Validation of XCO2 and XCH4 retrieved from a portable Fourier transform spectrometer with those from in situ profiles from aircraft-borne instruments

Column-averaged dry-air mole fractions of carbon dioxide (XCO2) and methane (XCH4) measured by a solar viewing portable Fourier transform spectrometer (FTS, EM27/SUN) have been characterized and validated by comparison using in situ profile measurements made during the transfer flights of two aircraft campaigns: Korea-United States Air Quality Study (KORUS-AQ) and Effect of Megacities on the Transport and Transformation of Pollutants at Regional and Global Scales (EMeRGe). The aircraft flew over two Total Carbon Column Observing Network (TCCON) sites: Rikubetsu, Japan (43.46 N, 143.77 E), for the KORUS-AQ campaign and Burgos, Philippines (18.53 N, 120.65 E), for the EMeRGe campaign. The EM27/SUN was deployed at the corresponding TCCON sites during the overflights. The mole fraction profiles obtained by the aircraft over Rikubetsu differed between the ascending and the descending flights above approximately 8 km for both CO2 and CH4. Because the spatial pattern of tropopause heights based on potential vorticity values from the ERA5 reanalysis shows that the tropopause height over the Rikubetsu site was consistent with the descending profile, we used only the descending profile to compare with the EM27/SUN data. Both the XCO2 and XCH4 derived from the descending profiles over Burgos were lower than those from the ascending profiles. Output from the Weather Research and Forecasting Model indicates that higher CO2 for the ascending profile originated in central Luzon, an industrialized and densely populated region about 400 km south of the Burgos TCCON site. Air masses observed with the EM27/SUN overlap better with those from the descending aircraft profiles than those from the ascending aircraft profiles with respect to their properties such as origin and atmospheric residence times. Consequently, the descending aircraft profiles were used for the comparison with the EM27/SUN data. The EM27/SUN XCO2 and XCH4 data were derived by using the GGG2014 software without applying air-mass-independent correction factors (AICFs). The comparison of the EM27/SUN observations with the aircraft data revealed that, on average, the EM27/SUN XCO2 data were biased low by 1.22 % and the EM27/SUN XCH4 data were biased low by 1.71 %. The resulting AICFs of Published by Copernicus Publications on behalf of the European Geosciences Union. 5150 H. Ohyama et al.: Validation of EM27/SUN XCO2 and XCH4 using aircraft data 0.9878 for XCO2 and 0.9829 for XCH4 were obtained for the EM27/SUN. Applying AICFs being utilized for the TCCON data (0.9898 for XCO2 and 0.9765 for XCH4) to the EM27/SUN data induces an underestimate for XCO2 and an overestimate for XCH4.

dry air mole fractions of CO2 and CH4 from a single portable, low-resolution near infrared solar absorption EM27/SUN Fourier transform spectrometer at the Rikubetsu and Burgos total carbon column observing network (TCCON) sites with in situ aircraft measurements.
The presented work represents one of the first documented examples of in situ validation of greenhouse gas measurements from a portable spectrometer of this type and therefore contributes significantly to the value of such measurement techniques.
The Authors have taken rigorous steps to ensure the robustness of the comparisons by demonstrating the stability of the portable instrument in terms of its instrument line shape and comparison of retrievals to the Tsukuba TCCON site, and by choosing which aircraft data to compare to, informed by the effect of large scale dynamics on the tropopause height in the case of the Rikubetsu comparison and by transport of regional emissions for Burgos. The manuscript is well written and follows a logical narrative.
All important steps are outlined, and assumptions appropriately justified. I would strongly recommend publication of the manuscript subject to some minor alterations outlined below.
We thank you for reading our paper carefully and providing valuable comments. We have added some descriptions for clarification and revised our manuscript according to your comments. Please see our specific responses below.

Specific comments
At the end of sections 3.1 and 3.2, and elsewhere in the manuscript particularly Table 2, the terms uncertainty and error are used interchangeably. The error in a measurement should refer to the difference between that measurement and the true value of the measurand whereas the uncertainty describes the range about the measurement in which the true value most likely lies. In the context of this work, the term uncertainty should be used. For further information I refer the authors to the BIPM Guide to the Expression of Uncertainty in Measurement.
We have revised the text to exclusively use the term uncertainty, unifying the two terms (i.e., uncertainty and error).
To aid with the understanding of the choice of aircraft profile used for the Rikubetsu comparison it would be helpful if the radiosonde lapse rate derived tropopause heights (or a subset thereof) and the GGG derived value were plotted on Figs 1 (b) and (c) or Fig 2 (a), and the GGG determined tropopause height included in Table 1.
We have added the tropopause heights from the radiosonde lapse rate and the GGG2014 in Figs. 1b and 1c. In addition, the tropopause height from the GGG2014 has been included in Table 1. Figure 1 (b) seems to be missing data from the ascent profile between just above the surface and approximately 3 km. It would also aid the interpretation if Figures 1 and 2, (b) and (c) included an indication of the transition from aircraft data to a priori in the composite profile.
As you pointed out, there is no description of the missing data from the ascent profile.
We have added the following sentence in : "There are missing data due to instrumental calibrations, especially between 0.24 and 2.78 km of the CO2 ascent profile (Fig. 1b)." Additionally, we have added the following sentences in Sect.
3.4 (lines 412-416): "When calculating aircraft XCO2 and XCH4 values, the missing data were linearly interpolated. We note that, provided that the missing data between 0.24 and 2.78 km of the CO2 ascent profile were substituted by the descent profile in the corresponding altitude range, the difference between the XCO2 values from the linear interpolation and the substitution was less than 0.1 ppm." Regarding the transition from aircraft data to the a priori profile, we have added the composite profiles in Figs. 1b, 1c, 3b, and 3c.
It should be made clearer that the EM27 results presented in Table 4 are before the derived airmass independent correction factor has been applied.
We have added the following sentence in the caption of Table 4: "The air mass independent correction factors derived in this study are not yet applied to the EM27/SUN data." Has the GGG2014 airmass dependent correction factor also been applied to the EM27 retrievals presented?
Yes. We have revised the description related to the correction factors in Sect. 2.1 (lines 143-148) as follows: "The GGG2014 software includes air mass independent and air mass dependent correction factors for the TCCON data. The air mass independent correction factors (AICFs) were not utilized (i.e., they were set to one) because we separately determined them for EM27/SUN in this study. Meanwhile, we used the same air mass dependent correction factors (ADCFs) as those applied to the TCCON data, and their validity is evaluated in Sect. 3.3." In addition, we have added the following sentences in Sect. 3.3 (lines 370-386): "As described in Sect. 2.1, we applied the GGG2014 ADCFs to the EM27/SUN retrievals.
The ADCF is a coefficient tied to a symmetric basis function (Eq. A12 in Wunch et al. (2011a)) representing spurious diurnal variation, and the values derived from the TCCON data at multiple sites are -0.0068 ± 0.0050 for XCO2 and 0.0053 ± 0.0080 for XCH4 (Wunch et al., 2015). To assess the relevance of applying the ADCFs derived from the TCCON data to the EM27/SUN data, we derived the ADCF for our EM27/SUN, such that the difference between the EM27/SUN and TCCON retrievals in Burgos that were individually averaged into 10 min bins is minimized while taking into account a coefficient for correcting the mean bias between EM27/SUN and the TCCON data. The derived ADCFs are -0.0064 ± 0.0004 for XCO2 and 0.0034 ± 0.0007 for XCH4 (the uncertainties were estimated as 1s standard deviations of daily ADCFs derived from four days side by side observations in Burgos). The ADCFs for XCO2 show good agreement between the EM27/SUN and the TCCON, while those for XCH4 show a slightly larger difference. Considering that the ADCFs for our instrument are consistent with those for the TCCON data within the uncertainties and that the ADCFs have the possibility to vary with the seasons and sites (Wunch et al., 2015), we conclude that the use of the mean ADCFs derived from the TCCON data is a reasonable choice." References: