Measurement of atmospheric carbon dioxide abundance on board unmanned aircraft – challenges and applications In this dissertation the COmpact Carbon dioxide analyser for Airborne Platforms (COCAP) is presented. COCAP measures the abundance of carbon dioxide (CO2) in ambient air as well as air temperature humidity and pressure and is specifically designed for the use on board small unmanned aircraft systems (UASs). Accurate CO2 measurements are ensured by extensive calibration in an environmental chamber by regular calibration in the field and by chemical drying of sampled air. In addition the analyser is equipped with a custom built lightweight thermal stabilisation system that reduces the influence of ambient temperature changes on the CO2 sensor by two orders of magnitude. The robustness of COCAP under varying environmental conditions has been verified through a series of tests both in the lab and in the field. As a first application of the newly developed instrument COCAP was used to constrain the nocturnal carbon dioxide emission of an ecosystem based on the nocturnal boundary layer (NBL) budget method. The NBL budgets were calculated from a series of CO2 profiles measured by COCAP on board a UAS during the course of two nights. The fluxes obtained in the pilot study are plausible and insensitive to experimental uncertainties. Given the versatility and moderate cost of UASs and their minimal infrastructure requirements this innovative sampling technique makes the NBL budget method for the quantification of surface fluxes more accessible and cost effective. This work demonstrates how the potential of UASs for measuring trace gases in theatmosphere can be exploited thus opening up new possibilities for atmospheric research.