Abstract. Ultraviolet solar irradiance (290-385 nm) was analyzed in Valencia and IntroductionUltraviolet solar radiation has a significant influence on the Earth's atmosphere. In the stratosphere the absorption of ultraviolet radiation causes an increase of the air temperature in the upper atmospheric layers and is the origin of the existence and variability of stratospheric ozone. On the surface, UV solar radiation represents a small part of the spectrum, although it is an important part because of its biological and photochemical effects. Thus a clear awareness of its availability and its variation, both temporal and spatial, is of great importance in many research areas: (1) Several studies have been conducted to quantify the attenuation of UV radiation by clouds. Factors such as cloud location, percent cover, cloud optical thickness, liquid water content, and particle distribution make it difficult to develop a quantitative relationship relating cloud properties to the attenuation of UV radiation. Some authors [Zavodska and Reichrt, 1985] classify the daily total and UV radiation measurements using the cloud amount as a parameter. Cloud cover is not a very objective measurement because it may be related to very different sky conditions, depending on the relative Sun-cloud position. In this sense, Estupi•an et al. [1996] showed that for the same cloud cover, reductions of up to 99% or increases of up to 27% of UV radiation may be produced with respect to the same measurement on clear days in terms of whether or 4759

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Abstract-We present a preliminary study of some optical properties of atmospheric aerosols over the area of Valencia, Spain, a coastal Mediterranean city. Measurements of spectral direct irradiance in the 300-1100 nm range were taken simultaneously at three sites: rural-continental, rural-coastal, and urban-coastal, all located within a 50 km radius of the city of Valencia. The irradiance measurements were obtained using three Li-cor 1800 spectroradiometers provided with radiance limiting tubes with field of views (FOVs) of 4.7 . The measurements were made under clear sky conditions during a field campaign carried out in the summer of 1998. In order to avoid the uncertainties associated with the determination of the water vapor content and the other atmospheric constituents, the analysis of the spectral aerosol optical thickness (AOT) values was limited to the 400-670 nm spectral band. From the values of the spectral AOT, both the Angstrom coefficients and the aerosol size distributions were obtained. The results show the great dependence of the optical aerosol characteristic on the direction of the prevailing winds (maritime or continental) in this area.

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