Mario BLUMTHALER Division for Biomedical Physics, Innsbruck Medical University Mullerstr. 44, A-6020 Innsbruck, Austria
Abstract. The intensity of solar UV radiation at the Earth's surface is highly variable. The most important parameters for cloudless conditions are in the sequence of their significance: solar zenith angle, total ozone content of the atmosphere, amount and type of aerosols, albedo of the surrounding and altitude above sea level. Based on measurements, the effects of these parameters are discussed individually. Furthermore, clouds usually attenuate the irradiance, only in exceptional cases they can lead to an increase over short time periods. The attenuation by clouds is less strong for UV radiation compared with radiation from the whole spectral range.
The ambient levels of solar UV radiation at the Earth's surface play an important role for the whole biosphere, because UV radiation can trigger a large number of effects on living organisms. Furthermore, solar UV radiation is important for photochemistry in the lower troposphere, as several chemical reactions are driven by the absorption of UV radiation. UV radiation also interacts with materials usually leading to changes in the molecular structure. All these effects are a consequence of the relatively high photon energy of UV radiation due to the short wavelengths. In contrast, the absolute amount of energy of solar UV radiation at the Earth's surface is relatively small, compared with the total energy emitted from the sun and received at the Earth.
The absolute level of the UV radiation depends on the intensity of the sun as the source, on astronomical and geographical parameters, on the characteristics of the Earth's atmosphere and on the local conditions of the ground in the surrounding of the measurement station.
The spectrum of solar radiation is primarily defined by the emission from the sun, which is close to an emission of a black body with a temperature of about 5800° Kelvin. When the radiation is passing through the outer part of the solar atmosphere it gets the high fine structure due to absorption by the molecules of the gas. These so called "Fraunhofer lines" appear in all measurements of the solar spectrum, and the smaller the used bandwidth the higher structure of the absorption lines can be observed. This extraterrestrial solar spectrum at the top of the Earth's atmosphere is further modulated in the annual course by the changing difference between sun and Earth. This leads to a variation of the intensity of all wavelengths by about ±3.5% with the maximal value on about 3 January and the minimal one on about 5 July.
Within the Earth's atmosphere, absorption and scattering processes modify the extraterrestrial spectrum. As a consequence of the scattering processes the radiation is separated in a direct and a diffuse component, and this separation is strongly dependent on wavelength.
The direct component is reduced by extinction processes (scattering and absorption) in the atmosphere and can be described with the extinction law of Beer
Io is the intensity outside the atmosphere, I is the intensity at the surface, m denotes the air mass, which is the length of the path of the direct beam through the atmosphere relative to the vertical path length. For the assumption of a plane-parallel atmosphere m depends on the solar zenith angle (SZA) with m=1/cos(SZA), which is a good approximation to the real situation for SZA<85°. Finally, x is the extinction coefficient. The index i indicates the different scattering and absorption processes. In the UV range the most important processes are scattering on molecules (Raleigh scattering), absorption by gases and scattering and absorption by aerosols and clouds. The most important absorbing gas is ozone, which absorbs strongly below 300 nm (Hartley region) and more weakly up to about 330 nm (Huggins band). Further atmospheric gases which absorb in the UV range are nitrogen dioxide and sulphur dioxide, which can be significant in polluted urban environments. Scattering on molecules depends strongly on wavelength (proportional to ^"4), whereas scattering on aerosols only weakly depends on wavelength (about proportional to Scattering on water droplets in clouds is almost independent on wavelength in the UV range.
The usual measurement quantity for solar radiation is "irradiance", which is the energy per time and wavelength interval through a horizontal surface. Therefore the unit is Wm-2nm-1. The combined diffuse and direct irradiance on a horizontal surface is called global irradiance. The share of diffuse irradiance on global irradiance increases with decreasing wavelength and with increasing SZA and further increases with higher amounts of scattering aerosols. As a consequence, in the UV range more than half of global irradiance can be diffuse.
In the following sections the variability of ambient levels of solar global irradiance is discussed based on measurements, which are carried out with spectroradiometric and broadband UV measurements under a large variety of environmental conditions and which allow deriving quantitative estimates of the effects of the influencing parameters. In general, such information could be retrieved also from radiative transfer calculations, but the variability of the input parameters has to be estimated from actual measurements. Therefore, radiative transfer model calculations are a helpful tool, but they cannot replace actual measurements. Most of the broadband measurements of solar UV radiation presented here are carried out with detectors which simulate the human erythema action spectrum  and which indicate therefore directly the erythemally-effective irradiance (GER). For the discussion of the effects of the various parameters on solar UV radiation, these measurements of GER can be interpreted as representative for the UVB range (280 nm to 315 nm), although a small contribution of the UVA range (315 nm to 400 nm) is included in these measurements.
The systematic discussion of the various parameters, which affect solar UV radiation, needs to exclude cloudiness in the first step. Then the most important parameter determining ambient UV levels is the solar zenith angle. In the UVB range the second important parameter is the total ozone content of the atmosphere. In urban environments with local air pollution the amount of aerosols is the next most important parameter. The reflectivity of the ground (albedo) becomes a further important parameter, if the terrain is covered by snow. Finally, the altitude above sea level has a great influence, where usually several other parameters are interacting at the same time. The following detailed discussion of these parameters will show the quantitative effect of each of these parameters individually.
According to Beer's law (see introduction) an increasing air mass as a consequence of increasing SZA will reduce the direct component of global irradiance. This becomes evident in the diurnal course of UV radiation as well as in the seasonal course, when the SZA is smaller at noon time in summer compared to winter. Furthermore, from this a latitudinal gradient arises, because smaller SZA at noon time occur when going from the pole towards the equator.
JUNGFRAUJOCH (3567 m)
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