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. In a 3.5-year long study, the long-termperformance of a mobile, solar absorption Bruker EM27/SUN spectrometer, usedfor greenhouse gas observations, is checked with respect to a co-locatedreference Bruker IFS 125HR spectrometer, which is part of the Total CarbonColumn Observing Network (TCCON). We find that the EM27/SUN is stable ontimescales of several years; the drift per year between the EM27/SUN and theofficial TCCON product is 0.02 ppmv for XCO2 and 0.9 ppbv forXCH4, which is within the 1σ precision of the comparison,0.6 ppmv for XCO2 and 4.3 ppbv for XCH4. The bias betweenthe two data sets is 3.9 ppmv for XCO2 and 13.0 ppbv forXCH4. In order to avoid sensitivity-dependent artifacts, the EM27/SUNis also compared to a truncated IFS 125HR data set derived fromfull-resolution TCCON interferograms. The drift is 0.02 ppmv forXCO2 and 0.2 ppbv for XCH4 per year, with 1σprecisions of 0.4 ppmv for XCO2 and 1.4 ppbv for XCH4,respectively. The bias between the two data sets is 0.6 ppmv forXCO2 and 0.5 ppbv for XCH4. With the presented long-termstability, the EM27/SUN qualifies as an useful supplement to the existingTCCON network in remote areas. To achieve consistent performance, such anextension requires careful testing of any spectrometers involved byapplication of common quality assurance measures. One major aim of theCOllaborative Carbon Column Observing Network (COCCON) infrastructure is toprovide these services to all EM27/SUN operators. In the framework of COCCONdevelopment, the performance of an ensemble of 30 EM27/SUN spectrometers wastested and found to be very uniform, enhanced by the centralized inspectionperformed at the Karlsruhe Institute of Technology prior to deployment.Taking into account measured instrumental line shape parameters for eachspectrometer, the resulting average bias across the ensemble with respect tothe reference EM27/SUN used in the long-term study in XCO2 is0.20 ppmv, while it is 0.8 ppbv for XCH4. The average standarddeviation of the ensemble is 0.13 ppmv for XCO2 and 0.6 ppbv forXCH4. 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 XCO2 and2.2 ppbv for XCH4. As indicated by the executed long-term study onone device presented here, the remaining empirical calibration factor deducedfor each individual instrument can be assumed constant over time. Thereforethe application of these empirical factors is expected to further improve theEM27/SUN network conformity beyond the scatter among the empiricalcalibration factors reported above.

atmospheric measurement techniques

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