Abstract
Abstract. At the end of 2015, a CO2/CH4/CO cavity ring-down spectrometer (CRDS) was installed at the Izaña Global Atmosphere Watch (GAW) station (Tenerife, Spain) to improve the Izaña Greenhouse Gases GAW Measurement Programme, and to guarantee the renewal of the instrumentation and the long-term maintenance of this program. We present the results of the CRDS acceptance tests, the raw data processing scheme applied, and the response functions used. Also, the calibration results, the implemented water vapor correction, the target gas injection statistics, the ambient measurements performed from December 2015 to July 2017, and their comparison with other continuous in situ measurements are described. The agreement with other in situ continuous measurements is good most of the time for CO2 and CH4, but for CO it is just outside the GAW 2 ppb objective. It seems the disagreement is not produced by significant drifts in the CRDS CO World Meteorological Organization (WMO) tertiary standards. The more relevant contributions of the present article are (1) determination of linear relationships between flow rate, CRDS inlet pressure, and CRDS outlet valve aperture; (2) determination of a slight CO2 correction that takes into account changes in the inlet pressure/flow rate (as well as its stability over the years), and attributing it to the existence of a small spatial inhomogeneity in the pressure field inside the CRDS cavity due to the gas dynamics; (3) drift rate determination for the pressure and temperature sensors located inside the CRDS cavity from the CO2 and CH4 response function drift trends; (4) the determination of the H2O correction for CO has been performed using raw spectral peak data instead of the raw CO provided by the CRDS and using a running mean to smooth random noise in a long water-droplet test (12 h) before performing the least square fit; and (5) the existence of a small H2O dependence in the CRDS flow and of a small spatial inhomogeneity in the temperature field inside the CRDS cavity are pointed out and their origin discussed.
Highlights
A CO2/CH4/CO cavity ring-down spectrometer (CRDS) was installed at the Izaña Global Atmosphere Watch (GAW) station (Tenerife, Spain) at the end of 2015 in order to improve the Izaña Greenhouse Gas (GHG) GAW Measurement Programme, and to guarantee the long-term maintenance of this program
At the end of 2015, a CO2/CH4/CO CRDS was installed at the Izaña Global GAW station to improve the Izaña GHG GAW Measurement Programme and guarantee its long-term maintenance
A correction for CO2 that takes into account the inlet pressure had to be incorporated in order to achieve a rms residual of around 0.02 ppm, which is the value we obtain with the Izaña Observatory (IZO) NDIR-based measurement systems
Summary
A CO2/CH4/CO cavity ring-down spectrometer (CRDS) was installed at the Izaña Global Atmosphere Watch (GAW) station (Tenerife, Spain) at the end of 2015 in order to improve the Izaña Greenhouse Gas (GHG) GAW Measurement Programme, and to guarantee the long-term maintenance of this program. The CRDS technique (Crosson, 2008) has improved considerably the stability and precision in the raw measurements compared to those of older techniques (e.g., non-dispersive infrared analyzers, NDIRs, gas chromatography, GC, with a flame ionization detector, FID, and GC with a reduction gas detector, RGD); the required frequency of use of calibrating/reference gases to achieve the GAW data quality objectives (DQOs) is much lower. This spectrometric technique does not require chromatographic gases (e.g., carrier gas, makeup gas, and FID gases for maintaining the flame), which are expensive and require great logistics efforts at remote stations. We very briefly describe in Appendix C a few more novelties in the Izaña GHG Measurement Programme since the WMO/IAEA Meeting on Carbon Dioxide, Other Greenhouse Gases and Related Tracers Measurement Techniques that took place in the year 2015 (GGMT-2015)
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