Lidar techniques

As discussed in connection with astronomical hnaging (Sect. 6.7), sodium fluorescence lidars have foimd important application for providing laser guide-stars for allowing an effective use of adaptive optics to enhance image quality through the turbulent atmosphere [10.89]. 

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 

As can be seen, all unknown parameters such as N y r), cTb and JCext(f) are eliminated upon division and only the absorption cross-sections at the two frequencies have to be known from laboratory measurements for an evaluation of the gas concentration N r) as a function of the distance R. It can also be seen that the method does not even require a uniform particle distribution, since any bumps in the curves due to particle clouds would be cancelled out. This is important since gaseous pollutants and particles frequently occur together. 

Mapping of a cross-section of an SO2 plume from a paper mill obtained by DIAL measurements. By multiplying the integrated SO2 content by the wind velocity normal to the vertical section a total flow of 230kg/h of SO2 was found [10.95] 

Mercury is a further gas which can have a geophysical origin apart from anthropogenic ones. It is the only pollutant which is present in the atmosphere in atomic form. Compared to molecules with a multitude of rotational-vibrational transitions, mercury in contrast exhibits only a single 

In order to measure the concentration of gaseous pollutants with lidar techniques, resonance absorption can be used in a similar way to the long-path-absorption method that was described above. However, in the lidar technique no retro-reflector is required and the scattering from particles is 

In Fig. 10.23 the absorption spectrum for SO2 in the wavelength region around 300 nm is shown. In practical DIAL measurements of SO2 the laser 

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Na is likely deposited in the upper atmosphere by meteors along with other metals (Clemesha et al., 1981) and distributed by solar winds (Happer et al., 1994). This atomic layer is "eaten away" at its bottom by chemical reactions (e.g. molecule and aggregate formation). Fe, Al, Ca are more abundant than Na, but the D2 transition is so strong that it provides the largest product of column density CNa and transition cross section, nominally 10 — 10" atoms/cm. The layer has been studied mostly with Lidar technique (Blamont and Donahue, 1961 Albano et al., 1970 Bowman et al., 1969 Sarrazin, 2001). 

Edner H, Paris GW, Sunesson A, Svanberg S. 1989. Atmospheric atomic mercury monitoring using differential adsorption LIDAR technique. Appl Opt 28 921. 

Spinhime, J. D., J. A. Reagan, and B. M. Herman, 1980. Vertical distribution of aerosol extinction cross section and inference of aerosol imaginary index in the troposphere by lidar technique, J. Appl. Meteorol., 19, 426-438. 

Application of LIDAR techniques to the measurement of nitrogen species has been very limited. Sensitivity seems to be the limiting factor. For N02 a detection limit of about 100 ppbv in the lower troposphere is reported for a resolution of 50 m along the laser path (16). 

There are two categories of remote sensing, active and passive. Passive techniques utilise electromagnetic radiation emitted from or transmitted through the atmosphere, the radiation source being for example the black body emission from the earth s surface or solar and stellar irradiances. The most critical part of a passive remote sensing instrument is its detector. In contrast, active remote sensing systems have their own radiation source and a detector, for example, radar and lidar techniques. 

Philbrick C.R. (2002). Overview of Raman lidar techniques for air pollution measurements in lidar remote sensing for industry and environmental monitoring. SPIE Proceedings, 4484, 136-150. 

A. Iwasaki, N. Akozbek, B. Ferland, Q. Luo, G. Roy, C.M. Bowden, S.L. Chin, A Lidar technique to measure the filament length generated by a high-peak power femtosecond laser pulse in air, Applied Physics B 76, 231 (2003)... 

J.P. Wolf, UV-DIAL-Lidar Techniques for Air Pollution Monitoring, in Encyclopedia of Analytical Chemistry , Ed. R.A. Meyers, Vol 3, pp 2226-2247, J. Wiley Sons, New-York (2000)... 

At that time no data was available for the thermochemical properties of NaO, so the suggestion was based on what turned out to be excellent chemical intuition. The first thermochemicaJ determination of the formation enthalpy of NaO was made in 1970, and it showed that Chapman s mechanism was exquisitely exoergic. Since the early nightglow observar tions and Chapman s analysis, the number of observations has increased exponentially due to the advent of rocket- and space-borne instrumentar tion, and the introduction of lidar techniques. These observations will be summarized in the following section. 

The advent of new intense laser sources allowed probing of additional metals, such as Fe, Ca and Ca+, and K, using the lidar techniques. All of the investigated layers exhibit sporadic events. Von Zahn and coworkers have improved the time resolution of their experiments to 1 s, allowing them to observe individual meteor trails and the associated local metal atom density increases. ... 

Gas phase metal ions have been observed at mid-latitudes by several investigators and appear to be related to the layers of metallic atoms present in the upper mesosphere and lower thermosphere. Meteoric ablation is believed to be the dominant source of these metals (Plane, 1991). The presence of atomic sodium (Na) was first detected from its noctural emission by Slipher (1929) and is discussed by Chapman (1939). In recent decades several other metallic atoms (e.g., K, Li, Fe, Ca, etc.) and ions (e.g., Mg+, Fe+, A1+, Na+, Ca+) have been identified. These species are present in permanent layers from 85 to 110 km, which have been studied in detail using resonance fluorescence lidar techniques (see e.g., Granier et al., 1989 Bills and Gardner, 1990 Qian and Gardner, 1995). 

Figure 7.15. Evolution between 21 30 and 23 00 of the Ca+ density (expressed in number of ions per cubic centimeter) measured by resonant lidar technique between 92 and 106 km on October 27, 1983. The time integration for the lidar observation is 8 minutes. From Granier et a/.(1989).

J.P. Wolf, H.J. Kolsch, P. Rairoux, L. Woste, Remote detection of atmospheric pollutants using differential absorption LIDAR techniques, in Applied Laser Spectroscopy, ed. by W. Demtroder, M. Inguscio (Plenum, New York, 1991), p. 435... 

The standard lidar technique to measure O3 in the stratosphere fails in the presence of heavy concentrations of aerosols, e.g. after a volcanic emption. Under... 

In the Raman lidar technique, both the elastically and inelasticaUy scattered radiation contents are recorded. Elastically scattered radiation depends on both molecular and particulate species in contrast, inelastic scattering depends only on molecular scattering. The ratio of the two signals yields a parameter called the aerosol scattering ratio (ASR), which constitutes a rough measure of the concentration of aerosols. An example for an ASR measurement is shown in Figure 28.27. From the available data one can extract both the extinction and back-scattering coefficients. 

F.J. Lehmann, S.A. Lee, C.Y. She Laboratory measurements of atmospheric temperature and backscatter ratio using a high-spectral-resolution lidar technique. Opt. Lett. 11, 563 (1986)... 

E. Zanzottera Differential absorption lidar techniques in the determination of trace pollutants and physical parameters of the atmosphere. Crit. Rev. Anal. Chem. 21, 279 (1990)... 

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 requirements for very sharp and well-defined laser frequencies when using the DIAL teclmique can be relaxed by using a gas correlation lidar technique [10.108], where the back-scattered radiation from a broadband transmitter is split up in two channels one supplied with a cell filled with optically thick gas which only passes the off-resonance components for the signal, and one channel where the total signal (on and off) is detected. If the cell gas is present in the atmosphere, a differential lidar effect is obtained using many absorption lines simultaneously (compare Fig. 6.77). This technique... 

H. Edner, G.W. Paris, A. Sunesson, S. Svanberg Atmospheric atomic mercury monitoring using differentizd absorption lidar techniques. Appl. Opt. 28, 921 (1989)... 

R. Ferrara, B.E. Maserti, H. Edner, P. Ragnarson, S. Svanberg, E. Wal-Under Mercury emi ions into the atmosphere from a chlor-alkali complex measm ed with the lidar technique. Atmos. Environ. 26A, 1253 (1992)... 

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