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.