Abstract
Abstract. Although the global methane (CH4) concentration has more than doubled since pre-industrial times, local emission sources are still poorly identified and quantified. Instruments onboard satellites can improve our knowledge about the methane global distribution owing to their very good spatial coverage. The IASI (Infrared Atmospheric Sounding Interferometer) instrument launched on the European MetOp-A platform is a Fourier transform spectrometer which measures the thermal infrared radiation emitted by the Earth and its atmosphere. In this paper, we present the first global distribution of methane total columns (mostly sensitive to the middle troposphere) from the IASI spectra using the methane ν4 absorption band. The retrieval spectral range was set in order to minimize possible spectroscopic issues. Results are discussed in terms of error budget and vertical sensitivity. In addition, we study the gain of information on surface methane concentrations provided by using the ν3 band, which is partly covered by IASI on the short-wave end of the spectra (extending to 2760 cm−1), where solar reflection contributes significantly.
Highlights
Despite the fact that methane (CH4) is the second most important anthropogenic greenhouse gas in our atmosphere, large uncertainties still exist on the location and intensity of the emission sources
The inversion model is based on the Optimal Estimation Method (OEM) (Rodgers, 2000) implemented in the Atmosphit software developed at the “Universite Libre de Bruxelles”
The retrieval was set-up for partial columns of 3 km thickness, extending from the ground up to 21 km. These partial columns were chosen to fully characterize the measurement even though the different levels are correlated through the Sa matrix. It follows from the shape of the averaging kernels that the IASI measurements have a maximum sensitivity to methane in the middle troposphere, between approximately 4 and 10 km and between 7 and 14 km for tropical regions
Summary
Observations from space which offer a very good spatiotemporal coverage, are useful to improve our knowledge of the relative strengths of the methane sources and sinks. Designed for meteorological purposes, IASI provides information on different trace gases important for climate monitoring (CO2, H2O (Herbin et al, 2009), CH4) and atmospheric chemistry, such as CO (George et al, 2009; Turquety et al, 2009), O3 (Boynard et al, 2009), HNO3 (Wespes et al, 2009) and NH3 (Clarisse et al, 2009). It allows the detection of minor trace gases during exceptional events such as fires (Coheur et al, 2009) or volcanic eruptions (Clarisse et al, 2008).
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