5.3. Detectability of Changes in Ground-Level Irradiance

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.

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.

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