Interactive comment on “ Variability and budget of CO 2 in Europe : analysis of the CAATER airborne campaigns – Part 1 : Observed variability ” by I

* Referee 1: The study reports very interesting results about the CO2 concentration distribution in Western and Eastern Europe through two surveys carried out on May 2001 and October 2002 from 300 m up to 4000 m. As far as this referee knows, not such wide campaigns studying the distribution of CO2 have been previously made in Europe. The study characterizes the distribution of measurements and links the obC10156

the proper representation of the vertical gradients in atmospheric inversion models. However, even if most of the data needed to exploit the measurements are included, the text fails at drawing conclusions from them. Some key questions raised by the authors are not dealt properly (for example Section 6). Furthermore, some pieces of the manuscript appear to be out of context, such as Section 2.3.4.; which reports the instrument used to analyze the Radon-222 concentration but any of the results are commented latter in the paper; or Figure 9 that only the way how it has been calculated is explained in the text and not the conclusions extracted from it.
Other aspect that I am concerned about relates to the back-trajectories analysis dealt in Section 3.2 and 3.3. The back-trajectory analysis is used to explain the differences of the measured CO2 mixing ratios along the campaigns in different regions/countries. However, any mention to the change of the altitude flight is done when comparing the data (when a change of altitude took place).
I would strongly recommend the authors to revise the aforementioned before its final publication. Furthermore, it would need a throughout reading to correct improper English sentences.
ABSTRACT Page 5666, line 11. Add "(±1standard deviation)" after "the mean CO2 concentration" Page 5666, line 13 and further occurrences: air masses (as two words). Page 5666, lines 13-14. The authors talk about pollution when measured high CO2 concentrations. As CO2 is not properly an atmospheric pollutant, I would suggest rewrite the sentence as "air masses get the signal of anthropogenic fluxes. . . from Benelux and Western Germany" Page 5666, line 20. I would remove "the so-called dirty thirty from WWF" as this given name is irrelevant to the description of the study in the Abstract section. Page 5666, line 28. Page 5667, lines 3, 8. I would remove the acronyms of the stations. The acronyms are not defined in the abstract and since they are mentioned further in the text, I would just keep the type of sites (ground stations; mountain sites. . .). Page 5666, line 28. Change "stations located near the ground" by "stations sampling within the PBL" Page 5667, line 1. Add "regional" after "local" I guess that the authors would say that the last minute of acquisition is kept to build the calibration curve. When the outside air is analyzed, it is also the last minute kept to calculate the actual concentration? Page 5672, line 25. LSCE is not defined before in the text (either in the author's affiliation addresses). Page 5672, line 25. I am not convinced that a figure of the CO2 instrument is needed. It seems, however, that the analyzer has been described previously in Filipi (2002 Discussion Paper off-context. I would suggest moving them at the last paragraph of the Introduction section or at the beginning of the results. Line 23. It is confusing the expression "in the following at two places". Please, rewrite it and be clearer. Page 5674, line 5. "The CO2 concentration was measured by gas chromatography in LSCE". Page 5674, line 20. Remove "due to rotation of the filter wheel". Page 5674, line 23. Write the name of the IR detector rather to just state the chemical abbreviation (PbSe). Page 5675-6. I would remove Section 2.3.4 as the Radon-222 measurements are not reported in the present study.
3. ORIGIN OF SAMPLED AIR MASSES. In this section is lacking a description where / at which altitude backtrajectories are computed. Which is the longitude/latitude range where the backtrajectories presented in Figures 4, 5 and 6 are representative for? Are the release points defined as a single point? Or it is defined as a range of longitude/latitude? Which type of backtrajectories rae used? Please, describe better how backtrajectories were computed. It seems that the altitude of the flight changed along the campaigns. Changes in the altitude are important as there is a change of the measured concentration in altitude linked to the vertical mixing and then, the vertical spread of surface fluxes. Linked to this point, it would be useful to have an estimation of the boundary layer height along transects where the results are reported (mainly Section 3.2 and 3.3) as it would help the reader to know whether the measurements were taken within the BL or in the FT. Linked to that point, for the flight done on the 23th May (Figure 4a), there is a clear decrease of the CO2 concentration in the region from -2 • to 2 • E (∼360ppmv) compared to the region from -4 to -2 • E (∼373 ppmv), that is coincidental with a change of the altitude of the flight (the altitude of the flight was between 500 and 1000 masl in the -2 • to 2 • E region whereas it was between 0 and 500 m in the -4 to -2 • E one). Was the vertical structure of the atmosphere influencing on the CO2 horizontal distribution? Another example of the change of the CO2 concentration in the altitude was presented on the 26th May 2001 (Figure 4d). For that day, in the first paragraph of page 5678 is stated that "air masses coming from the west and north-west, which were more exposed to more urban areas are associated with CO2 C1961

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Interactive Discussion Discussion Paper higher by 25 ppm above this minimum". I am not really convinced that this difference is only related to the advection of polluted air masses as the backtrajectories show that air masses were always above 2000 m, then, uncoupled with the surface fluxes. In these cases it is difficult to make a clear statement that the CO2 variability is only related to changes of the air masses origin rather than CO2 concentration sampled at higher altitudes have more concentration compared to air masses sampled below, close to vegetation which is uptaking CO2 by photosynthesis during the growing season/daytime when the campaign was undertaken. Page 5677, line 9. Remove "(37%)" as the percentage is stated before. 4. RELATIONSHIP BETWEEN CO AND CO2 IN POLLUTED AIR MASSES Again, as the authors are mainly reported the CO2 measurements with the aim to understand the underlying flux, I think that it is inappropriate to talk about polluted air masses. I would suggest titling the Section "RELATIONSHIP BETWEEN CO AND CO2 IN ANTHROPOGENIC INFLUENCED AIR MASSES". Other aspect that I am concern is that the authors are comparing the CO/CO2 ratios obtained from measurements with the annual national inventories. In one hand, anthropogenic fluxes show a strong diurnal cycle that is smoothed with the annual inventories. In the other hand, authors are comparing the national inventory even only a small region of one particular country is sampled. If the authors have no access to inventories with more temporal resolution, they can make a weighted mean of the countries that a sampled air mass is influenced for and maybe they can match better the observations with the inventories. To all R2, an estimation of its significance is lacking (add p-values to all R2). Page 5681, lines 5-10. Remove sentences from "In pollution loaded. . . (Palmer et al., 2006), after (e.g. Levin and Karstens, 2007) in page 5680, line 26. In lines 26 (page 5680) and 11(5681) the adjective complicated appears twice. Remove this adjective and explain better the constraints of the application of this method. Page 5682, lines 5-11. Why referring to the NDVI maps and not the terrestrial fluxes shown in Figure 1 when talking about the photosynthetic activity? Page 5682, line 28. What do you mean "which can be >0 in the North and <0 in the south")? 5. COMPARISON  show a strong variability" by "shows a large range". I would also add the range of CO2 measured mixing ratios in the PBL and in the FT for both campaigns in the text. Page 5684, line 15. Change "profil" by "profile". Page 5684, line 17. Change "has encountering airmasses...West" by "was sampling air masses coming from the South, East and West as well.". Page 5684, line 22. Add "s" to "observation" Page 5684, lines 25-26. Remove the first sentence and start the paragraph "Both PUY and SCH stations are located on the top of mid-elevation mountains..." Page 5685, line 3. Change "polluted" by "influenced by the valley processes" 6. ANALYSIS OF THE VER-TICAL VARAIBILITY Page 5685, line 21. Change "comparing" by "compared" Page 5685, line 24. Reported the mean and standard deviation values for CAATER 2 with the same significant numbers as CAATER 1 (two decimals are given in the first campaign, only one in the second). Same in page 5686, line 2, give the CO2 jump with two decimals. Page 5686, lines 6-11. Why the air masses are here separated depending on its origin from the LMDZ model and not from the HYSPLIT analysis presented in the current study? Page 5686, line 11. Add " (Figure 12)" after CAATER 1. Page 5686, line 11. I would suggest the "CO2 range" rather than "the CO2 variability" as the authors are afterwards stating the range of the measured values and not any estimation of the vari-

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Interactive Discussion Discussion Paper ability. Page 5686, line 11. Change "high" by "large". Page 5686, line 23. Add "( Figure  13)" after CAATER 2. Page 5686, line 23. I would suggest the "CO2 range" rather than "the CO2 variability" as the authors are afterwards stating the range of the measured values and not any estimation of the variability Page 5686, line 29. When talking about an advanced fall in the west part in Europe compared to the East, why not contrast this hypothesis with the surface fluxes show in Figure 1? From the NEE map on Figure  1e, it is observed that South/West Europe fluxes are ∼0gCm-2d-1, whereas positive fluxes are shown in Eastern Europe. Page 5687, lines 2-5. The same about when talking about CO2 emissions. Why only comment the national inventories from UNFCC? Figures 1 c and 1f show the anthropogenic fluxes (with the NEE). Why not discuss this fact using the provided information? Page 5687, lines 10-15. I don't see the point that to understand a measured gradient in October 2002 more regular flights are needed and then, the CARBOEUROPE network is essential to understand this gradient. Do the authors think that this gradient is persistent through all seasons? I think that in the current study the authors present a huge amount of information (meteorological parameters, NEE, anthropogenic and oceanic fluxes; backtrajectories analysis, etc.) and they should be able to point the observed gradient. Which are the mean fluxes in that area? How is the vertical stability? CONCLUSIONS Page 5687, line 25. I don't know if averages of CO2 measured in both campaigns are comparable since different flight paths were followed. Maybe a sentence of the mean surface fluxes of the underlying paths should be mentioned. Page 5688, lines 9. A "c" is lacking in bac-trajectories. TABLES Table 4. It lacks " • C" in the Temperature row for the CO analyzer Table 5. It would be nice to have an additional column with the "mean" inventories slopes for each flight. FIGURES Figure 1. "(for the days of the campaigns) Figure 2. Last sentence in figure caption "Longitude (horizontal scale) is given in • E and latitude (vertical scale) in • N. Figure 11. CO2 in subscript. Figure 12. A way to express the y-axis units for the left panel would be "z/zi". Right panel is missing axis labels. Figure 12. CO2 in subscript in the left panel x-label. Right panel is missing axis labels. C1964