Experimental investigation on collection efficiencies of airborne particulates by raindrop

Rain is an effective way to clean atmospheric particulate matter out of the air, which is significant for human health. Such particulate pollution mitigation by nature lacks a thorough investigation using a controlled laboratory experiment. We develop a new experimental method that can simulate the interaction of airborne micron particles with free-falling droplets at terminal velocity to quantify the particle collection efficiency by a single droplet. This method enables a direct measurement and statistical analysis of the effect of droplet size and particle mass concentration on both particle and mass collection efficiencies without restrictions on particle types. Our findings reveal that both mass and quantity collection efficiencies increase as droplet size decreases and are insensitive to the particle mass concentration. For polydisperse particles, larger droplets are relatively prone to capture fine particles. The size distribution of collected particles is attributed to the rear capture mechanism.