Long-Path Absorption Techniques

The principle of long-path absorption techniques is illustrated in Fig. 10.17. A laser beam is transmitted continuously into the atmosphere against a corner-cube retro-reflector (Fig. 6.21) that is placed at a distance of up to 10 km. The reflected beam is received by an optical telescope that is placed at the site of the laser and is directed towards the retro-reflector. The received light intensity is measured photo-electrically as a function of the laser wavelength. The absorption spectrum of the atmosphere between the laser and the retro-reflector is then recorded and the mean concentrations Nj of pollutant molecules can be determined using the Beer-Lambert relation 

Here PtCt ) is the received light intensity and Po(t ) is the intensity that would have been received in the absence of atmospheric absorption. a u) is 

In a variant of the long-path absorption technique, radiation from a diode laser or a light-emitting diode is transmitted fibre-optically to re- 

The absorption spectrum of the atmosphere between the laser and the retro-reflector is then recorded and the mean concentrations Nj, of pollutant molecules can be determined using the Beer-Lambert relation 

The IR region can also be reached by difference-frequency generation between two diode lasers operating in the red and near-IR regions [10.80]. For increased efficiency, a periodically poled litliium niobate (PPLN) crystal (Sect. 8.6) can be used. A set-up for such measurements is shown in Fig. 10.19 

The recordings in Figs. 10.19 and 10.20 were obtained by averaging a large number of fast sweeps. In this way the detrimental influence of atmospheric turbulence, etc. can be eliminated, as in doas measurements. If the optical absorption is very small an increased signal-to-noise ratio can be obtained if a modulation teclmique is employed. If the modulation frequency imposed on the diode-laser drive current is low compared to the absorption linewidth, the driver wavelength is modulated and the signal can be picked up by a lock-in amplifier as illustrated in Fig. 10.21a. The technique is referred to 

Set-up for sum-frequency generation between a blue and a red diode laser to achieve the 254 nm mercury line upper left). The mercury absorption spectra recorded at low pressure for natural isotopic mixture (a) and isotopically enriched samples in (6) and in (c), are also included in the right part 

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In addition, CH3 and OH radical concentration profiles are depicted in Fig. 9 which were measured using the in situ long path absorption technique described in Ref. 61(a). The OH radical profile was determined from... 

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]. 

In a variant of the long-path absorption technique, radiation from a diode laser or a light-emitting diode is transmitted fibre-optically to remotely located multi-pass absorption cells (Sects. 6.5.6, 9.2.1) and the partially absorbed beam is sent back to the measurement system also using fibre-optics. Using such a system, wliich operates at short IR wavelengths (1 to 2 pm) at which... 

A. Minato, T. Kobayashi, N. Sugimoto Laser long-path absorption technique for measuring methane using gas correlation method. J. Appl. Phys. 

Mercury and those elements (antimony, arsenic, bismuth, germanium, lead, selenium, tellurium and tin) which form volatile covalent hydrides may be separated from the matrix by vapour generation. The use of tin(II) chloride to generate elemental mercury and its subsequent aeration into a long-path absorption cell with silica windows has been described elsewhere in this book, as has the use of sodium borohydride to produce hydrides which are swept to a flame or heated tube for atomisation. This approach is far more successful for mercury than for the other elements, as the hydride generation technique is subject to interference from a large number of transition metals and oxyanions. 

Pure diacetyl, dibutyryl, and p Tuvic acid have all been reported to produce ozone on photolysis in oxygen. First attempts to verify this result with use of the long-path infrared technique were frustrated by appearance in the spectrum of a band of methanol which masked the absorption resulting from ozone. This difficulty was overcome by the use of the following technique ... 

As has been shown in various experimental studies the excimer laser photolysis/ laser long path absorption/cw LIF technique developed in our laboratory within the LACTOZ program provides reliable information on branching ratios and rate coefficients in hydrocarbon oxidation under simulated NOx rich tropospheric conditions. As an systematic extension of our previous work on the alkane series Cl - Ce in the present period it was aimed to study ... 

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... 

Both the a-X and b-X transitions have long been known from absorption by the oxygen in the earth s atmosphere, the source of radiation being the sun and the very long path length of oxygen overcoming their extreme weakness. For laboratory observation of these transitions, and particularly for accurate determination of absolute absorption intensity, CRDS has proved to be an ideal technique. 

FIGURE 11.51 Correlation between measurements made by the mass spectrometry-derivatization technique and long-path UV absorption in rural Colorado for lower NOt conditions (adapted from Mount et at., 1997a). 

Although the focus of this chapter is tropospheric HO measurements, it is worthwhile to mention techniques that have proven useful in the laboratory or in other regions of the atmosphere. As a small molecule in the gas phase, HO has a much-studied and well-understood discrete absorption spectrum in the near UV (29), shown in Figure 1, that lends itself to a variety of absorption and fluorescence techniques. The total atmospheric HO column density has been measured (30-32) from absorption of solar UV radiation, observed with a high-resolution scanning Fabry-Perot spectrometer. Long-path measurements of stratospheric HO from its thermal emission spectra in the far infrared have been reported (33-35). Long absorption paths in the atmospheric boundary layer have been used for HO detection from its UV absorption (36-42). 

Atmospheric molecules such as 02, Os, NO and NOz are inherently reactive because of the free radical nature of their electronic structures. In addition, there are literally hundreds of free radical species produced in the atmosphere via either photochemical or dark reactions of various hydrocarbons [1,2,27]. Clearly, an important prerequisite to laboratory studies of atmospheric chemistry is the ability to generate key free radical species in a clean fashion. Some representative techniques for generating the major free radical reactants, i.e., HO, HOO, R, RO and ROO (R = alkyl or other organic group), in combination with a long path IR absorption cell-chemical reactor are described below. 

GOME is a nadir sounding spectrometer which observes the up-welling radiance from the top of the atmosphere and the extra-terrestrial solar irradiance between 240 and 790 nm. The resolution of the measurements is chosen to be suitable for the application of the differential optical absorption spectroscopy (DOAS) technique, which was developed for long-path measurements and zenith sky observations (e.g. Platt and Pemer, 1980 Mounter a/., 1987 Eisinger etal., 1997). 

Code for measurement techniques WC, wet chemical CL, chemiluminescence (N0 + 03), material for N02 to NO conversion indicated LPA long-path optical absorption. 

Hiibler, G., D. H. Ehhalt, H. W. Patz, D. Perner, U. Platt, J. Schroder, and A. Tonnisson (1982). Determination of ground level OH concentrations by a long path laser absorption technique. In Physico-chemical Behaviour of Atmospheric Pollutants (B. Versino and H. Oh, eds.), Proc. Eur. Symp., 2nd, Varese, Italy, pp. 2-9. Reidel, Dordrecht, Holland. 

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