Hans Schrijver

Retrieval of carbon monoxide, methane, and nitrous oxide from SCIAMACHY measurements

The atmospheric spectrometer SCIAMACHY to be launched on board ESAs Envisat satellite in 2000 will measure UV, visible and IR spectra from nadir, limb and occultation with spectral resolution between 0.2 and 1.4 nm. SCHIAMACHYs channel 8 covering the wavelength range 2265-2380 nm will allow the global determination of concentrations of methane, carbon monoxide and nitrous oxide. Sensitivity studies using the most recent values for the instrument parameters show that the minimum values for the accuracies for total vertical columns are of order 5 Dobson units (DU) for carbon monoxide, 3 DU for methane, and 6 DU for nitrous oxide, for a 1 s SCIAMACHY nadir observation. The detection of the IR spectra features novel InGaAs detectors, specially developed for the SCIAMACHY project. While providing the required sensitivity in this wavelength domain, these detectors are limited by noise levels that vary strongly from pixel to pixel. This poses special challenges to the retrieval of molecule concentrations from the measured detector signals. Ways to overcome this problem are discussed.

SCIAMACHY advanced data retrieval algorithm development

Abstract The atmospheric chemistry instrument SCIAMACHY is a joint German-Dutch-Belgian contribution to the European Space Agency environmental satellite ENVISAT scheduled for launch in mid 2001. Instrument performance and calibration measurements recently carried out under thermal vacuum conditions have confirmed predicted performance. Algorithm development at SRON using the UV and visible part of the solar spectrum for ozone profile retrieval and the near infrared for carbon monoxide and methane retrieval has shown good progress. In particular, new avenues have been explored for the development of a fast operational algorithm for the retrieval of ozone profiles, including a new forward model based on perturbation theory that is faster than the current operational algorithm, coupled to a new inversion scheme with no use of a-priori ozone profile information. The near-infrared retrieval of CO and CH 4 is expected to yield higher accuracy data products based on the optimization of retrieval using micro-windows. New and advanced SCIAMACHY data products will be piloted in the Netherlands SCIAMACHY Data Centre that complements the operational ENVISAT payload data segment.

Error analysis of a heterodyne submillimeter sounder for the detection of stratospheric trace gases.

Heterodyne submillimeter detection techniques represent an important development in the field of remote sensing of atmospheric composition. The disclosure of this wavelength region by new low-noise detectors and multichannel high-resolution spectrometers leads to expectations of improved accuracy and vertical resolution of the vertical composition profiles derived from these measurements. Because of the low-noise levels of newly developed receivers, special care is required to ensure that fundamental limitations of the components used do not contribute to systematic errors exceeding the random errors. Operated in an upward-looking geometry, the sensitivity of the retrieval algorithm to noise and instrumental errors can be rather high, and hence instrumental limitations could induce large uncertainties in the derived atmospheric information. Instrumental uncertainties typical for a passive heterodyne sounder are quantified, and their effects on the accuracy of the derived vertical mixing ratio profiles are presented.

Noise-related limits on the detectability of concentration variations of CH4 and CO with SCIAMACHY

We present results of a sensitivity analysis of CO and CH4 observations in the infrared with the spectrometer SCIAMACHY. 2D simulations of trace gas dispersion are used to predict the concentration distributions of the two target gases. Changes in the concentrations are converted into changes in incoming flux to the instrument by simulating the photon scattering through the atmosphere by using several radiative transfer programs, and the HITRAN database. The incoming flux is transferred to measured detector pixel counts and noise employing the SCIAMACHY instrument simulation software as developed in SRON. The computed noise is translated to uncertainties in the determination of concentrations using the Cramer-Rao formalism. We show that for earth albedo values above 10% and realistic noise values the CH4 concentrations can be determined with a precision better than 1%; for CO the error can be higher than 10% in unfavorable conditions. As a by-product, the sensitivities for measuring H2O and N2O are also given.