ABSORPTION BANDS The many absorption bands at 2.3  m ( cm -1 ) and the one band near 1.6  m (6000 cm -1 ) will be considered (Figure 1). Other spectral regions in the solar absorption region contain methane absorptions that are too weak to be useful. The 2.3  m bands contain a dense number of methane lines belonging to the , and bands; at 1.6  m, the 2 3 band is the major component. On the basis of band strength, the 2.3  m bands provide stronger absorption but meanwhile be more susceptible to aerosols. The absorptions at 1.6  m are less dense and less interfered, and the band strength is distributed among many fewer absorption lines. Therefore, there are some isolated lines in that band that have strong peak absorptions that are comparable to the absorptions in the stronger bands at 2.3  m. The absorptions in both spectral regions have absorptions that are about the right strength for accurate measurements of methane absorptions in a nadir observation. Figure 1. Transmittance of the methane bands (see RADIATIVE TRANSFER MODEL). GOSAT also uses the 1.6  m band for methane retrieval. They have updated the methane absorption line list in that band over HITRAN 2008 database. The updated list revises the absorption strengths of the strong lines and includes new weak lines. Figure 2 shows the change in the atmospheric transmittance due to the updated list, which can be as large as 5% in the band region and is crucial to an extremely accurate measurement of methane concentrations. Figure 2. Change in the transmittance of the methane band due to the updated line list from HITRAN 2008 in the 1.6  m band. ABSTRACT. Methane is the second most important anthropogenic greenhouse gas in the terrestrial atmosphere. Thus any attempt to understand its impact on climate change requires knowledge of its sources and sinks, which may be derived from high precision satellite measurements. Spaceborne measurements of methane are compared and contrasted for two near infrared bands at 1.6  m and 2.3  m. First, we evaluate the spectroscopic line parameters from HITRAN 2008 database and a newer methane line list used by GOSAT. Then, a vectorized radiative transfer model is employed to estimate the weighting functions of the proposed instrument for the CH 4 bands for both radiance and linear polarizations. We found that the degree of linear polarization has a strong sensitivity below 10 km, which serves as an excellent candidate for locating surface sources and sinks. Measuring Column Averaged Methane Dry Air Mole Fraction from Space Geoffrey C. Toon £, King-Fai Li †,, Xi Zhang †, Charles E. Miller £, Linda R. Brown £, Yuk L. Yung † Geoffrey C. Toon £, King-Fai Li †,#, Xi Zhang †, Charles E. Miller £, Linda R. Brown £, Yuk L. Yung † £ NASA’s Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA † Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA Measuring Column Averaged Methane Dry Air Mole Fraction from Space Geoffrey C. Toon £, King-Fai Li †,, Xi Zhang †, Charles E. Miller £, Linda R. Brown £, Yuk L. Yung † Geoffrey C. Toon £, King-Fai Li †,#, Xi Zhang †, Charles E. Miller £, Linda R. Brown £, Yuk L. Yung † £ NASA’s Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA † Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA # Correspondence author address: AGU 09’ A41C-0107 INTRODUCTION Methane is the second most abundant anthropogenic green-house gas in the Earth’s atmosphere [IPCC, 2007], accounting for ~4 – 9% of the greenhouse effect on Earth. Thus any attempt to understand and quantify climate change requires knowledge of methane’s sources, sinks and distribution within the atmosphere. Unfortunately current Earth-based methane sensor networks such as those operated by NOAA are geographically sparse and prone to spatial bias. Both of these issues are shortcomings that a space-based observation program would be perfectly suited to remedy. Recently, a global map of methane has been reported from the SCIAMACHY measurement, but which has led to great controversies whether there are aerobic sources of methane [Frankenberg et al., 2008]. Obviously, independent satellite measurement is tempted. SCIAMACHY uses methane’s 1.6  m and 2.3  m near-infrared bands for retrieving the tropospheric methane at a resolution 0.26  m but with a coarse spatial resolution 240 km x 30 km. SCIAMACHY reports the averaged methane dry air mole column fraction, XCH 4, using CO 2 at 1.6  m as a proxy for the air column. JAXA’s GOSAT satellite, launched in 2009, measures the lower atmospheric methane through the 1.6  m band and a thermal infrared band at 8  m with Fourier Transform Spectrometers, both at a spectral resolution 0.27  m and a spatial resolution 10.5 km. Previous studies of the optical properties of the 1.6  m and 2.3  m bands have ignored the effects of polarizations. Polarization due to scattering by air molecules and aerosols in the lower atmosphere would provide better constraints on the concentrations of the trace gas and aerosols. Here we shall examine the linear polarization of these bands. RADIATIVE TRANSFER MODEL The transmittance shown in Figures 1 and 2 are calculated from a novel linearized vector radiative transfer model VLIDORT [Spurr, 2006]. VLIDORT has been developed for the numerical computation of the Stokes vector in a multiply elastically scattering multilayer medium. This model uses the discrete ordinate method to approximate the multiple scatter integrals and has the pseudo- spherical capability. It is also fully linearized: along with the polarized radiance field, it will deliver analytic weighting functions with respect to any atmospheric and/or surface properties. Figure 3 shows the atmospheric profiles used to calculate the transmission spectra. The model atmosphere has methane and aerosols only. The temperature and methane profiles are from the 1976 US Standard Atmosphere. The aerosol extinction coefficients,  a, at 0.75  m and 1.55  m were measured by SAGE- III and that at 2.45  m was measured by HALOE/UARS. They are annually averaged in Figure 3. Atmospheric settings in the radiative transfer model. VLIDORT calculates the Stokes’ vector ( I, Q, U and V ). The transmittance shown above applies only to the radiance I. The degree of linear polarization is defined by P = Q / I. Figure 4 shows the corresponding P in the methane bands. It is clear that P maximizes at wavelengths where the absorption is strong. Both have a background value ~-0.3% in the clear atmosphere. Figure 4. Degree of linear polarization in clear atmospheres in the two methane bands WEIGHTING FUNCTIONS The sensitivity of the measurements are characterized by the weighting functions K. Here we define K as the normalized Jacobians of I and P such that the maximum value is always 1. The derivatives are estimated by the 3- point Lagrangian interplation method, with the perturbations -20%, 0% (i.e. unperturbed reference) and +20% applied at each wavenumber and altitude. Notice that the derivatives must be divided by the thicknesses of the atmospheric layers to cancel the air-mass column effect in the radiative calculations. Figure 5 shows the band-averaged weighting functions of I ( K I ’ ) and P ( K P ’ ) for the methane bands. They peak near the surface but K I ’ has very broad altitude dependence. In contrast, K P ’ has a sharp peak below 10 km resulted due to scattering, which would greatly enhance the accuracy of locating the surface sources and sinks. Figure 5. Weighting Functions of the methane bands. REFERENCES (b) K P ’ (a) K I ’