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An imbalance of surface energy fluxes using the eddy covariance (EC) methodis observed in global measurement networks although all necessary correctionsand conversions are applied to the raw data. Mainly during nighttime,advection can occur, resulting in a closing gap that consequently should alsoaffect the CO2 balances. There is the crucial need for representativeconcentration and wind data to measure advective fluxes. Ground-based remotesensing techniques are an ideal tool as they provide the spatiallyrepresentative CO2 concentration together with wind components withinthe same voxel structure. For this purpose, the presented SQuAd (Spatiallyresolved Quantification of the Advection influence on the balance closure ofgreenhouse gases) approach applies an integrated method combination ofacoustic and optical remote sensing. The innovative combination of acoustictravel-time tomography (A-TOM) and open-path Fourier-transform infraredspectroscopy (OP-FTIR) will enable an upscaling and enhancement of ECmeasurements. OP-FTIR instrumentation offers the significant advantage ofreal-time simultaneous measurements of line-averaged concentrations forCO2 and other greenhouse gases (GHGs). A-TOM is a scalable method toremotely resolve 3-D wind and temperature fields. The paper will give anoverview about the proposed SQuAd approach and first results of experimentaltests at the FLUXNET site Grillenburg in Germany. Preliminary results of the comprehensive experiments reveal a mean nighttimehorizontal advection of CO2 of about10 µmol m−2 s−1 estimated by the spatially integratingand representative SQuAd method. Additionally, uncertainties in determiningCO2 concentrations using passive OP-FTIR and wind speed applying A-TOMare systematically quantified. The maximum uncertainty for CO2concentration was estimated due to environmental parameters, instrumentalcharacteristics, and retrieval procedure with a total amount of approximately30 % for a single measurement. Instantaneous wind components can bederived with a maximum uncertainty of 0.3 m s−1 depending on sampling,signal analysis, and environmental influences on sound propagation. Averagingover a period of 30 min, the standard error of the mean values can bedecreased by a factor of at least 0.5 for OP-FTIR and 0.1 for A-TOM dependingon the required spatial resolution. The presented validation of the jointapplication of the two independent, nonintrusive methods is in the focus ofattention concerning their ability to quantify advective fluxes

atmospheric measurement techniques

Line-averaging measurement methods to estimate the gap in the CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; balance closure – possibilities, challenges, and uncertainties