vvEPA
United States
Environmental Protection
Agency
Environmental Sciences Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S2-80-201 May 1981
Project Summary
Ambient Air Non-Methane
Hydrocarbon Monitor
Darrell Burch
A real-time monitor has been de-
veloped for measuring non-methane
hydrocarbons (NMHC) in ambient air.
The monitor consists of two basic
instruments, a methane monitor and a
flame-ionization detector (FID). The
methane monitor, which is based on
gas-filter correlation techniques, makes
use of the infrared absorption charac-
teristics of methane to measure its
concentration. A slight interference in
the measurement of methane by H2O
vapor in the sample air is minimized
using an electronic correction derived
from a simultaneous measurement of
the H2O concentration. The flame-ion
ization detector measures the concen-
tration of the total hydrocarbons
(THC), including methane. The con-
centration of non-methane hydrocar-
bons is obtained by subtracting the
methane concentration from the THC
concentration. The noise-equivalent
concentrations (peak-to-peak) of the
methane monitor and the FID are ap-
proximately 50 ppb and 5 ppb of
carbon, respectively. The estimated
uncertainty in the measurement of a
typical low-level NMHC concentration
is between 20 ppb and 50 ppb. Con-
centrations as high as 70 ppm can be
measured.
This Project Summary was devel-
oped by EPA's Environmental Sci-
ences Research Laboratory, Research
Triangle Park, NC, to announce key
findings of the research project that is
fully documented in a separate report
of the same title (see Project Report
ordering information at back).
Introduction
Hydrocarbons in the atmosphere play
an important role in the production of
photochemical smog and are of great
interest to atmospheric chemists and to
those concerned with air quality. Meth-
ane, the most abundant atmospheric
hydrocarbon, is essentially non-reactive
at the normal ambient concentrations of
a few ppm; thus, this gas does not
contribute significantly to atmospheric
photochemistry. It follows that the
quantity of most interest is the concen-
tration of the other hydrocarbons, com-
monly called non-methane hydrocarbons
(NMHC). For some detailed studies, it
may be desirable to know the concen-
tration of certain hydrocarbon species,
but for many purposes, it is sufficient to
know the sum of the concentrations of
all the NMHC's.
No convenient and reliable method
has yet been developed for routine
monitoring of NMHC's. One widely used
instrument, the flame-ionization detector
(FID), is capable of measuring the con-
centrations of all the hydrocarbons,
frequently called total hydrocarbons
(THC). However, this quantity is not the
one of most interest because it includes
the inert methane, which may constitute
from 30 to 90% of the THC.
The Ford Aerospace and Communica-
tions Corporation-Aeronutronic Division
of Newport Beach, California, with the
support of the Atmospheric Chemistry
and Physics Division of the Environ-
mental Sciences Research Laboratory,
Research Triangle Park, North Carolina,
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has developed a system for measuring
the concentration of non-methane
hydrocarbons in an air sample. This
program involves coupling a commer-
cially available FID to measure the THC
concentration with a custom-designed
methane monitor. The NMHC concen-
tration is equal to the difference between
the concentrations measured by the two
instruments. The methane monitor
uses gas-filter correlation techniques
and contains a set of multiple-pass
optics to give an 845 cm absorption
path. Water vapor in the air sample
interfers with the methane measure-
ment giving a false reading that depends
on the partial pressure of the H20 vapor.
An infrared H2O monitor built as an
integral part of the methane monitor
measures the H2O concentration; this
measurement is used to correct the
apparent methane concentration. A
diaphragm pump circulates either sam-
ple air, bottled zero-gas or bottle span
gas through the monitors. The span
calibrations of both the methane monitor
and the FID are quite stable. Slight drifts
in the zero settings of both instruments
make it necessary to flush the sample
chambers with zero-gas approximately
once each hour if the most accurate
results are required. The estimated
uncertainty in a measurement of a
typical NMHC concentration below 1
ppm (parts per million of carbon) is
between 20 and 50 ppb (parts per billion
of carbon).
Tests and Performance
Results of a few of the tests performed
on the entire instrument with the meth-
ane monitor and FID coupled together
are listed below. Output signals were
recorded separately for each monitor
and are expressed in terms of ppm of
methane. The sample cell of the methane
monitor was operated at 2-atm pressure
and 50°C.
Performance of Methane
Monitor Plus FID
Sensitivity and Noise
(Peak-to-peak noise level with 3 sec
electronic time constant)
Methane
Monitor: 0.05 ppm
FID: 0.005 ppm
Linearity
Methane Linear for concentrations
Monitor: less than 10 ppm, only a
slight deviation from lin-
earity for concentrations
between 10 to 20 ppm.
FID: Not checked carefully, prob-
ably linear to beyond 20 ppm.
Interferences
Methane (Without automatic correc-
Monitor: tion) 2.5 percent H20 pro-
duces interference corre-
sponding to approximately
+2.4 ppm. (With automatic
correction) No interference
for 1 percent H20. Less than
+0.1 ppm for lower H2O con-
centrations; approximately
0.2 ppm at 3 percent H20.
FID: No significant interferences
by normal atmospheric con-
stituents.
Additional topics covered in the main
report include: (1) the use of a Perma-
Pure Dryer as a means to reduce H20
interference in methane measurement;
(2) the modification of an FID to increase
stability; and (3) the electrical and
optical designs used in instrument
fabrication.
Conclusions
An instrument consisting of a methane
monitor combined with a FID can be
designed and built with adequate sensi-
tivity and accuracy to monitor ambient
NMHC concentrations under most con-
ditions of interest. The low concentra-
tions of hydrocarbons make it necessary
that both of the instruments be very sen-
sitive and stable. It is desirable that the
instruments be kept in a temperature
controlled room and the sample air be
drawn in from the outside through a
heated line. Care must be exercised to
avoid contaminating air samples and
calibration gases or losing hydrocarbons
on the walls of the gas handling system.
Activated charcoal filters in the fuel line
and combustion-air line of the FID
remove any residual hydrocarbons in
these gases and lead to greatly improved
instrument stability. A single pump of
the proper design can circulate the air
sample through both the methane moni-
tor and the FID. No changes in the
hydrocarbon concentration of an air
sample appear to take place when it
passes through a diaphragm pump with
the interior Teflon coated and properly
cleaned copper tubing heated to approx-
imately 50°C.
A standard sensor (combustion cham-
ber plus electronics) for a FID provides
adequate sensitivity when operated
with a convenient fuel mixture of 40%
H2 + 60% He. The short-term peak-to-
peak noise (period less than 10 sec) can
be made less than the equivalent of 5
ppb of methane for the FID and less than
50 ppb of methane for the methane
monitor. Longer-term drift of the zero-
settings of the instruments normally
leads to uncertainties in the measure-
ments that are somewhat larger than
those imposed by noise unless the drift
is accounted for by flushing the sample
chambers once every few minutes with
zero-gas. Stability of the methane moni-
tor is improved greatly by controlling the
temperatures of the sample cell, band-
pass filter, and gas-filter cell of the
methane monitor.
Interference by H2O in the air limits
the accuracy of the methane monitor
unless most of this gas is removed from
the air before it enters the methane
sample cell. One acceptable method of
accomplishing this is to pump the air
through a Perma-Pure Dryer before it
enters the methane sample cell. Air
going to the FID should by-pass the
dryer to avoid possible adsorption of
some of the complex hydrocarbons on
the walls of the dryer. The H20 inter-
ference can also be accounted for by
measuring the H20 concentration in the
methane sample cell and applying a
correction based on interference data
obtained previously with samples of
H2O plus clean air.
Recommendations
Additional tests should be carried out
with the instrument under a variety of
laboratory and field conditions to gain
more information about the detailed
performance. After these tests have
been completed, a prototype instrument
should be designed and built to operate
on the same basic principles as the
present instrument. This instrument
should include a FID to measure THC
concentrations and a methane monitor
that employs gas-filter correlation tech-
niques.
The following features and procedures
are recommended for the prototype
instrument. Many of these features are
included in the present instrument and
have proven to be desirable; others are
recommended as a result of knowledge
gained while assembling and testing
the present instrument. Important fea-
tures of the present instrument that are
not mentioned below should be included.
1. Package both the FID and methan^
monitor into a single unit.
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2. Use the combustion chamber from a
commercially available FID with
combustion fuel of 40% H2 and 60%
He.
3. Pump sample air through heated
lines to both the methane monitor
and the FID with a single diaphragm
pump capable of producing pressures
up to 5 atm. Split the gas flow so that
the gas to the FID does not pass
through the methane sample cell.
Include in the line to the FID a small
"delay tank" so that at a given time
the FID is sampling air that entered
the inlet line over the same period of
time as the air in the methane sample
cell.
4. Pass the air to the methane sample
cell through a dryer such as a Perma-
Pure Dryer to remove most of the
H20 vapor and thus reduce the inter-
ference by this gas in the measure-
ment of methane concentration.
5. Employ activated charcoal filters, or
some substitute, in the lines for the
combustion-air and fuel.
6. Operate the sample cell of the meth-
ane monitor between 3 atm and 5
atm to increase sensitivity and reduce
interference due to residual H20.
This also improves the efficiency of a
dryer similar to the Perma-Pure
Dryer.
7. Decrease the volume of the methane
sample cell and use multiple-pass
optics in the cell to obtain a sample
path length between approximately
8m and 15m.
8. Shape the sample cell to reduce the
volume while passing approximately
the same amount of radiation in the
monitoring beam as the present
instrument.
9. Control the temperatures of: (a)
methane gas-filter cell, (b) spectral
bandpass filter for methane monitor,
(c) methane sample cell, and (d)
regulators, valves, tubing, etc., that
are parts of the FID. Heat gas lines,
including the dryer, to approximately
50°C; it is not necessary to control
the temperature of these lines.
10. Include electronics to measure di-
rectly the difference between the
output signals of the FID and methane
monitor; this voltage is proportional
to the NMHC concentration.
Da rre I I Burch is with Ford Aerospace & Communications Corporation Newport
Beach, CA 92660.
William McClenny is the EPA Project Officer (see below).
The complete report, entitled "Ambient Air Non-Methane Hydrocarbon Monitor,"
(Order No. PB 81 120-008; Cost: $6.50, subject to change) will be available
only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
; US GOVERNMENT PRINTING OFFICE. 1W1 -757-012/7121
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Agency Cincinnati OH 45268
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