METO 621 CHEM Lesson 7

Albedo 200 – 400 nm

Solar Backscatter Ultraviolet (SBUV) The previous slide shows the albedo of the earth viewed from the nadir. Note that the y axis is on a log scale. The albedo shows a very low minimum near 260 nm, but has risen to about 20% at 330 nm and above. The large dip in the albedo matches the absorption cross section for ozone, shown in the next slide. The SBUV method takes advantage of the relation between the rapid change in the albedo and the ozone cross section to derive an altitude profile for ozone between 30 and 60 km altitude.

Solar Backscatter Ultraviolet (SBUV)

SBUV is a satellite spectrometer designed to retrieve the altitude profile of ozone between 25 and 50 km. Above about 25 km the absorption due to Rayleigh scattering is small, and we can ignore multiple scattering. Let the solar zenith angle be q 0., and the angle between the viewing direction from the spacecraft and nadir be q We define the column amount of ozone at an altitude z to be X(z). Let the atmospheric pressure at z be P(z) If we choose wavelengths below 300 nm, then all of the solar radiation is absorbed before it reaches the tropopause, hence there is no upward radiation from the ground.

Solar Backscatter Ultraviolet (SBUV)

dI/F S vs altitude, 245 to 305 nm

Solar Backscatter Ultraviolet (SBUV) Q( ) is called the normalized radiance. Note that    and  can be calculated knowing the orbital parameters. The next slide shows the same figure as the previous slide, only for Q. The upper parts of each wavelength curve now lie on top of each other.

dQ vs altitude, 245 to 295 nm

Solar Backscatter Ultraviolet (SBUV) The next figure plots dQ/dz for a single wavelength. Also plotted is the dQ/dz that would be obtained if there were no ozone absorption i.e. Rayleigh scattering only. The horizontal line is the altitude at which  = 1.0 The area B is almost equal to area C, hence the area within the curve (A+B) is equivalent to the area under the Rayleigh curve down to the horizontal line (A+C). The area A+C can be simply related to the pressure at the horizontal line. But the optical depth for ozone absorption is much greater than that for Rayleigh scattering. Hence Q at each wavelength can be related to the column density of ozone at an optical depth of one, versus the pressure altitude at that point. For wavelength chosen, Q=.00122, pressure altitude=.00135

dQ versus altitude for 275 nm

Contribution function and resolution The shape of the curve shown in the previous slide is called the contribution function, and the half width in altitude is a measure of the resolution of the method. However, it should be noted that the contribution at high altitudes follows the increase in pressure as the altitude becomes smaller. It is only when the optical depth due to ozone approaches one, that the contribution begins to fall off. If we subtract the contribution functions for two consecutive wavelengths then we get the next figure. The resolution of this contribution function is much smaller. It should be noted that there is a limit to the resolution that can be obtained – the scale height of ozone.

dQ vs altitude, for 265 and 275 nm

‘Difference’ for wavelengths, 245 to 285 nm