Observations and Modeling Studies of Atmospheric Radical Chemistry on the Greenland Ice Sheet

Chemical processing of pollutants, such as methane, hydrocarbons, and mercury, is an important aspect of the Arctic system. These pollutants, which most often originate at lower latitudes, react with various atmospheric radical species potentially forming ozone and aerosol, as well as transforming elemental mercury into more toxic compounds that can enter the arctic ecosystem. In addition, HNO3 deposited onto the snow has the potential to undergo chemical transformation to gas phase NOx above the snow pack. Despite the important role of atmospheric chemistry for the fate of air pollutants in the arctic our understanding of the underlying processes is incomplete. In particular the influence of snow and firn on the levels of OH/HO2 and halogen radicals is not well understood.

Motivated by observations unusually OH/HO2 radical ratios levels as well as indirect evidence that reactive halogens are present on the Greenland ice sheet, two field campaigns have been conducted at Summit, Greenland in 2007 and 2008 to study air and snow properties with a particular focus on halogen and HOx chemistry over snow. The Greenland ice sheet provides a large area for the chemical processing and can also serve as a model for snow-covered locations that are not influenced by marine air. In addition, the high altitude of the ice sheet allows it to be influenced and influence polluted air in the mid-troposphere.

Here we will present the results of our observations of ozone, BrO, and a large number of other species during the two GSHOx field campaigns in 2007 and 2008. Our measurements identified unexpectedly high BrO mixing ratios of up to 4ppt with typical levels in the range of 1-2ppt. In order to understand the chemical and physical processes occurring during these field experiments we have developed a new model for snow physics and chemistry. This model has been coupled to the boundary layer model MISTRA, which includes detailed multiphase chemistry in the atmosphere, with the goal of understanding how chemical species evolve with time in the interstitial air and to quantify the interplay between the chemistry in and above the snow. Both observations and modeling results show the strong impact of snow and firn on the chemistry ambient air.