Laboratory study of the collection efficiency of submicron aerosol particles by cloud droplets. Part II – Influence of electric charges

Abstract. A new In-Cloud Aerosol Scavenging Experiment (In-CASE) has been developed to measure the collection efficiency (CE) of submicron aerosol particles by cloud droplets. Droplets fall at their terminal velocity through a one-meter-high chamber in a laminar flow containing aerosol particles. At the bottom of the In-CASE's chamber, the droplet train is separated from the aerosol particles flow and the droplets are collected in an impaction cup whereas aerosol particles are deposited on a High Efficiency Particulate Air (HEPA) filter. The collected droplets and the filter are then analysed by fluorescence spectrometry since the aerosol particles are atomised from a sodium fluorescein salt solution (C20H10Na2O5). In-CASE fully controls all the parameters which affect the CE – the droplets and aerosol particles size distributions are monodispersed, the electric charges of droplets and aerosol particles are known and set, while the relative humidity is indirectly controlled via the chamber's temperature. This paper details the In-CASE setup and the dataset of 70 measurements obtained to study the impact of the electric charges on CE. For this purpose, droplets and particles charges are controlled through two charging systems developed in this work – both chargers are detailed below. The droplet charge varies from −3.0 × 104 ± 1.4 × 103 to +9.6 × 104 ± 4.3 × 103 elementary charges while the particle charge ranges from the neutralisation to −90 ± 9 elementary charges depending on the particle radius. A droplet radius of 48.5 ± 1.1 μm has been considered for four particle dry radii between 100 and 250 nm. These new CE measurements have been compared to the correlation of Kraemer and Johnstone (1955) and the extended model of Depee et al. (2019) where thermophoresis and diffusiophoresis are also considered since the measurements have been performed at a relative humidity level of 95.1 ± 0.2 %. As a result, both models adequately describe the electric charge influence on the measured CEs. Furthermore, the effect of the image charge (Jackson, 1999) in the electrostatic forces is measured.