EPA/600/A-96/067
UNCERTAINTY ESTIMATE FOR OPEN-PATH REMOTE SENSING
OF FUGITIVE EMISSIONS
James Flanagan
Center for Environmental Monitoring and Quality Assurance
Research Triangle Institute
Research Triangle Park, NC 27709
Richard Shores, Susan Thorneloe
Air Pollution Prevention and Control Division
National Risk Management Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
ABSTRACT
Open-path remote sensing techniques such as Fourier transform infrared (FTIR)
spectrometry offer a powerful approach for determining emission rates from line and area
pollution sources. Typically, a photon beam downwind of the source intersects the pollutant
plume, and a characteristic such as optical absorption is measured. The path-integrated
concentration is calculated from this measurement. The emission rate can then be estimated using
a tracer gas reference or by dispersion modeling.
Open-path monitoring has important advantages over conventional methods for measuring
fugitive emissions. However, a different set of design and quality assurance (QA) considerations
must be addressed in developing the measurement and data analysis protocols. Failure to consider
the method's unique characteristics can impair the accuracy and precision of the results obtained
from the calculation of fugitive emissions. This paper will discuss the following quality-related
issues, which were found to be critical in calculating fugitive emissions:
•	length and location of the optical path relative to the source,
•	placement of the tracer gas release point, and
•	meteorological measurements.
INTRODUCTION
The Air Pollution Prevention and Control Division (APPCD) conducted a greenhouse gas
(GHG) measurement program during the summer of 1995. The purpose of this program was to
develop a better estimate of GHGs being emitted from anaerobic lagoons commonly used for the
treatment of human, animal, and industrial wastes. This measurement program was undertaken by
EPA because of the scarcity of field measurements to confirm estimated GHG emission rates from
these sources. Data from the program can also be used to estimate the level of uncertainty that
can be assigned to these emission rates.
The APPCD Quality Assurance Staff conducted an audit at one of the GHG measurement
sites, a waste water treatment facility in Texas. The waste water treatment facility, shown in
Figure 1, included one aerobic and four anaerobic lagoons. The anaerobic lagoons were
approximately 200 by 300 feet, aligned north to south. The total anaerobic lagoon area, including
berms between and around the lagoons, was approximately 1000 by 350 feet. The FTIR beam
was 570 feet long and was placed 175 feet downwind of lagoon No. 4. Only the anaerobic

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lagoon emissions were measured at this site.
The tracer gas reference technique was used to estimate emission rates. The auditors utilized
a second tracer gas to assess the effects of location, wind speed, and direction on the calculated
emission rates. This paper presents the results of the audit, as they relate to the open-path FTIR
method.
METHODS
The GHG emission estimates were determined using open-path FTIR, supplemented by tracer
gas (SF6) releases and meteorological measurements. SF6 was released from between lagoons
No. 2 and 3, as shown in Figure 1. The emission rates for GHGs (ERoHO) were calculated from
the known SF6 release rate (ER^. The measured concentrations of GHGs (Cqho) and SF6 (C^)
were determined as shown in equation (1):
ER,
ER
SF6
'GHG
GHG
(1)
'SF6
The FTIR was operated continuously, but valid data were acquired only when the
meteorological conditions were acceptable. Meteorological equipment was also installed and
operated at the site. Acceptable meteorological conditions were defined by the wind speed and
direction. Wind speed indicated that there was sufficient mixing of the plume, and wind direction
indicated that the entire plume was captured by the FTIR beam. The acceptable meteorological
criteria had been defined before beginning the field measurement program.
The audit evaluated the FTIR data by release of an audit tracer gas that was different than the
tracer gas used and not emitted by the lagoons. The audit tracer gas was released to the
atmosphere through a dry gas meter connected to a rotameter. The rotameter was used to set the
approximate release rate and the exact release rate was calculated from the change in volume over
time, as indicated by the dry gas meter. The audit tracer release rate was maintained nearly
constant during the audit. The average release rate from all five locations was calculated to be 0.5
g/sec with a coefficient of variation of 9.7%.
Audit tracer gas was released from five locations within the waste water treatment lagoon
area: four comers and the center (SE, SW, CTR, NE, NW). Audit tracer gas was released from
only one point at a time. The release rate of the audit tracer gas was known only to the auditors.
The emission rate of audit tracer gas was calculated using equation (1), the same equation used to
calculate the GHG emissions. The calculated emission rate of audit tracer was then compared to
the known release rate.
RESULTS
It was found that releasing the audit tracer gas as near as possible to the SF6 release location
(CTR) resulted in good agreement between the calculated emission rates and the known release
rates. The FTIR beam was located downwind (North) of all the release points. Therefore, the
2

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release points on the upwind side (SE & SW) were farthest from the IR beam; the downwind
points (NE & NW) were nearest the IR beam. Releases from points upwind of the SF6 release
location (SE & SW) were biased low. Releases from points downwind of the SF6 release location
(NE & NW) were biased high. The variability of the downwind locations was also markedly larger
than for releases upwind, both on absolute and relative bases. These data are summarized in
Table 1.
Table 1. Results of Audit Tracer Gas Measurements
Location
Number of
Valid
Observations
Calculated Audit
Tracer Emission
Rate
g/sec
Bias*
					
Std. Error of
Estimate for
Audit Tracer
Emission Rate
g/sec(%)
Center (CTR)
5
0.49
-2.0%
0.044 (9%)
Upwind (SE)
12
0.32
-36%
0.178 (13%)
Upwind (SW)
3
0.19
-62%
0.007 (4%)
Downwind (NE)
7
8.8
+1660%
3.32 (38%)
Downwind (NW)
4
1.5
+200%
0.37 (25%)
*Bias is calculated assuming an average audit tracer gas release rate of 0.5 g/sec with a coefficient
of variation ([s/mean] x 100) of 9.7 %.
The quality of the meteorological data, particularly wind direction, can be critical. On one
occasion, tracer gas was released while the measurement contractor indicated that the
meteorological conditions were acceptable. As previously discussed, the wind direction must be
such that the entire plume is captured within the IR beam; however, during this release of audit
tracer, the FTIR analysis did not indicate any audit tracer gas. Further investigation showed that
the wind direction sensor had been installed with a bias of 25 degrees. This bias was great enough
to cause the audit tracer plume to miss the IR beam entirely, even though the SF6 tracer plume
was being captured completely. Emissions from portions of the lagoon near this audit tracer
release point were undoubtedly also being lost.
The auditing reference for a true north was determined using solar noon. Another reference
to a true direction was obtained from a National Oceanic and Atmospheric Administration
(NOAA) weather station near the field site. The NOAA wind direction data confirmed that there
was a significant bias in the wind direction data. The wind direction sensor alignment was
corrected during the audit.
3

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DISCUSSION
Placement of the reference tracer release point and of the IR beam is important in minimizing
bias in the total measurement. The audit tracer data in the table above show excellent agreement
when the audit tracer gas is released near the SF6 tracer (CTR location); however, the upwind and
downwind release points are biased low and high, respectively. This can introduce a significant
bias in the overall estimate of emissions from the facility. A crude estimate of how much this bias
would affect the emission rate for the entire facility was calculated based on a simple weighted
average using the upwind (SE & SW), center (CTR), and downwind (NE & NW) results. This Is
shown in equation (2).
ERavg . 0-32+0-19*2(0.49) *8.8 ¦» 1.5 . , „
6
This represents a total bias approaching 300%.
The effect of distance (between the tracer source and the IR beam) is to cause the portions of
the lagoon nearest the IR beam to be greatly overrepresented in the calculated emission rate for
the source. This distance effect would also cause the portions of the lagoon farthest from the IR
beam to be underrepresented, but this is a much smaller effect, and may be within experimental
error. Figure 2 provides a graphical representation of the calculated emission rates versus
distance from the IR beam. Moving the beam downwind helps to linearize the distance effect,
which would reduce the distance-dependent bias seen here.
There is also a major effect when meteorological sensors are not properly installed and
oriented. Accurate determination of wind speed and direction is critical in the calculation of
accurate emission rates. These meteorological measurements (speed and direction) should be
assured with the same level of certainty as is given to the FTIR measurements. In particular,
missing portions of the source or tracer gas emissions due to improper orientation can introduce
bias and degrade precision.
CONCLUSION
FTIR data quality can be improved by more appropriate placement of the IR beam. Based on
audit tracer results, moving the IR beam farther downwind would have produced a marked
reduction in bias for this site. Increased downwind distance would also provide a better mixed
plume, which would reduce the variability in concentration. The IR beam should also be long
enough to accommodate normal wind shifts. Meteorological sensors should be carefully aligned
and referenced to the orientation of the source. Finally, spreading the tracer gas release points
over more of the lagoon area should also improve the resultant data quality. However, only a
site-specific check with a second tracer that evaluates plume capture throughout the area source
can verily that the siting is acceptable.
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U1
Infra-
red
beam
fence
bom
Legend
0 - Ethylene Release Points
• • SFg Release Points
V
troretlector
' 1 fv1;	-
x	' >w
fence
fence

Truck
©
\
Met Tower
fence
>, * T *<-1 Tt
/*> ¦
berm
fence
<=>
South
Wind
Plant and Equipment
Area
fence
Figure 1. An overview of the waste water treatment facility and
location of the IR beam.
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200
•r "v,:
% 15 * v*
Average ER = 1.97 g/sec
^ 1' V< *
x .v,: f *¦
/ r. « 4	? ' V '
kActual E.R. = 0.50 g/sec
600
DISTANCE TO IR BEAM (ft)
1000
Figure 2. Graphical representation of calculated audit tracer
emission rates versus distance from the IR beam.
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MD,.D, „TD -d 11Q TECHNICAL REPORT DAT' npQfi1 qAODQ
NRMRL-RTP-P 119 (Ptanenodhuouctionson thertvene btfor iifti.¦
1. REPORT NO. MlM-. 2. 1111
EPA/600/A-96/067 1111
iiniiiiiiiiii
UnceV* aintyT£ sEtimate for Open-path Remote Sensing
of Fugitive Emissions
S. REPORT OATE
6. PERFORMING ORGANIZATION CODE
7. AUTHORISE
James Flanagan (RTI), and Richard Shores and
Susan Thorneloe (EPA, NRMRL-RTP)
B. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME ANO ADDRESS
Research Triangle Institute
P. O. Box 12194
Research Triangle Park, North Carolina 27709
10. PROGRAM ELEMENT MO,
11. CONTRACT/GRANT NO.
68-D3-0045
12. SPONSORING AGENCY NAME ANO ADORESS
EPA, Office of Research and Development
Air Pollution Prevention and Control Division
Research Triangle Park, NC 277U
13. TYPE OF REPORT AND PERIOD COVERED
Published paper; 8/95- 5/96
14. SPONSORING AGENCY CODE
EPA/600/13
i§. supplementary notes aPPCD project officer is Richard C. Shores, Mail Drop 91, 919-
4983. Presented at AWMA Conference, Research Triangle Park, NC. 5/7-9/96,
is. abstract The paper discusses three quality-related issues, found to be critical in
calculating fugitive emissions: (1) length and location of the optical path relative to
the source; (2) placement of the tracer gas release point; and (3) meteorological
measurements. Open-path remote sensing techniques, such as Fourier transform
infrared (FTIR) spectrometry, offer a powerful approach for determining emission
rates from line and area pollution sources. Typically, a photon beam downwind of
the source intersects the pollutant plume, and a characteristic such as optical ab-
sorption is measured. The path-integrated concentration is calculated from this
measurement. The emission rate can then be estimated using a tracer gas reference
or by dispersion modeling. Open-path monitoring has important advantages over con-
ventional methods for measuring fugitive emissions. However, a different set of
design and quality assurance considerations must be addressed in developing the mea-
surement and data analysis protocols. Failure to consider the method's unique char-
acteristics can impair the accuracy and precision of the results obtained from the
calculation of fugitive emissions.
17. KEY WORDS AND DOCUMENT ANALYSIS
*. DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
c COSATi Field/Group
Pollution Infrared Spectroscopy
Estimating
Probability
Emission
Remote Sensing
Fourier Transformation
Pollution Control
Stationary Sources
Uncertainty
Fugitive Emissions
Open-path Remote Sen-
sing
13 B
14G
12A
14B
IB. DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS (ThisReport)
Unclassified
21. NO. OF PAGES
20. SECURITY CLASS (This page)
Unclassified
73. PRICE
EPA Fotm 1220-1 (3-73)
q

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