vvEPA
Air and Radiation EPA420-P-04-001
April 2004
Nr-002b
United States
Environmental Protection
Agency
Conversion Factors for
Hydrocarbon Emission
Components
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EPA420-P-04-001
Revised April 2004
Conversion Factors for
Hydrocarbon Emission Components
NR-002b
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.
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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 lonization 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
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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
2- Stroke
Gasoline [2]
4- Stroke
Gasoline [2, 3]
Diesel [4]
LPG [5]
CNG [5]
TOG/THC
1.044
1.043
1.070
1.099
1.002
NMOG/THC
1.035
0.943
1.054
1.019
0.049
NMHC/THC
0.991
0.900
0.984
0.920
0.048
VOC/THC
1.034
0.933
1.053
0.995
0.004
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.
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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, JJ. (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] ARE, (1991), "Proposed Reactivity Adjustment Factors for Transitional Low-Emission
Vehicles," Technical Support Document, Sept., 27, 1991.
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