Challenge of measuring tropospheric ozone columns from satellites

Absorbing a large part of the aggressive UV-radiation, the remote stratospheric ozone layer protects life on the surface. Tropospheric ozone, on the other hand, is a toxic molecule affecting human health, crops and ecosystems. It is thus necessary to monitor both layers separately on a global scale, but measuring tropospheric ozone from space remains a real challenge. BIRA-IASB keeps investing its expertise in validation and data assimilation techniques to help solve this challenge.

The challenge of measuring tropospheric ozone columns from satellites

Satellite instruments which scan the Earth’s limb provide vertical ozone profiles in the stratosphere but not in the troposphere. Other satellite instruments scan the nadir (i.e. downwards) but their measurements are accurate only when integrated over the whole altitude range, delivering so-called total columns.

By subtracting the stratospheric contribution from the total ozone column, it is possible in principle to evaluate the tropospheric ozone column. This operation may seem simple but is quite difficult in practice because these instruments never look at exactly the same places and times. Direct subtraction between different satellite instruments is not satisfactory, and better results are sought by combining these observations with models through data assimilation systems.

Using chemical data assimilation to connect different observations

Chemical data assimilation consists in combining an atmospheric model with experimental data to provide analyses of atmospheric composition - including ozone – on a regular grid and at regular times. BIRA-IASB contributes to the monitoring of tropospheric ozone with two distinct approaches: by using its own assimilation system, BASCOE, and by independent validation of the assimilation system used by the European Copernicus Atmosphere Monitoring Service (CAMS).

BIRA-IASB contributed to two attempts for an indirect approach in 2021 and 2022. In the so-called “residual method”, observations from the high-resolution satellite instrument TROPOMI were used for the total ozone columns, and observations of stratospheric ozone by the NASA limb sounder MLS were assimilated into BASCOE to allow accurate matches with the locations of the TROPOMI measurement.

The difference between TROPOMI and BASCOE output provides the tropospheric ozone column. CAMS uses another approach where the assimilation system takes center stage: all satellite observations are directly assimilated, providing analyses of ozone at many different levels in both the troposphere and the stratosphere.

While intercomparisons between tropospheric ozone columns from different methods are encouraging, significant differences in some latitude ranges remain. The determination of tropospheric ozone columns, a key-parameter for air quality monitoring, thus remains a challenge in which BIRA-IASB keeps investing its expertise in validation and data assimilation techniques.

 

Sources:

Global maps data assimilation techniques TROPOMI BASCOE and CAMS
Global maps of the amount of ozone in the troposphere (expressed as the vertical column up to the tropopause, in Dobson Units), in May 2020, using two data assimilation approaches: TROPOMI-BASCOE residuals developed by DLR and BIRA-IASB (top panel); and direct assimilation by the Copernicus Atmosphere Monitoring Service (bottom panel).