Article Index


Organisation of the Joint Research Project (JRP)


JRP Number:  ENV59

JRP Title:       Traceability for atmospheric total column ozone (ATMOZ)

JRP Timing:     Start:   01/10/2014

                        End:    31/09/2017


JRP Participants:



Short name

Organisation full name




SFI Davos

Schweizerisches Forschungsinstitut für Hochgebirgsklima und Medizin in Davos (PMOD/WRC)



Funded JRP-Partner





Funded JRP-Partner


Cesky Metrologicky Institut Brno

Czech Republic


Funded JRP-Partner


Physikalisch-Technische Bundesanstalt



Funded JRP-Partner





Unfunded JRP-Partner


Kipp & Zonen BV



REG-Partner I


University La Laguna



REG-Partner II


Aristotele University of Thessaloniki



REG-Partner III


Aristotele University of Thessaloniki



Participants in Work Packages (WP)


Work Package Name

Active JRP-Participants (WP leader in bold)


Network instrument characterisations

PTB, Aalto, CMI, VSL,SFI Davos, Kipp, REG(ULL)


New Array Spectroradiometer Developments



Traceability of Total Column ozone

VSL,Aalto, PTB, SFI Davos, REG(UniHB), REG(ULL)


Creating Impact

SFI Davos, All


JRP Management and Coordination

SFI Davos, All




Project Background

Since the 1970s, it has been known that human-produced chlorofluorocarbons (CFCs) have led to recurring massive losses of total ozone in the Antarctic (ozone hole), which have also been recently observed in the Arctic, while in middle-latitudes, moderate ozone depletion has been observed. The Montreal Protocol on Substances that Deplete the Ozone Layer (a protocol to the Vienna Convention for the Protection of the Ozone Layer) is an international treaty designed to protect the ozone layer by phasing out the production of numerous substances believed to be responsible for ozone depletion. The Montreal protocol and its amendments have been successful in reducing the emission of ozone-depleting substances. Nevertheless, a recovery of the ozone layer has not been observed so far, and model projections have shown the recovery to not occur before the middle of the 21st century.

Atmospheric ozone has been defined an essential climate variable in the global climate observing system (GCOS) of the WMO. Careful long-term monitoring of the global ozone layer is still crucial in verifying the successful implementation of the Montreal Protocol and its amendments on the protection of the ozone layer, with the eventual recovery of the ozone layer to pre-1970s levels.


Project Objectives

Dobson and Brewer spectrophotometers are the main instruments used to monitor the ozone layer, and have been in use since the 1920s and 1980s respectively. Due to a lack of new validated instruments, both systems are still in use, even though Dobson spectrophotometers are no longer being manufactured and state-of-the-art array spectrophotometers would be available as alternatives.

There is therefore a need for an improved characterisation and calibration of the Dobson and Brewer instruments, particularly by involving the reference instruments of each network. This will have an impact on the whole global observing network by disseminating improved ozone measurements with known uncertainties and should also assist the development of an eventual replacement of the traditional Dobson spectroradiometers which require substantial manpower to operate (manual operation of the instrument).

Scientific and technical objectives

The JRP aims to address the problem areas above by the following activities:

  • Characterisation of the spectral bandwidth and wavelength of the World Reference and European Reference Dobson spectrophotometers in the range 300 nm to 350 nm (Task 1.1).
  • Characterisation of spectral bandwidth, wavelength, out-of-range stray light and temperature characterisation of several Brewer spectrophotometers (Task 1.2, Task 1.3).
  • Development of a field suitable tuneable radiation source using novel accordion gratings in the UV range for the bandwidth and wavelength characterisation of Dobson spectrophotometers with a wavelength uncertainty of 0.05 nm or less (Task 1.4).
  • Development a dedicated high resolution array spectroradiometer system (UV-PSR) for direct spectral solar UV irradiance measurements in the range 300 nm to at least 350 nm with UV LED based radiation source for field stability control (Task 2.2).
  • Organisation of and participation at two international field campaigns with the characterised reference Brewer and Dobson spectrophotometers in addition to newly available array spectroradiometer systems (Pandora, UV-PSR,...) in order to validate and assess the total column ozone retrieval and respective uncertainties (Task 2.3, Task 4.2).
  • Determination a high resolution extraterrestrial solar spectrum using a Fourier-transform spectrometer and medium resolution array spectroradiometers with an absolute uncertainty of ±2 % over the wavelength range 310 nm to 350 nm (Task 3.1).
  • Production of improved measurements of ozone-absorption cross-sections in the Huggins-band and a detailed uncertainty budget (Task 3.2).
  • Develop uncertainty models for correlated quantities with application to the total column ozone retrieval in order to determine a comprehensive uncertainty budget for Dobson, Brewer, and array spectroradiometers (Task 3.3, Task 3.4).


Expected results and potential impact

The improved characterisation and calibration of the Dobson and Brewer instruments, particularly by involving the reference instruments of each network, will have an impact on the whole global observing network by disseminating improved ozone measurements with known uncertainties. The development and thorough characterisation of novel spectroradiometer systems employing new techniques will demonstrate the capabilities of these new instruments and will pave the way for including these instruments in the global ozone monitoring network, in view of an eventual replacement of the traditional Dobson spectroradiometers which require substantial manpower to operate (manual operation of the instrument) and are not being manufactured anymore.

Scientific Impact:

  • Produce validated total ozone values from solar radiation measurements for different instrument characteristics and spectral ranges, which is essential to eventually replace ageing instrumentation with state-of-the-art instruments.
  • Develop array spectroradiometers and validate total ozone retrieval procedures to determine atmospheric total ozone.

Social Impact: The stratospheric ozone layer protects from adverse effects on humans, the biosphere and infrastructures by shielding the Earth surface from too high levels of ultraviolet radiation. The unequivocal observation of a recovery of total ozone by traceable ozone measurements as developed in this JRP will be an important health aspect and would provide significant social benefits to the whole population with respect to solar UV induced skin deceases such as melanoma skin cancers and eye cataracts.

Environmental Impact: Stratospheric ozone is a key atmospheric trace gas and its determination can give significant information as to transport processes in the upper atmosphere. Furthermore, the traceable determination of total column ozone with uncertainties of less than 1 % are necessary to be used as benchmark datasets to validate satellite retrieved tropospheric trace gases (e.g. remote determination of tropospheric pollution).



WP 1: Network Instrument Characterizations

The aim of this work package is to improve the consistency among the network instruments, the Dobson and the Brewer spectrophotometers, presently used for the atmospheric total ozone column (TOC) measurements, by top-level optical characterisation of their instrumental properties using the newest methodologies and measurement setups developed at the funded JRP-Partners.

The problem to be solved: Results of the ozone measurements using the two types of network instruments have shown discrepancies, which have been partly traced back to uncharacterised instrumental features. In particular, the oldest type of the network instruments in use, the Dobson spectrophotometer, shall profit from an accurate wavelength calibration and slit function characterisations, for example, by means of, wavelength tuneable lasers and monochromatic sources. Newer type of the network instruments, the Brewer spectrophotometers, are impaired by an insufficient stray light suppression (single-monochromator Brewer) as well as temperature dependencies requiring dedicated characterisations and corrections.

This WP will resolve these issues by:

  • Characterisation of several Dobson instruments operated in the European TOC network for the slit function and wavelength assignment. The acquired data will enable an accurate determination of the ozone measuring wavelengths and the corresponding ozone absorption coefficient.
  • Characterisation of the single monochromator (type MKII and MKIV) Brewer spectrophotometers for the out-of-range and in-range spectral stray light. This is one of the critical properties of the instruments limiting their performance in the TOC network measurements.
  • Temperature and wavelength characterisation under controlled conditions of one of the reference double-monochromator Brewer instruments of the RBCC-E, to determine its ozone weighted temperature coefficient and spectral bandpass.
  • Ad hoc characterisation of up to 3 network instruments according to the guidance received from the Steering Committee (to be established in D4.1.1) and COST Action ES1207.
  • Developing portable devices and methods for in-situ characterisation of Dobson spectrophotometers.



Description of Task 1.1

Characterisation of reference Dobson instruments (PTB, CMI)

The aim of this task is to characterise reference instruments of the TOC network of the Dobson spectrophotometers using the metrological facilities and competence of the JRP-Partners. In particular, three spectrophotometers will be characterised in terms of the wavelength assignment and their bandpass function. Currently, the consistency among the instruments is maintained based on the well-defined assembly and operating procedures. The optical characterisations within this task shall offer a metrological basis for the consistency of the measurements in the TOC network.


Description of Task 1.2

Characterisation of Brewer spectrophotometers in the TOC network (Aalto, PTB, Kipp)

The aim of this task is to characterise network Brewer spectrophotometers, specifically the out-of-range stray light of single monochromator Brewer spectrophotometers of type MKII and MKIV operated in the TOC network. Since these instruments use single-monochromator design, the stray light suppression has a significant influence on the ozone measurement for large ozone airmasses found in the Polar Regions where ozone depletion is of great concern. The characterisation of the instruments with respect to the spectral stray light properties will enable a quantification of the effect as well as a subsequent correction of the measurement data. The PTB and Aalto characterisations will be carried out as parallel activities within this task.


Description of Task 1.3

Characterisation of the reference RBCC-E Brewer (REG(ULL), PTB, Kipp, SFI Davos, Aalto)

The aim of this task is to characterise two Brewer spectrophotometers: One of the three reference Brewers of the RBCC-E operated at Izaña TOC measurement station and the travel standard Brewer of Kipp, responsible for the TOC calibration of network Brewer instruments. The characterisation will include the spectral and thermal characteristics. Brewer spectrophotometers are operated at ambient temperatures and use internal procedures to determine a temperature coefficient to account for temperature variations. This task will determine the temperature coefficient under controlled conditions in a climate chamber under static and transient conditions and compare it to the original value.


Description of Task 1.4

Portable tuneable radiation source for field instrument characterization (CMI, VSL)

The aim of this task is to develop a portable tuneable radiation source for in field characterisation of Dobson instruments slit function. The slit function is a critical parameter when comparing spectral measurements made by different instruments as the spectrum measured is the convolution of the spectrum being measured and the Dobson instrument slit function that, in general, is wavelength dependent. In order to characterise in field the Dobson instruments’ slit function the JRP-Consortium will develop a tuneable portable source able to cover the wavelengths of interest with the necessary high resolution, irradiance level and stability. The source will be tested during the RBCC-E field campaign in D2.4.3.



WP 2: New Array Instruments Developments

The aim of this work-package is to develop, characterise, and operate array spectroradiometers to measure direct solar UV irradiance in order to retrieve total column ozone from the full solar UV spectrum. This activity is a continuation of the activity performed in JRP ENV03 SolarUV (D2.3.1, D2.5.2).

The problem to be solved: Current ground-based instruments use only a few selected wavelengths, typically four, to retrieve the total column ozone in the atmosphere, making them very sensitive to wavelength misalignments and uncertainties in the ozone x-sections. Carefully characterised and calibrated Instruments that use the full solar spectrum between 300 nm and 350 nm to retrieve the ozone content are expected to be less sensitive to these uncertainties and provide a better constrained total ozone value from a larger parameter set.

This WP will resolve these issues by:

  • Characterising existing array spectroradiometer systems for spectral in-range and out-of-range stray light, wavelength to pixel assignment, linearity and noise equivalent irradiance. Finally, the instruments will be absolutely calibrated to measure the direct spectral solar UV irradiance spectra in the spectral range from 300 nm to 350 nm with a resolution of better than 1 nm. Developing a UV array spectroradiometer system optimised for direct solar irradiance measurements, based on the commercialised version of the precision solar spectroradiometer (PSR).
  • Perform a field validation campaign in Izaña (D2.3.3) collocated with Brewer and/or Dobson spectroradiometers to assess the total ozone retrieval by these instruments.


Description of Task 2.1

Characterisation of array spectroradiometer systems (PTB, VSL, SFI Davos, REG(ULL))

The aim of this task is to characterise, correct and calibrate UV array spectroradiometers in order to enable the direct solar UV irradiance measurements with uncertainties required for the retrieval of the total column ozone. Key instrument characteristics including spectral stray light, bandwidth, wavelength assignment and linearity will be determined and taken into account, following the experiences gained in JRP ENV03 Solar UV (D2.3.1). Quantifying these effects is not only fundamental for calculating the measurement uncertainty but it also allows correcting for the instrument imperfections and, thus, decreasing the overall uncertainty of measurements.


Description of Task 2.2

Development of a direct irradiance UV spectroradiometer system (SFI Davos, CMI, PTB)

The aim of this task is to develop a highly reliable and stable UV spectroradiometer system based on the Precision Solar Spectroradiometer (PSR) developed and commercialised at SFI Davos. SFI Davos will characterise the final assembled instrument in Task 2.1 (D2.1.1, D2.1.3, D2.1.4)) with the help of the devices available at PTB, VSL, and SFI Davos. The instrument will use a reference light source based on a UV-LED system for stability checking of the direct entrance port. The goal is to have an instrument with a relative stability of 1 % per year in the wavelength range 300 nm to at least 350 nm and a spectral resolution of 0.5 nm or less. The stray-light rejection will be at least 10-5.


Description of Task 2.3

Field Validation campaign at the Izaña Observatory (REG(ULL), SFI Davos)

(Start Sep 2015, end Sep 2017)

The aim of this task is to hold a field validation and intercomparison campaign in order to compare the total column ozone determined by array spectroradiometers with current state-of-the-art reference spectroradiometers for measuring direct solar UV irradiance and total column ozone. Due to the necessity of clear-sky stable conditions for direct solar irradiance measurements, a high altitude measurement site in southern Europe will be selected in D2.3.1. In order to perform a successful calibration of the instruments using the Langley-plot technique, a site on a high mountain station will be selected to have stable atmospheric conditions. The ozone community will be invited to participate in order to obtain reference values for total column ozone (WP4).


Description of Task 2.4

Participation in RBCC-E Campaign (REG(ULL), SFI Davos, CMI, Kipp)

The aim of this task is to participate at the annual RBCC-E total ozone and UV calibration campaign for transferring the ozone calibration to participating Brewer and Dobson spectrophotometers.




WP 3: Traceability of Total Column Ozone

The aim of this work-package is to determine a comprehensive uncertainty budget for the atmospheric total column ozone measurement by ground-based instruments, taking into account all parameters affecting the retrieval of this atmospheric species.

The problem to be solved: Atmospheric total column ozone is currently retrieved by various methodologies using different measurement procedures, wavelength regions and ozone absorption cross-sections. In particular, the effect from cross-correlation when using different wavelength regions on the retrieval uncertainty will have to be assessed in order to develop an uncertainty model valid for discrete wavelength ozone retrievals as applied to Dobson and Brewer spectrophotometers and array spectroradiometers using high resolution solar spectra.

Another source of uncertainty in the ozone retrieval is the use of ozone absorption cross-sections. Both Brewer and Dobson spectrometers operate in the near UV (305-340 nm) where the Huggins ozone absorption band has distinct absorption minima and maxima (whose differences are exploited in a differential retrieval to derive ozone amounts). This band is also the spectral region where ozone cross-sections have strong temperature dependence so that uncertainties in the assumed atmospheric temperatures (or neglect of temperature variation in the retrieval as in the standard WMO Brewer and Dobson retrieval) add to the retrieval error (Vanicek et al., 2003).

In the WMO standard retrieval of Brewers and Dobsons to date Bass Paur ozone cross-sections from the 1980s are used. In the meantime newer measurements like those from Brion as well as the very recent data from the University of Bremen group (are available. These newer ozone cross-sections have an estimated error of about 2 % or worse (depending on the spectral region). However, the error budgets reported generally do not include all potential uncertainties and correlations and are rather too optimistic. In this work package a detailed error budget will be provided for the relevant cross-section data that are currently used in remote sensing, which can be then translated into error contribution to the ozone retrieval. Secondly, the new ozone cross-section data of the Bremen cover a wide spectral range and a large dynamic range (on the order of 107). As part of this work improved ozone cross-sections in the Huggins band will be provided to further reduce the uncertainties in the near UV spectral region.



Description of Task 3.1

Validation of high resolution solar reference spectra (SFI Davos, PTB, REG(ULL))

The aim of this task is to validate the high resolution solar spectrum used as reference for the retrieval of total column ozone from ground-based direct solar UV spectra. The prime candidate for this task will be the extra-terrestrial spectrum constructed in JRP ENV03 SolarUV D2.4.1. Since the retrieval of total column ozone will be performed in the wavelength range 300 to 350 nm, the validation will be restricted to this range. The goal is to validate this spectrum with a relative uncertainty of 1 % over this wavelength range, e.g. the absolute irradiance level is not relevant for the ozone retrieval. The raw measurements will be based on the direct solar irradiance spectra measured during the Task 2.3 field measurement campaign organised at the Izaña Observatory for atmospheric research, Tenerife, Canary Islands.


Description of Task 3.2

Assessed and validated ozone absorption cross-sections (REG(UniHB), PTB)

The aim of this task is to provide a detailed uncertainty budget of the most relevant ozone absorption cross‑sections that are currently available. In addition new measurements of the Huggins ozone band in the near UV with an improved experimental setup will be conducted at REG(UniHB) in order to further reduce the uncertainties in that spectral region.


Description of Task 3.3

Task 3.3 Uncertainty models for correlated quantities (Aalto, VSL, SFI Davos, REG(ULL))

The aim of this task is to develop two methods to calculate uncertainties from correlated input variables, without a-priori knowing the correlation matrices: The first method will be applied to the double-ratio technique used by Dobson and Brewer spectrophotometers to retrieve TOC from 4 discrete wavelengths; the second method will retrieve the uncertainties of TOC from measurements of the full solar spectrum, using Differential Optical Absorption Spectroscopy (DOAS) techniques.

Total column is retrieved from direct solar irradiance measurements using retrieval methodologies based on the Beer-Lambert Law, with varying complexity, essentially due to the number of free parameters in the description of the atmosphere and the number of spectral measurements. There are various practices on how to calculate the uncertainty for the derived total column ozone that propagate from the uncertainties of the input variables. However, to get realistic uncertainty estimates, one has to account for correlations between the uncertainties at different wavelengths. This is problematic if the correlations are not known, which is typically the case. Monte Carlo simulations by randomly varying the input variables and neglecting correlations is a method often used. However, the uncorrelated uncertainties behave like noise and average out in the calculation, which strongly underestimates the uncertainty. On the contrary, if we assume full correlation of the uncertainties at different wavelengths, the uncertainty is underestimated too, because the spectral shape is not affected. In practice, the situation is typically between the two extremes presented. A method is needed that gives limits or an estimate for the uncertainty taking into account that correlations may exist, although unknown.


Description of Task 3.4

Task 3.4: Comprehensive Uncertainty budget for Total Column Ozone (SFI Davos, REG(ULL), REG(UniHB), Aalto, VSL)

The aim of this task is to produce a comprehensive uncertainty budget for ground-based total column ozone retrieval from Dobson, Brewer and array spectroradiometer measurements, taking into account cross-correlations between measurements at different wavelengths.



WP 4: Creating Impact

The aim of this workpackage is to support the dialogue between the JRP-Consortium and interested parties including; collaborators, stakeholders, the end-user community, academia, commercial companies, standards organisations and legislative bodies. The JRP results will be disseminated to the various interest groups via a number of mechanisms described below.


Description of Task 4.1

Knowledge Transfer (SFI Davos, All JRP-Partners, REG(ULL), REG(UniHB))

The aim of this task is to ensure that the results achieved by the JRP are adequately and appropriately communicated to the stakeholders and end-user community and that input and feedback is obtained from this community.

The scientific results of this project will be disseminated through the following activities:

  • International Conferences attended by JRP-Partners

  • Peer-Reviewed Publications

  • Standards & Committees

  • Website

  • Newsletter “UVNews”

  • Guideline documents


Description of Task 4.2

Training (SFI Davos, All JRP-Partners, REG(ULL), REG(UniHB))

The aim of this task is to provide targeted training to the stakeholder community.

The following activities will be undertaken:

  • On-demand characterisation of network.

  • Training during the RBCC-E Field Campaign

  • Two stakeholder workshops (in conjunction with COST Action ES1207 2016 and RBCC-E Brewer campaign 2017 in El Arenosillo Spain.


Description of Task 4.3

Exploitation (SFI Davos, All JRP-Partners, REG(ULL), REG(UniHB))

The aim of this task is to ensure the best exploitation of knowledge developed in the JRP in terms of devices and software.