A High-Flow Turbulent Cloud Chamber

A large laboratory facility (cloud chamber) has been built to study cloud formation under reproducible conditions. The chamber was designed to assess the influence of anthropogenic aerosols on the microstructure of marine clouds in coastal Western Europe. For this reason the supersaturations in the chamber are low, in the order of 0.1%, typical for coastal marine stratus. The very large size (30 m3 ) and flow (30 m3/min) of the chamber allow unperturbed use of conventional cloud instrumentation like Forward Scattering Spectrometer Probe (TSI Inc.) and high-flow cascade impactors for chemical analysis of aerosol and droplets. The performance of the cloud chamber was tested with laboratory-generated submicron-sized ammonium sulphate aerosol with a lognormal size distribution and varying the number concentrations. It was found that the sulphate particles above a threshold size of 0.07 or 0.1 /-Lm in diameter (dependent on the settings) grew into droplets (became "activated"), which corresponds according to the Kohler theory to supersaturations of 0.26% and 0.15%, respectively. Estimates of the supersaturation in the chamber from the measured liquid water content (LWC) gave the same value. Tests showed that LWC, droplet spectra, and activation threshold were stable for hours and reproducible from day to day