Air and Radiation EPA420-R-05-015 

December 2005 

NR-002c 

Conversion Factors for 

Hydrocarbon Emission

Components 
EPA420-R-05-015 

December 2005 

Conversion Factors for

Hydrocarbon Emission Components

NR-002c

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 

The purpose of this memorandum is to document the conversion factors for reporting 

hydrocarbon emissions in different forms. The general forms are total hydrocarbons (THC), 

total organic gas (TOG), nonmethane hydrocarbons (NMHC), nonmethane organic gas 

(NMOG), and volatile organic compounds (VOC), all defined in the introduction below. For 

reporting hydrocarbon emissions from nonroad equipment, it is helpful to provide an accepted 

means to estimate the hydrocarbons in the different forms. This is not a substitute for full 

speciation of hydrocarbon emissions. 

Introduction 

Hydrocarbon emissions can be reported in a variety of styles depending on the end use of 

the emission estimates and the measurement technique used in the underlying data. Not all 

emissions are measured for all engines, so a conversion from the most common measurement 

type to others is needed to supply an estimate in terms required by the user. 

Most hydrocarbon emissions data from mobile sources is measured as total hydrocarbon 

(THC). THC is the measured hydrocarbon emissions using a Flame Ionization Detector (FID) 

calibrated with propane. The FID is assumed to respond to all hydrocarbons identically as it 

responds to propane in determining the concentration of carbon atoms in a gas sample. Most 

hydrocarbons respond nearly identically as propane with notable exceptions being oxygenated 

hydrocarbons such as alcohols and aldehydes commonly found in engine exhaust. 

Because alcohols and especially aldehydes are chemically reactive and therefore ozoneforming hydrocarbons, the California Air Resources Board defined a measurement that adds the 

THC and the oxygenated components into a new measurement called total organic gas (TOG). 

[1] The oxygenated components are measured by collecting aldehydes on dinitrophenylhydrazine impregnated filter traps and alcohols in chilled water impingers. The aldehydes 

and alcohols are extracted and measured using chromatography to determine emission rates. 

Each mole of aldehydes and alcohols is added by weight as formaldehyde and methanol. 

Methane is an organic gas that is orders of magnitude less reactive than other 

hydrocarbons, so it is often excluded from emission estimations. The methane is measured by 

chromatographically separating the methane from the THC and analyzing the concentrations 

using a FID calibrated specifically for methane. The methane emissions are subtracted from the 

THC and TOG emission estimations to produce a nonmethane hydrocarbon (NMHC) and a 

nonmethane organic gas (NMOG) emission estimate. Some newer instruments can measure the 

NMHC directly however leading to lower uncertainty. 

Some hydrocarbons are less ozone-forming than other hydrocarbons, so EPA has 

officially excluded them from the definition of regulated hydrocarbons called volatile organic 

compounds (VOC). This definition excludes methane, ethane, acetone, and compounds not 

commonly found in large quantities in engine exhaust like chlorohydrocarbons from 

NR-002c December 2005 1 
consideration as VOC. For this work, the definition of VOC is the result of subtracting methane 

and ethane from the TOG emission estimates. Although acetone is not subtracted, it is present in 

smaller quantities compared to methane and ethane, and will have a negligible effect on the 

results. 

Conversion Factors 

Exhaust Emissions 

Because all studies to date have measured THC, all other hydrocarbon types will be 

given as a proportion of THC. The ratios given in the table below were derived from those 

studies that measured methane, ethane, and aldehydes. Alcohols are only found if the fuel 

contains alcohols, so they would have been considered if data were available. 

The hydrocarbon speciation data from nonroad engines is sparse. The 2-stroke engine 

conversions are derived from the study of only one moped engine while the 4-stroke engine 

results are an averaged result of 11 lawnmower engines studied. The diesel results are the 

average of two late 70s and early 80s vintage on-highway truck engines. The factors for 

compressed natural gas (CNG) and liquid petroleum gas (LPG) engines were estimated from 

data collected using on-highway light-duty vehicles equipped with catalysts. Nonroad 

equipment does not use this technology but no emissions data from nonroad CNG and LPG 

engines was available. 

NR-002c December 2005 2 
Table for Conversion Factors for Hydrocarbon Exhaust Emission Results 

Engine Type TOG/THC NMOG/THC NMHC/THC VOC/THC 

2-Stroke 

Gasoline [2] 1.044 1.035 0.991 1.034 

4-Stroke 

Gasoline [2, 3] 1.043 0.943 0.900 0.933 

Diesel [4] 1.070 1.054 0.984 1.053 

LPG [5] 1.099 1.019 0.920 0.995 

CNG [5] 1.002 0.049 0.048 0.004 

Crankcase and Evaporative Emissions 

For non-tailpipe emissions (i.e., crankcase and evaporative emissions) with fuels other 

than compressed natural gas, we will assume: 

THC = VOC = NMHC = TOG = NMOG 

Hence, all ratios will be 1.000. 

For engines using natural gas fuels (CNG), we will assume that the relationship of the 

non-tailpipe HC emissions (i.e., crankcase and evaporative emissions) will be: 

THC = TOG 

and 

VOC = NMHC = NMOG = 0 

Hence, the ratio of TOG to THC will be 1.000 while the other ratios will be 0.0. 

Conclusions 

The conversion factor is applied at the end of the model calculation of total 

hydrocarbons, known as THC. Emission factors are generated most typically as THC, so the 

conversion to other hydrocarbon forms is provided for the user. 

Newer data are becoming available and should be used to eventually augment and verify 

the data currently used in this report. Changes in engine technology needed to meet nonroad 

engine regulations that might affect the speciation profiles should also be considered, given 

NR-002c December 2005 3 
available data. Acetone should also be explicitly excluded as a VOC when the newer data are 

analyzed. 

References 

[1] Air Resources Board (1996), ACalifornia Non-methane Organic Gas Test Procedure,@

California Environmental Protection Agency, Last Amended June 24, 1996. 

[2] Hare C.T. and White, J.J. (1991), AToward the Environmentally-Friendly Small Engine, 

Lubricant, and Emission Measurement Issues@, SAE-911222. 

[3] Gabele, P., (1997), AExhaust emissions from four-stroke lawn mower engines,@ Journal of the 

Air & Waste Management Association, pp 642-649, vol.47, Sept., 1997. 

[4] Springer, Karl J. (1979), ACharacterization of Sulfates, Odor, Smoke, POM and Particulates 

from Light and Heavy-Duty Engines -- Part IX,@ Ann Arbor, Michigan: U.S. Environmental 

Protection Agency, Office of Mobile Sources. Publication no. EPA-460/3-79-007. 

[5] ARB, (1991), AProposed Reactivity Adjustment Factors for Transitional Low-Emission 

Vehicles,@ Technical Support Document, Sept., 27, 1991. 

NR-002c December 2005 4