F:\Denver_O3_2008\Reports\2010_Sens_OSAT\draft#1\Exec_Sum1.doc August 2008 ES-1 

EXECUTIVE SUMMARY 

Due to violations of the 0.08 parts per million (ppm) 8-hour ozone National Ambient Air Quality 

Standard (NAAQS) based on 2005-2007 air quality data, in November 2007 the Denver 

Metropolitan Area (DMA) reverted to an 8-hour ozone nonattainment area. This requires the 

DMA to develop an 8-hour ozone State Implementation Plan (SIP) that demonstrates the area 

will achieve the 0.08 ppm 8-hour ozone NAAQS by 2010. The Denver Regional Air Quality 

Council (RAQC), in consultation with the Colorado Department of Health and Environment 

(CDPHE) Air Pollution Control Division (APCD), contracted with ENVIRON International 

Corporation and their subcontractor Alpine Geophysics, LLC to develop the photochemical 

modeling databases necessary to demonstrate that the DMA will achieve the 0.08 ppm 8-hour 

ozone NAAQS by 2010. 

OVERVIEW OF APPROACH 

The Comprehensive Air-quality Model with extensions (CAMx; www.camx.com) was set up for 

a June-July 2006 episode on a 36/12/4 km grid with the 4 km domain focused on Colorado. 

Meteorological inputs were prepared using the MM5 meteorological model whose results and 

evaluation are discussed by McNally and co-workers (2008). An initial emissions inventory was 

prepared using the SMOKE emissions modeling system and a preliminary 2006 base case was 

performed. A preliminary model performance evaluation was conducted and diagnostic 

sensitivity tests performed to identify an optimal model configuration for simulating ozone 

formation in the DMA (Morris et al., 2008b). A revised final CAMx 2006 base case (Run 17) 

simulation was performed and a comprehensive model performance evaluation was conducted 

(Morris et al., 2008c). Although there were some model performance issues on some of the 

modeling days during the June-July 2006 episode, usually due to an ozone underestimation bias, 

on a vast majority of the modeling days the ozone model performance achieved EPA’s model 

performance goals that along with the other model performance metrics indicated that the model 

was simulating the observed ozone sufficiently well for use in making ozone projections. 

Furthermore, on most days the model reproduced the observed VOC/NOx ratios in Denver quite 

well suggesting that the model is simulating the same chemical regimes as observed as well. 

2010 BASE CASE OZONE PROJECTIONS 

The procedures given in EPA’s 8-hour ozone modeling guidance were used to project current 

year 8-hour ozone Design Values (DVC) to obtain projected future year 2010 8-hour ozone 

Design Values (DVF) at each of the DMA monitoring sites (EPA, 2007). These procedures use 

the 2006 and 2010 base case modeling results in a relative fashion whereby modeled relative 

response factors (RRFs) are used to scale the current year 8-hour ozone Design Value (DVC) to 

obtain the projected future year 8-hour ozone Design Value (DVF): 

DVF = DVC x RRF 

For the Denver 2010 ozone projections, with one exception, the DVCs were based on the 8-hour 

ozone Design Values from the 2005-2007 period (i.e., the three year average of the fourth 
 F:\Denver_O3_2008\Reports\2010_Sens_OSAT\draft#1\Exec_Sum1.doc August 2008 ES-2 

highest daily maximum 8-hour ozone concentration at each monitor). The exception to this was 

for the Fort Collins West (FTCW) monitor that started monitoring in 2006 so that the two year 

average of the fourth highest daily maximum 8-hour ozone concentrations was used from 2006-

2007 for FTCW. 

Table ES-1 summarizes the projected 8-hour ozone Design Values (DVF) at the DMA 

monitoring sites for the 2010 base case simulation using the CAMx 2006 and 2010 base case 

modeling results and EPA recommended default ozone projection procedures described above. 

The maximum projected 8-hour ozone Design Value is 84 ppb and occurs at both the Rocky 

Flats North (RFNO) and Fort Collins West (FCTW) monitoring sites (see column 5 in Table ES1). As this value is 84 ppb or lower, then the 2010 base case modeling results pass the modeled 

attainment demonstration test. However, because the maximum projected 8-hour ozone Design 

Values lie between 82 and 87 ppb, then a WOE analysis is required. Although the EPA 8-hour 

ozone projection procedure is to truncate the final projected DVF for comparisons with the 

NAAQS, in column 6 of Table ES-1 we present the DVFs to the nearest tenth of a ppb before 

truncation. In this case we see that the projected 2010 base case DVFs at RFNO and FTCW are 

both 84.9 ppb. 

Also shown in Table ES-1 are the RRFs and the cut-off thresholds used in selecting days and 

number of days used in calculating the RRF. The EPA desire to use at least 10 modeled days is 

satisfied using the Denver June-July 2006 modeling period. In order to achieve that many 

modeled days, the cut-off threshold had to be reduced from 74 ppb to 78 ppb depending on the 

monitor, with the RFNO and FTCW monitors using a 78 and 76 ppb thresholds, respectively. 

The level of ozone reductions in the projected Design Values appears to be greater the further 

away from central Denver the monitor resides. In fact, ozone is estimated to increase very 

slightly at the monitors in or immediately downwind of the urban core. This is due to the 

reductions in on-road mobile sources NOx emissions that increase ozone in the urban core. The 

ozone increases are due to less ozone titration due to reduction in the primary emitted NOx 

emissions and/or less inhibition effect that high NOx concentrations have on ozone formation. 

As one moves away from the Denver urban core, the ozone increases between 2006 and 2010 

turn into no change and then to ozone decreases. The distance from the Denver urban core when 

the ozone increases change to ozone decreases varies by day due to changes in emissions (e.g., 

weekday versus weekend day) and changes in meteorology. The RFNO monitor lies near the 

modeled ozone increase-to-decrease cross over distance, which explains why the model 

projected 2010 Design Value is relative insensitive to the changes in emissions from 2006 to 

2010 at this site (85.0 to 84.9 ppb, a 0.1 ppb reduction); of the 10 days used to construct the RRF 

for RFNO there are some days of ozone increases and some with ozone decreases. At the FTCW 

monitoring site, on the other hand, the model is more responsive (1.1 ppb ozone reduction) as it 

is an area where the modeled ozone changes either stay the same or are reduced between 2006 

and 2010. 

Figure ES-1 display the results of EPA’s unmonitored area analysis for the 2010 base case. 

DVCs in excess of 80 ppb are estimated to the south, west and northwest of Denver stretching to 

Fort Collins and then west of Fort Collins (Figure ES-1, left). In fact, the unmonitored area 

procedure estimates that there are current-year DVCs in excess of the 85 ppb NAAQS occurring 

in 12 grid cells to the west of the Fort Collins monitoring sites. The projected DVFs for the 2010 

base case (Figure ES-1, right) have greatly reduced the spatial extent of the DVFs in excess of 80 

ppb occurring to the south, west and northwest of Denver and the 12 cells with DVCs exceeding 
 F:\Denver_O3_2008\Reports\2010_Sens_OSAT\draft#1\Exec_Sum1.doc August 2008 ES-3 

the 85 ppb NAAQS have been reduced by half to 6 grid cells in the 2010 base case emissions 

scenario. EPA stresses that the unmonitored area analysis is much more uncertain than the 

modeled attainment test at the monitors. And whereas additional emissions controls would likely 

be needed to eliminate continued violations at the monitor, such actions may not be appropriate 

for the unmonitored area analysis. 

Table ES-1. Current-year (DVC) and projected future-year (DVF) 8-hour ozone Design Values 

using the CAMx 2006 and 2010 base case modeling results. 

2005-07 2010 Base Case 

Site ID Monitor Name County DVC DVF DVF RRF Cutoff #days 

80013001 Welby Adams 70.0 70 70.2 1.0042 77.0 11 

80050002 Highland Arapahoe 78.0 77 77.3 0.9916 78.0 14 

80130011 S. Boulder Creek Boulder 81.0 80 80.8 0.9976 78.0 10 

80310002 Denver - CAMP Denver 56.0 56 56.0 1.0017 78.0 10 

80310014 Carriage Denver 74.0 74 74.1 1.0022 78.0 10 

80350004 Chatfield State Park Douglas 84.0 83 83.4 0.9934 78.0 11 

80410013 USAF Academy El Paso 73.0 72 72.0 0.9873 75.0 10 

80410016 Manitou Springs El Paso 74.0 73 73.7 0.9966 74.0 10 

80590002 Arvada Jefferson 79.0 79 79.2 1.0026 78.0 10 

80590005 Welch Jefferson 75.0 75 75.0 1.0004 78.0 10 

80590006 Rocky Flats North Jefferson 85.0 84 84.9 0.9994 78.0 10 

80590011 NREL Jefferson 82.0 82 82.3 1.0039 78.0 11 

80690011 Fort Collins - West Larimer 86.0 84 84.9 0.9874 76.0 10 

80691004 Fort Collins Larimer 74.0 73 73.0 0.9878 76.0 12 

81230009 Greeley - Weld Tower Weld 78.0 77 77.7 0.9964 75.0 10 

GTH161 Gunnison Gunnison 68.0 67 67.8 0.9984 74.0 10 

ROM206 Larimer Larimer 76.0 75 75.2 0.9903 77.0 10 

ROM406 Larimer Larimer 76.0 75 75.2 0.9903 77.0 10 

Figure ES-1. Current-year interpolated 8-hour ozone Design Values (DVC; left) and projected 

future-year 8-hour ozone Design Values (DVF; right) for the 2010 base case simulation. 
 F:\Denver_O3_2008\Reports\2010_Sens_OSAT\draft#1\Exec_Sum1.doc August 2008 ES-4 

2010 SENSITIVITY TEST RESULTS 

Sixteen (16) 2010 emissions reductions sensitivity tests were conducted with the CAMx 

modeling system. Most of these emission reduction sensitivity tests reduced VOC and/or NOx 

emissions from a specific source category either just within the Denver nonattainment area 

(NAA) or within the entire state of Colorado. Ozone projections were made at each of the 

monitoring sites for each of the 2010 sensitivity tests. We also performed the unmonitored area 

analysis for each sensitivity test to better understand the spatial extent of any ozone benefits or 

adverse effects. All sensitivity tests modified emissions from the 2010 base case emissions 

scenario. 

Table ES-2 displays the results of the 2010 sensitivity tests in terms of VOC, NOx and CO 

emission reductions from the 2010 base case, changes in projected 8-hour ozone Design Values 

at the key RFNO and FTCW monitoring sites, maximum difference in 2010 8-hour ozone 

Design Values anywhere in the DMA and maximum difference in daily maximum 8-hour ozone 

concentrations anywhere in the DMA and on any of the modeling days. 

Mobile Source Emissions: The first three sensitivity tests examined the sensitivity of 2010 

ozone projections to on-road mobile source emissions. Reducing on-road mobile sources VOC 

emissions 20% in the DMA reduces the projected DVFs at RFNO and FTCW by 0.2 and 0.1 

ppb, respectively. The 7 psi RVP gasoline in on-road mobile source gasoline vehicles reduces 

the DVFs by 0.1 ppb at both monitors. And the zero percent ethanol penetration scenario in the 

on-road and non-road mobile source gasoline engines increases the DVF at RFNO by 0.1 ppb, 

and has no effect at FTCW. 

Oil and Gas VOC Emissions: VOC emissions from O&G sources in the NAA were reduced by 

20% (b1-sens04) and 40% (b1-sens04d) in two of the 2010 emissions sensitivity tests. The O&G 

VOC emission reductions had little effect at the RFNO monitor, but reduced the projected DVF 

at the FTCW monitor by 0.1 and 0.2 ppb, respectively. The spatial maps of differences in the 

DVFs show a large area of ozone benefits due to the O&G VOC reductions centered on the 

O&G production area in Weld County. The RFNO monitor is right at the edge of this benefits 

area. Note that the Denver EAC SIP modeling of the June-July 2002 episode saw more transport 

from the Weld County O&G production area down to the RFNO monitor, so these results are 

partly an artifact of the meteorological conditions of the June-July 2006 modeling period. The 

O&G VOC emissions clearly have an effect on ozone formation in the Fort Collins area. In fact, 

the 6 grid cells west of Fort Collins that are projected to still violate the 0.08 ppm 8-hour ozone 

NAAQS in the 2010 base case (Figure ES-1, right) are reduced to 4 and 3 grid cells in the 20% 

and 40% O&G VOC emission reduction sensitivity scenarios. Clearly VOC emission reductions 

from O&G sources in Weld County would benefit ozone attainment in the Fort Collins area and 

likely elsewhere in the Denver NAA under other meteorological conditions. 

Combined VOC & NOx Sensitivity Simulations: Sensitivity simulations b1-sens04b and b1-

sens04c looked at combined VOC and NOx emissions reductions from area, non-road point and 

O&G emissions in the NAA. Although both simulations reduced VOC emissions by 20%, NOx 

emissions were reduced by 20% in sens04b and by 30% in sens04c allowing us to isolate the 

effects of the NOx controls. Several of the other sensitivity tests also allow us to isolate the 

effects of the VOC and NOx controls in these two sensitivity tests for each source category. The 

b1-sens04b 20% VOC/NOx emissions reduction scenario reduces the DVF at RFNO by 0.5 ppb. 

This is due to reductions in the RFNO ozone DVF of approximately 0.1 ppb from area source 
 F:\Denver_O3_2008\Reports\2010_Sens_OSAT\draft#1\Exec_Sum1.doc August 2008 ES-5 

VOC (b1-sens07), 0.1 ppb from O&G VOC (b1-sens04), 0.2 ppb from non-road VOC (b1-

sens06) and 0.2 ppb from point and O&G source NOx (b1-sens05). An additional 0.2 ppb ozone 

reduction in the RFNO DVF is obtained when the NOx reduction is increased from 20% to 30%. 

At the FTCW monitor, the effects of the NOx emission reductions alone are even greater. The 

20% VOC/NOx reduction gives a 1.1 ppb reduction in the ozone DVF at the FTCW monitor; 

increasing the NOx reduction by another 10% increases the ozone reduction at the FTCW 

monitor by another 0.5 ppb (total 1.6 ppb reduction). This suggests that a majority of the ozone 

benefits at FTCW are due to the NOx emission reductions. Although as noted above, VOC 

emission reductions from O&G sources in the NAA are also beneficial for reducing ozone in the 

Forth Collins area. With the exception of a couple grid cells of isolated ozone increases, the 

effects of the combined VOC/NOx controls are wide-spread reductions in ozone throughout the 

DMA. 

State-Wide Sensitivity: The state-wide sensitivity tests produce nearly the same ozone benefits 

at DMA monitors as the controls in the NAA alone. This is seen most clearly by comparing b1-

sens05 with b1-sens08 that examine a 20% reduction in NOx emissions from point and O&G 

sources in, respectively, the NAA and Colorado. They produce the same ozone reduction at 

RFNO (0.2 ppb) and the state-wide reduction produces slightly more ozone reduction at FTCW 

(0.6 ppb) than the NAA controls alone (0.5 ppb). At this time we have only evaluated the effects 

of the state-wide emission reduction sensitivity tests within the DMA. There are likely more 

ozone benefits due to the Colorado state-wide emission reductions outside of the DMA that may 

be important given the new lower (March 2008) ozone NAAQS. 

Bark Beetle Sensitivity: The effects of accounting for the Bark Beetle infestation on biogenic 

emissions have small effects on the DVFs in the DMA (0.1 ppb reduction). Thus, the 2010 

Denver ozone projections are not affected by the Bark Beetle infestation. 

The 2010 emissions sensitivity tests show higher ozone sensitivity to reducing NOx emissions 

than reducing VOC emissions. Although there are small areas of ozone increases due to NOx 

emissions reductions in the Denver urban core, and at the locations (grid cells) of some point 

sources, the overall ozone reduction benefits of the NOx controls outweigh the ozone increases. 

Furthermore, the locations of the highest ozone increases due to NOx reductions are monitoring 

sites with low ozone concentrations. Although the ozone benefits of VOC reductions do not 

seem as great as those from NOx reductions, VOC emissions reductions do reduce ozone 

somewhat and do not exhibit any ozone increases as seen with the NOx emissions reductions. In 

particular, the VOC emissions reductions from O&G sources have ozone benefits in the Fort 

Collins area and particular the key FTCW ozone monitor. 
 F:\Denver_O3_2008\Reports\2010_Sens_OSAT\draft#1\Exec_Sum1.doc August 2008 

Table ES-2. Results of the Denver 2010 emission sensitivity tests.

Emissions (TPD) % Anthro (%) 

DV Ozone 

(PPB) 

G

Ozo

Test Description CO VOC NOx CO VOC NOx RFNO FTCW Ma

2006.a3 

Current Year 8-Hour Ozone Design Value 85 86 

b1 2010 Base Case -386.0 -42.2 -50.1 -10.2% -5.3% -5.6% 84.9 84.9 b1-sns01 20% VOC On-Road NAA 0.0 -22.8 0.0 0.0% -3.0% 0.0% -0.2 -0.1 0.1 b1-sns02 

Evap VOC On-Road in NAA (7 psi RVP) -46.9 -9.8 -0.3 -1.4% -1.3% 0.0% -0.1 -0.1 0.1 

b1-sns03 0% Ethanol in NAA 323.3 -3.8 -2.0 9.5% -0.5% -0.2% 0.1 0.0 0.4 b1-sns04 20% VOC O&G in NAA 0.0 -48.2 0.0 0.0% -6.4% 0.0% 0.0 -0.1 0.0 b1-sns04b 

20% VOC & NOx Ar, Pnt, Non-Rd and O&G in NAA 0.0 -72.5 -41.3 0.0% -9.6% -4.9% -0.5 -1.1 1.8 

b1-sns04c 

20% VOC & 30% NOx Ar, Pnt, Non-Rd, O&G in NAA 0.0 -72.5 -62.0 0.0% -9.6% -7.4% -0.7 -1.6 2.8 

b1-sns04d 40% VOC O&G in NAA 0.0 -96.3 0.0 0.0% -12.7% 0.0% -0.1 -0.2 0.1 b1-sns05 20% NOx Pnt & O&G NAA 0.0 0.0 -20.6 0.0% 0.0% -2.5% -0.2 -0.5 0.3 b1-sns06 20% VOC Non-Rd in NAA 0.0 -12.7 0.0 0.0% -1.7% 0.0% -0.2 -0.1 0.0 b1-sns07 20% VOC Area in NAA 0.0 -7.5 0.0 0.0% -1.0% 0.0% -0.1 -0.1 0.0 b1-sns08 20% NOx Pnt & O&G CO 0.0 0.0 -78.0 0.0% 0.0% -9.3% -0.2 -0.6 1.1 b1-sns09 20% VOC O&G in CO 0.0 -67.2 0.0 0.0% -8.9% 0.0% 0.0 -0.1 0.1 b1-sns10 

20% VOC & NOx Point & O&G in CO 0.0 -77.5 -78.0 0.0% -10.2% -9.3% -0.3 -0.6 0.9 

b1-sns11 

20% NOx Point & O&G in NAA + 20% NOx Pawnee 0.0 0.0 -23.0 0.0% 0.0% -2.7% -0.2 -0.5 0.3 

b1-sns12a 

Effects of increase in Bark Beetle 2006 to 2010 -8.4 -87.8 -0.3 -0.2% -11.6% 0.0% 0.0 -0.1 0.1 

b1-sns12b 

Effects of 2010 Bark Beetle infestation -21.1 -233.5 -0.8 -0.6% -30.8% -0.1% -0.1 -0.1 0.1 
F:\Denver_O3_2008\Reports\2010_Sens_OSAT\draft#1\Exec_Sum1.doc August 2008 ES-7

2010 OZONE SOURCE APPORTIONMENT MODELING 

The Anthropogenic Precursor Culpability Assessment (APCA) version of the CAMx ozone 

source apportionment was applied using the 2010 base case inventory with the emissions 

segregated into 8 source categories and 11 source regions. The source categories are presented 

in Table ES-3 and the geographic source regions are given in Table ES-4. Ozone source 

apportionment is obtained for each source group, which consist of a source region and source 

category (e.g., on-road mobile sources from the 7-County Denver Metro area). As the 

contributions of ozone from initial concentrations (IC) and boundary conditions (BC) are always 

obtained, this results in ozone source apportionment to 90 separate source groups in the Denver 

ozone source apportionment modeling (90 = 8 x 11 + 2). 

The CAMx ozone source apportionment uses reactive tracers that operate in parallel to the host 

model. For each source group, there are four tracers corresponding to the source group’s VOC 

and NOx concentrations (Vi and Ni) and ozone formed that is attributable to the source groups 

VOC concentrations (O3Vi) or NOx concentrations (O3Ni). In the original Ozone Source 

Apportionment Technology (OSAT) ozone source apportionment approach implemented in 

CAMx, when ozone is formed in a grid cell it is attributable to a source group based on the 

relative contributions of the source groups VOC or NOx concentration to the total VOC or NOx 

concentration in that grid cell based on a determination of whether the ozone formed was under 

VOC-limited or NOx-limited conditions. Thus, in OSAT the O3V and O3N reactive tracers 

indicate how much of the ozone is formed under VOC-limited versus NOx-limited conditions. 

This results in OSAT assigning ozone to biogenic VOCs, which is not necessarily control 

strategy relevant information as they are uncontrollable. The APCA version of source 

apportionment only assigns ozone formed to biogenic (uncontrollable) sources when it is due to 

the interaction of biogenic VOC with biogenic NOx. When ozone is formed under VOC-limited 

conditions due to the interaction of biogenic VOC with anthropogenic NOx, a case where OSAT 

would assign it to the biogenic VOC (O3V) source group, APCA redirects the assignment to the 

anthropogenic NOx (O3N) source group. Thus, with APCA the O3V and O3N tracers no longer 

represent ozone formed under VOC-limited and NOx-limited conditions. 

The source apportionment results were analyzed at each of the ozone monitor sites in the DMA. 

At each monitor location, for each day, the 8-hour average ozone results for each period over 70 

ppb were averaged to develop a composite contribution. 

Example displays for a high ozone day (July 29th) at the Rocky Flats North monitor are presented 

in Figure ES-2. APCA ozone source apportionment modeling results for other days and other 

monitors are presented in Appendix C. The results show significant day-to-day and monitor-tomonitor variations. Figure ES-2a presents the ozone results including the boundary conditions, 

that is, the contribution from sources outside the 12 km domain. Of the 76.1 ppb of ozone 

estimated at the monitor, ~48 ppb (two-thirds) was transported into the 12 km domain and ~18 

ppb (one-third) was attributed to sources in the seven-county Denver Metro area. Figure ES-2b 

presents the same results as ES-2a, but without the boundary conditions plotted and the vertical 

scale expanded to better resolve the source region contributions. This figure shows that of the 

~18 ppb from Metropolitan Denver sources, ~10 ppb was from motor vehicles, ~5 ppb was from 

non-road mobile sources with the balance from other sources. Figure ES-2c presents the ozone 

formed that is attributable to anthropogenic NOx concentrations, whereas Figure ES-2d presents 

the ozone formed attributable to anthropogenic and biogenic VOC concentrations. These two 
F:\Denver_O3_2008\Reports\2010_Sens_OSAT\draft#1\Exec_Sum1.doc August 2008 ES-8

figures suggest that emission reductions from anthropogenic NOx sources will be more effective 

at reducing ozone in the model than reductions from anthropogenic VOC sources, although both 

VOC and NOx controls will reduce ozone. The Fort Collins West monitor shows similar 

contributions as the Rocky Flats North monitor except the highest contributions are from sources 

in the Larimer/Weld County source region and oil and gas sources from Larimer/Weld county 

have a large contribution. 

The source apportionment results vary by day and by location. However, several overall trends 

emerge, namely: 

• Regional ozone transport into the 12 km domain is the largest contributor, often 

accounting for more than two-thirds of the total ozone; 

• At the Denver Metropolitan monitors the largest contributors are Denver Metropolitan 

metro area motor vehicle and non-road sources; 

• At the Fort Collins and Greeley monitors, the largest contributors tend to be Larimer and 

Weld County motor vehicles, non-road sources and oil and gas sources, and Denver 

Metropolitan sources; 

• The majority of the ozone formed is attributable to anthropogenic NOx emissions. 

In interpreting these results it is important to keep in mind that these source apportionment 

results are based on the Denver SIP 2006 modeling episode and are meteorologically dependent. 

For instance, the source apportionment modeling in support of the Denver Early Action Compact 

using a 2002 ozone episode showed more impact of sources in Northern Colorado into the 

Denver Metropolitan area. 
F:\Denver_O3_2008\Reports\2010_Sens_OSAT\draft#1\Exec_Sum1.doc August 2008 ES-9

Figure ES-2a. Rocky Flats North source apportionment for 29 July including boundary 

conditions. 

Figure ES-2b. Rocky Flats North source apportionment for 29 July excluding boundary 

conditions. 
F:\Denver_O3_2008\Reports\2010_Sens_OSAT\draft#1\Exec_Sum1.doc August 2008 ES-10

Figure ES-2c. Rocky Flats North source apportionment for 29 July attributable to 

anthropogenic NOx emissions. 

Figure ES-2d. Rocky Flats North source apportionment for 29 July attributable to 

anthropogenic and biogenic VOC emissions. 
F:\Denver_O3_2008\Reports\2010_Sens_OSAT\draft#1\Exec_Sum1.doc August 2008 ES-11

CONCLUSIONS OF 2010 OZONE PROJECTIONS AND 

EMISSIONS SENSITIVITY TESTS 

The 2010 emissions reduction sensitivity tests and 2010 ozone source apportionment modeling 

provide consistent results. The source apportionment modeling estimates that when daily 

maximum 8-hour ozone concentrations are greater than 70 ppb then approximately two-thirds of 

the ozone in the DMA is coming from outside of the Denver 12 km modeling domain (roughly 

outside of Colorado and adjacent states). Most of the remainder one-third of the ozone comes 

from sources within the Denver 9-county NAA. For the Rocky Flats North monitoring site on 

July 29, 2006, the 2010 ozone source apportionment results suggest that half of the locally 

generated ozone comes from on-road mobile sources, with ~1/4 from non-road mobile source, 

1/8 from EGU point sources and the remainder 1/8 from area, non-EGU and oil and gas (O&G) 

sources from the 7-County Denver area. For the Forth Collins West monitoring sites a similar 

breakdown in source categories is seen only sources from Larimer/Weld Counties contribute 

more than sources from the 7-County DMA and O&G emissions from Larimer/Weld Counties 

are major contributors as well. 

The APCA ozone source apportionment attributes most of the ozone formed to anthropogenic 

NOx emissions rather than anthropogenic and biogenic VOC emissions. This suggests that 

anthropogenic NOx control may be a viable path toward reducing ozone concentrations. The 

2010 sensitivity modeling also suggests that NOx emissions reductions are effective at reducing 

ozone concentrations. However, the 2010 emissions sensitivity tests also saw areas where the 

NOx emission reductions resulted in ozone increases. These areas include isolated grid cells at 

the locations of some point sources and within the Denver urban core. The extent of the ozone 

increases due to the NOx controls in the Denver urban core varied day-to-day due to changes in 

emissions (e.g., weekday versus weekend day) and meteorology. However, the overall benefits 

for reducing ozone from the NOx controls out weigh the adverse effects of the ozone increases, 

although care should be taken in the level and types of NOx emissions controlled to limit the 

adverse effects. Although the VOC emission reductions do not produce as large ozone reduction 

as the NOx controls at some key sites (e.g., Fort Collins West), they always reduce ozone or 

have no effect and no adverse effects of VOC emissions reductions are seen. 

Thus, an ozone reduction path using either NOx and/or VOC emission reductions appear to be a 

viable paths in the Denver area. The VOC emission reductions always reduce ozone or have no 

or very little effect. Although the ozone reductions due to the NOx controls are larger and more 

widespread, there are also local ozone increases due to the NOx controls that need to be 

considered.