Abstract
Abstract. Measurements of gas phase soluble bromide in the boundary layer and in firn air, and Br− in aerosol and snow, were made at Summit, Greenland (72.5° N, 38.4° W, 3200 m a.s.l.) as part of a larger investigation into the influence of Br chemistry on HOx cycling. The soluble bromide measurements confirm that photochemical activation of Br− in the snow causes release of active Br to the overlying air despite trace concentrations of Br− in the snow (means 15 and 8 nmol Br− kg−1 of snow in 2007 and 2008, respectively). Mixing ratios of soluble bromide above the snow were also found to be very small (mean <1 ppt both years, with maxima of 3 and 4 ppt in 2007 and 2008, respectively), but these levels clearly oxidize and deposit long-lived gaseous elemental mercury and may perturb HOx partitioning. Concentrations of Br− in surface snow tended to increase/decrease in parallel with the specific activities of the aerosol-associated radionuclides 7Be and 210Pb. Earlier work has shown that ventilation of the boundary layer causes simultaneous increases in 7Be and 210Pb at Summit, suggesting there is a pool of Br in the free troposphere above Summit in summer time. Speciation and the source of this free tropospheric Br− are not well constrained, but we suggest it may be linked to extensive regions of active Br chemistry in the Arctic basin which are known to cause ozone and mercury depletion events shortly after polar sunrise. If this hypothesis is correct, it implies persistence of the free troposphere Br− for several months after peak Br activation in March/April. Alternatively, there may be a ubiquitous pool of Br− in the free troposphere, sustained by currently unknown sources and processes.
Highlights
The discovery in 1998 that NOx was photochemically produced in snow at Summit, Greenland (Honrath et al, 1999) lead to the realization that sunlit snow is a chemically dynamic environment
This paper focuses on measurements of soluble ions in the gas phase, associated with aerosol, and in snow made by the University of New Hampshire, with particular attention paid to Br−
In 2007 the mixing ratio of soluble nitrite approximately 1 m above the snow consistently varied from minima of 1–3 ppt at night to maxima near 10 ppt shortly after local noon (Fig. 1)
Summary
The discovery in 1998 that NOx was photochemically produced in snow at Summit, Greenland (Honrath et al, 1999) lead to the realization that sunlit snow is a chemically dynamic environment. At Summit it soon became apparent that a large number of very active compounds were produced in the snow pack and released to the overlying air (see the 2002 special issue of Atmospheric Environment on snow photochemical investigations at Summit, volume 36, issues 15–16). Calculations with a photochemical box model indicated that the snow to air fluxes of H2O2, HCHO and HONO enhanced their mixing ratios significantly and should result in extremely elevated mixing ratios of HOx (OH plus HO2) just above the snow at Summit in summer (Yang et al, 2002). Sjostedt et al (2007) suggested that the good agreement between measured and predicted HOx indicated that no large sources or sinks of HOx at Summit were being overlooked, but the large underprediction of OH pointed to poor
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