Experimental Study of Collection Efficiencies between Submicron Aerosols and Cloud Droplets

Abstract

Collection efficiency E experiments for aerosol particles scavenged by cloud droplets were carried out in the newly built Collision Ice Nucleation Chamber (CLINCH). Pure water droplets having radii between 12.8 and 20.0 mm were allowed to fall freely and to collide in a laminar flow with lithium metaborate particles having radii between 0.05 and 0.33 mm. At the bottom of the chamber, the droplets and the particles captured were collected using a cup impactor. The collected solution was analyzed for the scavenged aerosol mass by inductively coupled plasma mass spectrometry. Evaporation of droplets was taken into account since the relative humidity inside the chamber was below 100%, resulting in final theoretical droplet sizes between 4.2 and 17.6 mm. The resulting experimental measurements were compared with theoretical values to see their correlation. The authors found an experimental trend similar to theory, as well as the ''Greenfield gap'' at the particle radius of 0.24 mm (E 5 0.038) for the smallest cloud droplet size investigated in this study. The experimental values of collection efficiency found herein agree with those from theory within one order of magnitude, similar to previous studies reported in the literature.

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

“…It is expected that a higher aerosol concentration will increase the chance of collision between particles and droplets, which will increase the value of the ECR, but will not affect CE, which is normalized. The most similar experimental conditions to ours are those of Ladino et al (2011 used similar droplet (radius of 12.8-20 µm) and particle sizes (radius of 0.05-0.33 µm). Experiments were conducted under RH conditions similar to our high-RH experiments (88 ± 2 %).…”

“…This could explain why the Greenfield gap is highly pronounced in the data in Fig. 6, while it is more pronounced in the data of Lai et al (1978) and Ladino et al (2011).…”

“…It has been shown theoretically by Wang et al (1978), Grover and Pruppacher (1985) and Ladino et al (2011), and experimentally by Grover et al (1977), that CE increases with decreasing RH. This is because a lower RH leads to an increase in the evaporation rate of the droplet, which strengthens the phoretic forces.…”

“…Droplets were generated at 30 Hz. This is a frequency much lower than previously used in other experimental works using cloud droplets (e.g., 1000 Hz in Ladino et al, 2011) where analysis was performed on a bulk basis. This rate yielded both a collection signal with PALMS and minimized possible dropletdroplet collisions inside the chamber.…”

“…The main difference between the two studies is the droplet charge, which has a stronger impact on the electro-scavenging force. Ladino et al (2011) used droplets with high charges, 5 × 10 4 elementary charges per droplet (Claudia Marcolli, personal communication, 2014), which are 2 orders of magnitude higher than the one used in this study. The higher droplet charge explains the higher CE values compared to those determined in this study.…”