Review of Maier et al, “Uncertainty of continuous ΔCO-based ΔffCO2 estimates derived from ¹⁴C flask and bottom-up ΔCO/ΔffCO2 ratios” by John Miller

General comments

Overall, this is a very good study focused on developing high frequency proxies (here CO) for the estimation for the recently added fossil content of atmospheric CO₂ measurements. The writing is generally very good and the figures are excellent, reflecting the analysis itself. The pdf has numerous comments but I will highlight two here:

1. Although this is clear in the title, use of “Delta(CO)-based DffCO2 estimates”, when used without explanation, has the potential to be highly misleading because these estimates (the atmospheric data-based ones the paper shows to be trustworthy) are still based on D¹⁴C I’m not exactly sure of the solution, but perhaps you can employ nomenclature/notation that identifies such values as ‘calibrated’ by ¹⁴C.
2. I have a few questions about the TNO inventory that could benefit by a bit more investigation and explanation. First, it appears that TNO includes biofuels such as wood.  But what about ethanol and biodiesel?  Generally, can the fossil components of the TNO inventory be isolated for a more direct comparison with ¹⁴C-based observations?  Second, in investigation of the point source impacts for Heidelberg, can you transport the non-point-source sectors to see how much the match to data is improved – i.e., is the mismatch mainly due to the ratio of (dilute) point to area sources in TNO or mainly due to incorrect emission ratios for the area sources?

Specific comments

Suggested edits and comments are embedded in the manuscript .pdf.  Blue highlights indicate those that are language oriented and yellow for science/conceptual issues.