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.