Central California Ozone Study (CCOS) - Desert Research Institute

Sun
Direct solar
radiation
Scatterd
sky radiation
Flat Plate Radiometer
The irradiance has a response factor of cos(θ) while the spherically integrated actinic flux
does not. It is possible make estimates of spherically integrated actinic flux from irradiance
measurements but the uncertainties may be at least ±40%. A complete discussion of the differences
between actinic flux and irradiance is given in Madronich (1987).
Another difficulty in calculating photolysis rate parameters is that the wavelength
dependent response of radiometer for the measurement of UVB and global solar radiation do not
match the product (action spectrum) of any particular species’ absorption cross sections (σλ) and
quantum yields (φλ). For example a typical UVB radiometer response function is similar but not
identical to the action spectrum of ozone,
Response
(arbitrary units)
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Figure 3.
250 275 300 325 350
Wavelength (nm)
Figure 3. The response function of a Yankee brand UVB radiometer in arbitrary units as a function
of wavelength.
The UVB radiometers can be used to provide an estimate of the photolysis rates of ozone to
produce excited oxygen atoms, O(1D), and a small fraction of these react to produce HO radicals.
Global solar radiometers react to a broader spectrum of radiation at higher wavelengths;
their response function is more like the action spectrum of NO2. For these reasons we propose to
take advantage of existing networks of UVB and solar radiation measurements and to augment
these with our own spectrally resolved actinic flux measurements.
C-18