Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Typical uncertainties quoted for UV spectroradiometers range from 5 to 15%, depending on the
quality of the instrument (McKenzie et al.,1993; Koskela, 1994; Gardiner and Kirsch, 1995; Bais et al.,
1997; Webb et al., 1998). When dealing with the shorter wavelength portion of the UV-B region, the
uncertainty in measured spectral irradiance increases significantly because of the decreasing signalto-noise
ratio and increasing error related to wavelength calibration in some instruments. Other
instrumental factors-such as stray light, temperature sensitivity, angular response, wavelength
instability, and degradation of optical components-may further reduce the reliability of data produced
by spectroradiometers (Gardiner and Kirsch, 1993, 1995; Seckmeyer and Bernhard, 1993; Slaper et
al., 1995; Groebner et al., 1996; Bais, 1997). A large part of this uncertainty appears to arise from the
absolute calibration standards and procedures, which can be as high as 6%. For the detection of
relative changes or trends, however, instrument stability over time is the most important parameter.
Even in this case, the uncertainty of the relative measurements performed by a given instrument
operating continuously at the same location can be significantly less than the uncertainty in absolute
irradiance. It is possible to achieve a relative uncertainty of 2-5% over a period of several years under
special circumstances (Kerr, 1997). Most measurement records fail to achieve this level of stability,
however.
Numerous studies during the past decade have established relationships between solar UV radiation
and atmospheric parameters identified previously. The effects of changes in ozone on UV irradiance
can be detected more easily than the ozone change itself because they are magnified several times
by strong absorption, which increases dramatically with decreasing wavelength. Consequently, even
the effect of small ozone changes-on the order of a few percent-can be detected in principle
(Madronich, 1992; Bais et al., 1993, 1997a,b), provided the absolute
irradiance remains above the
detection limits of the sensor.
The effect of a 1% decrease in
ozone on UV irradiance can be
detected at wavelengths
shorter than about 300 nm,
where the corresponding
increase in UV irradiance is
greater than 4% (Bodhaine et
al., 1997; Fioletov et al., 1997).
In this spectral region,
unfortunately, most single
monochromator
spectroradiometers suffer from
stray light (Gardiner and Kirsch,
1995; Bais et al., 1996). Only a
small subset of UV
spectroradiometers in the
existing worldwide network can
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Figure 5-1: Relationships among UVery, erythemal
weighting factor, ground irradiance, and erythemally
weighted irradiance at 30°N in July and January.