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. The combination of two well-established methods, of quadrocopter-borne air sampling and methane isotopic analyses, is applied to determine thesource process of methane at different altitudes and to study mixing processes. A proof-of-concept study was performed to demonstrate thecapabilities of quadrocopter air sampling for subsequently analysing the methane isotopic composition δ 13C in the laboratory. Theadvantage of the system compared to classical sampling on the ground and at tall towers is the flexibility concerning sampling location, and inparticular the flexible choice of sampling altitude, allowing the study of the layering and mixing of air masses with potentially different spatial originof air masses and methane. Boundary layer mixing processes and the methane isotopic composition were studied at Polder Zarnekow inMecklenburg–West Pomerania in the north-east of Germany, which has become a strong source of biogenically produced methane after rewetting thedrained and degraded peatland. Methane fluxes are measured continuously at the site. They show high emissions from May to September, and a strongdiurnal variability. For two case studies on 23 May and 5 September 2018, vertical profiles of temperature and humidity were recorded up to analtitude of 650 and 1000 m, respectively, during the morning transition. Air samples were taken at different altitudes and analysed in thelaboratory for methane isotopic composition. The values showed a different isotopic composition in the vertical distribution during stableconditions in the morning (delta values of − 51.5 ‰ below the temperature inversion at an altitude of 150 m on 23 May 2018 and at analtitude of 50 m on 5 September 2018, delta values of − 50.1 ‰ above). After the onset of turbulent mixing, the isotopic composition wasthe same throughout the vertical column with a mean delta value of − 49.9  ±  0.45 ‰. The systematically more negative delta valuesoccurred only as long as the nocturnal temperature inversion was present. During the September study, water samples were analysed as well formethane concentration and isotopic composition in order to provide a link between surface and atmosphere. The water samples reveal high variabilityon horizontal scales of a few tens of metres for this particular case. The airborne sampling system and consecutive analysis chain were shown to providereliable and reproducible results for two samples obtained simultaneously. The method presents a powerful tool for distinguishing the source processof methane at different altitudes. The isotopic composition showed clearly depleted delta values directly above a biological methane source whenvertical mixing was hampered by a temperature inversion, and different delta values above, where the air masses originate from a different footprintarea. The vertical distribution of methane isotopic composition can serve as tracer for mixing processes of methane within the atmospheric boundarylayer.

Studying boundary layer methane isotopy and vertical mixing processes at a rewetted peatland site using an unmanned aircraft system