Summary
The firn air "archive" enables reconstruction of atmospheric composition through the past century when many significant changes occurred in the global atmosphere. The large available air samples allow measurements of several trace gases and isotopes that are not normally possible with ice cores. The air age resolution can also be narrower than for ice cores since firn air doesn’t undergo bubble enclosure. Reconstruction of the atmospheric change of a compound from measurements of its profile in the firn is achieved by diffusion modelling, constrained by atmospheric tracers. In most cases, minimal chemical impacts from artefacts and contamination during the enclosure and residence of air in firn or during its extraction are found. However, large fractionations in isotopic ratios are evident and must be corrected by modelling diffusion. Careful site selection and a variety of firn air locations are necessary to produce atmospheric records that are sufficiently accurate, long and well resolved in time to constrain trace gas budgets. Atmospheric records reconstructed from Law Dome and South Pole firn air will be presented that are improving estimates of the emissions of methane, halomethanes and carbon monoxide from natural and anthropogenic sources.