Retrieval of tropospheric methane from MOPITT measurements: algorithm description and simulations

Abstract

Tropospheric concentrations of methane have been increasing at a rate of approximately 1%/year, though recent measurements suggest some slowing in this trend. Increased concentrations of methane, a greenhouse gas, will have significant consequences for tropospheric chemistry and climate on a global scale. Characterization of the spatial and temporal variability of methane is one goal of the MOPITT (Measurement of Pollution In The Troposphere) instrument included on the EOS Terra satellite. This instrument includes spectral channels designed to measure methane total column with approximately 1% precision with a spatial resolution of approximately 22 X 22 km. Retrieval of the methane total column will be accomplished by the MOPITT instrument from measurements of solar radiation reflected at the earth's surface. Gas correlation radiometry will be used to separate the spectral signature of methane in the upwelling radiance from features produced by other trace gases. The retrieval algorithm is based on maximum likelihood and uses an initial guess profile and methane total column variance estimates provided by aircraft in-situ measurements. In this talk, we will describe features of the retrieval algorithm in detail and present results of retrieval simulations conducted to test the sensitivity of the retrieval algorithm to various sources of error.