Quantifying Uncontrolled Landfill Gas Emissions from Two Florida ...

315 ppb. The number of calculated nitrous oxide concentrations that failed to meet the DQI
accuracy criterion in each data subset was recorded.
Overall, 180 data subsets were analyzed from this field campaign. Based on the DQI criterion set
forth for precision of ±10 percent, all of the data subsets were found to be acceptable for a
completeness of 100 percent. The range of calculated relative standard deviations for the data
subsets from this field campaign was 0.36 to 17.4 ppb, which represents 0.11- to 5.5-percent
RSD.
Each data point (calculated nitrous oxide concentration) in the data subsets was analyzed to
assess whether or not it met the DQI criterion for accuracy of ±25 percent (315 ± 79 ppb) for
path lengths less than 50 meters, ±15 percent (315 ± 47 ppb) for path lengths between 50 and
100 meters, and ±10 percent (315 ± 32 ppb) for path lengths greater than 100 meters. A total of
1569 data points were analyzed, and 1482 met the DQI criteria for accuracy, for a total
completeness of 94 percent.
5.2.3 Inter-comparison Study of OP-FTIR and OP-TDLAS Instruments
Operational difficulties were encountered in the field that resulted in scaling back the study to
the extent that the results are not considered reliable. Whenever two different types of
instruments are used (i.e., OP-FTIR and OP-TDLAS), field interlaboratory comparison is
recommended. This is to ensure that there is no potential bias between measurements. At each
survey area, the instruments were to be located diagonally across the survey area and operated
for ~30 minutes to collect methane concentration data across the same optical path.
The study was performed for Site #1 using the same optical path with a distance of ~200 meters.
Data were collected with the OP-FTIR for approximately 10 minutes and with the OP-TDLAS
for about 20 minutes. Overlapping data for the comparison were available for only 7 minutes. If
these data are fitted to a linear regression, the results indicated the OP-TDLAS is approximately
40% greater than the concentrations measured with the OP-FTIR. The very limited number of
measurements (n = 7) and the poor regression coefficient of determination (r 2 = 0.205) raise
question as to the validity of the inter-comparison results.
Although the instruments were deployed along an identical beam path, the difference in the
concentrations measured with both instruments may be due to a difference in the height of the
scanners, resulting in a difference in the height of the beam paths. The OP-FTIR scanner mount
is higher than the OP-TDLAS scanner mount, resulting in the OP-FTIR optical beam path being
higher than the OP-TDLAS beam path. The higher concentrations measured with the OP­
TDLAS may be due to the fact that the OP-TDLAS beam path was located closer to the surface
of the landfill cell. The field team personnel have extensive experience with use of both OP­
FTIR and OP-TDLAS instruments and have observed and documented through other studies that
the two instruments exhibit good comparability. (U.S. EPA, 2004; U. S. EPA, 2005a; U.S. EPA,
2005c; U.S. EPA, 2005e, U.S. EPA, 2005f. U. S. EPA, 2007).
Immediately prior to this field study, Boreal Laser, Inc. performed a bench top calibration
experiment with the OP-TDLAS instrument in a laboratory environment. The experiment
consisted of inserting a known concentration of methane into a calibration cell, and comparing
5-4