February 26, 1997


Dear Addressee:

     This letter is to inform you of a public workshop to be held
March 19 and 20, 1997, for the purpose of presenting and
discussing planned revisions to MOBILE5a, the highway vehicle
emission factor model.  The workshop will be held from 8:30 am to
5:00 pm March 19, and 8:30 am to 3:00 pm March 20, at Powsley
Auditorium in the Morris Lawrence Building of Washtenaw Community
College in Ann Arbor, MI.  The workshop site is about 5 miles
from the National Vehicle and Fuel Emission Laboratory, and about
2 miles north of I-94 (a map is enclosed for your use).

     MOBILE5a, the current release of highway vehicle emission
factor model, was issued in March 1993.  It was used by States
and other parties to develop emission inventories and emission
reduction plans required as part of the State Implementation Plan
(SIP) process for nonattainment areas.  The Office of Mobile
Sources (OMS) would like to have the next version of the model,
MOBILE6, released for use in the summer of 1998.

     While many of the planned revisions to the model are simply
updates to various estimates, refined on the basis of more and
more recent test data, there are a number of other, more
significant changes also planned.  We would like to encourage you
and members of your staffs and constituent organizations to plan
on attending the workshop in order for us to obtain the broadest
feedback from stakeholders and users of the model.

     This workshop is intended to provide an opportunity for
interested parties to hear more about OMS's plans with respect to
updating and revising the model, and to solicit input and
reaction to those plans.  Another MOBILE6 workshop will be held
late in 1997, and will be focused more on the analyses that were
performed, their results, and the implications of those results
for emission factor estimates.

     As described in the Federal Register notice of this
workshop, which can be viewed at the EPA Web site (see
http://www.epa.gov/OMSWWW/models.htm), there will also be one
presentation on the second day (March 20) relating to EPA's plans
for development of a nonroad mobile source emission inventory
model.  Development of that model is still at a very early stage,
and input from stakeholders and potential users of that model
will have a direct impact of how EPA proceeds with further
development.

     This workshop is taking place at an early stage in MOBILE6's
development, thus we do not expect to present significant results
(data and analyses).  Rather, the focus will be on planned data
collection and analysis activities, the likely implications for
modelers of the changes that are planned, and presentation of
proposals for changes in the input data structure and
requirements and the output produced by the model.

     For more information, please see
http://www.epa.gov/OMSWWW/models.htm.  We will be adding
information to this site before the workshop, and encourage you
to make use of the information that will be presented there both
before and after the workshop.  Thank you for your continuing
interest in this program, and we hope to see you at the workshop.


Sincerely,

Philip A. Lorang, Director
Assessment and Modeling Division
Office of Mobile Sources