Novel instruments for measuring vapour pressure
Aerosols often act as the seed around which clouds form, yet much remains to be learned about how they grow in the atmosphere. Henrik Pedersen and Aurelien Dantan tell us about their work in developing new instruments for measuring saturation vapour pressure and its wider relevance to understanding the influence of aerosols on the climate system.
The starting point of cloud formation is typically the presence of an aerosol, essentially a collection of particles suspended in the atmosphere, which provides a kind of seed around which a cloud can then form and condense. Primary aerosols are emitted into the atmosphere from a variety of different sources, as a result of both natural processes and human activity. “For example aerosols are formed by sea spray and by the emission of volcanic ash. Aerosols can also come from human sources, such as factories or cars,” outlines Henrik Pedersen, Associate Professor in the Department of Physics and Astronomy at Aarhus University. Once these aerosols are in the atmosphere, they are then transformed by chemical reactions. “Secondary aerosols are produced from small molecules in the atmosphere. They grow bigger as a result of molecular processes,” continues Professor Pedersen. “The diameter of a particle can grow from the sub-nanometre scale to micrometres.”
A wide variety of different factors can influence the rate of this growth, among the most important of which is a substance’s saturation vapour pressure (SVP). The SVP of a substance can be thought of as the point of equilibrium between the rate of evaporation and the rate of condensation, which is currently measured in a static way. “Current stateof-the art methods of measuring SVP work by establishing an equilibrium situation, where you have a substance and a vapour at the same temperature and pressure,” says Professor Pedersen. Accurate and reliable measurements of SVP are essential to gaining a deeper understanding of how aerosols grow in the atmosphere and their wider impact on the climate system, for example in affecting the earth’s radiative balance. “If you don’t know the SVP, then you cannot really describe how this condensation works in aerosol formation,” stresses Professor Pedersen.
New measurement method
As Principal Investigators of a research project based at Aarhus University, Professor Pedersen and Professor Dantan now aim to develop a new, dynamic method for measuring a substance’s SVP. This involves cooling a substance, letting it approach thermal equilibrium and stabilise with its surroundings, and following the pressure during this process. “We don’t depend on reaching thermal equilibrium,” explains Professor Pedersen. The project’s agenda also includes developing a means of measuring very low pressures, in a range relevant to aerosol formation. “The best commercially available absolute pressure sensors are relatively limited in range, down to a few millipascals. Many of the substances relevant to aerosol formation in the atmosphere have a SVP that lies below this,” says Professor Dantan. “We are therefore developing our own sensors to achieve better sensitivities.”
The goal of helping chemists and atmospheric scientists gain more knowledge about the SVP of different substances is a major motivation here, while researchers are also looking to develop new techniques and methods. This is about pushing beyond current capabilities in search of improved instruments. “We’re trying to beat the best commercially available absolute pressure sensors,” says Professor Dantan. Researchers in the project are developing a measuring instrument using very small nanostructures which vibrate when they interact with a gas, pushing the boundaries of science to achieve higher sensitivities. Even very tiny vibrations of these structures can be measured effectively using laser
the sub-millipascal range, and another paper has recently been published on how the instrument was built, and how SVPs are actually measured. “At the moment we are both measuring the actual pressure of certain substances, and we are developing the method further. So we are trying to probe the limits of the methods and get to even lower pressures,” says Professor Pedersen. The aim is not to reach a specific figure or target, but rather to develop the ability to measure pressure in a lower range, which will help atmospheric scientists build a fuller picture of how aerosols grow. “Measuring vapour pressures for atmospheric substances will be an important step forward in terms of understanding how aerosols grow,” stresses Professor Pedersen.
“ The best commercially available absolute pressure sensors are relatively limited in range, down to somewhere between 5-10 millipascals. Many of the substances relevant to aerosol formation in the atmosphere have a saturation vapour pressure that lies below this.”
interferometry, says Professor Dantan. “We can detect vibrations of these drums as small as 10 -12 metres – a fraction of the size of an atom –quite well using a laser. Because we can detect these very small vibrations, we can then detect very small pressure changes,” he outlines. Validating these improvements is a challenge however, as there are no other absolute pressure sensors available, so the project team is working with known gases to build a fuller picture. “We use a commercially available sensor, with a 5 millipascal sensitivity, to check the response of our own pressure sensor,” continues Professor Dantan. “We’re working to measure this sensitivity more precisely.”
The project team are exploring ideas to further improve the performance of these sensors and reach even lower pressures. In one recent paper, the researchers showed that the sensors can measure pressure in
Very low pressures
This research also holds wider relevance beyond the atmospheric science field, to both the industrial and academic sectors. Measuring very low pressures can be highly challenging, as the gas itself may be difficult to identify, circumstances in which Professor Dantan says absolute pressure sensors can play a valuable role. “An absolute pressure sensor can give you a measure, without needing to guess what gas is there,” he explains. The absolute pressure sensors under development in the project could be an important tool in calibration, in providing a kind of yardstick for other sensors, a topic Professor Dantan plans to explore further. “We are collaborating with a Canadian company. They are very interested in these kinds of sensors, as they could be relevant as a calibration standard for many applications beyond atmospheric science,” he continues.
A NoVel iNsTrumeNT for THe Accur ATe AND Direc T me A suremeNT of sATur ATioN VAPor Pressures of lowVol ATile subsTANces
Project objectives
The project aims at developing an instrument allowing for the direct and accurate determination of saturation vapor pressures of low-volatility substances using novel methods and state-of-the-art optomechanical absolute pressure sensors.
Project funding
This project is funded by the Independent Research Fund Denmark (DFF).
contact Details
Assoc. Prof. Aurélien Dantan Department of Physics and Astronomy, Aarhus University, Denmark e: dantan@phys.au.dk
Assoc. Prof. Henrik B. Pedersen Department of Physics and Astronomy, Aarhus University, Denmark e: hbp@phys.au.dk w: https://phys.au.dk/forskning/ forskningsomraader/atomar-molekylaer-ogoptisk-fysik/optomechanics-group/
R. V. Nielsen, M. Salimi, J. E. V. Andersen, J. Elm, A. Dantan and H. B. Pedersen, A new setup for measurements of absolute vapor pressures using a dynamical method: Experimental concept and validation, Rev. Sci. Instrum. 95, 065007 (2024)
M. Salimi, R. V. Nielsen, H. B. Pedersen and A. Dantan, Squeeze film absolute pressure sensors with sub-millipascal sensitivity, Sensors Actuators A 374, 115450 (2024)
Aurélien Dantan Henrik B. Pedersen
from Université Pierre et Marie Curie in Paris in 2005. After postdoctoral stays in France and Denmark he has been an associate professor at Aarhus University since 2013. His research interests include quantum optics, cavity quantum electrodynamics, optomechanics and the application of nanomechanical systems for sensing and metrology.
Henrik b . Pedersen obtained his PhD from Aarhus University in 1999 and worked as postdoc in Israel and Germany. He became associate professor at Aarhus University in 2007. His research interests include molecular processes, ion trapping, atmospheric physics, non-linear mechanics, and new instrumentation for sensing.
