Gas-particle Interactions Research Articles

Dynamic method to measure partition coefficient and mass accommodation coefficient for gas‒particle interaction of phthalates

The particle-gas partition coefficient (Kp) and mass accommodation coefficient (α) are two parameters characterizing the gas-particle interaction of semi-volatile organic compounds (SVOCs). Most of the available methods for measuring Kp require equilibrium at the chamber outlet, implying substantial preliminary testing. The need to separate gas-phase and particle-phase SVOCs also reduces the method accuracy. Few studies measuring α for indoor-related SVOCs are available, and they usually ignore the wall loss of SVOCs, resulting in reduced measurement accuracy. To overcome these deficiencies, we developed a dynamic method coupling a laminar flow tube chamber and an SVOC mass transfer model. Using the interaction between gas-phase di-2-ethylhexyl phthalate (DEHP) and (NH4)2SO4 particles (with diameters in the range of 10–600 nm) as an example, experiments were performed to evaluate the effectiveness and accuracy of the dynamic method. For the experimental conditions investigated (temperature = 25 °C and relative humidity <10%), gas-particle interaction between DEHP and (NH4)2SO4 particles is governed by surface adsorption because (NH4)2SO4 particles are in solid state. In this case, gas-particle partitioning should be characterized by the surface-area-normalized partition coefficient (KpA). KpA and α were measured to be 260 ± 80 m and 0.20 ± 0.05, respectively. Both are consistent with results reported in the literature. The method applicability for other SVOC-particle combinations and the improvement of method accuracy require further study.Copyright © 2019 American Association for Aerosol Research

Investigation of hydrodynamics of binary solids mixture spouted beds using radioactive particle tracking (RPT) technique

The hydrodynamic and mixing behavior of binary solids mixture spouted beds with particles of the same size but different densities have been investigated experimentally in gas–solid spouted beds for the first time, using an advanced non-invasive radioactive particle tracking (RPT) technique. The RPT experiments have been performed for different composition of binary mixture at different superficial gas velocity. The binary solids mixture used in the experiment consists of glass beads and steel particles of densities 2500 and 7400kg/m3, respectively. The RPT experiments were carried out for each one of the solids phases on an individual basis, and then the statistical averages of each flow fields are combined together to formulate the entire picture of the binary solids mixture flow field. It was found that segregation always takes place in the spout due to the dissimilar behavior between the different solids phases in terms of the solids velocity field and turbulent parameters. The results also demonstrated that for the hydrodynamics of binary solids mixture spouted beds which have particles of similar size but different densities, the particle–particle interaction plays an important role, but it does not dominate the gas–particle interaction, and each contributes to the extent of the mixing and segregation phenomena inside the bed. The results and findings of our work are valuable in understanding the hydrodynamics of the binary solids mixture encountered in the gas–solid spouted beds of TRISO nuclear fuel particles and provide benchmark data to validate computational fluid dynamics (CFD).

Unified theory for a sheared gas–solid suspension: from rapid granular suspension to its small-Stokes-number limit

A non-perturbative nonlinear theory for moderately dense gas–solid suspensions is outlined within the framework of the Boltzmann–Enskog equation by extending the work of Saha &amp; Alam (J. Fluid Mech., vol. 833, 2017, pp. 206–246). A linear Stokes’ drag law is adopted for gas–particle interactions, and the viscous dissipation due to hydrodynamic interactions is incorporated in the second-moment equation via a density-corrected Stokes number. For the homogeneous shear flow, the present theory provides a unified treatment of dilute to dense suspensions of highly inelastic particles, encompassing the high-Stokes-number rapid granular regime ($St\rightarrow \infty$) and its small-Stokes-number counterpart, with quantitative agreement for all transport coefficients. It is shown that the predictions of the shear viscosity and normal-stress differences based on existing theories deteriorate markedly with increasing density as well as with decreasing Stokes number and restitution coefficient.

Full-scale CFD investigation of gas-particle flow, interactions and combustion in tangentially fired pulverized coal furnace

Investigations suggest the need for better understanding of reactive gas-particle turbulent flow phenomena in full-scale energy systems. Numerical study was done in 350 MWe utility boiler tangentially fired furnace to clarify selected issues, such as turbulence modulation, particles dispersion, energy transfer between phases, combustion process and flame, by using an in-house developed combustion code. Numerical experiments demonstrated remarkable complexity of flow and interphase exchange. Maximal decrease in average turbulence kinetic energy of 33% due to dispersed phase was predicted for representative monodispersed coal; augmentation obtained for large particles could become attenuation due to the particles size change during combustion. Grinding fineness of polydispersed coal affected the flow, combustion and flame considerably. Fine grinding (R90 = 48.40%) provided ascending flame, higher furnace exit temperature and decrease in turbulence energy, compared with coarse grinding (R90 = 73.85%). Combustion of each particle size class of coal is completed at different vertical levels, influencing the flame position. Diagrams based on numerical predictions were proposed to enable efficient estimations of combustion and flame characteristics in the case-study furnace, for various coal qualities and mass fractions and changed distributions of coal particle size classes over the burner tiers, while necessity for further investigation was pointed out as well.

Influence of Particle Surface Area Concentration on the Production of Organic Particulate Matter in a Continuously Mixed Flow Reactor.

Organic particulate matter (PM) was produced at different particle surface area concentrations S in a continuously mixed flow reactor (CMFR). The apparent PM yield from the dark ozonolysis of α-pinene increased from 24.5 ± 0.7% to 57.1 ± 0.6% for an increase in S from 0.55 to 2.87 × 103 μm2·surface cm-3·volume. The apparent yield saturated for S > 2.1 × 103 μm2 cm-3. There was hysteresis in the apparent yield for experiments of increasing compared to decreasing S. The relative timescales of gas-particle interactions, gas-wall interactions, and thereby particle-wall cross interactions could explain the results. The PM carbon oxidation state and oxygen-to-carbon atomic ratio decreased from -0.19 to -0.47 and 0.62 to 0.51, respectively, for increasing S, suggesting that greater partitioning of semivolatile organic species into the PM contributed to the increased PM yield. A thorough understanding of the role of gas-wall interactions on apparent PM yield is essential for the extension of laboratory results into predictions of atmospheric PM production, and comparative results from CMFRs and batch reactors can be informative in this regard.

Planetesimal Population Synthesis: Pebble Flux-regulated Planetesimal Formation

Abstract We propose an expression for a local planetesimal formation rate proportional to the instantaneous radial pebble flux. The result—a radial planetesimal distribution—can be used as an initial condition to study the formation of planetary embryos. We follow the idea that one needs particle traps to locally enhance the dust-to-gas ratios sufficiently, such that particle gas interactions can no longer prevent planetesimal formation on small scales. The locations of these traps can emerge everywhere in the disk. Their occurrence and lifetime is subject to ongoing research; thus, here they are implemented via free parameters. This enables us to study the influence of the disk properties on the formation of planetesimals, predicting their time-dependent formation rates and the location of primary pebble accretion. We show that large α-values of 0.01 (strong turbulence) prevent the formation of planetesimals in the inner part of the disk, arguing for lower values of around 0.001 (moderate turbulence), at which planetesimals form quickly at all places where they are needed for proto-planets. Planetesimals form as soon as dust has grown to pebbles (mm to dm) and the pebble flux reaches a critical value, which is after a few thousand years at 2–3 au and after a few hundred thousand years at 20–30 au. Planetesimal formation lasts until the pebble supply has decreased below a critical value. The final spatial planetesimal distribution is steeper compared to the initial dust and gas distribution, which helps explain the discrepancy between the minimum mass solar nebula and viscous accretion disks.

DEM/CFD Simulations of a Pseudo-2D Fluidized Bed: Comparison with Experiments

The present work investigates the performance of a mesoscopic LaGrangian approach for the prediction of gas–particle flows under the influence of different physical and numerical parameters. To this end, Geldart D particles with 1 mm diameter and density of 2500 kg/m 3 are simulated in a pseudo-2D fluidized bed using a Discrete Element Method (DEM)/Large-Eddy Simulation (LES) solver called YALES2. Time-averaged quantities are computed and compared with experimental results reported in the literature. A mesh sensitivity analysis showed that better predictions regarding the particulate phase are achieved when the mesh is finer. This is due to a better description of the local and instantaneous gas–particle interactions, leading to an accurate prediction of the particle dynamics. Slip and no-slip wall conditions regarding the gas phase were tested and their effect was found negligible for the simulated regimes. Additional simulations showed that increasing either the particle–particle or the particle–wall friction coefficients tends to reduce bed expansion and to initiate bubble formation. A set of friction coefficients was retained for which the predictions were in good agreement with the experiments. Simulations for other Reynolds number and bed weight conditions are then carried out and satisfactory results were obtained.

Experimental and Eulerian-Lagrangian-Lagrangian study of binary gas-solid flow containing particles of significantly different sizes

The coexistence of large particles (such as biomass or coal particles) and fine particles in gas-solid flow is common. In this study, an Eulerian-Lagrangian-Lagrangian method (EMMS-DPM-DEM) was developed to simulate the binary gas-solid flow containing particles of significantly different sizes, where fine particles were simulated using coarse grained discrete element method and the motion of large particles was captured using discrete element method. Experiments on density segregation of large particles were also carried out to validate the developed simulation method. It was shown that EMMS-DPM-DEM can predict the density segregation process of large particles reasonably well. Furthermore, the density segregation mechanism of large particles in a dense fluidized bed was explained at different scales, i.e., the Archimedes principle at macroscale, the entrainment and the global circulation pattern due to bubble motions at mesoscale and the particle-particle and gas-particle interactions at microscale. The method proposed here can be directly used to study the hydrodynamics of biomass thermochemical conversion in fluidized beds, although it was validated using segregation experiments of coal particles.

Influence of gas flow oscillation on the efficiency of particulate solids treatment in a circulating fluidized bed

Apparatuses with fluidized bed are widely used in the solid fuel energetic, chemical and other industries. In particular, fluidized bed furnaces play the increasing role for thermo-chemical treatment and burning the solid fuels including bio-fuels. The use of pulsating gas flows is one of the possible ways to intensify the process of gas-particle interaction. However, the efficiency of the way is not always obvious, and new approaches are needed for preliminary estimation of its advantages. The method of mathematical modeling is used to solve the above problem. The model is based on the set of differential equations of a single particle motion in the upstream gas flow with variable with time velocity, and with variable with time particle properties due to this or that thermo-physical or chemical process of gas-particle interaction. The study of influence of the process parameters on its efficiency is done by numerical methods. The model of the effect of oscillations in the gas flow on the efficiency of particle processing in a circulating fluidized bed has been developed. The necessary time for enough particle treatment is compared for different amplitude and frequency of gas flow oscillation. The most considerable gain in time is being reached for the strong dependence of the reaction rate constant on the velocity of flow around the particle. In this case the optimum frequency of oscillations exists that give the maximum gain in time. The growth of the amplitude leads to the monotonic increase of the gain in time, or gives the optimum amplitude. Transition to the pulsating gas flow in a circulating fluidized bed reactor leads to considerable gain in time of particle treatment only in the case of strong dependence of the reaction rate constant on the velocity of flow around the particle. In this case the optimum frequency and amplitude of oscillation exist that gives the maximum gain in time.

Numerical Study on Particle–Gas Interaction Close to the Substrates in Thermal Spray Processes with High-Kinetic and Low-Pressure Conditions

In thermal spray processes, the interaction between the gas jet and the particulate feedstock can affect the coating build-up mechanisms considerably. In particular under high-kinetic and low-pressure conditions, small particles are subjected to rapid deflection and velocity changes close to the substrate. In this work, numerical studies were carried out to investigate the interaction between gas and particles in the substrate boundary layers (BL). Typical conditions for suspension plasma spraying (SPS), plasma spray-physical vapor deposition (PS-PVD), and aerosol deposition (AD) were taken as a basis. Particular importance was attached to the consideration of rarefaction and compressibility effects on the drag force. Typical Stokes numbers for the different thermal spray processes were calculated and compared. Possible effects on the resulting coating build-up mechanisms and microstructure formation are discussed. The results show that just for larger particles in the SPS process the laminar flow attached to the particles begins to separate so that the drag coefficients have to be corrected. Furthermore, slip effects occur in all the investigated processes and must be considered. The comparison of calculated Stokes numbers with critical values shows that there is a disposition to form columnar microstructures or stacking effects depending on the particle size for PS-PVD and SPS, but not for AD.

Nanoparticle–nanobubble interactions: Charge inversion and re-entrant condensation of amidine latex nanoparticles driven by bulk nanobubbles

HypothesisThe stability of colloidal suspensions can be influenced by supersaturation of the supporting electrolyte with gas. It has been proposed that this effect can be attributed to the formation of nanobubbles on the surface of the colloidal particles, in turn influencing DLVO forces. While previous interpretations have focused primarily on van der Waals interactions, probing positively charged particles can provide complementary insight into electrostatic interactions. ExperimentsHigh-power water electrolysis creates an aqueous solution supersaturated with oxygen and hydrogen. We study the ability of this solution to influence the electrophoretic properties of positive nanoparticles as a function of the particle-gas ratio. Both the ζ-potential and the effective hydrodynamic diameter of the resulting nanoentities were studied using dynamic light scattering for a range of nanoparticle sizes. FindingsGas-saturated solution interacts strongly with positive nanoparticles by decreasing and ultimately reversing the sign of their ζ-potential, which we attribute to the nucleation of negatively charged bubbles at the solid-liquid interface. This leads to re-entrant condensation of the particles near their point of zero charge, as directly observed via an increase in hydrodynamic diameter and macroscopic aggregation. These results indicate that modulation of electrostatic interactions can be the dominant mechanism for gas-particle interactions in these systems.

A novel approach to correlate heat transfer and pressure fluctuation in gas–solid spouted bed

Statistical analysis of pressure fluctuations in spouted beds has been used as a well-established diagnostic tool to determine bed and flow characteristics because of its smooth operation. However, in many recent and conventional applications of spouted beds such as drying, coal gasification, catalytic conversion, biomass treatment, and chemical vapor deposition, direct estimation of the heat transfer rate from the solid bed to the gas or vice versa has proven to be difficult. A variance and spectral analysis of pressure fluctuation is extended here to characterize the heat transfer phenomena in a spouted bed. In the present study, zirconia and alumina were used as the bed materials, and argon and nitrogen were used as the spouting gases. Experiments were conducted at various heating rates for different superficial gas velocities for a range of temperatures up to 300°C. Significant changes in the gas density and viscosity with different extents of heat transfer were observed to affect the momentum diffusivity and gas–particle interaction, which in turn led to local pressure fluctuations, causing the bed to behave differently. In the present work, a novel approach is proposed to establish a link between local pressure fluctuation and the extent of heat transfer in the bed. This method shows potential for correlation of the statistics of pressure fluctuation with the thermal properties of individual solids and gases. Thus, the technique can be extended to many industrial applications for the indirect estimation of the extent of heat transfer and prediction of unknown thermal properties of products in solids or gases.

Two-dimensional CFD–DEM simulation of vertical shock wave-induced dust lifting processes

This paper conducts a numerical study of particle dispersion by vertical shock waves using a combination of computational fluid dynamics and the discrete element method (DEM). The Magnus force is important for particle dispersion, and the DEM approach can consider the rotation of individual particles and provide a detailed analysis of particle–particle and particle–wall interactions. Simulations are conducted and the results are compared with those of previous experiments, showing that the models can capture the particle dispersion process and shock wave geometry inside the mixed gas–particle region. During the particle dispersion process, the contact force due to the particle–particle interactions causes the particles to move upward after the shock wave passes and then decreases rapidly due to the low collision frequency in the particle cloud. The Magnus force is initially lower and has little effect until the contact force decreases, when it becomes dominant and maintains the particles’ upward movement. It is mainly driven by the gas rotation above the dust layer, and the effect of particle rotation is relatively small in comparison. The gas velocity gradient above the dust layer in the particle dispersion region is caused by gas–particle interactions. The particle dispersion region becomes wider and thinner over time, meaning that the gas velocity gradient above the dust layer becomes small and hence that the Magnus force (which is driven by the gas rotation) also becomes small. In contrast, the downward drag force remains constant during the vertical motion because it is primarily affected by the velocity difference between the particles and the gas. This means that the drag and Magnus forces eventually balance, and the particles stop rising.

Unstructured continuum–kinetic solver with fluid–particle interaction for in-space propulsion contaminant dispersal

A new parallel generalized unstructured grid solver for prediction of rarefied gas dynamics is presented. The method is based on the direct numerical solution of the semi-classical Boltzmann equation using a discrete velocity method (DVM) and supports accurate prediction of non-continuum flows in near-vacuum rarefied flow regimes. For in-space propulsion applications which involve mixed continuum–rarefied environments, the flow regimes are solved separately through the use of DVM and hybrid RANS/LES solution procedures, respectively, on the same computational domain while interacting with one another. The solver also supports coupled Lagrangian particle transport and coalescence/breakup modeling to enable prediction of plume flow impingement and contaminant dispersal through mixed continuum–rarefied environments around spacecraft. Such simulation capabilities are essential for the prediction of spacecraft environments concerning plume impingement pressure, heating, and contamination on spacecraft components such as solar panels and sensitive instruments. Example applications include venting and plume flow environments from spacecraft main engines and attitude control thrusters firing in near-vacuum conditions. The framework upon which the solvers are developed is introduced along with a detailed description of the direct Boltzmann solution and collision integral evaluation procedures. The accuracy of standalone kinetic and coupled continuum–kinetic predictions is validated by comparison against experimental data for shock structure using various different intermolecular interaction models. Results are presented to demonstrate the merits of the capability for prediction of high-speed gas–particle interaction through complex moving shock systems, and thruster plume flow impingement with liquid droplet tracking in near-vacuum conditions.

Thermophoretic force on nonspherical particles in the free-molecule regime.

The present paper is devoted to studying the thermophoresis of a nonspherical convex particle suspended in a gas with nonuniform temperature distribution in the free-molecule regime. Based on the gas kinetic theory and the assumption of a rigid-body collision for the gas-particle interaction, analytical expressions for the thermophoretic forces are obtained for several typical nonspherical bodies, including cylinders, spheroids, needles, disks, and cuboids. The orientation dependences of the thermophoretic forces and thermophoretic velocities are evaluated based on these expressions. It is found that the influence of the pitching effect of the nonspheres can be significant. The expressions for the orientation-averaged thermophoretic forces are also obtained under the assumption of a uniform particle orientation distribution.

Investigation of erosion behaviors of sulfur-particle-laden gas flow in an elbow via a CFD-DEM coupling method

In the production and gathering process of natural gas with the high sulfur content, due to the variations in temperature, pressure, and other factors, sulfur dissolved in the gas may be precipitated as a solid particle in the gathering pipeline. Sulfur particles carried by high-speed flow impact elbow of pipelines, thus causing equipment malfunctions and even failures. In this study, a CFD-DEM-based erosion prediction model for gas-particle two-phase flow was proposed based on the consideration of the gas-particle, particle-particle and particle-wall interactions. The effects of secondary flow, vortices and particle trajectories on rare erosion scars were investigated. In addition, four kinds of polyhedral particles were modeled based on DEM framework to simulate erosion behaviors based on the consideration of particle shapes. The results indicate the V-shaped erosion scar is caused by the secondary collision. Two adjacent obvious erosion scars on the upper part of V-shaped scar are caused by direct collisions and sliding collisions. From the inlet to the outlet of the elbow, the turbulence intensity near the two sides of the wall increases and the secondary flows and vortices appear. Particle trajectories are affected by complex flow fluid, which causes rare erosion scars on the side walls of the downstream straight pipe near the elbow outlet. With the increase in the particle sphericity, the erosion rate decreases firstly and then increases. When the sphericity is <0.77, the erosion rate is mainly affected by the impact velocity and impact angle; when the sphericity is larger than 0.77, the influence of impact concentration on the erosion rate is more obvious.

Visualization of Projectile Flying at High Speed in Dusty Atmosphere

Considering a spacecraft that encounters particle-laden environment, such as dust particles flying up over the regolith by the jet of the landing thruster, high-speed flight of a projectile in such environment was experimentally simulated by using the ballistic range. At high-speed collision of particles on the projectile surface, they may be reflected with cracking into smaller pieces. On the other hand, the projectile surface will be damaged by the collision. To obtain the fundamental characteristics of such complicated phenomena, a projectile was launched at the velocity up to 400 m/s and the collective behaviour of particles around projectile was observed by the high-speed camera. To eliminate the effect of the gas-particle interaction and to focus on only the effect of the interaction between the particles and the projectile's surface, the test chamber pressure was evacuated down to 30 Pa. The particles about 400μm diameter were scattered and formed a sheet of particles in the test chamber by using two-dimensional funnel with a narrow slit. The projectile was launched into the particle sheet in the tangential direction, and the high-speed camera captured both projectile and particle motions. From the movie, the interaction between the projectile and particle sheet was clarified.

Global distribution of particle phase state in atmospheric secondary organic aerosols

Secondary organic aerosols (SOA) are a large source of uncertainty in our current understanding of climate change and air pollution. The phase state of SOA is important for quantifying their effects on climate and air quality, but its global distribution is poorly characterized. We developed a method to estimate glass transition temperatures based on the molar mass and molecular O:C ratio of SOA components, and we used the global chemistry climate model EMAC with the organic aerosol module ORACLE to predict the phase state of atmospheric SOA. For the planetary boundary layer, global simulations indicate that SOA are mostly liquid in tropical and polar air with high relative humidity, semi-solid in the mid-latitudes and solid over dry lands. We find that in the middle and upper troposphere SOA should be mostly in a glassy solid phase state. Thus, slow diffusion of water, oxidants and organic molecules could kinetically limit gas–particle interactions of SOA in the free and upper troposphere, promote ice nucleation and facilitate long-range transport of reactive and toxic organic pollutants embedded in SOA.

Micromechanical analysis of flow behaviour of fine ellipsoids in gas fluidization

The macro-scale flow behaviour of granular materials in gas fluidization is governed by particle-particle and gas-particle interactions, which are affected significantly by particle size and shape. Understanding the micro-scale flow structure of fine non-spherical particles is essential for process design, optimisation and control. In this work, the combined approach of computational fluid dynamics for gas phase and discrete element method for particles is used to study flow and force structures of fine ellipsoids in gas fluidization. The results reveal that fine particles have vortex flow structure in fluidized beds, and the vortex flow becomes more significant particularly for oblate particles. The microscopic structure analysis demonstrates that when aspect ratio deviates from 1.0, the packed beds could experience compaction, and in expanded beds, the bed expansion ratio increases. The effect of particle size is investigated, showing that with the decrease of particle size, the mean coordination number decreases due to more significant role of van der Waals force. Ellipsoids exhibit higher orientation order which varies greatly with gas velocity and particle size. In fluidized beds, ellipsoids tend to flow in small projected area to the fluid flow direction to reduce the flow resistance. The bed expansion criteria established in the literature are confirmed still valid for ellipsoids, but the gas velocity range for expanded bed interval is much wider than spheres.

Surface area-dependence of gas-particle interactions influences pulmonary and neuroinflammatory outcomes.

BackgroundDeleterious consequences of exposure to traffic emissions may derive from interactions between carbonaceous particulate matter (PM) and gaseous components in a manner that is dependent on the surface area or complexity of the particles. To determine the validity of this hypothesis, we examined pulmonary and neurological inflammatory outcomes in C57BL/6 and apolipoprotein E knockout (ApoE−/−) male mice after acute and chronic exposure to vehicle engine-derived particulate matter, generated as ultrafine (UFP) and fine (FP) sizes, with additional exposures using UFP or FP combined with gaseous copollutants derived from fresh gasoline and diesel emissions, labeled as UFP + G and FP + G.ResultsThe UFP and UFP + G exposure groups resulted in the most profound pulmonary and neuroinflammatory effects. Phagocytosis of UFP + G particles via resident alveolar macrophages was substantial in both mouse strains, particularly after chronic exposure, with concurrent increased proinflammatory cytokine expression of CXCL1 and TNFα in the bronchial lavage fluid. In the acute exposure paradigm, only UFP and UFP + G induced significant changes in pulmonary inflammation and only in the ApoE−/− animals. Similarly, acute exposure to UFP and UFP + G increased the expression of several cytokines in the hippocampus of ApoE−/− mice including Il-1β, IL-6, Tgf-β and Tnf-α and in the hippocampus of C57BL/6 mice including Ccl5, Cxcl1, Il-1β, and Tnf-α. Interestingly, Il-6 and Tgf-β expression were decreased in the C57BL/6 hippocampus after acute exposure. Chronic exposure to UFP + G increased expression of Ccl5, Cxcl1, Il-6, and Tgf-β in the ApoE−/− hippocampus, but this effect was minimal in the C57BL/6 mice, suggesting compensatory mechanisms to manage neuroinflammation in this strain.ConclusionsInflammatory responses the lung and brain were most substantial in ApoE−/− animals exposed to UFP + G, suggesting that the surface area-dependent interaction of gases and particles is an important determinant of toxic responses. As such, freshly generated UFP, in the presence of combustion-derived gas phase pollutants, may be a greater health hazard than would be predicted from PM concentration, alone, lending support for epidemiological findings of adverse neurological outcomes associated with roadway proximity.Electronic supplementary materialThe online version of this article (doi:10.1186/s12989-016-0177-x) contains supplementary material, which is available to authorized users.