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Validation of Odin/SMR \(CO\) with ACE-FTS

The Sub-Millimeter Radiometer (SMR) instrument onboard the Swedish-led Odin satellite was launched on 20th February 2001. The SMR has 5 receivers, A1 (541-558 Hz), A2 (486-504 Hz), B1 (563-581 Hz), B2 (547-564 Hz), C (118.25-119.25 Hz). Odin/SMR measures CO, from the thermal emission line corresponding to the J=5→4 rotational transition at 576.268 GHz. The vertical resolution of the CO measurements are 3–7 km.

In this study the new dataset which is part of the Odin/SMR version 3.0 level 2 data has been used. A frequency correction has been applied to the spectra before retrieval is performed which recovered a part of the data set that was affected by the malfunctioning phase-lock loop (PLL). The PLL is used in the front end of the instrument for CO measurements. In this process a basic frequency correction was applied to the spectra and the retrieval was set up using this new spectra and a new version of the Atmospheric Radiative Transfer Simulator (ARTS) operational processing system which has been developed for SMR retrievals. In the next step a broadening correction was applied. The figure below shows the amount of data recovered from level 1 data using this method denoted as version 3.0 level 2.

amt-13-5013-2020-f07-1.png

Comparison with ACE-FTS

A total of 12 925 coincident CO measurements between ACE-FTS and SMR were found between February 2004 (first available ACE-FTS CO measurements) and March 2019. The figure below shows spatial and temporal average comparison between ACE-FTS and Odin/SMR. The relative difference decreases almost monotonically with altitude, from a maximum of +15 % at 50 km to a minimum of almost −15 % at the highest altitudes. In particular, the difference decreases more quickly between 50 and 60 km from 15 % to near 0 %. SMR and ACE-FTS are in extremely good agreement with each other between 60 and 75 km, with a relative difference close to zero. The difference then slowly decreases with altitude up to about −15 % at 90 km and finally increases almost imperceptibly between 90 and 100 km.

amt-13-5013-2020-f10-3.png

Reference

Grieco, F., Pérot, K., Murtagh, D., Eriksson, P., Forkman, P., Rydberg, B., Funke, B., Walker, K. A., and Pumphrey, H. C.: Recovery and validation of Odin/SMR long-term measurements of mesospheric carbon monoxide, Atmos. Meas. Tech., 13, 5013–5031, https://doi.org/10.5194/amt-13-5013-2020, 2020.