QUANTIFICATION OF BIOLOGICAL EFFECTIVENESS OF UV RADIATION - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 52

Biologically weighted spectroradiometry

Spectroradiometry is the most fundamental radiometric technique (see Seidlitz
and Krins, this issue). Because the irradiance of environmental UV varies with time,
time has to be considered as another variable:
O

(2)

³

E eff ( t ) E O , t S O d

O O

with E O ( O ,t) = the terrestrial spectral UV irradiance at a certain time. The biologically
effective dose (BED) H eff (in ( J/m²) eff ) is then obtained by integration over the time span
of interest:

dt

(3)

eff ³

H E t

eff

The advantages of weighted spectroradiometry lie in the principally attainable
high accuracy of the spectroradiometric data, the capability to identify influences by
various meteorological parameters, to apply a large variety of biological weighting
functions and to use the data for the evaluation of model calculations and trend
assessments. High demands are made on the instrument specifications, such as high
accuracy - especially at the edge of the solar spectrum in the UV-B range -, high stray
light suppression, high reproducibility and temperature stability. Frequent calibration
with standard lamps and by use of field intercomparison campaigns with other
spectroradiometers are indispensable 5,19,20 . On the other hand, because of the high
demands on instrument specification and the relatively large investment costs,
spectroradiometers are only installed at selected sites (e.g., UV-networks). To
interpolate the discrete measurements made in typically 4 to 15 min intervals under
unstable weather conditions, more or less correct assumptions have to be made using the
information from additional continuously monitoring broadband radiometers. Yet, given
even the utmost accuracy of physical spectral irradiances, due to experimental errors in
the biological action spectra, the resulting calculated E eff values may suffer from
substantial uncertainties.
Errors may occur from inappropriate usage of action spectra, such as action
spectra measured over too limited wavelength ranges. Although ozone depletion affects
only the UV-B edge of sunlight, depending on the tail of the action spectrum, the
responses to UV-A and even to visible light might be important to be included. Another
source of errors lies in the measuring errors of the action spectra, which may arise
especially in their long wavelength tails where the biological sensitivity decreases by
orders of magnitude. Furthermore and most importantly, the method assumes simple
additivity of the various wavelength components by incorporating a multiplicative
constant appropriate for each wavelength. This is based on the fact that most action
spectra have been developed with monochromatic radiation. If interactions occur - and
they have been reported for various biological effects (reviewed in ref. 21) - simple
additivity might be an insufficient basis for biologically weighted dosimetry and
provides not a suitable indicator for the biological responses to the solar radiation
reaching the surface of the Earth.