Abstract

Abstract. We present a full-mission data product of carbon monoxide (CO) vertical column densities using the 2310–2338 nm SCIAMACHY reflectance measurements over clear-sky land scenes for the period January 2003–April 2012. The retrieval employs the SICOR algorithm, which will be used for operational data processing of the Sentinel-5 Precursor mission. The retrieval approach infers simultaneously carbon monoxide, methane and water vapour column densities together with a Lambertian surface albedo from individual SCIAMACHY measurements employing a non-scattering radiative transfer model. To account for the radiometric instrument degradation including the formation of an ice-layer on the 2.3 µm detector array, we consider clear-sky measurements over the Sahara as a natural calibration target. For these specific measurements, we spectrally calibrate the SCIAMACHY measurements and determine a spectral radiometric offset and the width of the instrument spectral response function as a function of time for the entire operational phase of the mission. We show that the smoothing error of individual clear-sky CO retrievals is less than ±1 ppb and thus this error contribution does not need to be accounted for in the validation considering the much higher retrieval noise. The CO data product is validated against measurements of ground-based Fourier transform infrared spectrometers at 27 stations of the NDACC-IRWG and TCCON network and MOZAIC/IAGOS aircraft measurements at 26 airports worldwide. Overall, we find a good agreement with TCCON measurements with a mean bias b = −1.2 ppb and a 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 larger mean station bias of b = −9.2 ppb with σ = 8.1 ppb and for the MOZAIC/IAGOS measurements we find b = −6.4 ppb with σ = 5.6 ppb. The SCIAMACHY data set is subject to a small but significant bias trend of 1.47 ± 0.25 ppb yr−1. After trend correction, the bias with respect to MOZAIC/IAGOS observation is 2.5 ppb, with respect to TCCON measurements it is −4.6 ppb and with respect to NDACC-IRWG measurements −8.4 ppb. Hence, a discrepancy of 3.8 ppb remains between the global biases with NDACC-IRWG and TCCON, which is confirmed by directly comparing NDACC-IRWG and TCCON measurements. Generally, the scatter of the individual SCIAMACHY CO retrievals is high and dominated by large measurement noise. Hence, for practical usage of the data set, averaging of individual retrievals is required. As an example, we show that monthly mean SCIAMACHY CO retrievals, averaged separately over Northern and Southern Africa, reflect the spatial and temporal variability of biomass burning events in agreement with the global chemical transport model TM5.

Highlights

  • Carbon monoxide (CO) is an important atmospheric trace gas for the understanding of tropospheric chemistry and air quality

  • We found that the MOZAIC/IAGOS measurements are on average biased low compared to the TM5 model used as priori for the SCIAMACHY CO retrieval (15 % ± 25 %)

  • We found that for sufficient signal level and temporal coverage, SCIAMACHY can catch the seasonal variability of the CO total column amount as reflected by NDACC-IRWG, TCCON and MOZAIC measurements

Read more

Summary

Introduction

Carbon monoxide (CO) is an important atmospheric trace gas for the understanding of tropospheric chemistry and air quality. The Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY) was one of the first space-based instruments observing CO from the shortwave infrared (SWIR) range around 2.3 μm (Bovensmann et al, 1999) and it was fully operational from January 2003 until April 2012 when contact with its host ENVISAT (Environmental Satellite) was lost In this period, an almost continuous long-term record of more than 9 years of SWIR measurements in the 2.3 μm spectral range from space was recorded. The SCIAMACHY CO measurements were compared with corresponding MOPITT CO retrievals (de Laat et al, 2010a) and validated with CO observations of ground-based spectrometers (de Laat et al, 2010b) and MOZAIC/IAGOS aircraft measurements (de Laat et al, 2012) All these previous studies were dedicated to the early years of the mission before 2009.

Retrieval approach
Instrument calibration
20 Dec 2004–5 Jan 2005 14 675–14 912
Validation
Ground-based Fourier transform spectrometers
Potential data application
Findings
Summary and conclusions
Full Text
Published version (Free)

Talk to us

Join us for a 30 min session where you can share your feedback and ask us any queries you have

Schedule a call