New MS-711 Application
Aerosol and Precipitable water vapor retrievals with the MS-711 Spectroradiometer
by Dr. Mario Po, Research Scientist, EKO EU
As the impact of human activity on the climate continues to unfold, researchers worldwide work to fully understand the physical, chemical and biological variables that characterize the Earth’s climate. These variables are called ‘Essential Climate Variables’ or ECVs and can be found individually or as groups of linked variables. Experts at the Global Climate Observing System (GCOS) specify a total of 54 ECVs, distributed between the atmosphere, land and ocean.
ECV data provides the empirical evidence needed to understand and predict the evolution of climate, to guide mitigation and adaptation measures, to assess risks and enable attribution of climate events to underlying causes, and to underpin climate-based services.
Water vapor and aerosol concentrations in the atmosphere are some of the most important drivers of weather and climate. While many satellites retrieve information about these atmospheric constituents, a bottom-up approach is critical to validate the satellite data and to measure the actual radiative effect at the surface.
In a few ground based sensor networks, these variables are monitored through ‘Aerosol Optical Depth’ (AOD) and ‘Precipitable Water Vapor’ (PWV); allowing not only for the cross-checking of satellite measurements, but also a higher temporal resolution for assessing both short- and long-term trends. For researchers to accurately determine the Earths’ altering energy budget it is crucial that they are able to monitor these variables with low uncertainty, extended spatial coverage and temporal resolution.
Networks such as SKYNET and AERONET make use of narrow band filter radiometers to measure AOD and PWV in specific bands where the effect in the optical attenuation of the sun lights’ path is most pronounced and can be isolated from other optical effects in the atmosphere.
With the improvements in measurement accuracy of field spectroradiometers, the interest in pursuing retrievals through the use of ground based spectral irradiance measurements has grown. While the measurement of narrow wavelength bands with a rotating monochromator, or sets of band-pass and cut-off filters, sample a very specific and short part of the solar spectrum, grating spectrometers can separate light into several wavelength bins for instantaneous spectral measurement of a light source; providing additional insight, and levels of detail.
The continuous measurement of the direct normal, diffuse horizontal, and global horizontal components of spectral irradiance is key to many environmental science and solar energy applications. Likewise, spectral irradiance is modified by changing concentrations of atmospheric trace gases and aerosols, creating a need for continuous monitoring. Adding methods for atmospheric parameter retrievals based on spectral irradiance data sets will present added value to the atmospheric, environmental science, and solar energy, research communities.
In recent studies, researchers at the Izaña Atmospheric Research Center (IARC), part of the Agencia Estatal de Meteorologia (AEMET), have explored this potential with EKO spectroradiometers, developing a methodology to quantify atmospheric parameters based on ground-based solar spectral measurements.
The Research Center is situated at a high-altitude site, 2367m above sea level, on the Spanish island of Tenerife (28.3°N, 16.5°W), an ideal location for evaluating solar radiation sensors and performing ‘Langley’ calibrations. Furthermore, the site contains high-end instrumentation installed for atmospheric research. It is part of the AERONET network for aerosol monitoring and has been appointed as a Commission for Instruments and Methods of Observation (CIMO) test bed for aerosols and water vapor remote-sensing instruments by the World Meteorological Organization (WMO). The combination of location and instrumentation made the IARC an excellent location for the proper evaluation of atmospheric parameters retrieved with a spectroradiometer.
The EKO MS-711 grating spectroradiometer was installed at the facility, on an EKO STR-22G Sun tracker with collimation. The setup allowed researchers to measure direct normal spectral irradiance within the 300 and 1100 nm wavelength range with a 0.4nm step and <7nm Full width half maximum (FWHM).
The AOD retrieved with the EKO MS-711 using the IARC-AEMET retrieval method was compared against a sun photometer, based on a scanning filter radiometer, measuring discrete points at 340, 380, 440, 500, 675, 870, and 1020 nm bands.
High-quality AOD measurements were taken in the VIS and near-IR ranges with AOD differences within the World Meteorological Organization traceability limits of > 95% for 380, 440, 500, 675, and 870 nm. In the UV range, for 340 nm, lower performance was achieved, with 86 % traceability.
The PWV was retrieved at the 940nm band and the results were compared against the output of the same device in this band. Additionally, other reference techniques available at the Izaña Observatory, such as meteorological radiosondes, the Global Navigation Satellite System (GNSS) and Fourier Transform Infrared Spectrometry (FTIR). The PWV values closely lined up with all of the different techniques, providing a mean bias and standard deviation of -0.30 ± 0.89 mm, 0.02 ± 0.68 mm, -0.57 ± 0.68 mm, and 0.33 ± 0.59 mm, with respect to the RS92, GNSS, FTIR and AERONET sun photometer, respectively.
These results, achieved by the Izaña Atmospheric Research Center, confirm that the EKO MS-711 spectroradiometer performs accurate and precise enough measurements to provide reliable AOD and PWV data on a routine basis.
The implementation of AOD and PWV retrievals with the EKO MS-711 proves that high-quality grating spectroradiometers can simultaneously obtain key parameters used in atmospheric science but also in other applications such as the evaluation of solar cell and module performance.
Find out more about the MS-711 @ https://eko-eu.com/products/solar-energy/spectroradiometers/ms-711-spectroradiometer
For more details on this study, visit:
- European Geosciences Union, Atmospheric Measurement Techniques. Volume 13, Issue 5 ‘Aerosol retrievals from the EKO MS-711 spectral direct irradiance measurements and corrections of the circumsolar radiation’ @ https://amt.copernicus.org/articles/13/2601/2020/
- MDPI, Remote Sensing. Volume 13, Issue 3 ‘Water Vapor Retrievals from Spectral Direct Irradiance Measured with an EKO MS-711 Spectroradiometer—Intercomparison with Other Techniques’ @ https://www.mdpi.com/2072-4292/13/3/350