Experimental setup of a CSTR experiment. Soot particles are produced and conditioned before atmospheric ageing is mimicked in the aerosol chamber. The impact of the ageing process is monitored by various measurement instruments.
Soot in the clouds Current models of cloud formation typically don’t include soot particles, as they are hydrophobic at the time they are produced, but what if there is a process in the atmosphere that makes these particles hydrophilic? We spoke to Dr Amewu Mensah about her work investigating how soot particles behave in the atmosphere and its wider relevance to climate modelling. The combustion process results in the production of soot, a particle that can represent a significant threat to human health. While large quantities of soot are generated across the world, it remains hard to understand what happens to soot particles when they are emitted into the atmosphere, a topic central to Dr Amewu Mensah’s research. “A perfect combustion process would result in the production of just CO2 and water – but perfect combustion never happens, so there are some left-overs. Soot forms as a result of the combustion process,” she explains. As the Principal Investigator of a research project based at ETH Zurich, Dr Mensah is investigating how these soot particles behave in the atmosphere, an important consideration in the context of wider concern around climate change. “How will atmospheric conditions evolve in future? The scientific community uses cloud models in this, and has parameterized many kinds of processes,” she says.
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Cloud condensation nuclei There is still a significant level of uncertainty around cloud formation and whether soot particles are involved however. Many aerosol particles float around in the atmosphere,
contribute to cloud formation. “Is there a process, while soot particles are flying around in the air, that could turn them from being initially hydrophobic – where they repel water – to hydrophilic, and therefore enable them to contribute to cloud formation? Will consideration of this process improve our representation of clouds in models?” she asks. While it is known that soot particles can gain a greater affinity for water by merging with something that is hydrophilic, for example salt, Dr Mensah is looking at other ways in which they can change. “Is there any way that these soot particles might become hydrophilic without merging with a pre-existing hydrophilic particle?” she continues. A process called heterogeneous oxidation is of great interest here. When an aerosol particle meets ozone gas in the atmosphere the two components are in different physical states, which is why the subsequent reaction is called heterogeneous oxidation. “I’m investigating whether ozone, at atmospheric conditions, is capable of oxidising the surface of a soot particle to such an extent that it will become hydrophilic,” explains Dr Mensah. One major challenge for researchers here is that these particles won’t necessarily survive for a long time within a small volume, so Dr Mensah
says it’s necessary to speed up the reaction. “Normally when we speed up reactions in the lab we put in a lot more ozone and a lot more soot particles, by a factor of a thousand or more,” she says. “While in the atmosphere there may be around fifty ozone molecules per billion air molecules, in the lab we sometimes do 50 per million, or even 50 into 100,000.”
CSTR approach This approach can be effective if the speed of the process under investigation is directly proportional to the concentration of the molecules. However, if the relationship is more complex then experimental results may not be entirely reliable, an issue of which Dr Mensah is well aware. “We tried to figure out a way to allow this reaction to take place at atmospheric conditions. That means at atmospheric ozone concentrations and particle concentrations,” she outlines. This relates specifically to the boundary layer, around the first 300 metres of the atmosphere above ground level; researchers are using an experimental technique involving a continuous flow stirred tank reactor (CSTR) to mimic environmental conditions. “In the CSTR, we managed to age soot particles at atmospherically relevant particle and ozone concentrations, as well as atmospherically relevant temperature and
humidity conditions, for up to 16 hours and beyond,” says Dr Mensah. Dr Mensah is keen to encourage researchers to use this experimental technique in their experiments. “This will help researchers to determine if their current way of thinking about the relationship between particle concentration and reaction speed is really correct,” she explains. “There may be different reactions at higher concentrations than you would get at low concentrations.”
Climate modelling The results of these experiments show that if soot particles are exposed to ozone at atmospheric background concentrations for long enough then they become hydrophilic and can in fact act as CCN. This holds significant implications for our understanding of how the climate is likely to evolve in future. “Clouds have a major impact on our climate and temperature budget,” says Dr Mensah. The project’s findings are being shared with climate modellers, which Dr Mensah hopes will help improve the representation of clouds. “Implementing soot as a potential CCN has a major impact on the representation of cloud cover in climate models,” she points out. “The impact of this on the climate depends on the type of the cloud. The really high clouds –
“CCN are the nuclei on which cloud droplets can form. When the conditions are right in terms of temperature and water vapour concentration, cloud droplets will form in the sky and we will see clouds,” outlines Dr
Cloud Condensation Nuclei are the nuclei on which cloud droplets can form. When the conditions are right in terms of temperature and water vapour concentration, cloud droplets will form in the sky and we will see clouds. like dust, which play a major role in cloud formation when the conditions are right. “We can’t see these particles, because they are too small – but they are recognised by the water molecules in the air. Water molecules attach to these aerosol particles, and then they form a droplet,” outlines Dr Mensah. These initial particles, those that were too small to be visible to the human eye, are called cloud condensation nuclei (CCN).
Mensah. “It has long been thought that pure soot, without any coating from hydrophilic material, cannot act as CCN.” Cloud formation in the current global climate models does not always match satellite and radar measurements however, leading researchers to re-examine some of the underlying ideas behind them. For Dr Mensah, the question is whether soot particles can in fact act as CCN, and if they
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Aerosol ageing pathways in the atmophere. After emission aerosol particles are exposed to ageing processes before they can be washed out from the atmosphere. The three most important ones are coating with hydrophilic substances, heterogeneous oxidation, and in-cloud cycling. These processes can turn hydrophobic particles into CCN or INPs
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SPACE Soot Particle Ageing Cloud Effects
Project Objectives
Aerosol-cloud interactions, i.e. the impact of aerosol particles on clouds, spans a multidimensional space. The impact of soot particles on clouds is especially highly uncertain. Since soot is the second most important anthropogenic emission after CO2 in terms of climate forcing, SPACE aims to resolve the impact of individual atmospheric ageing processes on the cloud formation potential of soot particles.
Project Funding
Funded by the Swiss National Science Foundation (SNSF); http://p3.snf.ch/Project-161343
Contact Details
Dr. Amewu A. Mensah Head of Air Quality Environmental and Health Protection Service City of Zurich Zurich, Switzerland E: A.Mensah@alumni.ethz.ch W: https://www.youtube.com/watch?v=BnOsfaGKHeU W: https://www.youtube.com/watch?v=aUSKHFsSYfI Friebel, F. and Mensah, A.A. (2019b). “Ozone Concentration versus Temperature: Atmospheric Aging of Soot Particles” Langmuir, doi: 10.1021/acs. langmuir.9b02372. Friebel, F., Lobo, P., Neubauer, D., Lohmann, U., Dusseldorp, S.D.v., Mühlhofer, E. and Mensah, A.A. (2019). “Impact of Isolated Atmospheric Aging processes on the Cloud Condensation Nucleiactivation of Soot Particles.” Atmospheric Chemistry and Physics Discussions 2019: 1-29, doi: 10.5194/ acp-2019-504. Friebel, F. and Mensah, A.A. (2019a). “Aging aerosol in a well-mixed continuous-flow tank reactor: an introduction of the activation time distribution.” Atmospheric Measurement Techniques 12(5): 26472663, doi: 10.5194/amt-12-2647-2019.
the cirrus clouds – trap outgoing long-wave radiation and contribute to heating, but lower clouds actually contribute to cooling, because they prevent short-wave radiation from reaching the earth’s surface.”
Particle surface A particle’s surface area is another important consideration with respect to understanding its wider impact, a topic that Dr Mensah and her colleague Franz Friebel are also investigating in the project. A soot particle has carcinogenic substances on its surface, and the quantity depends on the surface area. “If a particle has a large surface area there are a lot of these substances, whereas if it is small then there are less,” explains Dr Mensah. Soot particles have a fairly unusual shape, so determining their surface area is a complex task. “It has historically proved challenging to determine the surface area of soot particles, or of other particles that can be found in low concentrations in the atmosphere,” continues Dr Mensah. “We have developed a new measurement technique which we are now in the process of patenting. We’ve proved that we can determine the surface area of soot particles and we want to continue our research and develop this technique further.”
There are also plans to bring this technique to the market, with a start-up company currently being established to build these instruments. This will help to demonstrate the long-term importance of continued scientific research to both commercial development and our ability to prepare for emerging challenges. “This research has led to positive outcomes that will be beneficial to society,” stresses Dr Mensah. The experimental technique developed in the project can be applied to a range of other atmospheric particles aside from soot as the latest experimental results show. “We are convinced that this new measurement technique will open a new horizon to the characterization of nanoparticles that are nonspherical or too low in concentration for the standard technique.” says Dr Mensah. Research can lead to commercial development and new products, yet it’s important to also give scientists the freedom to address abstract questions that may not lead to immediate results. While keen to translate her research into tangible outcomes, Dr Mensah also plans to continue with more exploratory research in future. “We will look at how we represent clouds better, how we represent soot particles in models better, and how will we be able to anticipate the impact of climate change due to these better models?” she outlines.
A fast adsorption of an Ozone monolayer within several minutes is followed by a slow reaction of the adsorbed Ozone with the soot particle surface which takes hours. The proceeding oxidation causes an increase in the hydrophilicity of the particles.
Dr. Amewu A. Mensah
Dr. Amewu A. Mensah was a senior scientist at ETH Zurich. She has investigated the chemical and physical nature of aerosol particles throughout her academic career. As the PI of the SPACE project, she has focused on the cloud formation potential of soot particles after atmospheric aging. She has now moved to a governmental position. As the Head of air quality of the city of Zurich, she is dedicated to the fast implementation of scientific results for the benefit of society.
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Like many chemical reactions, the oxidation of soot with Ozone is highly temperate-dependent. An increase from 5°C to 35°C results in a 5 times faster reaction rate. Understanding the reaction kinetic of aging processes is vital for assessing the climate impact of soot particles.
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