290 310 330 350 370 390 X (nm)

FIGURE 13.15 Normalized action spectra taking response = 1.0 at 300 nm (adapted from Madronich, 1992).

ozone here, it is also the region most impacted by decreases in stratospheric ozone.

As is the case for assessing trends in total column 03, there are a number of complexities in determining if there are associated trends in UV at the earth's surface (e.g., see Frederick, f990; Crutzen, f992; Cor-rell et al., 1992; Seckmeyer et al., 1994; and Madronich et al., 1998). For example, changes in cloudiness and increased aerosol particle concentrations can alter the radiation at the earth's surface (see Chapter f4). This is particularly true in highly polluted urban areas such as Mexico City (Galindo et al., 1995). Light scattering by aerosol particles may have decreased UV in the 280- to 320-nm range by 2-18% since the beginning of the industrial revolution, offsetting at least in part increased UV from ozone depletion in the Northern Hemisphere, where most of the industrial emissions have occurred (e.g., Liu et al., 1991; Sabziparvar et al., 1998). Similarly, Wenny et al. (1998) measured UV-B radiation at a mountain top and in a nearby valley in North Carolina and correlated decreasing UV-B intensities with increased aerosol optical depth in the layer of air between the two. However, increased UV at large solar zenith angles has also been predicted (see, for example, Davies, 1993; and Tsitas and Yung, 1996).

Toumi et al. (1994) also suggested there is a feedback between reduced stratospheric ozone and particles in that the increased UV due to ozone depletion may increase sulfate particle formation by increasing the concentrations of tropospheric OH.

In short, changes in aerosol concentrations over industrialized regions can complicate the interpretation of UV trends (e.g., see Justus and Murphey, 1994). The same is true of clouds, which play a major role in the

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