HR: 14:55h
AN: A53G-04    [Abstracts]
TI: Long-Term Continuous Isotopic Ratio Measurements of Atmospheric CO2 Using a 4.3 Micron Pulsed Quantum Cascade Laser
AU: * Nelson, D D
EM: ddn@aerodyne.com
AF: Aerodyne Research, Inc., 45 Manning Rd., Billerica, MA 01821, United States
AU: McManus, J B
EM: mcmanus@aerodyne.com
AF: Aerodyne Research, Inc., 45 Manning Rd., Billerica, MA 01821, United States
AU: Herndon, S C
EM: herndon@aerodyne.com
AF: Aerodyne Research, Inc., 45 Manning Rd., Billerica, MA 01821, United States
AU: Zahniser, M S
EM: mz@aerodyne.com
AF: Aerodyne Research, Inc., 45 Manning Rd., Billerica, MA 01821, United States
AB: Real time methods to monitor the stable isotopic ratios of carbon dioxide are needed to quantify the sources and sinks of this centrally important greenhouse gas. This is an extreme instrumental challenge since the ratios need to be measured with an accuracy of at least one part in ten thousand or 0.1 ‰. We use tunable infrared laser differential absorption spectroscopy with pulsed QC lasers (QC-TILDAS) to address this challenge. QC lasers are attractive optical sources due to: 1) absence of cryogenic fluids, 2) stable single mode spectral output, 3) optical simplicity. Our optical system employs a dual-cell arrangement with ambient air flowing through the sample cell and a reference gas flowing through the reference cell. Both 13C/12C and 18O/16O ratios can be obtained in a single spectral window near 2310 cm-1. Spectral analysis is applied to the ratio of the sample and reference spectra, canceling correlated noise components. Because the absorption lines of the isotopic species have different temperature dependences, temperature stability is critical. The optical system is temperature controlled and the two multipass cells are thermally coupled and individually monitored with a precision of 1 mK. The ratio technique compensates for temperature sensitivity, as well as for drifts in laser line width, frequency, tuning rate and power variation. The long term stability of the thermally stabilized dual-cell QCL system has been evaluated using the Allan variance technique. The Allan plot shows a 1-sec RMS noise of 0.2 per mil, and a minimum RMS noise of 0.03 per mil after 300 sec integration using a liquid nitrogen cooled detector. We have used this instrument to perform long term (more than 6 months) monitoring of ambient air from the roof top of our laboratory. The data show variations in R13 and R18 that follow diurnal cycles. Winter data show clear correlations with morning and evening signatures of local automobile traffic. Data collected during spring are distinctly different and seem to show evidence of plant respiration.
DE: 0315 Biosphere/atmosphere interactions (0426, 1610)
DE: 0322 Constituent sources and sinks
DE: 0325 Evolution of the atmosphere (1610, 8125)
DE: 0394 Instruments and techniques
DE: 1610 Atmosphere (0315, 0325)
SC: Atmospheric Sciences [A]
MN: 2007 Fall Meeting