Abstract
Abstract. From 10 May through 17 June 2007 and 6 June through 9 July 2008 intensive sampling campaigns at Summit, Greenland confirmed that active bromine chemistry is occurring in and above the snow pack at the highest part of the Greenland ice sheet (72°36´ N, 38°25´ W and 3.2 km above sea level). Direct measurements found BrO and soluble gas phase Br− mixing ratios in the low pptv range on many days (maxima < 10 pptv). Conversion of up to 200 pg m−3 of gaseous elemental mercury (GEM) to reactive gaseous mercury (RGM) and enhanced OH relative to HO2 plus RO2 confirm that active bromine chemistry is impacting chemical cycles even at such low abundances of reactive bromine species. However, it does not appear that Bry chemistry can fully account for observed perturbations to HOx partitioning, suggesting unknown additional chemical processes may be important in this unique environment, or that our understanding of coupled NOx-HOx-Bry chemistry above sunlit polar snow is incomplete. Rapid transport from the north Atlantic marine boundary layer occasionally caused enhanced BrO at Summit (just two such events observed during the 12 weeks of sampling over the two seasons). In general observed reactive bromine was linked to activation of bromide (Br−) in, and release of reactive bromine from, the snowpack. A coupled snow-atmosphere model simulated observed NO and BrO at Summit during a three day interval when winds were weak. The source of Br− in surface and near surface snow at Summit is not entirely clear, but concentrations were observed to increase when stronger vertical mixing brought free tropospheric air to the surface. Reactive Bry mixing ratios above the snow often increased in the day or two following increases in snow concentration, but this response was not consistent. On seasonal time scales concentrations of Br− in snow and reactive bromine in the air were directly related.
Highlights
Mid-day peaks of reactive gaseous mercury (RGM) in excess of 200 pg m−3 were highly correlated with solar radiation and nearly stoichiometric decreases in gaseous elemental mercury (GEM) (Brooks et al, 2011)
During several smaller mercury depletion events (MDEs) later in 2007 and early in the 2008 campaign BrO mixing ratios were found to increase with RGM (Brooks et al, 2011; Liao et al, 2011; Stutz et al, 2011)
The Greenland Summit Halogen-HOx Experiment (GSHOX) campaigns have confirmed that active Bry chemistry is occurring in and above the snow in the center of the Greenland ice sheet
Summary
In 1998 and 1999 separate research teams at Alert, Summit, South Pole, and Neumayer discovered that sunlight shining on polar snow caused production of NO and NO2 which were released to the overlying air (Ridley et al, 2000; Honrath et al, 1999; Jones et al, 2000; Davis et al, 2001); for details see Grannas et al (2007) These findings launched a large number of follow on investigations into snow photochemistry at many field sites and in laboratories (summarized in Grannas et al, 2007). Observed OH was about twice predicted values on average (Sjostedt et al, 2007) These investigators speculated that active bromine chemistry, well known to convert HO2 to OH, might account for the unexpected partitioning of HOx observed at Summit. Our observations have shown that this process occured at Summit during
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