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