Filtration characteristics of a fibrous filter pretreated with anionic surfactants for monodisperse solid aerosols

This study elucidates the effects of using an anionic surfactant-pretreated filter as an electret filter on the aerosol penetration. Anionic surfactants (sodium oleate, SO; sodium dodecyl sulfate, SDS) were used to pretreat polypropylene fibrous filters to make them negatively charged. Various factors, including the particle size (0.05–$0.5\mathrm{\mu }\mathrm{m}$), the aerosol charge state (Boltzmann-equilibrium charge, neutral and singly charge), the face velocity (0.1, 0.3, 0.5 and $1.0\mathrm{m}/\mathrm{s}$), the species of aerosol (sodium chloride, NaCl and aluminum oxide, Al${}_2$O${}_3$), the relative humidity (RH 30% and 70%) and the concentration of surfactant (0.01, 0.05 and 0.08 M) were considered to evaluate their effects on the aerosol collection characteristics.Experimental results from our study demonstrate that the aerosol penetrations through anionic surfactant-pretreated filters (ASPFs) were lower than through the untreated filter. The $0.3\mathrm{\mu }\mathrm{m}$-aerosol penetrations through an untreated, 0.01, 0.05 and 0.08 M SO-pretreated filters with Boltzmann-equilibrium charged aerosol were about 78%, 53%, 45% and 40%. The penetration of 0.3-$\mathrm{\mu }\mathrm{m}$ aerosol of 0.01, 0.05 and 0.08 M SDS-pretreated filters with Boltzmann-equilibrium charged aerosol were, 62%, 58% and 51%. Pretreating the filter with anionic surfactant did not change the structure of the filter and the mechanical capture force. The electric field measured by an electrofieldmeter of the ASPFs was larger than that of untreated filter obviously. These findings imply that pretreatment with anionic surfactant made the filters charged. The surface charge of the ASPFs increased with the anionic-surfactant concentration. The increase of the collection efficiency of the ASPFs is in the range of 5–30%, and that depends on the aerosol size and surfactant concentration. Comparing between the aerosol penetration through the ASPFs with singly charged aerosol and that through the untreated filter with neutral aerosol indicates that the penetration through the ASPFs with singly charged aerosol decreases by a factor of 1.3–12.5. In contrast with the penetration through the ASPFs to that through the untreated filter with neutral aerosol, the penetration reduction factor of the ASPFs is in the range of 1.1–1.9. As the results shown, the Coulombic capture force is dominant for the smaller aerosol $(<0.1\mathrm{\mu }\mathrm{m})$ and the dielectrophoretic capture mechanism works at larger aerosol size $(>0.2\mathrm{\mu }\mathrm{m})$. Additionally, the penetrations through the ASPFs increased with the face velocity. ASPFs performed better when the tested aerosol had a larger dielectric constant $(k)$ such as the penetration through 0.05 M SO-pretreated filter with $0.3\mathrm{\mu }\mathrm{m}{\mathrm{Al}}_2{\mathrm{O}}_3$ aerosol $(k=9.6)$ was 47% and that with NaCl aerosol $(k=5.9)$ was about 53%. RH has no effect on the aerosol penetration through the ASPFs.     

The Aethalometer calibration and determination of iron concentration in dust aerosols

This paper shows the use of the instrumental neutron activation analysis (INAA) technique to calibrate Aethalometer instruments’ response to iron oxides present in Saharan dust aerosol. The five samples selected for this calibration were collected with a seven-wavelength Aethalometer (model AE31) at the summit of Pico mountain (2225 m ASL) in the Azores Islands. These samples correspond to measurements taken between the 31 October and 5 November, 2001. Analysis of these samples by the INAA technique gave a total mass of $14.84\pm 0.70\mathrm{\mu }\mathrm{g}$ (Fe). Correlation of the analytical results with the optical measurements allowed the determination of the elemental iron calibration constant, $K_{\mathrm{Fe}}$ ($0.234\pm 0.022\mathrm{\mu }{\mathrm{m}}^4{\mathrm{m}}^2{\mathrm{g}}^{-1}$), which can be used in the determination of iron concentrations from multi-wavelength Aethalometer measurements. As an example of this, we used Aethalometer measurements during the 2001 event to calculate hourly average dust iron, $〈C_{\mathrm{Fe}}〉$, concentrations, which range from 0.00 and $1.77\mathrm{\mu }\mathrm{g}(\mathrm{Fe}){\mathrm{m}}^{-3}$.     

Particle loss in a critical orifice

Particle deposition in different regions of a critical orifice assembly was studied numerically and experimentally. The investigated orifice is an O’Keefe E-9 (O’Keefe Control Co.) orifice whose diameter is 0.231 mm and critical flow rate is 0.455 slpm. The orifice assembly has an inlet tube $(\text{inner diameter}=10.4\mathrm{mm},\text{length}=90\mathrm{mm})$ and outlet tube $(\text{inner diameter}=6.2\mathrm{mm},\text{length}=60\mathrm{mm})$. In the numerical study, axisymmetric, laminar flow field of the orifice assembly was obtained first by solving the Navier–Stokes equations. The diffusion loss of nanoparticles was then calculated by solving the convection–diffusion equation. Inertial impaction and interception loss of 2–$10\mathrm{\mu }\mathrm{m}$ particles was calculated by tracing particle trajectories in the flow field. In the experimental study, monodisperse NaCl (20–800 nm in aerodynamic diameter) and fluorescein-containing oleic acid (2–$10\mathrm{\mu }\mathrm{m}$ in aerodynamic diameter) particles were used to test particle loss in both diffusion- and inertial impaction-dominated regimes. The numerical results were compared with the experimental data and good agreement was obtained with the maximum deviation smaller than 10.4%.     

Collection of airborne spores by circular single-stage impactors with small jet-to-plate distance

Most of the commonly used bioaerosol samplers are single-stage impactors that meet the conventional Marple's design criteria: their non-dimensional jet-to-plate distance, $S/W$, is greater than the established threshold (1.5 for rectangular nozzles and 1 for the circular ones). Recent studies have shown that these samplers underestimate the concentration of airborne fungal spores because their cut-off size is about $2.5\mathrm{\mu }$m (Air-O-Cell and Burkard samplers) or greater while some fungal species produce spores of ca. 1.8–$2.5\mathrm{\mu }$m in aerodynamic diameter. In this study, we evaluated the single-stage circular-jet impactors with very small jet-to-plate distances ($S/W⪡1$). The laboratory and field data obtained with test particles of different sizes and different origin (biological and non-biological) demonstrated the feasibility of these “incorrectly designed” impactors for the spore collection and total enumeration (viable + non-viable spores). A decrease in the jet-to-plate distance resulted in a critical decrease of the impactor's cut-off size ($d_{50}$): from $2.5\mathrm{\mu }$m to about $1\mathrm{\mu }$m. This reduction of cut-off size makes such an impactor efficient for collecting spores of all fungal species ($⩾1.8\mathrm{\mu }$m) and even some bacterial species ($⩾1\mathrm{\mu }$m). Since the spore surface density across the circular deposit area was non-uniform, three sample reading procedures were evaluated: the entire area count, random partial count, and a partial count on a rectangular “diametric slice”. The collection efficiency data suggested that a relatively small jet-to-plate distance is likely to result in excessive shear forces in the impaction zone, thus enhancing the spore deaggregation and bounce. The coefficient of inter-sample variation of the field samples, collected by commercially available impactors with $S/W\approx 0.099$, did not exceed 50% for the total spore count. The highest variability was observed for Arthrospores, which were more aggregated than other types of fungi.     

Study on CO2 reforming of methane to syngas over Al2O3–ZrO2 supported Ni catalysts prepared via a direct sol–gel process

Ni-based catalysts supported on Al₂O₃–ZrO₂ (Ni/Al₂O₃–ZrO₂) were prepared by a direct sol–gel process with citric acid as the gelling agent. The evaluation of the catalyst prepared for methane reforming with CO₂ was carried out with thermal gravimetric analysis (TGA), infrared spectroscopy (IR), X-ray diffraction (XRD), microscopy analyses (SEM and TEM), temperature-programmed reduction (TPR) and in a micro-reactor system. The catalytic performance for CO₂ reforming of methane to synthesis gas in a continuous-flow micro-reactor under atmospheric pressure was investigated. TGA, IR, XRD and microscopy analyses show that the Ni particles have a nanostructure of around $5\mathrm{nm}$ and are uniformly dispersed on the Al₂O₃–ZrO₂ support, which exists as an amorphous phase. Catalytic tests using CO₂ reforming of methane to synthesis gas show that the catalytic activity increases with increasing metal loading, and the 20Ni/Al₂O₃–ZrO₂ (0.2 Ni/Al molar ratio) catalyst has excellent activity and stability, compared with that of the Al₂O₃ supported Ni catalyst, with 91.9% conversion of CO₂ and 82.9% conversion of CH₄ over $50\mathrm{h}$ at $1073\mathrm{K}$, atmospheric pressure, hourly space velocity of $11,200\mathrm{ml}{\mathrm{gcat}}^{-1}{\mathrm{h}}^{-1}$ and CH₄:CO₂:N₂ of 2:2:1. The excellent catalytic activity and stability is attributed to the very highly and uniformly dispersed small metallic Ni particles, the reducibility of the Ni oxides and an interaction between metallic Ni particles and the support Al₂O₃–ZrO₂.