This study provides an analysis of annual growth rates of atmospheric carbon dioxide, derived from long term (more than ten years) observations of column-averaged dry-air mole fractions of CO₂ (hereafter XCO₂) from 12 sites, covering four latitudinal regions: the Arctic, two midlatitude bands in the Northern Hemisphere, and the Southern Hemisphere. These measurement sites form part of the Total Carbon Column Observing Network (TCCON) of ground-based remote sensing measurements of greenhouse gases; the column concentrations are obtained from high spectral resolution Fourier Transform Infrared (FTIR) spectrometer observations of direct sunlight, by considering how much light is absorbed by the molecules of interest at specific wavelengths as it passes down through the atmosphere, and into the FTIR spectrometer via a solar tracker. The column-averaged abundances provided by TCCON are complementary to the near-surface concentrations of CO₂ measured by in-situ air sampling instruments (e.g. from the NOAA Global Monitoring Laboratory network), which benefit from being able to acquire data through day and night, albeit with greater sensitivity to any local emissions sources. The XCO₂ data obtained by TCCON are also widely used for validation of satellite data. All three of these data sources (in situ, TCCON, and satellite -- indirectly, via assimilation into the CAMS reanalysis) are considered by this study in their analysis of long term CO₂ growth rates.

For the analysis a subset of the TCCON sites are chosen, covering four different latitudinal bands: two in Northern mid-latitudes, where the greatest impact of fossil fuel emissions is expected; one in the high Northern latitudes, to investigate CO₂ growth rate in the Arctic; and one covering the whole of the Southern Hemisphere, where the greatest influence on CO₂ concentration growth is expected to be transport from the Northern Hemisphere. Three sites are selected for each of these regions, with the choice determined by the availability of data over at least a five year period leading up to 2020 whilst avoiding sites located in highly urbanised regions. The decision to define the whole Southern Hemisphere as a latitudinal band is justified by the relative lack of spatial variability in CO₂ in this area compared with that seen in the Northern Hemisphere. As well as the TCCON data, data from a single NOAA in situ site (Mauna Loa, commonly used for CO₂ growth rate studies owing to its geographically isolated high altitude location) and the CAMS reanalysis total column CO₂ product are also considered. The continuous year-round availability of the CAMS data is used in this study to assess the impacts of gaps in the TCCON data record on the calculated CO₂ growth rates, particularly in the case of the Northern high latitude sites where the low solar elevation angle during winter prevents the acquisition of solar spectra for a significant fraction of the year. Three different methods for calculating the CO₂ growth rate are presented and evaluated -- the Dynamic Linear Model (DLM) method is shown to be the least sensitive to gaps in the time series, as demonstrated by comparing growth rates calculated from CAMS data that has been down-sampled to the TCCON data availability for Eureka, a Northern high latitude site. This is a key finding for this study, as it demonstrates the best method to use when investigating growth rates from irregularly sampled time series data, such as that provided by satellites and other remote sensing techniques over regions of variable cloud cover.

The final part of the analysis concerns the interannual variations in CO₂ growth rates (calculated solely using the DLM method, based on the conclusions of the previous section), and how these compare between the different latitudinal regions. Overall, there is broad agreement in CO₂ growth rate between the four regions, despite year-to-year variability. The results show the significant impact of ENSO conditions on growth rate in all regions, with the strongest correlation between growth rate and ENSO strength occurring in the Southern Hemisphere TCCON and the Mauna Loa Observatory data. The spatial variability in correlation is likely due to differences in biospheric sensitivity to ENSO events in different parts of the world. The authors also note that the analysis is able to identify shifts in anthropogenic emissions, most notably in 2020 as a result of COVID-19 related reductions which were significant in the densely urbanised 30 to 40°N latitude band. The EDGAR emissions map show in Figure A5 confirms that the majority of anthropogenic CO₂ emissions occur in this band, demonstrating that CO₂ growth rates in the 30-40°N region are particularly sensitive to fossil fuel activity. The relatively small impact of the change in anthropogenic emissions during the COVID-19 affected period, compared with the influence of the strong and very strong El Nino and La Nina events, is an interesting outcome of this work which I think warrants further study.

Overall, this study provides a timely analysis of the now long-term XCO₂ data available from TCCON, which is only possible now thanks to the continuous dedicated effort over the past decade and more of those operating the sites and processing the data. The study provides a robust method for calculating growth rates from irregularly sampled time series data, the selection of which is well supported by thorough testing of different methods on a down-sampled reanalysis product, which can be used with confidence on other long-term atmospheric concentration datasets where it would be of interest to determine the growth rate. As more data -- both ground-based and satellite -- is acquired over longer time periods, it will be very interesting to see similar studies applying this methodology to different regions of scientific interest. I am happy for this paper to be published, subject to the following minor technical corrections and suggestions:

Line 27: I'm not sure "monotonically" is the best word to use here, given that it implies little-to-no variation in the rate of increase of CO₂ emissions -- I would suggest removing the word altogether, but emphasising that the emissions increase annually, e.g. "... emissions have generally increased each year, but with occasional declines...";

Line 109: "... in the Southern Hemisphere where CO₂ growth is largely influenced by transport from the Northern Hemisphere" -- please can you provide a reference supporting this statement?

Figure 3: this figure is a little hard to interpret -- for each year, I suggest plotting the "CAMS ds" bars next to the "TCCON" bars, since I think the key point is the comparison between these two. I would then have the "CAMS non-ds" data on the right of the three for each year, so that the reader can see the effect of the down-sampling more easily (I would also explicitly label these "CAMS non-ds" instead of just "CAMS"). So in summary, for each year, plot from left to right: TCCON, CAMS ds, CAMS non-ds;

Figure A3: it would be useful for the reader to have this referred to somewhere in the main text, I think Section 3.1 (Data Preparation) would be most appropriate.

General Comments: This manuscript by Pak et al. analyzes annual growth rates of column-averaged dry-air mole fractions of CO2 using data from TCCON, CAMS, and the Mauna Loa Observatory. The TCCON data are used for four regions, and the TCCON-derived growth rates are compared with in situ Mauna Loa Observatory measurements using three calculation methods: Monthly Mean (MM), Fourier Fit residuals (FF), and Dynamic Linear Model (DLM). The CAMS data are used to support the analysis by evaluating their consistency with TCCON-based growth rates and analyzing each method’s robustness towards missing data. The authors found the DLM approach to improve trend estimations and handle data gaps better, especially in high-latitude regions. Overall, this manuscript is well written, with a robust analysis of the methods and supported with figures as necessary. The manuscript can be accepted for publication after a few minor changes and clarifications.

Specific Comments:

L88 – Please define NOAA GML. It was defined later in L94.

L90 – Please state what value the gridded modeled total column CO2 product adds.

L95 – Please remove “specifically”.

L101 – Please italicize “a priori”.

L103 – Please define WMO.

L106 – Do the fossil fuel emissions occur only in the mid-latitude regions?

L110 – Why was the “at least five years of data” criterion used?

L112 – Please state the latitude range.

L113- Please define AWIPEV

L124 – All three TCON stations are in close proximity. Are there other other southern hemispheric TCCON sites that can be used?

L130 – Are the data publicly available or will they be at the time of publication? Please ensure that the data are publicly available so the reader can recreate the figures from the analysis should they choose to do so.

L143 – “This approach …” – The rest of this paragraph might look better in the data introduction portion of the manuscript.

L 144 – Please provide a citation/data reference publication for the CAMS Reanalysis product.

L146 – What are the possible pros and cons of using satellite and aircraft data vicariously to fill gaps?

L156 – “ publicly available dataset..” – except for Lauder?

L158 – What value is the quality flag set to for good quality data?

L161 – How much error does the weekly averaging of the data introduce?

L166 – Does the sample size difference impact the outcome?

L174 – “similar studies” Please cite.

L205 – Please consider adding quotes (dlmhelper to “dlmhelper”).

L205 – Please define DLM.

L208 – “only one harmonic is selected” – Why?

L216 – Do edge effects introduce higher uncertainty?

L218 – “at three sites” – located at different latitudes. How does this impact the results?

Page 12 – What are the cons of using the DLM method? What are the possible trade-offs, even though relatively few, of using this method? How much of an improvement does the DLM method provide over the FF and MM approaches?

L293 – What was the reason behind the sudden increase in CO2 growth rates in 2016? Why was the growth rate relatively lower in the 40-50 N region?

L297 – Please consider splitting into two sentences.

L299- Why was there an increase in the 40-50 N latitude band, but a decrease in the 30-40 N latitude band regions? L303 -  Why wasn’t the reduced fossil fuel activity observed anywhere else besides the 30-40 N? Is it only due to dense urbanization?

Table 1:

1. For Avg Measurement/year the sample size varies. Does this introduce uncertainty in the analysis?
2. What are the possible cons of comparisons for sites with uneven latitude windows, i.e., 40-50N, 30 -40N vs. 10-50S)?