United States Air and Radiation EPA420-P-03-002
Environmental Protection May 2003
Agency NR-002a
Conversion Factors for
Hydrocarbon Emission
Components
> Printed on Recycled Paper
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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
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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
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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 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.
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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
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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 Low-
Emission Vehicles," Technical Support Document, Sept., 27, 1991.
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