Accounting for Changes in Particle Charge, Dry Mass and Composition Occurring During Studies of Single Levitated Particles

Abstract

The most used instrument in single particle hygroscopic analysis over the past thirty years has been the electrodynamic balance (EDB). Two general assumptions are made in hygroscopic studies involving the EDB. First, it is assumed that the net charge on the droplet is invariant over the time scale required to record a hygroscopic growth cycle. Second, it is assumed that the composition of the droplet is constant (aside from the addition and removal of water). In this study, we demonstrate that these assumptions cannot always be made and may indeed prove incorrect. The presence of net charge in the humidified vapor phase reduces the total net charge retained by the droplet over prolonged levitation periods. The gradual reduction in charge limits the reproducibility of hygroscopicity measurements made on repeated RH cycles with a single particle, or prolonged experiments in which the particle is held at a high relative humidity. Further, two contrasting examples of the influence of changes in chemical composition changes are reported. In the first, simple acid-base chemistry in the droplet leads to the irreversible removal of gaseous ammonia from a droplet containing an ammonium salt on a time scale that is shorter than the hygroscopicity measurement. In the second example, the net charge on the droplet (<100 fC) is high enough to drive redox chemistry within the droplet. This is demonstrated by the reduction of iodic acid in a droplet made solely of iodic acid and water to form iodine and an iodate salt.

A review of experimental techniques for aerosol hygroscopicity studies

“…Therefore, the relative mass change of the particle due to any physical or chemical processing can be quantified by measurement of the DC voltage. Haddrell et al (Haddrell et al, 2012) discussed conditions when the assumption of constant charge may fail and proposed experimental strategies to minimize its occurrence.…”

Photochemistry of single particles using acoustic levitation coupled with Raman microspectrometry

“…So far, levitation‐based setups, i.e. optical levitation, electrodynamic balance and ultrasonic trapping, have been shown to be suitable and powerful techniques for wall‐less investigations. When coupled with micro‐Raman scattering, the composition, size and phase partitioning of different chemical components within a liquid or solid droplet can be investigated, and then, many fundamental physical and chemical processes at the single particle scale with in situ conditions have been unravelled .…”

Assessing the airborne survival of bacteria in populations of aerosol droplets with a novel technology

“…During formation of micro-droplets, the induction electrode produces an ion imbalance in the liquid jet formed at the tip of the DoD dispenser, resulting in a net charge on the droplet of opposed polarity to the induction electrode. The magnitude of the net charge induced to the droplets has been reported previously (less than 5 fC) [36] producing a chemically insignificant shift in the original ion concentration of the droplets (approx. 7 Â 10 26 % more sodium than chloride ions), but sufficient for the droplets to be confined by the electrodynamic potential in the centre of the EDT.…”