The analysis of isotope ratios is of increasing importance to study the sources and sinks of atmospheric trace gases and to investigate their chemical reaction pathways. In the frame of this thesis, the isotopic composition of methane trapped in firn air from an Arctic and an Antarctic sites has been measured. From firn profiles, and thanks to the recursive use of a physical model of diffusive gas transport through the firn, the evolution of atmospheric δ13 CH4 could be reconstructed for both sites. The compilation of existing data with this study results makes it possible to infer a δ13 CH4 evolution of about +0.03‰/an in the southern hemisphere and +0.007‰/an in the northern hemisphere, since 1950 AD. A sensible method for extraction and analysis of nitrous oxide in air occluded in polar ice has been developed and successfully tested, thus giving access to the N2O isotopic composition during pre-industrial era. The isotopic composition of N2O trapped in firn and ice from one Arctic and one Antarctic sites has been measured. The experimental profiles as well as the use of the diffusive model, also used for CH4, lead to the continuous evolution of the N2O isotopic signal in the atmosphere, since the industrial era. By combining both sites results, the evolution of δ15N2O, 1δ15N2O, 2δ15N2O et δ18(O)N2O could be inferred: it is about -2.7‰, -2.4‰, -3.0‰ et -2.0‰ since 1700 AD, respectively. A trend since 1970 AD could also be deduced. The isotopic enrichment observed in the case of methane and the isotopic depletion deduced for nitrous oxide confirm a major anthropogenic sources responsibility in the exponential growth of these gases concentration since pre-industrial times and allow to suggest more restrictive scenarios about involved sources and sinks. Finally, although an experimental set up dedicated to the analysis of carbon monoxide isotopic composition by continuous flow mass spectrometry was not sufficient enough to lead to a stable system, many improvements have been made in order to be able to measure the isotopic ratios of this gas, in a near future.
Evolution temporelle du méthane et du protoxyde d'azote dans l'atmosphère : contrainte par l'analyse de leurs isotopes stables dans le névé et la glace polaires.