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MTU GE 4250 - Sample Application of Passive DOAS

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11Sample Application of ‘Passive’ DOASFor almost a century, atmospheric trace gas abundances have been measuredby their absorption of sunlight. There are several possible measurement ge-ometries, which have specific applications, advantages, and drawbacks:• Direct sunlight (or moon and starlight)• Scattered sunlight in zenith view geometry (ZSL-DOAS)• Scattered sunlight in ‘off-axis’ view geometry• Scattered sunlight in ‘multi-axis’ view geometry (MAX-DOAS)In all passive DOAS applications, determination of the (effective) length ofthe light path is more difficult than for active DOAS (see Chap. 10). Themethods used to overcome this challenge through the calculation of air massfactors (AMF) were introduced in Chap. 9.Direct sunlight is (during daylight hours) readily available and allows arelatively simple calculation of the optical path (see Chap. 9). The practicalrealization of this approach requires a manual or automatic mechanism topoint the instrument to the sun (or moon or star). A famous application ofdirect light spectroscopy is the determination of the O3total column den-sity, which has been routinely performed at many stations worldwide by UVabsorption spectroscopy since the technique was introduced by S. Dobson dur-ing the 1920s (Dobson and Harrison, 1926; see Chap. 6). Clearly, all sunlightreaching the surface of earth has to traverse the entire atmosphere, picking upthe spectral signature of its constituents. Thus, it is straightforward to deter-mine the total column density of a trace gas from direct light measurements.However, there is little or no information about the altitude distribution of thespecies. The nighttime composition of the atmosphere can be probed by usingdirect moonlight or starlight. Nevertheless, there has been surprisingly littleuse of direct sunlight spectroscopy in the UV or visible range for the determi-nation of atmospheric species other than ozone (some recent measurementsare described in Sect. 11.1).Determination of atmospheric trace gases by scattered sunlight spec-troscopy has been used to probe the atmosphere for three quarters of a century.U. Platt and J. Stutz, Sample Application of ‘Passive’ DOAS. In: U. Platt and J. Stutz,Differential Optical Absorption Spectroscopy, Physics of Earth and Space Environments,pp. 429–494 (2008)DOI 10.1007/978-3-540-75776-411c Springer-Verlag Berlin Heidelberg 2008430 11 Sample Application of ‘Passive’ DOASInitially most of the studies concentrated on stratospheric species, in particu-lar ozone (see Chap. 9), almost invariably using the zenith view geometry. Theinfluence of tropospheric gases and aerosol was seen as a nuisance, merely re-ducing the accuracy of the measurements. To determination of stratosphericcomposition by passive, ground-based instrumentation makes the best pos-sible use of the available technology. This might have been the reason whythe analysis of tropospheric species by passive differential absorption spec-troscopy with other geometries such as MAX-DOAS came only recently intowidespread use. During the last few years, a multitude of innovative tech-niques and observation geometries for probing the atmosphere close to the in-strument emerged. These include the aforementioned MAX-DOAS approach(see Sects. 11.3.2 through 11.3.3), as well as also plume scanning schemes(see Sect. 11.3.8), imaging DOAS (IDOAS; see Sect. 11.3.9), and topographictarget DOAS.Another area of rapid development is the observation of atmosphericcomposition from space. While this technology started in the 1960s withthe mapping of the stratospheric ozone column (TOMS), the recent decadehas seen enormous development, marked by several pioneering instruments,which record complete spectra at sufficient spectral resolution (Δλ < 1nm)to allow DOAS retrieval of trace gas columns (e.g. Global Ozone Monitor-ing Experiment, GOME; Scanning Imaging Absorption Spectrometer for At-mospheric Cartography, SCIAMACHY; Ozone Monitoring Instrument, OMI;GOME-2; and Optical Spectrograph and Infrared Imager System, OSIRIS).11.1 Atmospheric Measurements by DirectLight SpectroscopyDirect light spectroscopy using radiation from celestial bodies (sun, moon,or stars) allows the direct determination of total trace gas column densities.There are essentially three advantages over scattered light schemes. First,the extension of the light path in comparison with the vertical path, theAMF (see Chap. 9), is given by 1/ cos ϑ, with ϑ denoting the zenith an-gle of the celestial body. Secondly, spectroscopic analysis of direct sunlightbenefits from the high intensity. Lastly, the ring effect is absent. In particu-lar, moonlight or starlight is so weak that, with present technology, there isno alternative to spectroscopy with direct light. Nevertheless, moonlight orstarlight allows the study of nighttime chemistry, for instance of moleculesthat are destroyed by sunlight, such as chlorine dioxide (OClO) or the nitrateradical (NO3).One disadvantage of direct light DOAS is that Fraunhofer signatures varywhen imaging different parts of the sun, as discussed in Chap. 9. Moreover,some manual or automatic mechanism to point the instrument to the sun(or moon/star) is required, which can make direct light measurements from11.1 Atmospheric Measurements by Direct Light Spectroscopy 431mobile carriers difficult. Nevertheless, direct light DOAS measurements fromaircraft and balloon were reported as outlined in Sect. 11.1.2.11.1.1 Ground-based Measurement of Atmospheric SpeciesSome of the most scientifically successful moonlight DOAS measurements werethose of the OClO column density, as for example performed at the McMurdoAntarctic station in 1986 by Solomon et al. (1987b) (Figs. 11.1 and 11.2).A clear increase of OClO with increasing direct light AMF (dashed line) isobserved. (Note that evening twilight measurements using scattered sunlightwere performed at the same time, as shown in Fig. 11.12.Direct moonlight measurements were also made in Kiruna, Sweden, byWagner et al. (2002b) in order to obtain improved absorption cross-sectionsof the oxygen dimer (O2)2or O4. The investigated O4bands were found toshow an increase of the peak absorption with decreasing temperature, rangingfrom ≈ 13%/100 K at 477.3 and 532.2 nm, ≈ 20% at 360.5 and 577.2 nm to≈ 33% at 380.2 and 630.0 nm. Moreover, with the exception of the band at380.2 nm, the O4absorption cross-sections were found to be somewhat largerthan previous


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