Design and performance of a FLOw TUbe System, FLOTUS, for the CERN CLOUD chamber

Abstract. The primary functions of the FLOw TUbe System (FLOTUS) are to provide the CERN CLOUD chamber with a source of a) pre-aged organic vapours and particles, and b) cloud condensation nuclei (CCN) of any chosen chemical composition and diameter. The quartz flow tube has 20 cm internal diameter and 60 L volume, and it operates between−45 °C and +100 °C, limited by the currently-installed thermoregulator. All the experimental conditions in FLOTUS are controlled independently of CLOUD: ultrapure air flow, trace-gases, ozone, UV illumination, humidity and temperature. The vapours and particles produced in FLOTUS can be transferred either directly to analysing instruments or else injected into the CLOUD chamber for further experiments with a comprehensive suite of analysing instruments. Computational Fluid Dynamics (CFD) calculations indicate that up to 32 % transfer efficiency from FLOTUS to CLOUD can be achieved for sticky vapours of 1 nm mobility diameter, despite a long transfer pipe (3.42 m) with two 90° bends. Special attention is given in the design to minimise turbulence in the quartz flow tube: it is mounted vertically to avoid transverse convective turbulence from heating by the UV lamps, and the entrance and exit geometries are narrow-angle cones. We have analysed the residence time distributions of brief injections of toluene assuming an axially-dispersed plug-flow-reactor model. We measure Péclet numbers between 8.2 and 73 for flow rates of 15 to 45 L min⁻¹, respectively, indicating that bulk transport of molecules dominates over diffusion and turbulence. Hydroxyl (OH) radicals are produced in FLOTUS by photolysis of ozone in the presence of water vapour. At 30 L min⁻¹ flow rate, we measure a mean OH exposure of up to 3·10¹¹ cm⁻³s, which corresponds to 3·10⁹ cm⁻³ mean OH concentration and is equivalent to several days OH exposure under ambient conditions. FLOTUS produces aerosol particles from diverse inorganic and organic precursors, with self-preserving, close to log normal size distributions characteristic of coagulation-dominated growth. Aerosol particles can be produced with geometric mean diameters between 8.0 and 136 nm and with geometric standard deviations between 1.16 and 1.43. High particle injection rates into the CLOUD chamber are achieved of up to around 20 cm⁻³s⁻¹. High-resolution aerosol mass spectra show that the organic aerosol composition varies with the precursor system and the OH exposure, and is in good agreement with ambient measurements of aged organic aerosol. These initial results establish FLOTUS as an important extension of the CERN CLOUD chamber for novel experiments involving aged organic vapours, cloud condensation nuclei and aerosol-cloud processing.