Spaceborne Measurements of the Column Averaged Methane Dry Air Mole Fraction

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. A possible OCO-like [Miller et al., 2007] spaceborne measurement of methane by using reflected sunlight from the ground in the near infrared bands of methane at 4100-4300 cm-1 and 5900-6100 cm-1 is discussed. At a resolution the same as OCO's (R~20,000) and a signal-to-noise ratio ~130 (one-half of OCO's), it is shown that the first band, which contains more spectral lines, has a Shannon information content at least a factor of 10 higher than the second band. It is also shown that by moving to near-IR wavelengths the sensitivity peak is shifted down towards the surface, thus allowing better observations of the sites of methane's sources and sinks. Preliminary analysis focuses on the requirements of spectral resolution and signal to noise needed for accurate retrieval of methane.