Laboratory studies of collection efficiency of sub-micrometer aerosol particles by cloud droplets on a single-droplet basis

K Ardon-Dryer,Y.-W Huang,D J Cziczo

doi:10.5194/acp-15-9159-2015

Abstract

Abstract. An experimental setup has been constructed to measure the collection efficiency (CE) of sub-micrometer aerosol particles by cloud droplets. Droplets of a dilute aqueous ammonium sulfate solution with an average radius of 21.6 μm fall freely into a chamber and collide with sub-micrometer polystyrene latex (PSL) sphere particles of known sizes and concentrations. Two relative humidity (RH) conditions, 15 ± 3 % and 88 ± 3 %, hereafter termed "low" and "high", respectively, were varied with different particles sizes and concentrations. After passing through the chamber, the droplets and aerosol particles were sent to the Particle Analysis by Laser Mass Spectrometry (PALMS) instrument to determine chemical compositions on a single-droplet basis. "Coagulated droplets" (droplets that collected aerosols) had mass spectra that contained signatures from both an aerosol particle and a droplet residual. CE values range from 2.0 × 10−1 to 1.6 for the low-RH case and from 1.5 × 10−2 to 9.0 × 10−2 for the high-RH case. CE values were, within experimental uncertainty, independent of the aerosol concentrations. CE values in this study were found to be in agreement with previous experimental and theoretical studies. To our knowledge, this is the first collection experiment performed on a single-droplet basis with atmospherically relevant conditions such as droplet sizes, droplet charges and flow.

Highlights

The interplay between aerosol particles and water droplets in the atmosphere, especially in clouds, influences both aerosol and cloud properties
Two relative humidity (RH) conditions were measured in this experimental work: low (15 ± 3 %) and high (88 ± 3 %). Consistent with these previous works, we find a higher collection efficiency (CE) values for the low-RH experiments, by as much as www.atmos-chem-phys.net/15/9159/2015/
Droplet size and charge conditions can counteract each other in the case of larger droplets with higher charge. We suggest this may explain the agreement found between the CE values measured in this study and those of Wang and Pruppacher (1977) and the disagreement between our values and those of Ladino et al (2011)

Summary

Introduction

The interplay between aerosol particles and water droplets in the atmosphere, especially in clouds, influences both aerosol and cloud properties. When an aerosol particle comes in contact with a water droplet, the interaction can result in a collision followed by coalescence of the two. This process is known as “collection” or “coagulation”. The collection process is considered an important mechanism that can “scavenge”, and thereby remove, aerosol particles from the atmosphere (Starr and Mason, 1966; Owe Berg et al, 1970; Hampl and Kerker, 1972; Pranesha and Kamra, 1996). Collection can affect cloud dynamics, the precipitation process and cloud lifetime, and thereby change the global radiation budget (Rasch et al, 2000; Croft et al, 2009)

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