Absorption shielding mechanism

Above the troposphere, in the stratosphere and mesosphere, the gas density becomes increasingly more important, and the thermal conditions become subject to complex variations, both temporal and spatial. At a height of 25-35 km, the oxygen molecules are split by the influence of solar UV radiation to form ozone in accordance with Chapman s mechanism, which is responsible for the absorption of 97% of harmful ultraviolet solar radiation. Devoid of this shield, life on the terrestrial surface would have been doomed to extinction. 

Experiments in which an unstabilized PP film was shielded by a benzophenone or benzotriazole type UVAs showed that both could protect the unstabilized PP film by UV absorption. However, a film shielded by a benzophenone-type UVA-containing film showed a significantly faster degradation than a film containing this stabilizer. This clearly demonstrates that their protection is not only by U V absorption but also by other mechanisms. In contrast, a film shielded with a benzotriazole type UVA is considerably more stable than the shielding film itself, which clearly indicates the dominating role of UV absorption as main stabilization mechanism for this type of UVA [15,16],... 

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