Abstract
Abstract. A new In-Cloud Aerosol Scavenging Experiment (In-CASE) has been conceived to measure the collection efficiency (CE) of submicron aerosol particles by cloud droplets. In this setup, droplets fall at their terminal velocity through a 1 m high chamber in a laminar flow containing aerosol particles. At the bottom of the In-CASE chamber, the droplet train is separated from the aerosol particle flow – 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 controlled, and the relative humidity is indirectly set via the chamber's temperature. This novel In-CASE setup is presented here as well as the first measurements obtained to study the impact of relative humidity on CE. For this purpose, droplets and particles are electrically neutralised. A droplet radius of 49.6±1.3 µm has been considered for six particle dry radii between 50 and 250 nm and three relative humidity levels of 71.1±1.3 %, 82.4±1.4 % and 93.5±0.9 %. These new CE measurements have been compared to theoretical models from literature which adequately describe the relative humidity influence on the measured CE.
Highlights
Every year, several billion tonnes of particulate matter are emitted in the atmosphere, originating mainly from oceans, soils, gas-to-particle conversion, evaporating clouds and from human activities (Jaenicke, 1993)
Dépée et al (2019) numerically evaluated the contribution of the electrostatic forces on the collection efficiency (CE) for a droplet of 50 μm radius with −1000 elementary charges and 5 elementary charges on the aerosol particles (APs). They calculate an increase in the CE due to the electrostatic forces by 42 % and 22 % for an AP radius of 50 and 300 nm, respectively. Close to these two AP radii, a rise of the CE by a factor of 3 and 4, respectively, is observed when the relative humidity goes from 93.5 ± 0.9 % to 71.1 ± 1.3 % (Fig. 10)
It is assumed that the contribution of the thermophoresis and the diffusiophoresis is of first order in the measurements and the electrostatic forces can be neglected in the observed increase in CE
Summary
Several billion tonnes of particulate matter are emitted in the atmosphere, originating mainly from oceans, soils, gas-to-particle conversion, evaporating clouds and from human activities (Jaenicke, 1993). APs play a key role in weather and climate They act on cloud formation, and their chemical composition, size distribution and number concentration affect the droplet size distributions and precipitation (Tao et al, 2012). Many studies revealed that radioactive material like caesium-137 isotopes can attach to the atmospheric APs and were transported over long distances on a continental scale both after the Chernobyl (Devell et al, 1986; Jost et al, 1986; Pölläen et al, 1997) and the Fukushima (Kaneyasu et al, 2012; Adachi et al, 2013) nuclear accidents in 1986 and 2011, respectively. With a halflife of up to 10 years, caesium-137 can remain for decades in the atmosphere – following resuspension cycles of the atmospheric APs – and jeopardise both ecosystems and human health
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