Air bubbles found in polar ice cores preserve a record of past atmospheric composition up to 800 kyr back in time. The composition of the bubbles is not identical to the ancient atmosphere, as it is influenced by processes prior to trapping, within the ice sheet itself, and during sampling and storage. Understanding of these processes is essential for a correct interpretation of ice core gas records. In this work we focus on transport processes in the porous firn layer prior to bubble trapping, and in situ cosmogenic radiocarbon ( 14 C) production in ice.

First, we present a review of firn air studies. We describe the firn air sampling process, the relevant physical characteristics of firn, the different mechanisms of air transport, and the effects of firn air transport on gas records. Second, we present a characterization of the firn air transport properties of the NEEM deep drilling site in Northern Greenland (77.45 o N 51.06 o W). The depthdiffusivity relationship needs to be reconstructed using reference tracers of known atmospheric history. We present a novel method of characterizing the firn transport using ten tracers simultaneously, thus constraining the effective diffusivity better than the commonly used single-tracer method would. A comparison between two replicate boreholes drilled 64m apart shows differences in measured mixing ratio profiles that exceed the experimental error, which we attribute to lateral inhomogeneities in firn stratigraphy. We find evidence that diffusivity does not vanish completely in the lock-in zone, as is commonly assumed. Six state-of-the-art firn air transport models are tuned to the NEEM site; all models successfully reproduce the data within a 1 Gaussian distribution. We present the first intercomparison study of firn air models, where we introduce diagnostic scenarios designed to probe speciffc aspects of the model physics. Our results show that there are major di erences in the way the models handle advective transport, and that dffusive fractionation of isotopes in the firn is poorly constrained by the models.