Abstract
Abstract. A simple empirical CO2 model (SECM) is presented to estimate column-average dry-air mole fractions of atmospheric CO2 (XCO2) as well as mixing ratio profiles. SECM is based on a simple equation depending on 17 empirical parameters, latitude, and date. The empirical parameters have been determined by least squares fitting to NOAA's (National Oceanic and Atmospheric Administration) assimilation system CarbonTracker version 2010 (CT2010). Comparisons with TCCON (total carbon column observing network) FTS (Fourier transform spectrometer) measurements show that SECM XCO2 agrees quite well with reality. The synthetic XCO2 values have a standard error of 1.39 ppm and systematic station-to-station biases of 0.46 ppm. Typical column averaging kernels of the TCCON FTS, a SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric CHartographY), and two GOSAT (Greenhouse gases Observing SATellite) XCO2 retrieval algorithms have been used to assess the smoothing error introduced by using SECM profiles instead of CT2010 profiles as a priori. The additional smoothing error amounts to 0.17 ppm for a typical SCIAMACHY averaging kernel and is most times much smaller for the other instruments (e.g. 0.05 ppm for a typical TCCON FTS averaging kernel). Therefore, SECM is well suited to provide a priori information for state-of-the-art ground-based (FTS) and satellite-based (GOSAT, SCIAMACHY) XCO2 retrievals. Other potential applications are: (i) near real-time processing systems (that cannot make use of models like CT2010 operated in delayed mode), (ii) "CO2 proxy" methods for XCH4 retrievals (as correction for the XCO2 background), and (iii) observing system simulation experiments especially for future satellite missions.
Highlights
Our current knowledge about atmospheric CO2 concentrations and surface fluxes at regional scales over the globe comes primarily from ground-based in situ measurements of air sampling networks and tall towers
We present a simple empirical CO2 model (SECM), which addresses these needs but can be used for various other applications
We presented a simple empirical model (SECM), which can be used to simulate atmospheric CO2 background concentrations in form of mixing ratio profiles and XCO2
Summary
Our current knowledge about atmospheric CO2 concentrations and surface fluxes at regional scales over the globe comes primarily from ground-based in situ measurements of air sampling networks and tall towers. Current satellite and ground-based XCO2 retrieval techniques require more or less realistic estimates of true atmospheric concentrations. In order to prove SECM’s usability as a priori information for state-of-the-art satellite and ground-based XCO2 retrievals, we analyze the smoothing error introduced when using SECM instead of CT2010
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