Building the COllaborative Carbon Column Observing Network (COCCON): long-term stability and ensemble performance of the EM27/SUN Fourier transform spectrometer

Abstract

In a 3.5-year long study, the long-term performance of a mobile, solar absorption Bruker EM27/SUN spectrometer, used for greenhouse gas observations, is checked with respect to a co-located reference Bruker IFS 125HR spectrometer, which is part of the Total Carbon Column Observing Network (TCCON). We find that the EM27/SUN is stable on timescales of several years; the drift per year between the EM27/SUN and the official TCCON product is 0.02 ppmv for XCO 2 and 0.9 ppbv for XCH 4 , which is within the 1σ precision of the comparison, 0.6 ppmv for XCO 2 and 4.3 ppbv for XCH 4 . The bias between the two data sets is 3.9 ppmv for XCO 2 and 13.0 ppbv for XCH 4 . In order to avoid sensitivity-dependent artifacts, the EM27/SUN is also compared to a truncated IFS 125HR data set derived from full-resolution TCCON interferograms. The drift is 0.02 ppmv for XCO 2 and 0.2 ppbv for XCH 4 per year, with 1σ precisions of 0.4 ppmv for XCO 2 and 1.4 ppbv for XCH 4 , respectively. The bias between the two data sets is 0.6 ppmv for XCO 2 and 0.5 ppbv for XCH 4 . With the presented long-term stability, the EM27/SUN qualifies as an useful supplement to the existing TCCON network in remote areas. To achieve consistent performance, such an extension requires careful testing of any spectrometers involved by application of common quality assurance measures. One major aim of the COllaborative Carbon Column Observing Network (COCCON) infrastructure is to provide these services to all EM27/SUN operators. In the framework of COC-CON development, the performance of an ensemble of 30 EM27/SUN spectrometers was tested and found to be very uniform, enhanced by the centralized inspection performed at the Karlsruhe Institute of Technology prior to deployment. Taking into account measured instrumental line shape parameters for each spectrometer, the resulting average bias across the ensemble with respect to the reference EM27/SUN used in the long-term study in XCO 2 is 0.20 ppmv, while it is 0.8 ppbv for XCH 4 . The average standard deviation of the ensemble is 0.13 ppmv for XCO 2 and 0.6 ppbv for XCH 4 . In addition to the robust metric based on absolute differences, we calculate the standard deviation among the empirical calibration factors. The resulting 2σ uncertainty is 0.6 ppmv for XCO 2 and 2.2 ppbv for XCH 4 . As indicated by the executed long-term study on one device presented here, the remaining empirical calibration factor deduced for each individual instrument can be assumed constant over time. Therefore the application of these empirical factors is expected to further improve the EM27/SUN network conformity beyond the scatter among the empirical calibration factors reported above.

Separation of Methane Emissions From Agricultural and Natural Gas Sources in the Colorado Front Range

“…Three COCCON (Collaborative Carbon Column Observing Network;Frey et al, 2019) type EM27/SUN from the Karlsruhe Institute of Technology, Germany, measured CH 4 and oxygen (O 2 ) at each of the three sites. The deployment period was from 14 to 23 March 2015, of which five clear days provided simultaneous measurements from all four instruments: 14, 15, 16, 22, and 23 March.…”

“…The deployment period was from 14 to 23 March 2015, of which five clear days provided simultaneous measurements from all four instruments: 14, 15, 16, 22, and 23 March. Three COCCON (Collaborative Carbon Column Observing Network;Frey et al, 2019) type EM27/SUN from the Karlsruhe Institute of Technology, Germany, measured CH 4 and oxygen (O 2 ) at each of the three sites. In addition, the University of Colorado mobile Solar Occultation Flux (CU mobile SOF) instrument measured C 2 H 6 and NH 3 in Eaton, CO. All instruments use direct sunlight to measure the trace gas absorption along the direct solar beam and infer the vertical column density (VCD) of an absorber gas above the site.…”

Advancing Scientific Understanding of the Global Methane Budget in Support of the Paris Agreement

“…The Total Column Carbon Observing Network of ground-based spectrometers (Wunch et al, 2011) is used to diagnose any biases in GOSAT that result from the complexity of retrieval algorithms (e.g., Buchwitz et al, 2017); however, they may be unquantified in regions not covered by the network, which operates around 30 stations worldwide. Therefore, expanded surface in situ and column measurements, potentially through the development of lower-cost instruments (e.g., Frey et al, 2019), and improved vertical profiling are needed to improve quantification and correction of these biases and therefore extend the utility of XCH 4 measurements for understanding surface processes. Despite any direct bias correction, all modeling studies using these data sets should attempt indirect assessment of these biases by (i) using independent calibrated data, where possible, and (ii) when using data derived from the CO 2 proxy method, investigate the systematic uncertainty in derived emissions that is imposed by the modeled CO 2 fields.…”

“…A complete observing system would include a dense network of lower-cost in situ sensors combined with a network of classical instruments. These include measurements of isotopic ratio and coemitted species, recently developed ground-based remote sensing instruments (e.g., Frey et al, 2019) and data from satellite-based instruments that are deployed in an urban targeting mode (as planned for by, e.g., GOSAT-2).…”

The Atmospheric Imaging Mission for Northern Regions: AIM-North

“…Northern high-latitude TCCON sites in Europe such as Sodankyl€ a (67.4 N, 26.6 E) and others will also contribute to AIM-North validation. Additional XCO 2 , XCH 4 and XCO validation can be carried out with lower resolution FTSs, and there are a growing number of such instruments in Canada and Europe as well as one in Alaska (Frey et al 2018).…”