Experimental characterization of UV radiative transfer through the snowpack

The transmission of sunlight within the snowpack is of crucial importance in many environmental processes, such as radiative transfer and terrestrial energy balance, ecology, microbiology and photochemistry. The albedo of freshly fallen snow is typically very high (up to 98%), but due to rapidly changing climatic conditions it can decrease even significantly as a result of metamorphism processes of snow grains and increasing concentration of impurities (such as mineral dust or black carbon). The resulting increase in radiation absorption acts in a positive feedback mechanism on the energy balance of our planet, as it results in an intensification of the melting process and a consequent increase in the Liquid Water Content (LWC) of the snow, which in turn decreases its reflectivity.

In addition, the radiation (not only the visible spectrum, but mainly the UV component) passing through the most superficial layers of the snowpack is capable of both influencing the ecosystem through photosynthesis processes of algae and promoting photochemical reactions involving contaminants that may be present, including mainly mercury, bromine, iodine and lead.

Specifically, the wavelengths of UV-A (320-400 nm) and UV-B (290-320 nm) can cause weaker chemical bonds to be broken and chemical compounds to be released into the boundary layer.

Although some studies have been carried out in the recent past to characterize the transmission of solar UV-VIS radiation within the snowpack, these measurements have been performed in too small numbers and without a common approach. Rather, numerical models and simulations are largely used to this purpose, but this approach is often based on strong assumptions and approximations and include a multiplicity of parameters to which it is often difficult to assign an accurate value. The lack of experimental data and dedicated studies results in a significant gap in scientific research on snow.

In any case, an experimental approach is particularly challenging as well as complicated. Indeed, the solar UV radiation (below 300 nm) that manages to reach the surface of our planet is itself very weak, being already attenuated by the Earth’s atmosphere. This process of signal attenuation is therefore to be taken into account more when the intention is to study the mechanism of radiative transfer within the cryosphere.

Consequently, photodetectors with a large detection area are required to accurately assess its effects deep within the snow. Simultaneously, these sensors must be transparent to visible and infrared wavelengths to prevent unintended heating of the surrounding snowpack, ensuring that the physical system under investigation remains unaltered. At present, sensors with such properties are not commonly found commercially. However, the limited number of photodetectors that could be used for this purpose have a small surface area or operating range that does not allow their reliable use in snow and ice.

Framed within a collaboration between the Instrumental Optics Laboratory and CIMAINA, novel ad-hoc devices are being designed. They consist of a type II heterojunction based on oxides of nickel (NiO) and titanium (TiO2), realized via a sol-gel procedure (dip coating) from specific metal precursors.