G2.21

G2.21   Current spectroscopic databases contain uncertainties specifically effecting TCCON retrievals of CH₄ and CO₂

Gap detailed description

The shape of the calculated absorption depends on the spectroscopic data and the line shape model used. Both, the spectroscopic data and the line shape model, have a direct impact on the retrieved dry mole fractions of CH₄ and CO₂.

In the TCCON retrieval, isolated lines are assumed and the Voigt line shape, which is a convolution of a Gaussian (Doppler broadening) and a Lorentzian (pressure broadening), is used. The reason for the TCCON retrieval using the very basic Voigt line shape is that the spectroscopic databases provide almost all data needed for the calculation (not provided are the temperature dependence of the shift, self-broadening and H2O broadening). The calculation of the shapes of isolated lines should include speed dependence and Dicke narrowing, but the spectroscopic databases do not provide any data in this regard. Hence, instead of isolated lines the line shape model should include line mixing, but also for this problem HITRAN does not provide the relevant data. A further refinement of the retrieval would be to add the calculation of speed dependent and Dicke narrowed line mixing profiles.

Spectroscopic uncertainties are present in all spectral windows used for the TCCON retrieval and, more specifically, spectroscopic uncertainties are known to increase co-retrieved O₂, which serves as an internal standard to calculate XCO₂ (CO₂/O₂) and XCH₄, (CH₄/O₂), thus increasing the uncertainty of the CO₂ and CH₄ products.

Activities within GAIA-CLIM related to this gap

This gap is not addressed within GAIA-CLIM.

Gap remedy(s)

Remedy #1

Specific remedy proposed

So far TCCON uses a scaling factor to account for the uncertainties. Since the scaling factor has been found to be the same and constant over time for all sites, this approach works well. However, for higher precision laboratory measurements as well as better line shape, models are required to consider this. 

Measurable outcome of success

Retrieval of averaged mixing ratios without applying a scaling factor as well as spectral fit residuals at the noise level (without systematic features).

Achievable outcomes

In TCCON the O₂ volume mixing ratio can be regarded as constant in the atmosphere and therefore retrieved O₂ columns divided by pressure could be used as an internal standard to check the network consistency that is if the retrieval is sufficiently precise.

Relevance

Since the use of the current scaling factor allows to retrieve correct values for CO₂ and CH₄, the relevance is limited.

Timebound

We expect that within the next five years better spectroscopic data will be available. Scientists working on the line shape models include J.M. Hartmann, H. Tran (L.I.S.A., Paris) and G. Toon (NASA-JPL).

Gap risks to non-resolution