5.3. Detectability of Changes in Ground-Level Irradiance
Figure 5-1: Relationships among UVery, erythemal weighting factor, ground
irradiance, and erythemally weighted irradiance at 30°N in July and January.
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Our ability to detect future changes in solar UV irradiance attributable to
increases in aircraft emissions depends on two factors. The first factor is
the size of the expected change relative to natural variability and long-term
changes from other human causes. The second factor is the accuracy and stability
with which we are capable of making measurements over long periods of time.
The detection of a long-term change in UVery depends on the size of the expected
change, the length of the data record over which the change occurs, and the
variability of UVery. Weatherhead et al. (1998) show that a data record of at
least 15 years is necessary to distinguish a statistically significant (2-sigma)
long-term change of 5% per decade in UVery from the natural variability present
at most sites. A long-term change of 5% over 50 years (1% per decade to 2050)
would just be detectable at the best sites. The other factor is whether existing
instrumentation is able to detect expected long-term changes. Presently, many
different instruments devoted to monitoring changes in solar UV during forthcoming
decades are deployed over the globe. These instruments are either spectroradiometers
or broadband and narrowband detectors operating in the UV region. Spectroradiometers
are the most suitable instruments because they provide detailed measurements
of global spectral irradiance, which can be used to assess the importance of
various UV-controlling factors. Because of the weak intensity of UV irradiance
relative to other parts of the solar spectrum, however, it is only in recent
years that the quality of such measurements has become sufficient to detect
interannual changes in UV irradiance that are attributable to year-to-year changes
in ozone over relatively short (< 10 years) measurement records (Kerr and McElroy,
1993; McKenzie et al., 1993; Kerr and McElroy, 1994; Gardiner and Kirsch, 1995;
Groebner et al., 1996; Bais et al., 1997; Gurney, 1998). The quality of other
types of broadband instruments cannot yet be considered sufficient to detect
these small long-term UV changes (Leszczynski et al., 1996; Mayer and Seckmeyer,
1996; Blumthaler, 1997; Weatherhead et al., 1997; WMO, 1997).
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 signal-to-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
Figure 5-2: UVery as a function of latitude for the 1992
background atmosphere for January, April, July, and October.
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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 achieve the necessary
uncertainty limits at these short wavelengths. If changes in column ozone from
aviation are larger than 1%, longer wavelengths may be adequate for detection.
In contrast to ozone changes, UV attenuation by aerosols and clouds varies
weakly with wavelength, except (occasionally) when clouds contain ozone in significant
amounts. The effects of aerosols and clouds on ground-level UV irradiances have
been studied with the aid of model simulations as well as by measurements (Frederick
and Snell, 1990; Bais et al., 1993; Frederick and Steele, 1995; Blumthaler et
al., 1996; Bodeker and McKenzie, 1996; Kylling et al., 1997). Although an increase
in aerosols will generally lead to a reduction in UV irradiance via backscattering
to space, an increase in radiation is possible, depending on the relative altitudes
of the aerosol and ozone layers and the solar zenith angle (Davies, 1993; Tsitas
and Yung, 1996). Temporal variations in cloudiness and, to a lesser degree,
aerosols lead to large variations in UV irradiance measured at a fixed site.
This degree of natural variability will complicate attempts to detect small
changes in UV irradiance related to potential changes brought about by future
aviation.
Thus, it appears that some existing instrumentation is capable of detecting
future changes in UV caused by changes in ozone and perhaps by aerosols and
clouds. With the current state of technology, properly maintained instruments
should be able to detect UV irradiances that differ from the unperturbed state
by 2-5% or more. Whether it would be feasible to distinguish UV changes caused
by aircraft emissions from changes caused by the natural background variability
in atmospheric parameters depends on the relative magnitude of these variations
and the duration of the data sets.
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