Hello @tiloz and everyone else,
Sorry for my late reply, I was on leave yesterday.
As @Scott already pointed out, the UM ancillary file type (fieldsfile) is a stacked sequence of 2D data, preceded by headers (global headers first and then field-related headers).
The data follows the structure outlined by Scott and, as he suggested, it is recommended (and easier) to replace the data of an existing ancillary file having the same structure of the needed one.
@tiloz I think if you could give more details about the specific co2 structure you’re looking for (if you already know it), or otherwise give details about the experiment you are trying to simulate, we should be able to come up with the best co2 ancillary structure for it and how to create it.
Cheers
Davide