USE AND EVALUATION OF BIOLOGICAL SPECTRAL UV WEIGHTING FUNCTIONS FOR THE OZONE REDUCTION ISSUE - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 71

1.3. BSWF PREDICTIONS RELATED TO OZONE REDUCTION
Most action spectra and BSWF decrease in relative effectiveness with increasing
wavelength. The example of the two BSWF shown in Figure 2 shows them plotted both
on logarithmic and linear vertical axes. On linear axes, the two BSWF do not look so
very different, but the logarithmic plot brings out the large difference in slopes which
turns out to be very meaningful in many circumstances.
When these two spectra are used as BSWF to predict the increase in daily
biologically effective solar radiation against ozone column reduction at a temperate
latitude location (40° latitude) at the summer solstice, the two BSWF result in different
levels of effective radiation for a given amount of ozone reduction, i.e., greatly different
RAF values (Figure 3a,b). The steeper spectrum yields a more pronounced increase.
Also, plotted on the same graphs on the ordinate is the daily biologically effective solar
radiation at different latitudes without ozone reduction at the time of year of maximum
solar radiation (smallest solar zenith angles). The steeper spectrum used as BSWF in
this calculation results in a much larger latitudinal gradient in biologically effective
solar UV than does the less steep spectrum. Thus, the slopes of BSWF with increasing
wavelength are quite sensitive in these calculations.
1.4. DISCREPANCIES BETWEEN BSWF USED IN EXPERIMENTAL DESIGN
Apart from predicting RAF values and latitudinal values of biologically effective UV
(as in Figure 3), BSWF are also used to calculate how much radiation is to be added
with UV-emitting lamp systems in experiments designed to simulate the extra UV
radiation resulting from ozone depletion. The BSWF used makes a substantial
difference in how much lamp UV is added in the experiments. This is illustrated in a
brief scenario that follows. However, in this illustration instead of the Cen and Björn
spectrum, a commonly used BSWF, the generalized plant spectrum of Caldwell [9] is
used. This spectrum and the Setlow spectrum are plotted on logarithmic ordinate scales
in Figure 4 and it is clear these two spectra do not differ nearly as much as the BSWF of
Cen & Björn from that of Setlow (Figure 2).
Yet, these two spectra still yield considerable differences when employed in the
calculations. These differences are portrayed in a scenario where one assumes one
BSWF is appropriate, but in reality another BSWF is better suited. This is plotted as the
"errors'' caused by using one BSWF (the generalized plant spectrum) when another (the
Setlow DNA BSWF) is more appropriate are shown in Figure 5. The "RAF error'' is that
associated with calculating the increase in biological effective radiation with ozone
column reduction (essentially, the RAF as shown in Figure 3).