" We are grateful to Dr. Sasha Madronich for these calculations.

" We are grateful to Dr. Sasha Madronich for these calculations.

values for these parameters in carrying out photolysis calculations for particular locations. This is especially true for unusual situations such as high particle and/or surface-level ozone concentrations or unusual geography such as mountains, which shield the light at large solar zenith angles (e.g., Castro et al., 1997).

All the calculated actinic fluxes discussed so far refer to a cloudless sky. The effects of clouds are complex, in that they reduce the direct radiation at the earth's surface from the sun but, at the same time, can increase the total actinic flux directly above the cloud due to scattering from the top surface of the cloud. Madronich (1987) has treated the case of large uniform clouds of various optical depths which, however, are sufficiently large that the cloud completely diffuses both the reflected and the transmitted light. The actinic flux above the cloud then becomes a combination of the incident light (a combination of direct sunlight and

TABLE 3.13 Percentage Change of Calculated Actinic Flux at the Earth's Surface Using Best Estimate Albedos as a Function of Solar Zenith Angle and Selected Wavelengths When Model Aerosol Concentrations Are Either Zero or Doubled"

Wavelength (nm)

Actinic flux change (%)

340-345 No aerosol Double 400-405 No aerosol Double

540-550 No aerosol Double

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