Additional Analysis and Data for Ozone Recommendations – June 24, 2009 Page 1 

Supplemental Analysis for Recommended 8-Hour Ozone Designations 

The following analyses and data are provided in response to EPA questions and are additive to the 

State of Colorado Technical Support Document (TSD) for the Recommended 8-Hour Ozone 

Designations, dated March 9, 2009. 

Factor #2: Emissions Data (pages 12 and 13 of TSD)

The TSD provided summary NOx and VOC emissions for the 9-County Nonattainment Area for 

thirteen source categories. The EPA has requested that Electric Generating Units (EGUs) be 

separated out as a separate source category in Tables 1-2 and 1-3 of the TSD. EPA further 

requested summary emission totals for each source category in the 9-County area and 

surrounding area. Tables 1 and 2 provide the requested information. 

Table 1: 2006 NOx and VOC emission inventory for 9-County Nonattainment Area 

Table 2: 2006 NOx and VOC emission inventory for counties surrounding the 9-County 

Nonattainment Area 
Additional Analysis and Data for Ozone Recommendations – June 24, 2009 Page 2 

Factor #3: Population Density and Degree of Urbanization (pages 17, 18 and 19 of TSD)

The TSD provided a number of maps from the US Census Bureau, Denver Regional Council of 

Governments, and North Front Range Metropolitan Planning Organization that graphically 

indicated population density in the 9-county nonattainment area and surrounding counties. The 

EPA has requested that we supplement the maps with population density data at the county level. 

In Table 3, the Division has prepared a table indicating county level population, land area and 

calculated population density 

Table 3: County-level Population and Calculated Population Density 
Additional Analysis and Data for Ozone Recommendations – June 24, 2009 Page 3 

Factor #4: Traffic and Commuting Patterns (pages 20 thru 23 of the TSD)

The TSD included a number of major roadway maps (developed by CDOT) that included annual 

average daily trips (2007). The EPA requested that traffic data be provided in table format by 

county and include growth projections for years 2010, 2015 and 2020. Table 4 provides the 

requested information except that the data is expressed as vehicle miles travelled (VMT) rather 

than average daily trips. 

Table 4: County-level Annual Average Vehicle Miles Travelled 

The TSD provided a projection of the number of workers commuting between the Denver Region 

and the surrounding counties (Figure 1-17, based on 2000 Census). The EPA requested 

information on the number of workers commuting between counties within the Denver Region, 
Additional Analysis and Data for Ozone Recommendations – June 24, 2009 Page 4 

specifically Adams, Arapaho, Boulder, Broomfield, Clear Creek, Denver, Douglas, Gilpin and 

Jefferson Counties. Table 5 provides detail on the number of workplace trips for counties within 

the Denver Region. 

Table 5: Number of Trips between Residence and Workplace for Counties within the Denver 

Region 

Factor #6: Meteorology (page 29 of TSD)

The TSD includes a HYSPLIT back trajectory analyses (map) of the four highest days for each year 

in 2006-2008 for the Fort Collins West, Rocky Flats and Chatfield monitors. The EPA has 

requested that the elevation of the back trajectory points be included to better understand 

interpretation of the map. Figure 1 includes a revised back trajectory map that includes the 

elevation contours in color. The trajectory point altitudes above ground level do not necessarily 

reflect the deep vertical mixing that is often occurring. While HYSPLIT may show trajectories 

following the terrain, the atmosphere may in fact be deeply mixed during the transit period. 
Additional Analysis and Data for Ozone Recommendations – June 24, 2009 Page 5 

Figure 1: Hourly Back Trajectories Points for FTCW, RFLAT and CHAT Monitoring Sites 

Associated with above map (Figure 1), the EPA has also requested that the trajectory points within 

the domain be numerically characterized to better understand the relative spatial density of areas 

around the 9-County Region. Figure 2 provides a count of back trajectory points in each grid cell 

to help identify relative densities of back trajectory points. 
Additional Analysis and Data for Ozone Recommendations – June 24, 2009 Page 6 

Figure 2: Trajectory Points per Grid Cell (analyses based on Figure 1) 

Similar to the HYSPLIT back trajectory analyses in Figure 1 (highest ozone days for each year in 

2006-2008 for the Fort Collins West, Rocky Flats and Chatfield monitors), the EPA has requested a 

similar back trajectory map for days with ozone values between 76ppb to 79 ppb. Figure 3 

provides the back trajectory analysis for high ozone days (76-79 ppb) at the Fort Collins West, 

Rocky Flats and Chatfield monitors for years 2006-2008. 
Additional Analysis and Data for Ozone Recommendations – June 24, 2009 Page 7 

Figure 3: Hourly Back Trajectories Points for FTCW, RFLAT and CHAT Monitoring Sites 

The above analysis (76-79 ppb) indicates back trajectory patterns very similar to Figure 1, which 

depicts the back trajectories for the highest days in 2006-2008. It is difficult to visually ascertain 

differences between the back trajectory plots, so a comparison of the number of back trajectory 

points per grid cell is necessary to quantify the spatial differences. In Figure 4 below, the number 

of back trajectory points per grid cell are based the 76-79 ppb back trajectory analysis. In 

comparing Figures 2 and 4, it appears that the distribution of plus “+” signs appear to be pretty 

similar around the three metro-area monitoring sites. 
Additional Analysis and Data for Ozone Recommendations – June 24, 2009 Page 8 

Figure 4: Trajectory Points per Grid Cell (analyses based on Figure 3 back trajectories) 

Expanded Meteorology Conclusions: (page 30 of TSD)

The EPA requested additional explanation of any relationship between elevation and ozone 

concentrations for counties on the edge of existing non attainment area. The following paragraphs 

respond to the issue: 

Decades of weekly ozonesondes in Boulder, recent aircraft profiles of ozone over the Front Range, 

and research in other places in the United States -show that ozone concentrations in the boundary 

layer often increase with altitude above ground. One of the main reasons is that ozone near the 

ground is diminished by reactions with nitric oxide emitted near the surface by mobile and point 

sources. Ozone at ground level is also reduced to some extent by oxidation reactions with 

vegetation and other materials on the surface. Ozone near the top of the boundary layer may also 
Additional Analysis and Data for Ozone Recommendations – June 24, 2009 Page 9 

be elevated because of complex re-circulation effects, residual layer processes, and prolonged 

residence times that allow for an accumulation of ozone aloft. 

High ozone concentrations are possible in the higher terrain of the Front Range. It is known that 

individual concentrations in excess of the new standard have been measured at NOAA’s Niwot 

Ridge Tundra monitor at 11,500 feet in Boulder County (located in the existing 8-hour ozone nonattainment area) and a short-term exploratory monitor operated by the United States Forest 

Service (USFS) for one season at Kenosha Pass in Park County. The Niwot Ridge Tundra site uses 

an "equivalent" analyzer, but to our knowledge the NOAA air monitoring does not meet the QA/QA 

requirements as set forth in 40CFR58, Appendix A. At Kenosha Pass, the USFS used the 2B-Tech 

analyzer. This monitor is not designated as a "reference" or "equivalent" analyzer as set forth in 

40CFR53, and the monitoring effort did not meet the QA/QA requirements as set forth in 

40CFR58, Appendix A. Presently, there is no federal reference method data that show that ozone 

concentrations are in violation of the standard in Clear Creek, Gilpin, or Park Counties 

In response to the possibility of elevated ozone in the higher elevations or the Front Range 

foothills, where public exposure to elevated ozone is of particular concern, the Division has added 

two ozone monitors, one is located at Aspen Park (elevation 8,095 feet - near Conifer) and the 

other is located in Rist Canyon (elevation 6,750 feet - west of Fort Collins). Both monitors started 

operations this spring and as data is gathered over the summer, we should have a better sense of 

the relationship between ozone concentrations and elevation in the Front Range foothills. 

In addition to the long-term federal reference method ozone monitor located near Longs Peak at 

an elevation of about 9000 feet in Rocky Mountain National Park, the National Park Service (NPS) 

also operates a portable ozone monitor located along Trail Ridge Road in Larimer County just 

above tree line. The monitor was in operation for a part of a summer in 2008, and is back online 

for data collection this summer. The data from this site is not of federal reference method quality, 

but it should provide us with more information on ozone concentrations at high elevation sites. 

While it is certainly possible that high concentrations may occur at high altitudes in these 

counties, it is important to note that the primary source for this ozone is most likely the urbanized 

area of the plains to the east. Anthropogenic emissions from these mountain areas are expected to 

have an insignificant contribution to NAA ozone 

Factor #7: Geography/Topography (page 31 of TSD)

The EPA requested a revised 3-D map of Denver Basin (see Figure 1-25, TSD dated March 9, 2009) 

with less vertical exaggeration. In Figure 5, the topographic elements of the Denver Basin are 

defined with color variations indicating differences in elevation. The gray line depicts the current 

ozone non-attainment area and the names of major towns/cities are identified. 
Additional Analysis and Data for Ozone Recommendations – June 24, 2009 Page 10 

Figure 5: Topographic Illustration of Physical Barriers that define the Denver Basin 
Additional Analysis and Data for Ozone Recommendations – June 24, 2009 Page 11 

Expanded Summary Conclusions for Denver MetroArea/North Front Range 8-hour Nonattainment 

Area: (page 35 of TSD)

The preceding additional analysis and data provides information in support for retention of the 

existing boundary for the Denver Metro Area/North Front Range (DMA/NFR) Ozone 

Nonattainment Area. In response to the potential inclusion of additional areas that would expand 

the existing Ozone Nonattainment Area, the following considerations are offered: 

(1) Extend the existing nonattainment area boundary to the north to include all portions 

of Larimer and Weld Counties: 

The northern portions of Larimer and Weld Counties are rural and sparsely populated with most 

areas having a population density fewer than five people per square mile, as indicated in the 

Figure 1-8 of the TSD. There are only three stationary point sources with ozone precursor air 

pollutant emissions above 100 tons/year located north of the existing nonattainment area 

boundary. Expanding the nonattainment area to include these three point sources would not 

enhance the States regulatory authority, although any future major modifications to these facilities 

would be affected. 

The estimated 2006 emissions (all source categories) for the northern portion of Larimer County 

are: NOx ≈ 2,936 tons/year and VOC ≈ 2,968 tons/year. The estimated 2006 emissions (all source 

categories) for the northern portion of Weld County are: NOx ≈ 279 tons/year and VOC ≈ 864 

tons/year. More detail on emissions data can also be found in the TSD under Factor #2 Section. 

Depending on the severity of the future ozone nonattainment area classification, requirements 

associated with the existing ozone nonattainment area may change from voluntary to mandatory, 

such as vehicle Inspection/Maintenance Program. Consequently, the potential inclusion of these 

rural areas into the nonattainment area could result in requiring residents with vehicles to be 

subject to mandatory vehicle inspections. The emission reduction benefit associated with a 

mandatory I/M program targeting rural residents often located far from an inspection station is 

negligible. 

The above back trajectory analysis (reference Figures 2 and 4) indicates that the northern 

portions of Larimer and Weld Counties are infrequent contributors to air quality in the DMA/NFR 

nonattainment area. This is indicated by the very low number of trajectory points in the grid cells 

over the northern portions of Larimer and Weld Counties. 

In summary, the inclusion of the northern portions of Larimer and Weld Counties into the ozone 

nonattainment area is not warranted because of sparse population, low degree of urbanization, 

low precursor emissions, and infrequent contributions to air quality in the DMA/NFR. 

(2) Expand the existing nonattainment area boundary further west to include Clear 

Creek and Gilpin Counties: 

The counties of Clear Creek and Gilpin are lightly populated areas located in high elevation 

mountainous terrain outside of the existing ozone nonattainment area. Based on the information 
Additional Analysis and Data for Ozone Recommendations – June 24, 2009 Page 12 

in Table 3, above, the estimated 2007 population density for Clear Creek and Gilpin Counties are 

23.8 and 34.3 people per square mile respectively. The total estimated 2007 population residing 

in Clear Creek and Gilpin Counties is 9,408 and 5,144 people respectively. Compared to the 3.27 

million (2007) people residing in the existing DMA/NFR nonattainment area, these two counties 

represent less than 0.5 percent of the total population for the area. 

The combined ozone precursor emissions (NOx and VOC) for both counties are about 4,000 

tons/year from all source categories. There are no stationary point sources in Clear Creek or 

Gilpin Counties with ozone precursor emissions over 100 tons/year. 

The estimated 2007 annual average vehicle travel for Clear Creek and Gilpin Counties is 1,980,603 

and 192,315 vehicle miles travelled (VMT), respectively, based on the information provided in the 

above listed Table 4. The total estimated 2007 annual average VMT for DMA/NFR nonattainment 

area is about 82,470,000. The VMT for Clear Creek and Gilpin Counties represent about 2.6 

percent of the total VMT in the metro area. The higher VMT identified in Clear Creek County is 

associated with the I-70 corridor and some from the mountain casinos. Most of the mountain 

casino vehicle traffic originates from Jefferson County as listed in the above Table 5 which is 

located in the existing ozone nonattainment area. 

The above back trajectory analysis (reference Figures 2 and 4) indicates that Clear Creek and 

Gilpin Counties are infrequent contributors to air quality in the DMA/NFR nonattainment area. 

This is indicated by the low number of trajectory points in the grid cells over Clear Creek and 

Gilpin Counties. 

In summary, the inclusion of Clear Creek and Gilpin Counties into the ozone nonattainment area is 

not warranted because of low population, low degree of urbanization, very low precursor 

emissions, and infrequent contributions to air quality in the DMA/NFR. 

(3) Expand the existing nonattainment area boundary both east and west to include the 

urbanized portions of Elbert County (Towns of Elizabeth and Kiowa) and Park 

County (Town of Bailey): 

The counties of Elbert and Park are lightly populated areas outside of the existing ozone 

nonattainment area. Based on the information in Table 3 above, the estimated 2007 population 

density for Elbert and Park Counties are 12.5 and 7.7 people per square mile respectively. The 

total estimated 2007 population residing in Elbert and Park Counties is 23,093 and 17,006 people 

respectively. Compared to the 3.27 million (2007) people residing in the existing DMA/NFR 

nonattainment area, these two counties represent less than 1.3 percent of the total population for 

the area. 

Information from the State Demography Office indicates that the 2007 population estimates for 

the towns of Elizabeth and Kiowa are 1,456 and 610 persons respectively. Bailey is an 

unincorporated town that is not tracked as a municipality by the State Demography Office, 

although a Google search yielded population data for 2009 indicating that 9,080 people reside in 

the Bailey ZIP code (80421). 
Additional Analysis and Data for Ozone Recommendations – June 24, 2009 Page 13 

The combined ozone precursor emissions (NOx and VOC) for Elbert and Park Counties are about 

2,700 tons/year and 1,900 tons/year from all source categories respectively. There are no 

stationary point sources in Elbert or Park Counties with ozone precursor emissions over 100 

tons/year. 

The estimated 2007 annual average vehicle travel for Elbert and Park Counties is 644,878 and 

606,542 vehicle miles travelled (VMT) respectively based on the information provided in the 

above listed Table 4. The total estimated 2007 annual average VMT for DMA/NFR nonattainment 

area is about 82,470,000. The VMT for Elbert and Park Counties represent about 1.5 percent of 

the total VMT in the metro area. 

The above back trajectory analysis (reference Figures 2 and 4) indicates that Elbert and Park 

Counties are infrequent contributors to air quality in the DMA/NFR nonattainment area. This is 

indicated by the very low number of trajectory points in the grid cells over Elbert and Park 

Counties, particularly over the urbanized areas of concern. 

In summary, the inclusion of the urbanized areas of Elbert and Park Counties into the ozone 

nonattainment area is not warranted because of low population, low degree of urbanization, very 

low precursor emissions, and infrequent contributions to air quality in the DMA/NFR. 

(4) Expand the eastern nonattainment area boundary to include Morgan County: 

Morgan County is a rural area outside of the existing ozone nonattainment area. Based on the 

information in Table 3 above, the estimated 2007 population density for Morgan County is 22.2 

people per square mile. The total estimated 2007 population residing in Morgan County is 28,577. 

Compared to the 3.27 million (2007) people residing in the existing DMA/NFR nonattainment 

area, Morgan County represents less than 0.9 percent of the total population for the area. 

The ozone precursor emissions for Morgan County are NOx ≈ 7,794 tons/year and VOC ≈ 2,270 

tons/year from all source categories. The NOx emissions from one Electric Generating Unit 

represent over half of the total NOx in the county. There are five stationary point sources in 

Morgan County with ozone precursor emissions over 100 tons/year. 

The estimated 2007 annual average vehicle travel for Morgan County is 1,072,728 VMT based on 

the information provided in the above listed Table 4. The I-76 corridor represents a substantial 

total of the VMT in Morgan County. The total estimated 2007 annual average VMT for Denver 

Metro/North Front Range nonattainment area is about 82,470,000. The VMT for Morgan County 

represents about 1.3 percent of the total VMT in the metro area. 

The above back trajectory analysis (reference Figures 2 and 4) indicates that Morgan County is an 

infrequent contributor to air quality in the DMA/NFR nonattainment area. This is indicated by the 

very low number of trajectory points in the grid cells over Morgan County. 

In summary, the inclusion of Morgan County into the ozone nonattainment area is not warranted 

because of low population, low degree of urbanization, low precursor emissions, and infrequent 

contributions to air quality in the DMA/NFR.