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
EU Research
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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