united states                             Office of Transportation and Air Quality
            Environmental Protection                                        EPA420-F-03-043
            Agency                                                     ,T     ,   ~,0
                                                                      November 2003
                            Questions and Answers
What is EPA releasing?

      EPA is releasing a final version of MOBILE6.2, a computer model for estimating
      emissions from highway vehicles, including cars, trucks and motorcycles. This model
      replaces a draft version that was made available in October 2002. MOBILE6.2 is now our
      official model for estimating emissions of particulate matter (PM) and air toxics. It may
      also be used to estimate carbon monoxide (CO), hydrocarbons (HC) and oxides of
      nitrogen (NOx) for official EPA purposes.

How can I access the final version of MOBILE6.2?

      EPA has posted the final version of the materials needed for running MOBILE6.2 on the
      EPAis MOBILE6 Web site at: www.epa.gov/otaq/m6.htm.
How does the final version of MOBILE6.2 differ from the draft version?

       The final version of MOBILE6.2 includes a further reduction in future CO emissions to
       better account for the effect of new emission standards on car and light truck emissions.
       This is explained in more detail below.

       The model also includes some minor technical changes to make it easier to run the model
       on different computers. Finally, the model includes a change to the carbon dioxide (CO2)
       emissions methodology to better estimate atmospheric CO2. As explained below, the CO2
       portion of the MOBILE6.2 model will continue to be considered i draftT . EPA is not
       approving this part of the model in the final version of MOBILE6.2.

Why were new CO rates developed?

       In the 2002 versions of MOBILE6 and MOBILE6.2, we assumed that upcoming
       improvements in the HC and NOx emission standard for light duty vehicles would have
       no effect on the vehicles! basic CO emission rates. Now that vehicles certified to the
       National Low Emitting Vehicle (NLEV) and Tier2 standards are available, new data
       suggest that the changes in the HC and NOx emission standards for NLEV and Tier 2
       vehicles will affect CO emission rates. This difference is important enough that EPA has
       decided to change the basic emission and deterioration rates for CO emissions in
       MOBILE6 for vehicles certified to the NLEV and Tier 2 emission standards.

How does the new model compare to previous models for estimating CO emissions?

       The new model shows reduced CO emissions for vehicles certified to the NLEV and Tier
       2 standards. In most states, NLEV vehicles begin to enter the fleet in 2001. There is no
       change in CO emissions for years prior to the phase-in of these vehicles. The effect of this
       change on fleet-wide CO emissions is fairly small.

How does the new model compare to previous models for estimating PM and air toxics?

       EPAis PARTS model was created in  1995 to estimate PM emissions from highway
       vehicles. EPAis MOBTOX model was created in 1993 and updated in 1999 to estimate
       highway vehicle air toxics emissions. The final version of MOBILE6.2 officially replaces
       these models.

       The final MOBILE6.2 algorithm is not a major update to the PARTS and MOBTOX
       emission factors. However, MOBILE6.2 is easier to use than the previous models, and
       like the draft version, the finalMOBILE6.2 reflects new particulate regulations that came
       into effect since PARTS was released. Also, the vehicle fleet and activity data have been

Why were new particulate and air toxics models developed?

       Combining the particulate matter and air toxics models with MOBILE6.0 gives users a
       single, consistent way to model all the major highway vehicle pollutants. It eliminates
       significant duplication of technical material between the models, which reduces errors
       and speeds model updates.

       The new tools also address major problems with the previous models, particularly by
       updating the particulate emission factors to account for new regulations and significantly
       reducing the number of steps required to model air toxics.

       Combining updated PARTS and MOBTOX models with MOBILE was a prominent
       recommendation of the National Academy of Science Research Councilis review of
What kinds of particulate emissions can be calculated with the new model?

       MOBILE6.2 estimates highway vehicle emissions of direct particulate matter from
       vehicle exhaust, brake and tire wear, and particulate precursor emissions of gaseous
       sulfur dioxide (SO2) and ammonia (NH3). For diesel fueled vehicles, exhaust particulate
       matter is broken down into four constituents: lead particulate, organic carbon particulate,
       elemental carbon particulate, and sulfate particulate. For gasoline fueled vehicles the two
       carbon particulate components are combined.

       MOBILE6.2 does not include estimates of fugitive road dust emissions. These can be
       estimated using emission factors developed separately by EPAis Office of Air Quality
       Planning and Standards (OAQPS). Note, these fugitive dust factors have also been
       updated and the new factors are being approved for SIP and conformity use. More
       information is available on OAQPS's Web site at: www.epa.gov/ttn/chief/ap42/chl3/.

What kinds of air toxics emissions can be calculated with the new updated model?

       MOBILE6.2 estimates highway vehicle emission factors in milligrams per mile for
       mobile source air toxics. MOBILE6.2 estimates emission factors for benzene, 1,3-
       butadiene, formaldehyde, acetaldehyde, acrolein, andMTBE. All of these compounds
       except MTBE were identified as risk drivers in the recent National-Scale Air Toxics
       Assessment. The model also can estimate emissions of other toxics based on user
       provided information. Emission factors are reported according to whether they are
       exhaust, diurnal, hot soak, running loss, resting loss or refueling loss emissions.

How does the release of a final MOBILE6.2 affect EPAis policies on the use of MOBILE6
in SIPs and conformity determinations?

       The effect of MOBILE6.2 on SIP and conformity policy varies depending on what
       pollutant is being analyzed.  Please see the separate  SIP and conformity policy memo for
Does the final release of MOBILE6.2 mean that SIPs and conformity determinations will
be required for air toxics?

       No, SIPs and air quality designations are only required for criteria air pollutants. The
       criteria pollutants are ozone, carbon monoxide, particulate matter, lead, sulfur dioxide
       and nitrogen dioxide. Transportation conformity determinations are required only in
       nonattainment and maintenance areas for ozone, carbon monoxide, particulate matter and
       nitrogen dioxide.

What does the update of the air toxics model mean for analyses under the National
Environmental Policy Act?

       While MOBILE6.2 is EPA's best available tool for quantifying toxics emissions from
       highway vehicles, its availability has no direct bearing on the administration of the
       National Environmental Policy Act (NEPA). The Department of Transportation has
       responsibility for implementing NEPA for its projects, and it has not yet developed a
       policy on how mobile source air toxics should be addressed in NEPA analyses.

How does MOBILE6.2 estimate particulate emissions?

       MOBILE6.2 estimates emission factors using vehicle fleet and activity information from
       MOBILE6 and emission rate information from PARTS, supplemented with information
       about future vehicle emission standards. MOBILE6.2 calculates SO2 and sulfate
       particulate emissions via the method used in PARTS, but instead of using hard-coded fuel
       sulfur values, MOBILE6.2 bases the calculation on user-supplied fuel sulfur levels of
       gasoline and diesel fuel.

       It is important to realize that the PM emission rate estimates of MOBILE6.2 are not as
       sophisticated as those for HC, CO, and NOx. For example vehicle speed has little effect
       on the gram per mile estimates of PM emissions, and there are no effects of temperature
       or I/M programs.

       Additional detail regarding the particulate emission rate calculations in MOBILE6.2 is
       provided in the technical paper i MOBILE6.1 Particulate Emission Factor Model -
       Technical Description! (EPA420-R-03-001, January 2003). (Note, we have changed our
       model numbering since this paper was written. Despite the title, the report correctly
       describes the particulate matter data and algorithms in the final version of MOBILE6.2.)

How does MOBILE6.2 estimate toxics emissions?

       For the air toxics directly estimated in the model (benzene, 1,3-butadiene, formaldehyde,
       acetaldehyde, acrolein, and MTBE), MOBILE6.2 applies toxic fractions to the total
       organic gas (TOG) gram per mile emission factors to calculate air toxic emission factors.
       The model adjusts the fractions to account for the difference between laboratory data and
       in-use driving. The model also accounts for differences in exhaust toxic fractions between
       different vehicle technologies and normal and high emitting vehicles in calculating
       emission rates. Moreover, the model accounts for the impacts of specific fuel parameters
       as in EPAis Complex Model for Reformulated Gasoline.

       For other toxics, the user can enter emission factors or air toxic ratios as fractions of
       VOC, fractions of TOG, or fractions of PM, and the model will calculate average vehicle
       class and fleet in-use emission factors.

       MOBILE6.2  uses the same algorithms as MOBTOXSb to estimate toxic to TOG ratios
       for benzene,  1,3-butadiene, formaldehyde, acetaldehyde, and MTBE. However, while the
       base TOG emission rates  in MOBTOXSb incorporated elements of MOBILE6,
       significant revisions to emission rates were made subsequent to the development of
       MOBTOXSb and prior to release of MOBILE6. As a result, MOBILE6.2 estimates
       somewhat higher emission factors in base years, with a convergence in emission factors
       by 2020. These differences are discussed in more detail in the report, i Technical
       Description of the Toxics Module for MOBILE6.2 and Guidance on the Its Use for
       Emission Inventory Preparation! (EPA420-R-02-029, November 2002).

How does the new model compare to previous models for estimating fuel  economy and CO2

       While the new model is a final model for HC, CO, NOx, PM and air toxics, the fuel
       economy and CO2 estimates produced by the model are still considered draft. The fuel
       economy estimates in todayis version of the model are the same as those in the October
       2002 draft. However, the  CO2 estimates are now calculated with a different algorithm that
       assumes all carbon in the fuel is eventually converted to atmospheric CO2. The details of
       this algorithm are explained in the EPAis fall 2003 draft technical report i Updating Fuel
       Economy Estimates in MOBILE 6.2.1

Where can I get  more information on MOBILE6.2?

       Technical reports describing the details of the MOBILE6.2 calculations are posted on the
       MOBILE6 Web site at: www.epa.gov/otaq/m6.htm. A user guide is also posted on this
       site. If, after reviewing these materials, you still have questions about MOBILE6.2, please
       contact us by email at mobile@epa.gov.