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Tropospheric Ozone Assessment from space
Tropospheric Ozone Assessment Report (TOAR) is an international effort aiming to assess and document the global distribution, variability and trends in tropospheric ozone and associated impacts on human health, ecosystems, and climate.
The first TOAR phase (2014-2019) highlighted several discrepancies between satellite data that limited confidence in estimates of the burden, short-term variability, and long-term changes of ozone in the free troposphere. A potential cause for these discrepancies comes from the fact that satellite measurements acquired by different sounding techniques differ in spatiotemporal sampling, uncertainties, and vertical sensitivity to ozone.
In view of the second TOAR assessment (TOAR-II, 2020-2025), this finding motivated the Committee on Earth Observation Satellites (CEOS) to coordinate a cross-agency research activity on harmonising tropospheric ozone data from multiple satellites.
In this context BIRA-IASB took the responsibility to harmonise the spatiotemporal and vertical perspective of sixteen key satellite ozone data records.
What is the effect of data harmonisation?
Four harmonisation methods were developed:
- two for ozone profile data retrieved by optimal estimation, and
- two for tropospheric ozone column data retrieved by residual methods.
Harmonised profile data were anchored to ozone sounding measurements, recently homogenised also as part of TOAR-II. The effects of data harmonisation on the global distribution, seasonal cycle, and long-term changes in free tropospheric ozone were studied.
The largest reduction of discrepancy between datasets was obtained when using modelled global ozone fields (here from the Copernicus Atmosphere Monitoring Service CAMSRA) as a transfer standard to constrain the harmonisation process. A major achievement of this data harmonisation research is the better mutual consistency of the various satellite data sets, with a reduction of their dispersion by about 10-40 % depending on the dataset involved and with strong spatiotemporal dependencies.
Moreover, this work provides evidence that, beyond differences in spatiotemporal and vertical representativeness, differences in measurement uncertainty likely remain a major issue to improve the assessment of the spatial distribution and temporal evolution of free tropospheric ozone from satellite observations.
