Breakthrough in high-precision measurement of particulate matter thanks to Dutch technology
The Dutch SPEX instrument means a breakthrough in high-precision measurement of the properties of particulate matter. That is the conclusion reached by the Leiden astronomer Gerard van Harten in his doctoral thesis, which he defends on 8 December.
The Dutch SPEX instrument is being developed within a consortium composed of Leiden University, SRON (Netherlands Institute for Space Research) and Dutch industry partners for conducting detailed measurements on particulate matter, both from the ground and via satellite. SPEX’s new measurement technique turns out to be so accurate that SPEX instruments can collect crucial information on how particulate matter influences the climate and health. This is the most important conclusion drawn in Leiden astronomer Gerard van Harten’s thesis.
Tiny particles in the atmosphere can either heat or cool the climate. These effects are still not very well understood, and they constitute the most important source of unpredictability in climate models. That is why detailed measurements of these particles are necessary. Not just to measure the quantity of particulate matter, but especially to also determine the typical size of the particles and what they are composed of. A similar characterisation is also necessary to study the effects that particulate matter caused by air pollution have on health.
To get a good picture of the effects of particulate matter on a global scale, local surface measurements, and especially satellite measurements, are necessary. It is for this purpose that the SPEX instrument is being developed. It captures sunlight scattered by dust particles and uses this to measure the spectrum and polarisation, which contain information about the properties of the particles. The polarisation measurements have to be carried out with the utmost precision before the measurements can be completely interpreted.
During his doctoral research, Van Harten worked on a completely new method for measuring polarisation. The method is implemented using a number of small glass plates, making the satellite instrument relatively cheap both to construct and to launch, due to its light weight. It does not need to incorporate any moving parts, either. In his thesis, Van Harten demonstrates that this new measurement method is also extremely accurate. In the lab in Leiden, the first prototype immediately met the strictest requirements for a climate-related mission. Van Harten explains, ‘Compared with current satellite instruments, SPEX is many times more accurate. This is a tremendous leap forward.’
The principle of measurement behind SPEX comes from astronomy. Using polarisation measurements taken from Earth, NASA scientist James Hansen discovered in the 1970s that clouds on the planet Venus are composed of highly concentrated sulphuric acid. With the help of advanced instruments on large telescopes, Leiden astronomers are attempting to conduct similar measurements on exoplanets. Exoplanets are planets that orbit stars other than the sun. After Hans shifted the focus of his research to the climate of our own planet, these measurements proved to be crucial for Earth, as well. Now for the first time, the SPEX instrument can start collecting the necessary information about the effects of particulate matter on climate change. That is why Hansen, who has since become a famous climatologist, is also serving on Van Harten’s PhD committee.
The first SPEX measurements of particulate matter had already been conducted by Van Harten, with an instrument specially built for a ground-based station, commissioned by the National Institute for Public Health and the Environment. These measurements already provided a wealth of new information and possibilities. The next step towards putting the device on a satellite is now being taken by SRON: in the summer of 2015 the precision-calibrated SPEX instrument will fly on a NASA research plane. ‘These flights are the ultimate test case for eventual observations from a satellite,’ says Van Harten. ‘In my future position at NASA’s JPL institute, I will be very interested in seeing those results.
(3 December 2014)