APPENDIX C

Somerset County Maintenance Plan for the 

1-hour Ozone Standard

Technical Support Document

Mobile Source Highway Emissions Inventory

Executive Summary

                                                       Prepared for:



Mobile Sources Section, Bureau of Air Quality

Pennsylvania Department of Environmental Protection

PO Box 8468

Harrisburg, PA 17105-8468



                                                       Prepared by:



Michael Baker, Jr., Inc.

1304 Concourse Drive

Linthicum, MD  21090

410-689-3400





July 2007

 Somerset County: Mobile Source Emissions Inventory

Executive Summary

July 2007

Table of Contents

  TOC \o "1-4"  INTRODUCTION	  PAGEREF _Toc126628300 \h  1 

Methodology	  PAGEREF _Toc126628301 \h  1 

Inventory Submission Materials	  PAGEREF _Toc126628302 \h  2 

DATA SOURCES AND ANALYSIS TOOLS	  PAGEREF _Toc126628303 \h  3 

Analysis Tools	  PAGEREF _Toc126628304 \h  3 

Traffic Data Source/Fleet Data Inputs	  PAGEREF _Toc126628305 \h  4 

Traffic Growth Assumptions	  PAGEREF _Toc126628306 \h  4 

I/M and Fuel Parameters	  PAGEREF _Toc126628307 \h  5 

Weather Data	  PAGEREF _Toc126628308 \h  6 

Other Inputs	  PAGEREF _Toc126628309 \h  6 

RESOURCES	  PAGEREF _Toc126628310 \h  8 

 

List of Tables

  TOC \h \z \c "Table"    HYPERLINK \l "_Toc126628311"  Table 1:  
Summary of Appendices	  PAGEREF _Toc126628311 \h  2  

  HYPERLINK \l "_Toc126628312"  Table 2:   Summary of Inventory Analysis
Tools	  PAGEREF _Toc126628312 \h  3  

  HYPERLINK \l "_Toc126628313"  Table 3:   Somerset County VMT Growth
Assumptions	  PAGEREF _Toc126628313 \h  5  

 

Summary of Appendices

Appendix C-1: 	Mobile Source Highway Emissions Inventory – An
Explanation of Methodology

Appendix C-2:	Somerset County Ozone (VOC, NOx) Emission Results 

(By Year, Functional Class and Vehicle Type)

Appendix C-3: 	MOBILE6.2 Input Parameter Summary

Appendix C-4: 	MOBILE6.2 Sample Input Files

Appendix C-5: 	Traffic Growth Forecasting System

INTRODUCTION

This technical document supports the 1-hour ozone standard maintenance
plan for Somerset County, Pennsylvania.  Somerset County was part of the
Johnstown nonattainment area for the 1-hour ozone national ambient air
quality standard.   The county has attained this standard, which was
withdrawn by USEPA and replaced in June 2004 with the more stringent
8-hour ozone standard.   In accordance with the Clean Air Act, USEPA's
November 29, 2005 Final Rule to Implement the 8-hour Ozone National
Ambient Air Quality Standards and other EPA guidance, former 1-hour
ozone nonattainment and maintenance areas that were not designated as
nonattainment of the 8-hour ozone standard and that are attaining that
standard, must submit a section 110(a)(1) maintenance plan. 

The maintenance plan includes the development of an emissions inventory
of ozone precursors (VOC and NOx) for a base year and projection years. 
The emissions inventory is used to demonstrate that projected emission
levels are sufficient to maintain the ozone standard.  U.S. EPA’s
Redesignation Guidance states, “A state may generally demonstrate
maintenance of the NAAQS by either showing that future emissions of a
pollutant or its precursors will not exceed the level of the attainment
inventory, or by modeling to show that the future mix of sources and
emission rates will not cause a violation of the NAAQS.”  This
document provides the base year and future year projections of mobile
(highway) vehicle miles of travel (VMT) and emissions (ozone precursors
VOC and NOx) that are needed to demonstrate maintenance for the region. 

Methodology  

Guidance documents from EPA were used to develop the base and future
year emissions inventories for Somerset County.  They include:

Policy Guidance on the Use of MOBILE6 for SIP Development ad
Transportation Conformity, US EPA Office of Air and Radiation, dated
January 18, 2002.

Technical Guidance on the Use of MOBILE6 for Emission Inventory
Preparation, US EPA Office of Air and Radiation, and Office of
Transportation and Air Quality, dated January 2002.

User’s Guide to MOBILE 6.1 and MOBILE6.2, Mobile Source Emission
Factor Model, EPA420-R-02-028, dated October 2002.

Mobile source emission factors were calculated using EPA’s MOBILE6.2
emission model.  The methodologies used to produce the emission results
conform to the recommendations provided in EPA’s Technical Guidance. 
A mix of local data and national default (internal to MOBILE6.2) data
has been used for this submission.  Local data has been used for the
primary data items that have a significant impact on emissions.  These
include:

VMT and speeds

Vehicle type mixes

Vehicle age distributions 

Hourly distributions 

Temperatures/humidity 

Inspection/Maintenance parameters 

Fuel program characteristics.  

For this submission, most local data inputs to the analysis process
reflect the latest planning assumptions based on 2005 data; however the
2002 base year utilizes 2002 data assumptions consistent with past
inventory efforts.  Traffic forecast growth rates were updated in 2005
and current assumptions on control strategies (e.g.
Inspection-Maintenance programs) and fuel characteristics are assumed. 
The analysis methodology is consistent with past statewide inventory
efforts including the 2002 and 2005 National Emissions Inventory (NEI)
submissions.  A detailed methodology is provided in Appendix C-1. 
Detailed information on the Commonwealth’s Traffic Growth Forecasting
System is provided in Appendix C-5.

The following sections address the key input data sources and analysis
tools used for the Somerset County inventory.  The final section
summarizes the inventory emission results for all applicable ozone
pollutants.

Inventory Submission Materials  

To complement this document, attachments have been provided with
additional detail regarding the analysis methodology, the MOBILE6.2
input parameters, and the output VMT and emission results for Somerset
County.  These include:

Table   SEQ Table \* ARABIC  1 :   Summary of Appendices

Appendix	Title	Description

C-1	Mobile Source Highway Emissions Inventory – 

An Explanation of Methodology	Provides summary of methodology used to
calculate the VMT and emissions.

C-2	Somerset County Ozone (VOC, NOx) Emission Results (By Year,
Functional Class and Vehicle Type)	Provides county by functional class
emission tables and county by vehicle type emission tables for a summer
weekday in 2002, 2004, 2009, 2018

Also includes annual 2002 results for VOC, NOx, CO

C-3	MOBILE6.2 Input Parameter Summary	Provides summary of input
parameters related to traffic data sources, fuel, weather, I/M, and
other MOBILE6.2 related parameters.

C-4	MOBILE6.2 Sample 

Input File	Provides examples of the MOBILE6.2 input files.

C-5	Traffic Growth Forecasting System	Provides detailed explanation of
the Commonwealth’s revised methodology for VMT forecasting based on
socioeconomic and demographic projections.



DATA SOURCES AND ANALYSIS TOOLS 

This section provides a summary of the key input data and analysis tools
used for producing the Somerset County emissions inventory.  The key
elements to the modeling protocol are described in the sections below. 
A more detailed description of the analysis process and tools is
provided in the methodology report in Appendix C-1 and C-5. 

Analysis Tools  

The Somerset County inventory analysis utilizes several key
software/programs for producing the county emissions totals.  These
tools are outlined in Table 2.

Table   SEQ Table \* ARABIC  2 :   Summary of Inventory Analysis Tools

Tool	Purpose

MOBILE6.2	Produces emission factors for each pollutant in Grams/Mile for
VOC and NOx

PPSUITE	Processes the highway data, Calculates hourly congested speeds
for each state roadway segment, Prepares MOBILE6.2 input files,
Processes MOBILE6.2 output files

Custom SCC Output Reformatting Program	Prepares output database files by
EPA’s Source Classification Codes (SCC)



The Somerset County inventory reflects the highway mobile source
emission estimations using EPA’s MOBILE6.2 emission model.  The latest
version of MOBILE is a major revision based on new test data and
accounts for changes in vehicle technology and regulations.  In
addition, the model includes an improved understanding of in-use
emission levels and the factors that influence them resulting in
significantly more detailed input data.  As compared to previous MOBILE
versions, MOBILE6.2 has a significant impact on the emission factors,
benefits of available control strategies, effects of new regulations,
and corrections to basic emission rates.  

PPSUITE represents an enhanced version of the Post Processor for Air
Quality (PPAQ) software system that has been used for previous inventory
and conformity submissions in Pennsylvania.  The software has undergone
significant revisions to ensure consistency with the MOBILE6.2 emissions
model.  PPSUITE plays a key role in the development of hourly roadway
speed estimates, which are supplied as input to the MOBILE6.2 model. 
The software is also used to prepare the MOBILE6.2 input shell and to
process the MOBILE6.2 outputs. 

Custom programs have also been prepared to translate the VMT and
emission results into a database file indexed by EPA’s Source
Classification Code (SCC).  This code represents combinations of vehicle
type groupings, functional classes, and time periods.  

Traffic Data Source/Fleet Data Inputs  

The 2002 and 2005 PENNDOT Roadway Management System (RMS) data serve as
the primary highway data sources for the county and functional class VMT
estimates.  The data source has been updated to provide a “snapshot”
of the regional roadway system and volumes for 2002 and 2005.  The data
includes the average annual daily traffic volumes for all state roadways
in Pennsylvania.  To account for additional local roadway VMT and to
ensure consistency with reported HPMS totals, the 2002 and 2005 VMT
totals are adjusted to match the 2002 and 2005 HPMS VMT totals reported
to FHWA.  Although 2005 is not an analysis year for this maintenance
plan, the data is used as a basis for all future forecasts.

Based on the requirements for ozone inventories, traffic volumes on each
RMS roadway segment must be adjusted to a summer weekday.  The daily and
monthly seasonal factors are developed from data contained in the
documents, 2002 Pennsylvania Traffic Data and 2005 Pennsylvania Traffic
Data, as prepared by PENNDOT’s Bureau of Planning and Research.  The
seasonal and daily factors provided in these documents are based on
statistical analyses of 2002 and 2005 traffic counts taken at permanent
and in-pavement ATR (automatic traffic recorder) locations throughout
the state.  Based on these seasonal traffic volumes, the PPSUITE
software calculates unique congested speeds for each roadway segment
during an ozone summer weekday scenario.

 

PPSUITE calculates congested speeds by hour of the day for each roadway
segment and provides the information as input to the MOBILE6.2 software.
 To disaggregate the daily RMS volumes to hourly volumes, auto and truck
hourly pattern data from PENNDOT’s traffic data reports are used to
determine the temporal variations in traffic volumes.

Vehicle mix patterns are calculated for each county/functional class
grouping utilizing a combination of 2002 and 2005 RMS truck percentages
and MOBILE6.2 default vehicle mix distributions.  The development of
vehicle type pattern data input to the MOBILE6.2 software is described
in more detail in Appendix C-1.  The distribution of vehicles to fuel
type (diesel, gas) is determined from the MOBILE6.2 default diesel sales
fractions.

Vehicle age distributions are input to MOBILE6.2 for each county based
on registered vehicles that reflect July 1 summer conditions.  These
distributions reflect the percentage of vehicles in the fleet up to 25
years old and are listed by the 16 composite MOBILE6 vehicle types. 
Vehicle age distributions have been developed from the PENNDOT Bureau of
Motor Vehicles Registration Database.  2002 registration data is used
for the 2002 and 2004 analysis years, and 2005 registration data is used
for all future analysis years.  Due to insufficient data, only data for
light-duty vehicles are used as local inputs.  The heavy-duty vehicles
use the internal MOBILE6.2 defaults.

Traffic Growth Assumptions  

Traffic growth forecasting plays a pivotal role in estimating future
year emissions for the region.  This inventory utilizes county-specific
growth rates from a PENNDOT study completed in 2005.  As part of that
study, a statewide traffic growth forecasting system was developed that
incorporates traffic data from PENNDOT’s Traffic Information System
and socioeconomic forecasts.  The study data and statistical models were
recently updated with 2006 socioeconomic forecasts and the latest
available HPMS VMT trends.

The study and methodology are  documented in the report “Statistical
Evaluation of Projected Traffic Growth, Traffic Growth Forecasting
System: Final Report, March 14, 2005” and contains the following
improvements over past approaches:

 

Strong statistical basis and consistent with state of the art.

Thorough documentation of approach vs. alternatives.

Increased robustness through the inclusion of both county-level
historical traffic trends from the RMS databases and county-based
demographic projections from an independent 3rd party (Woods and Poole
Economics, Inc.).

Incorporates socio and economic data (households, mean household income)
and a relative measure of transportation capacity (lane miles per
capita).

Expedient and inexpensive update process.  

The results of the study have been shared between PENNDOT, PaDEP, and
other Interagency Consultation Group members, including the PA
Conformity Work Group (which includes EPA, FHWA, and representatives
from larger MPOs within the state).

The resulting forecasting system includes the development of VMT
forecasts and growth rates for four functional classifications in each
Pennsylvania county: urban interstate, urban non-interstate, rural
interstate, and rural non-interstate.  The forecasts use statistical
relationships based on historic HPMS VMT trends and future county
socioeconomic projections from Woods and Poole Economics, Inc.  The
model results from county-based model HH are used.  This is a
statistical model incorporating historical VMT trends, socio and
economic data (households, mean household income), and a relative
measure of transportation capacity (lane miles per capita), which
produce the upper boundary of VMT forecasts.     .  From an air quality
perspective, the use of the upper boundary is conservative, and provides
for factors outside PENNDOT or PaDEP control (e.g., potential
socio-economic growth above current forecasts, fleet age distributions,
vehicle type distribution) that influence motor vehicle emissions. 
Table 3 summarizes the assumed projected growth of VMT for future
analysis years.

Table   SEQ Table \* ARABIC  3 :   Somerset County VMT Growth
Assumptions

Analysis

Year	Total Growth 

From 2002	Annualized Growth From Previous Analysis Year

2002	-----	-----

2004	3.0 %	1.5 %

2009	3.6 %	0.5 %

2018	9.8 %	0.6 %



I/M and Fuel Parameters  

The Inspection Maintenance (I/M) program inputs to the MOBILE6.2 model
are based on past and current programs within the county.  For the 2002
analysis year, no I/M, ATP, or gas cap checks are assumed; however years
beyond 2003 include Pennsylvania’s statewide program.

The Pennsylvania inspection and maintenance (I/M) program was upgraded
and expanded throughout the state with a phase-in period starting in
December 2003 and fully implemented by June 2004.  The program test
requirements vary by region and include on-board diagnostics (OBD)
technology that uses the vehicle’s computer for model years 1996 and
newer to download potential engine problems that could effect emissions.
 The program, named PAOBDII, is implemented in the Philadelphia,
Pittsburgh, and South Central / Lehigh Valley Regions.  The Northern
Region receives gas cap and visual inspections and the other 42 counties
in the Commonwealth receive a visual inspection.  Vehicles subject to
the program include 1975 and newer model year gasoline cars and light
duty trucks up to 9,000 pounds GVW.  New model years are exempt for the
first year.  The county of registration determines which inspections are
required.  

Somerset County is one of the 42 counties in the Commonwealth in which
vehicles receive a visual inspection.  The program expanded to these
counties in December 2003.  There were no prior emission inspection
programs in these counties.  The program parameters include:

Model Years	Program Parameters

1975 & newer 	ATP



The summer weekday RVP values are consistent with values used for past
inventory efforts.  These assumptions, as well as the I/M program and
fuel parameters, are summarized in Appendix C-3.

Weather Data  

Updated weather information has been obtained from the National Climatic
Data Center to calculate the minimum and maximum temperatures and
absolute humidity data inputs to the MOBILE6.2 model.  These assumptions
are consistent with the 2002 and 2005 NEI inventory submissions and are
described below. 

For each of the 10 days with highest 8-hour ozone concentrations between
2000 and 2002, minimum and maximum temperatures and barometric pressure
values were obtained for each of the weather station locations in
Pennsylvania.  In addition, relative humidity values are obtained for
3-hour intervals (8 intervals during a 24 day period) for each of the
exceedence days.  Absolute humidity values are then calculated from the
relative humidity and barometric pressure for each of the 8 daily
intervals using EPA’s M6HUMID program. The lowest of these 8 values is
determined to be the absolute humidity for that day.  Per EPA guidance,
the average of the minimum and maximum temperatures for all 10 days and
the lowest of the absolute humidity values are used as the inputs to
MOBILE6.2 for the summer scenario analysis runs.

Other weather data required by MOBILE6.2 are assumed as the program
defaults.  These include the cloud cover, peak sun, and sunrise/sunset
options. 

Other Inputs  

Federal Programs

Federal vehicle emissions control and fuel programs are incorporated
into the MOBILE6.2 software. The programs include:

The Federal Motor Vehicle Control Program (FMVCP) including the National
Low Emission Vehicle Program (NLEV) and federal Tier II / Low Sulfur
Fuel Program;

Emissions standards for medium and heavy duty vehicles in 2002, 2004 and
2007;

Stage II and Onboard Refueling Vapor Recovery (ORVR).

Note: Pennsylvania considers emissions from refueling operations an area
source category. While MOBILE6.2 is employed to calculate emissions
factors for that source category, refueling emissions are not included
in highway vehicle emissions estimations.

State Programs   

The MOBILE6.2 software also allows for modeling of permissible state
activities control emissions from highway vehicles. 

In 1998, Pennsylvania promulgated the New Motor Vehicle Emissions
Control Program. This program created the Pennsylvania Clean Vehicles
Program that required new light-duty highway motor vehicles offered for
sale within Pennsylvania to be certified by the California Air Resources
Board (CARB) under that Board’s Low Emitting Vehicle (LEV) program.
The Pennsylvania Clean Vehicles Program does not require the California
Zero Emitting Vehicle (ZEV) requirement.

As a compliance alternative to the Pennsylvania Clean Vehicles Program,
automakers could elect to comply with the National Low Emission Vehicle
(NLEV) program. The NLEV program began phase-in starting in model
year1999 and was the compliance option for automakers. The Pennsylvania
rules allowed the NLEV compliance option for the duration of
Pennsylvania’s participation in the NLEV program. The Commonwealth’s
participation in NLEV was until model year 2006, thus requiring
automakers to comply with the Pennsylvania Clean Vehicles Program (i.e.
California LEV requirements…currently the LEV II program) for model
years 2006 and beyond. Pennsylvania is in the process of amending the
Pennsylvania Clean Vehicles Program to extend the compliance start date
to model year 2008. The modeling results assume a 2008 start year for
the Pennsylvania Clean Vehicles Program. Benefits from this program are
realized in 2008 and beyond. 

The methodology employed to estimate the benefits of adopting CA LEV II
emissions standards is consistent with the approach provided in the
EPA’s June 2002 Guidance (Modeling Alternative NLEV Implementation and
Adoption of California Standards in MOBILE6, June 5, 2002). The approach
assumes:

Tier2 phase-in schedule consistent with MOBILE6.2 defaults

LEVII phase-in schedule as provided in EPA’s June 2002 Guidance

Assumes PZEV and AT-PZEV vehicles share the same evaporative emissions
as LEVII/Tier2 vehicles

Appendix C-4 contains the modified MOBILE6.2 files used to model the
above assumptions. They are consistent with the June 2002 EPA guidance.

The federal Tier 2 / Low Sulfur Fuel Program took effect in 2004 and in
practicality supplanted the NLEV program for new light-duty vehicles. 

RESOURCES 

Draft Emissions Inventory Guidance for Implementation of Ozone and
Particulate Matter National Ambient Air Quality Standards (NAAQS) and
Regional Haze Regulations, EPA, June, 2003

Consolidated Emissions Reporting, Federal Register, June 10, 2002

2002 Pennsylvania Traffic Data,  PENNDOT Bureau of Planning and
Research, 2002.  

2005 Pennsylvania Traffic Data,  PENNDOT Bureau of Planning and
Research, 2005.  

User’s Guide to MOBILE6.1 and MOBILE6.2, Mobile Source Emission Factor
Model, EPA420-R-02-028, dated October 2002.

Technical Guidance on the Use of MOBILE6 for Emission Inventory
Preparation, US EPA Office of Transportation and Air Quality, January
2002.

Policy Guidance on the Use of MOBILE6 for Emission Inventory
Preparation, US EPA Office of Air and Radiation, January 18, 2002.

Modeling Alternative NLEV Implementation and Adoption of California
Standards in MOBILE6, EPA, June 5, 2002.

Statistical Evaluation of Projected Traffic Growth, Traffic Growth
Forecasting System: Final Report, Michael Baker Jr., Inc., March 14,
2005.

	

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