Investigation of actinic flux in mountainous areas

The general goal of the proposed project is to improve our understanding of the influence of topography and of three dimensional (3-D) radiation effects on the ultraviolet actinic flux and its implications for photolysis in the lower troposphere under conditions representative for the alpine region. The investigations will be through case studies and will focus on cloudless conditions, a situation very favorable for photochemical pollution. Two approaches will be taken in this investigation: An experimental approach will consist of measurements of the actinic flux, radiance, irradiance and of additional parameters of the surroundings (3-D aerosol characteristics, ground reflectivity, column ozone, aerosol optical depth) which have an influence on the actinic flux. These measurements will be performed within the scope of campaigns in Winter and in Summer at two different alpine locations, where simultaneous measurements will be performed at different altitudes. The theoretical approaches to calculate the actinic flux and the 3-D radiation field will be 3-D radiative transfer models as well as empirical algorithms. The models and algorithms will first be adapted to topographically structured terrain. They will then be validated using the experimental results to determine on the one hand the model input parameters and on the other hand to compare the model output with the measured irradiance, radiances and actinic flux. By using the aforementioned algorithms and models, simulations will be performed for some typical orographical locations and for possible changes in atmospheric particles and trace gases (mainly column ozone) concentrations and changes in surrounding conditions such as snow line and snow reflectance. Using measurements and the models and algorithms, the effect of the different parameters (albedo, aerosols, topography.) on actinic flux and photolysis rate will be quantified. The results of the project will be used to provide advice on how to best account for 3-D radiation effects in photochemical modeling in mountainous areas. Beside the improvements in modeling and measuring methods, the results will also serve to inform the wider UV community such as the biologists, the chemists and medical scientists.

The repartition of the solar radiation and ultraviolet radiation in mountainous region is very complex because of large changes in altitude, of the inclination of the slopes, of obstruction in the valleys by the surrounding mountains which may obstruct the sun and also because of large changes in ground reflectance which may only be in the order of 3% in the UV for snow free soils but around 70 to 95% over snow covered grounds. The general goal of the present project was to improve our understanding of the influence of topography and of three dimensional (3- D) radiation effects on the ultraviolet actinic flux (the radiative flux from all directions on a volume of air) and its implications for photolysis in the lower troposphere under conditions representative for the alpine region. The investigations were done through case studies and focussed on cloudless conditions. We choose two approaches: an experimental approach and a model oriented approach: The experimental approach consisted of measurements of the actinic flux, radiance, irradiance and of additional parameters of the surroundings (aerosol characteristics, reflectivity, column ozone, aerosol optical depth) which have an influence on the actinic flux. These measurements were performed within the scope of campaigns in Winter and in Summer in the region of Innsbruck and in the region of Sonnblick. Simultaneous measurements of actinic flux were performed the first time at different altitudes. We were able to improve the measurement methods and instrumentation on one side and to improve 3-dimensional modelling on the other side. Regarding the experimental improvements achieved, an improvement of all sky camera imaging system was achieved which now allows a quantification of the amounts of clouds, expressed as percentage amount of the sky covered by clouds (`total cloud cover`). The theoretical approaches to calculate the actinic flux and the 3-D radiation field was 3-dimensional radiation modelling as well as 1 dimensional radiation modelling. We found a solution to deal with the pixel discontinuity problem and were able to considerably reduce the calculation time of the 3-D model. The results using both approaches showed a strong dependence of UV radiation, UV actinic flux and photolysis rate on altitude. Increases in UV radiation of up to 100%/1000m and increases in actinic flux and photolysis rate of up to 200% /1000m or more were found when the valley pixels were shaded. Beside the altitude effect, the topography especially leads to strong inhomogeneities in the UV radiation, actinic flux and photolysis rate by shading effect which may reduce these quantities by more than 100%. The effect of the ground reflectance on these quantities was less strong and in the order of 10%.