Monitoring and assimilation of early TROPOMI total column carbon monoxide data in the CAMS system

The Tropospheric Monitoring Instrument (TROPOMI) on the Copernicus Sentinel 5 Precursor (S5P) satellite, launched in October 2017, provides a wealth of atmospheric composition data, including total columns of carbon monoxide (TCCO) at high horizontal resolution (7 km x 7 km) retrieved from the shortwave-infrared (SWIR) part of the solar spectrum. TROPOMI offline TCCO data (V1.0.2) have been included in the data assimilation system of the Copernicus Atmospheric Monitoring Service (CAMS) to assess the quality of the data and to carry out first assimilation tests with the data for the period 28 January to 3 May 2018. Even though S5P was still in its commissioning phase until 24 April 2018 the TROPOMI TCCO data were of good enough quality for these tests.

The TROPOMI data successfully capture the global TCCO distribution as given by the CAMS analysis that routinely assimilates thermal infrared (TIR) TCCO retrievals from the Measurement of Pollution in the Troposphere (MOPITT) instrument and the Atmospheric Sounding Interferometer (IASI). However, there are some biases between the TROPOMI data and the CAMS CO analysis, particularly in the northern hemisphere (NH) at low solar elevations north on 40⁰N where TROPOMI has higher TCCO values than CAMS. Relative to CAMS and averaged over the period from 28 January to 3 May 2018, the TROPOMI data have a bias of 0.17±0.27 10¹⁸ molec cm^-2 in the NH, -0.07 ± 0.19 10¹⁸ molec cm^-2 in the Tropics and 0.009 ± 0.12 10¹⁸ molec cm^-2 in the southern hemisphere (SH).

When TROPOMI data are assimilated in the CAMS system they lead to increased CO values in the Extratropics and lower values in the Tropics, with TCCO changes of up to 30% at high northern latitudes. Even though the TROPOMI data are total columns their assimilation has a large impact on the vertical structure of the CAMS CO analysis, because of different sensitivities of the TROPOMI SWIR and MOPITT and IASI TIR retrievals to CO in the atmosphere

The assimilation of TROPOMI TCCO improves the fit to European Global Atmosphere Watch surface CO observations over Europe and to In-service Aircraft for a Global Observing System aircraft profiles in several areas. Particularly noteworthy is that the assimilation of TROPOMI TCCO leads to increased CO values in the lower troposphere over Europe, where the long standing low bias of the CAMS system is reduced. The fit to data from the Total Carbon Column Observing Network is degraded in the NH, but improved at Lauder in the SH.