The high charge fraction of flame-generated particles in the size range below 3 nm measured by enhanced particle detectors

Abstract

Charging in flames significantly affects the properties of the resultant particles produced because of its influence in almost all stages of particle formation. The charging characteristics of flame-generated sub-3nm particles were investigated with three enhanced particle detectors including a high resolution differential mobility analyzer (DMA) coupled with an electrometer, a particle size magnifier coupled with a butanol-based condensation particle counter (PSM-bCPC), and an atmospheric pressure interface time-of-flight mass spectrometer (APi-TOF). Up to 95% of the flame-generated sub-3nm particles were charged at a sampling height of 5mm above the burner, indicating the existence of a strong ionization process in the investigated flame. This high fraction of charged particles contradicts the classical charging theories, which predict <1% charge fraction for particles below 3nm. Positively and negatively charged sub-3nm particles generated from a blank flame were dominated by organic ions and nitrate ions respectively. The flame-generated ions play an important role during titanium dioxide (TiO2) nanoparticle synthesis, as shown by the attachment of nitrate species on Ti-containing particles observed by the APi-TOF. The effects of the sampling height and precursor feed rate were also investigated.