An experimental setup has been constructed to measure the collectionefficiency (CE) of sub-micrometer aerosol particles by cloud droplets.Droplets of a dilute aqueous ammonium sulfate solution with an average radiusof 21.6 μm fall freely into a chamber and collide withsub-micrometer polystyrene latex (PSL) sphere particles of knownsizes and concentrations. Two relative humidity (RH) conditions,15 ± 3 % and 88 ± 3 %, hereafter termed "low" and"high", respectively, were varied with different particles sizes andconcentrations. After passing through the chamber, the droplets and aerosolparticles were sent to the Particle Analysis by Laser Mass Spectrometry(PALMS) instrument to determine chemical compositions on a single-dropletbasis. "Coagulated droplets" (droplets that collected aerosols) had massspectra that contained signatures from both an aerosol particle and a dropletresidual. CE values range from 2.0 × 10 to 1.6 for the low-RHcase and from 1.5 × 10 to 9.0 × 10 for thehigh-RH case. CE values were, within experimental uncertainty, independent ofthe aerosol concentrations. CE values in this study were found to be inagreement with previous experimental and theoretical studies. To ourknowledge, this is the first collection experiment performed on asingle-droplet basis with atmospherically relevant conditions such as dropletsizes, droplet charges and flow.
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