CERN Accelerating science

FLOTUS: a new addition to the CLOUD experiment

The Flow TUbe System (FLOTUS) during its installation at CLOUD, November 2022. Credit: Maximilien Brice, CERN



Atmospheric aerosols are tiny solid or liquid particles that are suspended in the Earth's atmosphere. Aerosol particles can have a large influence on climate since they scatter incoming sunlight and provide the seeds for cloud droplets. However, their formation mechanisms as well as their interactions with clouds are poorly understood and they remain the main source of uncertainty for anthropogenic climate change [1]. Since the startup of CLOUD in 2009, CERN has joined the mission to improve our understanding of climate change, by uncovering the mechanisms responsible for aerosol formation in the atmosphere and helping to reduce the uncertainties of their impact on the climate.

The CERN CLOUD experiment

Every cloud droplet needs a seed particle for water vapour to condense on. These cloud condensation nuclei (CCN) are aerosol particles above a size of around 50 nm, which have a variety of different origins. Some of these particles are directly emitted into the atmosphere – such as dust or biomass burning – and others are formed by gas-to-particle conversion. The latter process is known as nucleation and is responsible for around half of global CCN, and almost all those in the upper troposphere [2]. 

The CERN CLOUD experiment ( investigates the formation of aerosol particles in our atmosphere, including a unique focus on the influence of atmospheric ions generated by cosmic rays. The main part of the CLOUD experiment is the CLOUD chamber. It is a 26 cubic metre stainless-steel vessel, in which an air parcel at various atmospheric conditions can be simulated. The CERN CLOUD chamber is the world’s first and, so far, only aerosol chamber that is sufficiently clean to carry out controlled aerosol formation experiments under atmospheric conditions, where the active vapours are present at concentrations of one part per trillion by volume (pptv) or below.

The CLOUD experiment has been in operation for more than a decade and has since published numerous important advances in high-impact journals ( However, like every aerosol chamber experiment, the CLOUD chamber has one shortcoming – as opposed to the real atmosphere, it has walls. The CLOUD chamber walls provide a limit to the lifetime of aerosols and sticky vapours that is equivalent to a pristine ambient environment. Therefore, any chemical reaction or process observed in the CLOUD experiment so far has to take place on a timescale of within around an hour or so.

In the ambient atmosphere, however, certain processes take place over several hours or even days. The CLOUD experiment has recently been upgraded to tackle this limitation. A novel piece of equipment named the FLow TUbe System (FLOTUS) has been designed and built at CERN to allow the investigation of new particle formation properties in CLOUD with atmospherically-aged trace gases. 

A new extension to the CLOUD experiment – the FLow TUbe System FLOTUS

When trace gases are exposed to the real atmosphere, they undergo various chemical reactions, changing their chemical properties over time. This mainly happens via hydroxyl (OH) radicals “attacking” the gas molecules. FLOTUS is designed to simulate these aging processes. The main part of FLOTUS is a 3 m quartz tube of 20 cm diameter, surrounded by high intensity UV lamps. When trace gases, together with ozone and water vapour, are passed through FLOTUS, the UV light creates high concentrations of OH radicals which react with the trace gas. By varying ozone and water vapour concentrations, as well as the UV light intensity and temperature, the amount of OH radicals the trace gases are exposed to can be varied between levels equivalent to 1 hour and up to 10 days of atmospheric exposure time. These atmospherically aged vapours are then injected into the CLOUD chamber, where their chemical composition, as well as their new particle formation properties can be analysed with the full suite of CLOUD instruments.

Figure 1: Schematic of the CERN CLOUD experiment with its new FLow TUbe System (FLOTUS) to the right. FLOTUS is used to pre-age trace gases by exposing them to high levels of OH radicals before injecting them into CLOUD, where their aerosol particle formation properties can be analysed. A pick-off pipe transfers a small amount of the aged vapours to a mass spectrometer, which can directly measure the chemical composition of the aged gases at the exit of FLOTUS. Schematic by Jasper Kirkby.

The design work of FLOTUS, led by Serge Mathot (CERN EN-MME-DI), as well as its manufacturing were conducted here at CERN. FLOTUS has its own gas system and an independent temperature control, allowing operation between -40°C and +100°C. It has six UV lamps that can be piloted individually. The outlet of FLOTUS is connected to the CLOUD chamber via a remotely controlled gate valve. Additionally, a pickoff-pipe guides a fraction of the flow towards the roof of the chamber, where a mass spectrometer analyses the aged gases directly.

FLOTUS installation

The welding of flanges to FLOTUS and its installation at CLOUD came with a unique set of challenges, due to the extreme fragility of the quartz tube – in particular the delicate (1.5 mm thick) glass transition cylinder at each end that matches the thermal expansion coefficient of quartz to kovar and stainless steel. This huge technical challenge was brilliantly solved by Didier Lombard (CERN EN-MME). The tube had to be lifted, brought into a vertical position, and then inserted and lowered into its thermal housing box. In order to avoid contamination, not a single piece of plastic or glue is being used in CLOUD or anywhere in its upstream systems. Consequently, the FLOTUS gas system, the transfer pipe, as well as the quartz tube itself, had to be made leak-tight and connected relying solely on direct transitions from quartz glass to metal. In an immense team effort under the guidance of Didier, the fragile quartz tube was successfully installed and mounted in its thermal housing. The last nerve-wrecking undertaking – the connection of the long, rigid transfer line from the fragile end-piece of FLOTUS to the CLOUD chamber itself – was flawlessly executed by Louis-Philippe De Menezes and his colleagues of the EP-DT gas team in the days afterwards.

Figure 2: FLOTUS pre-assembly in the workshop shortly before the installation at CLOUD. The fragile quartz tube is being lifted into its thermal housing box for the first time to make last alignments for a perfect fit.

Figure 3: The FLOTUS quartz tube is thoroughly cleaned before its installation in its thermal housing box (upper left corner). Credit: Wiebke Scholz, University of Innsbruck.

Figure 4: Didier Lombard makes last alignments to bring the FLOTUS setup in its final position. The delicate glass transition section is in the lower right hand corner of the photograph. Credit: Maximilien Brice, CERN.

During the following weeks, first tests of the performance of the new system were completed. While the technical performance of FLOTUS (in particular, concentrations of 1010 OH radicals per cm3) was confirmed, the detailed physics integration of FLOTUS with CLOUD (optimisation of operating conditions and other parameters) remains to be done. An experimental campaign is planned to take place in April and May 2023, which will be dedicated to this work.

Summing up, an incredible task has been successfully accomplished – FLOTUS is installed at CLOUD, and it works at design specifications. With this remarkable accomplishment of adding this unique piece of equipment, CLOUD has opened a new window on truly reproducing atmospheric conditions in the chamber and looks forward to further advances in our understanding of aerosol particle formation in the atmosphere and their influence on climate change.


[1] Szopa, S., et al. "Short-lived climate forcers Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change ed V Masson-Delmotte et al." (2021).

[2] Gordon, H., et al. Causes and importance of new particle formation in present-day and pre- industrial atmospheres. J. Geophys. Res. Atmos., doi: 10.1002/2017JD026844 (2017).