Ozone spectroscopy with a CO(2) waveguide laser

Performance Characterization of a Fully Transportable Mid-Infrared Laser Heterodyne Radiometer (LHR)

A fully transportable laser heterodyne radiometer (LHR), involving a flexible polycrystalline mid-infrared (PIR) fiber-coupling system and operating around 8 µm, was characterized and optimized with the help of a calibrated high temperature blackbody source to simulate solar radiation. Compared to a mid-IR free-space sunlight coupling system, usually used in a current LHR, such a fiber-coupling system configuration makes the mid-infrared (MIR) LHR fully transportable. The noise sources, heterodyne signal, and SNR of the MIR LHR were analyzed, and the optimum operating local oscillator (LO) photocurrent was experimentally obtained. The spectroscopic performance of the MIR LHR was finally evaluated. This work demonstrated that the developed fully transportable MIR LHR could be used for ground-based atmospheric sounding measurements of multiple trace gases in the atmospheric column. In addition, it also has high potential for applications on spacecraft or on an airborne platform.

Transportable mid-infrared laser heterodyne radiometer operating in the shot-noise dominated regime

A transportable laser heterodyne radiometer (LHR) based on an external cavity quantum cascade laser, operating in the mid-infrared (mid-IR) around 8 µm, was developed for ground-based remote sensing of multiple greenhouse gases. A newly available novel flexible mid-IR polycrystalline fiber was first exploited to couple solar radiation, real-time captured using a portable sun-tracker, to the LHR receiver. Compared to free space coupling of sunlight, the technique usually used nowadays in the mid-IR, such fiber coupling configuration makes the LHR system readily more stable, simpler, and robust. Operation of the LHR with quasi-shot-noise limited performance was analyzed and experimentally achieved by optimizing local oscillator power. To the best of our knowledge, no such performance approaching the fundamental limit has been reported for a transportable LHR operating at a long mid-IR wavelength around 8 µm. CH₄ and N₂O were simultaneously measured in the atmospheric column using the developed mid-IR LHR. The experimental LHR spectrum of CH₄ and N₂O was compared and is in good agreement with a referenced Fourier-transform infrared spectrum from the Total Carbon Column Observing Network observation site and with a simulation spectrum from atmospheric transmission modeling.

Temperature Dependence of Air-Broadening and Shift Coefficients of O3 Lines in the nu1 Band

High-resolution Fourier transform absorption spectra of ozone broadened by dry air have been recorded at a number of temperatures from -63&deg;C to 29&deg;C. Using a multispectrum nonlinear least-squares procedure, we fit 29 of these spectra simultaneously to determine the air-broadening and shift coefficients and their temperature dependences for 450 lines in the 9-&mgr;m region; most of these belong to the nu1 band. Partial air-broadening results were obtained for 104 additional lines, and room-temperature self-broadening coefficients were also determined for most of the 554 lines measured. These results cover a wide range of rotational quantum numbers, particularly in the R branch, with J" </= 55 and Ka" </= 12. The variation of the retrieved broadening and shift parameters with the rotational quantum numbers has been examined; particularly interesting behavior of the broadening coefficients is noted as the value of Ka" approaches that of J". The broadening and shift coefficients compare well with previous room-temperature measurements in the nu1 and other bands. The temperature-dependence results are also consistent (within the stated uncertainties) with the few previous measurements of the temperature dependence of air- and N2-broadening coefficients in other O3 bands, but disagree with the mean value given in the HITRAN compilation. Copyright 1997Academic Press

Measurements of pressure-broadening coefficients of NO and O3 using a computerized tunable diode laser spectrometer

Foreign-gas broadening coefficients have been measured for selected lines of ozone in the 9.2-microm region and for several R -branch lines of nitric oxide in the 5.4-microm region using a computerized tunable diode laser spectrometer. The data analysis showed the importance of fitting a Lorentzian line shape out to several times the halfwidth to obtain a correct value of the broadening coefficient. The measured broadening coefficients of nitric oxide were in good agreement with those obtained by Abels and Shaw. The results of the analysis of eleven lines in the nu(1) band and five lines in the nu(3) band of ozone show a transition-dependent broadening coefficient. The average value of the foreign-gas broadening coefficients for the measured nu(1) and nu(3) lines are 0.075 and 0.073 cm(-1) atm(-1), respectively.

AFGL atmospheric absorption line parameters compilation: 1980 version

A new version of the AFGL atmospheric absorption line parameters compilation is now available. Major modifications since the last edition of 1978 include the updating of the strongest bands of water vapor, updated line positions for carbon dioxide, improved ozone parameters in the 5- and 10-microm regions, and updated and additional data for methane in the 3.5- and 7.7-microm regions. The atlas now contains over 159,000 rotational and vibration-rotation transitions from 0.3 to 17,880 cm(-1).

Laser absorption spectrometer: remote measurement of tropospheric ozone

The laser absorption spectrometer (LAS) is an airborne instrument able to perform remote measurement of trace atmospheric gases. The instrument uses a pair of carbon dioxide laser-heterodyne receiver systems that are directed downward from an airplane and determines gas concentration by the differential absorption method. Configured to measure tropospheric ozone, the LAS has been used in conjunction with another instrumented aircraft to test the accuracy of this method of measurement. A complete description of the instrument is presented, and the results of extensive flight testing are summarized.

High sensitivity point monitoring of ozone, and high resolution spectroscopy of the nu(3) band of ozone using a tunable semiconductor diode laser

Accurate measurements of the wavenumbers of more than 100 lines in the nu(3) band of ozone have been made using a tunable semiconductor diode laser and a temperature-stabilized germanium etalon. The accuracy for the line centers is estimated to be within 3-10 MHz and compares well with the accuracy attained using more complicated heterodyne techniques. The relevance of these results to ozone monitoring instruments based on CO(2) lasers is discussed. The tunability of the diode laser is also exploited to monitor ambient concentrations of ozone in the presence of interfering molecular species. A detection sensitivity of ~0.5 ppb of ozone is demonstrated.

Pressure and temperature dependence of Freon-12 absorption coefficients for CO(2) waveguide laser radiation

Freon-12 absorption coefficients for the P(22) through P(48) CO(2) waveguide laser transitions of the 00 degrees 1-(10 degrees 0, 02 degrees 0)(I) band are presented. For pressures below 10 Torr and a spectral resolution of 0.1 MHz, the absorption coefficients show considerable spectral structure within +/-250 MHz of the CO(2) laser line center; but individual absorption lines are only resolved for pressures below 100 microm. The absorption coefficients around the CO(2) waveguide laser lines are shown to be spectrally structureless as well as pressure independent above 20 Torr. The temperature dependence of the absorption coefficients are presented for the first time, and they are shown to be linearly dependent on temperature.

Ozone Monitoring with an Infrared Heterodyne Radiometer

Measurements of the total burden and of the concentration versus altitude profiles of ozone have been made with a ground-based heterodyne radiometer at Pasadena, California. The measurements were made in the 9.5-micrometer wavelength region, where a strong ozone infrared absorption band exists. The radiometer measured solar absorption at selected wavelengths, with a spectral resolution of 0.001 reciprocal centimeter, equivalent to the half-width of an ozone absorption line at the 10-millibar altitude level. A carbon dioxide laser served as the local oscillator. This technique can be used to gather important data on both tropospheric and stratospheric ozone, which are not readily accessible with other remote-sensing techniques.