Introduction to the JRP

The Joint Research Project "Traceability for surface spectral solar ultraviolet radiation" will significantly enhance the reliability of spectral solar UV radiation in the wavelength range 300 nm to 400 nm measured at the earth surface by developing new methods of observation (techniques and devices) to provide traceable solar UV irradiance measurements with an uncertainty of less than 2 %. This activity is essential to unambiguously quantify decadal changes in solar UV radiation due to the expected changes in the global climate system.

The project will shorten the traceability chain of the solar UV measurements to the SI unit and to reduce the associated transfer uncertainties. The goal is to approach uncertainties in the field comparable to those currently achieved only for primary spectral irradiance scale realisations at NMI level, i.e., at the level of 1 %. To provide traceable solar UV irradiance measurements with an uncertainty of less than 2 %, the portable reference spectroradiometer known as ‘QASUME’ will be fitted with an improved global entrance optic and newly developed solid-state detectors. A Fourier transform spectroradiometer will be adapted for spectral reference spectroradiometer.

To support the use of cost-effective array spectroradiometers in UV monitoring networks (as replacements for current UV filter Radiometers), significant progress needs to be achieved in the characterisation of these devices. New characterisation techniques and post-correction methods will be developed to determine and correct the stray light, linearity, and wavelength scale of array spectroradiometers. This will be supported by the design and construction of novel array spectroradiometers with improved stray light characteristics based on band pass filters and micro-electro-mechanical systems (MEMS) such as digital micro mirror devices.

The dissemination of the improved irradiance traceability and the developed tools and methods for using array spectroradiometers in the solar UV range will occur by a large field intercomparison of spectroradiometers organised at the end of the JRP at the World Radiation Centre, in Davos, Switzerland. Participants from the end-user community involved in solar UV measurements will be invited to the field intercomparison. Participating spectroradiometers from the end-users will be characterised and calibrated by the facilities developed in this JRP to provide traceability of spectral solar UV irradiance at this new level of uncertainty to the wider European UV monitoring community.

Motivation of the JRP

The quantification of UV radiation at the earth surface requires accurate measurements of global solar spectral UV irradiance in order to understand long-term trends in this parameter.

Long-term trends in surface solar radiation due to atmospheric induced changes (termed global dimming and global brightening), have demonstrated decadal changes of the order of 2 % per decade over Europe. These changes are currently explained by changes in the transparency of the atmosphere (aerosols) and possibly long-term changes in clouds (cloud fraction and cloud opacity). The effects on UV radiation have not yet been quantified due to the difficulty of observing these small changes over such long time scales. Future changes in UV radiation due to atmospheric changes are expected to be of the same order of magnitude and require measurements with significantly lower uncertainties of around 1 % to 2 % to detect such decadal changes.

The current knowledge on spectral solar UV radiation is limited to very few places worldwide where spectral solar UV monitoring instruments are located. Large-scale deployment of such instruments is limited by the required manpower and infrastructure to guarantee an adequate level of uncertainty. UV monitoring at additional locations is required to better understand the relationship between UV radiation and its influencing factors and to validate radiative transfer models and satellite-derived UV estimates.

Scanning spectroradiometers measure the solar irradiance spectrum sequentially, which requires several minutes of scanning time. The interpretation is thus rendered difficult in the case of fast varying atmospheric conditions. Also, the slow scanning speed does not allow fast sampling rates, limiting the number of measurements available per day.

Currently available array spectroradiometers are not suitable for solar UV measurements without complex correction methodologies due to the large dynamic range of the solar UV radiation between 300 and 400 nm and the significant stray light contamination of these instruments.

Due to the large decrease in UV radiation below 330 nm over many orders of magnitude due to the absorption of atmospheric ozone, wavelength accuracies of better than 0.05 nm are required to reach nominal uncertainties of 1% to 2% in the wavelength range 300 nm to 400 nm. Furthermore, a high dynamic range over at least five orders of magnitude is necessary to cover the wavelength range between 300 nm and 400 nm.

The rapid temporal variation of UV radiation due to atmospheric conditions (e.g. clouds) requires fast-scanning spectroradiometers with stray light rejection of at least 10^-6 to reliably sample the whole solar UV spectrum between 300 nm and 400 nm.

Fourier Transform Spectroradiometers (FTS) hold the promise of recording solar UV spectra within a very short time span (typically less than 1 minute) and with a wavelength uncertainty below 0.02 nm due to the very nature of their measurement process.

The first major task of the JRP will considerably improve the uncertainties associated with current state of the art scanning spectroradiometers to reach uncertainties of ± 1 % to ± 2 % for spectral solar UV irradiance measurements over the wavelength range 300 nm to 400 nm.

The second major activity of the JRP will optimise array spectroradiometers and develop characterisation methods for CCD and diode-array spectroradiometers and similar fast-scanning spectroradiometers. In future these devices will be used as cost effective network instruments for solar UV radiation measurements, replacing current filter radiometers (narrow and broadband). Investigations will include: Stray light rejection which is particularly problematic at very short wavelengths below 320 nm, improved wavelength characterisation particularly in the UV range, and linearity of the devices including the electronics. Finally a methodology for uncertainty analysis of solar UV measurements using array spectroradiometers will be developed to account for the properties of the instruments, including the correlations among the spectral values. The objective is to achieve uncertainties under network conditions of ± 5 % with these fast-scanning spectroradiometers.