Concentration measurements of nitric oxide (NO), nitrogen dioxide (NO2), nitrous acid (HONO), nitric acid(HNO3), formaldehyde (HCHO), hydrogen peroxide (H2O2), formic acid (HCOOH) and acetic acid (CH3COOH) wereperformed in air filtered through the pore spaces of the surface snowpack (firn air) at Summit, Greenland, in summer2000. In general, firn air concentrations of NO, NO2, HONO, HCHO, HCOOH, and CH3COOH were enhancedcompared to concentrations in the atmospheric boundary layer above the snow. Only HNO3 and H2O2 normallyexhibited lower concentrations in the firn air. In most cases differences were highest during the day and lowest duringnighttime hours. Shading experiments showed a good agreement with a photochemical NOx source in the surface snow. Patterns of H2O2, CH3COOH, and HNO3 observed within the surface snow-firn air system imply that the number of molecules in the snow greatly exceeded that in the firn air. Deduced partitioning indicates that the largest fractions of the acids were present at the ice grainair interface. In all cases, the number of molecules located at the interface was significantly higher than the amount in the firn air. Therefore, snow surface area and surface coverage are important parameters, which must be considered for the interpretation of firn air concentrations.