The role of the snowpack on the fate of alpha-HCH in an atmospheric chemistry-transport model.

A dynamic snowpack module was implemented in the Danish Eulerian Hemispheric Model Persistant Organic Pollutants (DEHM-POP), an atmospheric chemistry-transport model designed to study the environmental fate of persistent organic pollutants in the Northern Hemisphere. The role of the snowpack on the fate of alpha-hexachlorocyclohexane (alpha-HCH) was investigated by making simulations both with and without the formation of a snowpack and comparing model results with data from 21 air monitoring sites. The inclusion of a dynamic snowpack module in the DEHM-POP model generally improves the fit between modeled and observed alpha-HCH air concentrations for the winter and spring seasons and the overall correlation coefficient between predicted and observed concentrations are improved at 8 of the sites. The predicted snowpack concentrations are in good agreement with the few available snow measurements from the Arctic. The presence of a snowpack increases surface boundary layer air concentrations of alpha-HCH at midlatitudes, while the effect is more pronounced in the Arctic due to the longer periods of snow cover. The results indicate that the snowpack module in DEHM-POP acts as a fast-exchanging temporary storage medium for alpha-HCH, as significant fractions were rapidly revolatilized back into the atmosphere following deposition with snowfall, although the current parametrization for vapor-exchange probably over emphasizes this process. Nonetheless, increased air concentrations observed between March and May ("spring maximum events"; SME) at several high latitude monitoring stations are also predicted by the model. The model results indicate that the SMEs are associated with the revolatilization of previously deposited chemical from the snowpack, following a reduction in the capacity of the snowpack to retain alpha-HCH with increasing temperatures toward the end of the winter period, rather than the actual melting of the snowpack. The SMEs are not predicted at all the Arctic monitoring sites by the model, and the significance of the snowpack in controlling these in the model is, therefore, open to question given the uncertainties in the snow-air partition coefficient (K(sa)) and the reliance of the model on a one-layer snowpack rather than a multilayered snowpack.