U.S. Environmental Protection Agency

November 15, 1999

Document Released for Stakeholder Review and Comment

Determination of Methane Offsets as a Function of Mileage
for Light-Duty Cars and Trucks

Report Number EPA420-P-99-029

The Office of Mobile Sources, Assessment and Modeling Division
announces the release of "Determination of Methane Offsets as a
Function of Mileage for Light-Duty Cars and Trucks" for
stakeholder review and comment.  This document EPA420-P-99-029
(also known as M6.EXH.006) is available at the MOBILE6 section of
the OMS Web Site (http://www.epa.gov/oms/m6.htm)

Previous reports document how the MOBILE6 emission factor model
will allocate vehicle exhaust emissions between engine start
(start emissions) and travel (running emissions). This split
allows the separate characterization of start and running
emissions for correction factors such as fuel effects and ambient
temperature. It also enables a more precise weighting of these
two aspects of exhaust emissions for particular situations such
as morning commute, parking lot and freeway driving.

Because methane does not contribute significantly to ozone
formation, MOBILE attempts to separate it from total hydrocarbons
in estimating vehicle emissions. This document describes
methodologies for calculating this methane "offset" based on the
separation of start and running emissions proposed for MOBILE6.
The procedure follows the scheme of modeling the in-use
deterioration of emissions as a function of accumulated mileage.
For model year 1981-1993 light-duty cars and trucks, stratified
into carbureted versus fuel-injected, the method parallels that
used for total hydrocarbons, carbon monoxide and oxides of
nitrogen. This analysis utilizes actual methane data from
emissions tests conducted on vehicles from those model years.

For pre-1981 model years, data of the type used in the newer
vehicles is not available. Therefore, procedures are described
for estimating methane as a function of mileage using existing
data in combination with methods that are applied in MOBILE
version 5.

Comments on this report and its proposed use in MOBILE6 should be
sent to the attention of Phil Enns. Comments may be submitted
electronically to mobile@epa.gov, by fax to (734)214-4939, or by
mail to MOBILE6 Review Comments, US EPA Assessment and Modeling
Division, 2000 Traverwood Drive, 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 November 15,1999.
Comments will be accepted for sixty (60) days. 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 follow-up document.

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