Absorption long-path

Long-path absorption measurements for [HO ] have been primarily developed by researchers in Germany (122,123,124,125). This approach measures the uv absorption by HO over a pathlength up to several kilometers. The result is an HO concentration spatially averaged over the... 

Dorn, H.-P., U. Brandenburger, T. Brauers, and M. Hausmann, A New In-Situ Laser Long-Path Absorption Instrument for the Measurement of Tropospheric OH Radicals, J. Atmos. Sci., 52, 3373-3380 (1995a). 

Dorn, H.-P., R. Neuroth, and A. Hofzumahaus, Investigation of OH Absorption Cross Sections of Rotational Transitions in the A2 L +, r = 0 - X2II, c" = 0 Band under Atmospheric Conditions Implications for Tropospheric Long-Path Absorption Measurements, J. Geophys. Res., 100, 7397-7409 (1995b). 

Mount, G. H., and J. W. Harder, The Measurement of Tropospheric Trace Gases at Fritz Peak Observatory, Colorado, by Long-Path Absorption OH and Ancillary Gases, . /. Atmos. Sci., 52, 3342-3353 (1995). 

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. 

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... 

A new in situ long-path absorption instmment for the measurement of tropospheric OH radicals. J. Atmos. Sci. 52, 3373-3380. 

The large reactor diameter favours the application of long path absorption spectroscopy in theUV,VISandIR. ... 

Hofzumahaus, A., Dom, H.-P., Callies, J., Platt, U., and Ehhalt, D., Tropospheric OH concentration measurements by laser long-path absorption spectroscopy. Atmos. Environ. 25A, 2017 (1992). 

Johnston, P. V. and R. L. McKenzie (1984). Long path absorption measurements of tropospheric N02 in rural New Zealand. Geophys. Res. Lett. 11, 69-72. 

For a single-pass FM spectrometer working at 150 GHz the best operating condition is to minimise background, by controlling power reflection within and at each end of a long-path absorption cell, and to apply a modulation depth of 240 kHz p-p at a sample pressure 8-13 Pa. By contrast in a cavity spectrometer, the optimum modulation depth is governed by the cavity width, rather than the sample linewidth (Section 2.4). 

New high resolution (0.01 nm) measurements of the absorption cross-sections of NO2 were performed in the 300-500 nm range at 293, 240 and 220 K using a coolable 5m long path absorption cell (Malicet). Experiments were carried out at very low NO2 pressures in order to reduce absorption contributions of the dimer, N2O4. A definite temperature effect (up to 6 %) was observed in the structured region of the spectrum. 

The kinetics of the self-reaction (6) and the cross reactions (7) and (8) were studied by conventional flash photolysis [5, 11] and later by laser flash photolysis [4] combined with UV long path absorption spectroscopy. Rate constants for reaction (6) to (8) were obtained ... 

The oxidation of alkanes, alkenes and simple aromatics at 293 K under NOx rich tropospheric conditions has been studied using laser pulse initiation combined with cw laser long path absorption/LIF for the detection of OH and NO2. In the case of aliphatic hydrocarbons the absolute yield and the kinetics of the formation of these products have been found to be sensitive indicators for the reaction behaviour of the oxy radicals RO. In combination with mechanistic simulations rate constants for individual reactions as well as branching ratios have been derived, which permit the evaluation of the compound specific NO/NO2 conversion factors (NOCON - factors) for the first oxidation steps. In the case of benzene and toluene oxidation the results indicate that reaction of the primary formed X cyclohexa-dienyl radical (X = Cl, OH) with O2 is the dominant pathway, although the rate coefficients were found to be lower than 2 x 10" cmVs. 

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 ... 

As the cold-vapor mercury sample is already in the atomic state, there is no need of an atomizer, per se. The vapor, transferred directly from the cell or desorbed as a plug from a heated amalgamation trap, is commonly swept into a moderately heated (resistance wound heating to 200°C) 10 cm quartz T-tube located within the optical beam of a conventional AA spectrometer. Attenuation of an intense electrodeless discharge lamp line source at 253.7nm is used as a measure of the absorption. Alternatively, dedicated continuum source AA-based spectrometers fitted with long path absorption cells (30 cm) are frequently used to increase sensitivity and detection limit. 

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... 

Fig. 10.17. Long-path absorption measurement of atmospheric pollutants 320...

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-... 

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