A review of microfl uidic concepts and applications for atmospheric aerosol
science
Andrew R. Metcalf *, Shweta Narayan , and Cari S. Dutcher
Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota, USA
A B S T R A C T
Microfluidics is used in a broad range of applications, from biology and medicine to chemistry and polymer science, because this versatile platform enables rapid and precise repeatability of measurements and experiments on a relatively low-cost laboratory platform. Despite wide-ranging uses, this powerful research platform remains under-utilized by the atmospheric aerosol science
community. This review will summarize selected microfluidic concepts and tools with potential applications to aerosol science. Where appropriate, the basic operating conditions and tunable parameters in microfluidics will be compared to typical aerosol experimental methods. Microfluidics offers a number of advantages over larger-scale experiments; for example, the small volumes of sample required for experiments open a number of avenues for sample collection that are accessible to the aerosol community. Filter extraction, spot sampling, and particle-into-liquid sampling techniques could all be used to capture aerosol samples to supply microfluidic measurements and experiments. Microfluidic concepts, such as device geometries for creating
emulsions and developments in particle and droplet manipulation techniques will be reviewed, and current and potential microfluidic applications to aerosol science will be discussed.
Aggregation- and rarefaction-effects on particle mass deposition rates
by convective-diffusion, thermophoresis or inertial impaction: Consequences
of multi-spherule ‘ momentum shielding’
Daniel E. Rosnera and Pushkar Tandonb
aSol Reaction Engineering Group, Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut, USA; bCorning Inc., Corning, New York, USA
A B S T R A C T
At the same total spherule volume fraction in a gaseous mainstream, we predict the significant alteration of mass deposition rate attending extensive aggregation—illustrating our methods and results here not only for a mainstream of single-sized cluster aggregates, but also for coagulation-aged (near log-normal) distributions of large fractal-like aggregates (Ng D O(103), Df D 1.8 (DLCAs) or Df D 2.1 (RLCAs)) compared to isolated spherule deposition in the same environment. Because of their drastically different sensitivities to aggregation, we consider, sequentially, the particle transport mechanisms of either: ordinary isothermal convectivediffusion, thermophoresis (to a cooled solid target) or inertial impaction (without rebound). Using a rather general formulation (which incorporates Knudsen transition effects expected at elevated pressures) but neglecting direct “interception” effects, we find that for, say, Df D 2.1, N D O(103), Kn1: D mfp/R1 D 1, if convective-diffusion (with Sc >> 1) were the dominant
mechanism then mainstream aggregation would decrease expected mass deposition rates to much larger targets by somewhat more than one decade. However, for thermophoresis aggregation would increase deposition rates by approximately somewhat more than one decade, and, for, say, “eddy impaction” (in a fully turbulent duct flow) aggregation would increase deposition rates by as much as nearly 1.5 decades. Physically, these large aggregation enhancement-ratios for deposition by thermophoresis or particle Inertial Impaction are attributed to drag reduction (per spherule) associated with “momentum shielding”—analogous
to the aerodynamic advantages that birds, fish, bicyclists, runners,… experience when “in formation”. Using this approach, other impaction geometries and Knudsen number situations are also readily treated, as well as more “compact” even porous (Df D 3) aggregate populations. These predictive methods, illustrative results, and conclusions are expected to be useful to investigators seeking to maximize (or minimize) particle deposition rates on solid targets by exploiting control over the spherule aggregation process in the mainstream. As an important corollary, our methods also enable the quantitative deconvolution of aggregated aerosol sampling data, i.e., correcting for the systematic distortion (falsification) of sampled aggregate size distributions, pdfw(N), brought about by the size-dependent capture coefficients associated with momentum-shielding (nearly power-law: Smom » Nk) for each of the mechanisms considered here (C-D, T-P, or E-I; Section 5)). As demonstrated in Section 6.3, while we expect Log-Normal-type distributions to retain their shape, we predict the systematic correction factors needed to obtain the mean and median aggregate sizes (N and Ng) that must have existed in the mainstream (see Equation (30)). These correction factors become quite signifi- cant for each of the mechanisms (especially thermophoresis and impaction) when the mainstream aggregate size-spread is large (e.g., sg > ca. 2) and the pressure is high enough to cause Kn1 to drop to O(1). For completeness, the systematic consequences of the appreciable effective size of N >> 1 cluster aggregates, briefly discussed in Section 6.2, will need to be included, especially for the deposition of Df < 2 fractal-like aggregates on targets not much larger than the aggregates themselves (e.g., depth filter fibers,…). However, for capture by sufficiently large targets a noteworthy conclusion is that, of the distinct aerosol transport mechanisms considered here, isothermal convective-diffusion stands out as the only mechanism for which isolated spherules will deposit more efficiently than large-N cluster aggregates (when compared in the same flow environment
at the same mainstream spherule volume fraction).
Fine particles sampled at an urban background site and an industrialized coastal
site in Northern France— Part 2: Comparison of offl ine and online analyses
for carbonaceous aerosols
V. Crenna,*, A. Chakrabortya, I. Fronvala, D. Petitprezb, and V. Riffault a aSAGE–Departement Sciences de l’Atmosphere et Genie de l’Environnement, IMT Lille Douai, Universite de Lille, Lille, France; bPC2A, UMR
CNRS-Lille1, Villeneuve d’Ascq, France
A B S T R A C T
Particulate matter was sampled in Northern France during two summer and winter periods at both an urban background site (Douai, DO) and an industrialized coastal site (Grande-Synthe, GS). Ambient levels of particulate carbonaceous species and Polycyclic Aromatic Hydrocarbons (PAH) were measured by real-time measurements and via collection and analysis of offline filters (F). The comparison between online organic matter (OM) measured by an Aerosol Mass Spectrometer (AMS) and organic carbon (OC) determined by an offline thermal-optical method showed good linear trends in wintertime GS (r2 D 0.82 while only 0.50 in summer), and DO (r2 D 0.86 in summer and 0.92 in winter). However, significant differences were observed between analytical methods and sites with OCAMS/OCF ratios decreasing from 0.80 in DO during winter to 0.20 for GS in summer,
suggesting that a large part of OM could be in the PM1–PM2.5 fraction. The simultaneous measurements of Black Carbon (BC) and Elemental Carbon (EC) concentrations in PM2.5 were also well correlated at both sites with r2 D 0.61 –0.97 and slopes between 0.6 and 0.8. PAHs were analyzed in PM2.5 and also measured online by AMS in PM1 . Their wintertime concentrations were highly correlated in DO (r2 D 0.98) and to a lesser degree in GS (r2 D 0.67). r2 values determined for comparison between online and offline parameters (OC and PAHs) in GS were lower than in DO, probably due to a more complex aerosol composition and a higher variability of the physical and chemical properties resulting from the coastal situation and diversity of emission sources in the vicinity of GS.
Hydrogen-assisted spark discharge generated metal nanoparticles
to prevent oxide formation
R. T. Hallberga, L. Ludvigssona, C. Pregera, B. O. Meullera, K. A. Dicka,b, and M. E. Messinga
aSolid State Physics and Nano Lund, Lund University, Sweden; bCentre for Analysis and Synthesis, Lund University, Sweden
A B S T R A C T
There exists a demand for production of metal nanoparticles for today’s emerging nanotechnology. Aerosol-generated metal nanoparticles can oxidize during particle formation due to impurities in the carrier gas. One method to produce unoxidized metal nanoparticles is to first generate metal oxides and then reduce them during sintering. Here, we propose to instead prevent oxidation by introducing the reducing agent already at particle formation. We show that by mixing 5% hydrogen into the nitrogen carrier gas, we can generate single crystalline metal nanoparticles by spark discharge from gold, cobalt, bismuth, and tin electrodes. The non-noble nanoparticles exhibit signs of surface oxidation likely formed post-deposition when exposed to air. Nanoparticles generated without hydrogen are found to be primarily polycrystalline and oxidized. To demonstrate the advantages of supplying the reducing agent at generation, we compare to nanoparticles that are generated in nitrogen and sintered in a hydrogen mixture. For bismuth and tin, the crystal quality of the particles after sintering is considerably higher when hydrogen is introduced at particle generation compared to at sintering, whereas for cobalt it is equally effective to only add hydrogen at sintering. We propose that hydrogen present at particle generation prevents the formation of oxide primary particles, thus improving the ability to sinter the nanoparticles to compact and single crystals of metal. This method is general and can be applied to other aerosol generation systems, to improve the generation of size-controlled nanoparticles of non-noble metals with a suitable reducing agent.
Mass accommodation coeffi cients of fresh and aged biomass-burning emissions
Aditya Sinhaa, Rawad Salehb, Ellis S. Robinsonc, Adam T. Ahernc, Daniel S. Tkacikc, Albert A. Prestoc,
Ryan C. Sullivan c, Allen L. Robinsonc, and Neil M. Donahue c
aDepartment of Civil and Environmental Engineering, North Carolina State University, Raleigh, North Carolina, USA; bAir Quality and Climate Research Laboratory, College of Engineering, University of Georgia, Athens, Georgia, USA; cCenter for Atmospheric Particle Studies, Carnegie Mellon University, Pittsburgh, Pennsylvania, USA
A R T I C L E H I S T O R Y
Received 28 March 2017
Accepted 6 November 2017
A B S T R A C T
Most chemical transport models treat the partitioning of semi-volatile organic compounds (SVOCs) with the assumption of instantaneous thermodynamic equilibrium. However, the mass accommodation coefficients, a, of biomass-burning organic aerosol (BBOA) are largely unconstrained. During the FLAME-IV campaign, we thermally perturbed aged and fresh BBOA with a variable residence time thermodenuder and measured the resulting change in particle mass concentration to restore equilibrium. We used this equilibration profile to retrieve an effective a for components of BBOA that dictated this profile and found that the mass accommodation coefficients lie within the range 0.1 a 4 1. A simple plume dilution model shows a maximum of only a 7% difference between a dynamical and an instantaneous equilibrium partitioning model using our best-estimate value for a. This supports continued use of the equilibrium assumption to treat partitioning of biomass-burning emissions in chemical-transport models.
Numerical analysis of the dynamics of aerosol inertial collection and aggregation
on raindrops
Hui Zhua,b, Fengjiao Huaa, Yanming Kanga, and Yonghang Chena
aSchool of Environmental Science and Engineering, Donghua University, Shanghai, China; bDepartment of Built Environment and Energy Application Engineering, Guilin University of Aerospace Technology, Guilin, Guangxi, P. R. China
A B S T R A C T
A three-dimensional stochastic model is developed for predicting atmospheric aerosol collection and aggregation on the surface of a falling raindrop at its terminal velocity. Potential flow and viscous flow are assumed as the flow fields in the vicinity of the large and the small raindrops, respectively. The results show that hydrophobic coarse mode aerosols collected by either small raindrops ( dc < 100 mm) or large drops (dc > 100 mm) form aggregations on the surfaces of drops, and accumulation mode aerosols tend to be captured by the aggregations or hydrophobic coarse particles which have been collected by the drops, and this may significantly enhance the capability of the raindrop for fine aerosol collection. When the aggregation effect is considered in the calculation, fine aerosol efficiency can be promoted by one to two orders of magnitude. Therefore, fine particle collision efficiency by raindrops is underestimated by employing the classical dynamic theory which neglects the particle aggregation effect. However, the collection efficiency of coarse particles remains almost constant with the increase in the amount of particles collected by large drops, while there is only a slight increase in efficiency by small raindrops upon increasing in particle
concentration. This implies that the traditional limiting trajectory method can still be used for the calculation of coarse particle collection efficiencies by either small or large raindrops.
The initial stages of multicomponent particle formation during the gas phase
combustion synthesis of mixed SiO2/TiO2
Jiaxi Fanga, Yang Wanga,#,*, Juha Kangasluomab, Michel Attouib,c, Heikki Junninenb, Markku Kulmalab,
Tuukka Pet€aj€ab, and Pratim Biswasa,#
aAerosol and Air Quality Research Laboratory, Department of Energy, Environmental and Chemical Engineering, Washington University in St. Louis, St. Louis, Missouri, USA; bDepartment of Physical Sciences, University of Helsinki, Helsinki, Finland; cLISA Universite Paris Est, Diderot, Creteil, France
A B S T R A C T
The ability to properly scale the synthesis of advanced materials through combustion synthesis routes is limited by our lack of knowledge regarding the initial stages of particle formation. In flame aerosol reactors, the high temperatures, fast reaction rates, and flame chemistry can all play a critical role in determining the properties of the resulting nanomaterials. In particular, multicomponent systems pose a unique challenge as most studies rely on empirical approaches toward designing advanced composite materials. The lack of predictive capabilities can be attributed to a lack of data on particle inception and growth below 2 nm. Measurements for the initial stages of particle formation during the combustion synthesis of SiO2 and composite SiO2/TiO2
using an atmospheric pressure inlet time-of-flight mass spectrometer are presented. Both positively and negatively charged clusters can be measured and results show the presence of silicic acid species which grow through dehydration, hydrogen abstraction, and interactions with hydroxyl radicals. In the case of composite SiO2/TiO2 particle formation, new molecular species containing Ti atoms emerge. Tandem differential mobility analysis-mass spectrometry (DMA-MS) provided further insight into the size-resolved chemistry of particle formation to reveal that at each cluster size, further hydroxyl-driven reactions take place. From this we can conclude that previous assumptions on collisional growth from simple monomer species of SiO2 and TiO2 do not sufficiently describe the collisional growth mechanisms for particle growth below 2 nm.
Thermophoresis of a particle in a concentric cavity with thermal stress slip
Cheng Y. Li and Huan J. Keh
Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan
A B S T R A C T
The thermophoretic motion of a spherical particle situated at the center of a spherical cavity fi lled with a gaseous medium under a prescribed temperature gradient is studied analytically. The Knudsen number is small for the gas motion in the slip-fl ow regime, and the temperature jump, thermal creep, frictional slip, and particularly, thermal stress slip are allowed on the solid surfaces. After solving the equations of heat conduction and fl uid motion, an explicit formula for the migration velocity of the confi ned particle is obtained for different temperature conditions of the cavity with arbitrary values of the particle-to-cavity radius ratio and other parameters. Contributions from the thermoosmotic fl ow along the cavity wall and from the wall-corrected thermophoretic force to the particle velocity are equivalently important and can be linearly superimposed. With either or both of these contributions, the particle velocity in general is a decreasing function of the particle-to-cavity radius ratio and vanishes in the limit. The effects of the thermal stress slip at the solid surfaces to the migration velocity of the confi ned particle can be signifi cant and interesting, dependent on the thermal and interfacial properties of the particle and surrounding gas. The wall effect on the thermophoretic migration of the particle in a cavity is qualitatively different from that on the motion of the particle in a circular tube.
