Atmos. Chem. Phys. Discuss., 12, 23731-23757, 2012
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
The 2007–2011 evolution of tropical methane in the mid-troposphere as seen from space by MetOp-A/IASI
C. Crevoisier1, D. Nobileau1, R. Armante1, L. Crépeau1, T. Machida2, Y. Sawa3, H. Matsueda3, T. Schuck4, T. Thonat1, J. Pernin1, N. A. Scott1, and A. Chédin1
1Laboratoire de Météorologie Dynamique, UMR8539, CNRS, IPSL, Ecole Polytechnique, 91128 Palaiseau Cedex, France
2Center for Global Environmental Research, National Institute for Environmental Studies, Tsukuba, Japan
3Geochemical Research Department, Meteorological Research Institute, Tsukuba, Japan
4Atmospheric Chemistry Division, Max Planck Institute for Chemistry, Mainz, Germany

Abstract. Since July 2007, monthly averages of mid-tropospheric methane are retrieved in the Tropics over land and sea, by day and night, from IASI onboard MetOp-A, yielding a complete view of the geographical distribution, seasonality and long-term tendency of methane in the mid-troposphere. Retrieved methane displays a clear seasonal cycle of ~25 ppbv in the Northern Tropics, with a maximum in November and a minimum in April–May, a more complex cycle of ~15 ppbv in the Southern Tropics, and a south-to-north latitudinal variation of ~30 ppbv, in good agreement with regular aircraft measurements of the CONTRAIL program. Comparisons with CARIBIC aircraft measurements made at ~11 km yield an averaged difference between collocated IASI estimates and CARIBIC measurements of 7.2 ppbv with a standard deviation of 13.1 ppbv, and show that IASI captures well the evolution of mid-tropospheric methane. In particular, in 2007 and 2008, IASI shows an increase of mid-tropospheric methane in the tropical region of 9.5 ± 2.8 and 6.3 ± 1.7 ppbv yr−1 respectively, in excellent agreement with the rate of increase measured at the surface after almost a decade of near-zero growth. IASI also indicates a slowing down of this increase in the following years to ~2 ppbv yr−1, with the highest increase in 2010. Assuming that the recent evolution of methane is mostly due to an increase in surface emissions, IASI might indicate a decrease in tropical wetland emissions for the period 2009–2011, in agreement with decreasing tropical precipitation over this period, together with an increase in biomass burning emissions in 2010 in the Southern Tropics.

Citation: Crevoisier, C., Nobileau, D., Armante, R., Crépeau, L., Machida, T., Sawa, Y., Matsueda, H., Schuck, T., Thonat, T., Pernin, J., Scott, N. A., and Chédin, A.: The 2007–2011 evolution of tropical methane in the mid-troposphere as seen from space by MetOp-A/IASI, Atmos. Chem. Phys. Discuss., 12, 23731-23757, doi:10.5194/acpd-12-23731-2012, 2012.
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