Abstract
Abstract. Reactive halogens, and in particular bromine oxide (BrO), have frequently been observed in regions with large halide reservoirs, for example during bromine catalyzed coastal polar ozone depletion events. Much less is known about the presence and impact of reactive halogens in areas without obvious halide reservoirs, such as the polar ice sheets or continental snow. We report the first LP-DOAS measurements of BrO at Summit research station in the center of the Greenland ice sheet at an altitude of 3200 m. BrO mixing ratios in May 2007 and June 2008 were typically between 1–3 pmol mol−1, with maxima of up to 5 pmol mol−1. These measurements unequivocally show that halogen chemistry is occurring in the remote Arctic, far from known bromine reservoirs, such as the ocean. During periods when FLEXPART retroplumes show that airmasses resided on the Greenland ice sheet for 3 or more days, BrO exhibits a clear diurnal variation, with peak mixing ratios of up to 3 pmol mol−1 in the morning and at night. The diurnal cycle of BrO can be explained by a changing boundary layer height combined with photochemical formation of reactive bromine driven by solar radiation at the snow surface. The shallow stable boundary layer in the morning and night leads to an accumulation of BrO at the surface, leading to elevated BrO despite the expected smaller release from the snowpack during these times of low solar radiation. During the day when photolytic formation of reactive bromine is expected to be highest, efficient mixing into a deeper neutral boundary layer leads to lower BrO mixing ratios than during mornings and nights. The extended period of contact with the Greenland snowpack combined with the diurnal profile of BrO, modulated by boundary layer height, suggests that photochemistry in the snow is a significant source of BrO measured at Summit during the 2008 experiment. In addition, a rapid transport event on 4 July 2008, during which marine air from the Greenland east coast was rapidly transported to Summit, led to enhanced mixing ratios of BrO and a number of marine tracers. However, rapid transport of marine air from the Greenland east coast is rare and most likely not the main source of bromide in surface snow at Summit. The observed levels of BrO are predicted to influence NOx chemistry as well as impact HOx partitioning. However, impact of local snow photochemistry on HOx is smaller than previously suggested for Summit.
Highlights
The presence of reactive halogens and their impact on boundary layer chemistry has been well documented in the coastal Arctic and Antarctic (e.g. Barrie et al, 1988; Hausmann and Platt, 1994; Tuckermann et al, 1997; Saiz-Lopez et al, 2007) as well as lower latitudes (e.g. Saiz-Lopez et al, 2004; Peters et al, 2005; Read et al, 2008; Osthoff et al, 2008)
Using FLEXPART, a Lagrangian Particle Dispersion Model (LPDM) (Stohl et al, 2002), we evaluated the origin of air masses observed during the measurement periods and the potenital for sources of gas-phase bromine
While our correlation analysis is obviously not conclusive, we propose that local photolytic formation of reactive bromine at the surface, and most likely in the snow and firn, explains the bromine oxide (BrO) measured by our long-path Differential Optical Absorption Spectroscopy (LP-DOAS)
Summary
The presence of reactive halogens and their impact on boundary layer chemistry has been well documented in the coastal Arctic and Antarctic (e.g. Barrie et al, 1988; Hausmann and Platt, 1994; Tuckermann et al, 1997; Saiz-Lopez et al, 2007) as well as lower latitudes (e.g. Saiz-Lopez et al, 2004; Peters et al, 2005; Read et al, 2008; Osthoff et al, 2008). Reactive halogen chemistry has been observed near surface salt deposits, i.e. salt lakes and salt pans J. Stutz et al.: Longpath DOAS observations of surface BrO at Summit close to abundant sources of halides which, through multiphase chemical or biological processes, are released into the gas-phase as reactive halogen species. Recent observations of ClNO2 in Boulder (Thornton et al, 2010) point to the presence of halogens in the continental urban boundary layer. A recent study by Sjostedt et al (2007) proposed the presence of reactive bromine at Summit, Greenland, to explain the discrepancy between observed OH concentrations and those calculated by a highly constrained box model. Small amounts of chloride and bromide are found in surface snow at Summit (see Dibb et al, 2010), but it is to date unclear if reactive gaseous halogens are present at sufficiently high levels to have an influence on atmospheric chemistry
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