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Measurements and UV-B modeling

Ultraviolet-B radiation is monitored in Córdoba, Argentina (31o24' S, 64o11' W, 400 meters above sea level) using the Yankee Environmental Systems (YES) pyranometers model UVB-1. These are, to our knowledge, the first measurements obtained in Córdoba with high quality pyranometers. The UVB-1 is a precision radiometer that measures global solar UV-B radiation. Global radiation includes both light transmitted directly through the atmosphere and light scattered by atmospheric gases and particulate matter into the atmosphere. The instrument uses colored glass filters and a fluorescent phosphor to convert incoming UV-B radiation into green light, which is then precisely measured by a calibrated solid stated detector. The spectral response of the instrument is similar to the erythemal and DNA damage spectra, making it useful for biological impact studies such as erythemal dose rate studies.

Measurements of solar broad band UV-B irradiances are being carried out since November 1998. The site selected for the measurements represents semi-urban conditions and is along the most frequent wind direction (NE-SW) with reference to downtown. The equipments are mounted on a wide open area in the University Campus. We are also measuring and calculating Erythemally Effective Radiation (EER) which describes the effect of UV-B radiation on the skin (skin-reddening). To convert the UVB-1 instrument output voltage into either effective irradiance or CIE-defined erythemal irradiance in effective Wm^-2 the signal voltage must be multiplied by a conversion factor, provided by the manufacturer, for the solar zenith angle at the time the sample was taken.

As complement of the present study, simultaneous observations of total solar radiation (300-3000 nm) are taken using a YES Total Solar Pyranometer model TSP 700. Also, diffuse UV-B radiation is estimated using another UVB-1 pyranometer with a homemade device which consists in a shadow band approximately 15 cm above the detector to shadow it. The National Observatory of Córdoba provides the additional meteorological data.

Radiative transfer models are an important complement to measurements. Models are also an essential aid to identify the causes of observed UV changes, to carry out sensitivity studies, and ultimately to predict future UV environments under different atmospheric O3 reduction scenarios. The global UV-B measurements obtained in this work were compared with radiative transfer model calculations. For our model calculations we used the Sasha Madronich radiative transfer code (1995). This model calculates the direct UV-B irradiance, ultraviolet induced erythema and the photolysis rate constants for several species. The input parameters for the radiative transfer code are date and time, latitude and longitude, surface elevation, total column ozone amount and surface albedo. For our calculations, default temperature, ozone and aerosol profiles were used due to the lack of these data in Cordoba City. All the calculations were carried out under clear sky conditions. The surface reflectivity was assumed to be 5% for the spectral region concerned and for the environment of the measuring site.