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Federal Register / Vol. 86, No. 156 / Tuesday, August 17, 2021 / Rules and Regulations
of improvements resulting from existing control programs such as Tennessees I/M program. The commenter goes on to explain that additionally, when comparing monitor-level 4th high ozone maximum daily average MDA813
concentrations for receptors in the Middle Tennessee region, values that are used by EPA in determining ozone attainment and designations, not a single monitor has shown a decrease between 2014 and 2018. In fact, of the five monitors in the domain, . . . three show no change in 4th high MDA8
concentrations between the two years while the other two monitors show an increase of up to 3 parts per billion ppb in the MDA8 concentration observed. The commenter also points to an upward trend in highest concentrations across all monitors, in particular the maximum concentration exceeded 0.085 in 2018. The commenter also asserts that ozone is increasing in the Middle Tennessee Area based on EPAs Air Quality Index AQI and points to increases in the number of unhealthy days for sensitive groups and the maximum AQI value per year.
With respect to Hamilton County, the commenter claims that while current ozone NAAQS-related design values are below the standard in Hamilton County, recent observations in air quality in the region have shown a flat trend in air quality. The commenter goes on to explain that, when comparing monitor-level ozone MDA8 14
concentrations for receptors Hamilton County, Tennessee, values that are used by EPA in determining ozone attainment and designations, neither monitor has shown air quality improvement between 2015 and 2018.
In fact, . . . both monitors in the
domain . . . show no change in MDA8
concentrations between the two years with increases in value poorer air quality in the intermediate years.
Response A3: As discussed above in Response A1, EPA sets the NAAQS at levels protective of public health and welfare. With respect to ozone, the most recent 2015 8-hour ozone NAAQS is met if the annual 4th highest daily maximum 8-hour ozone concentration, averaged over three years, is equal to or less than 70 ppb. See, e.g., 80 FR 65292
October 26, 2015 containing an analysis of the most recent ozone NAAQS. In setting this standard, EPA
considered all of the components of the NAAQS indicator, averaging time, level, and form collectively, and determined that the standard provided the requisite protection of public health and welfare. See id.
EPA agrees with the commenter that the Middle Tennessee Area and Hamilton County are currently attaining all of the ozone NAAQS, including the current 2015 8-hour ozone standard.
EPA must evaluate these SIP revisions for consistency with CAA section 110l, which prohibits the Agency from approving revisions that would interfere with any applicable requirement regarding attainment or any other CAA
requirement. EPA reviews SIP revisions, like removal of the I/M program from Tennessees SIP, to determine whether they meet the applicable requirements of the CAA, including section 110a1, which requires SIPs to provide for implementation, maintenance, and enforcement of the NAAQS. See CAA
section 110k2, 3. EPA considers the status of an area attaining the NAAQS
when EPA evaluates whether a SIP
revision will interfere with attainment or maintenance of the NAAQS.15

In response to concerns about increasing ozone concentrations raised by the commenter, EPA evaluated the air quality trends in both Hamilton County and the Middle Tennessee Area.
The results of this analysis, discussed in detail in the April 2021 SNPRM, show that while both areas have observed yearly variability in measured ozone concentrations, there is not a strong increasing or decreasing trend in the ozone concentrations in either area since 2013. Both areas, along with several other areas in the southeastern United States, measured significantly higher ozone concentrations in 2012.
These high concentrations were primarily the result of meteorological conditions that were very conducive to ozone formation high temperature, low wind speed, and moderate relative humidity. Both areas have continued to attain the 2008 8-hour ozone NAAQS
and the 2015 8-hour ozone NAAQS after each standard became effective.16 EPA
uses a three-year design value to determine NAAQS compliance in order to account for the inherent yearly variability in ozone concentrations due to variations in meteorology, which can impact ozone levels during periods with similar emissions levels.
As shown in Table 1 below, the highest design value for the five ozone monitors in the Middle Tennessee Area is 72 ppb in 2014 using 20122014
data, 67 ppb in 2015 using 20132015
data, 67 ppb in 2016 using 20142016
data, 66 ppb in 2017 using 20152017
data, 67 ppb in 2018 using 20162018
data, 66 ppb in 2019 using 20172019
data, and 65 ppb in 2020 using 2018
2020 data. Starting with the 20132015
design values, the Areas design values do not indicate a strong increasing or decreasing trend and have remained below the 2015 8-hour ozone NAAQS.

TABLE 1MIDDLE TENNESSEE OZONE MONITOR DESIGN VALUES, ppb Monitor name
County
East Health/Trinity Lane Percy Priest Dam
Hendersonville
Fairview Middle School ..
Cedars of Lebanon

Davidson
Davidson
Sumner
Williamson
Wilson

Design value 20122014

Design value 20132015

Design value 20142016

Design value 20152017

Design value 20162018

Design value 20172019

Design value 20182020

70
72
66
67

65
67
62
62

66
67
67
61
64

65
64
66
60
63

66
67
66
60

65
65
66
60

64
65
65
60
60

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No valid design value due to incomplete data. The Cedars of Lebanon site had incomplete data in 2018 because there was an issue following the installation of a new monitoring shelter, and TDEC invalidated data collected before the issue was corrected. The East Health/Trinity Lane site had incomplete data in 2013.
In the June 11, 2020, NPRM 85 FR 35607, EPA inadvertently stated that the 20152017 Design Value was 66 ppb. The correct value is 65 ppb.
13 In its comments regarding the Middle Tennessee Area, the commenter appears to use the term MDA8 to refer to the maximum 8-hour daily average ozone concentration in a given year at a monitor.
14 In its comments regarding Hamilton County, the commenter appears to use the term MDA8 to refer to the ozone design value at a monitor. The design value at a monitor for the 8-hour ozone NAAQS is the annual 4th highest daily maximum
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8-hour ozone concentration averaged over three years.
15 Year to year changes in ozone levels result both from changes in precursor pollutant emissions and from fluctuations in meteorological conditions. This was taken into consideration in the development of the NAAQS and resulted in a protective standard that is based on a 3-year average of 4th maximums at an individual monitor.

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16 As shown in Table 1, 2014 is one of the years associated with attaining design values for the 2008
8-hour ozone NAAQS of 0.075 parts per million ppm. The 2008 8-hour ozone NAAQS was the applicable NAAQS for the 2015 ozone season. EPA
notes that the 2015 8-hour ozone NAAQS of 0.070
ppm was not in effect until October 1, 2015, and all design values, beginning with the 20142016
design value, attained the 2015 8-hour ozone NAAQS.

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