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MEMORANDUM

SUBJECT: Near-road NO2 Network and Data Analysis

FROM: Nealson Watkins, AQAD/AAMG (C339-02)

Adam Reff, AQAD/AQAG (C304-04)

TO: Docket No. EPA-HQ-OAR-2015-0486

This document presents information on the characteristics of the existing near-road NO2

monitoring network and the relationships between NO2 emissions, population, and traffic

with an analysis of near-road NO2 concentration data in support of the Revision to the 

Near-road NO2 Minimum Monitoring Requirements proposed rulemaking. The proposed 

revision would remove the existing network design requirements to install near-road NO2

monitors in Core Based Statistical Areas (CBSAs) having populations between 500,000 

and 1,000,000 persons, due by January 1, 2017. Current near-road NO2 monitoring data

indicate air quality in the near-road environment to be well below the National Ambient 

Air Quality Standards (NAAQS) for the oxides of nitrogen. In light of this information, 

and due to the correspondence between population, traffic, and expected NO2

concentrations in the near-road environment, it is anticipated that measured near-road 

NO2 concentrations in relatively smaller CBSAs (e.g., CBSAs with populations less than 

1,000,000 persons) would exhibit similar, and more likely lower concentrations than what 

is being measured in larger urban areas. As a result, the EPA is seeking to reduce 

additional burden on state and local air monitoring agencies by removing monitoring 

requirements in smaller CBSAs where measured NO2 air quality is expected to be well 

below the NAAQS. This document is intended to provide a review and analysis of 

emissions data, available monitoring site meta-data, population and traffic data, and NO2

monitoring data in order to provide the underlying rationale for the proposed rule. 

Background

On February 9, 2010, the EPA promulgated revisions to the NO2 NAAQS (75 FR 6474), 

which included a 1-hour standard with a 98th percentile form averaged over 3 years and a 

level of 100 parts per billion (ppb), reflecting the maximum allowable NO2 concentration 

anywhere in an area, while retaining the annual standard of 53 ppb. As part of the 2010 

NO2 NAAQS rulemaking, the EPA promulgated revisions to requirements for minimum 

numbers of ambient NO2 monitors that included new monitoring near major roads in 

larger urban areas, requirements to characterize NO2 concentrations representative of 
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wider spatial scales in larger urban areas (area-wide monitors), and monitors intended to 

characterize NO2 exposures of susceptible and vulnerable populations. A key objective to 

be met by requiring near-road monitoring was to measure maximum NO2 concentrations 

that may occur anywhere in an area, as the NO2 NAAQS review estimated that roadway 

associated exposures account for the majority of exposures to peak NO2 concentrations. 

Per 40 CFR part 58, Appendix D, near-road NO2 monitoring is currently required in each 

CBSA with a population of 500,000 or more persons to monitor a location of expected 

maximum hourly concentrations sited near a major road. An additional near-road NO2

monitoring station is required at a second location in any CBSA with a population of 

2,500,000 or more persons, or in any CBSA with a population of 500,000 or more 

persons that has one or more roadway segments with 250,000 or greater Annual Average 

Daily Traffic (AADT) counts. Based upon 2014 census estimates and data maintained by 

the U.S. Department of Transportation Federal Highway Administration on the most 

heavily trafficked roads in the U.S. 

(http://www.fhwa.dot.gov/policyinformation/tables/02.cfm), approximately 131 near-road 

NO2 sites are required within 108 CBSAs nationwide. The near-road NO2 monitoring 

network was originally required to be operational by January 1, 2013. However, in 2012,

the EPA proposed (77 FR 64244; October 19, 2012) and finalized, through a public 

notice and comment rulemaking, that the near-road NO2 monitoring stations would be 

installed in three phases spanning a 4-year period. The installation schedule for the nearroad NO2 monitoring network was then modified to reflect the following:

Phase 1: In CBSAs with a population of 1,000,000 or more persons, one nearroad NO2 monitor shall be reflected in the state Annual Monitoring Network Plan 

submitted July 1, 2013, and that monitor shall be operational by January 1, 2014.

Phase 2: In CBSAs where two near-road NO2 monitors are required (either 

because the CBSA has a population of 2,500,000 or more persons, or has a 

population of 500,000 or more persons plus one or more roadway segments 

having annual average daily traffic (AADT) counts of 250,000 or more), the 

second near-road NO2 monitor shall be reflected in the state Annual Monitoring 

Network Plan submitted July 1, 2014, and that monitor shall be operational by 

January 1, 2015. 

Phase 3: In CBSAs with a population of at least 500,000 persons, but less than 

1,000,000 persons, one near-road NO2 monitor shall be reflected in the state 

Annual Monitoring Network Plan submitted July 1, 2016, and the monitor shall 

be operational by January 1, 2017.
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The shift to this phased installation approach was finalized and published on March 14, 

2013 (78 FR 16184), to allow more time for states to establish the required monitoring 

sites and in order to synchronize installation with a schedule that would be consistent 

with available resources.

Near-road NO2 Monitoring Site Characteristics

In the preamble to the proposed 2010 NO2 NAAQS revision, it is stated that one of the 

largest factors affecting ambient exposures to NO2 above health benchmark 

concentrations are mobile source emissions (74 FR 34441). Further, in both the proposed 

(74 FR 34404) and final NO2 NAAQS revision rulemaking (75 FR 6474), it is stated that 

the near-road environment is believed to be where the greater population exposure risk to 

peak NO2 levels exists. As a result, near-road NO2 monitors were required to support the 

NAAQS. Near-road NO2 monitors were intended to be adjacent to the most highly 

trafficked roads within our nation’s urban areas, as these locations are where maximum

mobile source-derived NO2 concentrations were typically expected to occur. Required 

near-road NO2 monitors were to be installed with consideration of a number of factors 

known to influence NO2 concentrations in the near-road environment and using specific

siting criteria to ensure the monitors would be near roads where measured concentrations 

might represent the highest near-road NO2 concentrations for a given CBSA. These siting 

criteria are listed in 40 CFR part 58, Appendices D and E. Notably, near-road NO2

monitoring sites were required to be selected by accounting for AADT, fleet mix, 

roadway design, congestion patterns, terrain, and meteorology. Further, the sites were to 

be situated so that the monitor probe was as near as practicable to the outside nearest 

edge of the traffic lanes of the target road, but not located greater than 50 meters away 

from the edge of the traffic lanes. 

In support of the state and local air monitoring agencies responsible for installing these 

near-road NO2 monitoring sites, the EPA coordinated a near-road monitor siting study 

and subsequently produced the “Near-road NO2 Monitoring Technical Assistance 

Document,” for which the results and finished document, respectively, can be found on 

the EPA’s Ambient Monitoring Technology Information Center (AMTIC) website’s 

near-road NO2 page located at http://www3.epa.gov/ttn/amtic/nearroad.html. The “Nearroad NO2 Monitoring Technical Assistance Document” goes into detail on the 

importance and impact each of the individual factors (e.g., AADT, fleet mix, congestion 

patterns, terrain, and meteorology) have on the concentrations that might be measured at 

a near-road site. The EPA has been tracking many of these important factors, along with 

other physical information represented as site characteristics, pertaining to near-road NO2

monitoring sites as those sites have been installed and become operational. These metadata have been and will continue to be collected and updated as the near-road network 

becomes fully installed and operational. These meta-data are available at 
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(http://www3.epa.gov/ttn/amtic/nearroad.html). The meta-data presented and used in this 

document reflect data collected through December 2015, and include both partial and

complete meta-data on a total of 71 near-road sites. 

Upon review of the available near-road site meta-data (as of December 2015), it is 

evident that state and local air monitoring agencies have had success in identifying and 

securing access to property adjacent to highly trafficked road segments within CBSAs, 

pursuant to requirements in 40 CFR part 58. It is worth noting that there are thousands of 

road segments in a typical CBSA. The near-road network is appropriately focused on the 

most highly trafficked road segments, as shown by the distribution of near-road NO2 site 

placements relative to the highest trafficked roads in each respective CBSA in Figure 1. 

Notably, ~55 percent of the near-road sites are adjacent to one of the top 5 highest 

trafficked road segments in their respective CBSA; ~71 percent are adjacent to one of the 

top 10 roads; and ~91 percent are adjacent to one of the top 25 roads. 

Figure 1. Relative Annual Average Daily Traffic (AADT) rankings of the target roads of near-road 

monitoring sites within their individual Core Based Statistical Areas (CBSA).

In the “Near-road NO2 Monitoring Technical Assistance Document,” a metric was 

introduced that used both AADT and fleet mix, called Fleet Equivalent AADT (FEAADT) for use in comparing road segments for potential NO2 emissions with varying 

amounts of light-duty and heavy-duty vehicle make-ups. This comparison is important 

since heavy-duty diesel vehicles emit more NOX (which includes more directly emitted 

NO2 and more NO, which can be oxidized into NO2) than light-duty gasoline vehicles on 
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a per vehicle basis.

1 As a result, understanding the number of heavy-duty vehicles that 

make up total traffic volume was an important factor in determining where near-road 

NO2 monitors might be placed. The FE-AADT metric can be viewed as a stronger metric 

than AADT alone for use in comparing road segments that have varied amounts of total 

traffic (AADT) and heavy-duty vehicle counts when determining which road segments 

might be expected to have higher potential for a near-road site to measure peak NO2

concentrations. Figure 2 shows the distribution of near-road NO2 site placements relative 

to the road segments with higher FE-AADT values in each respective CBSA. Notably, 

~63 percent of the near-road sites are adjacent to one of the top 5 highest FE-AADT 

trafficked road segments in their respective CBSA; ~76 percent are adjacent to a top 10 

segment; and ~93 percent are adjacent to one of the top 25 segments. 

Another key piece of information to understand about near-road NO2 monitoring stations 

is the relative distance to their target road. The relative placement of near-road NO2

monitors to target road segments, in the horizontal, is governed in 40 CFR part 58, 

Appendix E, section 6.4(a), where it is stated that “...the monitor probe shall be as near 

as practicable the outside nearest edge of the traffic lanes of the target road segment, but 

Figure 2. Relative Fleet Equivalent Annual Average Daily Traffic (FE-AADT) rankings of the 

target roads of near-road monitoring sites within their individual Core Based Statistical Areas 

(CBSA).

shall not be located at a distance greater than 50 meters, in the horizontal, from the 

outside nearest edge of the traffic lanes of the target road segment.” As discussed in the 

 

1 Discussed in the Near-road NO2 Monitoring Technical Assistance Document at

http://www3.epa.gov/ttn/amtic/files/nearroad/NearRoadTAD.pdf.
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2010 NO2 NAAQS review’s Integrated Science Assessment (sections 2.5.4 and 4.3.6), 

the Risk and Exposure Assessment (section 7.3.2), and in the preamble to the final rule of 

the 2010 NO2 NAAQS revision (75 FR 6514), on-road, mobile source-derived NO2

exhibits a peak concentration on or very near the source road, and those concentrations 

decline over a variable but relatively short distance back to near area-wide or relative 

background concentrations. It was for this fact that near-road NO2 sites are required to be 

as near as practicable to their target roads. Upon review of the 71 near-road NO2 sites for 

which partial or complete meta-data are available (as of December 2015), it is evident 

that state and local air monitoring agencies were successful in placing their near-road 

monitoring stations as near as practicable to their target roads, pursuant to requirements 

in 40 CFR part 58. Figure 3 shows the distribution of the distances of near-road NO2

monitor probes relative to their respective target roads. Notably, ~59 percent of the sites 

are within 20 meters from their target road (which was a recommended target distance in 

the “Near-road NO2 Monitoring Technical Assistance Document”); ~87 percent are 

within 30 meters; and ~96 percent are within 40 meters. All sites are within 50 meters, as 

prescribed. 

Figure 3. Distribution of distances of near-road NO2 monitors to their respective target road. 
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Emissions, Population, and Traffic

In the preamble to the 2010 NO2 NAAQS proposal, it is stated that one of the largest 

factors affecting ambient exposures to NO2 above health benchmarks concentrations are 

mobile source emissions (74 FR 34441). A look at the NOX emissions inventory, and 

more specifically, the on-road mobile source contributions to the inventory that exist in 

individual CBSAs, can aid in understanding how mobile sources play a role in the 

potential exposure to NO2 for populations nationally, and in an individual CBSA. The 

2011 National Emissions Inventory (NEI) indicates that mobile sources account for about 

72.3 percent of the total NOX emissions inventory, approximately 8.7 million tons out of 

12 million tons, nationally. Figure 4 shows the breakdown between on-road, non-road, 

and all other NOX sources in the NEI, where on-road mobile sources account for ~49.6

percent of the national NOX inventory; non-road mobile account for ~22.7 percent; and 

all other NOX emission sectors account for ~27.7 percent. 

Figure 4. 2011 NOx national emissions inventory, segregated into non-road mobile, on-road mobile, and all 

non-mobile source sectors.

Further inspection of the NEI shows that at the CBSA level, on-road mobile source 

emission contributions increase with increasing CBSA population. Figure 5 shows the 

breakdown of NOX emissions by on-road, non-road, and all other emissions categories, 
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binned by CBSA population size. The CBSA population bins are sorted in

correspondence to the three phases by which the near-road NO2 network is being 

installed: greater than 2.5 million persons; greater than 1 million but less than 2.5 million 

persons; greater than 500,000 but less than 1 million persons; and finally, a bin for all 

areas with populations below 500,000 persons. 

Figure 5. 2011 NOx national emissions inventory, split by non-road mobile, on-road mobile, and all nonmobile source sectors, binned by CBSA population corresponding to the three phases by which the nearroad network is being installed, plus all CBSAs having less than 500,000 persons.

In Figure 5, on-road mobile source emission contributions to the binned CBSA-level 

inventories increase with increased CBSA populations. Notably, on-road mobile source

NOX emissions of CBSA-level inventories rise from ~28 percent in areas with 

populations less than 500,000 persons, to ~48.1 percent in CBSAs with populations 

between 500,000 and 1 million persons, to ~51.3 percent in CBSAs with populations 

between 1 million and 2.5 million persons, and finally up to ~55.3 percent in CBSAs 

having 2.5 million persons or more. These data confirm the major role that on-road 

mobile sources play in the potential ambient exposures of NO2 to the public, particularly 

in increasingly populated areas.
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A key reason behind requiring the near-road NO2 network was the recognition of the role 

of on-road mobile source emissions to potential public exposures, with particular 

emphasis on the fact that health research indicates that certain populations may 

experience relatively higher NO2 exposures with increased proximity to roads (74 FR 

34419). Because there is a correspondence between population and traffic, the EPA 

required near-road monitoring in relatively larger urban areas. One approach to 

understanding mobile source emissions impacts on potential measured NO2

concentrations and population exposure at the CBSA level is to look at the relationship 

between population and Vehicle Miles Traveled (VMT). VMT is a measurement of the 

total number of miles traveled by all vehicles in a specific area over a specific period of 

time. VMT data are typically collected or determined by state or local transportation 

agencies. These data are eventually passed on to the U.S. Department of Transportation, 

Federal Highway Administration (FHWA). The VMT data used in this analysis were 

obtained as part of mobile model inputs submitted to the EPA through the Emissions 

Inventory System (EIS).

Table 1 presents the VMT data that is plotted in Figure 6, with VMT data binned by 

CBSA populations that correspond to the three phases by which the near-road NO2

monitoring network is being installed, plus those areas having populations of 500,000 or 

fewer persons. In Table 1 and Figure 6 below, we see a positive correspondence between 

VMT and CBSA population, where VMT values increase with increasing CBSA 

populations.

Table 1. Mean and median VMT values, binned by CBSA populations 

corresponding to the three phases by which the near-road network is being 

installed, and all CBSAs having less than 500,000 persons.

CBSA Population Mean VMT Median VMT

Less than 500,000 1,607,649,834 1,318,583,764

500,000 to 1,000,000 4,398,168,987 4,102,617,866

1,000,000 to 2,500,000 7,881,014,062 8,523,556,781

2,500,000 and greater 20,924,181,690 16,565,022,565
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Figure 6. Plot of Vehicles Miles Traveled (VMT) of all CBSAs, nationwide, binned by CBSA population 

ranges corresponding to the three phases by which the near-road network is being installed plus all CBSAs 

having less than 500,000 persons. In each plot, the open circle is the mean, the central horizontal solid line 

is the median, and the box edges are the 25th and 75th percentiles. The whiskers extend to the highest value 

that is in between the box edge and 1.5*IQR (inter-quartile range – the distance between the box edges). 

The notches extend 1.58*IQR/sqrt(n) (where n = sample count) and give an indication of significant 

difference between the medians of the boxes. 

Since there is an understanding that near-road monitoring sites have been successfully

placed adjacent to the most highly trafficked roads in their respective CBSAs (as shown 

and discussed in the previous section), we can also use the AADT and FE-AADT counts 

of the target roads of the near-road NO2 network to visualize the relationship between 

CBSA population and the busiest roads in larger urban areas. Figures 7 and 8 plot 

volumes of traffic, using both AADT and FE-AADT, respectively, binned by CBSA 

populations corresponding to the three phases by which the near-road NO2 monitoring 

network is being installed. In these figures, the positive correspondence between traffic 

volumes and CBSA population are clearly discernable, where relative target road AADT 

and FE-AADT values increase with increasing CBSA populations.

The collective population and traffic data shown support the concept that increasingly 

higher populated CBSAs have increasingly more traffic throughout their urban areas. 

Further, higher populated CBSAs typically have more individual roads with relatively 

high traffic volumes than their lesser populated CBSA counterparts. As a result, it is 

evident that higher populated areas correspondingly have more vehicles, which in turn 
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increase the mobile source-derived emissions that lead to increased opportunity for 

higher NO2 concentrations particularly in the near-road environment. 

Figure 7. AADT counts of the target roads of near-road NO2 monitoring sites, binned by CBSA population 

ranges corresponding to the three phases by which the near-road network is being installed.
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Figure 8. FE-AADT counts of the target roads of near-road NO2 monitoring sites, binned by CBSA 

poupulaton ranges corresponding to the three phases by which the near-road network is being installed.

Measured NO2 Concentration Data Analysis

The intent of this analysis of collected near-road NO2 concentration data from the current 

near-road network is to determine whether NO2 concentrations in the near-road 

environment are relatively lower in CBSAs with lower populations compared to larger 

urban areas. The EPA currently estimates that there are 63 operational near-road NO2

sites, with variable data availability dependent upon start date and completeness starting 

in 2011 (a Gantt chart illustrating individual site length of operation and a discussion of 

site installation and operations is located in the appendix of this document). To date, no 

near-road NO2 monitor has recorded an annual mean value that exceeds the 53 ppb 

NAAQS level, nor has any monitor provided a single year 98th percentile 1-hour daily 

maximum concentration value above the 100ppb NAAQS level.2 

Figure 9 depicts the annual average and 98th percentile 1-hour daily maximum value of 

the four near-road sites that had complete data for 2013, while Table 2 provides the same 

statistics plus additional site specific CBSA and traffic meta-data for these four sites. The 

data presented in Figure 9 and Table 2 include near-road NO2 concentration data and 

CBSA and traffic related meta-data for the Boise, Des Moines, Detroit, and St. Louis 

CBSAs. The Detroit and St. Louis CBSAs have populations in excess of 2,500,000 

 

2 The design value for the 1-hour NAAQS standard is calculated using the 3-year average of the 98th

percentile 1-hour daily maximum concentrations for a site.
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persons, while both Boise and Des Moines have populations between 500,000 and 

1,000,000 persons. Viewing data in Table 2, we observe that the larger CBSAs have nearroad sites situated near roads with relatively higher traffic volumes. These 2013 data, 

although a small sample in terms of the number of sites, show that the near-road sites in 

the larger CBSAs measured higher concentrations of NO2 for both the annual mean and 

the 98th percentile 1-hour daily maximum value for 2013 than in the lesser populated 

CBSAs. This observation fits the concept that higher near-road NO2 concentrations are 

expected in relatively higher populated CBSAs. 

Figure 9. 2013 Near-road NO2 concentration summary statistics (in ppb) from the four sites that had a 

complete years’ worth of data, binned by CBSA population ranges corresponding to the three phases by 

which the near-road network is being installed.
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Table 2. Population, traffic, and NO2 concentration data (in ppb) for the four 

CBSAs that had a full year’s worth of data collected for 2013.

CBSA Population AADT FE-AADT

Annual 

Mean

(ppb)

98th % 1hr 

Daily Max 

(ppb)

Detroit 4,296,611 140,500 188,200 18 48.0

St. Louis 2,806,207 159,326 360,077 14 50.4

Boise 664,422 103,000 162,000 11 39.3

Des Moines 611,549 110,000 150,140 9 34.1

Figure 10 presents the annual mean and 98th percentile 1-hour daily maximum value of 

the 20 near-road sites that met data completeness requirements with their 2014 data, and 

Figure 11 presents the same statistics for the 42 near-road NO2 sites with complete data 

for 2015. These data are binned by CBSA population size corresponding to the three 

phases by which the near-road NO2 network is installed. Within each CBSA size bin, the 

data are ordered by annual averages, decreasing from left to right. In Figure 10, for the 

annual mean data and largely for the 98th percentile 1-hour daily maximum data, the 

highest concentrations tend to occur in the largest CBSAs. Most notable is that the NO2

data from the 500,000 to 1 million person size bin are among the lowest of all reported 

data. 

In Figures 10 and 11, we observe that there is overlap in the highest concentrations in one 

CBSA size bin to the lowest concentrations in a neighboring CBSA size bin, but the data 

presented generally support the concept that higher near-road NO2 concentrations are 

expected in higher populated CBSAs as compared to relatively lesser populated CBSAs. 

More specifically, there is some correspondence between CBSA population and NO2

concentrations, where NO2 concentrations increase with increasing CBSA population. It 

is also notable that none of the concentration data are threatening to exceed the annual or 

1-hour daily maximum standard levels of 53 ppb and 100 ppb, respectively. 
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Figure 10. 2014 Near-road NO2 concentration summary statistics (in ppb) from the sites that had at least 7200 hourly observations in AQS, binned by CBSA 

poupulation ranges corresponding to the three phases by which the near-road network is being installed.
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Figure 11. 2015 Near-road NO2 concentration summary statistics (in ppb) from the sites that had at least 75 percent completeness of their hourly observations in 

the 1st,2

nd , and 3rd calendar quarters of 2015 in AQS, binned by CBSA poupulaton ranges corresponding to the three phases by which the near-road network is 

being installed
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Conclusions

The evidence provided for consideration in the preceding sections leads to multiple key 

findings. First, the near-road NO2 monitoring sites are good representatives of the peak 

NO2 concentrations that can exist in the near-road environment, due to their proximity to 

top trafficked roads within their respective CBSAs. This is an important assertion, as 

having the whole of the near-road NO2 network be representative of expected peak, nearroad NO2 concentrations in a given CBSA allows for a fairer comparison of near-road 

data across CBSAs. 

Second, the analysis shows that higher populated CBSAs correspondingly have more 

vehicles, which in turn increases the availability of mobile source derived emissions that 

lead to increased opportunity for higher NO2 concentrations, particularly in the near-road 

environment. In short, higher populated CBSAs might expect to see relatively higher 

near-road NO2 concentrations than their lesser populated CBSA counterparts. Although 

this particular relationship between CBSA population size and measured or expected 

near-road NO2 concentrations is not absolute, the deviations are most likely due to a 

number of known factors including differences in near-road NO2 monitor target road

traffic volume, fleet mixes, congestion patterns, roadway design, terrain, and 

meteorology, along with some influence based upon the distance between the monitor 

and the road and background NO2 concentration differences, as were presented and 

discussed in the 2010 rulemaking and the “NO2 Near-road Monitoring Technical 

Assistance Document.” The influence of these factors is inherently part of the near-road 

NO2 network and were required to be considered in the near-road monitor siting process. 

It should be noted that the measured concentrations at every near-road NO2 site will 

always be influenced by any number of these factors to varying degrees in time. 

Third, the available data meeting 75 percent completeness criterion indicate that, to date, 

no near-road NO2 site has collected data show that NO2 concentrations are above or 

approaching the annual NO2 NAAQS of 53 ppb, nor the 100 ppb level of the 98th

percentile 1-hour daily maximum value. Notably, the highest annual mean value of any 

near-road NO2 monitor in 2014 was 27 ppb, found in the Los Angeles CBSA, while the 

highest annual value of the quarterly 2015 data is 26 ppb, in the Denver CBSA. The 

highest 98th percentile 1-hour daily maximum value of any near-road NO2 monitor in 

2014 is 69.6 ppb, in the Denver CBSA, while the highest value of the quarterly 2015 data 

is 72 ppb, in the New York CBSA. 

Finally, the analysis of the available near-road NO2 data from sites having largely a 

complete years’ worth of data in 2013 and 2014, and the 1

st

, 2

nd , and 3rd quarter analysis 

from 2015, indicate that the larger CBSAs tend generally to have higher measured nearroad NO2 concentrations than their lesser populated CBSA counterparts. This is true for 
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both forms of the NO2 NAAQS, although this correspondence is more strongly 

manifested and observable in the means (i.e., annual averages) of the data. In light of 

these results, the EPA believes the assertion that larger CBSAs generally can expect to 

have higher measured near-road NO2 concentrations than relatively smaller populated 

CBSAs is valid.
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Appendix

The first near-road NO2 site to begin operation is located in Detroit (AQS ID 26-163-

0093), which started as part of an EPA Office of Research and Development research 

study in 2011. The next near-road site to operate is in Boise, Idaho (AQS ID 16-001-

0023), which came online in the spring of 2012 as part of the near-road NO2 pilot study, 

the details of which can be found at http://www3.epa.gov/ttn/amtic/nearroad.html. While 

there were nine sites operational for some period of time during 2013, the bulk of sites 

came online in 2014, as the 1st phase of the near-road NO2 network was due to come 

online January 1, 2014.

Gantt chart showing the length of operations for 63 near-road NO2 sites, as of January 2016. The list is 

ordered alphabetically by CBSA name.

Of the nine near-road NO2 monitors operational and reporting data in 2013, only four of 

those sites operated for the full year (sites in Boise, ID; Des Moines, IA; Detroit, MI; and 

St. Louis, MO). The other five sites (Denver, CO; Hartford, CT; Boston, MA;

Minneapolis, MN; and Kansas City, MO) have data for at least 6 months or so of that 

year, but not enough for a sufficiently complete annual record. In 2014, 42 near-road NO2

sites reported data to AQS. Although Phase 1 near-road NO2 sites were supposed to begin 

operations on January 1, 2014, a number of state and local air agencies were not able to 
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begin operations on time for various reasons or had interruptions in operations, and as a 

result, a number of sites had start-up dates scattered throughout the year or experienced

data gaps. In 2015, the network continued to fill out, and as of January 2016, the EPA 

estimates that 63 near-road NO2 sites are operational, and the remainder of the required 

network is anticipated to be installed and operational during 2016. The EPA is tracking 

the installation and detailed characteristics of the near-road network, and provides these 

data, which is periodically updated, at http://www3.epa.gov/ttn/amtic/nearroad.html.