Spatially resolved quantification of the advection influence on the balance closure of greenhouse gases
Final Report Abstract
A closing gap for energy balance measurements using the eddy covariance (EC) method is observed although all necessary steps of data processing are applied to the raw data. Mainly during nighttime, an imbalance of surface energy fluxes measured by EC is observed probably caused by advective fluxes that consequently should also affect the balance of greenhouse gases (GHG’s) like CO2. The crucial need for representative concentration and wind data to measure advective fluxes was the motivation for the project SQuAd (Spatially resolved quantification of the advection influence on the balance closure of greenhouse gases). Line-averaging measurement methods are generally able to determine the required quantities. An innovative combination of ground-based remote sensing methods of acoustic tomography (A-TOM) and open-path Fourier transform infrared spectroscopy (OP-FTIR) enables the necessary transition from point to spatially resolved local scale. OP-FTIR instrumentation offers the significant advantage of real-time simultaneous measurements of line-averaged concentrations for CO2 and other GHG’s. A-TOM is a scalable method to resolve 3-D wind and temperature fields remotely. The principal objective of the project was to develop and test an innovative method combination of A-TOM and OP-FTIR to obtain spatially and temporally integrated and resolved information about wind components and CO2 concentration applied for a better understanding of advection processes. A joint experiment using A-TOM and OP-FTIR techniques as well as additional measurement equipment was carried out within the SQuAd project at the FLUXNET and ICOS site Grillenburg. The grassland test site for long-term EC measurements is located in the middle of a large clearing (40 hectare area) within the Tharandt Forest, 30 km away from Dresden in Germany. The special observation period was carried out in July, 2016. Two periods were of special interest due to high solar radiation during the day and the building up of a stably stratified boundary layer during nighttime. Advection occurs frequently under these conditions. The total area under investigation by two scanning passive OP-FTIR devices was approx. 120 m x 120 m. A-TOM was configured within this area and in such a way that spatially averaged wind velocities could be measured in two different heights above the ground (1.5 m, 3.0 m). Results of the comprehensive experiments reveal a mean night-time horizontal advection of CO2 of about 10 µmol m-2 s-1 estimated by the spatially integrating and representative SQuAd method. Additionally, uncertainties in determining CO2 concentrations using passive OP-FTIR and wind speed applying A-TOM are systematically quantified. The maximum uncertainty for CO2 concentration was estimated due to environmental parameters, instrumental characteristics, and retrieval procedure with a total amount of approx. 30 % for a single measurement. Instantaneous wind components can be derived with a maximum uncertainty of 0.3 m s^-1 depending on sampling, signal analysis, and environmental influences on sound propagation. Averaging over a period of 30 minutes, the standard error of the mean values can be reduced by a factor of at least 0.5 for OP-FTIR and 0.1 for A-TOM depending on the required spatial resolution. Possibilities to decrease these uncertainties include a higher redundancy and frequency of measurements as well as further methodological developments of the remote sensing methods. Simultaneously, two-dimensional numerical modeling of the atmospheric boundary layer was applied as an effective and flexible tool to determine the link between the measured exchange processes at one location and the source area of fluxes. The appropriate model development allows a generalization of the results obtained by measurement for other sites and longer periods. The presented SQuAd-approach offers the possibility to complement previous findings of multi-location, point-like measurements. Thereby, one advantage of the A-TOM/OP-FTIR-method is the combined measurement of wind components and temperature together with several GHG’s along similar paths and air volumes. Though, there are still tasks concerning the improvement of combined measurement methods within the SQuAd-approach, the project provides first examples of applying the new method to estimate a spatially representative advection during calm and stably stratified nighttime conditions at a grassland site.
Publications
- (2016): SQuAd – Approach for the Spatial Quantification of the Advection influence on the balance closure of greenhouse gases. AGU Fall Meeting, Abstract B13E-0660
Schütze, C., Barth, M., Hehn, M., Ziemann, A.
- (2017): Line-averaging measurement concept to determine CO2 advection – possibilities and uncertainties. Potsdam Greenhouse Gas Workshop “From Photosystems to Ecosystems”, Abstract P2017-6
Ziemann, A., Schütze, C., Starke, M.
(See online at https://doi.org/10.13140/RG.2.2.29614.56643) - (2017): Line-averaging measurement methods to estimate the gap in the CO2 balance closure – possibilities, challenges and uncertainties. Atmos. Meas. Tech., 10, 4165-4190
Ziemann, A., Starke, M., Schütze, C.
(See online at https://doi.org/10.5194/amt-10-4165-2017) - (2018): Application of Open-Path Fourier Transform Infrared Spectroscopy for atmospheric monitoring of a CO2 back-production experiment at the Ketzin Pilot site (Germany). Environ. Monit. Assess., 190:114
Sauer U., Borsdorf H., Dietrich P., Liebscher A., Möller I., Martens S., Möller F., Schlömer S., Schütze C.
(See online at https://doi.org/10.1007/s10661-018-6488-7)
