Virtual Tall Towers: First test results for the German Integrated Greenhouse Gas Monitoring System

Continuous accurate monitoring of greenhouse gas concentrations on a large number of tall towers (TT, approximately 100 m above ground level and higher) is increasingly becoming a tool to independently quantify regional emissions (top-down approach). However, installing or finding and equipping tall towers is costly. To densify the existing monitoring network as efficiently as possible, a secondary use of existing eddy-covariance flux stations (EC, typically 2 to 50 m above ground level depending on plant canopy height) as “virtual tall towers” (VTT) has been suggested. The basic idea is that the additional flux and turbulence information available at an EC station can be utilised to correct the near-surface concentration measurement, which is heavily influenced by local sinks and sources, towards an estimate that is more representative of the well-mixed part of the atmospheric boundary layer, and thus more indicative of regional emission sources. In the framework of the ITMS (https://www.itms-germany.de/) project, we aim to evaluate the feasibility of such methods on selected pairs of existing EC and TT stations in the ICOS (https://www.icos-cp.eu/) network. For this the EC gas analyzer needs to be calibrated directly or indirectly to the same reference as the TT gas analyzer, which is not commonly the case. Our first tests comprise one pair of an EC (DE-RuS, 2.5 m measurement height) and TT (AS-Jue, 120 m) stations approximately 5.6 km apart in Germany, as well a second pair on a single, tall tower in Sweden (SE-Svb) where EC measurements and shallow TT measurements are available on the same level (35 m). This latter unique situation allows us to estimate the higher TT measurement on the same tower (150 m) without requiring an additional calibration of the EC sensor. Together, the two test cases allow to separate calibration and spatial representativity issues on the one hand, from the actual VTT performance on the other hand. Our first results indicate that accurate, stable calibration at EC sites is crucial but difficult to achieve. In addition to an existing VTT technique (Haszpra et al. 2015), we also test a novel approach based on conditional sampling of eddies carrying air characteristic of the atmospheric boundary layer, building up on earlier work with temperature measurements (Graf et al. 2010).

Haszpra, L., Barcza, Z., Haszpra, T., Patkai, Z. and Davis, K.J., 2015. How well do tall-tower measurements characterize the CO2 mole fraction distribution in the planetary boundary layer? Atmospheric Measurement Techniques, 8(4): 1657-1671. https://doi.org/10.5194/amt-8-1657-2015

Graf, A. et al., 2010. Boundedness of Turbulent Temperature Probability Distributions, and their Relation to the Vertical Profile in the Convective Boundary Layer. Bound.-Layer Meteor., 134(3): 459-486. https://doi.org/10.1007/s10546-009-9444-9