BIAS: an algorithm for the retrieval of trace gas vertical columns from near-infrared earthshine measurements by the SCIAMACHY spectrometer

SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY) is an atmospheric chemistry instrument designed to measure global distributions of important trace constituents. It measures transmitted, reflected or scattered light from the Earth´s atmosphere in both limb and nadir viewing modes. The instrument will be launched in the year 2000 as part of the ESA (European Space Agency) ENVISAT-1 mission. The focus in this paper is on the retrieval of vertical column amounts of absorbing trace species from near infrared nadir earthshine measurements to be taken by SCIAMACHY (1.9 to 2.4μ, channels 7 and 8). From the atmospheric chemistry viewpoint, N₂O and CO are the most important trace absorbers in this spectral range; CH₄, H₂0 and CO₂ also absorb significantly in this region. BIAS (Basic Infrared Absorption Spectroscopy) is a straightforward least-squares spectral fitting algorithm developed for this type of retrieval. The plan of the paper is as follows. The SCIAMACHY instrument measurement capability is summarized. The BIAS algorithm and its application to SCIAMACHY near infrared nadir measurements is also described as are the requirements for BIAS in the context of SCIAMACHY operational data processing. A sensitivity study using BIAS is presented