Practical limitations of aerosol separation by a tandem differential mobility analyzer–aerosol particle mass analyzer

Abstract

A cavity ring-down spectrometer and condensation particle counter were used to investigate the limitations in the separation of singly and multiply charged aerosol particles by a tandem differential mobility analyzer (DMA) and aerosol particle mass analyzer (APM). The impact of particle polydispersity and morphology was investigated using three materials: nearly-monodisperse polystyrene latex nanospheres (PSL); polydisperse, nearly-spherical ammonium sulfate (AS) and polydisperse lacey fractal soot agglomerates. PSL and AS particles were easily resolved as a function of charge. For fresh soot, the presence of multiply charged particles severely affects the isolation of the singly charged particles. In cases where the DMA-APM was unable to fully resolve the singly charged particles of interest, the peak mass deviated by up to 13 % leading to errors in the mass specific extinction cross section of over 100 %. For measurements of non-spherical particles, non-symmetrical distributions of concentration as a function of mass were a sign of the presence of multiply charged particles. Under these conditions, the effects of multiply charged particles can be reduced by using a second charge neutralizer after the DMA and prior to the APM. Dilution of the aerosol stream serves to decrease the total number concentration of particles and does not remove the contributions of multiply charged particles.

Light source effects on aerosol photoacoustic spectroscopy measurements

“…Soot particle mass versus RH is shown in Fig. 1a; error bars represent the 1σ width of the Gaussian-shaped distribution [18]. The multiple points at low (≈ 5 %) and high (≈ 80 %) RH represent data collected across multiple days; solid gray line corresponds to average mass at low RH.…”

“…Ethylene delivery was controlled with a mass flow controller set such that the diffusion flame ran lean in an effort to physically remove larger particles bearing charges greater than +1 at the selected mobility diameter (350 nm) by shifting the size distribution to smaller particles; the reader is referred to the discussion in Radney and Zangmeister (2016) [18] for more details regarding the separation of particles bearing multiple charges using a tandem differential mobility analyzer-aerosol particle mass analyzer (DMA and APM, respectively). Experiments were made across multiple days and the flow of ethylene was adjusted such that soot with similar size distributions and C ext were obtained for each experiment.…”

“…The cavity-ring down spectrometer is identical to the one described in Radney and Zangmeister (2016) [18]. Briefly, light from the CW laser was injected into a high-finesse optical cavity until saturation (≈ 100 μs).…”

Response of real-time black carbon mass instruments to mini-CAST soot

“…However, for loose soot agglomerates, the DMA-CPMA may not completely separate singly charged particles (Olfert et al 2007). When multiply charged particles are present, the output function distribution is positively skewed (Radney and Zangmeister 2015). Since the distributions were symmetric, we did not consider multiply charged particles to affect the determined m p and effective density.…”

Methodology for quantifying the volatile mixing state of an aerosol

“…This assumption is also common in DMA measurements (Stolzenburg and McMurry 2008). Alternatively, another charge neutralizer can be used before the CPMA to reneutralize the mobility-classified particles making it easier to distinguish the charge state (Radney and Zangmeister 2016).…”