Office of Transportation EPA420-R-05-003 

and Air Quality March 2005 

EPA’s National Mobile Inventory 

Model (NMIM), A Consolidated 

Emissions Modeling System for 

MOBILE6 and NONROAD 
EPA420-R-05-003 

March 2005 

EPA’s National Mobile Inventory Model (NMIM), A 

Consolidated Emissions Modeling System for 

MOBILE6 and NONROAD 

Harvey Michaels


David Brzezinski


Rich Cook


Craig Harvey


Assessment and Standards Division


Office of Transportation and Air Quality


U.S. Environmental Protection Agency

TABLE OF CONTENTS 

ABSTRACT .................................................................. 3

INTRODUCTION AND OVERVIEW ............................................. 3

HOW NMIM WORKS ......................................................... 4

Temporal and Spatial Approximations for Efficient Running of National Inventories ... 5

NMIM Master and NMIM Worker: Distributed Processing ....................... 5

HOW NMIM RUNS MOBILE6 ................................................. . 6

Particulate Matter (PM) ................................................... 9

Evaluation Year and Month ................................................ 9

HOW NMIM RUNS NONROAD ............................................... . 10

POLLUTANTS FOR WHICH INVENTORIES ARE PRODUCED BY NMIM ............ 11

SOURCE CATEGORIES FOR WHICH INVENTORIES ARE PRODUCED BY NMIM .... 13

THE NMIM COUNTY DATABASE (NCD) ....................................... 16

Temperature and Humidity ............................................... 18

Fuel Properties ......................................................... 19

Vehicle Miles Traveled (VMT) ............................................ 20

Database Improvement .................................................. 20

ESTIMATION OF HAZARDOUS AIR POLLUTANT (HAP) EMISSIONS .............. 21

Gaseous HAPS ......................................................... 21

Polyaromatic Hydrocarbons (PAHs) ........................................ 22

Metals, Dioxins, and Furans .............................................. 22

SUMMARY ................................................................. 23

REFERENCES .............................................................. 24

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ABSTRACT 

This document describes EPA’s National Mobile Inventory Model (NMIM) and explains 

how it works. NMIM is a consolidated emissions modeling system for EPA’s MOBILE6 and 

NONROAD models. It generates county inventories using MOBILE6 and NONROAD at scales 

ranging from individual counties to the nation. Its primary improvements over MOBILE6 and 

NONROAD are 1) the inclusion of all the required county data for the nation in a single 

database, 2) a graphical user interface, 3) “shortcuts” for generating national inventories, 4) tools 

for aggregation and post-processing, 5) estimation of 33 HAPS and 17 dioxin/furan congeners, 

and 6) distributed processing capability to enhance performance. Previously, when national 

inventories have been constructed from MOBILE6 and NONROAD, the necessary input data 

have been widely scattered in disparate formats and have required additional specialized software 

to convert this data into input files for MOBILE6 and NONROAD, to run the models, to 

integrate the results into a final inventory, and to post-process the results into forms suitable for 

the national inventories. NMIM accomplishes all of these tasks in a single package. NMIM’s 

initial version in 2003 was intended to approximate the procedures previously used for 

calculating nationwide emissions for EPA’s National Emissions Inventory. EPA subsequently 

has added the capability of incorporating much more county-specific information into the 

database, including monthly average hourly temperature and humidity, fleet distributions, speed 

distributions, and diesel fractions for MOBILE6 and county-specific allocation and activity files 

for NONROAD. 

INTRODUCTION AND OVERVIEW 

This document describes EPA’s National Mobile Inventory Model (NMIM) and explains 

how it works.1 EPA’s National Mobile Inventory Model (NMIM) is a consolidated emissions 

modeling system for EPA’s MOBILE6 and NONROAD models. It was developed to produce, in 

a consistent and automated way, national, county-level mobile source emissions inventories for 

the National Emissions Inventory (NEI) and for EPA rule making. Previously, when national 

inventories have been constructed from MOBILE6 and NONROAD, the necessary input data 

have been widely scattered in disparate formats and have required additional specialized software 

to convert this data into input files for MOBILE6 and NONROAD, to run the models, to 

integrate the results into a final inventory, and to post-process the results into forms suitable for 

the national inventories. NMIM accomplishes all of these tasks in a single package. 

NMIM comprises a Java framework, graphical and command line user interfaces, the 

MOBILE6 and NONROAD models, a national county database, and postprocessing and 

aggregation capabilities. NMIM’s primary improvements over MOBILE6 and NONROAD are 

1) the inclusion of all the required county data for the nation in a single database, 2) a graphical 

user interface, 3) “shortcuts” for generating national inventories, 4) tools for aggregation and 

post-processing, 5) estimation of 33 hazardous air pollutants (HAPS) and 17 dioxin/furan 

congeners by ratio to various MOBILE6 and NONROAD output parameters, and 6) distributed 

processing capability to enhance performance. NMIM extends MOBILE6’s capabilities by 

3
producing inventories rather than just emissions factors. NMIM provides consistency across 

both models and all pollutants by using a single input database for MOBILE6 and NONROAD 

and for criteria pollutants and HAPS. 

However, NMIM is not a complete GUI front end for MOBILE6 and NONROAD, nor 

can it replace these models for all applications. Because it was designed primarily to generate 

national inventories, NMIM has features intended to speed its operation and minimize its output 

database size that also limit its applicability. For example, it will not produce hourly or bymodel-year output from MOBILE6. In addition NMIM, like NONROAD, does not produce 

emissions inventories for aircraft, locomotives, and commercial marine vessels.2 

This documentation begins with an overall explanation of how NMIM works, followed 

by the details of how it runs MOBILE6 and NONROAD. Next, it discusses the pollutant and 

source category inventories available from running NMIM. Then, it describes the NMIM County 

Database (NCD) and procedures for updating and improving it through the National Emission 

Inventory (NEI) process. Lastly, it explains how NMIM estimates various HAPS that are not 

direct outputs of MOBILE6 and NONROAD. 

HOW NMIM WORKS 

The NMIM user specifies a set of years and months, a geographic region (the whole US, 

any combination of whole states, or any combination of particular counties, including Puerto 

Rico and the Virgin Islands), a set of pollutants, and categories of on-road vehicles and non-road 

equipment. This collection of user requests is called a “run specification” or RunSpec, and can 

be saved in a file for later execution or for text editing. RunSpecs can be produced by the NMIM 

GUI or by using a text editor. NMIM RunSpecs can be executed from the GUI or from the 

command line. 

Based on the RunSpec and information in the NMIM county database (NCD), NMIM 

writes input files for the MOBILE6 and NONROAD models. NMIM then runs these models, 

reads their output files, performs additional processing if necessary, and puts the inventories into 

an output database. Additional processing includes multiplying MOBILE6 emission factors by 

vehicle miles traveled (VMT) and estimating emissions of some other pollutants (see below) as 

ratios to pollutant inventories generated by MOBILE6 and NONROAD. 

NMIM has post-processing capability that can be applied after the inventory is generated. 

This includes aggregation over months, roadway types, vehicle types, and equipment types. 

Output can go into a database table or into a tab-delimited text file. The format can be 

normalized (a pollutant on each line) or wide (a pollutant in each column.) NMIM can also 

process the output database into annual, monthly, or ozone season day (OSD) inventories and 

convert them into NEI Input Format Version 3 (NIF3), which is required for NEI submissions. 

The user’s post-processing selections can be made from the GUI and saved as a text “aggregation 

specification” file or AgSpec, which can be reloaded into the GUI, edited from the GUI or in a 

4
text editor, and run from the GUI or from the command line. 

Temporal and Spatial Approximations for Efficient Running of National Inventories 

NMIM employs two main techniques, adopted from previous NEIs, to make the 

production of national inventories tractable. The first is to assume that monthly time resolution 

is adequate for both meteorology and source activity and therefore to do twelve monthly runs 

instead of 365 daily runs. NMIM is designed to do only monthly runs and produces annual 

inventories by adding the twelve monthly inventories. 

The second technique is to group similar counties, allowing NMIM to do a single 

MOBILE6 or NONROAD run for the entire group. The county groups are defined in the 

CountyMap table of the NCD. One of the counties in each group is referred to as the 

representing county, since it represents the others, which are called represented counties. It 

should be noted that county grouping is not mandatory; NMIM allows the user to employ county 

groups or to run all counties individually. If time is available, running individual counties is 

superior, since it uses the most individualized data for each county. In either case, the output 

contains inventories for all the individual counties in the selected geographic region. 

As an example of county grouping, for some national runs that EPA has performed to 

date, counties were placed in the same county group for a particular year if they were in the same 

state and had the same fuel properties and control programs. This grouping was adapted from 

past NEIs. It assumed that if counties are in the same state, their temperatures are similar 

(California had two temperature groupings). The representing county was chosen as the one with 

the highest VMT, so that the temperature and relative humidity data is from a county with a 

significant share of mobile source activity. This scheme resulted in about 100 county groups for 

NONROAD and about 200 for MOBILE6 in 2002, representing the 50 states plus the District of 

Columbia, a total of more than 3000 counties. 

EPA does not recommend or supply any particular county grouping. If they choose to 

employ county groups, users must tailor them to suit the purposes of their inventories. As more 

county-specific data becomes incorporated into the NCD, the problem of choosing county groups 

becomes more difficult. On the other hand, as computing power increases, it becomes easier to 

run every county. 

NMIM Master and NMIM Worker: Distributed Processing 

As a way of further improving performance, NMIM may be run in a distributedprocessing mode, employing multiple computers. NMIM comprises two programs, Master and 

Worker. Master and Worker(s) each have a simple text configuration file which specifies the 

path to a shared folder through which they communicate. In standalone mode, one Master, one 

Worker, and the shared folder are on the same computer. In distributed mode, the Master is on 

one computer, multiple workers are on separate computers, and the shared folder, which they all 

5
have access to, is on yet another computer (although it could be on the same computer as the 

Master or one of the Workers). Therefore, to execute NMIM in distributed mode requires only 

an ordinary local area network (LAN) where all the NMIM computers have access to a single 

shared folder. 

In either mode, to execute a RunSpec requires a Master, one or more Workers, and a 

shared folder. When a RunSpec is executed, the Master 1) reads the RunSpec (which defines the 

inventory in terms of geography, time, pollutants and sources), 2) writes appropriate MOBILE6 

and NONROAD input files, 3) bundles these files together with any external data files needed, 

and 4) posts them as “TODO” files on the shared folder. A TODO file always contains the data 

for a single MOBILE6 or NONROAD run for one county for one month. A Worker, seeing a 

TODO file on the shared folder, copies it to its local work space, executes MOBILE6 or 

NONROAD as appropriate, and writes the results to a "DONE" file in the shared folder. The 

Master, seeing the DONE file, downloads it from the shared folder to its local workspace, 

extracts the MOBILE6 or NONROAD output file, processes it further as necessary, and inserts it 

into the output database specified in the RunSpec. 

HOW NMIM RUNS MOBILE6 

NMIM writes a MOBILE6 input file using data obtained from the NCD and executes 

MOBILE6 once for each month for each county (if the user chose the Geographic Representation 

option “County”) or once for each representing county (if the user chose the Geographic 

Representation option “County Group”). To produce an on-road inventory, NMIM multiplies the 

resulting emission factors by the VMT for each requested county, regardless of which 

Geographic Representation option was chosen. 

The MOBILE6 input files constructed by NMIM are designed to accommodate detailed 

user input and to use a consistent set of commands. In order to use consistent fuels data for all 

pollutants, the AIR TOXICS command is always used, even if no air toxics are requested by the 

user. To enable the AIR TOXICS command, MOBILE6 requires that at least one of the six air 

toxics that it models internally be specified in its input file. To fulfill this requirement, NMIM 

always inserts the command to model acrolein in the MOBILE6 input file, whether the user 

requests it or not. However, if the user does not request acrolein, it will not appear in the output 

table. (Note that acrolein was an arbitrary choice; any of the six air toxics would have served the 

same function.) The ALTITUDE, POLLUTANTS and EVALUATION MONTH commands are 

always explicitly used, rather than depending on MOBILE6 default settings and always appear in 

the MOBILE6 input files written by NMIM. The input files always use the HOURLY 

TEMPERATURES command, rather than MIN/MAX TEMPERATURE command. The hourly 

RELATIVE HUMIDITY command is always used, rather than the ABSOLUTE HUMIDITY 

command. The BAROMETRIC PRES command is always used, since this value interacts with 

the relative humidity values. Average speed distributions are always specified using the SPEED 

VMT command, rather than the AVERAGE SPEED command. 

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Gasoline fuel parameters require the maximum amount of detail to properly model toxic 

emissions. The OXYGENATE command is required when modeling air toxics, and supersedes 

the OXYGENATED FUELS command used to model winter oxygenate programs in MOBILE6. 

The OXYGENATE command requires that oxygen content be expressed as volume percent 

instead of weight percent, as required by the OXYGENATED FUELS command. GAS 

AROMATIC%, GAS OLEFIN% , GAS BENZENE%, E200 and E300 must always be specified. 

RVP OXY WAIVER command is always set to 1 (no waiver), because Reid vapor pressure 

(RVP) values from the fuel surveys are assumed to already account for any RVP effect from 

oxygenated fuels. The FUEL RVP command is always required. The GASOLINE SULFUR 

command and FUEL PROGRAM command Option 4 are always used to explicitly set the sulfur 

content of gasoline. The same gasoline sulfur content is used for both commands and for all 

years in the FUEL PROGRAM command, although it would normally have different sulfur 

values for different years. Since each MOBILE6 run covers only a single month in a particular 

calendar year, NMIM sets all possible sulfur values the same to avoid programming logic to 

determine which of the possible years to change. 

Some counties have local emission control programs. The basic information for these 

programs is stored in the NCD and is used to create the appropriate commands for the input file 

when needed. Inspection and maintenance (I/M) programs for counties are stored in external 

data files and accessed using the I/M DESC FILE command. 

In addition to the basic required information, NMIM can also include county specific data 

that is normally provided to MOBILE6 using external data files. Nearly any of the valid 

MOBILE6 commands can be used, including commands used to model local Low Emission 

Vehicle (LEV) phase-in programs and local natural gas vehicle fractions. Diesel sales fractions 

are stored in an external file and used to create the appropriate input command. 

Not all MOBILE6 commands are used by NMIM. The VMT FRACTIONS and VMT 

BY FACILITY commands are not needed, since these commands are only needed to create 

composite emission rates. NMIM converts all emission rate results from MOBILE6 to tons using 

the county specific VMT for each vehicle class and roadway type. Since all gasolines are 

explicitly defined, both the SEASON command and FUEL PROGRAM command Option 2 are 

never used. Options such as NO CLEAN AIR ACT can be specified as "global" NMIM inputs 

that affect all counties and all months. 

Only weekdays are modeled by NMIM. Commands that apply to weekend variations are 

not used. This simplification makes sense because most weekend differences in MOBILE6 are 

temporal distributions, so MOBILE6's emission factors at the day level are little affected by these 

differences. (Parameters that can differ between weekends and weekdays are hot soak duration 

distribution, start distribution, starts/day, soak distribution, and trip length distribution.) The 

major difference between weekdays and weekends is VMT, which is provided in the NCD by 

month, vehicle type, and roadway type. 

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MOBILE6 has only 4 facility or roadway types: freeways, arterials, ramps, and locals. 

Ramp speed is fixed at 34.6 MPH and local speed at 12.9 MPH. Distributions of average speeds 

are specified separately for freeways and arterials using the SPEED VMT command. If only a 

single MOBILE6 scenario were run, the same distribution of average speeds would be applied to 

all vehicle types. Separate specification of average speed distributions for all combinations of 

the twelve roadway types and all twenty-eight MOBILE6 vehicle types would require running 

168 MOBILE6 scenarios. 

In order to avoid running so many MOBILE6 scenarios, while retaining reasonable 

flexibility, NMIM groups vehicle class-roadway type combinations into 18 groups, shown in 

Table 1. These groups are those that have been used in past NEIs, and provide flexibility in 

assigning average speeds while limiting the number of MOBILE6 runs necessary to generate an 

inventory. Since nine of these combinations use the MOBILE6 freeway facility type and nine use 

the arterial facility type, a total of nine MOBILE6 scenarios are needed to model the eighteen 

vehicle class/roadway type combinations. Average speed distributions for each of these eighteen 

vehicle class/roadway type combinations can be specified for each county. 

Table 1. The 18 vehicle class-roadway type combinations in NMIM. 

M6Vtypes* Road Types M6 Ftype 

LDV Rural Interstate Freeway 

LDT Rural Interstate Freeway 

HDV Rural Interstate Freeway 

LDV Urban Interstate Freeway 

LDT Urban Interstate Freeway 

HDV Urban Interstate Freeway 

LDV Urban Freeways & Expressways Freeway 

LDT Urban Freeways & Expressways Freeway 

HDV Urban Freeways & Expressways Freeway 

LDV,LDT Rural Principal Arterial Arterial 

LDV,LDT Rural Minor Arterial Arterial 

HDV Rural Principal Arterial Arterial 

LDV,LDT Rural Major Collector Arterial 

LDV,LDT Rural Minor Collector, Rural Local Arterial 

HDV Rural Minor Arterial Arterial 

LDV,LDT Urban Principal Arterial, Urban Minor Arterial, Urban Collector Arterial 

HDV Rural Major Collector, Rural Minor Collector, Rural Local Arterial 

HDV Urban Principal Arterial, Urban Minor Arterial, Urban Collector Arterial 

* Reference MOBILE6.2 User Guide, Appendix B

LDV = MOBILE6 Vehicle Types 1 and 16.

LDT = MOBILE6 Vehicle Types 2-5.

HDV = MOBILE6 Vehicle Types 6-15.

8
1

2

3

4

5

6

7

8

9

10

11

12

Particulate Matter (PM) 

Because MOBILE6 can model only one particle size at a time, if both PM10 and PM2.5 

are desired, NMIM runs a tenth MOBILE6 scenario to obtain emission factors for the extra 

particle size. MOBILE6 separates exhaust particulates into SO4, organic carbon (OC), elemental 

carbon (EC), lead, tire wear, and brake wear. Of these, only SO4 depends on speed. If either 

PM10 or PM2.5 is requested, the results are obtained from the standard nine scenarios. If both 

PM10 and PM2.5 are requested, the nine scenarios are run for PM10. Since all SO4 is PM2.5, if 

both PM10 and PM2.5 are requested, the SO4 emission factor for both is taken from the nine 

scenarios that are sensitive to speed, and the tenth scenario is used to obtain the emission factors 

for all the other PM2.5 components. 

Evaluation Year and Month 

MOBILE6 specifies a calendar year and an evaluation month of either January or July. 

These two parameters determine the fleet composition for which emission factors are generated. 

For each month of a given inventory year, NMIM writes the MOBILE6 input file using the 

combination of calendar year and evaluation month shown in Table 2 below. 

Table 2. The MOBILE6 calendar years and evaluation months that are used by NMIM for each 

month of inventory year, Y. 

NMIM Month of MOBILE6 calendar year MOBILE6 evaluation month 

Inventory Year Y 

Y 1 

Y 1 

Y 1 

Y 7 

Y 7 

Y 7 

Y 7 

Y 7 

Y 7 

Y+1 1 

Y+1 1 

Y+1 1 

The reasoning behind this scheme is that the fleet composition in October, November, and 

December of year Y is more like that of January of year Y+1 than it is like July of year Y. This 

scheme does not cause a problem with fuel properties, because NMIM always looks up the fuel 

properties in the NCD for the inventory year and month being modeled. Control programs in 

MOBILE6 are always assumed to begin on January 1, but MOBILE6 assumes that these 

programs have no effect on that day, since the program has had no time to get started. Hence 

9
NMIM is not erroneously introducing next year's control programs by modeling October, 

November, and December as January 1 of the following year. 

HOW NMIM RUNS NONROAD 

NONROAD estimates monthly fuel consumption and emissions of total hydrocarbons 

(THC), carbon monoxide (CO), nitrogen oxides (NOX), sulfur dioxide (SO2), and particulate 

matter (PM). NMIM then processes the monthly results as needed to produce annual and ozone 

season day emissions. Additional pollutants are produced by NMIM as ratios to some of these 

outputs. 

The NONROAD Model reads a set of text instructions, known as an “opt file” (for 

options). NMIM creates this file from data in the NCD. As employed in NMIM, the opt file is 

limited to one state and specifies month and year, fuel properties, temperature, and the counties 

for which to calculate emissions, which may be all or a subset of the counties in the state. 

NONROAD internally produces emissions for the whole state and then allocates the emissions 

for each SCC to the requested counties. Output is produced only for the county or counties 

selected in the NMIM RunSpec. 

The NONROAD Model includes a group of files that specify equipment populations, 

emission factors, deterioration rates, activities, and allocations from the state to the county. 

County-specific allocation, population, seasonality, and activity files can be specified in the 

NCD. These files will override NONROAD's default files. 

The fuel properties required by the NONROAD Model are not the same as those in the 

NCD. The NONROAD Model requires “Oxygen Weight %” in its opt file. The conversion from 

NCD fuel properties to oxygen weight percent is performed by NMIM as follows: 

oxywtpct = 

etohvolume*0.3448*etohmktshare 

+ mtbevolume*0.1786*mtbemktshare 

+ tamevolume*0.1636*tamemktshare 

+ etbevolume*0.1533*etbemktshare 

These conversion factors are detailed under the OXYGENATE command in the MOBILE6 

User's Guide.3 

How NMIM converts from Total Hydrocarbons (THC) to other hydrocarbon (HC) species 

THC is the NONROAD Model's native output. The other HC species that can be 

requested from NMIM are listed and described in Table 3 below. The conversion from THC to 

the other HC species differs between exhaust and evaporative emissions. For NMIM's 

10
emissiontype classifications (exhaust, evaporative, refueling), NONROAD crankcase emissions 

are classified as exhaust. 

NMIM uses factors in the SCC table to convert NONROAD exhaust THC to the other 

hydrocarbon outputs (VOC, NMHC, TOG, and NMOG). For evaporative emissions except for 

FuelType=CNG, no conversion is necessary (i.e., VOC = NMHC = TOG = NMOG = THC). For 

evaporative CNG emissions, TOG = THC, and NMOG = NMHC = VOC = 0. 

POLLUTANTS FOR WHICH INVENTORIES ARE PRODUCED BY NMIM 

Hydrocarbons may be expressed in one of five forms, listed in Table 3 below. The 

conversion factors in the SCC table are those used in the MOBILE6 and NONROAD models and 

depend on fuel and engine type. 

Table 3. Hydrocarbon forms available from NMIM. (MOBILE6 User Guide). 

Hydrocarbon Form Includes Includes Includes Includes 

FID HC Methane Ethane Aldehydes 

Total Hydrocarbons (THC) Yes Yes Yes Partially 

Non-methane Hydrocarbons (NMHC) Yes No Yes Partially 

Volatile Organic Compounds (VOC) Yes No No Yes 

Total Organic Gases (TOG) Yes Yes Yes Yes 

Non-methane Organic Gases (NMOG) Yes No Yes Yes 

Table 4 lists all pollutants for which NMIM produces inventories. The pollutant codes 

are those specified by the NIF3.4 Numeric codes are Chemical Abstracts Service Registry 

Numbers5

 (CASRN) with the hyphens removed. 

In Table 4, a non-blank “Ratio to” column (MB for MOBILE6, NR for NONROAD) 

indicates that the pollutant is calculated by NMIM, after the MOBILE6 or NONROAD model is 

run, by ratio to the pollutant listed in the column. A blank “Ratio to” column indicates that the 

pollutant is calculated inside MOBILE6 or NONROAD. The ratio depends on source type, 

expressed as a source classification code (SCC), and fuel characteristics. The complete list of 

these ratios may be found in the SCC and SCCToxics tables of the NCD. Ratio units are 

g/gallon, g/mile, and g/g of PM or VOC. For on-road vehicles, naphthalene is ratioed to exhaust 

PM and to evaporative VOC. For non-road equipment, it is ratioed to exhaust PM10 only. SOA 

is present to provide input to REMSAD (Regional Modeling System for Aerosols and 

Deposition6

). 

The “Six HAPS” category represents the first HAPS studied for mobile sources. They are 

selected individually in the NMIM RunSpec. The 27 “Add’l. HAPS” (additional HAPS) are 

selected as a group in the NMIM RunSpec. The 17 dioxin/furan congeners are also selected as a 

group. Pollutants in Table 4 without a category listed may be selected individually in the NMIM 

RunSpec. All pollutants are output separately, even if they are selected as a group. 

11 
Table 4. List of pollutants for which inventories are produced by NMIM. (See text.)

Pollutant Ratio to 

Code PollutantName Category MB NR

CO Carbon Monoxide

HC Hydrocarbons (choice of five forms)

NOX Nitrogen Oxides

SO2 Sulfur Dioxide

PM10-PRI Primary PM10 (Filterables and Condensibles)

PM25-PRI Primary PM2.5 (Filterables and Condensibles) PM10*

NH3 Ammonia Gal*

75070 Acetaldehyde Six HAPS VOC

107028 Acrolein Six HAPS VOC

71432 Benzene Six HAPS VOC

106990 1,3-Butadiene Six HAPS VOC

50000 Formaldehyde Six HAPS VOC

1634044 MTBE Six HAPS VOC

100414 Ethyl Benzene Add’l. HAPS VOC VOC

100425 Styrene Add’l. HAPS VOC VOC

108883 Toluene Add’l. HAPS VOC VOC

110543 Hexane Add’l. HAPS VOC VOC

120127 Anthracene Add’l. HAPS PM10 PM10

123386 Propionaldehyde Add’l. HAPS VOC VOC

129000 Pyrene Add’l. HAPS PM10 PM10

1330207 Xylene Add’l. HAPS VOC VOC

16065831 Chromium (Cr3+) Add’l. HAPS Mile Gal

18540299 Chromium (Cr6+) Add’l. HAPS Mile Gal

191242 Benzo(g,h,i)perylene Add’l. HAPS PM10 PM10

193395 Indeno(1,2,3,c,d)pyrene Add’l. HAPS PM10 PM10

205992 Benzo(b)fluoranthene Add’l. HAPS PM10 PM10

206440 Fluoranthene Add’l. HAPS PM10 PM10

207089 Benzo(k)fluoranthene Add’l. HAPS PM10 PM10

208968 Acenaphthylene Add’l. HAPS PM10 PM10

218019 Chrysene Add’l. HAPS PM10 PM10

50328 Benzo(a)pyrene Add’l. HAPS PM10 PM10

53703 Dibenzo(a,h)anthracene Add’l. HAPS PM10 PM10

540841 2,2,4-Trimethylpentane Add’l. HAPS VOC VOC

56553 Benz(a)anthracene Add’l. HAPS PM10 PM10

7439965 Manganese Add’l. HAPS Mile Gal

7440020 Nickel Add’l. HAPS Mile Gal

83329 Acenaphthene Add’l. HAPS PM10 PM10

85018 Phenanthrene Add’l. HAPS PM10 PM10

86737 Fluorene Add’l. HAPS PM10 PM10

12
Pollutant Ratio to 

Code PollutantName Category MB NR 

91203 Naphthalene Add’l. HAPS PMVOC PM10 

1746016 2,3,7,8-Tetrachlorodibenzo-p-Dioxin Dioxin/furan Mile Gal 

19408743 1,2,3,7,8,9-Hexachlorodibenzo-p-Dioxin Dioxin/furan Mile Gal 

3268879 Octachlorodibenzo-p-dioxin Dioxin/furan Mile Gal 

35822469 1,2,3,4,6,7,8-Heptachlorodibenzo-p-Dioxin Dioxin/furan Mile Gal 

39001020 Octachlorodibenzofuran Dioxin/furan Mile Gal 

39227286 1,2,3,4,7,8-Hexachlorodibenzo-p-Dioxin Dioxin/furan Mile Gal 

40321764 1,2,3,7,8-Pentachlorodibenzo-p-Dioxin Dioxin/furan Mile Gal 

51207319 2,3,7,8-Tetrachlorodibenzofuran Dioxin/furan Mile Gal 

55673897 1,2,3,4,7,8,9-Heptachlorodibenzofuran Dioxin/furan Mile Gal 

57117314 2,3,4,7,8-Pentachlorodibenzofuran Dioxin/furan Mile Gal 

57117416 1,2,3,7,8-Pentachlorodibenzofuran Dioxin/furan Mile Gal 

57117449 1,2,3,6,7,8-Hexachlorodibenzofuran Dioxin/furan Mile Gal 

57653857 1,2,3,6,7,8-Hexachlorodibenzo-p-Dioxin Dioxin/furan Mile Gal 

60851345 2,3,4,6,7,8-Hexachlorodibenzofuran Dioxin/furan Mile Gal 

67562394 1,2,3,4,6,7,8-Heptachlorodibenzofuran Dioxin/furan Mile Gal 

70648269 1,2,3,4,7,8-Hexachlorodibenzofuran Dioxin/furan Mile Gal 

72918219 1,2,3,7,8,9-Hexachlorodibenzofuran Dioxin/furan Mile Gal 

CO2 Carbon Dioxide 

SOA Secondary Organic Aerosol VOC* VOC* 

* Ratios for the pollutants are in the NCD SCC table. All others are in the SCCToxics table. 

SOURCE CATEGORIES FOR WHICH INVENTORIES ARE PRODUCED BY NMIM 

NMIM’s output is always in terms of source classification codes (SCC), which are 

described in the SCC table. For on-road output, NMIM also distinguishes five emission types 

(exhaust, evaporation, refueling, brake wear, and tire wear). For non-road output, NMIM 

distinguishes three emission types (exhaust, evaporation, and refueling) and also reports the 

NONROAD power classes, which subdivide a given SCC by horsepower range. 

The VMT in the BaseYearVMT table is by the 28 MOBILE6 vehicle classes. In NMIM 

output, however, these 28 vehicle classes are aggregated into the 12 vehicle classes that 

correspond to SCC codes. These 12 vehicle classes are shown in Table 5. The M6VClass table 

defines the correspondence between these two sets of vehicle classes, which is shown in Table 

5a. 

13
Table 5. The twelve vehicle classes that correspond to SCCs. 

Class Description Class Description 

LDGV Light duty gasoline vehicles LDDT Light duty diesel trucks 

LDGT1 Light duty gasoline truck 1 2BHDDV Class 2b heavy duty diesel 

vehicles 

LDGT2 Light duty gasoline truck 2 LHDDV Light heavy-duty diesel vehicles 

HDGV Heavy duty gasoline vehicles, including MHDDV Medium heavy-duty diesel 

buses vehicles 

MC Motorcycles HHDDV Heavy heavy-duty diesel 

vehicles 

LDDV Light duty diesel vehicles BUSES Diesel buses 

14
Table 5a. The 28 MOBILE6 vehicle classes and the 12 vehicle classes corresponding to SCCs 

that are output by NMIM. 

M6 28 M6 12 SCC Description 

#

1 LDGV LDGV Light-Duty Gasoline Vehicles (Passenger Cars) 

2 LDGT1 LDGT1 Light-Duty Gasoline Trucks 1 (0-6,000 lbs. GVWR, 0-3750 lbs. 

LVW) 

3 LDGT2 LDGT1 Light-Duty Gasoline Trucks 2 (0-6,000 lbs. GVWR, 3751-5750 lbs. 

LVW) 

4 LDGT3 LDGT2 Light-Duty Gasoline Trucks 3 (6,001-8,500 lbs. GVWR, 0-5750 lbs. 

ALVW) 

5 LDGT4 LDGT2 Light-Duty Gasoline Trucks 4 (6,001-8,500 lbs. GVWR, 5751 lbs. 

and greater ALVW) 

6 HDGV2B HDGV Class 2b Heavy-Duty Gasoline Vehicles (8501-10,000 lbs. GVWR) 

7 HDGV3 HDGV Class 3 Heavy-Duty Gasoline Vehicles (10,001-14,000 lbs. GVWR) 

8 HDGV4 HDGV Class 4 Heavy-Duty Gasoline Vehicles (14,001-16,000 lbs. GVWR) 

9 HDGV5 HDGV Class 5 Heavy-Duty Gasoline Vehicles (16,001-19,500 lbs. GVWR) 

10 HDGV6 HDGV Class 6 Heavy-Duty Gasoline Vehicles (19,501-26,000 lbs. GVWR) 

11 HDGV7 HDGV Class 7 Heavy-Duty Gasoline Vehicles (26,001-33,000 lbs. GVWR) 

12 HDGV8A HDGV Class 8a Heavy-Duty Gasoline Vehicles (33,001-60,000 lbs. GVWR) 

13 HDGV8B HDGV Class 8b Heavy-Duty Gasoline Vehicles (>60,000 lbs. GVWR) 

14 LDDV LDDV Light-Duty Diesel Vehicles (Passenger Cars) 

15 LDDT12 LDDT Light-Duty Diesel Trucks 1 and 2 (0-6,000 lbs. GVWR) 

16 HDDV2B 2BHDDV Class 2b Heavy-Duty Diesel Vehicles (8501-10,000 lbs. GVWR) 

17 HDDV3 LHDDV Class 3 Heavy-Duty Diesel Vehicles (10,001-14,000 lbs. GVWR) 

18 HDDV4 LHDDV Class 4 Heavy-Duty Diesel Vehicles (14,001-16,000 lbs. GVWR) 

19 HDDV5 LHDDV Class 5 Heavy-Duty Diesel Vehicles (16,001-19,500 lbs. GVWR) 

20 HDDV6 MHDDV Class 6 Heavy-Duty Diesel Vehicles (19,501-26,000 lbs. GVWR) 

21 HDDV7 MHDDV Class 7 Heavy-Duty Diesel Vehicles (26,001-33,000 lbs. GVWR) 

22 HDDV8A HHDDV Class 8a Heavy-Duty Diesel Vehicles (33,001-60,000 lbs. GVWR) 

23 HDDV8B HHDDV Class 8b Heavy-Duty Diesel Vehicles (>60,000 lbs. GVWR) 

24 MC MC Motorcycles (Gasoline) 

25 HDGB HDGV Gasoline Buses (School, Transit and Urban) 

26 HDDBT BUSES Diesel Transit and Urban Buses 

27 HDDBS BUSES Diesel School Buses 

28 LDDT34 LDDT Light-Duty Diesel Trucks 3 and 4 (6,001-8,500 lbs. GVWR) 

SCC output also distinguishes twelve roadway types, listed in Table 6 below. The twelve 

roadway types are those used by the Federal Highway Administration’s (FHWA) Highway 

Performance Monitoring System (HPMS).7 These roadway types, in combination with the 

twelve vehicle types, result in 144 SCCs for on-road mobile sources. 

15
Table 6. Twelve Roadway Types 

Interstate: Rural Interstate: Urban 

Other Principal Arterial: Rural Other Freeways and Expressways: Urban 

Minor Arterial: Rural Other Principal Arterial: Urban 

Major Collector: Rural Minor Arterial: Urban 

Minor Collector: Rural Collector: Urban 

Local: Rural Local: Urban 

Emissions are estimated by the NONROAD Model for 214 SCCs. SCCs distinguish 

between equipment types, fuels (gasoline, diesel, liquified petroleum gas (LPG), and compressed 

natural gas (CNG)) and between two stroke and four stroke gasoline engines. In addition, 

NONROAD produces horsepower categories, and NMIM retains these in its output. 

Within NMIM and the NONROAD Model, the non-road SCCs are grouped into 12 

segments, listed in Table 7. 

Table 7. NONROAD Model equipment segments. 

Recreational Lawn/Garden Logging Oil Field 

Construction Agriculture Airport Support Pleasure Craft 

Industrial Commercial Underground Mining Railroad 

Any single SCC always falls under only one of these segments, corresponding to its most typical 

application, although it may be used in other segments as well. For example, skid steer loaders 

are in the construction segment, although they are also used in agriculture. Fuels are gasoline, 

diesel, LPG, and CNG. NMIM users must choose a segment and fuel; individual SCCs may not 

be selected. Output, however, is by individual SCC. 

THE NMIM COUNTY DATABASE (NCD) 

The NMIM County Database (NCD) contains all the county-specific information needed 

to run MOBILE6 and NONROAD. It also contains the list of pollutants and the ratios of HAPS, 

dioxins/furans, and some metals to various NONROAD and MOBILE6 outputs that are used to 

estimate inventories of these non-standard pollutants. This database is in MySQL, an open 

source data base management system that is available from www.mysql.com. The tables in the 

database are listed in Table 8. 

16
Table 8. Tables in the NMIM County Database. 

Table Contents 

BaseYearVMT VMT by year, county, M6VClass, and HPMSRoadType. 

County For each county, Federal Information Processing System (FIPS) codes for the 

county and state, altitude, beginning and end of ozone season, Stage 2 

information, Natural Gas Vehicle (NGV) fraction file name 

CountyMap The representing county for each county, one for NONROAD and one for 

MOBILE6. 

CountyMonth Defines the set of possible county-month combinations. 

CountyMonthHour Monthly average hourly temperature and humidity table used if “Use yearly 

weather data” is not selected or there is no data for the requested year in the 

CountyYearMonthHour table. 

County NRFile References to external NONROAD files pertaining to a county. 

CountyVMTMonth 

Allocation 

Mileage allocation factors for the twelve months of the year, by county. 

CountyYear Stage2 percent input to the NR model, plus external file references for 

MOBILE6 and NR. 

CountyYearMonth Gasoline, diesel, and natural gas fuel IDs for each county for each year and 

month. 

CountyYearMonth 

Hour 

Historical hourly temperature and relative humidity. 

DataSource Defines datasource identifiers used in other tables. 

Diesel Diesel sulfur content associated with each diesel ID. 

EmissionType Associates emission types (exhaust, evap, brake, tire) with EmissionTypeID 

used in other tables 

FileType Defines the set of valid external files and their 3-character extensions. 

Gasoline Detailed fuel properties associated with each gasoline ID. 

Hour Defines the hour identifiers. 

HPMSRoadType Defines the twelve HPMS road type identifiers. 

M6VClass Defines the 28 vehicle classes used in MOBILE6. These are the valid 

combinations of M6Vtype and fuel. 

M6VType Defines the fuel-independent vehicle types used in MOBILE6. 

NaturalGas Natural gas sulfur content associated with each natural gas ID. 

PollutantCode Associates NIF pollutant codes and pollutant names with PollutantCodeID used 

in other tables. 

SCC Associates with each SCCID an SCC code and description, and ratios for NH3, 

PM25, and for converting between HC forms. 

SCCToxics SCC and fuel property-dependent ratios for calculating HAPS, dioxin/furans, 

and metals. 

State Associates state names and abbreviations with state FIPS codes used in other 

tables. 

VMTGrowth The annual VMT growth rate for a M6VClass by county and year. 

VMTMonth 

Allocation 

Factors for allocating annual VMT to the 12 months, by M6VType and 

HPMSRoadType, used if there are no county-specific values in 

CountyVMTMonthAllocation. 

17
The main emphasis in putting together the database has been on historical years, for 

which data are available. The best year as of this writing is 2002, since states and local 

organizations submitted corrected information as part of the NEI process. However, future years 

are of interest for rulemaking and other purposes. For future years, refinery modeling of the 

impacts of anticipated fuel changes, such as phase-in of low sulfur fuels, is used to develop 

projected fuel properties. Items such as age and speed distributions are assumed to remain 

unchanged from the most recent historical year, unless future estimates for such data are 

submitted by states. NMIM uses 20-year average temperature and humidity data from the 

CountyMonthHour table for future years. Data for all years from 1999 to 2050 is present in the 

NCD, except for VMT. Users wishing to generate a future year on-road inventory may have to 

provide their own VMT data and insert it into the BaseYearVMT table of the NCD. For future 

year non-road inventories, the necessary estimates (e.g., of population and activity) are contained 

in the data files associated with the NONROAD Model. 

Temperature and Humidity 

The NCD contains monthly average hourly temperature and humidity for historical years 

in the CountyYearMonthHour table. It contains 20-year averages (1981 to 2000) of monthly 

average hourly temperature and humidity in the CountyMonthHour table. The user can select 

whether to use the historical or 20-year average data. When historical data are not present in the 

database, the program automatically uses the 20-year average. 

Because minimum and maximum temperatures occur at different hours each day, the 

minimum of the hourly averages will be higher than the average of the daily minima, and the 

maximum of the hourly averages will be lower than the average of the daily maxima. To avoid 

this narrowing of the daily temperature range, the monthly average of hourly temperatures is 

assumed to capture the daily temporal pattern and is mathematically stretched so that the low 

temperature equals the monthly average of the daily minima and the high temperature equals the 

monthly average of the daily maxima, producing a set of "adjusted monthly average hourly 

temperatures." The same procedure is applied to dewpoint. An adjusted monthly average hourly 

relative humidity is then calculated from the adjusted monthly average hourly temperatures and 

dewpoints. It is these adjusted hourly temperatures and relative humidities that are found in the 

CountyYearMonthHour table of the NCD. A similar procedure was used to generate the 20-year 

average temperatures and humidities in the CountyMonthHour table. 

The historical and 20-year average hourly temperatures and humidities were developed by 

Air Improvement Resources (AIR), Inc. for EPA under contract, from data obtained from the 

National Climatic Data Center. County temperatures were determined by weighting nearby 

temperature stations by their distance from the population-based centroid of each county. For 

Puerto Rico and the Virgin Islands, averages were calculated using the five calendar years 1999 

through 2003. 

18
MOBILE6 uses the hourly temperatures directly from the CountyYearMonthHour table. 

NONROAD does not use hourly temperatures, but rather daily minimum, maximum and average 

temperatures. For NONROAD, NMIM selects the minimum and maximum from all 24 hourly 

temperatures, but calculates the average from the hourly temperatures from 6 AM to 9 PM 

(hourly intervals 7 to 21). 

Fuel Properties 

The CountyYearMonth table contains gasoline, diesel, and natural gas fuel identifiers for 

each county for each month of each calendar year. On-road vehicles and non-road equipment 

have separate fuel identifiers. The identifiers are associated with fuel properties in the Gasoline, 

Diesel, and NaturalGas tables. 

For gasoline, the fuel properties were determined by the Eastern Research Group, Inc., 

(ERG) under contract to EPA using gasoline survey data.8 The fuel properties stored in the 

Gasoline table are: 

o Average Reid Vapor Pressure (RVP) 

o Average sulfur content 

o Maximum sulfur content 

o Ethanol (ETOH) percent by volume of ethanol blended gasolines 

o Ethanol blend market share 

o MTBE (methyl tertiary butyl ether) percent by volume of ether blended gasolines 

o MTBE blend market share 

o ETBE (ethyl tertiary butyl ether) percent by volume of ether blended gasolines 

o ETBE blend market share 

o TAME (tertiary amyl methyl ether) percent by volume of ether blended gasolines 

o TAME blend market share 

o Aromatic content 

o Olefin content 

o Benzene content 

o E200 (vapor percentage of gasoline at 200 degrees Fahrenheit) 

o E300 (vapor percentage of gasoline at 300 degrees Fahrenheit) 

The gasoline properties were derived from several surveys: 

o EPA's Reformulated Gasoline survey, 20009 

o EPA's Oxygenated fuel program summary, 200110 

o TRW (previously NIPER) fuel survey, 199911 

o Alliance of Automobile Manufacturers of America (AAMA) survey, 199912 

Market share for oxygenated gasolines was obtained from the EPA Oxygenate Type 

Analysis Tables13 and the Federal Highway Administration website.

14 

19
If the methyl tertiary butyl ether (MTBE) percent volume content was less than 0.1 

percent, MTBE content was set to be zero with a zero percent market share. Similarly, if the 

ethanol percent volume content was less than 0.1 percent, ethanol content was set to be zero with 

a zero percent market share. For any area which reported non-zero values for tertiary amyl 

methyl ether (TAME) or ethyl tertiary butyl ether (ETBE), the entire market share was set to be 

MTBE, since it was not possible to distinguish the market share between these specific 

oxygenates. 

All gasoline properties are area-wide averages, except for oxygenates, which are allowed 

to have market shares. Three fuels (winter, summer, and spring/fall) are determined for each 

county and assigned to months by season. Months representing seasons vary by location. 

Spring/fall gasoline properties are derived from summer and winter fuels by interpolation. 

Gasolines in Puerto Rico and the Virgin Islands were assumed to be similar to gasolines in 

Collier County, Florida. Details of how the fuel survey data were applied to individual states and 

counties are described in the ERG report to EPA.15 The NCD allows counties to have different 

NR and on-road gasolines, but none do as of this writing, because data have not been available to 

make this distinction. 

For diesel fuel and natural gas, the only fuel property stored is sulfur content, in the 

Diesel and NaturalGas tables, respectively. A sulfur content of 30 ppm for natural gas is used for 

all counties in all states for all years and months. Non-road and on-road diesel sulfur values tend 

to differ in the same county because they are subject to different regulations. In addition, there 

are two kinds of non-road diesel fuel: that used for pleasure craft and that used for all other nonroad segments. 

Vehicle Miles Traveled (VMT) 

VMT is the critical activity parameter for converting MOBILE6's emission factors into 

inventories. Historical VMT is derived from data from the Federal Highway Administration 

(FHWA). Unfortunately, these data are not gathered at the spatial and temporal scales needed by 

NMIM, nor for the same vehicle classes required by MOBILE6. The procedures for gathering 

these data and processing them into the required scales and vehicle classes are described in detail 

in the NEI documentation.16 States and local organizations may submit corrections to the VMT 

data. 

Database Improvement 

As part of the NEI process, States and regional and local organizations are asked to 

review the NCD and provide more accurate data, if available. This review includes the county 

data and also the external files that can be referenced by MOBILE6 or NONROAD, such as fleet 

distributions and equipment allocations. The development of this database improvement system 

promises to extend NMIM’s advantages beyond convenience to include the production of better 

mobile source inventories. Procedures for submitting improved data for inclusion in the NCD 

20
are contained in a technical memorandum posted with the Draft 2002 NEI.

17 This document is 

also useful as a comprehensive description of what county-specific data can go into the NCD. 

Considerable data has been received as part of the 2002 NEI process. 

ESTIMATION OF HAZARDOUS AIR POLLUTANT (HAP) EMISSIONS 

The hazardous air pollutants for which inventories are produced by NMIM are listed in 

Table 4 above. The “six HAPS” are produced internally by MOBILE6. In all other cases, as 

indicated in the “Pollutants” section above, HAP inventories are generated by ratios to various 

MOBILE6 and NONROAD outputs. 

HAPS are estimated using data sources and methods developed for the 1999 NEI for 

HAPS, version 3,18,19 with some modifications, described below. NMIM does not estimate HAP 

emissions for compressed natural gas (CNG) engines. 

HAPS are estimated in NMIM using one of three approaches: 

1) Gaseous HAPS – Apply toxic to VOC ratios to VOC estimates. 

2) Poly-Aromatic Hydrocarbons (PAHs) – Apply toxic to PM10 ratios to PM10 estimates. 

3) Metals, Dioxins and Furans – For NONROAD, multiply HAP gram per gallon emission 

factors by county level fuel consumption estimates. For MOBILE6, multiply HAP gram 

per mile emission factors by county level VMT estimates. 

The NCD SCCToxics table provides a complete listing of toxic ratios and emission factors for all 

SCCs and fuel combinations. The above approaches are described in more detail in the 

following sections. 

Gaseous HAPS 

NMIM uses the toxic to VOC ratios described in the documentation for the 1999 NEI for 

HAPS, version 3, and summarized in Volume 1, Appendix D, Table 1 of the documentation. 

Separate ratios are used for evaporative and exhaust emissions for each of the following four 

categories of gasoline blends: 

1. Baseline Gasoline. All cases that do not fall into categories 2-4 below. Ratios are in 

variables “ExhBaseGas” and “EvapBaseGas” in the SCCToxics table. 

2. WO (Winter Oxygenate) Gasoline / Ethanol or ETBE - Used where the fuel contains 

ethanol which is greater than or equal to 5% by volume or ETBE greater than or equal to 

5% by volume. Ratios are in variables “ExhEthGas” and “EvapEthGas” in the 

SCCToxics table. 

21
3. WO (Winter Oxygenate) Gasoline / MTBE / TAME - Used where the fuel contains 

MTBE which is greater than or equal to 12% by volume or TAME greater than or equal 

to 13% by volume. Ratios are in variables “ExhMTBEGas” and “EvapMTBEGas” in the 

SCCToxics table. 

4. RFG/MTBE/TAME - Used where the fuel is RFG and where the fuel contains oxygenate 

greater than 5% by volume and where the fuel contains MTBE which is less than 12% by 

volume or TAME less than 13% by volume. Ratios are in variables “ExhRFGGas” and 

“EvapRFGGas” in the SCCToxics table. 

In many cases, HAP profiles for specific non-road equipment and engine type 

combinations are available. However, for some equipment/engine type combinations, no 

speciation data are available. In such instances, default values for 2-stroke gasoline engines, 4-

stroke gasoline engines, and diesel engines are used. These default values represent an average 

fraction for various equipment types within an engine category. 

Polyaromatic Hydrocarbons (PAHs) 

All PAHs emitted in exhaust are estimated as fractions of PM10, although the data used 

to calculate mass ratios include both gas and particle phase PAH emissions. The data used to 

develop the PAH fractions are described in the documentation for the 1999 NEI for HAPS. 

Evaporative naphthalene emissions from on-road vehicles are estimated as a fraction of VOC. 

NMIM does not currently estimate evaporative naphthalene emissions for non-road equipment. 

Metals, Dioxins, and Furans 

On-Road 

NMIM estimates on-road emissions using g/mile emission factors developed for the 1999 

NEI for HAPS, version 3. 

Non-Road 

Metals 

The approach used by NMIM to estimate non-road county level metal emissions differs in 

a number of respects from the approach used in the 1999 NEI for HAPs, version 3. In the 1999 

NEI, nationwide metal emissions for gasoline engines were obtained by applying a mass per 

gallon emission factor by nationwide gasoline consumption from the NONROAD model. For 

diesel engines, a mass per brake-horsepower hour emission factor was multiplied by nationwide 

energy output. The resultant nationwide emission estimates were then spatially allocated to 

22
counties relative to the county proportion of PM10 emissions compared to the national PM10 

emissions, as obtained from the NONROAD model. 

In contrast, NMIM multiplies mass per gallon emission factors for both gasoline and 

diesel engines by county level fuel consumption in gallons to obtain a county level inventory 

estimate. For diesel engines, mass per brake-horsepower hour emission factors were converted 

to mass per gallon emission factors using the following equation: 

grams per gallon = grams per brake-horsepower hour (g/bhphr) * average fuel density 

(lb/gallon) / fuel consumption per brake-horsepower hour (lb/bhphr) 

where: 

average fuel density = 7.01 lb/gal 

fuel consumption per brake horsepower hour = 0.408 lb for engines less than 100 hp 

= 0.367 lb for engines greater than 100 hp 

The fuel consumption per brake horsepower hour estimates are from the NONROAD model.20 

NMIM looks up the gram per gallon metal emission factors for gasoline and diesel engines in the 

SCCToxics table of the NCD. 

Dioxins/furans 

Mass per gallon emission factors for dioxins and furans from non-road engines were 

calculated by multiplying the on-road vehicle emission factors in grams per mile by fleet average 

fuel economy estimates. The assumed fuel economy for gasoline vehicles was 21.5 miles per 

gallon; for diesel vehicles it was 7 miles per gallon. Resulting gram per gallon emission factors 

are contained in the NCD SCCToxics table. 

SUMMARY 

NMIM is a consolidated modeling system for MOBILE6 and NONROAD designed to 

produce national inventories. A standard input configuration has been designed to provide 

consistency between MOBILE6 and NONROAD and between criteria pollutants and HAPS. 

NMIM’s run specification can be constructed from its GUI or by a text editor. It can be executed 

from the GUI or from a command line. It is capable of standalone operation and can also take 

advantage of multiple computers on a LAN through a simple distributed processing scheme. 

Post-processing capabilities include the production of output in NIF3 format. The inclusion of a 

database improvement system in the NEI process promises to extend NMIM’s advantages 

beyond convenience to the production of better mobile source inventories. 

23
REFERENCES

1. This document is an update of a paper that was presented at the International Emission 

Inventory Conference (IEIC) at Clearwater, Florida in June 2004. At that conference, a training 

was given using source code NMIM20040512 and database County20040508. Since that time, 

bugs have been fixed, capabilities have been expanded, and the database has been improved. 

2. For a detailed explanation of how emissions were calculated for aircraft, commercial marine 

vessels, and locomotives for the Draft 2002 NEI, see 

ftp://ftp.epa.gov/EmisInventory/draftnei2002/mobile/nonroad/documentation/ 

3. User’s Guide to MOBILE6.1 and MOBILE6.2: Mobile Source Emission Factor Model, U.S. 

Environmental Protection Agency, Ann Arbor, MI, August 2003; EPA420-R-03-010. May be 

found as a link to http://www.epa.gov/otaq/m6.htm 

4. Details of the NIF3.0 format may be found as links to 

http://www.epa.gov/ttn/chief/nif/index.html#ver3. 

5. See EPA’s Substance Registry System: http://www.epa.gov/srs/ and the CAS Registry 

website: http://www.cas.org/EO/regsys.html. 

6. Information on REMSAD may be found at http://remsad.saintl.com/overview.htm. 

7. Information on the Federal Highway Administration’s (FHWA) Highway Performance 

Monitoring System (HPMS) is available at http://www.fhwa.dot.gov/policy/ohpi/hpms/. 

8. "National Mobile Inventory Model (NMIM) Base and Future Year County Database 

Documentation and Quality Assurance Procedures," prepared by Eastern Research Group, Inc., 

for U.S. Environmental Protection Agency, Office of Transportation and Air Quality, Ann Arbor, 

MI, 2003. 

9. “Reformulated Gasoline Survey Data for 2000,” U.S. Environmental Protection Agency, 

Office of Transportation and Air Quality, Ann Arbor, Michigan, 2000 Internet address: 

http://www.epa.gov/otaq/consumer/fuels/mtbe/oxy-95-00.pdf 

10. “U.S. EPA Oxygenated Fuel Program Summary, State Winter Oxygenated Fuel Program 

Requirements for Attainment or Maintenance of CO NAAQS,” U.S. Environmental Protection 

Agency, Office of Transportation and Air Quality, Ann Arbor, Michigan. October 2001. Internet 

address: http://www.epa.gov/otaq/regs/fuels/oxy-area.pdf 

11. Thompson, Ramo, and Woolridge (TRW) or National Institute for Petroleum and Energy 

Research (NIPER) Fuel Survey, 1999. 

12. “North American Gasoline and Diesel Fuel Survey,” Alliance of Automobile Manufacturers, 

1999. 

24
13. “U.S. EPA Oxygenated Fuel Program Summary, State Winter Oxygenated Fuel Program 

Requirements for Attainment or Maintenance of CO NAAQS,” U.S. Environmental Protection 

Agency, Office of Transportation and Air Quality, Ann Arbor, Michigan. October 2001. Internet 

address: http://www.epa.gov/otaq/regs/fuels/oxy-area.pdf 

14. Federal Highway Administration (FHWA) website for oxygenated fuel sale

percentage. Table MF-33E – Estimated Use of Gasohol and Table MF-21 – Motor-Fuel Use,

1999. Internet address: http://www.fhwa.dot.gov/ohim/hs99/mfpage.htm

15. "National Mobile Inventory Model (NMIM) Base and Future Year County Database 

Documentation and Quality Assurance Procedures," prepared by Eastern Research Group for 

U.S. Environmental Protection Agency, Office of Transportation and Air Quality, May 2003, 

(EPA contract No. 68-C00-112, Work Assignment 3-05). 

16. "Documentation for the Onroad National Emissions Inventory (NEI) For Base Years 1970-

2002," prepared for: Office of Air Quality Planning and Standards, Emission Factor and 

Inventory Group, U.S. Environmental Protection Agency, Research Triangle Park, NC 27711, 

prepared by: E.H. Pechan & Associates, Inc., 5528-B Hempstead Way, Springfield, VA 22151, 

January 2004. 

17. "Instructions to State and Local Agencies for Updating the County-Level Database from 

EPA’s National Mobile Inventory Model: Technical Memorandum," prepared by Maureen 

Mullen, E.H. Pechan and Associates, Springfield, VA, for Emission Factors and Inventory 

Group, Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency, 

Research Triangle Park, NC 27711, November, 2004. It is available on the web at 

ftp://ftp.epa.gov/EmisInventory/draftnei2002/mobile/nmim_related/nmim_technical_memorandu 

m_1104.pdf. 

18. Documentation for the Onroad National Emissions Inventory (NEI) For Base Years 1970-

2002 Prepared for: Office of Air Quality Planning and Standards Emission Factor and Inventory 

Group U.S. Environmental Protection Agency Research Triangle Park, NC 27711. Prepared by: 

E.H. Pechan & Associates, Inc., 5528-B Hempstead Way, Springfield, VA 22151, January 2004. 

http://www.epa.gov/ttn/chief/net/1999inventory.html#final3haps 

19. “Documentation for Aircraft, Commercial Marine Vessel, Locomotive, and Other Nonroad 

Components in the National Emissions Inventory, Volumes I and II,” prepared by Eastern 

Research Group for U.S. Environmental Protection Agency, October 7, 2003. Available from 

http://www.epa.gov/ttn/chief/net/1999inventory.html#final3haps 

20. Exhaust and Crankcase Emission Factors for Nonroad Engine Modeling—CompressionIgnition, U.S. Environmental Protection Agency, Office of Transportation and Air Quality, 

Report No. EPA420-P-02-016, November 2002. May be found on the web at 

http://www.epa.gov/otaq/nonrdmdl.htm#docs. 

25