A reactive Arctic surface layer ? The chemical connection between snow and the atmosphere

Slider image credit : Didier Voisin

The remote nature and conditions of the Arctic make it difficult to routinely observe chemical species in the atmosphere. Observational stations and measurement campaigns are therefore extremely valuable and provide insight into the chemical composition and processes occurring in the Arctic atmosphere. Measurements of chemical species such as ozone, mercury, chlorine, and bromine in the Arctic are of great scientific interest as they are involved in “depletion events” each year during spring. These events refer to the chemical destruction of ozone and mercury at the surface down to very low concentrations compared to background levels. Despite our current knowledge of Arctic atmospheric chemistry, great uncertainty still surrounds the processes driving the release, transport, and deposition of chemical species in the polar regions. In particular, understanding the links between snow, sea ice, and halogen emissions remains a challenge, and is crucial to predicting the potential impacts of a changing Arctic climate on polar atmospheric chemistry.

The goal of this study was to investigate how halogen emissions from coastal snow in the Arctic influence chemical reactivity (e.g., free radical concentrations) within the polar boundary layer. To do this, a 1-dimensional atmospheric chemistry and transport model was co-developed by researchers at IGE and UCLA. The newly developed model (Platform for Atmospheric Chemistry in 1D, PACT-1D) was used to study the role of halogen snow emissions on atmospheric chemistry during the spring 2009 OASIS campaign at Utqiagvik, Alaska (Figure 1). Using PACT-1D, we were able to simulate surface chemical observations from the campaign and show the importance of including halogen emissions from surface snow on boundary layer chemistry. As a result, we propose a promising approach of including halogen emissions from land-based snow in 3D regional and global models.

Our results show that the majority of modeled chlorine (Cl2) is confined to the lowest 15 m of the atmosphere (Figure 2). Cl2 is a highly photolabile species, photolyzing to form chlorine radicals (Cl) which are very reactive atmospheric oxidants. This suggests that the lowest 15 m of the polar springtime atmosphere is highly reactive due to high Cl2 concentrations and Cl oxidation chemistry. The Arctic boundary layer will therefore exhibit strong vertical gradients in chemical composition and reactivity very close to the surface. This has important implications for both observational and modeling activities in the polar regions. Firstly, surface measurements made within this highly oxidizing surface layer are not likely to be representative of the chemistry occurring just several meters aloft. In addition, stable thermal conditions can reduce atmospheric mixing of species between the inversion layer and above, resulting in a very heterogeneous vertical structure. Therefore, assumptions made about polar boundary layer chemistry, inferred from surface measurements, must carefully consider the impacts of both high chemical reactivity and limited vertical mixing on the true chemical behavior of the boundary layer. Secondly, this presents a difficult challenge for 3D models to capture such vertical variation close to the surface due to limitations on vertical model resolution. Most atmospheric chemistry models do not currently include detailed descriptions of land-based snow emissions, which are needed to accurately model Arctic boundary layer chemistry. Work at the IGE will continue to look at how 3D model representation of Arctic atmospheric chemistry can be improved.

Reference :
Ahmed, S., Thomas, J. L., Tuite, K., Stutz, J., Flocke, F., Orlando, J. J., et al. (2022). The role of snow in controlling halogen chemistry and boundary layer oxidation during Arctic spring : A 1D modeling case study. Journal of Geophysical Research : Atmospheres, 127, e2021JD036140. https://doi.org/10.1029/2021JD036140

Author : Shaddy Ahmed, IGE - 10/05/2022