ENVIRONMENTAL PROTECTION AGENCY
                                
                    [AMS - FRL - _____ - __]
                                
                     Air Pollution Control;
                 Motor Vehicle Emission Factors
                                

AGENCY:   Environmental Protection Agency (EPA)

ACTION:   Notice of Public Workshop

SUMMARY:  The Environmental Protection Agency is now in the
process of developing revisions and improvements to the highway
vehicle emission factor model (the MOBILE model).  The current
version of the model, MOBILE5a, was released for use March 26,
1993.  The next version of the model, MOBILE6, is tentatively
planned for completion early in 1998 and release for use in the
summer of 1998.  This notice announces the first public workshop
for the purpose of discussing issues raised by the pending
revisions to the model, and provides the first formal opportunity
for comment and reaction to the plans for data collection,
analysis, and proposed model revisions.  There will be at least
one additional MOBILE6 workshop, most probably to be held late
this year.  The workshop will also include a short presentation
concerning EPA's plans for development of a nonroad mobile source
emissions inventory model.

DATES:  The workshop will be held Wednesday, March 19 and
Thursday, March 20, 1997.  The times are from 8:30 am to 5:00 pm
March 19, and 8:30 am to 3:00 pm March 20.  All times are Eastern
Standard Time.

ADDRESSES:  The workshop will be held in Powsley Auditorium of
the Morris Lawrence Building, Washtenaw Community College, 400
East Huron River Drive, Ann Arbor, MI 48106.  Directions to the
workshop can be requested from the contact person listed below,
or from the EPA Technology Transfer Network (TTN) bulletin board
system (BBS), or through accessing the OMS World Wide Web (WWW)
site.  Information on how to electronically access this and other
workshop-related information appears immediately below.

FOR FURTHER INFORMATION CONTACT:  Ms. Betty Measley, U. S. EPA
Office of Mobile Sources, Assessment and Modeling Division,
Emission Inventory Group, 2565 Plymouth Road, Ann Arbor MI 48105. 
Telephone:   (313) 741-7903; fax (313) 741-7939.

This notice, as well as related information concerning the
workshop, may be found in the OMS section of the EPA TTN BBS.  To
access this information using the WWW:

     http://www.epa.gov/OMSWWW/models.htm
     ftp:      ftp.epa.gov   Chg Dir->pub/gopher/OMS

For those directly accessing the TTN BBS by modem connection:

     TTNBBS Dial-in:  (919) 541-5742  [Voice help:  (919) 541-5384]

Workshop-related files, including a copy of this notice, a map
showing the location of WCC, and later additional information as
described in the body of this announcement, will be found at the
OMS Section, Models & Utilities Subsection

SUPPLEMENTARY INFORMATION: Under Section 130 of the Clean Air Act
Amendments of 1990, EPA is required to review, and to revise as
necessary, the emission factors used to estimate emissions of
volatile organic compounds (VOC), carbon monoxide (CO), and oxides
of nitrogen (NOx) from area and mobile sources.  In the case of
highway vehicles, emission factors for these pollutants as a
function of various parameters are estimated using the highway
vehicle emission factor model, commonly referred to as MOBILE.
This model, which was first developed in the late 1970s, has been
revised, updated, and improved periodically since that time to
account for increasing data and analyses concerning in-use
emissions performance of highway vehicles, changes in vehicle and
emission control technology, changes in fuel composition,
strengthening of applicable emission standards, refinements to
applicable test procedures, and other items that affect emission
levels in use.

Section 130 of the Act requires that this emission factor review,
and revision as needed, be performed at least every three years.
As noted above, the current official version of the model,
MOBILE5a, was released in March 1993.  Since that time, one
interim update to the model has been developed, MOBILE5a_H,
released in November 1995.  While not involving revision and
update to the entire model, this version was developed to address
specific needs on the part of emission factor users.  MOBILE5a_H
incorporated a number of changes intended to improve the ability
of modelers, particularly States and local/regional governments,
in estimating the benefits of various innovative inspection and
maintenance (I/M) programs and to improve the accuracy of modeling
situations in which such programs change over time or different
programs are applied to different subsets of the covered fleet.

The time elapsed since the last complete revision to the model and
the additional test data and analyses available since that time
warrant another thorough update and revision to the model. OMS
plans significant changes not only to the underlying emission
factor estimates, but to how emission factors are modeled to
account for things such as separation of start and running exhaust
emissions, roadway facility type, average traffic speeds, and a
number of other important changes that will affect the input
information required to use the model as well as the type of
information produced by the model.  Thus, this first MOBILE6
workshop will present an overview of the more important model
revisions being planned.  The tentative agenda for this workshop
is discussed below.  Other aspects of the modeling of highway
vehicle emissions that are not specifically included within the
following discussion may also be briefly addressed in this
workshop; however, the agenda discussed below is intended to
illustrate the major areas of discussion for the workshop.

The workshop being announced by today's notice will span two
days.  In an effort to facilitate travel plans on the part of
attendees, a preliminary agenda for the two days is presented
below.  Note that the first day (March 19) is largely devoted to
"technical" issues involved in updating and revising the model,
while the second part-day (March 20) is focused more on "user
changes," meaning those revisions planned that will affect the
input data requirements and file structure and output changes. 
Many attendees will likely want to be present for both sessions,
however, some may find that they can limit their attendance to
one or the other days based on their specific interests and
needs.

Topics to be discussed on March 19.  The first day of the workshop
is planned to include presentations on the new facility-specific
speed correction cycles, start emissions and separation of start
from running exhaust emissions, technology fractions for future
model years, the findings of the in-use deterioration team,
effects of fuel oxygenates and sulfur content on emissions,
"real-time" diurnal evaporative emissions, and revisions to the
modeling of emissions from heavy-duty vehicles.  Each presentation
will be followed by a short discussion/question and answer period,
and there should be some time left at the end of the day for more
general open discussion of the material that has been presented.

New facility-specific driving cycles.  One area of concern with
respect to the accuracy of modeled emission factors has been the
methods used to correct emission estimates based on the Federal
Test Procedure (FTP), intended to represent overall urban area
driving and with an average speed of 19.6 mph, for other average
speeds.  EPA is designing a plan for a much different approach to
this issue that we hope to include in MOBILE6, which should both
improve the accuracy of emission estimates over the range of
travel speeds of interest and improve the integration of emission
factor modeling with transportation planning and modeling.  This
would represent a major departure from the approach taken in
previous versions of the model, and will result in significant
changes to both the input data requirements and output emission
factor estimates relative to earlier versions.

Start emissions and separation of start from running emissions.
MOBILE has used operating mode fractions (describing the portion
of overall vehicle miles travelled (VMT) by vehicles in
cold-start, hot-start, or stabilized operation) as an input to
provide exhaust emission factors in grams per mile that include
start emissions.  Based on the needs of the air quality and
transportation planning communities and the availability of data
suitable for this type of analysis, EPA is proposing to make two
major changes in this area: Provision of start emissions (in grams
per vehicle per start) and stabilized running exhaust emissions
(in grams per mile) at the option of the model user, and basing
start emission estimates on time that a vehicle has been off
(rather than simply "cold" and "hot" starts, start emissions will
be modeled as a function of time that vehicles have been off, or
"soak time").

Technology fractions.  Emissions from highway vehicles are
estimated on a fleetwide basis using information on the fractions
of each model years' fleet that use different technologies (e.g.,
fuel delivery systems, catalytic converter type).  Projecting
future year emission requires that projections of future
technology fractions by model year be included in the MOBILE
model.  A contractor working for EPA has developed such estimates
for future years, which will be presented and discussed.

In-Use Deterioration Team Findings.  A team within OMS has spent
considerable effort reexamining the extent of and causes of in-use
deterioration, or the increase in emissions over time as vehicles
accumulate mileage and components, including emission control
components, age and degrade in performance.  The team has worked
in cooperation with the In-Use Deterioration Work Group of the
Mobile Source Technical Advisory Subcommittee (a subcommittee to
the Clean Air Advisory Committee established under the Federal
Advisory Committee Act).  All available data on in-use emissions
performance over time have been used in an attempt to better
quantify the extent of in-use emissions deterioration.  An
overview of the findings of the team to date, and some of the
potential implications for the modeling of in-use deterioration of
emissions in MOBILE6, will be presented and discussed at this
workshop.


Fuel Sulfur and Oxygenate Content Effects.  EPA has known for
some time that other aspects of fuel (gasoline) composition,
beyond volatility as measured by Reid vapor pressure (RVP), have
an impact on emissions.  MOBILE5a included the ability for the
modeler to specify the effects of RVP, and of oxygenate type
(i.e., alcohol or ether blends) and content (% by wgt), on
emissions.  At this workshop, information on revising and
improving the modeling of oxygenate type and content on
emissions, and on the impacts of sulfur content on emissions,
will be presented and discussed.

Onboard Diagnostic System Effects.  With the introduction of
second generation onboard diagnostic systems (OBD-II) to the
light-duty fleet, EPA needs to develop methods for modeling the
impact of these systems on reducing in-use deterioration of both
exhaust and evaporative emissions, in both inspection/maintenance
(I/M) program areas and non-I/M areas.  Proposed approaches to
including such effects in MOBILE6, based in part on previous work
by the California Air Resources Board and recommendations made by
the Modeling Work Group of the Mobile Source Technical Advisory
Subcommittee, will be presented and discussed.

Heavy-Duty Vehicle Emission Estimates.  The estimation of in-use
emissions from heavy-duty vehicles is complicated by the fact that
such engines are regulated on a mass/work basis (grams per brake
horsepower-hour), while users of emission factors generally need
emissions on a mass/activity basis (i.e., grams per mile). This
necessitates the use of conversion factors to adjust g/bhp-hr
emissions to g/mi.  These conversion factors will be updated for
MOBILE6.  In addition, plans are to expand the number of vehicle
categories for which specific emission factors are estimated by
the model, replacing "heavy-duty gas vehicle" and "heavy-duty
diesel vehicle" emission factors with estimates specific to a
number of subcategories (e.g., by GVW class, with buses treated
separately).  EPA's plans for revisions in these areas will be
presented and discussed.

Fleet Characteristics.  In order to model emission factors for the
entire in-use fleet of highway vehicles, information on the total
numbers of vehicles by vehicle type, the registration
distributions by age of each vehicle type, and the annual mileage
accumulation rates by age of each vehicle type are required. While
modelers often substitute locality-specific data for the national
data that is included in MOBILE, particularly for registration
distributions by age, it is still important for national modeling
and estimation of the impact of new rules, standards, and test
procedures to update these types of information periodically.  EPA
has retained a contractor to develop more recent information on
fleet characteristics, including detailed information on the
various subclasses of heavy-duty gas and diesel vehicles, and
buses, for use in MOBILE6.  A progress report on results to date
and plans for incorporating such data into MOBILE6 will be
presented and discussed.

Real-Time Diurnal Emissions.  For gasoline-fueled vehicle types,
non-exhaust emissions are a significant portion of total emissions
of volatile organic compounds (VOC).  Non-exhaust, or evaporative,
emissions consist of diurnal emissions, hot soak (trip-end)
emissions, refueling emissions, and running and resting losses.
Diurnal emissions, generated during times that a vehicle is not
being driven and ambient temperatures are increasing, have in the
past been based on data obtained during one-hour forced heat
builds, with the temperature increase representative of an entire
eight-hour period being performed over one hour.  More recent
testing has shown that if the emissions are measured over longer
periods of time, more representative of the rate and duration of
temperature increases actually experienced by in-use vehicles, the
results are not the same.  To improve the accuracy of diurnal
emission estimates, and to provide users with the ability to
better model emissions over shorter periods of time than full days
(e.g., airshed models typically require emissions on an
hour-by-hour basis), MOBILE6 will incorporate so-called "real
time" diurnal emissions estimates and means of estimating such
emissions over shorter time periods and lesser temperature
increases characteristic of such shorter times.  Plans for
implementing this approach in MOBILE6 will be presented and
discussed.

Liquid Leaks, Trip Characteristics.  Plans are for the new version
of the model to have means of explicitly accounting for, and
estimating emissions due to evaporation of, liquid leaks of fuel.
Modeling liquid leaks explicitly in MOBILE would have an impact on
estimates for other non-tailpipe emission factors (diurnals, hot
soaks, running and resting losses), as the presumption is that
liquid leaks, if and when encountered, have been included within
these other emission source categories.  All of these evaporative
emission estimates are affected by trip characterisitics, or
travel patterns, such as average number of trips per day, miles
per day, miles per trip, and so forth.  EPA plans to have update
these trip characteristics in MOBILE6 on the basis of analysis of
data obtained from instrumented vehicles.

These are the main areas in which presentations are planned for
the first day of the workshop.  Results of test programs and data
analyses will be presented where available, and in all subject
areas plans for additional work and proposed revisions to the
model's treatment of each area will be discussed.

Topics to be discussed on March 20.  As noted above, the focus of
the presentations and discussion on the second day of the workshop
will be more toward changes that impact the input data
requirements and file structure and on proposed output changes.
The second day will also include a presentation concerning EPA's
plans for development of a nonroad emission inventory model.

Inspection/Maintenance (I/M) Programs and Credits.  One of the
more important aspects of the model from the perspective of many
States and local/regional entities is the modeling of the benefits
of various types of periodic I/M programs.  MOBILE5a_H was
released in 1995 to provide an interim tool for use in modeling
certain types of tests and combinations of programs that could not
be modeled adequately using MOBILE5a.  The increasing variety of
test types (e.g., idle tests, IM240 tests, use of remote sensing
devices in conjunction with other I/M programs, the ASM and BAR90
tests), the tendency toward greater use of multiple sets of
cutpoints (based, for example, on age of vehicle at time of test),
and the frequency with which a given area is using more than one
type of I/M program, whether simultaneously or sequentially, all
suggest that there is a need for changes in how the emission
benefits resulting from such programs are estimated and reflected
in MOBILE emission factors.  In MOBILE6, EPA is considering
significant changes to the means by which credits for I/M programs
are modeled.  EPA will present proposals for changes in the
modeling of I/M programs for discussion and comment.


Input/Output Structure Changes.  In past updates to the MOBILE
model, EPA has made a strong effort to maintain upward
compatibility of input data files used to run the model.  That is,
a MOBILE4.1 input file, for example, can be used to run MOBILE5a,
although some features of MOBILE5a have no corresponding feature
in MOBILE4.1.  This has been accomplished through adding new
options as either (i) additional permitted values assigned to
existing control flags, or (ii) additional optional variables
appended to the end (right side) of existing input file lines, set
up so that if they are missing (as would be the case if an input
file for an older MOBILE version not having that feature) this in
interpreted as "new option not to be included in modeling." The
extent of changes planned and proposed for MOBILE6 are such that
it will no longer be possible to maintain this "upward
compatibility" of input files.  The output files are also likely
to change significantly.

Because this is likely to be of great interest to State and
local/regional modelers in particular, EPA will devote one
presentation to specifically outlining all of the input and output
changes implied by the model revisions noted above, as well as
others not the subject of specific presentations at this workshop.
This information is still in the proposal stage, and the input of
and reaction from modelers at and after the workshop will assist
in determining the precise nature of these changes in MOBILE6.

Nonroad Model - Overview of Plans.  The final presentation at the
workshop will not be directly related to MOBILE6, but instead will
present an overview of EPA's plans for the development of a
nonroad mobile source emission inventory (as versus emission
factor) model.  Current nonroad inventory development practices
are based on EPA's Nonroad Engine and Vehicle Emissions Study
(NEVES), done under the 1990 Clean Air Act Amendment requirements,
and "Procedures for Emission Inventory Preparation, Volume IV:
Mobile Sources (EPA-450/4-81-026d (revised), 1992). In
consideration of the increased recognition of the importance of
emissions from nonroad sources in terms of overall emissions and
air quality, and the considerable practical difficulty of
implementing the current guidance, EPA is planning to develop a
SIP-related nonroad emissions inventory model to meet the needs of
the modeling audience.  EPA will present its plans and proposals
for development of a nonroad mobile source emission inventory
model, and will be especially interested in input from workshop
attendees as to their needs and preferences for such a model.
Specifically, EPA would like to know the types of
locality-specific input data (e.g., equipment populations) that
users of such a model would anticipate developing and using in
order to customize nonroad emission inventories for the geographic
domain of interest.  Such information obtained at the workshop
will assist EPA in determining the best appraoches to use in a
nonroad emission inventory model to maximize its utility.

Additional Information.  To the extent possible, EPA will post
material at the TTN BBS site described under "For further
information contact" above in advance of the workshop.  Those
planning to attend, and those interested in following the progress
of workshop planning more closely, should periodically visit the
workshop information site.  For example, some of the presentation
materials that will be used at the workshop will be posted in
advance to facilitate discussion and comment at the workshop.


______________     __________________________________________
    Date           Mary Nichols, Assistant Administrator
                                 Office of Air and Radiation