The ATMOS research group in the NANOMO unit, led by Nønne Prisle, Associate Professor at the University of Oulu, are trying to find out what kind of chemistry is happening in cloud droplets and tiny nanometer-sized aerosol particles in the atmosphere. This knowledge could eventually, hopefully, give us more accurate theoretical models to understand the ongoing climate change.
– The only thing that can halter climate change is to stop emitting CO2. Nønne Prisle is very, very clear on that. Even so, she says, if we want to take any other step to try to counter climate change, we really need to know what’s going on in the clouds since these processes could be quite critical.
The ATMOS team are using the beamline HIPPIE at MAX IV being so-called commissioning experts, which means that the experiment is done both to provide useful data but also to verify the capacity and capability of the beamline experimental station.
– We want to know how the surface separates itself chemically from the interior of solutions that are similar to those that make up the cloud droplets in the atmosphere, explains Nønne Prisle.
Such solutions are water combined with organic compounds, in this specific experiment a compound with soap-like characteristics to simulate the cloud droplets over an ocean. The organic compound comes from plants, fishes and other sea living animals, which when they die and dissolves ends up in the water droplets created by wind and waves that are soaring the oceans.
By using a liquid jet the team measures how the different compounds in the liquid solution disperse itself between the surface of the liquid and the bulk of the liquid. The liquid jet beam is 20 micrometres (mm) in diameter which is way bigger than a real cloud droplet which is somewhere between just 0,1 mm up to 1 mm in diameter when it forms. For comparison; a hair strand is approximately 80 mm.
When simulating the climate over a ten-year period maybe a hundred years into the future, scientists use models with many different parameters. One such parameter is the cooling effect of clouds. For these models to be accurate the scientists need to know how the cloud droplets form, how long they stay as droplets and what the factors are that can make them change the size and either evaporate or grow and rain out.
Nønne Prisle explains; – The clouds are cooling, so they generally counter the global warming from CO2. This means that when we relate increase in CO2 in the atmosphere to measured increase in temperature, we get a certain ”climate sensitivity”. But this number includes the damping effect of clouds. Clouds are short-lived, so changes in clouds happen on days and weeks, whereas CO2 and other greenhouse gasses stay for decades or centuries. If we have estimated wrongly how big the cooling of clouds is, it means the effect of the CO2 we already released, and of that which we will release in the coming years, could be even more powerful than what we currently believe. The amount of CO2 we think would lead to the Paris agreement 2-degree warming could lead to an unexpectedly warmer future if we have not properly decoded the cloud effect.
Another difference that needs to be accounted for in this research is the fact that the liquid jet beam is a cylinder and the droplet is a tiny sphere.
– We use our theoretical models to calculate the surface chemistry on cloud droplets. Then we ask the model to calculate what would happen if it was a jet beam (i.e. a cylinder) and compare that to the measurements we get here. This is the best way to validate what the model does, explains Nønne Prisle because we cannot – yet – directly measure most details about the droplets themselves.
Due to their chemical composition, organic compounds arrange themselves either on the surface of a cloud droplet or in the interior (bulk). Obviously, there is a multitude of different compounds making up a multitude of different solutions, possibly behaving quite differently. All of these would need to be studied to make the theoretical models take in to account every conceivable mixture of solutions in a cloud. So how do you choose what solutions you study if you have limited time and resources?
Jack Lin, postdoc in the ATMOS team, comes from the field of atmospheric measurements and can relate the results to observations in the atmosphere. He explains; – We choose solutions that are representative, with model compounds basically, since we don’t know the exact composition of the organic compounds present in the atmosphere.
– In this specific experiment, we study a soapy solution where the organic compounds strongly prefer to go to the surface, continues Jack Lin, and this is the extreme on one end of the scale.
And, as if on cue, soap bubbles start to build up in the container where the solution in the jet beam is collected. Given the reaction in the control room, this is not a wanted part of the experiment, and the beamline staff immediately starts to raise the pressure in the sample chamber to make it stop.
The team also study solutions at the other extreme where the compound strongly prefers to stay in the bulk. And then solutions that are in between those extremes. Picking the right ones is a very long process; Nønne Prisle spent half a year on literature studies when doing her thesis to learn which compounds have already been found and which have interesting properties to look at and, most importantly, which can be handled in the lab. With this knowledge as a platform and by reading up on new research the team has put together a portfolio of different compounds to be studied at MAX IV, either to confirm that they are understood or to find unexpected properties that can be studied more in detail.
Being a scientist is a slow and hard work, Nønne Prisle stresses, since it’s long hours for many days trying and trying to get the experiment working the way you anticipated. She’s very happy that the ATMOS team is composed of people with ambitions but also with complementary skills, talent and huge amounts of tenacity and patience. Not only when it comes to setting up and performing the actual experiment but even more so when it comes to analysing the data derived.
Kamal Raj is a PhD student in ATMOS and one of the key employees on Nønne Prisle’s ERC project SURFACE. Working with the data analysis he points to the XPS spectra at one of the data screens and explains; – Using the high brilliance X-rays at MAX IV gives us unprecedented possibilities to measure the very tiny differences in the XPS signals generated by the molecules depending on their location in the liquid solution jet beam.
Nønne elaborates; – This is important because these differences correspond to different behaviours in cloud droplets, which, according to our estimates could have a big impact in aerosols in nature and therefore need to be included in comprehensive climate models.
Kamal continues; – The first thing we do in the data analysis is to calibrate our data accounting for the fact that the beam energy, as well as the position of the beam and of the jet stream, differs between the measurements.
Next step is to compare the theoretical models with the experimental data to figure out what you have learned and find things to further study to get an even more complete understanding of the processes involved. Asked whether the map or the reality wins if they don’t give the same result, Nønne laughs and says: – That is a deep philosophical question! I’m half theorist and half experimentalist so I cannot say for sure where the truth lies, there can be flaws or misinterpretations in both. Both give a window to the truth, but not the complete picture.
This goes to show that science is a never-ending iterative process, building new knowledge by always questioning the results you get and by always being curious.
End of June the ATMOS team will do commissioning experiments at the beamline FinEstBeAMS to test the experimental setup they have been building together with colleagues in Oulu. Here, the ambition is to do measurements on droplets the size of 5–10 nanometres.
Going forward, the ATMOS team hope to do experiments at several other beamlines at MAX IV; Veritas (RIXS), Species (AP-XPS) and FlexPES (XPS) to take advantage of all the different techniques and thus get a more holistic view on the aerosols that surrounds us and that so profoundly impacts the climate of our planet.