Huggins bands

Measurements of ozone concentration in the ozone layer in the stratosphere are made in the less intense Huggins band to avoid complete absorption of the laser radiation. Again, the two or three wavelength DIAL method is used to make allowance for background aerosol scattering. A suitable laser for these measurements is the XeCl pulsed excimer laser (see Section 9.2.8) with a wavelength of 308 nm, close to the peak absorption of the Huggins... 

This channel has recently been observed in the 335-352 nm region of the Huggins band.49 Such a channel is not inconsistent with previously determined branching ratios in the Hartley band. These determinations typically rely on measuring the amount of 0(3P) as a function of time and inferring the channels present from the observed kinetics. Such a measurement accounts for the amount of 0(4D) versus 0(3P) formed but not which channels gives rise to these species. 

FIGURE 4.7 UV absorption of 03 at room temperature in the Hartley and Huggins bands. At the longer wavelengths, each curve has been expanded by the factor shown. (Adapted from Daumont et al., 1989.)... 

The Huggins bands are in the 300- to 360-nrn region, whereas the 440- to 850-nrn region represents the Chappuis bands. As seen in Figs. 4.7 and 4.8, both absorptions are much weaker than the Hartley bands. 

It should be noted that the foregoing considerations apply to the major absorption bands. In some cases, weaker absorption bands of the major greenhouse gases can be sufficiently weak to fall in the linear region. This is the case, for example, for light absorption by 03 in the Chappius band, even if the strong Hartley-Huggins band (see Chapter 4.B) is saturated (e.g., see Lacis et al., 1990). 

The absorption in the ultraviolet and visible regions consists of the Hartley bands (2000 to 3200 A), the Huggins bands (3000 to 3600 A), and the Chappuis bands (4400 to 8500 A). The absorption coefficients of these bands arc given in Figs. VI I2 andVI 12b. Figure VI 12c shows the absorption coefficients in the vacuum ultraviolet region. 

Fig. VHI-8. Photodissociation coefficients J of ozone and NOj in sec-1 as a function of altitude for an overhead sun and x = 45", respectively. Above 50 km 03 dissociates predominantly into O( D) + Ojf A) by photolysis in the Hartley band. Below 50 km the photolysis in the Chappuis and Huggins bands becomes progressively more important, producing 0(3P), 02(Jf3I ), and Oj( A). The extent of Oj( A) production is uncertain below 50 km. From Nicolet (738) for 03 and Shimazaki and Whitten (872b) for N02, reprinted by permission of Reidel and the American Geophysical Union.

The important bands for ozone are the Hartley bands between 2000 and 3000 angstroms in the ultraviolet, with a very intense maximum absorption at 2550 angstroms the Huggins bands, weak absorption between 3200 and 3600 angstroms the Chappius bands, a weak diffuse system between 4500 and 6500 angstroms in the visible spectrum and the infrared bands centered at 4.7, 9.6 and 14.1 microns, the latter being the most intense. 

At wavelengths greater than 310 nm, the Huggins bands correspond to the limit of the O3 ultraviolet absorption, and in the visible region (410— 850 nm) the Chappuis bands play an important role leading to the 03 photodissociation in the lower part of the atmosphere, troposphere, and lower stratosphere. 

Fig. VI-12, (a)Absorption coefficients of (), Huggins bands (3000 to 3600 A) and Chappuis bands (44(H) to 8500 A) k (atm" 1 cm" ), 0 C, base 10. From Griggs (425), reprinted with permission. Copyright 1968 by the American Institute of Physics. (h) Absorption coefficients of O, Hartley bands (2000 to 3000 A), k is in units of atm" 1 cm"1, 0 C, base 10. From Griggs (425), reprinted by permission. Copyright 1968 by the American Institute of Physics, (c) Absorption coefficients of O, in the region 1000 to 2200 A. k is in units of (atm"1 cm" ), 0°C, base e. From Tanaka et al. (961), reprinted by permission. Copyright 1953 by the American Institute of Physics.

The source of O( D) is 03 photolysis in the Hartley continuum, and possibly in the Huggins bands,... 

It is convenient to discuss three regions of the absorption spectrum, separately the Hartley band from 2000 to 3000 A., the Huggins band from 3000 to 3500 A., and the Chappuis band from 4000 to 7500 A. The results of the ozone absorption coefficients measurements are displayed in Figures 2, 3, and 4, respectively. The absorption coefficients measured by the authors are tabulated in Table I. 

Figure 3. Absorption coefficients of ozone in region of the Huggins band, 3000 to 3500 A.

The absorption of ultraviolet radiation by ozone in the Huggins and Hartley bands constitutes the principal source of heat in the stratosphere and mesosphere. The heating rate reaches 10 K/day near the stratopause on the average, with a maximum of about 15 K/day near the summer pole. The effect of the Huggins bands in the visible region becomes... 

Around 300 nm, the Hartley band becomes weak, and from 310 to 350 nm it blends with the temperature dependent Huggins bands. Figure 4.35 shows the importance of this temperature sensitivity, especially at long wavelengths. A careful calculation is required to account for the details of the spectral structure as a function of temperature. 

Figure 4-35. Spectral distribution of the ozone cross section in the Hartley band (200-300 nm), Huggins bands (310-350 nm), and Chappuis bands (410-750 nm).

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