United States

Environmental Protection

Agency

Air and Radiation EPA420-P-97-009

November 1997

NR-002

Conversion Factors for

Hydrocarbon Emission

Components

 Printed on Recycled Paper
EPA420-P-97-009

November 1997

Conversion Factors for

Hydrocarbon Emission Components

NR-002

Assessment and Standards Division

Office of Transportation and Air Quality

U.S. Environmental Protection Agency

NOTICE

This technical report does not necessarily represent final 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 final EPA decision, position, or regulatory action.
Conversion Factors for Hydrocarbon Emission Components

Report No. NR-002

November 24, 1997

Assessment and Standards Division

EPA Office of Transportation and Air Quality

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

hydrocarbons in the exhaust.

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
NR-002 November 24, 1997 2

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, and compounds not commonly

found in large quantities in engine exhaust like chlorohydrocarbons from consideration as VOC. 

For this work the definition of VOC is the result of subtracting methane and ethane from the

TOG emission estimates.

Conversion Factors

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

as a proportion of THC. The proportionalities 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.

Table for Conversion Factors for Hydrocarbon 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] 1.036 0.740 0.704 0.647

CNG [5] 1.002 0.049 0.048 0.004
NR-002 November 24, 1997 3

Conclusions

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

expressed as (THC). Emission factors are generated most typically as THC, so the conversion to

other hydrocarbon forms is provided for the user.

References

[1] Air Resources Board (1996), “California Non-methane Organic Gas Test Procedure,”

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

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

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

[3] Gabele, P., (1997), “Exhaust emissions from four-stroke lawn mower engines,” Journal of

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

[4] Springer, KarlJ. (1979), “Characterization of Sulfates, Odor, Smoke, POM and Particulates

from Light and Heavy-DutyEngines-- Part IX,” Ann Arbor, Michigan: U.S. Environmental

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

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

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