Surface UV radiation: Past, Present and Future

BAIS A, LUBIN D, .. PIACENTINI R D ET AL

Scientific Assessment of Ozone Depletion: 2006

World Meteorological Organization

Año: 2007; p. 1 - 54

·               New methods to quantify the aerosol optical properties have been developed. These properties have important influences on UV radiation. By combining spectral irradiance and radiance measurements, the effective single scattering albedo and optical depth of aerosols can be derived with reasonable accuracy at locations with moderate to high aerosol optical depth. Under low aerosol conditions, accurate determination of the wavelength dependence of the aerosol optical depth in the UV-B is restricted by calibration uncertainties and by the interference of ozone and SO2. ·               Although the Total Ozone Mapping Spectrometer (TOMS) instrument is no longer available, continuity of satellite-derived global UV data is maintained. The new Ozone Monitoring Instrument (OMI) onboard the National Aeronautics and Space Administration (NASA) Earth Observing System (EOS) Aura spacecraft was launched in July 2004 for continued global monitoring of ozone, other trace gases, and surface UV irradiance. ·               Tropospheric aerosols are responsible for the overestimation of UV irradiance from satellite instruments (e.g., TOMS) that use solar backscattered ultraviolet radiation to derive surface UV irradiance. Although at clean sites the agreement with ground-based measurements is good, over more polluted locations the bias can be as large as 40% because the lowermost atmosphere containing the absorbing aerosols is not adequately probed. The presence of clouds, and snow or ice cover, can also lead to significant biases. New algorithms have been developed to improve the parameterization of aerosol and snow and ice effects on satellite-derived surface UV irradiance, as well as of cloud effects using Advanced Very High Resolution Radiometer (AVHRR) and METEOSAT images, showing on average good agreement with ground-based UV observations. ·               Further improvements have been made in UV measuring instruments and techniques. For example, a transportable spectroradiometer was compared with instruments at more than 25 sites in Europe. The uncertainty of well-maintained spectroradiometers, however, could not be significantly reduced in recent years, mainly due to the remaining difficulties with lamp calibrations. Diode array and CCD spectrographs record the entire spectrum in a fraction of a second, allowing for better spectral characterization of cloud effects of UV radiation, but their intrinsic stray-light problem limits their use in the UV-B. Narrowband multifilter radiometers are now used in several networks providing more information than broadband radiometers and are a useful supplement to well-maintained spectroradiometers. Those that include rotating shadow-bands provide, in addition, estimates of the aerosol optical properties. ·               Algorithms have been developed for converting spectral irradiance to actinic flux, which is more relevant to atmospheric chemistry. This enables us to derive historic actinic fluxes from the 1990s, when the irradiance measurements became widely available. The resulting uncertainties in the actinic fluxes are between 5% and 15% under all sky conditions. ·               Model calculations incorporating only ozone projections show that UV levels will decrease over the next few decades. These calculations imply that UV irradiance is currently close to maximum and under this scenario they will revert to pre-1980 levels at midlatitudes between about 2040 and 2070, but later at southern high latitudes. However, other factors that influence UV are likely to dominate over these time scales. ·               Climate change will also influence surface UV irradiance through changes induced mainly to clouds and surface reflectivity. Aerosols and air pollutants are also expected to change in the future. These factors may lead to either increases or decreases in surface UV radiation, through absorption or scattering. If the projections for future ozone are correct, these factors are likely to dominate future changes in UV radiation.