The Greenland Summit Halogen-HO(x) (GSHOX) Campaign was performed in spring 2007 and summer 2008 to investigate the impact of halogens on HO(x) (= OH+HO(2)) cycling above the Greenland Ice Sheet. Chemical species including hydroxyl and peroxy radicals (OH and HO(2) + RO(2)), ozone (O(3)), nitrogen oxide (NO), nitric acid (HNO(3)), nitrous acid (HONO), reactive gaseous mercury (RGM), and bromine oxide (BrO) were measured during the campaign. The median midday values of HO(2) + RO(2) and OH concentrations observed by chemical ionization mass spectrometry (CIMS) were 2.7x10(8) molec cm(-3) and 3.0x10(6) molec cm(-3) in spring 2007, and 4.2x10(8) molec cm(-3) and 4.1x10(6) molec cm(-3) in summer 2008. A basic photochemical 0-D box model highly constrained by observations of H(2)O, O(3), CO, CH(4), NO, and J values predicted HO(2) + RO(2) (R = 0.90, slope = 0.87 in 2007; R = 0.79, slope = 0.96 in 2008) reasonably well and under predicted OH (R = 0.83, slope = 0.72 in 2007; R = 0.76, slope = 0.54 in 2008). Constraining the model to HONO observations did not significantly improve the ratio of OH to HO(2) + RO(2) and the correlation between predictions and observations. Including bromine chemistry in the model constrained by observations of BrO improved the correlation between observed and predicted HO(2) + RO(2) and OH, and brought the average hourly OH and HO(2) + RO(2) predictions closer to the observations. These model comparisons confirmed our understanding of the dominant HO(x) sources and sinks in this environment and indicated that BrO impacted the OH levels at Summit. Although, significant discrepancies between observed and predicted OH could not be explained by the measured BrO. Finally, observations of enhanced RGM were found to be coincident with under prediction of OH
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