GAW Report No. 205 - IGAC Project

CHAPTER 1 - INTRODUCTIONFigure 12 - Atmospheric composition measurements from UV-VIS-NIR backscatter instrumentsA disadvantage of solar backscatter sounders such as GOME and SCIAMACHY is that themeasurements require solar light, i.e., they can only be performed on the Earth’s sunlit side. Incontrast, satellite observations using thermal emission permit measurements during night as wellas day. Thermal emission measurements use the thermal infrared (TIR) spectral region tomeasure tropospheric trace gases (Figure 13), including tropospheric O 3 , CO, CH 4 , H 2 O, HDO andvolcanic SO 2 . Several experimental retrievals for species with weaker emission features, such asNH 3 and CH 3 OH, have been recently demonstrated with the TES and IASI instruments [Beer et al.,2008; Clarisse et al., 2009]. Retrieval of these species and others may become routine in thefuture with higher spectral resolution instruments. Measurement sensitivity in the TIR dependsprimarily on the thermal contrast between the Earth’s surface and the lowest layer of theatmosphere, which is typically highest in the mid to upper troposphere [Deeter et al., 2007]. Thus,these instruments provide a characterization of the total column concentration, but with lesssensitivity to boundary layer concentrations. Developing techniques to enhance sensitivity to theboundary layer, for example by integrating UV/visible and IR measurements is a major researchchallenge. Figure 14 shows the history of thermal emission instruments used to measureatmospheric composition.16