Abstract
Abstract. The ultraviolet index (UVI) is the most commonly used variable to inform about the level and potential harmful effect of ultraviolet (UV) radiation reaching the Earth's surface. This variable is derived from the output signal of UV radiometers applying conversion factors from calibration methods. This paper focused on the influence of the use of two of these methods (called one-step and two-steps methods) on the experimental UVI measured by a YES UVB-1 radiometer located in a midlatitude station, Granada (Spain) for the period 2006–2009. In addition, it also analyzes the deviation from the UVI values obtained when the manufacturer's calibration factors are applied. For this goal, a detailed characterization of the UVB-1 radiometer from the first Spanish calibration campaign of broadband UV radiometers at the "El Arenosillo" INTA station in 2007 was used. In addition, modeled UVI data derived from the LibRadtran/UVSPEC radiative transfer code are compared with the experimental values recorded at Granada for cloud-free conditions. Absolute mean differences between measured and modeled UVI data at Granada were around 5% using the one-step and two-steps calibration methods, indicating an excellent performance of these two techniques for obtaining UVI data from the UVB-1 radiometer. Conversely, the application of the manufacture's calibration factor produced a large overestimation (~14%) of the UVI values, generating unreliable alarming high UVI data in summer. Thus, the number of days with an extreme erythemal risk (UVI higher than 10) increased up to 46% between May and September at Granada. This percentage reduced to a more reliable value of 3% when the conversion factors obtained with the two-steps calibration method are used. These results evidence the need for a sound calibration of the broadband UV instruments in order to obtain reliable measurements.
Highlights
The well-known depletion of the stratospheric ozone layer in the past decades causes concern about increasing ultraviolet (UV) radiation at the surface (UNEP, 2006)
The UV index (UVI) is defined as 40 times UV erythemal radiation (UVER) according to the joint recommendation of the World Meteorological Organization (WMO), the World Health Organization (WHO), the United Nations Environmental Programme (UNEP), and the International Commission on Non-Ionizing Radiation Protection (ICNIRP) (WMO, 1998; WHO, 2002)
This study has shown that the actual conversion factor corresponding to the YES UVB-1 instrument localized at Granada is around 10% smaller than the calibration factor supplied by the manufacturer
Summary
The well-known depletion of the stratospheric ozone layer in the past decades causes concern about increasing ultraviolet (UV) radiation at the surface (UNEP, 2006). Anton et al.: Influence of the calibration on experimental UV index at a midlatitude site, Granada elevations reached at these latitudes throughout the year This fact together with the changes in the population’s habit associated with sun overexposure makes indispensable to obtain high-quality UVI measurements in this region in order to inform people and to reduce the risk of health damage. The intercomparison of broadband UV radiometers with respect to a reference spectroradiometer (traceable to international standards) provides a practical way for improving the accuracy and comparability of the measurements performed by different instruments (Leszczynski et al, 1995; Bais et al, 1999; Grobner et al, 2007; Vilaplana et al, 2009) In this context, it is highly interesting to study different calibration methods for measuring daily UVI in order to improve the accuracy of these values.
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