Carbon monoxide column retrieval for clear-sky and cloudy atmospheres: a full-mission data set from SCIAMACHY 2.3 μm reflectance measurements

Abstract

Abstract. We discuss the retrieval of carbon monoxide (CO) vertical column densities from clear-sky and cloud contaminated 2311–2338 nm reflectance spectra measured by the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) from January 2003 until the end of the mission in April 2012. These data was processed with the Shortwave Infrared CO Retrieval algorithm SICOR that we developed for the operational data processing of the Tropospheric Monitoring Instrument (TROPOMI) that will be launched on ESA’s Sentinel-5 Precursor (S5P) mission. This study complements previous work that was limited to clear-sky observations over land. Over the oceans, CO is estimated from cloudy-sky measurements only, which is an important addition to the SCIAMACHY clear-sky CO data set as shown by NDACC and TCCON measurements at coastal sites. For Ny-Ålesund, Lauder, Mauna Loa, and Reunion, a validation of SCIAMACHY clear-sky retrievals is not meaningful because of the high retrieval noise and the few collocations at these sites. This improves significantly when considering cloudy-sky observations, where we find a low mean bias b = ±6.0 ppb and a strong correlation between the validation data set and the SCIAMACHY data sets with a mean Pearson correlation coefficient r = 0.7. Also for land observations, cloudy-sky CO retrievals present an interesting complement to the clear-sky data set, which is less sensitive to the spatial representativeness of the satellite and validation measurement. For example, at the cities Teheran and Beijing the agreement of SCIAMACHY clear-sky CO observations with MOZAIC/IAGOS airborne measurements is poor with a mean bias of b = 171.2 ppb and 57.9 ppb because of local CO pollution, which cannot be captured by SCIAMACHY. The validation improves significantly for cloudy sky retrievals with b = 52.3 ppb and 5.0 ppb, respectively. This is due to a reduced retrieval sensitivity to CO below the cloud and so to the altitude range, which is mostly affected by strong local surface emissions. At the less urbanized region around the airportWindhoek, local CO pollution is less prominent and so MOZAIC/IAGOS measurements agree well with SCIAMACHY clear-sky retrievals with a mean bias of b = 15.5 ppb, but can be even further improved considering cloudy SCIAMACHY observations with a mean CO bias of b = 0.2 ppb. Overall the cloudy-sky CO retrievals from SCIAMACHY short wave infrared measurements present a valuable addition to the clear-sky only data set. Moreover, the study represents the first application of the S5P algorithm for operational CO data processing on cloudy observations prior to the launch of the S5P mission.

Measuring Carbon Monoxide With TROPOMI: First Results and a Comparison With ECMWF‐IFS Analysis Data

“…In the SWIR spectral range, the ocean surface is very dark (except for glint observation geometry) and so cloud‐free observations generally do not measure sufficient light to achieve a meaningful retrieval. The good agreement gives confidence in valuable TROPOMI CO data product for cloudy conditions as was already indicated for Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) observations by Borsdorff et al () and Gloudemans et al (). Our supporting information shows the difference between the a priori and a posteriori CO column densities.…”

“…The algorithm provides the vertically integrated column of CO including the corresponding averaging kernel for individual measurements (Borsdorff et al, ). The algorithm is extensively tested for measurements of the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY), which covers the same spectral region with the same spectral resolution as TROPOMI but with a lower signal‐to‐noise ratio, lower radiometric accuracy, and lower spatial resolution (Borsdorff et al, , ).…”

Air quality impacts of COVID-19 lockdown measures detected from space using high spatial resolution observations of multiple trace gases from Sentinel-5P/TROPOMI

“…This results in a large improvement in measurement sensitivity for relevant air quality products, including NO 2 , SO 2 , HCHO, and CHOCHO, thus enabling the study of rapid emission changes for even smaller sources as compared to previous instruments. For CO measurements, the daily global coverage of TROPOMI at a resolution of 7 × 5.5 km 2 represents a huge improvement to its predecessor SCIAMACHY (Bovensmann et al, 1999;Borsdorff et al, , 2017 with a spatial resolution of 120 × 30 km 2 .…”

Interannual Variation in Stable Isotopes in Water Vapor Over the Northern Tibetan Plateau Linked to ENSO

“…The retrieval used the approach described in Borsdorff et al. (2017) to account for instrument degradation. The latest retrieved full‐mission data of H 2 O and HDO from January 2003 to April 2012 were used in this study (Text ).…”