How to Convert CAMS CO₂ Multi-Level Data (Model & Pressure Levels) to Column-Mean CO₂ (XCO₂)?

Datasets and Usage CAMS - Datasets and Usage
1

In the CAMS Global Greenhouse Gas Reanalysis products (e.g., CAMS-EGG4), CO₂ is provided at both model levels and standard pressure levels as layer-resolved mixing ratios. I would like to compute a column-mean dry-air molar fraction (i.e., XCO₂) from these multi-layer fields, but I cannot locate an official description of the exact conversion method in the documentation.

Could anyone clarify the recommended procedure for converting multi-level CO₂ data into a single column-mean value?

In particular, I would like to understand:

  1. How to vertically weight CO₂ mixing ratios—should the averaging be pressure-weighted, mass-weighted, or use layer thickness derived from surface pressure and model definition?

  2. Whether the reported CO₂ values are already dry-air mole fractions, or whether water-vapor dilution needs to be explicitly removed when computing XCO₂.

  3. Whether the procedure differs between model-level fields and pressure-level fields, and if so, what the correct approach is for each case.

  4. Whether ECMWF provides an explicit formula, example script, or reference publication where this conversion is described.

Any official references, technical notes, peer-reviewed papers, or example implementations would be greatly appreciated, especially if they illustrate XCO₂ derivation directly from CAMS multi-layer CO₂ output.

2

Hi,
the data can be downloaded directly from the ADS under single-level chemical vertical integrals.

Thanks

3

Hi,

Thank you for your response. I am aware that the column-mean molar fraction of CO₂ can be downloaded directly from the single-level chemical vertical integrals product. My main question is about the calculation process: how is the multi-level CO₂ data converted into the column-mean molar fraction? If you know the procedure, I would greatly appreciate it if you could share it with me.

Thanks.