G2.33   Lack of in-depth understanding of random and systematic uncertainties of MAX-DOAS tropospheric O3 measurements

Gap detailed description

Although several studies have demonstrated the potential of multi-angular UV-Visible scattered light measurements of the MAX-DOAS and Pandora types to measure tropospheric O3, the analysis of uncertainties and the validation of the resulting measurements has generally been limited in scope. As a result, a comprehensive error budget and validation of tropospheric O3 retrieval from MAX-DOAS and Pandora measurements is currently lacking. Like for other MAX-DOAS measurements, the main uncertainties for O3 are related to the estimation of the effective photon light path, which is dependent on the aerosol content and optical properties. In addition, for O3, the interference with the strong O3 absorption taking place higher up in the atmosphere is potentially a significant source of systematic bias.

In addition to the lack of information content (G2.31) and consensus on retrieval approaches (G2.32), the lack of uncertainty characterization and validation of tropospheric O3 measurements from MAX-DOAS and Pandora instruments analysis limits the potential for network capabilities assessment.

Activities within GAIA-CLIM related to this gap

Assessment of both random and systematic uncertainties based on a literature review and on current findings of other projects such as NORS and FRM4DOAS.

Gap remedy(s)

Remedy #1

Specific remedy proposed

More studies addressing the characterization of uncertainties in tropospheric O3 measurements from MAX-DOAS type of instruments are necessary. This should include an assessment of both random and systematic uncertainties and validation with reference independent observations, which can be provided by ozonesonde data and/or in-situ surface O3 instruments. The CINDI-2 intercomparison campaign will be an ideal opportunity to support such an assessment.

Measurable outcome of success

A measure of success would be to provide a realistic traceability chain for tropospheric O3 measured with MAX-DOAS type instruments.

Achievable outcomes

Technological / organizational viability: medium.

Indicative cost estimate: medium (>1million).


An accurate understanding of the uncertainties in the MAX-DOAS measurements is an important prerequisite for developing strategies to further reduce the measurement uncertainty and for the validation of the MAX-DOAS data using other independent measurement techniques.


To develop this remedy will take approximately 2 years.

Gap risks to non-resolution

Identified future risk / impact

Probability of occurrence if gap not remedied

Downstream impacts on ability to deliver high quality services to science / industry / society

Uncertainties on tropospheric O3 MAX-DOAS retrieval uncertainties not estimated in a robust and consistent manner.


An incomplete assessment of the error budget on tropospheric O3 retrievals from MAX-DOAS observations will lead to a lack of robustness in the satellite and model validation studies which would use these measurements as correlative data.

Work package: