminary comments on this matter.
11) Does the subcommittee have an opinion on how "urban street canyons " or "urban
core " might be defined, perhaps quantitatively, and with regard to use in potential
rule language!
I have no preliminary comments on this matter.
\2)Does the Subcommittee agree with the stated considerations for selecting sites for the
pilot study!
The stated considerations are reasonable, although there relative weighting needs to
be defined because some considerations are more important than others. Assuming
resources will be limited, preference should be given to large urban areas with
sufficient exiting monitoring infrastructure to place the road-side measurements in
context.
13) Please comment on the minimum equipment/pollutant measurement complement that
should be deployed at each pilot study site and also the ideal equipment complement
that each site should or could have, respectively.
The minimum additional measurements should include NO/NOx, CO, black carbon,
meteorology (perhaps at a setback or other nearby representative location), and traffic
characterization (vehicle count, class, and speed). CO2 could also be useful. It would
be ideal to have each of these parameters (but certainly NCh) measured at a
representative "background" site (background from the perspective of the roadway).
The measurement matrix should take into consideration whether the data would be
used to evaluate any screening tool(s) developed to aid in site selection. Beyond this
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minimum list of measurements, others could be added to fulfill specific study
objectives. It is possible that research groups would be interested in adding
measurement which would leveraging the investment in site infrastructure, and it
would be great to accommodate this to the extent practicable.
14) Please comment on the saturation study design details.
The details are reasonable but it might be worthwhile to refine the saturation study
objectives and then revisit the study design details. For example, is it desired to
harmonize certain details within an urban area (or between areas, if possible) to more
clearly evaluate the impact of other details? In one design the saturation monitors
could be placed about the same distance from roadways, while in another design they
could be placed at different distances from the roadways. If distance from roadway is
a key parameter, then this could impact the data and its interpretation.
Inconsistencies in upwind/downwind siting could strongly influence the interpretation
of data from short-term saturation monitoring studies. If a screening tool was
developed, one objective might be the evaluation of this tool through careful design
of the saturation studies.
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Dr. Warren H. White
AADT, fleet mix, roadway design, congestion, terrain, and meteorology all affect
ambient concentrations, in complicated and interdependent ways. However, the
combined near-road effect of all these influences can be described with just a few degrees
of freedom. The framework outlined below is hardly new (e g. White, 1977), but seems
worth revisiting in light of the new rule.
The key to a simple description is that not much chemistry has a chance to occur
in the short time air spends near the road. A cross-road wind component of only 1 m/s,
for example, carries air from 150 m on one side to 150 m on the other in just 5 minutes.
On such a time scale the complex chemistry of smog formation can be considered
determined by the surrounding air, independent of the fresh emissions. More precisely,
the only reactions needing consideration are the rapid scavenging of 63 by NO
[1] O3+NO-»O2 + NO2,
and the rapid photolysis of NO2 to yield
[2] O2 + NO2 -> O3 + NO
after additional steps. These reactions leave unchanged the concentrations of odd oxygen
[Ox] = [O3] + [NO2] and nitrogen oxides [NOX] = [NO] + [NO2], and their relative rates
establish a photostationary state that is generally fairly well approximated in the
atmosphere:
[3][03][NO]/[N02] =
Since Ox and NOX are chemically conserved near the road, their concentrations
respond only to physical dilution and mixing. They can be modeled as the sum of a
variable contribution from roadway vehicle exhaust and a uniform background supplied
by the surrounding air. For given concentrations [Ox]o and [NOx]o at the monitor, the
reactive species can be expressed in terms of NO2:
[NO] = [NOX]0 - [N02] and [O3] = [Ox]0 - [NO2].
Substituted into the photostationary equilibrium [3], these identities yield a quadratic
equation in [NO2] that can be solved for [NO2] in terms of [Ox]o, [N0x]o, and k2/k|. The
following plots illustrate some features of the relationship.
The conservation of odd oxygen limits microscale NO2 maxima to the sum of
directly-emitted primary NO2 plus the reservoir of odd oxygen available in the
surrounding air. An important siting consideration is therefore the middle-scale ozone
background, which 1 did not see mentioned in the Study Approach or Charge Questions.
This background bounds the NO2 produced from primary NO emissions, contrary to the
impression one might get from statements such as this (FR v74, nl 34, 7/1 5/2009,
p34441): "However, since the rate of conversion of mobile source NO to NO2 ... is a
generally rapid process, (; e , on the order of a minute (ISA Section 2.2.2)), NO2 behaves
like a primary pollutant in the near-road environment, exhibiting peak concentrations on
or closely adjacent to roads."
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I will be happy to supply the spreadsheet used to generate Figures 1-3 to anyone
else who might like to play with it.
Reference:
White, Warren H. (1977) NOx-03 photochemistry in power plant plumes: comparison of
theory with observation. Environmental Science & Technology 11, 995-1000.
scenario
NO, = [NONO21, ppb
oxidant = [NO2O3], ppb
photostationary ratio = 03NO/MO:, ppb >
exhaust NO,/NO,
200
12345 determinants
range range range range range
50 50 SO SO 50 middle-scale background
10 10 10 10 10 temperature and photolysis rate
5% 10% 15% 20% 25% diesel fleet characteristics
scenario
— 1
— 7
3
4
•MHHHI.Q
100 200 300
NOX (ppb)
400
500
Figure 1. If oxidant background is 50 ppb (~ 50 ppb O3 PRB + <1 ppb NO2 PRB), then
even 25% NOj in the fleet exhaust and 200 ppb near-road NOX is not enough to make 100
ppb NO2.
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scenario
NO, = [NO+NO2], ppb
oxidant = [NO2+O3], ppb |
photostationary ratio = O3NO/NO2, ppb >
exhaust NO2/NO,
200
150
& 100
O
50
2345 determinants
range range range range range
100 100 100 100 100 middle-scale background
0 5 10 15 20 temperature and photolysis rate
10% 10% 10% 10% 10% diesel fleet characteristics
scenario
r ^
2
3
•mri--i.--i.---J-- A.
r-
100 200 300
NOX (ppb)
400
500
Figure 2. The most favorable condition for NC>2 at a typical background oxidant level (75
ppb Os + 25 ppb NC>2) is a dark sky (small photostationary ratio) to minimize NC>2
photolysis.
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scenario 1234
NO, - [NO+NO;], ppb range range range range
oxidant = [NO2-tO3], ppb § 75 100 125 150
photostationary ratio = O3NO/NO2, ppb " 10 10 10 10
exhaust NOz/NO, 10% 10% 10% 10%
200
5 determinants
range
175 middle-scale background
10 temperature and photolysis rate
10% diesel fleet characteristics
scenario
—1
2
3
M
5
100 200 300
NOX (ppb)
400
500
Figure 3. In the absence of elevated exhaust NO2/NOX ratios, background oxidant is
needed to convert the primary NO emissions.
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scenario 12345
NO, = [NO+NO2], ppb range range range range range
oxidant = [NOj+Oj], ppb « 75 100 125 150 175
photostationary ratio = O3NO/NO2, ppb > 10 10 10 10 10
exhaust NOj/NO, 10% 10% 10% 1096 10%
determinants
middle-scale background
temperature and photolysis rate
diesel fleet characteristics
500
400
300
200
100
NOX (ppb)
scenario
^m 1
2
3
4
5
tracer
Figure 4. Concentrations of conserved primary emissions like CO ("tracer") fall off more
rapidly with distance from the roadway than those of NC«2 do.
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Questions for EPA:
I am not convinced that a substantial near-road monitoring program forNC>2 and other
traffic-related species is a good use of Agency resources. I think it will be hard to
implement in a meaningful way, and I don't see great potential value in the data it will
produce. I recognize that the decision has been made already, and that I am not required
to understand the reasons behind it. I could better focus on our charge questions,
however, if 1 had answers for the following questions of my own.
1. What is meant by hourly NO2 concentrations - should they be equivalent to
actual arithmetic averages of instantaneous concentrations? Exhaust concentrations at a
near-road sampling inlet can vary greatly within a few seconds. In a given setting
(background oxidant levels and meteorology), NC>2 concentrations depend nonlinearly on
exhaust concentrations. Under these conditions an instrument's time response - and the
nature of any 'internal averaging' - requires careful characterization. The reliance on a
difference method (NOX-NO) further heightens the challenge for measurements near the
road, where the signal/noise ratio is least. Is the goal, as it was with the PM2 5 FRM, to
replicate the undefined and uncontrolled shortcomings of historical data that underlie
existing epidemiological analyses? Or is it to make an accurate measurement of N02?
2. How are concentrations from microscale locations to be linked to available
public health statistics for epidemiologic analyses? Data from neighborhood- or urban-
scale monitors have demonstrated utility for epidemiology because they are indicative of
typical exposures for identifiable populations large enough to generate routine public
health statistics. The numbers of residences near microscale monitors will be small, and
the vehicle occupants driving by them will be anonymous. Will site-specific panel
studies be required to connect the near-road data to health effects?
3. How large a slice of the monitoring pie is ultimately contemplated for near-road
monitoring? The Agency deserves great credit for recognizing the need "to support
measurement of multiple NAAQS pollutants" in calling this meeting. "Maximum
expected hourly concentrations" are likely to occur at different locations (with different
vehicle mixes and road characteristics) for different candidate species (e.g. NOX, CO,
black carbon, PMo i, and PMio). And health researchers will view the consequently
different pollutant mixes as an important environmental signals for epidemiological
analyses. Measuring different species at different sites would clearly be of little value for
anything more than a NAAQS-compliance determination. Are we looking at NCOR.E on
steroids, something like 75 x (number of traffic-related species) for the total number of
sites?
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Dr. Yousheng Zeng
General Comments
Near-road monitoring requirements: The purpose of near-road monitoring is to protect
the health of residents living near roadways. There should be a screening criterion: In a
particular CBSA, if there are no residents living within the 50-m corridor, near-road
monitoring should be exempted. Following a similar line of thinking, if there is only one
community within the 50-m corridor, the near-road monitor should be sited at this
community, and not necessarily at a location where the impact is highest. In this case,
other siting analysis is unnecessary.
The end-point of near-road monitoring: Normally when an ambient monitor shows
exceedance of NAAQS, state/local authorities are required to develop a State
Implementation Plan (SIP) to bring the area into attainment with NAAQS. The State
Implementation Plan will include some control measures to achieve attainment. If a near-
road NCh monitor shows exceedance of NAAQS, how will a non-attainment area be
delineated and what does EPA expect the state/local authority to do? Due to the nature of
significant concentration gradient along the roadways, the area with high NC>2
concentrations could be extremely small. What will be the basis for designating an area
as non-attainment area? The non-attainment is basically caused by mobile sources. In
some areas, it is largely attributable to vehicles passing through the area on the interstate
highways. What can the state/local authority do to achieve attainment? If the state/local
authority cannot do anything, what is the point of requiring this type of near-road
monitoring? EPA could conduct some studies and achieve attainment through regulations
on vehicle emission standards.
Charge Question 3.c
In urban areas, the road segments that have high AADT are commonly elevated
roadways. Requiring monitoring sites at-grade will either miss the plume from the
roadways or significantly limit the choices for the monitoring sites. As far as the vertical
location is concerned, the guidance document should consider the two factors - (1) the
monitor's probe intake should be in the general vertical area of the plume coming from
the roadways; and (2) the residence time for sample to travel from the probe intake to the
analyzer will meet the criteria (20 sec.), i.e., no extremely tall probe from the ground that
cause a long residence time. As long as these two criteria are met, there is no need to
specify whether the monitor needs to be at-grade.
Charge Question 4
In many traffic related air quality impact analyses (e.g., air quality analyses as part of
required NEPA process for highway projects), the CALINE3 and CAL3QHC models are
used. They are still listed as preferred models on the EPA SCRAM webpage. EPA should
evaluate these models along with AERMOD and provide guidelines on which model
should be used for siting near-road monitors.
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In this guidance document. EPA should explain if and how Ambient Ratio Method
(ARM) and Ozone Limiting Method (OLM) can be used in conjunction with AERMOD
model to convert freshly emitted NO to NO2. On June 28. 2010, EPA issued a memo
addressing these issues for more general NO2 modeling
(http://www.epa.gov/nsr/documents/20100629no2guidance.pdf). There should be some
consistence between these modeling policy memos and this guidance document to be
developed. If the guidance document identifies CALINE3 or CAL3QHC as allowed
model, it also should explain if and how ARM and OLM can be used.
Also see my response to Charge Question 6 on modeling.
Charge Question 5
A trailer-based transportable monitor will be very useful and practical for near-road
monitoring. It will be self contained (a generator, analyzer, zero air, calibrator, retractable
met tower, wireless modem, etc.) in a relatively small trailer. It can be pulled by a pick-
up truck to a candidate site for a day, a week, or a longer period of monitoring. It will be
moved to another candidate site. Once the candidate site screening is completed, the
trailer can be stationed in the chosen permanent site, blocked up and tied down to serve
as the permanent near-road monitor in that CBSA. The data generated by such a system
will have the same quality as fixed monitoring station. Compared to a motor vehicle
based monitor, the trailer-based unit offers comparable mobility at much lower cost, and
it can used as a fixed monitor at a permanent site for years. In terms of data quality and
comparability, the data generated by a FRM or FEM analyzer in the trailer-based monitor
has a higher quality and confidence level than the data generated by other screening
instruments (passive devices and portable instruments). Presumably there will be no
meteorological (met) instruments collocated with passive or portable devices. An analysis
of the data gathered by these devices will rely on met data from nearby met stations. For
near-road monitoring, the wind conditions will be relevant and extremely localized. The
analysis based on met data from some distance could be misleading.
Charge Question 6
Peak CO concentrations are expected in urban street canyons and/or urban cores,
especially at intersections where cars are idling in front of traffic light and the impact is
coming from more than one street. I am not familiar with typical NO2 concentrations in
this type of situation as compared to NO2 concentrations near major highway with heavy
traffic. I am sure this type of data is available. If NO2 concentrations in urban street
canyons are comparable to the NO2 concentrations near major highway, using one site to
serve the monitoring need for both CO and NO2 should be encouraged in the guidance
document. Otherwise, it would be infeasible to make a compromise between the two
needs and the monitoring for CO and NO2 should be addressed separately.
The CAL3QHC model is design to predict CO concentrations near road intersections. If
EPA has validated the model, should the modeling be sufficient for determination of
compliance with CO NAAQS and therefore no monitoring is required? In the recent SO2
NAAQS rule, EPA is changing its long-standing position of using monitoring data for
NAAQS attainment determination, and will use modeling for NAAQS attainment
determination. For the same rationale, using a modeling analysis to determine NO2 and
CO NAAQS attainment seems reasonable. Similar to (actually even worse than) the case
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of SO2, ambient CO and NCb concentrations have an extremely high spatial variability
near road. One monitor showing compliance with NAAQS at one street corner or road
segment does not mean that the NAAQS is attained at different street corner or road
segment. Modeling can cover a much larger space at a much lower cost. Even at the same
street corner or road segment, moving the monitor by one meter could make the
difference of attaining or not attaining NAAQS.
Charge Question 11
Before responding to Charge Question 11,1 would like to ask if monitoring these
pollutants with extremely high spatial variability in a micro-scale is a good idea. See my
response to Charge Question 6. If the answer is no, there is no need to spending resources
to develop definition of "urban street canyons" and "urban core" and associated guidance
for monitoring.
In case EPA wants to pursue monitoring at street locations with high traffic volume and
high spatial concentration variability, the following elements should be considered in
defining urban street canyons:
• Traffic information similar to the one for near-road monitoring (e.g., AADT,
posted speed limit, traffic light cycle)
• Street geometry
o Ratio of street side building height to the width of the street (H/W ratio).
Need to develop an approach to the treatment of (1) different heights of
buildings on the two sides of the street and (2) tiered buildings.
o One-way vs. two-way street (more plug flow in one-way and more
turbulent in two-way street).
o Is the street lined with trees on the sidewalk? Tree canopy may have an
effect of umbrella and trap portion of pollutants at the street level.
o Some way to normalize the H/W ratio with respect to number of traffic
lanes on the street. This factor may not be important because the effect
may have been incorporated by the combination of H/W ratio and the
traffic volume (e.g., AADT).
• Meteorological factors: the angle between the street and prevailing wind direction
(higher concentrations are expected if the angle is 90 degree).
In the context of ambient air quality monitoring rule, perhaps a set of cut-off values
reflecting the above mentioned elements can be used to define urban street canyons.
Charge Question 14.C.
The impact of mobile sources to ambient air quality is governed by two types of factors,
vehicle emissions and dispersion conditions. For compliance monitoring, the monitors
should be placed near the highest impact area, which means both emissions and
dispersion conditions are equally important. For the pilot study, however, factors
associated with dispersion (e.g., terrain, roadway design, extremely micro-scale
meteorological conditions) should be given more attention than factors related to the level
of vehicle emissions (e.g., AADT, fleet mix) because the emission rates can be
characterized well using current tools (e.g., MOVES), AND the emission rates are the
only parameter that impact ambient concentration in a linear or near linear fashion. If the
pilot study can provide better understanding of the dispersion, the impact of a higher
emission scenario can be anticipated or predicted by simply substituting the emission
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rates. The information derived from a more dispersion focused pilot study will be more
useful than an emission focused pilot study.
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