Laser Monitoring of the Atmosphere

A detailed understanding of our atmosphere, and of the various photochemical or collisional processes which determine the atmospheric composition, is of fundamental importance for mankind. Since in densely populated industrial areas air pollution has become a serious problem, the study of pollutants and their reactions with natural components in the atmosphere has become an urgent demand. Various techniques of laser spectroscopy can be successfully used in atmospheric and environmental research direct absorption measurements, laser-induced fluorescence, spontaneous Raman scattering, or coherent anti-Stokes Raman spectroscopy (CARS see Chap.9) can be utilized either for in situ measurements or for remote sensing. 

An obvious way to determine the concentrations n. of atomic or molecular constituents is the measurement of the total attenuation which a laser beam experiences along a known distance L through the atmosphere. Species with density n. and absorption cross section o (m) result in an absorption coefficient 

Besides absorption losses the laser light suffers Mie scattering by aerosols and Rayleigh scattering by molecules in the atmosphere. The total attenuation dl of a monochromatic laser beam at frequency w with intensity 1(0) = Iq is then dl = Iq - I(L) where 

In order to separate the contribution of a selected molecular species, the laser frequency cj has to be properly chosen to coincide with an absorption line of only one molecular species. If this is not possible, because there may be always overlapping bands from other molecules, the measurement has to be performed at several frequencies. Since the Rayleigh and Mie scattering cross sections do not differ appreciably in a frequency interval corresponding to the linewidth Ao) of a molecular absorption line, the absorption coefficient and with it the density n. of a specified absorbing molecular component can be determined by measuring the difference in attenuation - dI(a) + Ao)) when the laser is alternatively shifted into and out 

Because most polar molecules present in the atmosphere can be identified by their characteristic vibrational-rotational lines in the near infrared (called the fingerprint region of molecular spectroscopy) infrared lasers can be used. In particular multiline lasers, such as HF, DF, 6629 or CO lasers with their numerous lines are well suited for simultaneous detection of several atmospheric constituents [14.18], 

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E. D. Hinkley, ed.. Laser Monitoring of the Atmosphere, Springer-Vedag, Berlin, 1976. 

Volume 14 Laser Monitoring of the Atmosphere Editor E.D.Hinkley... 

H. Inaba In Laser Monitoring of the Atmosphere ed. by E.D. Hinkley, Topics Appl. Phys., Vol.l4 (Springer, Berlin, Heidelberg 1976)... 

We will discuss two active remote-sensing techniques for the atmosphere — the long-path absorption technique and the lidar technique. However, we will first consider a passive technique, in which lasers play an important part in the detection scheme. This optical heterodyne technique is even more frequently used for signal recovery in connection with the active optical remote-sensing methods. The field of laser monitoring of the atmosphere is covered in several monographs and articles [10.70-10.76]. 

The best characterized ion recombination systems are those involving irradiated rare gases. In the mid 1970 s rare gas-monohalide systems were extensively studied, due to their new-found application in u.v. exciplex lasers. Pulsed electron irradiation of these systems was the only major excitation method, as it allowed investigation under realistic laser pressure conditions of several atmospheres gas pressure. Typically these systems were investigated by monitoring of the time dependence of their characteristic peak fluorescence, as given in Table 12. 

The back scattering can be caused by the Raman process. Because of the weakness of this kind of scattering, high-power laser beams are normally required even for the monitoring of major atmospheric species. Here light back scattered with a characteristic Stokes frequency sliift is detected. The... 

The methods and means for ecological diagnostics make rapid strides among all the NDT and TD developing areas. To provide the atmosphere monitoring recently the good results were achieved in the development of surface-acoustics wave sensors (SAW), laser measuring systems, infrared detectors and systems based on other physical principles. 

It must also be emphasized that the major mass of a heterodispersed aerosol may be contained in a few relatively large particles, since the mass of a particle is proportional to the cube of its diameter. Therefore, the particle-size distribution and the concentration of the drug particles in the exposure atmosphere should be sampled using a cascade impactor or membrane filter sampling technique, monitored using an optical or laser particle-size analyzer, and analyzed using optical or electron microscopy techniques. 

Laser-induced electronic fluorescence. Two devices reported recently look very promising for continuous atmospheric monitoring. Sensitivities of 0.6 ppb for nitrogen dioxide and ppb for formaldehyde are claimed. Careful attention to possible interference from other species is necessary. Detection of the hydroxyl radical in air ( 10 molecules/cm ) has been claimed for this technique, but it has been pointed out that this concentration seems much too high, especially because the air had been removed fix>m the sunlight 6 s before analysis spurious effects, such as photolysis of the ozone in the air by the laser beam and two-photon absorption by water vapor, might have been responsible for the hydroxyl radical that was observed. 

The development of lasers has opened up several new techniques for monitoring pollutants in the atmosphere. Sensitivities down to the parts-per-billion range are claimed, and continuous monitoring is possible. The photoionization mass spectrometer has been developed as a sensitive detector for fi radicals in the gas phase. A high-resolution mass spectrometer coupled to a computer is capable of detecting up to 300 compounds in air, both in particulate form and in the gas phase. 

Schiff, H. I G. I. Mackay, and J. Bechara, The Use of Tunable Diode Laser Absorption Spectroscopy for Atmospheric Measurements, in Air Monitoring by Spectroscopic Techniques (M. W. Sigrist, Ed.), Chemical Analysis Series, Vol. 127, pp. 239-333, Wiley, New York, 1994b. 

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