Appendix C 

Mobile Source Emissions Modeling 

for Atlanta Ozone 

Reasonable Further Progress 

State Implementation Plan 

Mobile Source Emissions Modeling for Atlanta Ozone RFP Plan 

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Overview 

The Georgia Environmental Protection Division (EPD) worked with the Atlanta Regional 

Commission (ARC) to develop mobile1

 source emissions inventories for the purpose of 

establishing motor vehicle emission budgets (MVEB) for the Atlanta 8-Hour Ozone 

Reasonable Further Progress (RFP) State Implementation Plan (SIP) revision, hereinafter 

called the Ozone RFP Plan. These inventories reflect the most recent planning 

assumptions and emission factor model2

 available, and the use of updated travel demand 

and emissions calculation models. The ARC's travel demand and emissions estimation 

modeling process was employed to estimate on-road mobile source emissions. The 

mobile source emission inventories for the Ozone RFP Plan MVEB were calculated in a 

manner consistent with federal regulations for performing regional emissions analyses 

used in transportation conformity determinations. The alignment of methodologies for 

mobile source inventories and transportation conformity emissions analyses prevents 

spurious differences between motor vehicle emission budgets (i.e., the SIP's estimate of 

future mobile source emissions) and transportation conformity analyses that must 

conform to those budgets. 

The 20-county Atlanta ozone nonattainment area was originally classified as a "marginal" 

nonattainment area. With an attainment year of 2007, marginal 8-hour ozone areas were 

required to attain the National Ambient Air Quality Standards (NAAQS) by the 2006 

ozone season. Because monitoring data from the 2006 ozone season indicated that 

Atlanta had not attained the NAAQS, the area was reclassified3

 to "moderate." In ozone 

nonattainment areas with air quality classified as moderate or worse, a reasonable further 

progress demonstration, per Section 182(b) of the Clean Air Act (CAA), requires 

emission reductions equaling 15% within six years of the base year (i.e., by the end of 

2008 for the 8-hour ozone NAAQS). 

The initial 15% reduction must be in emissions of volatile organic compounds (VOC). 

Per 40 CFR Part 51.910(a)(1)(iii), Requirements for Preparation, Adoption, and 

Submittal of Implementation Plans, moderate areas of which a portion has an approved 1-

hour ozone 15% VOC Plan can choose to treat the nonattainment area as two parts, each 

with a separate RFP target, and may substitute reductions in nitrogen oxides (NOx) for 

VOC in the sub-area with the approved 15% Plan. The 15% reduction for the sub-area 

without an approved 15% VOC Plan must still be entirely VOC. Because photochemical 

air quality modeling indicates that NOx reductions are more effective than VOC 

reductions in reducing ozone concentrations in the region,4

 Georgia has chosen to rely 

solely on NOx emission reductions in the 13-county part of the Atlanta ozone 

nonattainment area with an approved 15% VOC Plan. 

1

 The term "mobile" is used to describe emissions from on-road motor vehicles. 

2

 MOBILE6.2.03 (http://www.epa.gov/otaq/m6.htm#m60) 3

 73 Federal Register 12013, March 6, 2008 

4

 See Appendix A, Sensitivity of ozone concentrations in Atlanta to NOx and VOC controls inside and 

outside the nonattainment area 

(http://www.gaepd.org/Files_PDF/plans/sip/app_a_nox_substitution_and_out_of_area_benefits.pdf) 
Mobile Source Emissions Modeling for Atlanta Ozone RFP Plan 

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For use in making transportation conformity determinations for the Atlanta 8-hour ozone 

nonattainment area, the Ozone RFP Plan will establish MVEB for the year 2008. In 

contrast with previous EPA guidance and with Georgia’s approved Post-1996 9% Plan 

and Post-1999 ROP5

 Plan, this Ozone RFP Plan does not rely on any emission reductions 

outside the nonattainment area. As determined through interagency consultation, the 

emission factors used reflect an assumption that ethanol-blend gasoline had 100 percent 

market share in summer 2008. Modeling assumptions were distributed to the interagency 

consultation partners on October 28, 2008. 

Mobile Source Emissions Inventory 

In order to develop the Ozone RFP Plan MVEB, 2002 Base Year and 2008 budget year 

VOC and NOx emission inventories for the 20-county eight-hour ozone nonattainment 

area were produced. Reference Figure C-1 for a map of the nonattainment area. The 

Base Year inventories were adjusted by removing VOC and NOx already scheduled for 

control by previous federal regulations on motor vehicles and gasoline volatility as 

required by Section 182(b)(1) of the CAA. 

According to the EPA's MOBILE6 policy guidance,6 "If SIPs are revised..., base 

year...motor vehicle emission inventories will need to be recalculated with the latest 

available planning assumptions... Base year...inventories should use the best data 

available for those years." The motor vehicle emissions inventories for years 2002 and 

2008 for the 20 counties that comprise the 8-hour ozone nonattainment area reflect the 

latest travel demand and emissions modeling processes used for transportation 

conformity purposes by the ARC, and latest available planning assumptions as 

documented in Appendix C, Exhibits 3 and 4. 

5

 The terminology EPA uses for progress towards attainment has been codified at 40 CFR Part 51.900: 

"reasonable further progress" (RFP) is used for the 8-hour ozone NAAQS, "rate of progress" (ROP) is used 

for the former 1-hour NAAQS. 

6 http://www.epa.gov/otaq/models/mobile6/m6policy.pdf
Figure C-1 

Atlanta Ozone Nonattainment Area 

Mobile Source Emissions Modeling for Atlanta Ozone RFP Plan 

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Mobile Source Emissions Modeling for Atlanta Ozone RFP Plan 

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Emissions Inventory - 20 County Eight-Hour Ozone 

Nonattainment Area

In support of Ozone RFP Plan MVEB development, the ARC produced nonattainment 

area mobile emissions inventories for the 2002 Base Year, the 2002 Adjusted Base Year, 

and the 2008 RFP budget year. The travel demand and emissions modeling process and 

the planning assumptions used for the Ozone RFP Plan are summarized below and in 

Appendix C, Exhibits 3 and 4. 

The ARC currently maintains a travel model for 20 counties that correspond to the eighthour ozone nonattainment area. The ARC travel demand model is designed to, at 

minimum, represent the state of the practice in travel demand modeling and to meet all 

modeling requirements specified in the federal Transportation Conformity Rule (40 CFR 

Parts 51 and 93). The Transportation Conformity Rule establishes a regulatory 

requirement that includes minimum specifications for travel models used to forecast 

vehicle activity for regional emission analyses in certain nonattainment and maintenance 

areas. These minimum specifications (see 40 CFR Section 93.122(b)) apply to 

metropolitan planning areas with an urbanized area population over 200,000 that are also 

serious, severe, or extreme ozone; or serious carbon monoxide; nonattainment areas. 

Atlanta met those conditions under the one-hour ozone standard, for which the area had 

been classified as serious (effective January 6, 1992) and severe (effective January 1, 

2004), before being redesignated to attainment of the one-hour ozone standard (effective 

June 14, 2005). Because travel model VMT are adjusted to count-based estimates before 

being used for emissions calculations, the biggest remaining difference between networkmodel-based activity and activity estimated using other "appropriate methods [40 CFR 

Section 93.122(d)]" is the level of speed averaging. Despite the fact that Atlanta's current 

ozone nonattainment classification (moderate) would not otherwise subject it to the 

modeling requirements of 93.122(b), EPA regulations7

 require the continued use of 

activity from network-based travel models for transportation conformity emissions 

estimates in Georgia. 

A full consultation process, including national peer review, and the ARC strategic travel 

demand model enhancement program guide all modifications to the travel demand model. 

As a result, all elements of the travel demand model are designed to support all technical 

and policy decisions that are required in developing a comprehensive, multimodal 

transportation plan and program in accordance with the Safe, Accountable, Flexible, 

Efficient Transportation Equity Act: A Legacy for Users (SAFETEA-LU), the 1990 

Clean Air Act Amendments, and the Transportation Conformity Rule. 

The previously most recent emissions inventories developed using ARC’s travel demand 

and emissions modeling process were completed as part of the transportation conformity 

 

7

 "In all areas not otherwise subject to paragraph (b) of this section, regional emissions analyses must use 

those procedures described in paragraph (b) of this section if the use of those procedures has been the 

previous practice of the MPO [40 CFR Section 93.122(d)]." 
Mobile Source Emissions Modeling for Atlanta Ozone RFP Plan 

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analysis for ARC's Envision6 2030 Regional Transportation Plan (RTP) and associated 

FY 2008-2013 Transportation Improvement Program (TIP); and the Gainesville-Hall 

MPO's 2030 RTP and associated 2008-2013 TIP; approved by the U.S. Department of 

Transportation (USDOT), in consultation with the EPA, on October 10, 2007. A number 

of enhancements to the travel demand model, described below, are reflected in those 

inventories and in the inventories developed for the Ozone RFP Plan MVEB. Some 

changes to the travel model were a result of a significant model recalibration initiated 

after the expansion of the modeling platform from a 13-county to a 20-county geography. 

Enhancements include updated population and employment estimates that reflect the 

expanded modeling platform, as well as structural changes within the model needed to 

recalibrate to its base year, 2000, including the following: 

• Mode choice model updated to improve performance of suburban intra-county trips, 

• Attraction share model incorporated into four-step modeling process, and 

• New commercial vehicle and truck model developed to improve traffic assignment 

for both passenger vehicles and truck traffic. 

The ARC’s emissions model has also been updated to reflect all travel model 

improvements resulting from expansion to the 20-county geography and subsequent 2000 

base year recalibration. The emissions modeling procedure requires input of travel model 

networks for the specific scenario years, in this case 2002 and 2008. External files that 

specify VMT adjustment factors and MOBILE6 emission factors are also required. 

Changes to these networks and external files include the following: 

• Network files reflect expansion of model system to cover the 20-county eight-hour 

ozone nonattainment area (2002 and 2008 travel model network years), 

• Highway Performance Monitoring System (HPMS) adjustment factors8 updated to 

reflect the latest 20-county travel model recalibration for year 2000, 

• Updated seasonal adjustment factors, and 

• Latest MOBILE6 emission factors. 

Travel Model Networks

To prepare the mobile source emissions inventories for establishing the Ozone RFP Plan 

MVEB, it was necessary to create a 2002 transportation network that accurately reflected 

the transportation system in place during the 2002 base year, as well as a transportation 

network for 2008. The year 2002 network needed for the baseline inventory was derived 

from the base year 2000 travel model network; no regionally significant projects were 

opened to traffic between 2000 and 2002. Year 2002 population and employment data 

was interpolated from the latest regional control demographic forecasts documented in 

Appendix C, Exhibit 4 

(http://www.gaepd.org/Files_PDF/plans/sip/exhibit_4_PopEmp_PlanAssumptions_0422

08.pdf). 

 

8

 The VMT from the travel demand model are adjusted based on the VMT estimates that Georgia DOT 

develops for HPMS. 
Mobile Source Emissions Modeling for Atlanta Ozone RFP Plan 

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The year 2008 network needed for the 2008 RFP MVEB inventories was derived from 

the 2010 travel model network. Year 2008 population and employment data were 

interpolated from the latest regional control demographic forecasts documented in 

Appendix C, Exhibit 4. 

A small portion of the Atlanta Urbanized Area extends into Hall County. In February 

2003, the Gainesville-Hall MPO (GHMPO) was designated for the Gainesville Urbanized 

Area; the planning boundary for the GHMPO covers Hall County in its entirety. Hall 

County is included in Atlanta's ozone nonattainment area. The ARC, by agreement with 

the GHMPO, performs the emissions modeling for Hall County. Travel model networks 

reflect the project listing as approved: 

• in ARC's Envision6 2030 RTP and 2008-2013 TIP; adopted by the ARC Board 

September 26, 2007; and received a positive conformity determination from USDOT 

October 10, 2007; and 

• in the GHMPO's 2030 RTP and 2008-2013 TIP; adopted by the GHMPO Board 

August 14, 2007; and received a positive conformity determination from USDOT 

October 10, 2007 

Project coding for network years 2002 (based on 2000 network) and 2008 (based on 2010 

network) was reviewed by state and local project sponsors in March and April 2007 as 

part of the development of the Envision6 and GHMPO 2030 RTPs and FY 2008-2013 

TIPs. Year 2002 and 2008 networks were run through a full travel model assignment 

procedure, based on the recalibrated model stream, to produce the VMT and speeds for 

the respective mobile source emission inventories. 

VMT Adjustment Factors

Traffic volumes produced by the travel model are adjusted within the emissions modeling 

process by applying a VMT adjustment factor, which is a combination of HPMS 

adjustment factors, used to reconcile travel model VMT to HPMS VMT estimates,9

 and 

seasonal adjustment factors used to convert the average annual daily traffic produced by 

the travel model to average summer-time daily traffic for the purposes of ozone precursor 

emissions modeling.10 

EPA requires11 that reasonable methods be used to estimate VMT on off-network (offmodel) roadways within the urban transportation planning area. Off-model VMT is travel 

that is accounted for within HPMS estimates, but not accounted for within the coded 

transportation network; it typically reflects travel on the local road system. EPA also 

recommends,12 for areas with travel demand models in place, that HPMS adjustments be 

 

9

 40 CFR Sections 93.122(a)(7) and 93.122(b)(3) 

10 Section 3.4.2.6 Procedures for Emission Inventory Preparation, Volume IV: Mobile Sources, EPA420-R92-009, USEPA Office of Air and Radiation, Office of Mobile Sources, 1992. 

11 40 CFR Section 93.122(a)(7) 

12 40 CFR Section 93.122(b)(3) 
Mobile Source Emissions Modeling for Atlanta Ozone RFP Plan 

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made based on comparison of base year VMT from the transportation model to base year 

HPMS VMT estimates. The ARC calculates HPMS adjustment factors by comparing 

HPMS VMT to travel model VMT by 12 HPMS functional classes for the 2000 

calibration year. The resulting factors are then applied to travel model VMT estimates 

for future analysis years. The following equation was used to calculate the HPMS 

adjustment factors: 

HPMS Adjustment Factori = 2000 HPMS VMTi / 2000 MODEL VMTi

(where i=HPMS functional class) 

To determine the "2000 HPMS VMT," average daily VMT for the year 2000 were 

grouped by the 12 HPMS functional classes for the 13 counties13 that correspond to the 

nonattainment area for the previous 1-hour ozone standard where a 15% VOC Plan is in 

place, and, separately, for the 7 counties14 that were not part of the 1-hour ozone 

nonattainment area, but are nonattainment under the 8-hour ozone standard, where there 

is not currently a 15% VOC Plan. County-level HPMS data by functional class was 

taken from the Georgia Department of Transportation's (Georgia DOT) Office of 

Transportation Data "445 Report." Georgia DOT's 445 Report provides information on 

mileage and VMT by route type and road system and contains county-specific State 

Route, County Road and City Street mileage and VMT broken down by functional 

classification. VMT by county and functional class were aggregated to total VMT by 

HPMS functional class. "2000 Travel Demand Model VMT" at the HPMS functional 

class level was derived from the recalibrated 2000 travel model network. HPMS 

adjustment factors for the 13- and 7- county areas are listed below. 

Note that for the HPMS adjustment factors for the 7-county area, urban and rural HPMS 

VMT were combined by HPMS functional classification prior to comparing with ARC 

travel model VMT. Travel model network coding for the 7-county area took place in 

early 2006 and reflects the latest version (from the 2000 Census) of the urbanized area 

definition. The 2000 HPMS data that is used as the base for calculation of the adjustment 

factors reflects the 1990 Census urbanized area definition. Georgia DOT did not update 

to the 2000 Census definitions until calendar year 2005, with their 2004 445 Report. In 

order to avoid a disconnect between travel model VMT in the 7 counties coded with 2000 

Census urban/rural designations and 2000 HPMS VMT that reflects the 1990 definition, 

the urban and rural VMT were combined by functional class before the factors were 

calculated. This produced four unique HPMS adjustment factors for the 7-county area, as 

opposed to 12 unique factors calculated for the 13-county area. 

13 (Cherokee, Clayton, Cobb, Coweta, DeKalb, Douglas, Fayette, Forsyth, Fulton, Gwinnett, Henry, 

Paulding, and Rockdale) 

14 (Barrow, Bartow, Carroll, Hall, Newton, Spalding, and Walton) 
Mobile Source Emissions Modeling for Atlanta Ozone RFP Plan 

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Table C-1 

HPMS Adjustment Factors – 13 Counties 

Functional Class 

Name 

Functional Class 

Number HPMS VMT Model VMT Factor 

 Rural Interstate 1 5,470,815 8,583,156 0.64 

 Rural Principal Arterial 2 3,578,882 3,579,075 1.00 

 Rural Minor Arterial 6 3,821,128 3,343,871 1.14 

 Rural Major Collector 7 3,510,608 4,021,609 0.87 

 Rural Minor Collector 8 1,191,162 1,121,165 1.06 

 Rural Local 9 3,322,148 5,199,188 0.64 

 Urban Interstate 11 35,934,531 35,043,828 1.03 

 Urban Other Freeway 12 5,765,979 2,422,195 2.38 

 Urban Principal Arterial 14 10,398,680 15,341,281 0.68 

 Urban Minor Arterial 16 22,101,606 18,118,598 1.22 

 Urbanized Collector 17 7,540,916 6,098,164 1.24 

 Urbanized Local 19 15,412,042 15,841,739 0.97 

Table C-2 

HPMS Adjustment Factors – 7 Counties 

Functional Class 

Name 

Functional Class 

Number HPMS VMT Model VMT Factor 

 Rural Interstate 1 4,160,787 2,426,269 0.85 

 Rural Principal Arterial 2 3,912,496 1,299,608 0.97 

 Rural Minor Arterial 6 3,843,626 1,784,578 0.97 

 Rural Major Collector 7 2,498,042 1,435,602 1.80 

 Rural Minor Collector 8 1,019,255 303,600 1.80 

 Rural Local 9 3,112,673 1,515,043 1.09 

 Urban Interstate 11 322,200 2,510,593 0.85 

 Urban Other Freeway 12 0 310,430 0.85 

 Urban Principal Arterial 14 2,874 1,979,977 0.97 

 Urban Minor Arterial 16 17,640 2,952,583 0.97 

 Urbanized Collector 17 0 216,380 1.80 

 Urbanized Local 19 20,796 1,353,590 1.09 

Summer (seasonal) adjustment factors are used to convert from average annual daily 

VMT to average summer-season daily VMT for the purposes of ozone precursor 

emissions modeling.15 Seasonal adjustment factors were the latest factors provided by 

the Georgia DOT Office of Transportation Data, on August 9, 2006. Summer-adjustment 

factors for the 13- and 7- county geographies are listed below. 

 

15 Procedures for Emission Inventory Preparation, Volume IV: Mobile Sources, Section 3.4.2.6, EPA420-

R-92-009, USEPA Office of Air and Radiation, Office of Mobile Sources, 1992. 
Mobile Source Emissions Modeling for Atlanta Ozone RFP Plan 

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Table C-3 

Summer Adjustment Factors 

Functional Class Name 13-County Factor 7-County Factor 

 Rural Interstate 1.05 1.04 

 Rural Principal Arterial 1.02 1.02 

 Rural Minor Arterial 1.01 1.01 

 Rural Major Collector 1.00 1.00 

 Rural Minor Collector 1.01 1.01 

 Rural Local 1.01 1.01 

 Urban Interstate 1.02 1.00 

 Urban Other Freeway 1.03 1.00 

 Urban Principal Arterial 0.99 1.00 

 Urban Minor Arterial 0.99 0.99 

 Urbanized Collector 0.97 1.00 

 Urbanized Local 0.97 0.97 

HPMS adjustment factors were multiplied by the seasonal adjustment factors for each 

HPMS functional classification to produce a set of VMT adjustment factors by HPMS 

functional classification for 13-county and 7-county portions of the 20-county 

nonattainment area, separately. VMT adjustment factors are listed below. 

Table C-4 

VMT Adjustment Factors 

Functional Class Name 13-County Factor 7-County Factor 

 Rural Interstate 0.67 0.89 

 Rural Principal Arterial 1.02 0.99 

 Rural Minor Arterial 1.16 0.98 

 Rural Major Collector 0.88 1.81 

 Rural Minor Collector 1.07 1.81 

 Rural Local 0.64 1.10 

 Urban Interstate 1.05 0.86 

 Urban Other Freeway 2.44 0.85 

 Urban Principal Arterial 0.67 0.97 

 Urban Minor Arterial 1.21 0.96 

 Urbanized Collector 1.21 1.80 

 Urbanized Local 0.95 1.06 

MOBILE6 Emission Factors

Emission factors were produced for 2.5 miles per hour (mph), then 3 mph to 65 mph, 

inclusive, in one mile per hour increments for VOC and NOx using EPA's 

MOBILE6.2.03 model. Mobile source emissions are calculated by multiplying the VMT 
Mobile Source Emissions Modeling for Atlanta Ozone RFP Plan 

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for each link16 in the travel model network by the corresponding VMT adjustment factor 

(by HPMS class), then by an emission factor that corresponds to the congested flow 

speed of the link. Link level emissions are then summed to the time of day, and daily 

level, for reporting purposes. 

Emission factors reflect two types of programs, the federally mandated motor vehicle 

emission control programs and the locally specific parameters for Atlanta. 

Federal Programs Regional Parameters

National Low Emission Vehicle Standards Average Fuel Volatility 

Heavy-Duty Vehicle Emission Standards Vehicle Registration Information 

Tier 1 Emission Standards Ambient Weather Conditions 

Tier 2 Emission Standards17 Inspection & Maintenance Program 

Low-Sulfur Georgia Gasoline18

MOBILE6 estimates emissions for four types of "driving cycle" - arterial/collector, 

freeway, ramp, and local road. It is assumed that all VMT by motor vehicles will occur 

in one of these four driving cycles. Each driving cycle implies different assumptions 

about vehicle activity and different emission estimates in MOBILE6. The definitions of 

the different driving cycles from EPA's Technical Guidance on the Use of MOBILE6.2 

for Emission Inventory Preparation19 are given below. 

• Freeway Driving Cycle 

In MOBILE6, "freeway" VMT refers to driving that occurs on roadways that do not have 

traffic signals, that usually have limited access (via converging ramps) and have free flow 

speeds greater than 50 miles per hour. These roadways are usually divided and have 

more than one lane in each direction. This definition does not include short sections (less 

than two miles) of a roadway between signals, but could include longer roadway 

segments that effectively act as freeways. 

• Arterial/Collector Driving Cycle 

In MOBILE6, "arterial/collector" VMT refers to driving that occurs on roadways that 

have signalized traffic control. These roadways are not freeways, because they have 

traffic signals, but they may be divided, multiple lanes, one-way and have high free-flow 

speeds. However, traffic will be stopped periodically by traffic signals and will be 

 

16 A link refers to a section of the modeled road network in which none of the describing attributes change 

(e.g., link "A" is a segment of a highway that has 4 lanes with speed limit of 55; if the number of lanes 

changes, then a new link should be coded). 

17 (beginning with 2004 models) 

18 when different from federal sulfur levels 

19 http://www.epa.gov/otaq/models/mobile6/420r04013.pdf
Mobile Source Emissions Modeling for Atlanta Ozone RFP Plan 

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further affected by access to the roadway by driveways and un-signalized intersections. 

Even in free flow, the driving on arterial/collector roadways will be characterized by 

cruising periods interrupted by traffic signals. 

• Local Roadway Driving Cycle 

In MOBILE6, "local roadway" VMT refers to driving on roadways that are not normally 

considered as part of the traffic network. These roadways do not have traffic lights and 

rarely have more than one lane in each direction. They usually allow vehicle parking on 

the roadway surface and traffic control is handled via stop/yield signs. Speed limits are 

normally 30 miles per hour or less. The driving cycle used in MOBILE6 to model local 

roadways is fixed at an average speed of 12.9 miles per hour. Driving on local roadways 

is characterized by extremely low speeds and frequent stops at intersections. 

• Freeway Ramp 

In MOBILE6, "freeway ramp" VMT refers to the access roadways for freeways. It 

includes both traffic entering the freeway and exiting. Driving on freeway ramps is 

characterized by rapid acceleration from stop or low speeds to freeway speeds and 

decelerations from freeway speeds to low speeds or stop. Freeway ramp activity is not 

included in the MOBILE6 freeway roadway definition. Therefore, all freeway activity 

must include a corresponding freeway ramp activity to account for acceleration and 

deceleration to and from freeway speeds. 

Many areas do not explicitly account for freeway ramps as a separate roadway type so 

EPA developed a default ramp fraction (8% of freeway travel) for MOBILE6 users to 

account for this. This approach is not necessary for Atlanta's nonattainment area freeway 

emission estimates because freeway ramps have been explicitly defined in the ARC 

highway network. Freeway ramps have been defined as separate facilities stratified by the 

type of design and speed, high, medium and low. High-speed ramps represent ramps that 

connect freeway-to-freeway travel, such as I-285 to I-85N, while the lower speed ramps 

are for access to a freeway from an arterial or egress from a freeway to an arterial. The 

high-speed ramps in the ARC model have significantly higher free-flow speeds than the 

lower speed ramps and are not characterized by rapid acceleration or by deceleration 

from freeway speeds to low speeds or stops. Based on guidance from EPA, emissions 

for high-speed freeway ramps are estimated using non-ramp freeway emission factors 

while emissions for lower speed freeway ramps are estimated using freeway ramp 

emission factors. 

In MOBILE6, only arterial/collector and freeway emissions are speed sensitive. Emission 

factors were produced for 2.5 miles per hour and, in one mile per hour increments, 

between 3 mph and 65 mph, inclusive, for these two driving cycles. The all-vehicles 

composite emission factors were used to develop the mobile source emissions inventories 

for the MVEB. 
Mobile Source Emissions Modeling for Atlanta Ozone RFP Plan 

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Emission factors are applied to travel model VMT based on the appropriate driving cycle 

and congested flow speed. The mapping of MOBILE6 roadway type to both HPMS 

functional class and ARC roadway facility type20 used for the MVEB emissions 

modeling is shown in Tables C-5 and C-6, below. The appropriate MOBILE6 roadway 

type can be determined by using the facility type definitions in the highway network 

except for the rural other principal arterial. EPA believes that facilities with the HPMS 

classification of rural other principal arterial should be modeled using a combination of 

the freeway and ramp emission factors. EPA has recommended that the freeway 

emission factors be applied to 92% of the VMT while the ramp emission factors be 

applied for the remaining 8% of the VMT for these facilities. This is the only case where 

a combination of emission factors by driving cycle is used for the same functional 

classification. In the ARC highway networks, facilities with rural other principal arterial 

HPMS classifications have different facility types, ranging from expressway to different 

types of arterials. Within the ARC emissions modeling procedure, the HPMS functional 

classification, facility type, and free-flow speed for each link are checked to determine 

the appropriate MOBILE6 driving cycle for this category. Only links with a facility type 

of arterial or collector, an HPMS classification of rural other principal arterial, and a freeflow speed greater than 50 mph, will have the VMT adjusted based on the above criteria. 

Facilities with a freeway facility type and an HPMS classification of rural other principal 

arterial will not be adjusted. Table C-5 lists the mapping of HPMS functional classes to 

MOBILE6 driving cycles and Table C-6 lists the mapping of the ARC facility types to 

the MOBILE6 driving cycles. 

20 ARC uses a more refined breakdown of roadway types for travel demand modeling to enable a more 

accurate estimate of roadway volumes and speeds. ARC roadway types are referred to as Facility Types. 
Mobile Source Emissions Modeling for Atlanta Ozone RFP Plan 

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Table C-5 

HPMS Functional Classifications 

Mapped to MOBILE6 Driving Cycles 

HPMS Highway Functional System MOBILE6 Driving Cycle 

Rural interstate Freeway and freeway ramp 

Rural other principal arterial Freeway and freeway ramp 

Rural minor arterial Arterial/collector 

Rural major collector Arterial/collector 

Rural minor collector Arterial/collector 

Rural local Local 

Urban interstate Freeway and freeway ramp 

Urban other freeways Freeway and freeway ramp 

Urban other principal arterial Arterial/collector 

Urban minor arterial Arterial/collector 

Urban collector Arterial/collector 

Urban local Local 
Mobile Source Emissions Modeling for Atlanta Ozone RFP Plan 

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Table C-6 

Listing of ARC Facility Type Classifications 

by MOBILE 6 Driving Cycles

Facility Type Classification Code Area Type MOBILE6 Driving Cycle 

Interstate/Freeways 1 Urban/Rural Freeway 

Parkway 2 Urban/Rural Freeway 

HOV Buffer Separated 3 Urban/Rural Freeway 

HOV Barrier Separated 

High Speed Ramps/CD 

Roads 

4 

5 

Urban/Rural 

Urban/Rural 

Freeway 

Freeway 

Principal Arterial – Class I* 12 Rural Arterial/collector or Freeway 

and Ramp 

Principal Arterial – Class I 12 Urban Arterial/collector 

Principal Arterial – Class II* 13 Rural Arterial/collector or Freeway 

and Ramp 

Principal Arterial – Class II 13 Urban Arterial/collector 

Minor Arterials – Class I 14 Rural Arterial/collector 

Minor Arterials – Class I 14 Urban Arterial/collector 

Minor Arterials – Class II 15 Rural Arterial/collector 

HOV Arterials 16 Urban/Rural Arterial/collector 

Major Collector 17 Urban/Rural Arterial/collector 

Minor Collector 18 Urban/Rural Arterial/collector 

Medium Speed Ramp 6 Urban/Rural Ramp 

Low Speed Ramp 7 Urban/Rural Ramp 

Loop Ramp 8 Urban/Rural Ramp 

Off Ramp w/Intersection 9 Urban/Rural Ramp 

On Ramp w/Intersection 10 Urban/Rural Ramp 

Planned Ramps 

w/Intersections 

19 Urban/Rural Ramp 

Planned Directional Ramps 20 Urban/Rural Ramp 

Centroid Connectors 0 Urban/Rural Local 

*If HPMS classification is Rural Other Principal Arterial and free-flow speed is >50 mph - 92% 

of VMT allocated to freeway while 8% allocated to ramp 

Specific variables within the MOBILE6 inputs for the MVEB inventories are 

documented below. 

• Ambient Weather Conditions 

MOBILE6 allows relative humidity and temperature to be input for each hour of the day, 

with a single entry for barometric pressure. Hourly humidity and temperature inputs, and 

daily barometric pressure, were derived from National Weather Service Local 

Climatological Data from the 10 days with the highest ozone concentrations for the threeyear period 2000-2002. The hourly humidities and temperatures were averaged across 
Mobile Source Emissions Modeling for Atlanta Ozone RFP Plan 

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the 10 days. The daily average barometric pressures for the 10 days were also averaged. 

See Exhibit 1 for details.21

• Fleet Age 

Separate, area-specific registration distributions by age for the 13- and 7-county parts of 

the Atlanta ozone nonattainment area, developed using registration data obtained from 

R.L. Polk & Company,22 were modeled. In keeping with EPA guidance,23 the 

MOBILE6 default distribution was used for the heaviest (Class 8B) heavy-duty vehicles. 

• Gasoline24

The 2002 Base Year and the 2008 RFP budget year mobile source emissions inventories 

for the Atlanta 8-hour ozone nonattainment area reflect the effects of the low-sulfur and 

low RVP gasoline required by state rule in and near the nonattainment area. In 2002, 

sulfur and RVP were 150 parts per million average and 7.0 pounds per square inch, 

respectively. For the 2008 RFP budget year, Phase 2 Georgia Gas (30 ppm sulfur, 7.0 psi 

RVP) with 100% market share ethanol blend (10% by volume) and an RVP waiver was 

assumed. Per Georgia's Gasoline Marketing Rule, 391-3-1-.02(2)(bbb), this waiver 

allows the RVP of ethanol blend gasoline to rise as high as 8.0 psi. 

• Vehicle Inspection and Maintenance Program 

The vehicle inspection and maintenance (I/M) program in place in 1990 was modeled for 

the nonattainment area Adjusted Base Year mobile source inventory. This was a 

decentralized program for 12-years-old-and-newer cars and light trucks in Cobb, DeKalb, 

Fulton, and Gwinnett counties. That annual program consisted of an idle test and a check 

for tampering of three items: air pump system, catalytic converter, and fuel inlet 

restrictor. For 2002 Base Year and 2008 mobile source emissions modeling, the 

enhanced I/M program25 in place in the 13-county part of the nonattainment area was 

modeled. This annual program requires onboard diagnostics system checks on 1996 and 

newer model year cars and light trucks, 2-mode ASM tests on 25-year-old through 1995 

21 http://www.gaepd.org/Files_PDF/plans/sip/exhibit_1_hourly_temp_and_humidity.pdf 22 Registration data was from R. L. Polk & Co.'s National Vehicle Population Profile ® current as of 

October 2002 and from R. L. Polk & Co.'s TIPNet ® current as of March 2003. See Exhibit 2, 

http://www.gaepd.org/Files_PDF/plans/sip/exhibit_2_2002_registration_distributions.pdf, for more details 

on the registration distributions by age. 

23 "EPA encourages and recommends the use of actual locality-specific...registration distributions by age in 

the development of SIP emission inventories. One exception to this would be areas having relatively few 

local HDDV registrations, but significant interstate trucking activity within the local area. Such areas may 

want to retain and use the [MOBILE] national registration distributions." Section 2.2.3.6, User's Guide to 

MOBILE5. 24 In 1999, Georgia's two-phase gasoline sulfur control program limited average sulfur in gasoline sold in 

the 13-county Atlanta area and in 12 surrounding counties to 150 parts per million (ppm). In addition, 

there is a seasonal (June 1 to September 15) 7.0 pounds per square inch (psi) Reid vapor pressure (RVP) 

cap on gasoline sold in this Phase 1 area. In 2003, Phase 2 of Georgia's gasoline rule reduced average 

sulfur to 30 ppm and added 20 additional counties outside the 20-county ozone nonattainment area to the 

sulfur and (seasonal) RVP control program. 

25 See Rules for Enhanced Inspection and Maintenance, Chapter 391-3-20, 

http://www.gaepd.org/Files_PDF/rules/rules_exist/391-3-20.pdf
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model year vehicles; a check for catalytic converter tampering on 25-year-old through 

1995 model year vehicles; and a gas cap pressure test on all subject vehicles. 

Additional detail related to development of emission factors used for the Ozone RFP Plan 

emissions inventory is provided in Appendix C, Exhibit 3 

(http://www.gaepd.org/Files_PDF/plans/sip/exhibit_3_PlanAssumptions_102808.pdf). 

Emission Inventory Files 

The MOBILE6 input variables are specified in the input files. Exhibit 5 contains 

abbreviated versions of the MOBILE6.2 input files used to model link-level, 

nonattainment area emission factors for the 2002 Base and Adjusted Base Years. There 

is a separate scenario in each input for each speed, with only the speed varying. To 

conserve space, most of the speed scenarios are omitted from these inputs: 

http://www.gaepd.org/Files_PDF/plans/sip/exhibit_5_base_and_adj_base_inputs_ozone_

rfp.pdf

Exhibit 6 contains abbreviated versions of the input files used to calculate emission 

factors for the 2008 nonattainment area MVEB inventories: 

http://www.gaepd.org/Files_PDF/plans/sip/exhibit_6_2008_inputs.pdf

Exhibit 7 is a compressed file containing the complete MOBILE6.2 input files 

(filename.IN), plus the output (filename.TXT), post-processed output (filename.PRN) 

and supporting files (filename.D) used in modeling the mobile source emission 

inventories for the Ozone RFP Plan MVEB (all these files are in ASCII text format): 

http://www.gaepd.org/Files_PDF/plans/sip/exhibit_7_mobile_files_ozone_rfp.zip

Loaded network files that list VMT, speeds, FIPS26 code, facility type code, HPMS 

functional class code, VOC and NOx, etc., by time of day for each network link are 

produced by the emissions calculation model. These files were imported into database 

tables and daily emissions were then summarized. The data tables and the SQL queries 

used to summarize 2002 and 2008 link-level emissions are contained in a compressed 

Microsoft Access file available here: 

http://www.gaepd.org/Files_PDF/plans/sip/02_and_08_network_database.zip

File size: 27 megabytes compressed, 74 megabytes expanded. 

 

26 Federal Information Processing Standard, a numerical code indicating state and county.