Air and Radiation EPA420-R-05-014 

December 2005 

NR-001c 

Temperature Corrections for 

Nonroad Exhaust Emissions 
EPA420-R-05-014 

December 2005 

Temperature Corrections for 

Nonroad Exhaust Emissions

NR-001c

Assessment and Standards Division 

Office of Transportation and Air Quality 

U.S. Environmental Protection Agency 

NOTICE

This technical report does not necessarily represent fi nal EPA decisions or positions.

It is intended to present technical analysis of issues using data that are currently available.

The purpose in the release of such reports is to facilitate the exchange of

technical information and to inform the public of technical developments which

may form the basis for a fi nal EPA decision, position, or regulatory action.
Purpose 

This report documents how the final NONROAD2005 emission inventory model 

accounts for the effects of temperature on four-stroke exhaust emissions. RVP and temperature 

also affect both vapor displacement, which is covered in NR-013b (ARefueling Emissions for 

Nonroad Engine Modeling@) as well as diurnal emissions, which are covered in NR-012c 

(ANonroad Evaporative Emission Rates@). 

Background 

NONROAD includes effects of ambient temperature on exhaust emissions. The method 

used in NONROAD is taken directly from the MOBILE5b highway vehicle emission factor 

model. Only effects on 4-stroke gasoline engine exhaust hydrocarbons (HC), carbon monoxide 

(CO), and oxides of nitrogen (NOx) are included. We are not aware of any data that could be 

used to estimate temperature effects on the other pollutants in NONROAD, so no such effects 

are currently modeled. 

Approach 

Temperature Effects on Four-Stroke Exhaust Emissions 

The details of the temperature correction method from the MOBILE5b model are as 

follows: 

TCF = EXP [ A * (Tambient -75) ] 

where: 

TCF = multiplicative Temperature Correction Factor 

Tambient = Ambient Temperature, in degrees Fahrenheit 

"A" = The coefficient "A" is taken from MOBILE5b for "Bag 2" effects on uncontrolled 

light-duty gasoline vehicles (LDGV's). "Bag 2" refers to the hot stabilized portion of the 

test sequence, so no cold-start effects are present. The value of "A" in the above equation 

depends on high or low temperature (relative to 75F) and on pollutant (HC/CO/NOx). 

Values for A are: 

NR-001c December 2005 1 
"A" Tambient<75 Tambient>75 

HC (4-stroke) - 0.00240 +0.00132 

CO (4-stroke) +0.00158 +0.00375 

NOx (4-stroke) - 0.00892 - 0.00873 

These coefficients are applied only to pre-1968 model year vehicles in MOBILE, 

including both LDGVs and LDGTs. These coefficients for pre-1968 model year vehicles are 

assumed to apply to all nonroad 4-stroke engines. Due to lack of data, there is no attempt in 

NONROAD to apply different temperature corrections for different generations of nonroad 4-

stroke engine technologies. 

For two-stroke engines, conditions differ significantly from those of on-road motor 

vehicles. Therefore, the 4-stroke corrections from MOBILE5b are not applied to 2-stroke or 

diesel engines in NONROAD. Due to lack of data for these engine types, temperature effects on 

exhaust emissions from 2-stroke and diesel engines are not currently included in 

NONROAD2005. 

NR-001c December 2005 2