United States       Air and Radiation      EPA420-F-98-040
           Environmental Protection              September 1998
x>EPA      MOBILES Information
           Sheet #7

           MOx Benefits of Reformulated
           Gasoline Using MOBILESa


                \               Information Sheet #7
               /        NOx Benefits  of Reformulated
                             Gasoline  Using MOBILESa

                              September 30, 1998
   US EPA, NVFEL, AMD, 2000 Traverwood Drive, Ann Arbor, Michigan 48105 (734) 214-4902
I.     Introduction

      MOBILES Information sheets are a series of documents intended to give users detailed
information about techniques that can be used to more accurately model highway mobile source
emissions and avoid potential  errors or resolve limitations to modeling capabilities. When
referring to MOBILES in this and other Information Sheets, we will mean the latest officially
released version of the MOBILE model, unless otherwise specifically noted. At the time of this
Information Sheet, two versions of the model were actively being used. The most current version
of the model, MOBILESb, was released in October of 1996 while the other version, MOBILESa,
was released in March of 1993.

      All of the MOBILES Information Sheets will be made available on the EPA Office of
Mobile Sources site on the World Wide Web (WWW). The WWW site is available to users 24
hours a day, 7 days a week, and may be reached at "http://www.epa.gov/OMSWWW/".

      If you would like to be added to our mailing list, please follow the instructions included
in the last section of the Information Sheet.  This only need be done once. You do not need to
send in this information if you are already on our mailing list.

II.    Problem Description

      When the original MOBILES was released in December 1992 and updated in March 1993
(MOBILESa), the final rules for federal Reformulated Gasoline (RFG) had not yet been finalized.
As a result, the original MOBILES (and MOBILESa) RFG benefits were based on the proposed
rule, which did not include NOx benefits. When MOBILESb was released in October 1996, it
included updated estimates of RFG benefits, including the effect of RFG on NOx emissions.
Most States, however, had already completed most of their emission estimates using MOBILESa.
Any area can properly determine the NOx benefits of RFG as described in the final rule by using

MOBILES Information Sheet #7                                         September 30,1998

MOBILESb. This is the most accurate method available and is the recommended approach for
calculating the benefits of RFG. However, some areas requested a method to calculate the
appropriate reductions in NOx emissions without repeating their inventory analysis. Although it
is not the preferred approach, a solution is offered below for those areas who are unable to repeat
their inventory analysis using the MOBILESb model.

III.    Remedy

       MOBILESb includes the effect of the final RFG rules on NOx emissions.  When possible,
users should make all highway vehicle estimates using the latest version of the MOBILE model.
Users must never directly compare vehicle emission estimates based on MOBILESa results with
other estimates based on MOBILESb. This policy is discussed in the memorandum to the EPA
Regional Office Air Directors dated August 11, 1997, "Summary of Comments on and Guidance
for Use of MOBILESb" from Philip A. Lorang, the Director of the Assessment and Modeling
Division of the Office of Mobile Sources available on our web site at:


       However, users calculating NOx benefits from federal Reformulated Gasoline (RFG) and
who are unable to repeat their inventory analysis using the MOBILESb model  could use
MOBILESb results to adjust inventories based on MOBILESa results if the input parameters of
the MOBILESa and MOBILEb runs are carefully matched. The estimate can be made following
three steps:

       Run MOBILESb with and without RFG by using the RFGFLAG (Section in the
       updated September 1996 MOBILES User Guide).

      Calculate the difference in NOx emissions ("without RFG" minus "with RFG" results).
       Divide this difference by the MOBILESb results without RFG.  This is the fractional NOx
       RFG benefit.

      Multiply the NOx emissions, based on MOBILESa, by the fractional NOx RFG benefit.
       This will be the estimate of NOx emission benefit from RFG.

        The MOBILESb runs must use the same local parameters as were used in the original
MOBILESa runs. When preparing submissions for EPA, users must always consult with the
appropriate EPA Regional office to determine the amount of aggregation of scenarios for the
MOBILESb runs that will be allowed. Depending on the variety of conditions modeled in the
inventory, more than one fractional NOx RFG benefit based on MOBILESb may be needed.  For
example, areas which calculate inventories separately by county would likely need to calculate at
least a separate fractional NOx benefit determined for each county.  Alternatively, inventories
that are aggregates of MOBILESa runs that used the same fleet characteristics (i.e., registration

MOBILES Information Sheet #7                                            September 30,1998

distributions) would be good candidates for adjustment using a single, MOBILESb based,
fractional NOx RFG benefit. The fractional NOx benefit should be relatively insensitive to
differences in speed, temperature and operating mode.

IV.    Additional Information/ Listserver Subscription

The Emission Inventory Group, Assessment and Modeling Division, EPA Office of Mobile
Sources sponsors a listserver named EPA-MOBILENEWS. A listserver is an electronic mailing
list service that will send messages to the all of the persons with e-mail addresses who have
"subscribed" to that particular listserver.  This listserver has largely replaced physical mailings of
information to parties interested in the MOBILE model, as well as others interested in general
mobile source emission factor and inventory information (including particulate emissions and the
PART emission factor model, and nonroad mobile sources).

To subscribe to the EPA-MOBILENEWS listserver send the following message:


where FIRSTNAME and LASTNAME is your name (for example: Fred Flintstone) to the EPA
List server address. The list server e-mail address is:

             li stserver@unixmail. rtpnc. epa.gov

Your  e-mail address will then be added to the list of subscribers and a confirmation message will
be sent to your e-mail address.  Whenever a message is posted to the EPA-MOBILENEWS
listserver by the listserver owner (The Assessment and Modeling Division of the EPA Office of
Mobile Sources), a copy of that message will be sent to every person who has subscribed.

You can remove yourself from the list by sending another message to the listserver address.  This
message must be sent from the same e-mail address that you used to subscribe, and must contain
the message:

             unsubscribe EPA-MOBILENEWS

If you are interested in highway vehicle emission factors, including particulate matter, the
MOBILE and PART emission factor models, and non-road vehicle emission factors and
modeling,  as well as guidance issued from OMS on the use of these models and development of
emission inventories for all mobile sources, the Emission Inventory Group encourages you to
take advantage of the EPA-MOBILENEWS listserver as the best and most efficient way to get
up-to-date information about these issues.
MOBILES Information Sheet #7                                            September 30,1998

We recognize that some parties interested in these topics may not be able to subscribe to the
listserver, and will continue to mail paper copies of information to those parties on request. If
you cannot subscribe to the EPA-MOBILENEWS listserver at this time and wish to continue to
receive paper copies of information by regular mail, please mail or fax your current information
(name, company name, mailing address, phone and fax numbers) to us at the address below and
clearly indicate that you wish to continue receiving paper copies of all information distributed
over the listserver. Send this request to:

       MOBILE/PART Mailing Lists
       Assessment and Modeling Division
       2000 Traverwood
       Ann Arbor, MI 48105

or fax the information to:     734/214-4939

and note "Attention:  MOBILE/PART Mailing Lists" on the fax.
MOBILES Information Sheet #7                                             September 30,1998