Detection of Long Term Change

Long-term ground-based UV spectral irradiance measurements must be carefully made and analyzed to preclude variations due to clouds that could be mixed into UV trend estimates, or whose variability can mask the detection of small changes. If ground-based data are filtered for cloud-free observations, then UV-B changes caused by the variability in ozone amounts are easily observed in multi-year data records. Aerosols and other forms of pollution can also produce apparent changes in UV irradiance that masks the effect of ozone changes. These changes can be taken into account if measurements are made simultaneously within the UV-B range (e.g., 305 nm) and outside of the ozone absorbing range (e.g., 324 nm). The lack of ability to separate aerosol and pollution effects from ozone-induced changes limits the usefulness of broadband instruments (300 nm - 400 nm) for understanding the observed irradiance changes.

Radiometric and wavelength calibration of spectrometers used for irradiance trend estimates must be carefully maintained to detect the relatively small changes caused by ozone and aerosols. Making accurate spectral measurements can be difficult, since the natural UV spectrum at the ground changes by several orders of magnitude from 300 nm to 400 nm. A slight wavelength misalignment can cause significant errors in the measured UV-B irradiance amount. Wavelength misalignment is less important for broadband wavelength integrated quantities, such as erythemal irradiance. Equally difficult are measurements of UV irradiance ratios at both small (near noon) and large SZAs (near sunrise or sunset) because of the large dynamic range required. The use of sensitive CCD spectrometers and multiple neutral density filters can mitigate the intensity problem at the expense of some additional calibration. Experience to date with the backthinned CCD Pandora spectrometers indicates both the spectrometers and the optics are stable with field calibration and suited for both short- and long-term measurements.

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