| Atmospheric methane concentrations have reached a new high at 1845 ± 2 ppb, accounting
for an increase of 256 % since pre-industrial times (WMO, 2016). In the last ten years,
methane has been on the rise again at rates of ∼0.3%/year (e.g., Bader et al., 2016), after a
period of stabilization of about 5 years. This recent increase is not fully understood due to
remaining uncertainties in the methane budget, influenced by numerous anthropogenic and
natural emission sources. In order to examine the cause(s) of this increase, we focus
on the two main methane isotopologues, i.e. CH3D and 13CH4. Both CH3D and
13CH4 are emitted in the atmosphere with different ratio depending on the emission
processes involved. As heavier isotopologues will react more slowly than 12CH4, each
isotopologue will be depleted from the atmosphere at a specific rate depending on the
removal process. Methane isotopologues are therefore good tracers of the methane
budget.
In this contribution, the first development and optimization of the retrieval strategy of
CH3D as well as the preliminary tests for 13CH4 will be presented and discussed
, using FTIR (Fourier Transform infrared) solar spectra collected at the Eureka
(80.05 ˚ N, -86.42 ˚ E, 610 m a.s.l.) and Toronto (43.66˚ N, -79.4˚ E, 174 m a.s.l.)
ground-based sites. Mixing ratio vertical profiles from a Whole Atmosphere Community
Climate Model (WACCM v.4, Marsh et al., 2013) simulation developed by Buzan et
al. (2016) are used as a priori inputs. A discussion on the type of regularization
constraints used for the retrievals will be presented as well as an evaluation of available
spectroscopy (primarily the different editions of the HITRAN database, see Rothman et al.,
2013 and references therein). The uncertainties affecting the retrieved columns as
well as information content evaluation will be discussed in order to assess the best
strategy to be employed based on its altitude sensitivity range and complete error
budget.
Acknowledgments
Whitney Bader has received funding from the European Union’s Horizon2020 research
and innovation programme under the Marie Sklodowska-Curie grant agreement n˚ 704951,
and from the University of Toronto through a Faculty of Arts & Science Postdoctoral
Fellowship Award.
References
Bader, W., Bovy, B., Conway, S., Strong, K., Smale, D., Turner, A. J., Blumenstock, T.,
Boone, C., Coulon, A., Garcia, O., Griffith, D. W. T., Hase, F., Hausmann, P., Jones,
N., Krummel, P., Murata, I., Morino, I., Nakajima, H., O'Doherty, S.,
Paton-Walsh, C., Robinson, J., Sandrin, R., Schneider, M., Servais, C., Sussmann, R.
and Mahieu, E.: Ten years of atmospheric methane from ground-based NDACC
FTIR observations, Atmos. Chem. Phys. Discuss., 1–31, doi:10.5194/acp-2016-699,
2016.
Buzan, E. M., Beale, C. A., Boone, C. D. and Bernath, P. F.: Global stratospheric
measurements of the isotopologues of methane from the Atmospheric Chemistry
Experiment Fourier transform spectrometer, Atmos. Meas. Tech., 9(3), 1095–1111,
doi:10.5194/amt-9-1095-2016, 2016.
Marsh, D. R., Mills, M. J., Kinnison, D. E., Lamarque, J.-F., Calvo, N. and Polvani, L.
M.: Climate Change from 1850 to 2005 Simulated in CESM1(WACCM), J. Clim., 26(19),
7372–7391, doi:10.1175/JCLI-D-12-00558.1, 2013.
Rothman, L. S., Gordon, I. E., Babikov, Y., Barbe, A., Chris Benner, D., Bernath, P. F.,
Birk, M., Bizzocchi, L., Boudon, V., Brown, L. R., Campargue, A., Chance, K.,
Cohen, E. A., Coudert, L. H., Devi, V. M., Drouin, B. J., Fayt, A., Flaud, J.-M.,
Gamache, R. R., Harrison, J. J., Hartmann, J.-M., Hill, C., Hodges, J. T., Jacquemart,
D., Jolly, A., Lamouroux, J., Le Roy, R. J., Li, G., Long, D. A., Lyulin, O. M.,
Mackie, C. J., Massie, S. T., Mikhailenko, S., Müller, H. S. P., Naumenko, O. V.,
Nikitin, A. V., Orphal, J., Perevalov, V., Perrin, A., Polovtseva, E. R., Richard, C.,
Smith, M. A. H., Starikova, E., Sung, K., Tashkun, S., Tennyson, J., Toon, G. C.,
Tyuterev, V. G. and Wagner, G.: The HITRAN2012 molecular spectroscopic database,
J. Quant. Spectrosc. Radiat. Transf., 130, 4–50, doi:10.1016/j.jqsrt.2013.07.002,
2013.
World Meteorological Organization, Greenhouse gas bulletin, N˚ 12, October 2016. |