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Ultraviolet radiation

A long-term decrease in the stratospheric ozone has been detected by various ground-based and space-borne instruments. The ozone decline is expected to produce an increase of the ultraviolet radiation at the ground. Although some results showing evidence of the influence of the ozone decrease on UV have been reported, due to the presence of other factors able to influence the transfer of the ultraviolet radiation in the atmosphere, to the limited accuracy of the measurements of ultraviolet irradiance, and to the relatively short time interval over which UV measurements are available, the detection of the long-term behavior of the ultraviolet irradiance at the surface of the Earth is still problematic. The first analyses of the long-term behavior of the UV-B irradiance, based on the measurements of the United States network of Robertson-Berger UV-B radiometers over the period 1974–1985, showed an unexpected decrease of the erythemal dose. These results pointed out the need to maintain well calibrated instruments and with continuous oversight, and have led scientists to speculate causes due to other atmospheric parameters, such as clouds, tropospheric ozone, and aerosols, that may influence the UV-B irradiance.
 
Satellite estimates of UV radiation show a large bias (10-20%) with respect to ground-based observations. Thus, ground-based high quality measurements of UV radiation, especially if combined with the observations of other relevant parameters, are particularluy needed.
 
Lampedusa is the southernmost location of Italy, where the high levels of UV and solar radiation are expected, and is particularly indicated for this type of investigations.
 
Measurements are carried out at Lampedusa with a Brewer MK III, and with an UV-MFRSR. The observations show that aerosol play a significant role in the modulation of the UV radiation.

The bottom figure shows two UV spectra measured by the Brewer at the same solar zenith angle, for different total ozone (affecting mostly wavelengths shorter than 320 nm) and aerosol optical depth (whose effect is dominant for wavelengths longer than 320 nm). The upper graph shows the ratio between the irradiance of May 27 and May 18. The values of total ozone and aerosol optical depth at the time of the spectral measurements are reported below.

Date Optical depth at 500 nm Total ozone (Dobson units)
May 18 0.65 293.5
May 27 0.23 347


Figure 2: erythemal irradiance derived from the Brewer UV spectra, at the mean Sun-Earth distance and at the solar
zenith angles of 30°, versus the aerosol optical depth at 415 nm. A linear fit to the data is also displayed

As a result, the erythemal irradiance shows a marked dependence on the aerosol amount. The figure shows the dependence of the erythemal irradiance on the aerosol optical depth at 415 nm, derived from observations carried out in 1999. It is estimated that an aerosol layer of optical depth 0.5 at 415 nm produces a reduction of the erythemal irradiance as large as 25 and 27% at solar zenith angles of 30° and 60°, respectively.