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

Martin Kunz,Christoph Gerbig,Rainer Gasche,Marcus Schumacher,Richard H Grant,Benjamin Wolf,Frank-Thomas Koch,Jost V Lavric,Matthias Zeeman

doi:10.5194/amt-13-1671-2020

Abstract

Abstract. 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 calm and 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 µmolm-2s-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.

Highlights

The terrestrial biosphere plays a major role in the carbon cycle
The curves illustrate that deconvolution amplifies noise in the data by a factor of 7 if no averaging is applied (τ = 1 s)
If more than 100 samples are averaged (τ ≥ 100 s), the difference between original and deconvoluted data becomes negligible and the uncertainty of the average due to noise is lower than 0.5 μmol mol−1

Summary

Introduction

The terrestrial biosphere plays a major role in the carbon cycle. It has taken up approximately one-quarter of the carbon emitted by human activities since 1750 (Ciais et al, 2013), but the future development of this land sink under a changing climate is uncertain. EC measurements often misrepresent nighttime fluxes (Goulden et al, 1996; Gu et al, 2005). This is related to the stable stratification that often develops close to the surface at night. There is currently no generally accepted solution for how to obtain reliable measurements of nighttime fluxes using the EC technique (Gu et al, 2005; Aubinet et al, 2010; Hayek et al, 2018)

Methods

Results

Conclusion