ABSORPTION BANDS

1. AGU 09’ A41C-0107 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 † £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 Email address: kfl@gps.caltech.edu ABSTRACT. Methane is the second most important anthro-pogenic 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. The feasibility of spaceborne measurements of methane is investigated for two near-IR 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 CH4 bands for both radiance and linear polarizations. We found that the degree of linear polarization has a strong sensitivity below 10 km, which may facilitate for locating surface sources and sinks. ABSORPTION BANDS The many absorption bands at 2.3 m (4100 - 4300 cm-1) and the one band near 1.6 m(6000 cm-1) will be considered (Figure 1). Both spectral regions have absorptions that are about the right strength for accurate measurements of methane in a nadir observation. 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 then2+2n4, n1+2n4 and n3+n4bands; at 1.6 m , the 2n3 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. 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 [Lyulin et al., 2009; Nikitin, private communication] over HITRAN 2008 database [Rothman et al., 2009]. 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 the design of the experiment with the expected precision (< 0.6%) previously discussed. Figure 2. Change in the transmittance of the methane band due to the updated line list [Lyulin et al., 2009; Nikitin, private communication] from HITRAN 2008 in the 1.6 m band. 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 2004. Figure 3. Atmospheric settings in the radiative transfer model. VLIDORT calculates the Stokes’ vector (I, Q, U and V) [Goody & Yung, 1989]. 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 (KI’) and P (KP’) for the methane bands. These weighting functions correspond to ~0.1% of the unperturbed spectra. They peak near the surface but KI’ has very broad altitude dependence. In contrast, KP’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. INTRODUCTION Methane accounts for~4 – 9% of the greenhouse effect on Earth [IPCC, 2007]. The seasonal cycle of surface methane varies with latitudes; it is ~0.6% and ~2.6% at the equator and in mid-latitudes [Butler et al., 2004]. Thus, to understand and quantify climate change, we have to know about methane’s sources, sinks and its distributions with precisions better than ~0.6%.Unfortunately, current Earth-based methane sensor networksare geographically sparse, especially in Southern Hemisphere [Butler et al., 2004]. Both of these issues areshortcomings that a space-based observation programcould remedy. For example, ESA’s SCIAMACHY uses methane’s 1.6 m and 2.3 m near-infrared bands for retrieving the tropospheric methane at a resolution ~1.4 nm but with a low spatial resolution 30 x 60 km2. In these two bands, the reflected sunlight is measured during daytime and the thermal spectra is negligible. SCIAMACHY reports the averaged methane dry air mole column fraction, XCH4, using CO2 at 1.6 m as a proxy for the air column [Buchwitz et al., 2004]; the expected precision is ~0.5% (1-). Recently, the global map of methane reported from SCIAMACHY has led to great controversies whether there are aerobic sources of methane [Frankenberg et al., 2008] and is still under debate. JAXA’s GOSAT, 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 ~0.2 cm-1 and a spatial resolution 10.5 km [Kasuya et al., 2009]. Data from GOSAT is not yet available to public. 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 examine the linear polarization of these bands. SUMMARY Satellite-based measurements of methane have to have precisions better than ~0.6%. Two methane near-IR bands (1.6 m and 2.3 m) are currently used for methane retrievals. We re-examined the transmisson spectra of the 1.6 m-band based on HITRAN2008 line table with an updated methane line list and found that the relative change can be as large as ~5%, crucial to achieve the required precision. We calculated the weighting functions of the two bands with linear polarizations. Both radiance and polarizations have the greatest sensitivity near the surface. But due to scattering, their weighting functions are different, which provides additional information on the vertical distribution of methane. The S:N ratio of the instrument must be better than 1000:1. REFERENCES Buchwitz et al. (2004), Proc. SPIE 5235, Remote Sensing of Clouds and the Atmosphere VIII, Schafer et al. (Eds.), 375; Butler et al. 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