United States

Environmental Protection

Agency

Air and Radiation EPA420-P-03-002

May 2003

NR-002a

Conversion Factors for

Hydrocarbon Emission

Components

 Printed on Recycled Paper
EPA420-P-03-002

May 2003

Conversion Factors for

Hydrocarbon Emission Components

NR-002a

Assessment and Standards Division

Office of Transportation and Air Quality

U.S. Environmental Protection Agency
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 ozone-forming 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 dinitro- phenylhydrazine 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, 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 onhighway 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] 1.099 1.019 0.920 0.995

CNG [5] 1.002 0.049 0.048 0.004
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, Karl J. (1979), “Characterization 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), “Proposed Reactivity Adjustment Factors for Transitional LowEmission Vehicles,” Technical Support Document, Sept., 27, 1991.