Surface flux estimates derived from UAS-based mole fraction measurements by means of a nocturnal boundary layer budget approach

The carbon exchange between ecosystems and the atmosphere has a large influence on the Earth system and specifically on the climate. This exchange is therefore being studied intensively, often using the eddy covariance (EC) technique. EC measurements provide reliable results under turbulent atmospheric conditions, but under stable conditions – as they often occur at night – these measurements are known to misrepresent exchange fluxes. Nocturnal boundary layer (NBL) budgets can provide independent flux estimates under stable conditions, but their application so far has been limited by rather high cost and practical difficulties. Unmanned aircraft systems (UASs) equipped with trace gas analysers have the potential to make this method more accessible. We present the methodology and results of a proof of concept study carried out during the ScaleX 2016 campaign. Successive vertical profiles of carbon dioxide dry air mole fraction in the NBL were taken with a compact analyser carried by a UAS. We estimate an average carbon dioxide flux of 12 μmol m−2 s−1, which is plausible for nocturnal respiration in this region in summer. Transport modelling suggests that the NBL budgets represent an area on the order of 100 km2