February 18, 1998


Document Released for Stakeholder Review and Comment

Soak Length Activity Factors for Start Emissions

Report Number M6.FLT.003

     The Office of Mobile Sources, Assessment and Modeling Division,
announces the release of "Soak Length Activity Factors for Start
Emissions" for stakeholder review and comment. This document
M6FLT003.PDF/M6FLT003.WPD is available at the MOBILE6 section of the
OMS Web Site (http://www.epa.gov/omswww/models.htm).

     In addition to the standard daily emission estimates which are
calculated by MOBILE5, MOBILE6 will compute and report emission
factors for start, running, and evaporative emissions by hour.  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.  To compute hourly emission factors, the model
requires estimates of hourly vehicle activity.  In particular,
M6FLT003 describes the methodology used to calculate soak length
activity factors for start emissions and lists the activity factors
that MOBILE6 will use.

     The soak length activity factors for start emissions were
developed based on data from two instrumented vehicle studies that
collected considerable data on vehicle usage.  Based on these data,
three distributions were developed.  These are: (1) estimates for the
number of starts per day, (2) a distribution of starts by hour, and
(3) the 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 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