Examining the role of sea ice and meteorology in Arctic boundary layer halogen chemistry

Abstract

Given the ubiquitous nature of ice, chemistry taking place on ice surfaces has a substantial effect on the environment, particularly in the polar regions. The return of sunlight to the polar regions releases halogen radicals (e.g. Br, Cl and their oxides, e.g. BrO) generated from salts on ice surfaces. These radicals fundamentally alter the chemistry of the Arctic boundary layer through processes such as boundary-layer ozone depletion events and mercury deposition events. Current understanding of the chemical processes involved in Arctic halogen chemistry is inhibited by a lack of knowledge about the ice surfaces on which this chemistry is thought to take place, as well as the sparsity of long-term field observations of this chemistry and its effects. This dissertation addresses both needs through a combination of laboratory experiments and long-term field studies. First, we use X-ray absorption computed micro-tomography at the Advanced Photon Source to image brine distributions within laboratory grown mimics of sea-ice features. These experiments showed that when brine is introduced to ice via wicking of brine from a saline surface, the resulting brine distribution is heterogeneous, with brine existing in distinct regions within the sample, rather than evenly spreading over the sample surface. To examine the horizontal and vertical extent of halogen chemistry in the Arctic boundary layer, we conducted long-term measurements of BrO at Barrow, Alaska using Multiple-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS). We developed a method to reduce these measurements to timeseries of near-surface and total column amounts of BrO. These measurements showed that the vertical distribution is highly variable, ranging from shallow layer events confined to the lowest 200 m, to distributed column events, which have lower mixing ratios of BrO, but are more distributed throughout approximately the lowest kilometer of the atmosphere. We find that the observed vertical distributions of BrO are influenced by atmospheric stability. We found minimal influence of wind speed on either lower-tropospheric bromine activation (LT-VCD) or the vertical distribution of BrO, while examination of seasonal trends and the temperature dependence of the vertical distribution support the finding that atmospheric stability affects the distribution of BrO. While shallow layer events have higher concentrations of halogens, distributed column events tend to have higher overall amounts of activation, implying that in situ near surface measurements may be insufficient to constrain the role of environmental parameters in the activation of halogens. Examination of multiple years of data at Barrow, Alaska shows that time spent in first year ice (FYI) areas is weakly linearly correlated (R=0.38) with the activation of BrO. However, examining annual averages of BrO shows that despite the non-linear relationship between time in FYI areas and BrO, time spent in FYI areas still influences the interannual variability of BrO.