MEMORANDUM

DATE:     October 11, 1996

SUBJECT:  Release of MOBILE5b

FROM:     Philip A. Lorang, Director
          Assessment and Modeling Division

TO:       Regional Air Directors

Introduction

     This memorandum announces the release of an interim update
to the current highway vehicle emission factor model, MOBILE5a
(March 26, 1993).  This interim update to the model is MOBILE5b
(September 14, 1996).  A copy of the program and related files on
diskette, and a paper copy of the revised Chapter 2 for the
User's Guide to MOBILE5, is attached for your use and
information.  The program and all related files are also
available through the Technology Transfer Network (TTN) computer
bulletin board system (BBS); detailed information on the file
names, contents, and location appear later in this memo.

     The remainder of this memo explains what changes have been
made in MOBILE5b, relative to MOBILE5a; discusses the reasons why
this interim model update is being released at this time; and
discusses the situations in which use of MOBILE5b is allowed. 
The use of MOBILE5b is not being required at this time.  As part
of this process, the Office of Mobile Sources (OMS) is seeking
input from model users (including, but not limited to, EPA
Regional Offices, State and local air quality agencies, and State
and regional/local transportation and planning agencies) as to
when and under what conditions use of MOBILE5b should be
required.

Changes Made in MOBILE5b

     The changes that have been made in this interim update to
the model fall into two broad categories:  Programming the
effects of regulations that have been finalized since the release
of MOBILE5a, and improving the modeling of various innovative
inspection and maintenance (I/M) program designs and options. 
Most of the improved I/M program modeling features were made
available previously in MOBILE5a_H; however, that version of the
model was very complicated to use and did not provide the
flexibility that is included in MOBILE5b.  Details of the
specific changes in MOBILE5b follow:

     Final Regulations.  The Final Rule for onboard refueling
vapor recovery (OBVR) systems is reflected in MOBILE5b.  While
OBVR system requirements could be modeled by the user in
MOBILE5a, a number of inputs were required, and no provisions
were made for phase-in of the requirements.  In MOBILE5b, OBVR
system requirements are built-in, do not require additional user
input, and the nine-year phase-in period before all light-duty
gas vehicles and light-duty gas trucks are required to have
onboard VRS is modeled.  The Final Rule for detergent gasoline
additives has also been finalized.  The effects of this rule on
emissions is accounted for by MOBILE5b with no user input
requirements.  Finally, the revised Final Rule for reformulated
gasoline (RFG) is reflected in MOBILE5b.  The impact of the RFG
rule is that, starting in calendar year 2000, a reduction in NOx
emissions of about 6.8% for some vehicles (i.e., gasoline-fueled
vehicles equipped with three-way catalysts), under summer
conditions, is modeled as resulting from the new requirements.

     Inspection and Maintenance (I/M) Program Options.  There are
a number of ways in which MOBILE5b allows modelers to more
readily estimate the impacts of various I/M program options.  As
noted, many of these were available through use of the MOBILE5a_H
version of the program; however, MOBILE5b makes these options
easier to model, with less likelihood of error, and further
expands the flexibility provided to the model user.  Retest-based
hybrid I/M programs, pressure checks, and purge checks can be
easily modeled using MOBILE5b.  Technician training and
certification (TTC) credits are also readily obtained using
MOBILE5b.  The Acceleration Simulation Mode (ASM1 and ASM2) tests
are now options for test type.  Specific test-and-repair program
effectiveness values can now be supplied by the modeler,
replacing the 50% credit reduction that was coded into MOBILE5a. 
The phase-in of benefits for I/M programs during their first
cycle of operation (first year for annual programs, first two
years for biennial programs) is correctly modeled in MOBILE5b,
where MOBILE5a and 5a_H did not provide correct credits under
those circumstances.  The credit files for estimating the
benefits of all types of I/M programs have been streamlined and
modified ("smart" credits) in such a way as to minimize the
complexity of the input files and thus leading to reduction in
the number of errors that are likely.

     Miscellaneous Changes.  Two other changes have been included
in MOBILE5b that will benefit many model users.  One of these is
the reactivation of idle emission factor calculations.  This is
based on the "MOBILE5 Information Sheet #2" algorithm, in which
exhaust emission factors at an average speed of 2.5 mph (the
minimum speed for which emission factors can be calculated) are
converted to grams/hour and used as estimated idle emissions. 
Post-processing will no longer be required to develop these
values, as setting the correct flag to request idle emissions
will produce those numbers as part of the program output.  The
other is an expansion in the range of calendar years (CY) for
which emission factors can be estimated, from CY 2020 in MOBILE5a
to CY 2050 in MOBILE5b.  This is particularly useful in examining
the full benefits of regulations being implemented in the 1990s
and beyond, as the previous limit of CY 2020 was not sufficient
to model complete fleet turnover for regulations taking effect in
1995 and later model years.

     What is Not Included in MOBILE5b.  In my November 10, 1994
memo "Planned Release of MOBILE5b," I noted several developments
that had taken place that affected in-use emission estimates.  Of
these, most were based on implementation of new regulations
(OBVR, RFG, and detergent gasoline, as described above), and are
included in MOBILE5b as it is being released now.  One change
that was discussed in the November 10, 1994 memo is not included
in MOBILE5b, however.  This is the inclusion of driving patterns
(e.g., high speeds, steep accelerations and decelerations) that
are not part of the Federal Test Procedure (FTP).  These non-FTP
("off-cycle") driving patterns and the emissions increases
associated with such driving behaviors are a subject of
continuing work.  As requested by the Modeling Work Group (part
of the Mobile Source Technical Advisory Subcommittee of the Clean
Air Act Advisory Committee, established under provisions of the
Federal Advisory Committee Act), we have decided not to include
our first approximation of these effects in MOBILE5b.  They will
be included in MOBILE6, which will undergo a more thorough
peer/outside review process than has been characteristic of
earlier versions of the model.  The Modeling Work Group
recognized the need to supply States and other model users with
an interim model update containing the features detailed above,
and so agreed that OMS could and should release MOBILE5b without
the "non-FTP effects," which will likely be the subject of much
comment and revision before their inclusion in a later model
update (MOBILE6).

Why is MOBILE5b Being Released Now?

     In light of the recent passage of the National Highway Bill,
and the need for many States to recalculate their 15% VOC
Reduction Requirements, OMS believes that this interim update to
the MOBILE model will be useful for many parties in their
modeling efforts.  OMS would like to stress that the use of
MOBILE5b is optional -- it is not required.  It is being provided
as a tool that may be useful in specific situations.  OMS
recommends that MOBILE5b be used by those areas required to
recalculate their 15% VOC reductions.  Additional guidance on
modeling the 15% VOC Reduction Requirement for 1999 is being
released under separate cover.  Based on the changes included in
MOBILE5b, as described above, the following guidance is offered
as to who should use MOBILE5b now:

     The use of MOBILE5b is not approved for use in any State
Implementation Plan (SIP) submissions (other than the 15%
recalculations as discussed above), nor for any transportation
conformity findings, during the 60 day comment period described
in the following section..  OMS wants to understand the
implications of approving such uses of MOBILE5b, and wants to be
able to provide comprehensive guidance on issues that arise with
respect to consistency between SIPs prepared using one version of
the model and conformity determinations made using another
version, among other issues, before approving more widespread use
of MOBILE5b.  This is discussed below.

Seeking Comment on When and Whether Use of MOBILE5b Should be
Required and/or Allowed

     The release of a new version of the MOBILE model unavoidably
raises a number of issues and questions.  OMS has determined, for
the reasons outlined above, that release of this interim update
to the MOBILE model is warranted by the need on the part of a
number of States to recalculate their 15% VOC reduction plans and
the complexity of modeling a number of I/M programs (e.g., those
that have not yet completed a full testing cycle before the
emission factor evaluation date, those with various types of
hybrid programs) using the earlier MOBILE5a and 5a_H versions of
the model.  However, as discussed above, OMS also wants to
understand the implications of approving the use of MOBILE5b for
SIP submittals and conformity findings, and to provide
comprehensive guidance on issues that arise with respect to
consistency between SIPs prepared using one version of the model
and conformity determinations made using another version, among
other issues, before approving more widespread use of MOBILE5b.

     Thus, we are seeking comment for the next sixty (60) days
from all affected parties (EPA Regional Offices, State and
local/regional air quality and transportation planning agencies,
Department of Transportation) on how additional guidance on the
need for and use of this version of the model should be handled.

     For example, we would appreciate input from affected parties
on how the release of MOBILE5b will impact the State
Implementation Plan (SIP) process, what complications may arise
from use of this version of the model, and how the release of
this version of the model will impact conformity evaluations.  We
want this release to be a beneficial tool for those who need it,
or who choose to use it for other reasons, while not
unnecessarily disrupting other ongoing related work.  We want
users of the model to inform us of when and under what conditions
it will be advantageous for them to use MOBILE5b, and when and
under what conditions it will not be advantageous.

     Reaction and comment from model users over the next 60 days
will assist OMS in developing and providing additional guidance
on the use of MOBILE5b between now and the scheduled release of a
completely new version of the model, MOBILE6, in the summer of
1998.  Please provide any comments addressing the issues raised
in this section of this memo to our office.  Comments may be
submitted by mail, phone, or E-mail.

Attachments

cc:  R. Schoeneberg, FHwA (w/atts)
     J. Shrouds, FHwA (w/atts)
     T. Helms, OAQPS (w/atts)
     D. Mobley, OAQPS (w/atts)
     L. Audette, OMS/RSPD
     L. Cook, OMS/RSPD
     G. MacGregor, OMS/RSPD