Spectral measurement of aerosols from UV to NIR as part of the MAPP project (EURAMET)

Atmospheric aerosols arise from natural phenomena, such as salt from sea spray or sand from deserts.
They also arise from anthropogenic sources including the burning of waste or fossil fuels. Depending on their nature they can cool or heat the Earth’s surface, affect cloud formation and weather patterns. Ground-based instruments can help determine their nature but require improved metrology to determine their influence on climate change.

Challenges

Reducing the production of greenhouse gases is essential to keep global temperatures less than 1.5°C above pre-industrial levels. However, atmospheric aerosols also have a significant impact, contributing the largest uncertainty to estimates of climate change. Particles from manmade sources, such as waste incineration, tend to be dark, trapping heat and warming the Earth. With the exception of wildfires, natural aerosols, such as volcanic sulfuric particles, are generally whiter and reflect sunlight causing cooling. Models suggest the latter have counteracted around half of all greenhouse gas effects since the 1880s. Thus they have been recognised as “Essential Climate Variables” by the Global Climate Observing System.

Information on how all particles in a column of atmosphere absorb or scatter light are provided by radiometers using remote sensing techniques. These measure the “Aerosol Optical Depth” (AOD) from the ground to the “Top of the Atmosphere” (ToA) where no aerosols exist. They are calibrated against a ToA value that is extrapolated from ground-based measurements, performed at high-altitude locations during clear weather, and then relocated to their operational monitoring sites, at which time metrological traceability is lost.

In 2021 a new reference spectrum was developed that provided ToA values based on satellite observations – the “Spectral Solar Irradiance Sensor-1 Hybrid Solar Reference Spectrum” (TSIS-1 HSRS) with very reduced uncertainties relative to previous satellite-determined solar spectra. This removed the need for ground-based extrapolations, opening the possibility of calibrating instruments in the laboratory with measurements fully traceable to the SI.

Impact

For forty years Gigahertz-Optik have supplied cutting-edge instrumentation, software, knowledge and metrological solutions for optical measurements in LED manufacturing, lasers, photomedicine, imaging sensors, solar and ozone applications and many more.
The BTS sensor covering 300 nm – 1050 nm and 950 nm – 2150 nm allowed spectral measurements above 1700 nm to which it was limited before. Gigahertz-Optik have now integrated the new software into this spectroradiometer, and the company acknowledge the MAPP project, PMOD/WRC and PTB in helping to calibrate and validate it for AOD measurements. The work performed has provided significantly increased confidence in measuring the radiative properties of aerosols and their impact on climate. The AODs retrieved using laboratory calibrated instruments also showed excellent agreement with filter radiometers belonging to two atmospheric networks – with acceptance limits well within those defined by the World Meteorological Organization.