Active radical chemistry has been observed in the air above sunlit snow surfaces. Past observations of hydroxyl radicals in Greenland showed evidence of unrecognized chemical processes leading to OH concentrations different from those predicted in constrained box models. To investigate if halogen chemistry could explain the observations of unusual hydroxyl radical levels, the Greenland Halogen and Hydrogen Radical Photochemistry Experiment (GSHOX) was performed at Summit Greenland in the summers of 2007 and 2008.

In this special section the following results of the GSHOX experiment will be presented.

• meteorological and radiative conditions that impact atmospheric chemistry at Summit;
• direct evidence for the presence of reactive bromine;
• indirect evidence of halogen chemistry through hydrocarbon clock;
• sources of bromine and bromide on the Greenland ice sheet;
• impact of halogen chemistry on OH/HO₂ chemistry;
• impact of halogen chemistry on mercury chemistry;
• NO_x and bromine chemistry in the firn;
• influence of firn chemistry on atmospheric chemistry.