U.S. Environmental Protection Agency
^^ t PAA                                     Office of Transportation and Air Quality
                                                                     EPA420-S-03-005
                                                                       March 19, 2003
                                 MOBILE6.2 Policy Update

       On November 12, 2002, the U.S. Environmental Protection Agency (EPA) posted an
updated version of the MOBILE model, dubbed MOBILE6.2, on this Web site. EPA would like
to clarify the relationship between MOBILE6 and MOBILE6.2, and how either version of the
model can be used for official State Implementation Plan (SIP) and transportation conformity
purposes at this time.

       As background, EPA announced the release of MOBILE6 for official SIP and
transportation conformity purposes on January 29, 2002 (67 FR 4254). MOBILE6 estimates
motor vehicle emission rates for hydrocarbons (HC), oxides of nitrogen (NOx) and carbon
monoxide (CO). MOBILE6.2 is  an updated version of MOBILE6 that adds the capability of
estimating particulate matter (PM), carbon dioxide (CO2), and air toxics emission rates to
MOBILE6. Because it is an updated version of MOBILE6, MOBILE6.2 also estimates emission
rates for HC, NOx, and CO. MOBILE6.2 includes one new feature (i.e., the ability to use hourly
humidity input), and some updates and corrections to methods used to estimate HC, NOx, and
CO emissions. These changes make MOBILE6.2 more reliable and easier to use than MOBILE6,
but they do not significantly change the HC, NOx, or CO emission rates estimated by the
MOBILE model.

       Given that the HC, NOx, and CO emission rates are not significantly different between
MOBILE6 and MOBILE6.2, EPA will accept the  use of either version of the model in respect to
ozone and CO for SIPs and conformity determinations. The release of MOBILE6.2 does not start
a new conformity grace period for ozone or CO conformity determinations, and it does not
trigger  the need for any new ozone or CO SIPs. Guidance given in the January 18, 2002, EPA
memo,  "Policy Guidance on the  Use  of MOBILE6 for Ozone and CO SIP Development and
Transportation Conformity," continues to apply in respect to ozone and CO SIPs  and conformity
determinations, regardless of whether MOBILE6 or MOBILE6.2 is used.

       States may choose to continue using MOBILE6 in ozone or CO SIPs or conformity
determinations that have already begun. However, given the improvements made in MOBILE6.2
and new improvements made to  the model on an ongoing basis, we strongly encourage states to
use the latest version of the MOBILE6 model posted on the Web site whenever any new ozone or
CO SIP or conformity analysis is initiated.

       MOBILE6.2 is not yet approved with respect to PM SIP development or conformity
determinations. We are developing guidance on the use of MOBILE6.2 for PM. This guidance
will be  released when MOBILE6.2 is approved with respect to PM for SIPs and conformity.

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