G2.26 Poorly understood uncertainty in ozone cross-sections used in the spectral fit for DOAS, MAX-DOAS and Pandora data analysis

Gap abstract: 

The uncertainty in the ozone absorption cross-sections is one of the main systematic error sources in the remote sensing of atmospheric ozone using UV-visible spectroscopy techniques. It is a structured random effect in that even though the uncertainty can be considered as primarily a systematic error source, the actual error is dependent on atmospheric temperature which varies across the annual cycle and with synoptic conditions. Presently the uncertainty in total column ozone due to uncertainty in absorption cross-sections is assumed to be around one to a few per cent but it is poorly quantified. If the same cross-sections are used in satellite observations and ground-based observations, one source for non-consistency can be excluded from the comparison allowing a relative rather than absolute comparison, but this is not always the case. In addition, when the uncertainties related to ozone cross-sections and their temperature dependencies are well characterized, this effect can be included in the error budget of ozone observations. It may be possible that this also improves the retrieval itself.

Part I Gap description

Primary gap type: 
  • Knowledge of uncertainty budget and calibration
Secondary gap type: 
  • Parameter (missing auxiliary data etc.)
ECVs impacted: 
  • Ozone
User category/Application area impacted: 
  • Operational services and service development (meteorological services, environmental services, Copernicus Climate Change Service (C3S) and Atmospheric Monitoring Service (CAMS), operational data assimilation development, etc.)
  • Climate research (research groups working on development, validation and improvement of ECV Climate Data Records)
Non-satellite instrument techniques involved: 
  • UV/VIS zenith DOAS
  • UV/VIS MAXDOAS
  • Pandora
  • This gap represents the top-level coordination and harmonisation activity required across the general spectroscopic measurement field, therefore G2.26 should be addressed in parallel with G2.37.

Detailed description: 

The ozone absorption cross-section is one of the main systematic error sources in the remote sensing of atmospheric ozone using UV-visible spectroscopy techniques. The uncertainty in the cross-sections can be considered as a systematic error source, although the actual error depends on atmospheric temperature, and thus it can be considered as a pseudo-random (or structured random) error, as mentioned in the deliverable D4.3 ‘Uncertainty Budget’ of the EC FP7 project NORS . Presently the uncertainty in total column ozone due to uncertainty in absorption cross-sections is assumed to be around one to a few per cent (WMO GAW report 218 , NORS_D4.3_UB.pdf). In general, when the uncertainties related to ozone cross-sections and their temperature dependencies are well characterized, this effect can be included in the error budget of ozone observations. It is also possible that by including the (correlated) uncertainty to the retrieval algorithm, this would improve the retrievals as well.

The recent WMO IGACO-O3/UV activity ACSO (Absorption Cross Sections of Ozone) performed a thorough evaluation of the existing cross-sections and their impact on ground-based and satellite ozone retrievals. In particular, cross-sections studied were Bass and Paur (published in 1985), Brion, Daumont Malicet (published in 1995) and Serdyuchenko et al. (2014). The outcome of the ACSO study was that the latest Serdyuchenko et al. cross-sections are recommended to be used for ground-based Brewer and Dobson instruments. However, these cross-sections were not recommended to be used for satellite retrievals due to a deficiency in the signal-to-noise ratio close to 300nm. From the perspective of satellite validation, it would be beneficial if the same cross-sections were used by both satellites and ground-based instruments such that at a minimum a relative comparison were possible. However, if different absorption cross-sections are used in the satellite validation, it is important to understand what type of differences they cause in the validation. Related to GAIA-CLIM, it is to be noted that neither Pandora nor any other DOAS or MAX-DOAS instruments were included in the ACSO study.

Operational space missions or space instruments impacted: 
  • Copernicus Sentinel 4/5
  • MetOp
  • Polar orbiters
  • Geostationary satellites
  • UV/VIS nadir
  • Passive sensors
Validation aspects addressed: 
  • Geophysical product (Level 2 product)
  • Time series and trends
  • Calibration (relative, absolute)
  • Spectroscopy
Gap status after GAIA-CLIM: 
  • After GAIA-CLIM this gap remains unaddressed

A literature study leading to a summary of the findings including a recommendation of how this should be applied with regard to DOAS, MAX-DOAS and Pandora instruments has been undertaken in GAIA-CLIM but this does not close the gap.

Part II Benefits to resolution and risks to non-resolution

Identified benefitUser category/Application area benefittedProbability of benefit being realisedImpacts
Using the same cross-sections consistently with satellite and ground based instrumentation improves comparison by reducing the uncertainty in one critical factor.
  • Operational services and service development (meteorological services, environmental services, Copernicus services C3S & CAMS, operational data assimilation development, etc.)
  • Climate research (research groups working on development, validation and improvement of ECV Climate Data Records)
  • High
More reliable ozone products. Improved validation by improving the data consistency (removing one source of discrepancy in the respective data analyses).
Understanding the uncertainties of the cross-sections improves the error characterization of the ground based instrument.
  • Operational services and service development (meteorological services, environmental services, Copernicus services C3S & CAMS, operational data assimilation development, etc.)
  • Climate research (research groups working on development, validation and improvement of ECV Climate Data Records)
  • Medium
Improved error characterization of the ozone products
Understanding the uncertainties of the cross-sections may improve the retrieval results if correctly taken into account in the data processing (e.g. correlated errors).
  • Operational services and service development (meteorological services, environmental services, Copernicus services C3S & CAMS, operational data assimilation development, etc.)
  • Climate research (research groups working on development, validation and improvement of ECV Climate Data Records)
  • Medium
Potentially improved ozone products and their uncertainties.
Identified riskUser category/Application area at riskProbability of risk being realisedImpacts
Error characterization is missing one component
  • Operational services and service development (meteorological services, environmental services, Copernicus services C3S & CAMS, operational data assimilation development, etc.)
  • Climate research (research groups working on development, validation and improvement of ECV Climate Data Records)
  • Medium
Incomplete error characterization causing potential decrease in data quality.

Part III Gap remedies

Gap remedies: 

Remedy 1: Improved understanding of the effects of differences in ozone cross-sections

Primary gap remedy type: 
Research
Secondary gap remedy type: 
Technical
Proposed remedy description: 

It is necessary to study in-depth what impact the differences in the ozone cross-sections recommended for Dobson and Brewer instruments and the ones used for satellite retrievals have on the retrieved ozone amount when applied within the DOAS data analysis technique. This would be best achieved via a simulation study using the operational Pandora retrieval algorithm with alternative cross-sections of ozone but should also be added to the list of follow-up studies for the CINDI-2 intercomparison exercise. However, preliminary information should also be obtained from a literature study in consultation with the Brewer and Dobson communities and some original quantitative analyses. The analysis may be expected to lead to recommendations for future processing of measurements to be taken up by those networks operating these instruments. The analysis may also require additional dedicated measurements at a small number of sites to support the characterisation.

Relevance: 

Starting from the results achieved within the ACSO study, the study proposed here will help to understand the uncertainties caused by different sets of ozone cross-sections used within the data analysis and how this impacts on the overall measurement uncertainty.

Measurable outcome of success: 

If the difference in the end product (total column ozone) is quantifiable with regard to which of the different ozone cross-sections have been used within the retrieval, then this can be applied to better compare the ozone data measured by satellites with ground-based data sets while both satellite and ground-based observations still use their preferred ozone cross-sections for the data analysis.

Expected viability for the outcome of success: 
  • Medium
Scale of work: 
  • Single institution
  • Consortium
Time bound to remedy: 
  • Less than 3 years
Indicative cost estimate (investment): 
  • Low cost (< 1 million)
Indicative cost estimate (exploitation): 
  • No
Potential actors: 
  • EU H2020 funding
  • Copernicus funding
  • ESA, EUMETSAT or other space agency
  • Academia, individual research institutes
References: 

  • Bass A.M., and R.J. Paur, The ultraviolet cross-sections of ozone: I. The measurements in Atmospheric ozone (Ed. C.S. Zerefos and A. Ghazi), Reidel, Dordrecht, Boston, Lancaster, pp. 606-610, 1985.
  • Serdyuchenko, A., V. Gorshelev, M. Weber and J.P. Burrows, New broadband high- resolution ozone absorption cross-sections, Spectroscopy Europe, 23, 14-17. Available at: http://www.spectroscopyeurope.com/articles/55-articles/3082-new-broadban... resolution-ozone-absorption-cross-sections, 2011.
  • Serdyuchenko, A., V. Gorshelev, M. Weber, W. Chehade and J.P. Burrows, High spectral resolution ozone absorption cross sections - Part 2: Temperature dependence, Atmospheric Measurement Techniques, 7, 625-636, doi:10.5194/amt-7-625-2014, 2014.