Research on Chemical Intermediates

, Volume 20, Issue 3–5, pp 525–556 | Cite as

The use of tunable diode laser absorption spectroscopy for atmospheric measurements

  • H. I. Schiff
  • G. I. Mackay
  • J. Bechara
Article

Summary: Present Status and Future Developments

We have attempted to describe how tunable diode laser absorption spectroscopy can be applied to measurements of clean and polluted air. Pb salt diodes emit in the infrared region where most gases of atmospheric interest have their strong absorptions, providing the basis for a universal system. The high monochromaticity of these diodes have made TDLAS the most specific method available for unequivocal identification of atmospheric species, a particularly useful property for complex mixtures in polluted air or in automobile stack emissions and for serving as a comparison standard for other, less definitive methods. The combination of modulation and long-path techniques have permitted sensitivities in the parts per trillion range, required for measurements in remote, clean air or for minor, but important constituents in complex mixtures. The rapid tuning rate of these diodes, permitting real-time measurements in the fractions of second range, has been exploited in measurements from fast-flying aircraft, for eddy correlation flux measurements and for studying transients in automobile exhaust. Interesting advances are being made in the use of TDLAS systems for making real-time isotope ratio measurements.

Considerable improvements have been made in the operating temperatures and tuning ranges of commercially available diodes which have permitted the use of liquid N2 cryostats or Dewars. A miniature He cryostat cooling system has also been recently developed which weighs 7 kgm and requires only 75 watts of electrical energy. Commercial systems are now available which can fit in a standard instrument rack or small aircraft, and which can be operate automatically, without attendance for long periods of time. The price of these systems have also declined significantly.

The main disadvantage of the TDLAS for atmospheric measurements is the limited number of species that can be measured by the same diode. This limitation can be mitigated to some extent by the use of multiplexing techniques which permit simultaneous operation of a number of diodes. The tuning range of modern laser diodes are continuously being extended and is now sufficient in many cases to encompass strong absorption lines from 4 or 5 gases.

The high specificity of TDLAS depends on the molecule possessing resolved rotational-vibronic structure. Roughly speaking, this limits the method to molecules having less than 10 atoms or those having symmetrical structures. Extension to larger, asymmetric molecules has not yet been utilized although it is possible that fast scan techniques can be used to measure larger molecules which do not have completely resolved ro-vibrational spectra.

Another limitation is that the best specificity and sensitivity is achieved when measurements are made at sub-atmospheric pressure. This necessitates the use of sampling cells which can introduce surface effects for reactive and unstable molecules. The pressure broadening of some molecules, such as N20, CH4, O3, HCI, NO, HNO3 and NH3, are however, sufficiently small that measurement at atmospheric pressure is possible with open paths.

Increased interest is being shown in the application of laser diodes made of group III–V compounds to gas measurements. The attractive features of these diodes are that they operate at or near room temperature, while some are relatively inexpensive and of relatively high quality and power due to their wide-spread use in communication and consumer electronic industries. They operate in the near infrared region where absorption coefficients are some two three orders of magnitude lower than those in the fundamental, mid-infrared region. But high frequency modulation techniques can be used to regain some of the sensitivity so that, at present, detection limits approach 1–10% of the sensitivity achievable from the more complex and more expensive mid-infrared systems. In addition both the optical and electronic components are more readily available, less expensive, and much smaller which permits construction of relatively inexpensive, compact instruments. For a certain limited number of species, where ultra-high sensitivity is not required the NIR systems will provide advantages of size, simplicity and cost.

For a more universal and sensitive system the Pb salt, mid-infrared diode systems will continue to provide a superior and highly specific measurement system. Improvements both in quality and performance of the Pb-salt diodes and in the hardware and software to operate the system will result in continuing improvements in sensitivity, reliability and performance of these systems as well as in cost reduction.

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Authors and Affiliations

  • H. I. Schiff
    • 1
  • G. I. Mackay
    • 1
  • J. Bechara
    • 1
  1. 1.Unisearch Associates Inc.ConcordCanada

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