Action spectra can be applied to estimate the biological and chemical impacts of ozone depletion and related UV increases. The fractional increases in biologically-active UVR are amplified compared to the changes in total ozone in the atmosphere. The degree of amplification, or "radiation amplification factor (RAF)", is defined by equation (4):
where (UVbiogeo)2 and (UV^^! are the "weighted" irradiances that correspond, respectively, to total ozone amounts (03)1 and (03)2. The weighted irradiance is simply the integrated cross-product of the solar spectral irradiance and the action spectrum [3,8,11,12,80,235]. For small changes in total ozone, the RAF corresponds approximately to the % increase in UVbi0ge0 that would occur with a 1% decrease in ozone. Modeling studies using the Antarctic action spectra for phytoplankton photoinhibition have indicated that RAFs fall in the 0.2 to 0.8 range, which signifies a moderate sensitivity to ozone depletion . Based on the close similarity between action spectra for Antarctic and Northern mid-latitude sites shown in Figure 3, it is likely that the mid-latitude RAFs are also in this range. By comparison, the RAF for generalized DNA damage, which applies to UV damage to bacterioplankton (see section 220.127.116.11), is close to 2 . The lower sensitivity of phytoplankton photosynthesis to ozone depletion can generally be attributed to factors such as the occurrence of mycosporine-like amino acids or carotenoids in cells that help shield the DNA from UV-B damage, as well as repair mechanisms that involve longer-wavelength UV-A radiation that is little affected by ozone depletion [8,11,12,88]. RAFs for various photoreac-
tions of CDOM fall in the 0.2 to 1.1 range , indicating that they are approximately as sensitive to changes in the stratospheric ozone layer as the effects on health, plants, and tropospheric photolysis.
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