G2.32    Better characterization of the different MAX-DOAS tropospheric O3 retrieval methods needed

Gap detailed description

The potential of MAX-DOAS and similarly designed instruments to measure tropospheric O3 have only been demonstrated in a limited number of pioneering investigations. In these studies, experimental retrieval methods have been applied which are based on Optimal Estimation (OE) schemes (Liu et al., 2006; Irie et al., 2011) or on more simple approaches such as the modified geometrical approximation used in Gomez et al. (2014) to infer free-tropospheric O3 concentration from a high-altitude site. More work is necessary to better characterize the different possible approaches to tropospheric O3 retrieval from multi-axis scattered light measurements in both UV and visible ranges.

Similar to the lack of information content analysis (see G2.31), the lack of consensus on retrieval methods 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

To address this gap we will undertake an investigation of the different retrieval approaches suitable to measure tropospheric O3 from MAXDOAS-type instruments and provide an overview of the state-of-the-art understanding of the different methods. 

Gap remedy(s)

Remedy #1

Specific remedy proposed

More in-depth studies are needed to investigate the different retrieval methods. Ideally this should be conducted in a coordinated way, e.g. as part of an instrument intercomparison experiment such as the CINDI campaign (Piters et al., 2012). A suitable combination would be to first develop possible strategies within GAIA-CLIM and then to test these as part of the upcoming CINDI2 intercomparison campaign during September 2016. With most of the active MAX-DOAS research groups involved, this campaign will provide an ideal opportunity for a retrieval technique study for tropospheric O3 MAX-DOAS observations.

Measurable outcome of success

A measure of success would be if we can provide in-depth characterisations of the different retrieval methods and their advantages and disadvantages for the retrieval of tropospheric O3 from MAX-DOAS measurements.

Achievable outcomes

Technological / organizational viability: medium.

Indicative cost estimate: medium (>1million).


The characterisation would then provide the necessary information for the scientific community to make a better informed decision on which retrieval approach to choose and to aim at increased homogenisation of tropospheric O3 data provided by different research groups.


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

Existing methods for the retrieval of tropospheric O3 from MAX-DOAS observations not assessed and possibilities for further improvements/new methods not 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: