New paper published in Atmospheric Measurement Techniques Discussion entitled “Tropospheric CH4 signals as observed by NDACC FTIR at globally distributed sites and comparison to GAW surface in-situ measurements” by Sepúlveda et al.

In this paper, we extend the Sepúlveda et al. (2012) study, which is limited to a subtropical site, to a set of nine globally-distributed  NDACC FTIR stations covering polar, mid-latitudinal, and subtropical regions and we focus on the quality of the lower free tropospheric CH4 FTIR data. We slightly change our previous profiling retrieval in order to minimise potential humidity interferences at the humid sites. We demonstrate that the retrieved lower tropospheric CH4 mole fractions  can be significantly affected by CH4 variations in the upper troposhere lower stratosphere (UTLS) caused by tropopause altitude shifts. We show that this dependency on UTLS variations can be significantly reduced by an aposteriori correction method. We compare our retrieved tropospheric CH4 signal to the surface in-situ CH4 GAW data. To do so, we filter the GAW data to get the regional-scale signal. We show that both the remote sensing and in-situ GAW data observe very similar lower tropospheric regional-scale CH4 signals.
The good agreement is demonstrated for the different time scales that are interesting for CH4 source/sink research: daily, seasonal, and long-term biannual mean evolution. In addition, we demonstrate that both networks observe consistent latitudinal CH4 gradients.

Figure 1: CH4 mole fraction seasonal cycle obtained by NDACC FTIR (red stars) and in-situ GAW (black squares) stations.

Reference:
E. Sepúlveda, M. Schneider, F. Hase, O. E. García, A. Gómez-Pelaez, S.Dohe, T. Blumenstock, and J. C. Guerra. Long-term validation of total and tropospheric column-averaged CH4 mole fractions obtained by mid-infrared ground-based FTIR spectrometry. Atmos. Mes. Tech., 5:1425-1441, 2012

More information about the study:

Sepúlveda, E., Schneider, M., Hase, F., Barthlott, S., Dubravica, D., García, O. E., Gomez-Pelaez, A., González, Y., Guerra, J. C., Gisi, M., Kohlhepp, R., Dohe, S., Blumenstock, T., Strong, K.,  Weaver, D., Palm, M., Sadeghi, A., Deutscher, N. M., Warneke, T., Notholt, J., Jones, N., Griffith, D. W. T., Smale, D., Brailsford, G. W., Robinson, J., Meinhardt, F., Steinbacher, M., Aalto, T., and Worthy, D.: , Atmos. Meas. Tech. Discuss., 7, 633-701, doi:10.5194/amtd-7-633-2014, 2014.