***E.O. 12866 Review

– Draft

– Do Not Cite, Quote or Release During Review***

2

- 1 CHAPTER 2 : DEFINING THE OZONE A IR QUALITY PROBLEM

Overview This section provides overviews of ozone precursor emissions and at mospheric chemistry (section 2.1); ambien t ozone concentrations (section 2.2 ) ; ambient ozone mon itoring in the U.S. (section 2.3 ); and available evidence and information related to background ozone (section 2.4).
2.1 Emissions and Atmospheric Chemistry Ozone is formed through photochemical reactions of precursor gases and is not directly emitted from specific sources. In the str atosphere, ozone occurs naturally and provides protection against harmful solar ultraviolet radiation. In the troposphere, near ground level, ozone forms through atmospheric reactions involving two main classes of precursor pollutants: volatile organic com pounds (VOCs) and nitrogen oxides (NO X ). Carbon monoxide (CO) and methane (CH 4 ) are also important for ozone formation over longer time periods (US EPA, 2013, section 3.2.2)

. Emissions of ozone precursor compounds can be divided into anthropogenic and natu ral source categories , with n atural sources further divided into biogenic emissions ( from vegetation, microbes, and animals ) and abiotic emissions ( from biomass burning, lightning, and geogenic

sources )

. Anthropogenic sources, including mobile sources and power plants, account for the majority of NO X and CO emissions. Anthropogenic sources are also important for VOC emissions, though in some locations and at certain times of the year (e.g., southeastern states during summer) the majority of VOC emissions comes from vegetation (US EPA, 2013 , section 3.2.1). In practice, the distinction between natural and anthropogenic sources is often unclear, as human activities may directly or indirectly affect emissions from what would have been considered natural sourc es during the preindustrial era. Thus, emissions from plants, animals, and wildfires could be considered either natural or anthropogenic, depending on whether emissions result from agricultural practices, forest management practices, lightning strikes, or other types of events (US EPA, 2013 , sections 3.2 and 3.7.1)

.***E.O. 12866 Review

– Draft

– Do Not Cite, Quote or Release During Review***

2

- 2 Rather than varying directly with emissions of its precursors, ozone changes in a nonlinear fashion with the concentrations of its precursors. NO X emissions lead to both the formation and destru ction o f ozone, depending on the local quantities of NO X , VOC, free radicals, and sunlight. In areas dominated by fresh emissions of NO X , radicals are removed, which lower s the ozone formation rate. In addition, the scavenging of ozone by reaction with NO is called “titration” and is often found in downtown metropolitan areas, especially near busy streets and ro ads, as well as in power plant plumes. This short

- lived titration results in localized areas in which ozone concentrations are suppressed compared t o surrounding areas, but which contain NO 2 that contributes to subsequent ozone formation further downwind. The NOx titration effect i s most pronounced in ur

ban core areas that have a high volume of mobile sou rce NOx emissions from vehicles. In areas with relatively low NO X concentrations, such as those found in remote continental areas and rural and suburban areas downwind of urban centers, ozone production typically responds linearly to NO X concentrations (e.g., ozone decreases with de creasing NO X emissions). Consequently, ozone response to reductions in NO X emissions is complex and may include ozone decreases at some times and locations and increases of ozone at other times and locations. As a general rule, as NOx emissions reductions occur , you c an expect lower ozone values to increase while the higher ozone values would be expected to decrease. NOx reductions are expected to result in a compressed ozone distribution, relative to current conditions (EPA, 2014a)

. The formation of ozone from precurs or emissions is also affected by meteorological parameters such as the inte nsity of sunlight and atmospheric mixing

. Major episodes of high ground

- level ozone concentrations in the eastern United States are often associated with slow

moving high pressure s ystems. High pressure systems during the warmer seasons are associated with the sinking of air, resulting in warm, generally cloudless skies, with light winds. The sinking of air results in the development of stable conditions near the surface that inhibit or reduce the vertical mixing of ozone precursors. The combina tion of inhibited vertical mixing and light winds min imizes the dispersal of p ollutants , allowing their concentrations to build up. In addition, in some parts of the United States (e.g., in Los Angeles), mountain barriers limit mixing and result in a higher frequency and duration of days with elevated ozone concentrations. Photochemi cal activity involving precursors is enhanced during warmer seasons because of the greater availa bility of sunligh t and hig her temperatures (US EPA, 2013, section 3.2).***E.O. 12866 Review

– Draft

– Do Not Cite, Quote or Release During Review***

2

- 3 Elevated wintertime ozone concentrations have recently been measured in mountain valleys in the Western U.S. (Schnell et al, 2009; Rappengluck et al., 2014; Helmig et al., 2014). Hourly ozone concentr ations during these winter events have been observed to reach 160 ppb. This phenomenon is believed to result from the combination of several factors: 1) strong wintertime inversions or “cold pools”, which trap air in a shallow layer close to the ground, 2 ) substantial emissions of NOx and VOC from nearby oil and gas operations, 3) high albedo of deep snow, which leads to enhanced UV intensity and photochemical activity, and 4) possible uncharacterized sources of radicals. These wintertime ozone events hav e currently only been observed in a limited number of locations in Wyoming, Utah, and Colorado. Events can last for multiple days and can occur several times a year, but do not occur every winter in these locations.
Ozone concentrations in a region are af fected both by local formation and by transport of

ozone and its precursors from upwind areas. Ozone transport occurs on many spatial scales including local transport between cities, regional transport over large regions of the U.S. and international/long

- range transport. In addition, ozone can be transfe rred into the troposphere from the stratosphere, which is rich in ozone , through stratosphere

- troposphere exchange (STE). These intrusions usually occur behind cold fronts, bringing stratospheric air with them and typically affect ozone concentrations in higher elevation areas (e.g. > 1500 m) more than areas at lower elevations ( U.S. EPA, 2013 , section 3.4.1.1) . The role of long

- range transp ort of ozone and other elements of ozone background are discussed in more detail in Section 2.4.
2.2 Spatial and Temporal Variations in Ambient Ozone Concentrations Because ozone is a secondary pollutant formed in t he atmosphere from precursor emissions, c oncentrations are generally more regionally homogeneous than concentrations of primary pollutants emitted directly from stationary and mobile sources ( US EPA, 2013 , section 3.6.2.1). However, variation in local emissions characteristics, meteorological con ditions, and topography can result in daily and seasonal temporal variability in ambient ozone concentrations, as well as local and national

- scale spatial variability. Temporal variation in ambient ozone concentrations results largely from daily and season al patterns in sunlight, precursor emissions , atmospheric stability, wind direction, and temperature***E.O. 12866 Review

– Draft

– Do Not Cite, Quote or Release During Review***

2

- 4 (US EPA, 2013, section 3.7.5)

. On average, ambient ozone concentrations follow well

recognized daily and seasonal patterns, particularly in urban areas. Sp ecifically, daily m ax imum 1

- hour ozone concentrations in urban areas tend to occur in mid

- afternoon , with more pronounced peaks in the warm mo nths of the ozone season than in the colder months (US EPA, 2013, Figures 3

- 54, 3

- 156 to 3

- 157). Rural sites also follow this general pattern, though it is less pronounced in colder months (US EPA, 2013, Figure 3

- 55). With regard to day

- to

- day variability, median maximum daily 8

- hour average (MDA8) ozone concentrations in U.S. cities from 2007 through 2009 were approx imately 47 ppb, with typical ranges between 35 to 60 ppb and the highest MDA8 concentrations above 100 ppb in several U.S. cities (as noted further below). In addition to temporal variability, there is considerable spatial variability in ambient

ozone conc entrations within cities and across different cities in the United States. With regard to spatial variability within a city, local emissions characteristics, geography, and topography can have important impacts. For example, as noted above, f resh NO emissi ons from motor vehicles titrate ozone present in the urban background air, resulting in an ozone gradient around roadways with ozone concentrations increasing as distance from the road increases ( US EPA, 2013 , section 3.6.2.1). Measured ozone concentration s are relatively uniform and well

- correlated within some cities (e.g., Atlanta) while they are more variable in others (e.g., Los Angeles) (US EPA, 2013, section 3.6.2.1 and Figures 3

- 28 to 3

- 36). Ozone concentrations also vary considerably across cities. Several cities had very high measured ozone concentrations in 2007 through 2009 when the maximum recorded MDA8 was 137 ppb in Los Angeles, and was near or above 120 ppb in Atlanta, Baltimore, Dallas, New York City, Philadelphia, and St. Louis (US EPA, 20 13, Table 3

- 10). These same cities also had high 98 th percentile ozone concentrations, with Los Angeles recording the highest 98 th percentile concentration (91 ppb) and many eastern and southern cities reporting 98 th percentile concentrations near or above 75 ppb. In contrast, somewhat lower 98 th percentile ozone concentrations were recorded in cities in the western United States outside of California (US EPA, 2013, Table 3

- 10).***E.O. 12866 Review

– Draft

– Do Not Cite, Quote or Release During Review***

2

- 5 Rural sites can be affected by transport of ozone o r ozone p re cursors from upwi nd urban areas and by local anthropogenic sources such as motor vehicles, power generation, biomass combustion, or oil and gas operations (US EPA, 2013, section 3.6.2.2)

. In addition, ozone tends to persist longer in rural than in urban areas due to lower rates of chemical scavenging in non

urban environments

. At higher elevations, increased ozone concentrations can also result from stratospheric intrusions (US EPA, 2013, sections 3.4, 3.6.2.2). As a result, ozone concentrations measured in some rural sites can be higher than those measured in nearby urban areas (US EPA, 2013, section 3.6.2.2)

. 2.3 Ozone Monitoring 2.3.1 Ozone Monitoring Network To monitor compliance with the National Ambient Air Quality Standards ( NAAQS ) , state and local environmental agenc ies operate ozone monitoring sites at various locations, depending on the population of the area and typical peak ozone concentrations. 1 All of the state and local monitoring stations that report data to the EPA Air Quality System ( AQS ) use ultraviolet (UV ) Federal Equivalent Methods (FEMs). In 2013, there were over 1,300 state, l ocal , and tribal ozone monitors reporting concentrations to EPA

. The “State and Local Monitoring Stations” ( SLAMS ) minimum monitoring requirements to meet the ozone design criteria are specified in 40 CFR Part 58, Appendix D. The requirements are bo th population and design value based

. 2 The minimum number of ozone monitors required in a Metropolitan Statistical Area (MSA) ranges from zero for areas with a population of at least 50,000 and under 350,000 with no recent history of an

ozone design value greater than 85 percent of the NAAQS, to four for areas with a pop ulation greater than 10 million and an ozone design value greater than 85 percent of the NAAQS. At least one site for each MSA, or Combined Statistical Area (CSA), must be sited to record the maximum concentration for that particular metropolitan area. Sin ce highest ozone concentrations

1 The minimum ozone monitoring network requirements for urban areas are listed in Table D

- 2 of Appendix D to 40 CFR Part 58. 2 A design value is a statistic that describes the air quality status of a given area relative to the level of the NAAQS. Design val ues are typically used to classify nonattainment areas, assess progress towards meeting the NAAQS, and develop control strategies. See http://epa.gov/airtrends/values.html (U, 2010, 677582) for guidance on how these values are defined.***E.O. 12866 Review

– Draft

– Do Not Cite, Quote or Release During Review***

2

- 6 tend to be associated with particular seasons for various locations, EPA re quires ozone monitoring during specific ozone monitoring seasons , which vary by state

. 3 Figure 2

- 1 shows the locations of the U.S. ambient ozone moni toring sites reporting data to EPA at any time during the 2009

- 2013 period . The gray dots that make up over 80% of the ozone monitoring network are SLAMS monitors, which are operated by state and local governments to meet regulatory requirements and provid e air quality information to public health agencies. Thus, the SLAMS monitoring sites are largely focused on urban and suburban areas. The blue dots highlight two important subsets of monitoring sites within the SLAMS network: the “National Core” (NCore) m ulti

- pollutant monitoring network and the “Photochemical Assessment Monitoring Stations” (PAMS) network. While the existing U.S. ozone monitoring network has a largely urban focus, to address ecosystem impacts of ozone, such as biomass loss and foliar inju ry, it is equally important to focus on ozone monitoring in rural areas. The green dots in Figure 2

- 1 represent the Clean Air Status and Trends Network (CASTNET) monitors, which are located in rural areas. There were about 80 CASTNET sites operating in 201 3, with sites in the eastern U.S. being operated by EPA and sites in the western U.S. being operated by the National Park Service (NPS). 4 In total, there were about 120 rural ozone monitoring sites operating in the U.S. in 2013.

3 The required ozone monitoring seasons for each state are listed in Table D

- 3 of Appendix D to 40 CFR Part 58. Revised monitoring seasons are being proposed along with the proposed revision of the ozone NAAQS level. 4 Additionally, the black dots represent “Special Purpose Monitoring Stations” (SPMS), which include about 20 rural monitors as part of the “Portable O 3 Monitoring System” (POMS) network operated by the NPS.***E.O. 12866 Review

– Draft

– Do Not Cite, Quote or Release During Review***

2

- 7 Figure 2
- 1. Map of U.S. ambient O3 monitoring sites reporting data to EPA during the

200 9
- 201 3 period. 2.3.2 Recent Ozone Monitoring Data and Trends To determine whether or not the ozone NAAQS has been met at an ambient monitoring site, a statistic commonly referred to as a “desi gn value” must be calculated based on three consecutive years of data collected from that site. The form of the existing ozone NAAQS design value (DV) statistic is the 3

- year average of the annual 4 th highest daily maximum 8

- hour ozone concentration in par ts per billion (ppb), with decimal digits truncated. The existing primary and secondary ozone NAAQS are met at an ambient monitoring site when the DV is less than or equal to 75 ppb. 5 In counties or other geographic areas with multiple monitoring sites, th e area wide DV is defined as the DV at the highest individual monitoring site, and the area is said to have met the NAAQS only if all monitoring sites in the area are meeting the NAAQS. Figure 2

- 2 shows the trend in the annual 4 th highest daily maximum 8

- h our ozone concentrations in ppb based on 910 “trends” sites with complete data records over the 2000 to

5 For more details on the data handling proced ures used to calculate design values for the existing ozone NAAQS, see 40 CFR Part 50, Appendix P.***E.O. 12866 Review

– Draft

– Do Not Cite, Quote or Release During Review***

2

- 8 2013 period

. The center line in this figure represents the median value across the trends sites, while the dashed lines represent the 25 th and 75 th perc entiles, and the bottom and top lines represent the 10 th and 90 th percentiles. Figure 2

- 3 shows a map of the ozone DVs (in ppb) averaged across the 2009

- 2011, 2010

- 2012, and 2011

- 2013 periods at all monitoring sites in the contiguous U.S. 6 The trend figure shows that the annual 4 th highest daily maximum values decreased for the vast majority of monitoring sites in the U.S. between 2000 and 2013. The decreasing trend is especially sharp from 2002 to 2004, when EPA implemented the “NO X SIP Ca ll”, a program designed to reduce summertime emissions of NO X in the eastern U.S., but has continued to decrease since then, in part due to ongoing reductions in mobile source NOx emissions. Within the overall downward trend, there are periodic short

- term increases. These variations from the overall trend are the result of inter

- annual variability in meteorological conditions.

Figure 2
- 2. Trend in U.S. A nnual 4 th H ighest D aily M aximum 8
- hour O zone

C oncentrations in ppb, 2000 to 201 3. Solid center line r epresents the median value across monitoring sites, dashed lines represent 25th and 75th percentile values, and top/bottom lines represent 10th and 90th percentile values.

6 All monitoring sites in Alaska, Hawaii, and Puerto Rico had DVs below 60 ppb.***E.O. 12866 Review

– Draft

– Do Not Cite, Quote or Release During Review***

2

- 9 Figure 2
- 3. Map of 8
- hour O zone D esign V alues in ppb , Averaged across the 2009
- 201 1 ,

2010
- 2012, and 2011
- 2013 P eriod s In addition to the DV described above, another ozone metric of interest is the W126 index value, which has been found to correlate with ozone

- related damage to plants and ecosystems (EPA, 2014c)

. The W126 metric is a sea sonal aggregate of daytime (8:00 AM to 8:00 PM) hourly ozone concentrations designed to measure the cumulative effects of ozone exposure on plant and tree species, with units in parts per million

- hours (ppm

- hrs). The W126 metric uses a logistic weighting f

unction to place less emphasis on exposure to low hourly ozone concentrations and more emphasis on exposure to high hourly ozone concentrations (Lefohn et al, 1988).

F igure 2

- 4 shows the trend in annual W126 concentrations in ppm

- hrs based on 900 “trends ” sites with complete data records over the 2000 to 2013 period

. The center line in this figure represents the median value across the trends sites, while the dashed lines represent the 25 th and 75 th percentiles, and the bottom and top lines represent the 10 th and 90 th percentiles. Figure 2

- 5 shows a map of the 3

- year average annual W1 26 concentrations in ppm

- hrs averaged across the 2009

- 2011, 2010

- 2012, and 2011

- 2013 periods at all monitoring sites in the***E.O. 12866 Review

– Draft

– Do Not Cite, Quote or Release During Review***

2

- 10 contiguous U.S. The general patterns seen in these figures are similar to those seen in the DV metric for the existing standard.

Figure 2
- 4. Trend in U.S. Annual W126 Concentrations in ppm
- hrs, 2000 to 2013 Solid

center line represents the median value across monitoring sites, dashed lines represent 25t h and 75th percentile values, and top/bottom lines represent 10th and 90th percentile values.***E.O. 12866 Review

– Draft

– Do Not Cite, Quote or Release During Review***

2

- 11 Figure 2
- 5. Map of 3
- year Average W126 Values in ppm
- hrs, Averaged Across the 2009
-

2011, 2010
- 2012, and 2011
- 2013 Periods

2.4 Background Ozone One of the asp ects of ozone that is unusual relative to the other pollutants with NAAQS is that, periodically, in some locations, an appreciable fraction of the observed ozone result s from

sources or processes other than local and domestic regional anthropogenic emissio ns of ozone precursors (Fiore et al ., 2002). Any ozone formed by processes other than the chemical conversion of local or regional ozone precursor emissions is generically referred to as “background” ozone. B ackground ozone can originate from natural sourc es of ozone and ozone precur sors, as well as from manmade international e missions of ozone precursors. Natural sources of ozone precursor emissions such as wildfires, lightning, and vegetation can lead to

ozone formation by chemical react ions with other na tural sources

. Another important component of background is ozone that is naturally formed in the stratosphere through interactions of ultraviolet light with molecular oxygen

. Stratospheric ozone can mix down to the surface at high concentrations in discre te events called intrusions , especially at higher

- altitude locations. The***E.O. 12866 Review

– Draft

– Do Not Cite, Quote or Release During Review***

2

- 12 manmade portion of the background includes any ozone formed due to anthropogenic sources of

ozone precursors emitted far away from the local area (e.g., international emissions). Fin ally, both biogenic and international anthropogenic emissions of methane, which can be chemically converted to ozone over relatively long time scales, can also contribute to global background

ozone levels. Away from the surface, ozone can have an atmospheric lifetime on the order of weeks. As a result, background ozone can be transported long distances in the upper troposphere and, when meteorological conditions are favorable, be available to mix down to the surface and add to the ozone loading fro m non - background sources.
The definition of background ozone can vary depending upon context, but it generally refers to ozone that is formed by sources or processes that cannot be influenced by actions within the jurisdiction of concern. In the Policy As sessment for the Review of the Ozone National Ambient Air Quality Standards (US EPA, 2014c), EPA identified three specific definitions of background ozone : natural background (NB) , N orth American background (NAB), and United S tates background (USB)

. Natura l background is the narrowest definition of background , and it is defined as the ozone that would exist in the absence of any manmade ozone precursor emissions. The other two definitions of background are based on a presum ption that the U.S. has little inf luence over anthropogenic emissions outside either our continental or domestic borders. North American background is defined as that ozone that would exist in the absence of any manmade ozone precursor emissions from North America. U.S. background is defin ed as that

ozone that would exist in the absence of any manmade emissions inside the United States. Modeling studies have estimated what background levels would be in the absence of certain sets of emissions by simply assessing the remaining ozone in a si mulation in which certain emissions were removed (Zhang et al. (2011), Emery et al. (2012), US EPA (2014c))

. This basic approach is often referred to as “zero

- out” modeling or “emissions perturbation” modeling. While the zero

- out approach has traditionally been used to estimate natural background, North American background, and U.S. background , the methodology has an acknowledged limitation. It cannot answer the question of how much of the existing observed ozone results from background sources or processes

.***E.O. 12866 Review

– Draft

– Do Not Cite, Quote or Release During Review***

2

- 13 A separate modeling technique can be used to estimate the contribution of background ozone and other contributing source terms to total ozone within a model. This approach, referred to as “source apportionment” modeling, has been described and evaluated in the peer

- reviewed literature (Dunker et al., 2002; Kemball

- Cook et al., 2009). Source apportionment modeling has frequently been used in other regulatory settings to estimate the “contribution” to ozone of certain sets of emissions (EPA 2005, EPA 2011). The source apportionment technique provides a means of estimating the contributions of each user

- identified source category to ozone formation in a single model simulation. This is achieved by using multiple tracer species to track the fate of ozone precur sor emissions (VOC and NO X ) and the ozone formation resulting from these emissions. The methodology is designed so that all ozone and precursor concentrations are tracked and apportioned to the selected source categories at all times without perturbing the inherent chemistry. The primary limitation of the source apportionment modeling is that its estimations of background ozone are explicitly linked to the emissions scenarios modeled and would change with different emissions scenarios.
2.4.1 Seasonal Mean Background O zone in the U.S. The ISA (US EPA 2013, section 3.4) previously established that background ozone concentrations vary spatially and temporally and that simulated mean background concentrations are highest at high

- elevation sites within the weste rn U.S. Background levels typically are greatest over the U.S. in the spring and early summer. EPA modeling presented in the Policy Assessment for the Review of the Ozone National Ambient Air Quality Standards (US EPA, 2014 c) focused on the months from Apr il to October. Figure 2

- 6 displays the spatial patterns of seasonal mean natural background ozone as estimated by a 2007 zero

- out scenario using the Community Multiscale Air Quality (CMAQ) model. Seasonal means are computed over those seven months. This fi gure shows the average daily maximum 8

- hour ozone concentration that would exist in the absence of any anthropogenic ozone precursor emissions at monitor locations. As shown, seasonal mean NB levels range from approximately 15

- 35 ppb (i.e., +/

- 1 standard deviation) with the highest values at higher

- elevation sites in the western U.S

. The median value over these locations is 24.2 ppb, and m ore than 50 percent of the locations have natural background levels of 20

- 25 ppb. The highest modeled estimate of seaso nal average, natural background, 8

- hr daily maximum ozone is 34.3 ppb at the high

- elevation CASTNET site***E.O. 12866 Review

– Draft

– Do Not Cite, Quote or Release During Review***

2

- 14 (Gothic) in Gunnison County, CO. Natural background ozone levels are higher at these high

elevation locations primarily because of natural stratospheri c ozone impacts and international transport impacts that increase with altitude (where ozone lifetimes are longer)

.

Figure 2
- 6. Map of 2007 CMAQ
- estimated Seasonal Mean of 8
- hour Daily Maximum

Ozone from Natural Background (ppb) based on Zero
- Out Modelin g Figures 2

- 7 and 2

- 8 show the same information for the NAB and USB scenarios. In these model runs, all anthropogenic ozone precursor emissions were removed from the U.S., Canada, and Mexico portions of the modeling domain (NAB scenario) and then only from the U.S. (USB scenario). The figures show that there is not a large difference between the NAB and USB scenarios. Seasonal mean NAB and USB ozone levels range from 25

- 50 ppb, with the most frequent values estimated in the 30

- 35 ppb range . The median s easonal mean background levels are 31.5 and 32.7 ppb (NAB and USB, respectively). Again, the highest levels of seasonal mean background ozone are predicted over the intermountain western U.S. Locations with NAB and USB concentrations greater than 40 ppb ar e confined to Colorado, Nevada, Utah, Wyoming, northern Arizona, eastern Califo rnia, and parts of New Mexico. The 2007 EPA modeling suggests that seasonal mean USB concentrations are on average 1

- 3 ppb higher than NAB background. These results were similar to those reported by Wang et al. (2009). From a seasonal mean perspective, background ozone levels are typically well

- below the NAAQS thresholds.***E.O. 12866 Review

– Draft

– Do Not Cite, Quote or Release During Review***

2

- 15 Figure 2
- 7. Map of 2007 CMAQ
- estimated Seasonal Mean of 8
- hour Daily Maximum

Ozone from North American Back ground (ppb) based on Zero
- out Modeling

Figure 2
- 8. Map of 2007 CMAQ
- estimated Seasonal Mean of 8
- hour Daily Maximum

Ozone from United States Background (ppb) based on Zero
- Out Modeling 2.4.2 Seasonal Mean Background O zone in the U.S. as a Proportion of Total O zone Another informative way to assess the importance of background ozone as part of seasonal mean ozone levels across the U.S. is to consider the ratios of NB, NAB, and USB to total modeled ozone at each monitoring location. Considering the propor tional impact of background ozone allows for an initial assessment of the relative importance of background and***E.O. 12866 Review

– Draft

– Do Not Cite, Quote or Release During Review***

2

- 16 non - background sources

. Because ozone chemistry is non

- linear, one should not assume that individual perturbations (e.g., zero

- out runs) are add itive in all locations. Figures 2

- 9 and 2

- 10 show the ratio of U.S. background to total ozone using the metric of the seasonal mean 8

- hr daily maximum ozone concentrations as estimated by both the zero

- out and source apportionment modeling methodologies. R ecall that the terms NB, NAB, and USB are explicitly linked to th e
zero - out modeling approach. For comparison, in Figure 2

- 11 we are extending the definition of USB to also include the source apportionment model estimates of the ozone that are attributable to sources other than U.S. anthropogenic emissions

. To preserve the original definition of USB, this second term will be hereafter referred to as “apportionment

- based USB”. As noted earlier , the advantage of the source apportionment modeling is that all o f the modeled ozone is attributed to various source terms without perturbing the inherent chemistry. T hus , this approach is not affected by the confounding occurrences of background ozone values exceeding the base ozone values as can happen in the zero - ou t modeling (i.e., background proportions > 100%). Consequently, one would expect the fractional background levels to be lower in the source apportionment methodology as a result of removing this artifact. When averaged over all sites, ozone from sources o ther than U.S. anthropogenic emissions is estimated to comprise 66 (zero

- out) and 59 (source apportionment) percent of the total seasonal ozone mean. The spatial patterns of USB and apportionment

- based USB are similar across the two modeling exercises. Bac kground ozone is a relatively larger percentage (e.g., 70

- 80%) of the total seasonal mean ozone in locations within the intermountain western U.S. and along the U.S. border. In locations where ozone levels are generally higher, like California and the east ern U.S., the seasonal mean background fractions are relatively smaller (e.g., 40

- 60%). The additional 2007 modeling confirms that background ozone, while generally not approaching levels of the ozone standard, can comprise a considerable fraction of total seasonal mean ozone across the U.S (EPA, 2014c).
2.4.3 Daily Distributions of Background Ozone within the Seasonal Mean As a first

- order understanding, it is valuable to be abl e to characterize seasonal mean l evels of background ozone . However, it is wel l established that background levels can vary substantially from day

- to

- day within the seasonal mean

. From an implementation perspective, the values of background ozone on possible exceedance days are a more meaningful***E.O. 12866 Review

– Draft

– Do Not Cite, Quote or Release During Review***

2

- 17 consideration. The Policy Assessment for the Review of the Ozone National Ambient Air Quality Standards (US EPA, 2014 c ) concluded that “anthropogenic sources within the U.S. are largely responsible for 4 th highest 8

- hour daily maximum O 3 concentrations ” based on modeling using a 2007 base year and the two distinct modeling methodologies described above. Figure 2

- 10 and 2

- 11 show the distribution of daily MDA8 apportionment

- based USB levels (absolute magnitudes and relative fractions, respectively) from the CAMx simulation

. The 2007 modeling shows that the days with highest ozone levels have similar distributions (i.e., means, inter

quartile ranges) of background ozone levels as days with lower values , down to approximately 40 ppb

. As a result, the proportion of total ozone that has backgroun d origins is smaller on high

ozone days (e.g., days > 60 ppb) than on the more common lower ozone days that tend to drive seasonal means . Figure 2

- 11 also indicates that there are cases in which the model predicts much larger background proportions, as sh own by the upper outliers in the figure. These infrequent episod es usually occur in relation to a specific event, and occur more often in specific geographical locations, such as at high elevations or wildfire prone areas during the local dry

season.
It s hould be noted here that EPA has policies for treatment of air quality monitoring data affected by these types of events. EPA’s exceptional events policy allows exclusion of certain air quality monitoring data from regulatory determinations if a State adeq uately demonstrates that an exceptional event has caused the exceedance or violation of a NAAQS. In addition, Section 179B of the Clean Air Act ( CAA ) also provides for treatment of air quality data from international transport when an exceedance or violati on of a NAAQS would not have occurred but for the emissions emanating from outside of the United States. Finally, CAA section 182(h) authorizes the EPA Administrator to determine that an area designated nonattainment can be treated as a “rural transport ar ea”. In accordance with the statute, a nonattainment area may qualify for this distinction if it meets the following criteria: 1) the area does not contain emissions sources that make a significant contribution to monitored ozone concentrations in the

area , or in other areas; and 2) the area does not include and is not adjacent to an MSA.***E.O. 12866 Review

– Draft

– Do Not Cite, Quote or Release During Review***

2

- 18 Figure 2
- 8. Map of Site
- Specific Ratios of U.S. Background to Total Seasonal M ean

O zone based on 2007 CMAQ Zero
- Out Modeling

Figure 2
- 9. Map of Site
- Specific Ratios of Apportionment
- Based U.S. Background to

S easonal M ean O zone based on 2007 CAMx Source Apportionment Modeling***E.O. 12866 Review

– Draft

– Do Not Cite, Quote or Release During Review***

2

- 19 Figure 2
- 10. Distributions of Absolute Estimates of Apportionment
- Based U.S.

Background (all site
- days), Binned by Modeled MDA8 from the 2007 S ource Apportionment Simulation

Figure 2
- 11. Distributions of the Relative Proportion of Apportionment
- Based U.S.

Background to T otal O zone (all site
- days), Binned by Modeled MDA8 from the 2007 Source Apportionment Simulation***E.O. 12866 Review

– Draft

– Do Not Cite, Quote or Release During Review***

2

- 20 2.5 R eferences Camalier, L. ; Cox, W.; Dolwick, P. (2007). The Effects of Meteorology on Ozone in Urban Areas and their use in Assessing Ozone Trends. Atmos Environ 41: 7127

- 7137. Dunker, A.M.; Yarwood, G; Ortmann, J.P.; Wilson, G.M. (2002). Comparison of source apportionment and so urce sensitivity of ozone in a three

- dimensional air quality model. Environmental Science & Technology 36: 2953

- 2964. Emery, C; Jung, J; Downey, N; Johnson, J; Jimenez, M; Yarwood, G; Morris, R. (2012). Regional and global modeling estimates of policy rele vant background ozone over the United States. Atmos Environ 47: 206

- 217. http://dx.doi.org/10.1016/j.atmosenv.2011.11.012

. Helmig, D., Thompson, C.R., Evans, J., Boylan, P., Hueber, J., Park, J.H. (2014). Highly elevated atmospheric levels of volatile org anic compounds in the Uintah Basin, Utah, Environmental Science & Technology, 48, 4707

- 4715. Henderson, B.H., Possiel, N., Akhtar, F., Simon, H.A. (2012). Regional and Seasonal Analysis of North American Background Ozone Estimates from Two Studies. Availab le on the Internet at: http://www.epa.gov/ttn/naaqs/standards/ozone/s_o3_td.html
Kemball

- Cook, S.; Parrish, D.; Ryerson, T; Nopmongcol, U.; Johnson, J.; Tai, E.; Yarwood, G. (2009). Contributions of regional transport and local sources to ozone exceedances in Houston and Dallas: Comparison of results from a photochemical grid model to aircraft and surface measurements. Journal of Geophysical Research

Atmospheres, 114: D00F02. DOI: 10.1029/2008JD010248. Lapina K., Henze D.K., Milford J.B., Huang M., Lin M., Fiore A.M., Carmichael G., Pfister G.G., Bowman K. (2014). Assessment of source contributions to seasonal vegetative exposure to ozone in the U.S. Journal of Geophysical Research

- Atmospheres, 119: DOI: 10.1002/2013JD020905. Lefohn, A. S.; Laurence, J. A.; Kohut, R. J. (1988). A comparison of indices that describe the relationship between exposure to ozone and reduction in the yield of agricultural crops. Atmos. Environ. 22: 1229

- 1240. Rappengluck, B., Ackermann, L., Alvarez, S., Golovko, J., Buhr, M., Fiel d, R.A., Soltis, J., Montague, D.C., Hauze, B., Adamson, S., Risch, D., Wilkerson, G., Bush, D., Stoeckenius, T., Keslar, C. (2014). Strong wintertime ozone events in the Upper Green River basin, Wyoming, Atmospheric Chemistry and Physics, 14, 4909

- 4934.
S chnell, R.C., Oltmans, S.J., Neely, R.R., Endres, M.S., Molenar, J.V., White, A.B. (2009) Rapid photochemical production of ozone at high concentrations in a rural site during winter, Nature Geoscience, 2, 120

- 122. U.S. Environmental Protection Agency (200 5). Technical Support Document for the Final Clean Air Interstate Rule Air Quality Modeling. Office of Air Quality Planning and Standards, Research Triangle Park, NC, 285pp. http://www.epa.gov/cair/technical.html. U.S. Environmental Protection Agency (2011 ). Air Quality Modeling Final Rule Technical Support Document. Office of Air Quality Planning and Standards, Research Triangle Park, NC, 363pp. http://www.epa.gov/airtransport/CSAPR/techinfo.html

. U.S. Environmental Protection Agency. (2013). Integrated Sc ience Assessment for Ozone and Related Photochemical Oxidants, U.S. Environmental Protection Agency, Research Triangle Park, NC. EPA/600/R

10/076. U.S. Environmental Protection Agency (2014a): Health Risk and Exposure Assessment for Ozone, final report. US EPA, OAQPS, 2014, RTP, NC, EPA

- 452/R

- 14

- 004a***E.O. 12866 Review

– Draft

– Do Not Cite, Quote or Release During Review***

2

- 21 U.S. Environmental Protection Agency. (2014b). Welfare Risk and Exposure Assessment for Ozone, final, U.S. Environmental Protection Agency, Research Triangle Park, NC. EPA

- 452 /P

- 14

- 005a. U.S. Environmental Protection Agency. (2014c). Policy Assessment for the Review of the Ozone National Ambient Air Quality Standards, U.S. Environmental Protection Agency, Research Triangle Park, NC. EPA 452/R

- 14

- 006. Wang, HQ; Jacob, DJ; Le Sager, P; Stree ts, DG; Park, RJ; Gilliland, AB; van Donkelaar, A. (2009). Surface ozone background in the United States: Canadian and Mexican pollution influences. Atmos Environ 43: 1310

- 1319. http://dx.doi.org/10.1016/j.atmosenv.2008.11.036

. Zhang, L; Jacob, DJ; Downey, NV; Wood, DA; Blewitt, D; Carouge, CC; Van donkelaar, A; Jones, DBA; M urray, LT; Wang, Y. (2011). Improved estimate of the policy

- relevant background ozone in the United States using the GEOS

- Chem global model with 1/2° × 2/3° horizontal resolution over North America. Atmos Environ 4

.