This thesis presents investigations into CO2 and CH4 measurements aboard commercial airliners, and potential applications of CO2 profiles from the In-service Aircraft for a Global Observing System (IAGOS) project for carbon cycle studies. The design of a high-accuracy continuous CO2/CH4/H2O analyzer suitable for use aboard commercial airliners has been accomplished based on improvements of a commercially available instrument using the cavity ring-down spectroscopy (CRDS) technique. In order to obtain dry air mixing ratios of CO2/CH4/H2O from direct measurements of wet air, water correction functions have been developed from laboratory experiments. The CRDS analyzer was flown without a drying system or any in-flight calibration gases during a campaign over the Amazon rain forest in Brazil in May 2009. To validate the continuous in situ CO2/CH4 measurements made by the CRDS analyzer against reliable discrete CO2/CH4 measurements from air samples taken with glass flasks during flight, weighting functions have been derived for both single flasks and paired flasks. The comparison results demonstrate that the CRDS analyzer is fully adequate for high-accuracy continuous CO2 and CH4 measurements without a drying system or in-flight calibrations during a field aircraft campaign. To investigate whether profiles from a commercial airliner program are regionally representative, given that these profiles are made near major cities and are potentially contaminated by local pollution, CO profiles over Frankfurt (Main) were assessed as a proxy for fossil fuel CO2 components in a modeling framework. An underestimation of CO enhancements in the planetary boundary layer (PBL) was found, and was attributed to an underestimation of fossil fuel emissions in the inventory. The analyses showed that measurements in the upper half of the PBL are regionally representative and can be useful for carbon cycle studies to constrain biospheric fluxes.