Figure 1.01

A typical vertical profile of ozone in the midlatitudes of the northern hemisphere. The stratosphere lies between the tropopause and stratopause (marked in red). Superimposed on the figure are plots of UV radiation as a function of altitude for UV-a (320-400 nm, cyan), UV-b (280-320 nm, green), and UV-c (200-280 nm, magenta). The width of the bar indicates the amount of energy as a function of altitude. The UV-c energy decreases dramatically as ozone increases because of the strong absorption in the 200-280 nm wavelength band. The UV-b is also strongly absorbed, but a small fraction reaches the surface. The UV-a is only weakly absorbed by ozone, with some scattering of radiation near the surface.

Figure 1.02

UV flux versus wavelength for various altitudes (top of the atmosphere, 30 km, 20 km, and the surface). The red line shows the surface flux for a 10 % reduction of ozone. The blue line shows the DNA action spectrum. The action spectrum is most simply defined as the biological effect as a function of wavelength, where a large value implies large biological damage.

Figure 1.03

Surface erythemal UV exposure for July 1988 as determined from the Total Ozone Mapping Spectrometer aboard the NASA Nimbus-7 satellite. Erythemal spectral exposure is principally responsible for sunburn. It is higher in the tropics than in the polar regions. This is because of the angle of the Sun and the amount of column ozone at different latitudes. Note the high erythemal exposures for the southwestern United States. The higher altitudes of Earth's surface and the lack of cloudiness increase the erythemal exposure for this region.

Figure 1.04

Illustration of the life cycle of an ozone molecule. In step 1, the oxygen molecule is photolyzed by extreme UV creating two oxygen atoms. In step 2, ozone is photolyzed by UV into and oxygen atom and an oxygen molecule, with a subsequent reaction of this oxygen atom with another oxygen molecule to reform ozone. In step 3, ozone can react with a chlorine atom (among others) to form ClO and O2. The ClO can subsequently react with an oxygen atom to form another oxygen molecule and a Cl atom. This "catalytic reaction" of step 3 results in the conversion of an O3 and an O atom into two oxygen molecules (O2) without affecting the original Cl atom.

Figure 1.05

Average total column ozone as measured by the Nimbus-7 Total Ozone Mapping Spectrometer (TOMS) plotted versus time and latitude. High levels of ozone are shown in orange and red, while low levels are shown in blue and violet. The units of measurement are Dobson Units (DU), a measure of the thickness of the ozone layer. The data is taken from 1979-1992.

Figure 1.06

Average number density of ozone as measured by the Nimbus-7 Solar Backscatter UltraViolet instrument (SBUV) plotted versus latitude and altitude. High levels of ozone are shown in orange and red, while low levels are shown in blue and violet. The units of measurement are Dobson Units per kilometer (DU/km). The SBUV measures the number of ozone molecules in layers while TOMS measures the total number of molecules from the surface to space. The SBUV data is taken from 1980-1989. The black arrows show the stratospheric circulation known as the Brewer-Dobson circulation. It is responsible for the observed equator-to-pole ozone distribution. The circulation varies by season and by hemisphere.

Figure 1.07

Distribution of CFC-12 (June-July 1992) as measured from the CLAES instrument aboard the UARS satellite. The volume mixing ratio (relative amount of CFC-12 per unit volume of air) is expressed in parts per trillion by volume. The pressure scale on the left-hand side is expressed in hecto-Pascals, with 1013 hPa being the average surface pressure (1 hPa equals 1 millibar). The blue arrows schematically display the mean circulation of the stratosphere for the northern summer months. Air is slowly transported into the stratosphere in the tropics (large blue arrow near the Equator at 16 km), and slowly carried into the upper stratosphere where CFC- 12 is destroyed by the suns UV radiation.

Figure 1.08

Vertical profiles of HCl (averaged between 55°5 and 55°N) between 1992 and 1998. The year tick marks are centered on January 1. Here, the volume mixing is expressed in parts per billion. Note the increase of HCl with altitude, which results from the breakup of CFCs by UV radiation. Also note the steady increase of HCl from 1992. This increase results from the CFC increases in the upper stratosphere over this period.

Figure 1.09

Total column ozone for October 1970, 1971, 1972, and 1979 (top panels) and 1993, 1994, 1996, and 1997 (bottom panels). The color scale on right hand side of the figure indicates the column ozone amount measured in Dobson Units. A Dobson Unit is equal to 1/100th of a millimeter. The thickness that the ozone layer would have if all of the overhead column were brought down to the surface, where a value of 300 Dobson Units is equal to 3 millimeters (about the thickness of two stacked pennies). 

Figure 1.10

Total column ozone for the Arctic during March (top) and the Antarctic during October (bottom). The Arctic ozone values are averaged over the polar regions northward of 63°N, while the Antarctic values are averaged over the region southward of 63°S. Note the strong downward trend of ozone over both polar regions.