Ozone and the Spectral Quality of Incident Sunlight

Atmospheric attenuation of the solar spectrum

Radiation emitted by the Sun ranges from gamma rays (10-18 m) to radio waves (107m). Fortunately, at sea level, solar radiation has been stripped of its most biologically damaging wavelengths by gases in the atmosphere, most notably water vapor and ozone. All X-rays, some UVC (100-280 nm) and some UVB (280-315nm) wavelengths are absorbed in the outermost atmospheric layers, the thermosphere and the mesosphere. The stratosphere contains 90% of the ozone in the atmosphere, and this ozone layer filters out the remaining UVC and most UVB (Figures 1 and 2). The shortest UVB wavelengths reaching the ground are 285-290 nm.

The attenuation of UVC and UVB is through the photochemical dissociation of ozone and its subsequent reformation from the diatomic and singlet oxygen products (O3 <--> O2 + O). UVA (315-400 nm), visible (400-750 nm), and infrared (750 to 1 x 106nm) wavelengths also reach sea level and are partially attenuated by atmospheric gases and aerosols, but are not affected by ozone concentrations.

Clouds play a dominant and complex role in reducing UVB intensities. Clouds do not attenuate UVB wavelengths as efficiently as UVA or visible light; therefore, while cloudy skies lower incident radiation intensities, they may enhance the ratio of UVB to higher wavelengths.

Ozone depletion

From the 1970s to the turn of the century, synthetic halogenated compounds (e.g., chlorofluorohydrocarbons,

Ozone (ppm)

Figure 1 Vertical distribution of ozone in the atmosphere and location of the ozone layer. Redrawn from US National Aeronautics and Space Administration (http://www.nasa.gov).

Ozone (ppm)

Figure 1 Vertical distribution of ozone in the atmosphere and location of the ozone layer. Redrawn from US National Aeronautics and Space Administration (http://www.nasa.gov).

225 250 275 300 325

Wavelength (nm)

Figure 2 Differences in spectral quality and intensity of UV radiation at the top of the atmosphere and at the Earth's surface. Redrawn from US National Aeronautics and Space Administration (http://www.nasa.gov).

225 250 275 300 325

Wavelength (nm)

Figure 2 Differences in spectral quality and intensity of UV radiation at the top of the atmosphere and at the Earth's surface. Redrawn from US National Aeronautics and Space Administration (http://www.nasa.gov).

CFCs) initiated declines of 3-6% in stratospheric ozone levels over tropical and temperate latitudes (60° N-60° S). More ozone was depleted over polar regions, with the largest-magnitude depletion (over 50%) still occurring seasonally over Antarctica (Figure 3). While the pollutant compounds are quite stable in the troposphere with long residence times (40-80 years), they eventually migrate into the stratosphere and disrupt the equilibrium of ozone dissociation and reformation described above (Figure 4). International compliance with the Montreal Protocol on Substances that Deplete the Ozone Layer has successfully limited the release of ozone-depleting substances, and ozone layer recovery to pre-1980 status is expected within the next 100 years.

Maximum area

Minimum (ozone)

35 30

X 20

Maximum area

Minimum (ozone)

1985

1995

2005

Figure 3 Maximum area (millions of km3) and minimum column ozone concentration (ozone) from 1979 to 2006 during the annual springtime ozone depletion cycle over Antarctica. Normal ozone concentrations are in the range of 350-400 Dobson units (DU). Data from US National Aeronautics and Space Administration (http://www.nasa.gov).

1975

1985

1995

200

180

3

160

O

140

c

o

120

2

100

c <u

o

80

c o

o

60

d)

c

40

N

20

o

0

Year

Figure 3 Maximum area (millions of km3) and minimum column ozone concentration (ozone) from 1979 to 2006 during the annual springtime ozone depletion cycle over Antarctica. Normal ozone concentrations are in the range of 350-400 Dobson units (DU). Data from US National Aeronautics and Space Administration (http://www.nasa.gov).

Ozone destruction and re-formation in equilibrium

Ozone destroyed, but does not re-form C

Figure 4 Simplified model of the photochemistry of ozone depletion. (a) Ozone dissociates when UVB is absorbed and re-forms to absorb more UVB. This sequence depicts the normal equilibrium state of this photochemical reaction. (b) Components of pollutant molecules, such as the chlorine shown here, can react with singlet oxygen producing compounds with very long dissociation rates, greatly slowing down the re-formation process of ozone by sequestering singlet oxygen molecules. Thus, ozone breaks down at a rapid rate and is not regenerated to maintain equilibrium concentrations.

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