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Rapid temperature change causes fractionation of isotopic gaseous species in air in firn (snow) by thermal diffusion, producing a signal that is preserved in trapped air bubbles as the snow forms ice. Using a model of heat penetration and gas diffusion in the firn, as well as the values of appropriate thermal diffusion constants, it is possible to reconstruct the magnitude of a particular paleoclimate change. Isotopic nitrogen in air serves as a convenient tracer for such paleoreconstruction, because the ratio 29N2/28N2 has stayed extremely constant in the atmosphere for ≥106 years. However, prior to this work no data were available for thermal diffusion of 29N2/28N2 in air, but only in pure N2. We devised a laboratory experiment allowing fractionation of gases by thermal diffusion in a small, tightly controlled temperature difference. A mass spectrometer was employed in measuring the resulting fractionations yielding measurement precision greater than was attainable by earlier thermal diffusion investigators.Our laboratory experiments indicate that the value of the thermal diffusion sensitivity (Ω) for 29N2/28N2 in air is +(14.7 ± 0.5) × 10−3 per mil/°C when the average temperature is –30.0°C. The corresponding value for 29N2/28N2 in pure N2 that we find is +(15.3 ± 0.4) × 10−3 per mil/°C at –30.6°C, in agreement with the previously available literature data within their large range of uncertainty. We find that an empirical equation, Ω = (8.656/TK − 1232/T K2) ± 3% per mil/°C, describes the slight variation of the sensitivity values for 29N2/28N2 in air with temperature in the range of –60 to 0°C. A separate set of experiments also described in this paper rules out adsorption as a candidate for producing additional temperature change-driven fractionation of 29N2/28N2 in the firn air. The combined newly obtained data constitute a calibration of the fossil–air paleothermometer with respect to isotopic nitrogen and will serve to improve the estimates of the magnitudes of past abrupt climate changes recorded in ice cores. [Copyright &y& Elsevier]