CH<sub>4</sub>, CO, and H<sub>2</sub>O spectroscopy for the Sentinel-5 Precursor mission: an assessment with the Total Carbon Column Observing Network measurements

Abstract

Abstract. The TROPOspheric Monitoring Instrument (TROPOMI) will be part of ESA's Sentinel-5 Precursor (S5P) satellite platform scheduled for launch in 2015. TROPOMI will monitor methane and carbon monoxide concentrations in the Earth's atmosphere by measuring spectra of back-scattered sunlight in the short-wave infrared (SWIR).S5P will be the first satellite mission to rely uniquely on the spectral window at 4190-4340 cm −1 (2.3 µm) to retrieve CH 4 and CO. In this study, we investigated if the absorption features of the three relevant molecules CH 4 , CO, and H 2 O are adequately known. To this end, we retrieved total columns of CH 4 , CO, and H 2 O from absorption spectra measured by two ground-based Fourier transform spectrometers that are part of the Total Carbon Column Observing Network (TCCON). The retrieval results from the 4190-4340 cm −1 range at the TROPOMI resolution (0.45 cm −1 ) were then compared to the CH 4 results obtained from the 6000 cm −1 region, and the CO results obtained from the 4190-4340 cm −1 region at the higher TCCON resolution (0.02 cm −1 ).For TROPOMI-like settings, we were able to reproduce the CH 4 columns to an accuracy of 0.3 % apart from a constant bias of 1 %. The CO retrieval accuracy was, through interference, systematically influenced by the shortcomings of the CH 4 and H 2 O spectroscopy. In contrast to CH 4 , the CO column error also varied significantly with atmospheric H 2 O content. Unaddressed, this would introduce seasonal and latitudinal biases to the CO columns retrieved from TROPOMI measurements. We therefore recommend further effort from the spectroscopic community to be directed at the H 2 O and CH 4 spectroscopy in the 4190-4340 cm −1 region.

Simulated retrievals for the remote sensing of CO<sub>2</sub>, CH<sub>4</sub>, CO, and H<sub>2</sub>O from geostationary orbit

“…As CO absorbs weakly, the uncertainty in XCO is only reduced by 47.2 %, which is significantly less than that for the other gases. The weak absorption of CO and the interferences from other strong absorbing gases such as CH 4 and H 2 O in the near-infrared are also noted by Buchwitz et al (2004) and Galli et al (2012). Our results corroborate Galli et al's findings that, due to interferences, the quality of the XCO retrieval is systematically influenced by the shortcomings in CH 4 and H 2 O spectroscopy.…”

The operational methane retrieval algorithm for TROPOMI

“…Another relevant source of errors to the CH 4 data product could be spectroscopic errors. This has been studied in detail by Galli et al (2012) and Checa-Garcia et al (2015). Note that a study is ongoing to improve the spectroscopic data for TROPOMI's SWIR spectral range (Loos et al, 2015).…”

“…The first guess layer subcolumns of these gases are calculated from input profiles of CH 4 and CO (from the global chemical transport model TM5; Houweling et al, 2014), and temperature, humidity and pressure profiles (from ECMWF forecast data). Molecular absorption features are calculated using appropriate spectroscopic databases (Tran et al, 2006;Rothman et al, 2009;Scheepmaker et al, 2012). Here, we evaluate the absorption cross sections on a 72-layer pressure-equidistant grid to account for the strong temperature dependence and pressure dependence of the cross sections.…”

“…We have attempted to include the most important contributions to the forward model error, except for errors due to the underlying spectroscopic database, which have been investigated elsewhere; see Galli et al (2012) and ChecaGarcia et al (2015). The simulations were computed using a line-by-line radiative transfer model, whereas the retrieval method uses the linear k-method.…”

“…HITRAN spectroscopic line parameters (Rothman et al, 2009) are available through HITRAN online (http://hitran.org), and the line parameters from Scheepmaker et al (2012) are available in their Supplement. Our retrieval algorithm retrieves three effective aerosol parameters, namely total column, size parameter and central height.…”