Comparison of MIR (NDACC) and NIR (TCCON) column-averaged CO from ground-based solar Fourier Transform Infrared (FTIR) measurements

Abstract:

As part of the Total Carbon Column Observing Network (TCCON), the Karlsruhe Institute of Technology operates a solar Fourier Transform Infrared (FTIR) spectrometer in Karlsruhe, Germany. TCCON provides precise measurements of column abundances of CO₂, CO, CH₄, N₂O, HDO, and H₂O from solar absorption spectra recorded in the near infrared spectral domain. The Karlsruhe TCCON spectrometer is an extension over the current TCCON instrumental setup, as a dichroic setup allows to record spectra in the near infrared (NIR) and mid infrared (MIR) spectral domain simultaneously. Measurements in the MIR are performed following the Network for the Detection of Atmospheric Composition Change - Infrared Working Group Retrieval Parameter guidelines (NDACC-IRWG).

Understanding differences in column abundances from both networks is of high interest for satellite validation using data from both networks yielding a wider spatial and temporal coverage. However, a direct comparison of long-term timeseries retrieved in different wavelength regions is not trivial due to the non-ideal sensitivity of the remote-sensing observation. Different choices of a priori profiles and differing averaging kernels influence the retrieved trace gas columns leading to e.g. a quasi-periodic seasonal bias between data products derived from different spectral regions. Moreover, deficients in spectroscopic parameters might induce additional systematic biases, which might depend on e.g. airmass, humidity or temperature.

We present a comparison between column abundances of Carbon Monoxide (CO) concentrations retrieved in the NIR and MIR. We analyse apparent seasonal differences due to differing sensitivities and estimate the bias due to different calibration factors and residual inconsistencies of spectroscopic data. Our study improves the comparability of TCCON and NDACC-IRWG measurements for future CO validations of satellite missions like the Sentinel 5 Precursor mission to be launched in 2016.