Synergetic use of IASI and TROPOMI for generating a tropospheric methane profile product

We present a method for the synergetic use of IASI (Infrared Atmospheric Sounding Interferometer) profile and TROPOMI (TROPOspheric Monitoring Instrument) total column data products. Our method uses the output of the individual retrievals and consists of linear algebra a posteriori calculations (i.e. calculation after the individual retrievals). We show that this approach is largely equivalent to applying the spectra of the different sensors together in a single retrieval procedure, but with the substantial advantage of being usable together with different individual retrieval processors, of being very time efficient, and of directly benefiting from the high quality and most recent improvements of the individual retrieval processors.

For demonstrating the method, we focus on atmospheric methane (CH₄) and use IASI profile products generated by the processor MUSICA (MUlti-platform remote Sensing of Isotopologues for the investigation of the Cycle of Atmospheric water). We perform a theoretical evaluation and show that the a posteriori combination method yields total column averaged CH₄ products (XCH₄) that have the same good sensitivity as the respective TROPOMI products and upper tropospheric and lower stratospheric (UTLS) CH₄ profile data with the same good sensitivity as the IASI product. In addition, the combined product offers sensitivity for the tropospheric partial column, which is not provided by the individual TROPOMI nor the individual IASI product. The theoretically predicted synergetic effects are verified by comparisons to CH₄ reference data obtained from collocated XCH₄ measurements at five globally distributed TCCON (Total Carbon Column Observing Network) stations, CH₄ profile measurements made by 24 individual AirCore soundings, and lower tropospheric CH₄ data derived from continuous observations made at two nearby Global Atmospheric Watch (GAW) mountain stations. The comparisons clearly demonstrate that the combined product can reliably detect XCH₄ signals and allows to distinguish between tropospheric and UTLS CH₄ partial column averaged mixing ratios, which is not possible by the individual TROPOMI and IASI products. We find indications of a weak positive bias of +1.7% +/- 1.2% of the combined lower tropospheric data product with respect to the references. For the UTLS CH₄ partial columns we find no significant bias and a scatter with respect to the references of below 1%. We also briefly demonstrate the possibility of generating a combined IASI + TROPOMI water vapour isotopologue ratio product (HDO/H₂O), which allows the detection of boundary layer HDO/H₂O ratios independently from free tropospheric ratios.

The approach has the particular attraction, that IASI and TROPOMI successor instruments will be jointly aboard the upcoming Metop Second Generation satellites (guaranteeing observations from the 2020s to the 2040s). There will be several 100,000 globally distributed and perfectly collocated observations (over land) of IASI and TROPOMI successor instruments per day, for which combined products can be generated in a computationally very efficient way.