FLOTUS: a new FLow TUbe System for the CERN CLOUD chamber

The CERN CLOUD experiment (Kirkby et al. 2011) investigates aerosol particle nucleation and growth under controlled atmospheric conditions. To extend its experimental capabilities, a new FLow TUbe System (FLOTUS) was recently developed as an external 60 litre flow tube directly coupled to the CLOUD chamber. FLOTUS consists of a quartz tube with conical entry and exit geometries for laminar-flow, following the approach of the Caltech Flow Tube Reactor (Huang et al. 2017). The 3 m long x 20 cm diameter quartz tube is mounted vertically to minimise convective turbulence. It is housed in a temperature-controlled enclosure with a gas system independent of the CLOUD chamber. Six separately-controlled ultraviolet lamps mounted inside the FLOTUS thermal housing enable in situ photochemical production of hydroxyl radicals (OH) from water vapour and O₃ up to extremely high concentrations of up to 10¹⁰ cm^-3.

The chemical composition and size distribution of particles generated in FLOTUS can be characterized either at a sampling point at the exit of FLOTUS or after transfer into the CLOUD chamber. We assessed the flow conditions inside the FLOTUS quartz tube and along the transfer line to the CLOUD chamber using computational fluid dynamics simulations with COMSOL, confirming laminar flow and well-defined transport of gases and particles in both the quartz chamber and the transfer line to CLOUD. We quantified OH production rates in FLOTUS using toluene attenuation experiments.

We have used FLOTUS to generate aerosol particles across a wide range of sizes between 10-150 nm and chemical compositions, which include sulfuric acid(–ammonia), highly oxygenated organic molecules (HOM, from α-pinene and isoprene), methanesulfonic acid, and other systems. We characterized the composition and size of particle populations produced in FLOTUS directly using aerosol mass spectrometry and mobility-based size distribution measurements, and after injection into CLOUD using a suite of state-of-the-art measurement instruments to determine particle size and chemical composition.

The controlled injection of freshly formed particles enables subsequent experiments in the CLOUD chamber under novel conditions, including studies of aerosol evaporation, cloud activation, aqueous-phase processing of aerosol, and surface chemistry, all under atmospheric conditions. FLOTUS represents an important technical advancement for the CLOUD experiment by decoupling particle formation from studies under different conditions in the CLOUD chamber, increasing the experimental flexibility and enabling systematic investigations of aerosol transport, fate, and cloud chemistry interactions.

 

Kirkby, Jasper, et al. "Role of sulphuric acid, ammonia and galactic cosmic rays in atmospheric aerosol nucleation." Nature 476.7361 (2011): 429-433.

Huang, Yuanlong, et al. "The Caltech Photooxidation Flow Tube reactor: design, fluid dynamics and characterization." Atmospheric Measurement Techniques 10.3 (2017): 839-867.