Differential optical absorption

Allen, H. C. Brauers, T. Finlayson-Pitts, B. J. Illustrating Deviations in the Beer-Lambert Law in an Instrumental Analysis Laboratory Measuring Atmospheric Pollutants by Differential Optical Absorption Spectrometry, /. Chem. 

Applications The differential optical absorption spectrometer has been used to monitor concentrations of gases or intermediate compounds such as SO, NO, O5, HCHO, HNO, CS, NO, and OH in the atmosphere.In atmospheric measurements with open paths of 100 to 1000 m, a detection limit of about 1 ppb can be achieved. In the emission measurements, the path length across the duct or the plume can range in meters. 

Edner, Hans, Anders Sunesson, Sune Svanberg, Leif Llneus, and. Svante Wallin. Differential Optical Absorption Spectroscopy System Used for Atmospheric Mercury Monitoring. Applied Optics 25 (1986), pp. 403-409. 

In the late 1970 s, a group of German researchers developed a novel instrument, the long-path length (e.g., 1-17 km) ultraviolet-visible differential optical absorption spectrometer, DOAS (Platt et al., 1979). Application of this instrument in field studies in remote... 

Perner, D., and U. Platt, Detection of Nitrous Acid in the Atmosphere by Differential Optical Absorption, Geophys. Res. Lett., 6, 917-920 (1979). 

Winer, A. M and H. W. Biermann, Long Pathlength Differential Optical Absorption Spectroscopy (DOAS) Measurements of Gaseous HONO, N02, and HCHO in the California South Coast Air Basin, Res. Chem. Intermed., 20, 423-445 (1994). 

Figures 4.35, 4.36, and 4.37 show the absorption spectra of the free radicals CIO, BrO, and IO, respectively (Wahner et al., 1988 DeMore et al., 1997 Laszlo et al., 1995). All have beautifully banded structures at longer wavelengths and large absorption cross sections, which allows one to measure these species in laboratory and atmospheric systems using differential optical absorption spectrometery (DOAS) (see Chapter 11.A.Id). However, as in the case of HCHO, adequate resolution is an important factor in obtaining accurate cross sections.

The reaction with BrO, however, is potentially important under some conditions. As discussed in Chapter 11, UV-visible differential optical absorption spectrometry (DOAS) has been used to measure BrO under these conditions (e.g., see Hausmann et al., 1993 Platt and Hausmann, 1994 and Tuckermann et al., 1997). Concentrations up to about 30 ppt have been measured. The rate constant for the BrO-DMS reaction is 2.6 X 10 13 cm3 molecule-1 s-1 at 298 K (Bedjanian et al., 1996) and gives with essentially unit yield DMSO and a bromine atom (Barnes et al., 1993 Bedjanian et al., 1996) ... 

O, UV absorption Chemiluminescence Differential optical absorption spectrometry... 

In this case, a is the differential optical absorption cross section for the absorption band. In practice, of course, there are many different absorbers, t, present at different concentrations and absorbing at different wavelengths over the path length L. 

A major advantage of DOAS is its high sensitivity for species that meet the requirement of having narrow absorption bands in the UV-visible. Furthermore, because the differential optical absorption coefficients are fundamental spectroscopic properties of the molecule, the measurements need not be calibrated in the field. 

Brauers, T., M. Hausmann, U. Brandenburger, and H.-P. Dorn, Improvement of Differential Optical Absorption Spectroscopy with a Multichannel Scanning Technique, Appl. Opt., 34, 4472-4479 (1995). 

Plane, J. M. C., and N. Smith, Atmospheric Monitoring by Differential Optical Absorption Spectroscopy, in Spectroscopy in Envi-... 

Platt, U D. Perner, G. W. Harris, A. M. Winer, and J. N. Pitts, Jr Observations of Nitrous Acid in an Urban Atmosphere by Differential Optical Absorption, Nature, 285, 312-314 (1980a). 

Platt, U Differential Optical Absorption Spectroscopy (DOAS), in Air Monitoring by Spectroscopic Techniques (M. W. Sigrist, Ed.), Chemical Analysis Series, Vol. 127, pp. 27-84, Wiley, New York, 1994. 

Stutz, J., and U. Platt, Numerical Analysis and Estimation of the Statistical Error of Differential Optical Absorption Spectroscopy Measurements with Least-Squares Methods, Appl. Opt., 35, 6041-6053 (1996),... 

Evidence for the contribution of the CIO + BrO interaction is found in the detection and measurement of OCIO that is formed as a major product of this reaction, reaction (31a). This species has a very characteristic banded absorption structure in the UV and visible regions, which makes it an ideal candidate for measurement using differential optical absorption spectrometry (see Chapter 11). With this technique, enhanced levels of OCIO have been measured in both the Antarctic and the Arctic (e.g., Solomon et al., 1987, 1988 Wahner and Schiller, 1992 Sanders et al., 1993). From such measurements, it was estimated that about 20-30% of the total ozone loss observed at McMurdo during September 1987 and 1991 was due to the CIO + BrO cycle, with the remainder primarily due to the formation and photolysis of the CIO dimer (Sanders et al., 1993). The formation of OCIO from the CIO + BrO reaction has also been observed outside the polar vortex and attributed to enhanced contributions from bromine chemistry due to the heterogeneous activation of BrONOz on aerosol particles (e.g., Erie et al., 1998). 

Pitts et al. (1985) first used differential optical absorption spectrometry (DOAS) to establish unequivocally that N02 injected into a mobile home forms HONO. Interestingly, the dependence of the rate of HONO generation on the N02 concentration was similar to that measured in laboratory systems, consistent with production in, or on, a thin film of water adsorbed on surfaces. A number of studies have confirmed that the behavior is similar to that in laboratory systems i.e., the rate of production of HONO increases with N02 and with relative humidity. Indoor levels of HONO as high as 8 ppb as a 24-h average and 40 ppb as a 6-h... 

The Frankfort LPA instrument (51-53) departs from both of these instruments in two principal ways it achieves the necessary path length within a 6-m folded-path cell, and it rapidly scans a narrow-band frequency-doubled dye laser across the spectral region of interest (the Qi(2) line group) in a process sometimes called differential optical absorption spectrometry (DOAS). The scanning rate is sufficient to ensure that the observed air volume is chemically and physically stationary during each scan (the baseline standard deviation is less than 2 x 10-4 for a 0.2-ms scan). The laser output is actively feedback-stabilized to provide a flat spectral baseline, and a detection limit better than 10"5 in optical density has been claimed. A summary of published LPA configurations is given in Table II. 

GASCOD-A uses the differential optical absorption spectroscopy (DOAS) technique. It operates in the UV-visible spectral region and enables the detection of the trace gases listed in Table 4. The deliveries of GASCOD-A are total amounts of the trace constituents at zenith and nadir as well as vertical profiles. 

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). 

Platt, U. and D. Pemer (1980) Direct measurements of atmospheric HCHO, HONO, O3, NO2, and SO], by differential optical absorption spectroscopy in the near UV. Journal of Geophysical Research 85 7453-7465. 

Note NA = not applicable FID = flame ionization detector DOAS = differential optical absorption spectroscopy. 

The direct accurate measurement of local OH concentrations has been one of the major technical challenges in atmospheric chemistry since the early 1980s. This goal was first achieved in the stratosphere (e.g., Stimpfle and Anderson, 1988), but the troposphere proved more difficult (Crosley, 1995). Nevertheless, early long-baseline absorption methods for OH were adequate to test some basic theory (e.g., Poppe et al., 1994). Current successful direct methods include differential optical absorption near-UV spectroscopy with long baselines (e.g., Mount, 1992 Dorn et al, 1995 Brandenburger et al., 1998), laser-induced fluorescence after expansion of air samples (e.g., Hard et al., 1984, >1995 Holland et al., 1995), and a variety of chemical conversion techniques (Felton et al, 1990 Chen and Mopper, 2000 Tanner et al., 1997). 

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