Statement on Development of and Schedule for MOBILE6
                      August 21, 1996

     Planning the timing of updates to guidance for the
estimating of mobile source emissions always requires EPA to
balance a number of competing considerations.  The Office of
Mobile Sources (OMS) wants to allow sufficient time for the
development of a scientifically sound, error-free, and user-
friendly emission factor estimation tool for our customers.  We
also want to inform them of significant new facts and other
information in time for their decisions and selections of control
options to be based on the best information available, rather
than on "old" estimates that have been modified by new data or
insights.  We want to avoid situations in which an honest dialog
about new data and tentative insights distracts attention from
State technical work and related decision-making.  Finally, as is
always true, the States want assurances that their State
Implementation Plans (SIPs) will be reviewed for approval by EPA
under the same guidance that was applicable during their
development.

     After consultation with the Office of Air Quality Planning
and Standards (OAQPS), OMS has determined that an appropriate
window for final release of MOBILE6 would be the Summer of 1998. 
Release of the final version of MOBILE6 in Summer 1998 implies
that significant portions of the proposed content of the model
would be provided (in non-software form) for stakeholder and peer
review comments beginning late this year or early in 1997. 
Draft, or "beta version," software for MOBILE6 would be made
available approximately March 1998.

     Following is a summary of some of the major areas of
revision planned for MOBILE6, and  a brief description of the
expanded process planned for stakeholder and peer review of the
model.

     Aspects of MOBILE being reexamined and likely to be revised. 
MOBILE6 is planned as an extensive update and reworking of the
model, and there are a large number of areas in which we plan to
make changes.  This list is not exhaustive, and covers only some
of the more important issues.

     In the realm of exhaust emissions, we will be examining non-
FTP (so called "off-cycle") driving behaviors and frequencies of
occurrence, and the effects of such driving on emissions.  This
is a very important area and is likely to substantially change
in-use emission estimates for all pollutants and all vehicle
types.  The work of the In-Use Deterioration Work Group, part of
the FACA Mobile Source Technical Advisory Subcommittee, will be
used in our efforts to refine and improve estimates of in-use
deterioration of emissions.  We plan to expand the emission
factor calculations for heavy-duty diesel vehicles (HDDVs),
providing emission factors by subcategory (e.g., light, medium,
heavy, buses) as is now done in the mobile source particulate
emission factor model.  Heavy-duty conversion factors (g/bhp-hr
to g/mi) will be updated.  We would like to separate start-related 
and running (stabilized) emissions for gasoline-fueled
vehicle categories.  The effects of onboard diagnostic systems
(OBD II) on exhaust emissions will be reconsidered.  If possible,
we intend to update and improve estimates of idle emission
factors, and to look at interactions of various correction
factors (temperature, fuel volatility, average speed).  More
extensive work based on development of, and testing of vehicles
over, new driving cycles based on newer data and specific to
highway facility types (limited access, major arterial, etc.) is
also planned.

     For non-exhaust (evaporative) emissions, we will be
incorporating real-time diurnal emissions estimates and the
effects of the new, more stringent evaporative emission test
procedure, and updating estimates of hot soak, resting loss, and
running loss emissions.  Trip characteristics (e.g., trip
frequencies, average trip lengths) will be revised on the basis
of newer data.  The effects of OBD II systems on evaporative
emissions will be considered.  Analysis of new data on rates of
functional purge/pressure test failures and associated emission
impacts will be reflected.

     In the area of modeling inspection and maintenance (I/M)
program effects, we will be incorporating credits for remote
sensing (RSD) programs, supporting additional emissions cutpoints
in estimating of program benefits, and including provisions for
other-than-50% credit discounts for test-and-repair programs. 
The effects of OBD II systems in conjunction with I/M programs
will be evaluated and modeled.  To the extent permitted by time
and availability of data, we would like to model the effects of
I/M programs applicable to heavy-duty vehicles.

     Fuels effects to be updated include impact of in-use sulfur
levels on emissions, updated effects of oxygenates on exhaust
emissions, and possibly including the "complex model" used in
support of the reformulated gasoline (RFG) rules in MOBILE. 
Fleet characterization data (registration distributions and
average annual mileage accumulation rates by vehicle type) will
be revised on the basis of more recent information.  A number of
other changes are planned to make the model more user-friendly
and to "clean up" sections of the code that have become obsolete
or superceded by later revisions.

     As can be seen from the number and scope of areas being
reexamined and revised, along with the fact that considerable
testing will be performed over the next 12 months or so to obtain
additional data for use in the model, it is not possible for us
to estimate the overall changes in emission factors that are
likely to be seen in MOBILE6.  It is quite possible that these
changes will be significant, particularly for calendar years of
evaluation of 2000 and beyond; yet at this time it is not
possible for us to estimate with any confidence where these
results will lead.  Many of our final decisions on how to revise
aspects of the model will be affected by vehicle testing and data
analysis work to be performed between now and next summer.

     Expanded opportunities for stakeholder and peer review of
proposed model revisions.  In addition to the many changes
discussed above, we have worked with the Modeling Work Group of
the Mobile Source Technical Advisory Subcommittee to develop
recommendations for obtaining significantly more outside input
and review during the development of MOBILE6.  In consideration
of the practical problems posed by attempting a outside review of
a full finished product, we are planning to have an ongoing
process in place by this fall and continuing throughout the
development period for MOBILE6 in which ample opportunity is
provided to all interested parties to comment on work as it is
completed.  The exact details of the process are still not final,
but it appears that the central mechanism will be establishment
of a specific electronic forum in which OMS will post analyses,
test program proposals, documentation, and other model-related
work, and invite review and comment from outside parties.  We
anticipate that this will result not only in a better product,
but also one that when released in final form should face less
criticism than has sometimes been the case for past releases.  Of
course, allowing time for comment, EPA response to comments, and
changes necessitated or suggested by commenters, will require
more time to implement properly, and this is accounted for in the
schedule that we have proposed for MOBILE6.

     Once we are ready to release MOBILE6 in final form, EPA will
have to articulate clear guidance regarding its use.  We
generally support the "no late hits" philosophy when it comes to
changes being made in EPA guidance to the States, but feel that
we should postpone specific decisions until we have a better
understanding of how MOBILE6 emission factors will differ from
those estimated using MOBILE5a, and are cognizant of  the status
that various related planning activities have reached by that
time.