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Abstract
Abstract. This article presents new measurements of the efficiency with which aerosol particles of accumulation mode size are collected by a 1.25 mm sized raindrop. These laboratory measurements provide the link to reconcile the scavenging coefficients obtained from theoretical approaches with those from experimental studies. We provide here experimental proof of the rear capture mechanism in the flow around drops, which has a fundamental effect on submicroscopic particles. These experiments thus confirm the efficiencies theoretically simulated by Beard (1974). Finally, we propose a semi-analytical expression to take into account this essential mechanism to calculate the collection efficiency for drops within the rain size range.
Highlights
Aerosol particles are important components of the atmosphere
Δ:ELPI; daaaa ∈ [7 nn ; 10 μm] (12 size classes) ω: thermocouple and capacitive hygrometer χ: APS; daaaa ∈ [0.5 μm ; 20 μm] (52 size classes) ε: high-efficiency particulate arresting (HEPA) filter γ: HEPA filter η: argon injection φ: synthetic air injection φ: knife gate valve τ: drop collector comparisons with Beard (1974) simulations will be direct. This model is interesting as we have previously shown that, for 2 mm diameter drops (Quérel et al, 2014b), it is the only one able to predict the sharp rise in the collection efficiency observed experimentally for submicroscopic particles, which is due to the eddies that develop within the wake of the drop
Quérel et al (2014b) showed that their efficiency measurements of submicron particles could only be explained by rear capture
Summary
Aerosol particles are important components of the atmosphere. They contribute significantly to the Earth’s energy budget by interacting with solar radiation directly as well as indirectly by serving as precursors to cloud formation (cloud condensation nuclei – CCN) which will interact with this radiation (Twomey, 1974). The physical properties of these particles in suspension within the atmosphere (size, concentration, affinity for water, etc.) are essential parameters for characterising air quality For these reasons, the scientific community has actively studied the physics of atmospheric aerosol particles. The secondary sources are associated with the gas-to-particle conversion of certain gases present in the atmosphere The size of these particles greatly varies and ranges from 1 nm to several hundred microns. There is a particle size range that has no efficient removal process and a very long atmospheric residence time This size range is referred to as the accumulation mode (Whitby, 1973) and comprises particles with a diameter between 0.1 and 2 μm. These particles can remain in the upper troposphere for several months (Jaenicke, 1988) and can be transported over long distances, crossing oceans and continents (Pruppacher et al, 1998)
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