10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Experimental Study of the In-Cloud Electroscavenging
ALEXIS DEPEE, Pascal Lemaitre, Anne Mathieu, Marie Monier, Andrea Flossmann, French Radioprotection and Nuclear Safety Institute
Abstract Number: 996 Working Group: Aerosol Physics
Abstract The scavenging of aerosol particles (AP) is of importance for understanding the atmospheric particles loading, which impacts both air quality and climate. In the case of a nuclear accident and the discharge of radioactive material, this understanding is a key to the forecast of radioactive dissemination and needed to protect populations. Far from the source, the AP scavenging is mainly due to the wet scavenging which combines two mechanisms: the AP removal by clouds (rainout) and the AP collection by raindrops (washout).
In most current models, the AP scavenging is often parameterized with empirical formulas issued from measurements including precipitation rates (Scott, 1982). No distinction is made regarding if scavenging occurred in the cloud or below in the precipitative region, while the microphysics effects which lead to the AP removal are significantly different. Indeed, there is no parameterization of the rainout whereas it has been shown both theoretically (Flossmann, 1998) and experimentally (Laguionie et al., 2014) that rainout dominates the wet scavenging.
The lack of a detailed model for in-cloud AP scavenging can be explained by the difficulty to measure AP collection efficiencies by micron-sized cloud droplets; and even more to disentangle the different processes implied in the collection. For radioactive aerosol particles, the electrostatic forces are one of the factors which could significantly impact the AP collection by clouds since AP and cloud droplets are charged. Thus, few theoretical works using the theory of mirror charges introduced by Jackson (1975) have been undertaken. However, no experimental data is available in the literature to validate (or not) those models.
The present study attempts to fill this data gap through a novel experiment called IN-Cloud ElectroScavenging Chamber (INCESC) inspired by the Collision Ice Nucleation Chamber (CLINCH, Ladino et al., 2011). The chamber mixes an online AP flow with a micron-size droplet jet to quantify the AP collection by droplets via a fluorescence analysis. A Differential Mobility Analyzer (DMA) is used to get a monodispersed AP size while the droplets are generated by a piezo-injector. A cooling system sets the chamber’s temperature to regulate the relative humidity. INCESC distinguishes itself from CLINCH by proposing a new system to control the charge of AP and droplets.
Finally, the measurements of the AP collection efficiencies by droplets for some AP and droplet charges and few AP radii are presented and compared to the theoretical model implemented for this purpose. These results reflect a next step towards the improvement of the AP scavenging model by strengthening the knowledge of the interaction between AP and clouds. A final stage will be to use these data to get a new parameterization of the in-cloud scavenging of heavily charged radioactive AP.
• Flossmann, A. I. (1998). Interaction of aerosol particles and clouds. Journal of the atmospheric sciences, 55(5), 879-887. • Jackson, J. D. (1975). Electrodynamics. Wiley-VCH Verlag GmbH & Co. KGaA. • Ladino, L., Stetzer, O., Hattendorf, B., Günther, D., Croft, B., & Lohmann, U. (2011). Experimental study of collection efficiencies between submicron aerosols and cloud droplets. Journal of the atmospheric sciences, 68(9), 1853-1864. • Laguionie, P., Roupsard, P., Maro, D., Solier, L., Rozet, M., Hébert, D., & Connan, O. (2014). Simultaneous quantification of the contributions of dry, washout and rainout deposition to the total deposition of particle-bound 7 Be and 210 Pb on an urban catchment area on a monthly scale. Journal of Aerosol Science, 77, 67-84. • Scott, B. C. (1982). Theoretical estimates of the scavenging coefficient for soluble aerosol particles as a function of precipitation type, rate and altitude. Atmospheric Environment (1967), 16(7), 1753-1762.