Interactive comment on “ Comparisons of CH 4 satellite GOSAT and ground-based FTIR measurements near Saint-Petersburg ( 59 . 9 N , 29 . 8 E ) ”

to validate satellite data using a reliable ground-based instrument, or to compare two equivalent instruments, GOSAT TANSO-FTS and FTIR of SPbU. In this paper, the authors cite many papers on GOSAT validation, and therefore, their aim looks like adding another validation of GOSAT data at high latitude using a reliable ground-based instrument. However, the authors replied to Referee #2, “Indeed, one of goals of our paper is not “introduction”, but “validation” of Saint-Petersburg measurement site.” I do not think it is good way to validate a ground-based instrument by using satellite data, because satellite measurements could contain more uncertainties and assumptions than ground based measurements. In particular, I do not agree to their opinion “the GOSAT data seem to be better validated than Saint-Petersburg ground-based measurements.” As also mentioned by the authors, no TCCON site is at higher latitude than 55 degrees, which means that no GOSAT data have ever been validated at higher latitudes than 55 degrees. I think the most (and almost the only) merit of this paper is to validate GOSAT data at such high latitude.


Introduction
Methane is the second most important anthropogenic greenhouse gas.Despite its low concentration in the Earth's atmosphere, CH 4 is responsible for about 15 % of the anthropogenic contribution to the greenhouse effect.Currently, there are networks for local flask ground and aircraft measurements within GAW (Global Atmosphere Watch) and NOAA CMDL/ESRL (Convey et al., 2003).Ground-based remote sensing optical measurements are made at the stations of international networks NDACC and TCCON.First global satellite data on the total methane content in the atmospheric column were obtained using the IMG/ADEOS equipment measuring the outgoing thermal radiation spectrum with high spectral resolution (Kobayashi et al., 1999).Further studies were carried out with satellite devices SCIAMACHY, AIRS, IASI, TES (Xiong et al., 2010;Sussmann et al., 2005;Razavi et al., 2009;Wecht et al., 2012).Despite extensive observation programs, CH 4 geographical distribution and its sources are not known sufficiently (Solomon et al., 2007).Regular global satellite methane measurements could lead to solutions of the problem.In January 2009, the GOSAT (Greenhouse gases Observing SATellite) was launched.It is a joint project of the Japanese Aerospace Exploration Agency and the National Institute for Environmental Studies in Tsukuba, Japan (Kuze et al., 2009).The satellite is designed to monitor global distributions of column contents of atmospheric CO 2 and CH 4 from space.Column average mole fractions of carbon dioxide, X CO 2 , and methane, X CH 4 , are retrieved from the data of TANSO-FTS (Thermal And Sensor for carbon Observation Nearinfrared Fourier Transform Spectrometer), which is a Fourier Transform Spectrometer for measurements of carbon-bearing gases in infrared range from the GOSAT satellite (Yoshida et al., 2011).
For validation of satellite observations of greenhouse gases, the special monitoring network TCCON (the Total Carbon Column Observation Network) was set up, which uses ground-based Fourier transform infra red (FTIR) spectroscopy of direct solar radiation for regular measurements of column contents of CO 2 , CH 4 and other climateforming gases (Wunch et al., 2011).Similar FTIR measurements are carried out also on the international ground-based network NDACC (Network for the Detection of Atmospheric Composition Change, http://www.ndsc.ncep.noaa.gov/).To obtain gas species contents, devices of the NDACC and TCCON networks usually use, respectively, middle and near IR spectral ranges.Sussmann et al. (2013) described an intercalibration of the measurements at both networks.
In Saint-Petersburg State University (SPbU), spectroscopic measurements of total column methane were started in 1991 (Mironenkov et al., 1996;Makarova et al., 2009).These measurements up to year 2009 were carried out using a solar IR grating spectrometer with resolution of 0.4-0.6 cm network TCCON (see above).They found substantially lower satellite values compared to those obtained from ground-based observations.Later, comparisons of X CO 2 and X CH 4 obtained with other retrieval algorithms for GOSAT and TCCON data gave better agreements between satellite and ground-based measurements (Notholt et al., 2012;Cogan et al., 2012;Yoshida et al., 2013).These comparisons were performed for ground-based observation sites located at latitudes lower than 55 • .Therefore, it is interesting to compare GOSAT and ground-based observations performed at higher latitudes and utilizing different retrieval algorithms.
In this paper, we compare X CH 4 obtained by the GOSAT satellite with ground-based FTIR spectroscopic observations near Saint-Petersburg in years 2009-2012, which are performed at latitude about 60 • N using modifications of retrieval algorithms designed for the international network NDACC.

Measurement and data processing
The FTIR measurement site of SPbU is located at the Peterhof campus (59.88 • N, 29.82 • E, 20 m a.s.l.), about 35 km southwest from the centre of Saint-Petersburg.The measurement tools include an automatic solar tracking system, solar flux input system, and an analog channel for cloud monitoring during the measurements.Observations are performed under a cloudless sky, or in large enough cloud cover breaks.Interferograms are usually recorded by an InSb detector for the optical path differences of 180 cm.Times of accumulation and averaging of ten scans are adjusted to obtain a single spectrum in about 12 min.
At least three popular computer programs (SFIT and PFOFIT for the NDACC and GFIT for TCCON network) exist for interpreting of ground-based FTIR observations.Comparisons of the first two algorithms and a new approach (Kozlov information operator) showed very close estimates of the methane total content using the same a priori information (see, for example, Senten et al., 2012).In the present study, we performed retrievals of total column contents of greenhouse gases in the atmosphere from the Introduction

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Full FTIR spectrometry using the standard software SFIT2 v 3.92 (Pougatchev et al., 1995;Rinsland et al., 1998;Hase et al., 2004) designed for the NDACC network.We used the optimal estimation technique in SFIT2 and retrievals of methane content profiles with their consequent integration.The main input data for SFIT2 are spectra of solar radiation (including related information on interferometer parameters), and a priori profiles of atmospheric trace gases and their variations.These profiles (recommended by NDACC) were created using WACCM (Whole Atmosphere Community Climate Model) for Peterhof latitude, longitude and altitude (Garcia et al., 2007).Vertical profiles of atmospheric pressure and temperature required for retrieving of greenhouse gases are taken from the nearest site of upper air soundings Voejkovo (see, for example, Weather Web, 2013), which is located 50 km eastward from Peterhof.Different infrared spectral intervals were used at the NDACC FTIR network for retrievals of the atmospheric column CH 4 content (Goldman et al., 1988;Schneider, 2005;Griesfeller et al., 2006;Wunch et al., 2007;Angelbratt et al., 2011;Sussmann et al., 2011Sussmann et al., , 2012;;Sepulveda et al., 2012).In the present study, we use the three spectral intervals (2613.7-2615.4, 2835.5-2835.8 and 2921.0-2921.6 cm −1 ) recommended by Sussmann et al. (2011), as well as four spectral intervals (2613.7-2615.4, 2650.6-2651.3, 2835.5-2835.8 and 2903.6-2904.03cm −1 ) recommended for a long time in the NDACC documentation and used by Sepulveda et al. (2012).Mean signal-to-noise ratios in these spectral bands are about 800.According to Sussmann et al. (2011), we used the HITRAN 2000 (with additions of 2001) database of spectroscopic line parameters (Rothman et al., 2003) for the above mentioned three spectral windows, and the HITRAN 2004 database (Rothman et al., 2005) for the other four windows.Random relative errors of individual X CH 4 measurements do not exceed 0.3-0.5 % according to error matrix calculations within the optimal estimation method implemented in the SFIT2 software.Under stable atmospheric conditions, variations of measured X CH 4 within spectra series and throughout the day do not generally exceed 1 %.Full  To increase the amount of compared data, we also analyzed the individual couples of ground-based and satellite X CH 4 values, for which the difference in dates of their measurements do not exceed two days.Figure 2 shows the corresponding pairs of X CH 4 SPB and X CH 4 GOS for both versions of the GOSAT satellite data.The solid line in Fig. 2 corresponds to X CH 4 SPB = X CH 4 GOS .One can see that almost all of the measured X CH 4 values for the GOSAT V01.xx data lie below the solid line in Fig. 2, while for the GOSAT V02.xx data the situation is different.
Table 3 shows the mean, median characteristics and standard deviations calculated for the ground-based and satellite data presented in Fig. 2. The mean and median values in Table 3 for both types of measurements are closer to each other for the GOSAT V02.xx data.The long-and short-dashed lines in Fig. 2 have shifts relative to the solid line according to the average δX CH 4 values from Table 3 for the GOSAT data versions V01.xx and V02.xx, respectively.The results presented above were obtained using the three spectral intervals listed in Sect. 2 and recommended by Sussmann et al. (2011).Similar estimations using another set of four spectral intervals (see Sect. 2) have small differences from the results considered above.Average differences between the two sets of data are about 0.2 % for our measurements in Saint-Petersburg.Average deviation (similar to that in Table 2) of GOSAT version V02.xx CH 4 data from ground-base FTIR measurements using four spectral intervals is δX CH 4 ∼ −1.6 ppb, and its standard deviation n ∼ 15.6 ppb.(Saitoh et al., 2012) to −39 ± 11 ppb (Tanaka et al., 2012).Values δX CH 4 for GOSAT version V01.xx data in Tables 1 and     3  average differences between GOSAT and ground-based FTIR X CH 4 as low as −0.3 or −0.4 %.Butz et al. (2011) also showed existence of systematic biases of −0.3 %.Yoshida et al. (2013) made comparisons of the GOSAT version V02.xx data with observations at TCCON ground-based network and found average biases of −5.9 ppb (−0.3 %) and standard deviations of 12.6 ppb (0.7 %).Our analysis of methane mole fraction from the GOSAT data version V02.xx (see Tables 2, 3 and Fig. 2) showed individual deviations between satellite and ground-based values in the range 0.01-1.8% and the average differences about 0.2 ± 0.8 %.Some differences from estimations by Yoshida et al. (2013) may be caused by substantial statistical errors (because of limited number of measurements at Saint-Petersburg).However, the magnitudes of average deviations in Tables 2 and 3 show that Saint-Petersburg FTIR observations using the retrieval algorithms from the NDACC network could give reasonable agreement with the GOSAT satellite data.Standard deviation of δX CH 4 values in Table 2 is 15.1 ppb (about 0.9 %), which is compatible with compound errors of both types of measurements and is slightly larger than the value 12.6 ppb obtained by Yoshida et al. (2013).

Discussion
Tables 1-3 show substantial standard deviations of ground-based FTIR X CH 4 values (up to 16-17 ppb).One should keep in mind that these measurements are carried out near the Saint-Petersburg megalopolis, so the total methane variability there might be higher than that for background measurements.Makarova et al. (2006) estimated that emission from Saint-Petersburg may contribute up to 2 % to the overall CH 4 column content.This enhanced variability of X CH 4 near Saint-Petersburg may contribute to some differences in average δX CH 4 and their standard deviations obtained in the present paper compared to the estimations by Yoshida et al. (2013).
In our study, we compared X CH 4 measured with GOSAT in ±3 • latitude and longitude vicinity of the ground-based observation site.We also tried to use ±1 • and ±5 • colocation criteria.In these cases, we obtained the same orders of magnitude for biases between ground-based and satellite measurements as those in Tables 1-3.Comparisons of ground-based and satellite FTIR methane mole fraction measurements do not take into account some characteristics that may influence the Introduction

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Full measurements and data processing, for example, differences in averaging kernels of remote sensing methods (Parker et al., 2011), or uncertainties in the parameters of fine structure of spectral lines (Chesnokova et al., 2011).Also due to a relatively small amount of sunny days for FTIR measurements near Saint-Petersburg, we should consider the present comparison as preliminary.

Conclusions
We compared the average over atmospheric column methane mole fractions, measured with FTIR spectroscopy from the Earth's surface at the Peterhof campus of Saint Petersburg State University (59.9 Full  Full  Full   Full Discussion Paper | Discussion Paper | Discussion Paper | −1 .Since January 2009, the Atmospheric Physics Department of SPbU started ground-based solar FTIR measurements using the Bruker IFS 125 HR interferometer giving high spectral resolution.Results of atmospheric trace gas retrievals in SPbU were described by Poberovskii et al. (2010), Polyakov et al. (2011), Virolainen et al. (2011) and Yagovkina et al. (2011).Morino et al. (2011) have performed a preliminary validation of X CO 2 and X CH 4 observed with the GOSAT satellite comparing them with measurements on the FTIR Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |

Figure 3
reveals histograms of differences δX CH 4 between pairs of measurements presented in Fig. 2. For the GOSAT data version V02.xx the deviations are almost symmetrical respective to zero in Fig. 3b, while Fig. 3a demonstrates systematic underestimation of the X CH 4 from the GOSAT V01.xx data compared to the ground-based FTIR measurementsDiscussion Paper | Discussion Paper | Discussion Paper | Morino et al. (2011) made a comparison of the GOSAT version V01.xx and FTIR spectroscopic X CH 4 measurements in years 2009-2010 at 9 stations of ground-based TC-CON network at latitudes from 45 • S to 53 • N.They found a systematic underestimation of satellite methane mole fractions with δX CH 4 ≈ −20 ± 19 ppb (−1.2 ± 1.1 %).Determinations of deviations between the GOSAT V01.xx data and CH 4 aircraft measurements in the troposphere gave δX CH 4 from −8 ± 10 ppb falls within the specified ranges, with the mean and median δX CH 4 ≈ −13 ± 26 ppb.This may be an indirect indication that optical X CH 4 obtained from the Earth's surface near Saint-Petersburg are compatible with methane column mole fractions measured with the TCCON network.Several algorithms for X CH 4 retrieval from GOSAT data were compared recently with ground-based FTIR spectroscopic observations.Parker et al. (2011) compared GOSAT X CH 4 measurements with data from TCCON network and with results of numerical modeling.They estimated relative deviations of individual satellite measurements to be ∼ 0.1-0.9% depending on latitude.Notholt et al. (2012) analyzed several different algorithms of methane retrievals and obtained standard deviations of differences between GOSAT and ground-based X CH 4 values in the range of 0.8-4 %.Schepers et al. (2012) studied influence of radiation scattering and cirrus clouds and obtained Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | from ground-based high-resolution FTIR measurements, Atmos.Meas.Tech., 5, 161-180, doi:10.5194/amt-5-161-2012,2012.Sep úlveda, E., Schneider, M., Hase, F., García, O. E., Gomez-Pelaez, A., Dohe, S., Blumenstock, T., and Guerra, J. C.: Long-term validation of tropospheric column-averaged CH 4 mole fractions obtained by mid-infrared ground-based FTIR spectrometry, Atmos.Meas.Tech., 5Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |
To compare X CH 4 measured near Saint-Petersburg from the Earth's surface and from aboard the GOSAT satellite, we found intervals of simultaneous measurements in years2009 -2012.For .Forthese time intervals, X CH 4 values measured with GOSAT in ±3 • latitude and longitude vicinity of the ground-based observation site were selected from the database of National Institute of Environmental Studies in Tsukuba, Japan (NIES, 2013).The ground-based X CH 4 values taken for the comparison were obtained at the lowest zenith angles of the Sun (usually ±3 h from local noon).We used only X CH 4 within the 95 % confidence interval around the mean values for the corresponding observation intervals.Because the GOSAT satellite X CH 4 are estimated for dry atmo-CH 4 for the GOSAT data version V01.xx compared to the ground-based measurements near Saint-Petersburg.Table 1 gives the daily mean X CH 4 SPB and X CH 4 GOS for matching dates of ground-based and GOSAT (data version V01.xx) observations, respectively.Unfortunately, the number of sunny days for FTIR spectroscopic observations and number of data pairs in Table 1 are very limited at high latitudes.In Table 1, the differences δX CH 4 = X CH 4 GOS − X CH 4 SPB are negative in most cases (up to −43 ppb, or ∼ −2.4 %).The average over Table 1 δX CH 4 = −13 and its standard deviation ∼ 26 ppb, which shows that GOSAT CH 4 data version V01.xx are about 0.7 % lower than the ground-based FTIR values.Also, Table 1 shows slightly larger standard deviations of X CH 4 GOS for the GOSAT V01.xx data than those for ground-based X CH 4 SPB .Introduction Sussmann et al. (2011)cussion Paper | Discussion Paper | Discussion Paper |3 Results of comparison sphere (without water vapor), the ground-based X CH 4 were also adjusted to the dry atmosphere using the data of reanalysis of meteorological information by the ECMWF European Centre(Dee et al., 2011)for the moments of time and coordinates of groundbased measurements near Saint-Petersburg.Figure1presents individual X CH 4 values from satellite and ground-based measurements using the three spectral intervals recommended bySussmann et al. (2011).In many cases, the dates of measurements with those methods do not match exactly.However, Fig.1shows systematically lower X

Table 1 .
Table2is the same as Table1, but for the GOSAT data version V02.xx.When several ground-based or satellite X CH 4 values were registered during a day, we used respective daily means in Tables1 and 2. Deviations δX CH 4 between GOSAT and ground-based measurements have different signs for different days in Table2, and vary from −21 ppm (−1.2 %) to 31 ppm (1.8 %), which is smaller than deviations for data version V01.xx in In most cases, the relative deviations are less than 1 % in Table2.The average difference is δX CH 4 = 3.7 ppb (or about 0.2 %) for the GOSAT CH 4 data version V02.xx and the ground-based FTIR measurements.Standard deviation of average δX CH 4 in Table2is 15.1 ppb, or less than 1 %.In addition, variability (standard deviation) of methane mole fractions is significantly smaller (6.5 ppb) for the GOSAT V02.xx data than for ground-based measurements (17 ppb) in Table2.
• N, 29.8 • E) in years 2009-2012 with similar observations with the Japanese GOSAT satellite (data versions V01.xx and V02.xx).Average difference between the GOSAT data version V01.xx and ground-based FTIR measurements from the Earth's surface is δX CH 4 ≈ −13 and their standard deviation ∼ 26 ppb, which is consistent with literature data about comparisons of this version of GOSAT data with the network of ground-based FTIR stations TCCON and with airplane insitu measurements.The same average differences for the GOSAT data version V02.xx are smaller (δX CH 4 /X CH 4 ≈ 0.2 %) and show that Saint-Petersburg FTIR observations could provide reasonable agreement with satellite data.Standard deviation of δX CH 4 values is 15.1 ppm (about 0.9 %), which is compatible with combined errors of both types of measurements.Relatively small amount of sunny days for FTIR measurements near Saint Petersburg requires further accumulation of the data of ground-based and satellite FTIR measurements and their comparisons.Introduction

Table 1 .
Daily average values of CH 4 mole fractions for matching dates of ground-based FTIR measurements near Saint-Petersburg and GOSAT data version V01.xx.VMR units are ppb.Date X CH 4 spb X CH 4 gos δX CH 4 δX/X ,% Introduction

Table 2 .
Same as Table 1, but for the GOSAT data version V02.xx.

Table 3 .
Average characteristics for the data shown in Fig.2.VMR units are ppb.

Table 3 .
Average characteristics (in ppb) for the data shown in Figure VMR units are ppb.априорный CH 4 SPB X CH 4 GOS δX CH 4