Evaluation of Particle Bounce in Various Collection Substrates to be Used as Vaporizer in Aerosol Mass Spectrometer

Abstract

The determination of the collection efficiency (CE) of particles during transport, vaporization, and ionization in the aerosol mass spectrometer (AMS), which uses vaporizer to evaporate non-refractory particles with subsequent ionization, is important for accurately quantifying the concentrations of chemical constituents. Particle bounce in the vaporizer can be considered as one of the most important parameters influencing the CE of particles. Substrates with various shapes (flat, cylindrical, reverse-conical, cup, trapezoidal, and reverse-T), materials (stainless steel, copper, tungsten, and molybdenum), pores with average sizes of 0.2, 1, 5, 20, and 100 mm, and mesh with a size of 79 mm, which can be a possible candidate for the vaporizer in the AMS, were constructed. Bounce fractions of sub-micrometer particles (polystyrene latex, oleic acid, and dioctyl phthalate) were determined using the differential mobility analyzer (DMA)-impactor technique under a constant impact velocity. For the porous substrate, the particle bounce fraction significantly decreased with increasing pore size and porosity, but there was an upper limit for the pore size above which the particle bounce fraction no longer decreased significantly (i.e., the rebounded particles successfully escaped from the pores). The mesh substrate also had a lower particle bounce fraction than the flat substrate. Among the tested materials, the copper substrate having the lowest hardness and elasticity had the lowest particle bounce fraction. In addition, the reverse-T shape substrate having more available surfaces for particle entrapment led to the reduction of particle bounce fraction. In terms of phase, the liquid particles had lower particle bounce fractions than the solid particles. Our results suggest that the vaporizer in the AMS should provide traps for multiple collisions of the rebounding particles with an appropriate porosity or mesh and should be made of lowhardness materials to minimize particle bounce.

Laboratory characterization of an aerosol chemical speciation monitor with PM2.5measurement capability

“…There is also uncertainty regarding the effect of the porous tungsten structure. Earlier (unreported) measurements in our lab showed that porosity helped reduce bounce, in agreement with the published results of Kang et al (2015). However, the porous structure can also provide a physical mechanism for collection of lower volatility material that can slowly decompose, desorb and in the case of organic matter (OM), form char.…”

“…Some previous studies have investigated the effects of vaporizer material and geometry on particle bounce. For example, a study by Kang et al (2015) showed that particle bounce decreases with increased porosity of the material and a cavity-shaped vaporizer, but no significant differences among reverse-conical, trapezoidal, and reverse-T shapes were observed. Following the work of Cross et al (2009), three types of single particle vaporization or detection events have been characterized following particle impaction on the standard vaporizer surface, (1) instantaneous and complete vaporization (Figure 4a), (2) partial vaporization, and (3) elastic bounce without any vaporization (null event) ( Figure 4b).…”

Evaluation of the new capture vaporizer for Aerosol Mass Spectrometers (AMS) through laboratory studies of inorganic species

“…However, d m = 300 nm (mobility diameter) of NaNO 3 corresponds to d va ∼ 680 nm (= d m × material density), which inadvertently exceeds the size range of 100 % lens transmission (d va ∼ 550 nm for a well-functioning standard lens, e.g. Knote et al, 2011). Thus a correction factor is required for correction of the NaNO 3 data for lens transmission losses, so that the corrected ratio can be interpreted as CE due to vapourizer bounce only.…”

Evaluation of the new capture vapourizer for aerosol mass spectrometers (AMS) through laboratory studies of inorganic species

“…A parameterization of CE for ambient particles based on composition has been used successfully in many environments (Middlebrook et al, 2012), but the remaining uncertainties on CE are thought to dominate the uncertainty of AMS concentration measurements (Bahreini et al, 2009) Efforts aiming to minimize the uncertainty of aerosol mass spectrometers have been conducted recently. Using the differential mobility analyser (DMA) coupled to an impactor to quantify the particle bounce fraction, Kang et al (2015) tested different metal materials, vapourizer porosities and shapes for particle bouncing and found copper, meshed/porous surface, as well as reversed T-shape materials best at reducing the particle bounce fraction. Each version can reduce the bounce fraction by around 10-50 % compared to the basic set up, indicating that CE can be improved through vapourizer design.…”