February 18, 1998


Document Released for Stakeholder Review and Comment

Soak Length Activity Factors for Hot Soak Emissions

Report Number M6.FLT.004

     The Office of Mobile Sources, Assessment and Modeling Division,
announces the release of "Soak Length Activity Factors for Hot Soak
Emissions" for stakeholder review and comment. This document
M6FLT004.PDF/M6FLT004.WPD is available at the MOBILE6 section of the
OMS Web Site (http://www.epa.gov/omswww/models.htm).

     MOBILE6 will compute and report hourly emission factors for
start, running, and evaporative emissions.  These outputs will be in
addition to the standard daily emission estimates which are currently
calculated by MOBILE5.  The hourly emission factors will allow the
MOBILE6 model to provide more precise output that accounts for the
time of day that vehicle emissions occur.

     The hot soak emissions are a function of the soak length
following the vehicle key off'. A soak can range in length from the
instantaneous time duration following the vehicle key off' to a
duration of 1 hour. A hot soak also requires at least 4 minutes of
prior vehicle operation to sufficiently warm the engine.

     This document describes the data and methodology used to
calculate soak length activity factors for hot soak emissions.  The
data used in the analysis were collected from two instrumented vehicle
studies.  The results of the analysis will be built into the MOBILE6
model as data parameters.  These results are shown in tabular form in
the document.  The results include three distributions.  These are:
(1) estimates for the number of hot soaks per day, (2) a distribution
of hot soaks by hour, and (3) the hot soak length distribution within
each hourly group.  In addition, the three distributions were
stratified by weekday and weekend.  These distributions will be used
with the corresponding vehicle hot soak emissions data in MOBILE6 to
estimate hourly emission rates.

     Comments on this report and its proposed use in MOBILE6 should be
sent to the attention of Ed Glover.  Comments may be submitted
electronically to mobile@epamail.epa.gov, or by fax to (313)741-7939,
or by mail to "MOBILE6 Review Comments", US EPA Assessment and
Modeling Division, 2565 Plymouth Road, Ann Arbor MI 48105.  Electronic
submission of comments is preferred.  In your comments, please note
clearly the document that you are commenting on including the report
title and the code number listed. Please be sure to include your name,
address, affiliation, and any other pertinent information.

This document is being released and posted on February 18, 1998.
Comments will be accepted for sixty (60) days, ending April 19, 1998.
EPA will then review and consider all comments received, and will
provide a summary of those comments and how we are responding to them
in the form of a followup document within 30 days after the close of
the comment period (by May 18, 1998).


Thank you for your continuing interest in the development of MOBILE6.

Sincerely,

Emission Inventory Group, Assessment and Modeling Division
US EPA Office of Mobile Sources