Effects of desert dust and ozone on the ultraviolet irradiance at the Mediterranean island of Lampedusa during PAUR II

[1] Observations of the ultraviolet spectral irradiance, tropospheric aerosols, and ozone were carried out at Lampedusa (35.5°N, 12.6°E), during May and June 1999, in conjunction with the Photochemical Activity and Ultraviolet Radiation modulating factors II experiment. Lampedusa is a small Italian island in the Mediterranean, about 100 km off the west coast of Tunisia, North Africa. During the campaign, aerosol transport from Sahara, mainly controlled by the development of high-pressure systems over North Africa, strongly affected the troposphere at Lampedusa. Aerosol optical depths at 415 nm ranged between 0.14 and 0.91. Dust aerosols with an optical depth of 0.5 at 415 nm produced a reduction of the measured irradiance at 350 nm by about 20%, at a solar zenith angle of 30°, and by about 25% at 60°. The occurrence of large aerosol optical depths was associated with low total ozone values, mainly due to the influence of surface pressure on the stratospheric vertical structure and transport. Thus the aerosol attenuation of the UV irradiance was large when low absorption by ozone occurred, and a large ozone absorption occurred when aerosol attenuation was small. Mostly because of their large variability, aerosols produce larger attenuation effects than ozone on the UV irradiance for wavelengths longer than 315 nm. By neglecting the influence of aerosols on the radiation amplification factors, calculation of the dependence of the UV irradiance on ozone can be significantly underestimated. For the erythemally weighted irradiance a value for the radiation amplification factor between −0.1 and 0.7, depending on the solar zenith angle, has been derived. A simple model to take into account the aerosol effect in the determination of the radiation amplification factor has been developed. By applying this model an erythemal radiation amplification factor between 0.8 and 1.25 is obtained. 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. When conditions similar to those of Lampedusa occur, i.e., when large aerosol loads may be present, and for certain meteorological conditions, the effects of the aerosol on the surface ultraviolet irradiance cannot be neglected.