G2.31    Lack of understanding of the information content of MAX-DOAS tropospheric O3 measurements

Gap detailed description

Retrieving tropospheric O3 from passive remote sensing observations is difficult because almost 90% of the total column O3 resides in the stratosphere. However, it has been shown that information on tropospheric O3 can be extracted from multi-angular observations of the sunlight scattered by the atmosphere, using the so-called MAX-DOAS technique or similarly designed instruments (see e.g. Irie et al., 2011; Liu et al., 2006; Gomez et al., 2014). Although these pioneering studies have demonstrated the feasibility of tropospheric O3 measurements from UV-Visible absorption measurements in both the Huggins (Irie et al., 2011) and Chappuis bands (Gomez et al., 2014), the information content of such measurements remains to be explored in depth in terms of altitude range, dependency on measurement geometry (in particular the number of viewing angles being sampled), dependency on atmospheric visibility (i.e. aerosol content), solar geometry, horizontal representativeness, etc.

This current lack of knowledge of the information content of MAX-DOAS tropospheric O3 measurements limits the assessment of the usability of the technique for large scale O3 monitoring. This gap is partially addressed within GAIA-CLIM.

Activities within GAIA-CLIM related to this gap

This gap is partially addressed within GAIA-CLIM – however, due to the limited available funding, we envisage to inform the wider community about the gap and to contribute towards resolving the gap, but we will not be able to remedy the situation solely within GAIA-CLIM.

Gap remedy(s)

Remedy #1

Specific remedy proposed

More studies are needed to investigate the potential of the MAX-DOAS remote-sensing technique for tropospheric O3 measurements. In particular, the information content of measurements must be analyzed in different spectral ranges (covering both Huggins and Chappuis O3 absorption bands) and a broad range of observation geometries and atmospheric conditions. These issues will be addressed during the CINDI-2 MAX-DOAS intercomparison campaign which will be held in September 2016 in Cabauw (the Netherlands).

Measurable outcome of success

One measure of success would be the greater availability of more accurate tropospheric O3 data based on MAX-DOAS measurements e.g. within NDACC. 

Achievable outcomes

Technological / organizational viability: medium

Indicative cost estimate: medium (>1million)


If the information content can be better defined, and thus provide us with a clearer picture of, for example, the vertical profile and altitude range of the measurements, then this will lead to better usability of the MAX-DOAS measurements made globally.


To develop and test this remedy will take about 2-3 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

Ability to retrieve tropospheric O3 vertical profiles/column densities from MAX-DOAS observations not assessed/investigated.


Satellite and model tropospheric O3 validation studies will not benefit from these potentially highly-relevant (global coverage; measurement frequency: every 20 minutes during daytime) correlative data sets.


Work package: