308.00 308.05 308.1 308.15 308.20 Wavelength (nm)

FIGURE 11.42 (a) Energy levels and some allowed transitions for the OH(X2n -> A22 + ) absorption, (b) a typical broadband laser emission line profile, and an OH reference spectrum with absorption lines in this region. (Adapted from Mount, 1992; and Dorn et al., 1995a.)

compared to the OH linewidth that six different OH lines can be measured simultaneously using a high-resolution spectrograph and linear photodiode array. In an open multipass White cell with base path of 38.5 m and a total path length of 1.85 km, a detection limit for OH of 8.7 x fO5 radicals cm~3 can be obtained (Brandenburger et al., 1998).

Open-beam double-pass instruments using a retro-Hector array to return the light beam back to the spectrograph are also in use (e.g., Mount, 1992; Mount and Harder, 1995; Harder et al., 1997a). Another variation is the use of a fast-scanning technique with pi cosecond pulse lengths and a photomultiplier detector, rather than the use of a photodiode array. For example, Armerding and co-workers (Armerding et al., 1994, 1995, 1996, 1997) tune the laser wavelength rapidly over the region of interest, covering up to ~13 cm~' in spectral width in 0.1 ms. Multiple scans, taken with a repetition rate of f.3 kHz, are averaged to improve the signal-to-noise ratio.

A typical example of such measurements was shown in Fig. 11.15.

A major advantage of this approach is that the fundamental spectroscopic parameters for OH, including the absorption cross sections for various transitions, are well known (e.g., see Mount, 1992; Dorn et al., 1995b), so that absolute concentrations of OH can be calculated based solely on the absorption spectra. Another major advantage is that the laser beams are expanded so that generation of OH in the beam itself by photolysis of ozone is not the problem that it has been in LIF measurements (vide infra).

A disadvantage is that the use of long paths gives average concentrations over the whole distance, over which there could be considerable variability. Folded paths using White cells provide a more restricted measurement distance, but reflection losses on the mirrors preclude increasing the total path length and hence detection sensitivity beyond a certain point. Whether such a system can be made sufficiently stable for aircraft use is also not clear. Finally, the sensitivity of such systems is in the fO5 to low 106 radical cm-3 range. While this is sufficient for daytime measurements when there are significant photolytic sources of OH, it is not adequate for nighttime measurements where much lower concentrations (likely of the order of fO4 cm-3 or less) are generated by such processes as PAN decomposition.

Laser-induced fluorescence (LIF). Laser-induced fluorescence measurements have been applied to the atmosphere since the suggestion of Baardsen and Ter-hune in 1972 that this method should be feasible. Figure 11.43 shows the energy levels and transitions involved in LIF measurements. OH is excited from its ground X2n state into the first electronically excited A2£ state. The v" = 0 to v' = 0 transition is around 308 nm and the v" = 0 to v' = 1 at 282 nm. Two schemes have been used: excitation using 282 nm into v' = f of the upper electronic state, or excitation using 308 nm into v' = 0 of the upper state. Collisional quenching deactivates some of the v' = 1 into v' = 0 in competition with fluorescence, mainly in the (1,1) band of the electronic transition (that is, from v' = 1 of the upper state into v" = 1 of the lower state). Collisional deactivation of v' = 0 then occurs in competition with fluorescence in the (0,0) band at 308 nm

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