Douglas S. Baer, Manish Gupta, Thomas G. Owano. Los Gatos Research, 67 East Evelyn Avenue, Suite 3, Mountain View, CA 94041. We report on the ...
DEVELOPMENT AND TESTING OF HIGH-PRECISION ANALYZERS FOR MEASUREMENTS OF N2O AND CO, AND FOR MEASUREMENTS OF CH4, CO2, AND H2O Douglas S. Baer, Manish Gupta, Thomas G. Owano Los Gatos Research, 67 East Evelyn Avenue, Suite 3, Mountain View, CA 94041
Introduction
Methane (CH4)
Nitrous Oxide (N2O)
We report on the continued development of two continuous-flow gas analyzers (Figure 1), based on cavity enhanced laser absorption spectroscopy (e.g. Off-Axis ICOS), which accurately measure important atmospheric constituents over a wide range of mixing ratios. One instrument (N2O/CO/H2O Analyzer) uses a tunable quantum cascade laser operating near 4.56 microns to record measurements of carbon monoxide, nitrous oxide. and water vapor in ambient air. The other instrument (GGA-EP), uses two tunable diode lasers operating near 1.6 microns to record measurements of carbon dioxide, methane, and water vapor in ambient air. Both instruments require low-power (< 200 W, steady state) and no cryogens, thus enabling long-term field deployment. Measurements recorded at leading atmospheric monitoring laboratories in the US, Europe, and Asia indicate that both instruments exceed the WMO goals for measurements of these gases.
The CH4 measurement precision of the GGA-EP was determined by measuring a constant gas source for > 19 hours. The data (Figure 4) shows a 10-second precision (1 ) of ±0.41 ppbv, which improves with averaging and yields a 1-hour precision of ±0.07 ppbv. The instrument temperature is controlled to within ± 6 mK and is stable, despite ambient fluctuations. The accuracy of the unit was confirmed to be better than ±0.79 ppbv by NOAA researchers (Figure 5). Note that the instrument simultaneously measures water vapor concentration to report a dry mole fraction.
The N2O measurement precision of the N2O/CO/H2O analyzer was determined by measuring a constant gas source for > 8 hours. The data (Figure 7) shows a 10-second precision (1 ) of ±0.30 ppbv. The accuracy of the unit was confirmed to be better than ±0.11 ppbv by NOAA researchers (Figure 6)., The instrument simultaneously measures water vapor concentration to report a dry mole fraction.
Figure 4: GGA-EP measurements of a constant methane source for > 19 hours.
Figure 1: Off-Axis ICOS Analyzers for N2O/CO/H2O (left) and CO2/CH4/H2O (right).
Carbon Dioxide (CO2)
The CO2 measurement precision of the GGA-EP was determined by measuring a constant gas source for > 19 hours. The data (Figure 2) shows a 10-second precision (1 ) of ±0.07 ppmv, which improves with averaging and yields a 1-hour precision of ±0.02 ppmv. Note that the instrument temperature is controlled to within ± 6 mK and is stable, despite ambient fluctuations. The accuracy of the unit was confirmed to be better than ±0.08 ppmv by NOAA researchers (Figure 3). Note that the instrument simultaneously measures water vapor concentration to report a dry mole fraction.
Figure 7: Allan deviation plot for the N2O/CO/H2O analyzer developed from > 8 hours of continuous measurements of a constant gas source.
Figure 5: Accuracy of the GGA-EP CH4 reading as determined by researchers at NOAA.
Carbon Monoxide (CO)
The CO measurement precision of the N2O/CO/H2O analyzer was determined by measuring a constant gas source for > 8 hours. The data (Figure 7) shows a 10-second precision (1 ) of ±0.45 ppbv. The accuracy of the unit was confirmed to be better than ±0.43 ppbv by NOAA researchers (Figure 6). Again, the instrument simultaneously measures water vapor concentration to report a dry mole fraction.
Figure 2: GGA-EP measurements of a constant carbon dioxide source for > 19 hours.
Conclusions We have developed a GGA-EP and mid-infrared Off-Axis ICOS analyzer for the accurate quantification of CO2/CH4/H2O and N2O/CO/H2O respectively. These analyzers are capable of: • Measuring CO2 with a precision (1 , 1 hr) and accuracy exceeding ±0.02 ppmv and ±0.08 ppmv respectively. • Measuring CH4 with a precision (1 , 1 hr) and accuracy exceeding ±0.07 ppbv and ±0.79 ppbv respectively. • Measuring CO with a precision (1 , 10 seconds) and accuracy exceeding ±0.45 ppbv and ±0.45 ppbv respectively. • Measuring N2O with a precision (1 , 10 seconds) and accuracy exceeding ±0.30 ppbv and ±0.11 ppbv respectively. • Both analyzers make simultaneous measurements of water vapor to report dry mole fractions. • Both analyzers consume < 200 Watts (steady-state) and do not require cryogens, enable long-term field operation. • Both analyzers can be used for mobile monitoring and flight studies without further modification
Acknowledgements LGR gratefully acknowledges A. Crotwell, E. Dlugokencky, P. Novelli, and B. Hall (NOAA Earth Systems Research Laboratory, Boulder, Colorado, USA) for providing independent verification of the analyzers against NOAA standards. Figure 3: Accuracy of the GGA-EP CO2 reading as determined by researchers at NOAA.
Figure 6: Accuracy of the N2O (left) and CO (right) reading of the N2O/CO/H2O analyzer determined by researchers at NOAA.
