TRANSFORMATION OF IRRADIANCE MEASUREMENTS INTO SPECTRAL ACTINIC FLUX FOR PHOTOLYSIS RATES DETERMINATION

The actinic flux is the only radiometric quantity suitable for photoly sis frequency determination. It is derived from solar spectral irradia nce measurements performed by a portable spectroradiometer in the 300- 850 nm wavelength range. The spectral irradiance is first divided into a direct part and a diffuse part, according to the atmospheric condit ions, and then converted into the corresponding actinic flux quantity. As an intermediary, the ratio T-dd of diffuse actinic flux to diffuse irradiance is calculated by the spherical harmonics radiative code wi th respect to wavelength, solar zenith angle, surface albedo, and aero sol properties. The results of extensive sensitivity studies of T-dd a s a function of the main atmospheric parameters are discussed and lead to the conclusion that aerosol optical depth is the major critical va lue for a precise T-dd determination. The global algorithm to transfor m spectral irradiance into actinic flux is finally applied for photodi ssociation rate calculations by convolution of the obtained actinic fl ux spectra with the absorption cross-sections and quantum yields of th e molecule studied. Photolysis rates of different atmospheric photooxi dants have been measured with this spectroradiometric method during th e summers of 1993 and 1994 in Brittany and in Portugal. The ozone and nitrogen dioxide photodissociation rates obtained present a good agree ment with the j(O-3) and j(NO2) actinometer results, for the same expe rimental conditions in Brittany.