Ultraviolet (UV) radiation, although only about 1% of the sun's total radiative output, initiates and controls the chemical, radiative, and dynamic processes which determine the state of the earth's middle and upper atmosphere. The UV spectrum is divided into UV-C (100 - 280 nm), UV-B (280 - 320 nm), and UV-A (320 - 400 nm) radiation. The UV-C radiation as well as the radiation at shorter wavelengths is absorbed in the upper and middle atmosphere.
When the solar extraterrestrial UV radiation enters the atmosphere, it interacts with the various atmospheric constituents. Scattering occurs in air molecules, aerosol particles and cloud particles. Absorption occurs in air molecules (ozone) and aerosol particles. The variability in the different constituents as well as of the position of the sun in the sky leads to the variability of the radiation measured on the ground.
The UV-B radiation (about 0.5% of the solar radiation) is responsible for most of the effects of sunlight on the body. It is the main cause of sunburn and tanning, as well as the formation of vitamin D3 in the skin, and it influences the immune system. UV-B radiation does not penetrate far into the body; most of it is absorbed in the superficial tissue layers of 0.1 mm depth. However, primary reactions in the superficial layers have consequences throughout the body (van der Leun and Gruijl, 1993). In general, sunburn can be avoid by sensible behaviour. The skin needs to adapt from its winter condition to the increased UV-B irradiance in summer. The avoidance of sunburn depends on going through this process carefully. In general, it has been accepted that the UV-B portion produces the more deleterious biological effects but the UV-A radiation has been shown to produce the same biological effects, most likely through alternative mechanisms (Treina et al., 1996).
The wavelengths responsible for carcinogenesis are determined by experimental observations. The erythemal action spectrum has been defined by the International Lighting Commission (commission internationale de l'\'eclairage, CIE), as a result of various experiments on the human skin (Mc Kinlay and Diffey, 1987). The erythemal action spectrum (see Fig. 1) confirms that the carcinogenesis effectiveness of the UV-B is much higher than at larger wavelengths. In many cases, the irradiance is multiplied with the erythemal action spectrum and integrated over the UV-A and UV-B wavelengths. The result is called erythemal UV irradiance.
Figure 1: Erythemal action spectrum CIE as a function of the wavelength
The atmospheric ozone content, the aerosol particles conditions, the surface albedo, the altitude and the cloud cover influence the UV radiation reaching the ground. In our project, these influences are estimated by analysing Swiss erythemal ultraviolet measurements. The results based on measurements are compared with transfer radiation calculations to better understand the contribution of the various influences. The development of a UV-Index forecast model for Switzerland is also part of the project.
The direct, diffuse and global erythemal UV radiation is measured by the SMI (Swiss Meteorological Institute) at Davos (1600 m a.s.l.) and Payerne (490 m a.s.l.). Two-minutes mean values are recorded since May 1995 by using UV-Biometer (Solar Light Co, model 501). These instruments are checked and calibrated by the Physikalisch- meteorologisches Observatorium and World radiation center (PMOD/WRC) at Davos (http://www.pmodwrc.ch)(see Fig. 2).
Figure 2. UV-Biometer measurements at Davos, March 1-8, 1996. Global (solid curve), direct (dotted curve), and diffuse (dashed curve) radiation. Total ozone amount at Arosa in Dobson units, see Total ozone homepage.
Analysis of measurements. Some results.
Radiative transfer calculations. Some results.
The no-snow and snow models based on clear-sky UV-Biometer measurements at Davos have
been used as basis for the UV-Index (40 * irradiance in W/m2) forecast of the Swiss
Meteorological Institute during summer 1997. These models participated also to the
UV-model intercomparison organized in the COST Action 713 (UVB forecasting). See the COST
713 page for the description of the comparison procedure (http://www.meteo.physik.uni-muenchen.de/strahlung/cost). The Swiss results are described in www_uv_cost.html.
e-mail: anne@atmos.umnw.ethz.ch