Development of an acoustic cloud condensation nuclei counter

Abstract

Theoretical development of an acoustic cloud condensation nuclei counter is presented. The proposed instrument will be able to count the number of aerosols present in a sample of air flowing through a growth chamber. The air condenses around aerosols and forms water droplets. The number of aerosols is determined by measuring the sound produced underwater by these droplets when the droplets strike a water surface at the bottom of the growth chamber with an impact velocity equal to either their terminal or maximum velocities. First, the terminal velocities of the droplets are calculated. Then, the maximum velocities that these droplets can sustain without breaking are calculated as a function of droplet diameter. Second, the sound due to droplet impact is estimated and experimentally verified. In this study the bubble sound is not considered because micro droplets falling with terminal velocities do not create bubbles. Also, when accelerated, the velocities are limited such that there is no bubble formation at impact. Finally, two approaches are considered for the instrument. One case uses one hydrophone to measure the total sound radiated by all the droplets. The other case is based on a grid of tiny hydrophones, each measuring a droplet impact sound.