Aerosol layering in free troposphere, its impact on modification of the UV irradiation over industrial site in southern Poland

Ceilometers are effective instruments to study atmospheric profiles. They are primarily designed to study cloud base height however are widely used to study aerosol optical properties profiles, aerosol backscatter or extinction coefficients or just atmospheric layering. In this work we utilized statistics of aerosol layers in free troposphere to explain differences between measured and modelled UV radiation at Raciborz station in southern Poland.

Raciborz is a town in southern Poland, close to the Czech Republic border. This area is affected by local urban and industrial pollutions from urban area of Silesia in South Western Poland and North Eastern Czech Republic. Remote locations of aerosols may also play important role in UV radiation modification. The observatory in Raciborz is equipped with CHM-15k Nimbus ceilometer, triple Sun-Sky-Lunar CIMEL ceilometer, and Kipp & Zonen UVS-E-T biometer.

Series of erythema dose rates were compared to that modelled by Tropospheric Ultraviolet-Visible (TUV) radiation transfer model. We used satellite (OMI) ozone, aerosol columnar properties (optical thickness at 340 nm, Angstrom exponent for 340-440nm range as well as single scattering albedo and asymmetry parameter at 440 nm) measured by CIMEL photometer as model input values. The model/observations differences are within +/- 5% range for 90% of measured doses during cloud free conditions. The median of model/observations ratio differences is 0.994. To explain revealed ratio variability a statistical, regression model was developed. A random forest approach applied to normalized erythema doses explained about 52% of model/observations ratio differences by aerosol characteristics in free troposphere.

Aerosol characteristics provided as statistical model input are: mean altitude of aerosol’s layer base and top, geometrical thickness of all layers and sum of aerosol layer backscatter intensity (in arbitral units) normalized by layer geometrical thickness. Aerosol layers are found throughout the year with the highest frequency in August (about 80%  days with at least one layer) and the lowest in November (about 10% days). For days with layers, the mean number of  the layers per day is less variable usually 2-3 layers in one day. The mean depth of the layer  is  larger in summer (1.3 km) than in winter (0.8 km) but the life time of the layer is similar of about 7-8 hours in both periods.

Acknowledgements

This work was supported by Polish National Science Center under grant no. 2017/25/B/ST10/01650.