Carbon monoxide total columns from SCIAMACHY 2.3 µmatmospheric reflectance measurements: towards a full-missiondataproduct (2003–2012)

[摘要] We present a full-mission data product of carbon monoxide (CO) vertical columndensities using the 2310–2338 nm SCIAMACHY reflectance measurementsover clear-sky land scenes for the period January 2003–April 2012.Theretrieval employs the SICOR algorithm, which will be used for operational dataprocessing of the Sentinel-5 Precursor mission. The retrieval approach inferssimultaneously carbon monoxide, methane and water vapour column densitiestogether with a Lambertian surface albedo from individual SCIAMACHYmeasurements employinga non-scattering radiative transfer model.To account for theradiometric instrument degradation including the formation of anice-layer on the 2.3 µm detector array, we considerclear-sky measurements over the Sahara as a natural calibrationtarget. For these specific measurements, we spectrally calibrate theSCIAMACHY measurements and determine a spectral radiometric offsetand the width of the instrument spectral response function asa function of time for the entire operational phase of the mission.We show that the smoothing error of individual clear-sky COretrievals is less than ±1 ppb and thus this errorcontribution does not need to be accounted for in the validationconsidering the much higher retrieval noise. The CO data product isvalidated against measurements of ground-based Fourier transforminfrared spectrometers at 27 stations of the NDACC-IRWG and TCCONnetwork and MOZAIC/IAGOS aircraft measurements at 26 airportsworldwide.Overall, we find a good agreement with TCCONmeasurements with a mean bias b = −1.2 ppb anda station-to-station bias with σ   = 7.2 ppb.The negative sign of the bias means a low bias of SCIAMACHY CO with respect to TCCON.For the NDACC-IRWG network, we obtain a largermean station bias of b  = −9.2 ppb withσ  = 8.1 ppb and for the MOZAIC/IAGOSmeasurements we find b  = −6.4 ppb withσ  = 5.6 ppb.The SCIAMACHY data set issubject to a small but significant bias trend of 1.47 ± 0.25 ppb yr⁻¹. After trend correction, the bias withrespect to MOZAIC/IAGOS observation is 2.5 ppb, with respectto TCCON measurements it is −4.6 ppb and with respect toNDACC-IRWG measurements −8.4 ppb. Hence, a discrepancy of3.8 ppb remains between the global biases with NDACC-IRWGand TCCON, which is confirmed by directly comparing NDACC-IRWG andTCCON measurements.Generally, the scatter of the individualSCIAMACHY CO retrievals is high and dominated by large measurementnoise.Hence, for practical usage of the data set, averaging ofindividual retrievals is required.As an example, we show thatmonthly mean SCIAMACHY CO retrievals, averaged separately overNorthern and Southern Africa, reflect the spatial and temporalvariability of biomass burning events in agreement with the globalchemical transport model TM5.

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