Interactive comment on “ Characteristics of the Greenhouse Gas Concentration Derived from the Ground-based FTS Spectra at Anmyeondo , Korea

General comments: First of all, we would like to strongly appreciate referee’s very constructive and valuable comments, suggestions, and feedbacks on the manuscript. On the basis of this, we have tried to address all the issues raised on this manuscript. We have discussed the feature of OASIS system and its performance in a detail manner in the revised manuscript. We have included other TCCON site data for comparison purpose, and also added some species such as CO, and CH4 derived from Anmyeondo FTS instrument. Comments: Section 2.2: What exactly does "operation is semi-automated" mean and how does it affect e.g. the number of measurements as


Introduction
Monitoring of greenhouse gases (GHGs) is a crucial issue in the context of the global climate change.
Carbon dioxide (CO 2 ) is one of the key greenhouse gas and its global annual mean concentration has been increased rapidly from 278 to 400 ppm since 1750, pre-industrial year (WMO greenhouse gas bulletin, 2016).Radiative forcing of atmospheric CO 2 accounts for approximately 65 % of the total radiative forcing by long-lived GHGs (Ohyama et al., 2015 and reference therein).Human activities, such as burning of fossil fuels, land use change, etc., are the primary drivers of the continuing increase in atmospheric greenhouse gases and the gases involved in their chemical production (Kiel et al., 2016 and reference therein).In the fact that it is a global concern for demanding accurate and precise longterm measurements of greenhouse gases.
In the field of remote sensing techniques, solar absorption infrared spectroscopy is an essential technique, which has been increasingly used to determine changes in atmospheric constituents.
Nowadays, a number of instruments deployed in various platforms (e.g., ground-based, space-borne) have been operated for measuring GHGs such as CO 2 .Our g-b FTS at the Anmyeondo station has been measuring several atmospheric GHGs such as CO 2 , CH 4 , CO, N 2 O, and H 2 O operated within the framework of the Total Carbon Column Observing Network (TCCON).XCO 2 retrievals from the g-b FTS have been reported at different TCCON sites (e.g, Ohyama et al., 2009;Deutscher et al., 2010;Messerschmidt et al., 2010Messerschmidt et al., , 2012;;Miao et al., 2013;Kivi and Heikkinen, 2016).TCCON achieves the accuracy and precision in measuring the column averaged dry air mole fraction of CO 2 (XCO 2 ), as about 0.25 % that is less than 1 ppm (Wunch et al., 2010), which is essential to get information about sinks and sources, as well as validating satellite products (Rayner and O'Brien, 2001;Miller et al., 2007).It is reported that the precision of CO 2 even 0.1 % can be achieved during clear sky conditions (Messerschmidt et al., 2010;Deutscher et al., 2010).The network aims to improve global carbon cycle studies and supply the primary validation data of different atmospheric trace gases derived from spacebased instruments, e.g., the Orbiting Carbon Observatory 2 (OCO-2), the Greenhouse Gases Observing Satellite (GOSAT) (Morino et al., 2011;Frankenberg et al., 2015).
The objective of this study is focused on the characteristics of XCO 2 concentration retrievals from g-b FTS spectra and is implement a preliminary comparison against OCO-2 over the Anmyeondo station.
The FTS spectra have been processed using the TCCON standard GGG2014 (Wunch et al., 2015) retrieval software.One of the interesting issues in this work is a new home made addition to our g-b FTS instrument that reduces the solar intensity variations from the 5% maximum allowed in TCCON to less than 2%.This paper presents introduction to instrumentation and measurement site, and next to that, provides results and discussion followed by conclusions.

Station description
The G-b FTS observatory was established in the Anmyeondo (AMY) station, which is located at 36.32˚ N, 126.19˚E, and 30 m above sea level.This station is situated on the west coast of the Korean Peninsula, which is 180 kilometres away from Seoul, the capital city of Republic of Korea.Figure 1 displays the Anmyeondo station.It is also a regional GAW (Global Atmosphere Watch) station that belongs to the Climate Change Monitoring Network of KMA (Korean Meteorological Administration).
The AMY station has been monitoring various atmospheric compositions such as greenhouse gases, aerosols, ultraviolet radiation, ozone, and precipitation since 1999.The total area of Anmyeondo is estimated to be ~87.96km 2 and approximately 1.25 million people reside in this island.Some of the residents over this area are engaged in agricultural activities.Vegetated areas consisting of mainly pine trees are located in and around the FTS observatory.The topographic feature of the area consists of low level hills, on average it is about 100 m above sea level.The climatic condition of the area is: the minimum temperature occurred on winter season with an average of 2.7 ˚C, and the maximum temperature is about 25.6 ˚C during summer season.In addition, the annual precipitation amount is estimated to be 1,155 mm; and the high amount of snows would be observed in winter.Such a observation site has been designated as part of TCCON site since August 2014.The AMY Station's on TCCON wiki page is kept available and can be found at: [https://tccon-wiki.Anmyeondo.edu]

G-b FTS instrument
Solar spectra are acquired by operating a Bruker IFS 125HR spectrometer (Bruker Optics, Germany) under the framework of TCCON.Currently, our g-b FTS instrument operation is semi-automated for taking the routine measurements under clear sky conditions.It is planned to make an FTS operation mode to be fully automated by this year.The solar tracker (Tracker A547, Bruker Optics, Germany) is mounted inside a dome.The tracking ranges in terms of both azimuthal and elevation angles are about 0 to 315 and -10 to 85 degrees, respectively, while the tracking speed is about 2 degrees per second.The tracking accuracy of ±4 minutes of arc can be achieved by the Camtracker mode.Under clear sky conditions, the dome is opened and set to an automatic-turning mode, in order that the mirrors are moved automatically to search for the position where the sunspot is seen by the camera.Then, the solar tracker is activated in such a way that the mirrors are finely and continuously controlled to fix the beam into the spectrometer.Figure 2 displays an overview of the general data acquisition system.This ensures that all spectra were recorded under clear weather conditions.The other important feature that has been made on the FTS spectrometer is the implementation of the interferogram sampling method (Brault, 1996), that takes advantage of modern analogue-digital converters (ADCs) to improve the signal-to-noise ratio.
The spectrometer is equipped with two room temperature detectors; an Indium-Gallium-Arsenide (InGaAs) detector, which covers the spectral region from 3,800 to 12,800 cm -1 , and Silicon (Si) diode detector (9,000 -25,000 cm -1 ) used in a dual-acquisition mode with a dichroic optic (Omega Optical, 10,000 cm -1 cut-on).A filter (Oriel Instruments 59523; 15,500 cm -1 cut-on) prior to the Si diode detector blocks visible light, which would otherwise be aliased into a near-infrared spectral domain.
TCCON measurements are routinely recorded at a maximum optical path difference (OPD max ) of 45 cm leading to a spectral resolution of 0.02 cm -1 .Two scans, one forward and one backward, are performed and individual interferograms are recorded.As an example, Figure 3 shows a single spectrum recorded on 4 October 2014 with a resolution of 0.02 cm -1 .A single scan in one measurement takes about 110 s.
Measurement setting for the Anmyeondo g-b FTS spectrometer of the Bruker 125HR model is summarized in Table 1.The pressure inside FTS is kept at 0.1 to 0.2 hPa with vacuum pump to maintain the stability of the system and to ensure clean and dry conditions.

Characterization of FTS-instrumental line shapes
For the accurate retrieval of total column values of the species of interest, a good alignment of the g-b FTS is essential.The instrument line shape (ILS) is retrieved from the regular HCl cell measurement that is an important indicator of the status of the FTS's alignment (Hase et al., 1999).The analyses of the measurements were performed using a linefit spectrum fitting algorithm (LINEFIT14 software) (Hase et al., 2013).Here, we have carried out experiments to investigate the influences of ILS.We showed the time series of the phase error (rad) (left panel) and modulation efficiency (right panel) in the HCl measurement using the source of light from tungsten lamp in the period of October 2013 to September 2017, which is depicted in Figure 4. Modulation amplitudes for well alignment should be controlled in a limit of 5 % loss at the maximum optical difference (Wunch et al., 2011).In our g-b FTS measurements, it is found that the maximum loss of modulation efficiency is less than 1 %, which is quite close to the ideal value.The phase errors are less than 0.0001.Hase et al. (2013) reported that this level of small disturbances from the ideal value of the modulation efficiency is common to all wellaligned instruments.This result confirmed that the g-b FTS instrument is well aligned and stable during the whole operation period.
We also confirmed that the ILS was not affected by the variable aperture during the operation of this system.The modulation efficiency and phase error were estimated to be 99.98 % and 0.0001 rad.Sun et al. (2017) reported the detailed characteristics of the ILS with respect to applications of different optical attenuators to FTIR spectrometers within the TCCON and NDACC networks.They used both lamp and sun cell measurements which were conducted after the insertion of five different attenuators in front of and behind the interferometer.In Sun et al. (2017), the ILS result was indicated by considering optical attenuator number 1 which is in good agreement with our findings.

Data processing
Within the TCCON standard retrieval strategy, we have derived the column-averaged dry-air mole fraction CO 2 (XCO 2 ) and other atmospheric gases (O 2 , CO, CH 4 , N 2 O, and H 2 O) using GFIT algorithm.
The spectral windows used for the retrieval of CO 2 and O 2 are given in Table 2.The TCCON standard GGG2014 (version 4.8.6)retrieval software was used to obtain the abundance of the species from FTS spectra (Wunch et al., 2015).The XCO 2 is the ratio of retrieved CO 2 column to retrieved O 2 column, Computing the ratio using Eq. ( 1) minimizes systematic and correlated errors such as errors in solar zenith angle, surface pressure, and instrumental line shape that existed in the retrieved CO 2 and O 2 columns (Washenfelder et al., 2006, Messerschmidt et al., 2012).Top panel of Fig. 6 depicts the time series of laser sampling error (LSE) obtained from InGaAs spectra at the Anmyeondo FTS station in the measurement period of February 2014 to December 2016.LSE is small and centered around zero in an ideal case.Slightly large LSE values were shown on 10 March, 2014 (see top panel of Fig. 7).On this date, we conducted the laser adjustment in FTS.
The X air is a useful indicator of the quality of measurements and the instrument performance.The X air would be unity for an ideal retrieval, however, due to spectroscopic limitations there is a TCCON wide bias and solar zenith angle (SZA) dependence.The retrieval of X air deviating more than 1% from the nominal value of 0.98 would suggest a systematic error.The time series of X air is shown in the bottom panel of Figure 5.The X air record reveals that the instrument has been stable during the measurement period.It shows that the values of X air are fluctuated between 0.974 and 0.985, and the mean value is 0.982 with a standard deviation of 0.0015 in which the scatter for X air is about 0.15 %.The low variability in time series of X air indicates the stability of the measurements.

Aircraft observation campaigns over Anmyeondo station
In this section, we have discussed a preliminary comparison results made between aircraft observations and g-b FTS over the Anmyeondo station.Here, FTS data were averaged over a time window of ± 30 minutes with respect to the aircraft measurement time.In addition, the averaging kernel of the FTS was applied into the aircraft data.The g-b FTS data were corrected for an airmass-dependent artefact for XCO 2 and XCH 4 , as well as calibrated with respect to TCCON common scaling factors.This scale factor was derived empirically using aircraft profiles over many TCCON sites in order to place the TCCON data on the WMO standard reference scales (Wunch et al. 2010) for both XCO 2 and XCH 4 .This comparison study will be useful for ensuring that the TCCON common scale factors can be applied to our g-b FTS data.The statistical results for XCO 2 and XCH 4 comparisons between aircraft and g-b FTS are summarized in Table 3.The mean absolute difference between FTS and aircraft were found to be -0.798± 1.734 ppm, the corresponding mean relative differences of -0.196 ± 0.427 % for XCO 2 , while the mean absolute difference of XCH 4 is -0.0079 ± 0.012 ppm, with a corresponding mean relative difference of -0.426 ± 0.632 %.These differences appeared on both species were consistent with the combined total errors of instruments.Wunch et al. (2010) reported that the uncertainties (2σ) of the TCCON common scale are approximately 0.2 % for XCO 2 and 0.4 % for XCH 4 .It is determined that our g-b FTS uncertainty was found to be within this range of uncertainties and can be calibrated against WMO standard scale.Here, we also include some results from the aircraft campaign conducted in 2016, which was operated by KORUS-AQ (Korea-U.S.-Air Quality) joint program aiming at advancing the ability to monitor air pollution from space. Figure 6 illustrates the results of XCO 2 and XCH 4 comparisons between the aircraft observation and TCCON sites data.Light blue diamond marks show for Anmyeondo station.
Our results laid within the indicated linear regression curves as with other TCCON sites.

OCO-2
Orbiting Carbon Observatory-2 (OCO-2) is NASA's first Earth-orbiting satellite, which was successfully launched on July 2, 2014 into low-Earth orbit.It is devoted to observing atmospheric carbon dioxide (CO 2 ) to get better insight for the carbon cycle.The primary mission is to measure carbon dioxide with high precision and accuracy in order to characterize its sources and sinks at different spatial and temporal scales (Boland et al., 2009;Crisp, 2008Crisp, , 2015)).The instrument measures the near infrared spectra (NIR) of sunlight reflected off the Earth's surface.Using a retrieval algorithm, it provides results of atmospheric abundances of carbon dioxide and related atmospheric parameters at the nadir, sun glint and targets modes.Detailed information about the instrument is available in different papers (Connor et al., 2008;O'Dell et al., 2012).In this work, we used the OCO-2 version 7Br bias corrected data.

Time series of g-b FTS XCO 2 , seasonal and annual cycle
The time series of XCO 2 along with retrievals of other trace gases such as XCO and XCH 4 from g-b FTS is presented in Figure 7 (panel a-c) during the period from February 2014 to December 2016.In such time series plots, each marker represents single retrievals, and the fitting curves of the retrieved values are also depicted (red solid line).We showed the seasonal cycle of XCO 2 , XCO, and XCH 4 in the time series using a fitting procedure described by Thoning et al. (1989).Standard deviations of the differences between the retrieved values and the fitting curves are 1.42 ppm, 11.0 ppb, and 10.3 ppb for XCO 2 , XCO, and XCH 4 , respectively.It is evident that all species have a feature of seasonal cycle.Year to year variability of XCO 2 is highest in spring and lowest during the growing season in June to September.Moreover, the behavior of seasonal cycle of XCO 2 at our site was compared with that of XCO 2 at Saga, Japan, which is discussed in later section.The atmospheric increase of XCO 2 from 2015 to 2016 was 3.65 ppm which is larger than the increase from 2014 to 2015.For the case of XCH 4 , its increase from 2015 to 2016 was 0.02 ppm which is higher than the increase from 2014 to 2015, whereas in XCO the rate of increment from year to year was found to be slightly decreased (see Table 4).
Moreover, the seasonal and annual cycles of XCO 2 derived from the g-b FTS were compared with insitu tower observations of CO 2 over the Anmyeondo station, which are presented in Figure 8. Regarding in-situ data, samples were collected using flask using non-dispersive infrared (NDIR) method at the altitude of 77 meters above sea level at Anmyeondo station (36.53 N, 126.32 E) (details about data are available at http://ds.data.jma.go.jp/jmd/wdcgg/).Nearly 97 % of in-situ data were taken during day time between 04:00 -08:40 UTC (13:00 -17:40 Korea Standard Time (KST)) so that the early morning and night time observations of CO 2 were almost neglected.In-situ CO 2 monthly means are generated by first averaging all valid event measurements with a unique sample date and time.The values are then extracted at weekly intervals from a smooth curve (Thoning et al., 1989) fitted to the averaged data and then these weekly values are averaged for each month.As can be seen in Figure 8, the overall patterns of seasonal and annual cycle of FTS XCO 2 tend to be similar with that of in-situ tower CO 2 over there.
This could suggest that the amplitude of seasonal cycle was likely to be driven by the imbalance of ecosystem exchange.

Correlations between XCO 2 and XCO
CO is co-emitted with CO 2 from combustion sources, leading to a significant positive correlation between them when combustion is a significant source of observed CO 2 .The midday peaks for each gas reflect the influence of anthropogenic emissions.To examine this effect, we have determined the correlations between ΔXCO and ΔXCO 2 at our site.In order to compute the correlations, first we have selected hourly averaged data for both XCO and XCO 2 that were recorded between 06:00 and 07:00 UTC (i.e 15:00 and 16:00 LST, local standard time), excluding summer data, and then calculated the anomalies by subtracting the hourly averaged data from the mean of the selected data during the measurement period of February 2014 to December 2016.Figure 9 depicts the relationship between hourly CO 2 and CO means of anomalies at Anmyeondo during the whole measurement period, excluding summer data.CO 2 and CO had a correlation of 0.50, and this suggests that there is an influence of combustion emissions on CO 2 .However, in a summer season, a negative relationship between them was identified at this site, with the small magnitude of correlation -0.22, and a correlation slope of -0.84.In Ohyama et al., (2015) paper, they derived the correlation coefficients and slopes of ΔXCO/ΔXCO 2 and ΔXCH 4 /ΔXCO in order to understand the short term variations of XCO 2 , XCO, and XCH 4 in summer seasons during July 2011 and December 2014 at Saga, Japan.The trajectories for the summer season were classified into three types, depending on the origin of the air masses.The trajectories for types I, II, and III relate to transport of air masses from the Asian continent (China), Southeast Asia, and the Pacific Ocean, respectively.The negative slope of the ΔXCO/ΔXCO 2 ratio for the type I (slope was -3.15 ppb ppm -1 ) gentler than for the type II (slope was -14.3 ppb ppm -1 ), which was due to the transport of the air masses that experience the strong biospheric uptake of CO 2 over the Asia.This argument could support for our analysis at Anmyeondo station.The slope that we obtained in our station is close to the slope reported in type III case in Ohyama et al., (2015) paper.
In Wang et al (2010), the diurnal cycles of CO 2 signal was dominated by the biospheric activity from May to September, with a maximum drawdown of 39 ppmv in daily CO 2 in the summer at rural station near Beijing.Biospheric activity, however, has little impact on CO except for the CO source from in situ oxidation of biogenic hydrocarbons.They obtained that the correlation between CO 2 and CO in summer was insignificant.The correlation slope gives the emission ratio of CO to CO 2 , which fluctuates with the sources of CO 2 , depending on different combustion types and biospheric activity.In our case, the correlation slope of CO to CO 2 was found to be 2.27 ppb ppm -1 during the whole measurement period excluding summer, which is smaller than the correlation slope reported in Hefei FTS station where it was estimated to be 5.66 ppb ppm -1 (Wang et al., 2017 and references therein), which are primarily attributed to the smaller emission in CO.

Comparison of Anmyeondo XCO 2 with nearby TCCON station
We presented the comparison of our FTS XCO 2 data with a similar ground-based high resolution FTS observation at Saga TCCON station (33.26 N,130.29 E) in Japan, which is the closest TCCON station to our site.Among those TCCON station, Rikubetsu, Tsukuba, and Saga are located in Japan (Morino et al., 2011, Ohyama et al., 2009, 2015) and Hefei is located in China (Wang et al., 2017).To demonstrate the comparison between them, we have shown the daily averaged XCO 2 of two stations during the period of 2014 to 2016 in Figure 10.As can be seen, variations of XCO 2 at the Saga station agreed well with Anmyeondo station.The daily averaged XCO 2 revealed the same seasonal cycle as that of our station.The lowest XCO 2 appeared in late summer (August and September), and the highest value was in spring (April).Ohyama et al., (2015) studied the time series of XCO 2 at Saga, Japan during the period from July 2011 to December 2014.They showed seasonal and interannual variations over there.The peak-to-peak seasonal amplitude of XCO 2 was 6.9 ppm over Saga during July 2011 and December 2014, with a seasonal maximum and minimum in the average seasonal cycle during May and September, respectively.In recent finding of Wang et al. (2017), the g-b FTS temporal distributions of XCO 2 at Hefei, China were reported.The FTS observations in 2014 to 2016 had a clear seasonal cycle XCO 2 reaches a minimum in late summer, and then slowly increases to the highest value in spring.The daily average of XCO 2 ranges from 392.33 ± 0.86 to 411.62 ± 0.90 ppm, and the monthly average value shows a seasonal amplitude of 8.31 and 13.56 ppm from 2014 to 2015 and from 2015 to 2016, respectively.The seasonal cycle was mainly driven by biosphere-atmosphere exchange.Butz et al., (2011) reported that the observations from GOSAT and the co-located ground-based measurements agreed well in capturing the seasonal cycle of XCO 2 with the late summer minimum and the spring maximum for four TCCON stations (Bialystok, Orleans, Park Falls, and Lamont) in the Northern Hemisphere.We inferred that the variation of XCO 2 over Anmyeondo station is in harmony with the variation pattern in elsewhere in mid-latitude Northern Hemisphere.

Comparison of XCO 2 between the g-b FTS and OCO-2
In this section, we present a comparison of XCO 2 between the g-b FTS and OCO-2 version 7Br data (bias corrected data) over Anmyeondo station during the period between 2014 and 2016.For making a direct comparison of the g-b FTS measurements against OCO-2, we applied the spatial coincidence criteria for the OCO-2 data within 3° latitude/longitude of the FTS station, as well as setting up a time window of 3 hours (maximum 3 h mismatch between satellite and g-b FTS observations).Based on the coincidence criteria, we obtained 13 coincident measurements, which were not sufficient to infer a robust conclusion.But it gives a preliminary result for indicating a level of agreement between them.
The comparison of the time series of XCO 2 concentrations derived from the g-b FTS and OCO-2 on daily medians basis are demonstrated during the measurement period between 2014 and 2016, as depicted in Figure 11.As can be seen in the plot, the g-b FTS measurement exhibits some gaps occurred due to bad weather conditions, instrument failures, and absences of an instrument operator.In the present analysis, the XCO 2 concentrations from FTS were considered only when its retrieval error was below 1.50 ppm (it is not shown here), which is the sum of all error components such as laser sampling error, zero level offsets, ILS error, smoothing error, atmospheric apriori temperature, atmospheric apriori pressure, surface pressure, and random noise.Wunch et al. (2016) reported that the comparison of XCO 2 derived from the OCO-2 version 7Br data against a co-located ground-based TCCON data that indicates the median differences between the OCO-2 and TCCON data were less than 0.50 ppm, a corresponding RMS differences less than 1.50 ppm.The overall results of our comparisons were comparable with the report Wunch et al. (2016).The OCO-2 product of XCO 2 was biased (satellite minus g-b FTS) with respect to the g-b FTS, which was slightly higher by 0.18 ppm with a standard deviation of 1.19 ppm, a corresponding RMS difference of 1.16 ppm.This bias could be attributed to the instrument uncertainty.In addition to that, we also obtained a strong correlation between them, which was quantified as a correlation coefficient of 0.94 (see Table 5).
Moreover, both instruments are generally agreed in capturing the seasonal variability of XCO 2 at the measurement station.As can be seen clearly from the temporal distribution of FTS XCO 2 , the maximum and minimum values are discernible in spring and late summer seasons, respectively.It was found that its mean values in spring and summer were 402.72 and 396.92 ppm, respectively (see Table 6).This is because the seasonal variation of XCO 2 is most likely to be controlled by the imbalance of the terrestrial ecosystem exchange, and this could explain the larger XCO 2 values in the northern hemisphere in late April (Schneising et al. 2008, and references therein).The minimum value of XCO 2 occurs in August, which is most likely due to uptake of carbon into the biosphere in associated with the period of plant growth.Furthermore, both instruments showed high standard deviations during summer, which are about 3.28 ppm in FTS and 3.77 ppm in OCO-2, and this suggests that the variability reflects strong sources and sink signals.

Conclusions
Monitoring of greenhouse gases is an essential issue in the context of the global climate change.
Accurate and precise continuous long-term measurements of the greenhouse gases (GHGs) are substantial for investigating their source and sinks.Nowadays, several remote sensing instruments operated on different platforms are dedicated for measuring GHGs.Greenhouse gases such as XCO 2 , XCH 4 , XH 2 O, XN 2 O measurements have been made using the g-b FTS at the Anmyeondo station since 2013.However, in this work, we focused on the measurements taken during the period of February 2014 to December 2016.The instrument has been operated in a semi-automated mode since then.The FTS instrument has been stable during the whole measurement period.Regular instrument alignments using the HCl cell measurements are performed.Thus, it guarantees the quality of the spectra.The TCCON standard GGG2014 retrieval software was used to retrieve XCO 2 , XCO, and others GHG gases from the g-b FTS spectra.
In this work, the g-b FTS retrieval of XCO 2 and XCH 4 were compared with aircraft measurements that were conducted over Anmyeondo station.We obtained the mean absolute difference between FTS and aircraft were found to be -0.798± 1.734 ppm, the corresponding mean relative differences of -0.196 ± 0.427 % for XCO 2 , while the mean absolute difference of XCH 4 is -0.0079 ± 0.012 ppm, with a corresponding mean relative difference of -0.426 ± 0.632 %.These differences appeared on both species were consistent with the combined total errors of instruments.The preliminary comparison results of XCO 2 between FTS and OCO-2 were also presented over the Anmyeondo station.The mean absolute difference of XCO 2 between FTS and OCO-2 was calculated on daily median basis, and it was estimated to be 0.18 ppm with a standard deviation of 0.19 with respect to the g-b FTS.This bias could be attributed with instrument uncertainty.Based on the seasonal cycle comparison, both the g-b FTS and OCO-2 illustrated a consistent pattern in capturing the seasonal variability of XCO 2 , with maximum in spring and minimum in summer.In summer and fall, plants flourish and CO 2 is most likely to be consumed by photosynthesis.However, in winter and spring, weak photosynthesis phenomenon would be expected to occur because of low plant flourishing and CO 2 reaches the highest values particularly in April.Therefore, the outcome of this study reflects the suitability of the measurements for improving the understanding of the carbon cycle, as well as evaluating the remote sensing data.

Figure 2 .
Figure 2. Photographs of the automated FTS laboratory.The Bruker Solar Tracker type A547 is mounted in the custom made dome.A servo controlled solar tracker directs the solar beam through a CaF 2 window to the FTS (125HR) in the laboratory.The server computer is used for data acquisition.PC1 and PC2 are used for controlling the spectrometer, solar tracker, dome, camera, pump, GPS satellite time, and humidity sensor.

Figure 3 .
Figure 3. Single spectrum recorded on 4 October 2014 with a resolution of 0.02 cm -1 .A typical example for the spectrum of XCO 2 is shown in the inset.

Figure 4 .
Figure 4. Phase error (rad) (left panel) and Modulation efficiency (right panel) of HCl measurements from the g-b FTS are displayed in the period from October 2013 to September 2017.Resolution = 0.015 cm -1 , Aperture = 0.8 mm.

Figure 5 .
Figure 5.Time series of LSE (top panel) and X air (bottom panel) from the g-b FTS during 2014-2016 is shown.Each marker represents a single measurement.

Figure 6 .
Figure 6.The comparisons of XCO 2 and XCH 4 between the aircraft observation and TCCON sites data are shown.The left side is XCO 2 and the right side is XCH 4 (light blue depicts for Anmyeondo station).

Figure 7 .Figure 8 .
Figure 7. Time series of XCO 2 , XCO, and XCH 4 from top to bottom panels (a-c), respectively in the period between February 2014 and December 2016 is given.Each marker indicates a single retrieval.Fitting curves (red solid lines) are also displayed.

Figure 10 .
Figure 10.Time series of XCO 2 retrieval from Anmyeondo FTS and Saga FTS in the period of February 2014 to December 2016 is depicted.

Figure 11 .
Figure 11.Left panel: The time series of XCO 2 from the g-b FTS (blue triangle) and OCO-2 (red triangle) over the Anmyeondo station from February 2014 to December 2016 are shown.Right panel: The linear regression curve between FTS and OCO-2 is shown.All results are given on daily medians basis.
Data recorded beyond these range of variations in cavity pressure and temperature were discarded in this analysis.Variance of the cavity pressure and temperature in flight result in variance in the CO 2 and CH 4 mixing ratios.The Picarro CRDS instrument has been regularly calibrated with respect to the standard gases within the error range recommend by World Meteorological Organization (CO 2 is 380.23 ± 0.1 ppm, CH 4 is 1.825 ± 0.001 ppm) Several aircraft observation campaigns over Anmyeondo station were carried out during the period between 2012 and 2016.However, a few numbers of aircraft data matched with the remote sensing instruments were available during this observation period.The total number of the aircraft measurements that matched with g-b FTS was only three and all those coincident observations were laid within a period of 2015.The g-b FTS retrieval of XCO 2 and XCH 4 were compared with aircraft measurements.
(Chen et al., 2010)ent station.The temperature and pressure of the gas sample have to be tightly controlled at 45 ℃ and 140 Torr in the CRDS, which leads to highly stable spectroscopic features(Chen et al., 2010).Any deviations from these values cause a reduction of the instrument's precision.

Table . 5
Summary of the statistics of XCO 2 comparisons between OCO-2 and the g-b FTS from 2014 to 2016 are presented.N -coincident number of data, R -Pearson correlation coefficient, RMS -Root Mean Squares differences.

Table . 6
Seasonal mean and standard deviations of XCO 2 from the g-b FTS and OCO-2 in the period between 2014 and 2016 are given below.