Measurement and modeling of spectral UV irradiance at Garmisch-Partenkirchen

Short summary:

• The main aim of this work was to combine experimental and theoretical methods to investigate the transfer of ultraviolet radiation through the Earth's atmosphere.
• Global spectral irradiance has been measured continuously over two years, using a Bentham DTM 300 double monochromator.
• The global irradiance data were supplemented by direct spectral irradiance measurements using a narrow-angle entrance optics on a sun-tracking device. These data were used to derive a time series of total ozone and aerosol optical depth which are the most important atmospheric parameters determining the UV irradiance under cloudless sky conditions.
• Using and adapting a radiative transfer model, it has been shown that, at unpolluted locations like Garmisch-Partenkirchen, global spectral irradiance can be simulated within about ±5%, using only total ozone and spectral aerosol optical depth as input to the model.
• The evaluated model is an ideal tool to study the effects of various parameters on UV irradiance because it provides the normalization which is required to separate certain effects. For example, to quantify the attenuation by aerosols, a measured spectrum can be compared with a spectrum simulated for the conditions at the measurement time (that is, the actual solar zenith angle, surface albedo, and total ozone), but for aerosol-free atmosphere. Such investigations are nearly impossible using measurements only because conditions which differ by only one of the parameters are hardly found, even in a data set as large as the one available here.
  
  
  • Aerosols have been shown to reduce global irradiance by typically 5-15%, even at an unpolluted site. In particular, it was found that the decrease of global irradiance scaled approximately linear with the aerosol optical depth, being about 20% for an optical depth of 1.
  • Enhanced albedo due to snow cover in winter was found to increase the global UV irradiance at Garmisch-Partenkirchen by about 10-15%, corresponding to an average effective albedo of 0.38. This relatively low value can be explained by large fractions of the surrounding area which are woodland or city.
  • The monthly averaged reduction of UVA irradiance due to clouds was calculated by normalizing each of the 60,000 measured spectra to the corresponding, model-simulated, cloudless sky irradiance.
  • Thick clouds were found to enhance absorption of UV irradiance by increasing the photon path within the cloud. A longer path corresponds to more absorption, and in an extreme case, the tropospheric pathlength was found to be enhanced by a factor of 10 which, concerning its effect on spectral UV irradiance, acted effectively like a doubling of total ozone. Although such effects occur only when the irradiance is low anyway, they have serious consequences for remote sensing methods which make use of scattered radiation to infer atmospheric parameters like total ozone.