Space-borne measurements of atmospheric CH4 by high-resolution near infrared spectrometry of reflected sunlight

Methane (CH4) is the second most important anthropogenic greenhouse gas in the terrestrial atmosphere. It constitutes about 20% of the anthropogenic climate forcing by greenhouse gases on Earth [IPCC, 2007]. The seasonal cycle of surface methane varies with latitude, going from ~0.6% at the equator to ~2.6% in the mid-latitudes [Butler et al., 2004]. Thus, to understand and quantify climate change due to CH4, we have to know its sources, sinks and spatiotemporal distribution with high precision. We introduce a strategy for measuring the CH4 mixing ratio profile from a space-borne platform. It employs high-resolution spectra of reflected sunlight taken simultaneously in the near-infrared (NIR) CH4 (1.6-μm and 2.3-μm) and O2 (0.76-μm) bands. The CH4 profile retrieval algorithm involves three major components. The first component is the forward model, which produces radiances and weighting functions in a scattering, absorbing, emitting atmosphere. As a starting point, the same model is used to generate the synthetic spectrum. The second component simulates the instrument's spectral resolution, spectral range as well as several instrument noise sources. The third component is an inverse method based on optimal estimation theory [Rodgers, 2000] for retrieving the CH4 profile from the synthetic spectra. We use the VLIDORT code [Spurr, 2006] to model the radiative transfer. We explore methods to characterize scattering by clouds and aerosols and evaluate the random and systematic errors in the retrievals.