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Concentration measurements of nitric oxide (NO), nitrogen dioxide (NO 2), nitrous acid (HONO), nitric acid (HNO3), formaldehyde (HCHO), hydrogen peroxide (H2O2), formic acid (HCOOH) and acetic acid (CH3COOH) were performed in air filtered through the pore spaces of the surface snowpack (firn air) at Summit, Greenland, in summer 2000. In general, firn air concentrations of NO, NO2, HONO, HCHO, HCOOH, and CH3COOH were enhanced compared to concentrations in the atmospheric boundary layer above the snow. Only HNO3 and H 2O2 normally exhibited lower concentrations in the firn air. In most cases differences were highest during the day and lowest during nighttime hours. Shading experiments showed a good agreement with a photochemical NOx source in the surface snow. Patterns of H 2O2, 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 grain-air 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. © 2004 Elsevier Ltd. All rights reserved.