Abstract
Abstract. Major limitations of our present knowledge of the global distribution of CO2 in the atmosphere are the uncertainty in atmospheric transport mixing and the sparseness of in situ concentration measurements. Limb viewing space-borne sounders, observing the atmosphere along tangential optical paths, offer a vertical resolution of a few kilometers for profiles, which is much better than currently flying or planned nadir sounding instruments can achieve. In this paper, we analyse the feasibility of obtaining CO2 vertical profiles in the 5–25 km altitude range from the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS, launched in August 2003), high spectral resolution solar occultation measurements. Two main difficulties must be overcome: (i) the accurate determination of the instrument pointing parameters (tangent heights) and pressure/temperature profiles independently from an a priori CO2 profile, and (ii) the potential impact of uncertainties in the temperature knowledge on the retrieved CO2 profile. The first difficulty has been solved using the N2 collision-induced continuum absorption near 4 μm to determine tangent heights, pressure and temperature from the ACE-FTS spectra. The second difficulty has been solved by a careful selection of CO2 spectral micro-windows. Retrievals using synthetic spectra made under realistic simulation conditions show a vertical resolution close to 2.5 km and accuracy of the order of 2 ppm after averaging over 25 profiles. These results open the way to promising studies of transport mechanisms and carbon fluxes from the ACE-FTS measurements. First CO2 vertical profiles retrieved from real ACE-FTS occultations shown in this paper confirm the robustness of the method and applicability to real measurements.
Highlights
Determining the spatial and temporal structure of surface carbon fluxes has become a major scientific issue during the last decade
With the accuracy obtained on pointing parameters, assuming errors on the temperature profile of the order 1 K standard deviation, assuming instrumental and model noise according to Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) measurements, and assuming a regularization matrix based on the MOZART model CO2 covariance matrix with degrees of freedom around 9, the retrieved CO2 error averaged over 25 occultations comes to less than 1 ppm bias with a standard deviation around 2 ppm
Coupled with column measurements from a nadir instrument, occultation measurements will bring useful constraints to the surface carbon flux determination (Pak, 2001; Patra, 2003), for example, by indicating what portion of the column measurement comes from the region below 5 km
Summary
Determining the spatial and temporal structure of surface carbon fluxes has become a major scientific issue during the last decade. In the so-called “inverse” approach, observed atmospheric concentration gradients are used to disentangle surface fluxes, given some description of atmospheric transport. Foucher et al.: Feasibility of CO2 profile retrieval from ACE-FTS from the surface, and the difference between atmospheric and surface mixing ratios is determined by the processes that transport surface air throughout the atmosphere, including advection, convection and eddy mixing (Shia, 2006). Because it takes several months to transport surface air to the lower stratosphere, the CO2 mixing ratio is lower and the seasonal cycle is different there as compared to the troposphere (Plumb, 1992, 1996; Shia, 2006). Results using synthetic spectra and first retrievals using real ACE-FTS data are presented and discussed
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