Observations of hydroxyl and peroxy radicals and the impact of BrO at Summit, Greenland in 2007 and 2008

Abstract. 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 + HO₂) cycling above the Greenland Ice Sheet. Chemical species including hydroxyl and peroxy radicals (OH and HO₂ + RO₂), ozone (O₃), nitrogen oxide (NO), nitric acid (HNO₃), nitrous acid (HONO), reactive gaseous mercury (RGM), and bromine oxide (BrO) were measured during the campaign. The median midday values of HO₂ + RO₂ and OH concentrations observed by chemical ionization mass spectrometry (CIMS) were 2.7 × 10⁸ molec cm⁻³ and 3.0 × 10⁶ molec cm⁻³ in spring 2007, and 4.2 × 10⁸ molec cm⁻³ and 4.1 × 10⁶ molec cm⁻³ in summer 2008. A basic photochemical 0-D box model highly constrained by observations of H₂O, O₃, CO, CH₄, NO, and J values predicted HO₂ + RO₂ (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 HO₂ + RO₂ 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 HO₂ + RO₂ and OH, and brought the average hourly OH and HO₂ + RO₂ 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.