Pollution of the snow surface with polycyclic aromatic hydrocarbons during the formation of frost

When analyzing chemical compositions of snow the high variability of content of polycyclic aromatic hydrocarbons (PAHs) in snow cover between snowfalls is observed. Researchers explain this by concentrating of snow. However, another mechanism of atmospheric contamination of the snow cover surface is possible. It may be a precipitation of fine crystals of PAHs from the atmosphere in the composition of cryohydrates, which can form aerogenic anomalies on the snow surface at formation of hoarfrost. The process starts in the atmosphere during the interaction of finely dispersed crystals of PAHs with cloud supercooled drops. This results in the cryogenic concentration of solid particles of PAHs by way of formation of solid eutectic mixture – cryohydrates, which are a two-phase system consisting of a fine mixture of crystals of solid particles and ice. Evidence of their manifestation is the presence of the Forel hatching on the surface of the facets of the hoarfrost crystals appearing due to the different optical density of alternating zones, which consist of interpenetrating domains of crystallized solid aerosols and ice. At the same time, due to the presence of temperature inversion over the snow cover and its drying effect on the near-snow layer of air, a stable mass transport of water vapor down to the snow cover is formed, which can initiate the flow of fine cryohydrates from the PAHs. Therefore, the growth of atmospheric ice crystals, begun in the surface atmosphere, continues on the snow surface during formation of hoarfrost, thus creating a special nano-relief of snow cover. The paper presents the results of observations of changes in the concentration of individual PAHs in the upper 18 mm layer of snow at accumulation of the surface hoarfrost during a long period between snowfalls. Some micro-morphological features of the forms of skeletal rime micro-crystals are shown, with which an increase in the nano-roughness of the snow surface is associated, as well as the manifestation of the signal of the aerogenic PAH anomaly on the snow surface. Since the conditions for the formation of surface hoarfrost occur more often than for snowfalls, the hoarfrost may be an informative object of testing when detecting hydrocarbon contamination of snow cover during the intervals between snowfalls