Mobility Size Distributions Research Articles

How big is a boulder? The importance of boulder definition choice in earth science research and river management

AbstractBoulders are globally widespread and influence processes across many landscapes including hillslopes, coasts, rivers and extra‐terrestrial settings. Boulders are described as particles, sufficiently large, that they have a disproportionate effect on the surrounding landscape. Moving beyond this conceptual definition, however, requires a somewhat arbitrary decision of how to define a minimum boulder size. The implications of boulder definition on study findings are rarely considered. We investigate the suitability of five boulder definitions, two based on fixed sizes: (1) 0.26 m and (2) 1 m, and three definitions which vary based on system characteristics: (3) grain mobility, (4) grain protrusion and (5) surface grain size distribution (> 84th percentile, D84). We consider the impact of definition on calculated boulder metrics, and, for the >1m and >D84 definitions, their association with channel and catchment characteristics across 20 boulder‐bed streams in northern Sweden. We also surveyed river managers responsible for restoring these rivers, to gain a practitioner insight on boulder size definition. We found that boulder definition matters; for metrics relating to the number or density of boulders, the >D84 and >1m size definitions were negatively correlated. Surveys indicated the importance of communicating boulder definition. We conclude that, whilst the best choice of boulder size definition will vary based on the questions of interest and techniques employed, evaluating the implications of the chosen boulder size definition and communicating the reasoning behind boulder definition choice is crucial.

Effective density and packing of compacted soot aggregates

The mass concentration of soot aggregates is often estimated from mobility size distributions using a mobility-based effective density, ρeff. This ρeff changes with aggregate morphology. In particular, the ρeff of a soot core increases when it becomes compacted by the surface tension of condensing coatings, such as combustion-related vapours, secondary aerosols, and cloud water. The extent of this compaction is a function of coating volume, up to an asymptotic limit of complete compaction. While complete compaction has previously been shown to correspond to a universal, scale-invariant packing factor for sufficiently large aggregates, it has not previously been explicitly quantified. Here, we critically reanalyze multiple datasets compiled from the literature on the ρeff of completely compact soot. We show that, regardless of the coating material, soot aggregates generally become completely compacted following a 5-fold increase in volume. The final aggregate shape is more simply described by ρeff than by mobility diameter or shape factor. Below 140 nm diameter (about 20 aggregate monomers), the compacted ρeff obeys a power law; above 140 nm, it reaches a constant value of 651 ± 8 kg m−3. This ρeff is 3 times larger than that of freshly-produced soot at 140 nm. We provide a parameterization of the compacted ρeff for the estimation of soot mass concentration after coating, or, conversely, for use as a benchmark to estimate the extent of aggregate restructuring. Our parameterization can be easily adapted to other nanoparticle aggregates whose material density is known.

Pushing nano-aerosol measurements towards a new decade – technical note on the Airmodus particle size magnifier 2.0

Abstract. Accurate measurement of the size distribution of sub-10 nm aerosol particles is still a challenge. Here we introduce a novel version of the Airmodus particle size magnifier (PSM 2.0), which is a condensation-particle-counter-based instrument with a sizing range of 1–12 nm. The extended size range compared to the earlier PSM version enables the direct detection of forming clusters and particles as well as the study of their growth processes without the challenges related to particle charging. It also gives an overlap between the activation size distribution measurements with the PSM and mobility size distribution measurements with conventional mobility particle sizers. We compared the performance of PSM 2.0 to that of a mobility particle size spectrometer, the original A10 particle size magnifier, and a Neutral cluster and Air Ion Spectrometer (NAIS) during field measurements. Also, calibration results were compared against the A10 instrument. The results show that PSM 2.0 is able to activate sub-2 nm clusters and that the concentration and size distribution between 2–12 nm compare well, especially with the NAIS.

Microfluidic-Aerosol Hyphenated Synthesis of Metal-Organic Framework-Derived Hybrid Catalysts for CO2 Utilization.

A new and efficient technique is developed by combining the hyphenated microfluidic- and aerosol-based synthesis with the coupled differential mobility analysis for the effective and continuous synthesis and simultaneous analysis of metal-organic frameworks (MOFs)-derived hybrid nanostructured products. HKUST-1, a copper-based MOF, is chosen as the representative to fabricate Cu-based hybrid catalysts for reverse water-gas shift (RWGS) reaction, an effective route for CO2 utilization. The effect of precursor concentration and carrier selection on the properties of the resulting products, including mobility size distribution, crystallization degree, surface area, and metal dispersion are investigated, as well as the correlation between the material properties of the synthesized catalysts and their catalytic performance in RWGS reaction in terms of conversion ratio/rate, selectivity, and operational stability. The results indicate that the continuous microfluidic droplet system can successfully synthesize MOF colloids, followed by the continuous production of MOF-derived hybrid materials through the tandem aerosol spray-drying-reaction system. High catalytic activity and low initiate temperature toward RWGS (turnover frequency = 0.0074s-1; 450°C) are achievable. The work facilitates the production and the designed concept of relevant MOF-derived hybrid nanostructured catalysts in the continuous synthesis system and the enhancement of applications in CO2 capture and utilization.

Comprehensive characterization of particulate matter emissions produced by a liquid-fueled miniCAST burner

Soot particles generated by a liquid-fueled miniCAST burner, supplied with diesel B7, was characterized for the first time over a total of 34 operating points covering an overall equivalence ratio ( ϕ) range of 0.103-1.645. To characterize the gas and particulate phase emissions, electron microscopy images, mobility-equivalent size distributions and mass concentration of the soot aggregates were recorded, and optical extinction/absorption coefficients of the exhaust were measured. The burner produces soot aggregates with geometric mean diameter size in the range 45-105 nm with primary particle size in the range 18-39 nm. The Ångström absorption exponent ( α) was evaluated in the range 400-1000 nm and was found to increase with ϕ and vary in the range 1.05-2.25. The interposition of a catalytic stripper in the sampling line was found to (i) flatten the shape of the size distribution of aggregates, (ii) oxidize most of the gas phase thus impacting optical extinction coefficients particularly below 400 nm and (iii) decrease α. The soot volume fraction ( fv) was determined by three independent methods: optical absorption, mass deposit and mobility size distribution combined with morphology data. fv evaluated from size mobility data accounting for morphological aspects agreed within 13% with fv measured optically and within 25% with fv evaluated from mass concentration measurements. The precise methodology developed to characterize engine-like soot particles produced by a liquid-fueled miniCAST can now be transposed to study other regular, renewable, and surrogate liquid fuels to investigate their physical and optical properties before considering their large-scale use.

Triggering conditions, runout, and downstream impacts of debris flows following the 2021 Flag Fire, Arizona, USA

Debris flows pose a serious threat to communities in mountainous areas, particularly in the years following wildfire. These events have been widely studied in regions where post-wildfire debris flows have been historically frequent, such as southern California. However, the threat of post-wildfire debris flows is increasing in many regions where detailed data on debris-flow physical properties, volume, and runout potential are sparse, such as the Southwest United States (Arizona and New Mexico). As the Southwest becomes more vulnerable to these hazards, there is an increasing need to better characterize the properties of post-wildfire debris flows in this region and to identify similarities and differences with nearby areas, particularly southern California, where there is a greater abundance of data. In this paper, we study the characteristics and downstream impacts of two post-wildfire debris flows that initiated following the 2021 Flag Fire in northern Arizona, United States. We gathered data regarding soil hydraulic properties, rainfall characteristics, watershed response, and debris-flow initiation, runout, volume, grain size, and downstream impacts during the first two monsoon seasons following the containment of the Flag Fire. We also applied established debris-flow runout and volume models that were developed in southern California to our study watershed and compared the output with observations. In the first monsoon season following the fire, there were two post-wildfire debris flows, one of which resulted in damage to downstream infrastructure, and one major flood event. We found that, while more intense rainfall is required to generate debris flows at our study site compared to southern California, burned watersheds in northern Arizona are still susceptible to debris flows during storms with low recurrence intervals in the first year following fire. During the second monsoon season, there were no major runoff events, despite more intense storms. This indicates that the temporal window for heightened debris-flow susceptibility at our study area was less than one year, due to the recovery of soil hydraulic properties and vegetation regrowth. We also found that the debris-flow properties at our study site, such as volume, mobility, and grain size distribution, may differ from those in other regions in the western United States, including southern California, potentially due to regional differences in rainfall characteristics and sediment supply. Differences in rainfall characteristics and sediment supply may have also influenced the performance of the debris-flow runout and volume models, which overpredicted the observed runout distance by 400 m and predicted a volume more than 17 times greater than what was observed.

Ions Generated from a Premixed Methane-Air Flame: Mobility Size Distributions and Charging Characteristics

ABSTRACT Chemical ionization in combustion systems forms concentrated ions of both polarities. Applying electric fields and plasmas to combustion systems has been shown to reduce emissions (such as particulate matter) potentially by controlling the ionic properties. Detailed flame-generated ion properties need to be characterized to better understand and predict the dynamics and roles of these ions in combustion and particle formation processes. In this work, we used a high-resolution differential mobility analyzer (HR-DMA) to map the mobility and size distributions of positive and negative ions generated from a premixed methane-air flat flame under atmospheric pressure. Measurements were conducted over a wide range of stoichiometric ratios (0.8 to 1.2) and heights above the burner (HAB, 7–42 mm). Positively charged ions are relatively stable over the entire range of experimental conditions, showing two major modes, one at 1.16 nm, and another at 1.42 nm. The mode corresponding to 1.42 nm showed a gradual increase in size with HAB, indicating the charging of hydrocarbon precursors. With the mobility values of the ions, we calculated their approximate mass values based on the empirical mobility–mass relationship and estimated the charging characteristics of particles in the flame. The ion profiles and particle charging characteristics obtained in this study will improve our understanding of electrostatic interactions in flame systems.

Dynamics of soot surface growth and agglomeration by enclosed spray combustion of jet fuel

Understanding the dynamics of soot formation and growth during combustion of jet fuel is essential for mitigation of aircraft engine emissions. Here, soot formation during enclosed spray combustion of jet fuel is investigated for its capacity to form soot with comparable characteristics to that from aircraft engines. For this, microscopy, scanning mobility particle, X-ray diffraction & Raman spectroscopy measurements and discrete element modeling (DEM) are employed along the flame centerline at various Effective eQuivalence Ratios (EQR). The DEM-derived mobility and primary particle size distribution dynamics are in excellent agreement with those measured at 5–63 cm height above the burner (HAB) for the measured temperature and soot volume fraction. At low EQR (1.46 and 1.59), soot surface growth stops at residence time, t = 4–7 ms, resulting in median soot primary particle diameters, d¯p, of ∼ 14 nm. At longer t (high HAB), agglomeration takes over increasing the median mobility diameter from 16 to 88 or 145 nm at EQR of 1.46 or 1.59, respectively, without altering d¯p and having the disorder over graphitic Raman band ratio, D/G = 0.9 ± 0.01 and a crystallite length, Lc = 1.24 ± 0.02 nm. In contrast, increasing EQR from 1.59 to 1.88, enhances soot surface growth, increases d¯p up to 23 nm and results in more graphitic soot having D/G = 0.8 ± 0.01 and Lc = 1.47 ± 0.01 nm. Furthermore, the D/G of soot is inversely proportional to its d¯p that is determined largely by surface growth.

Soot nanoparticle sizing in counterflow flames using in-situ particle sampling and differential mobility analysis verified with two-colour time-resolved laser-induced incandescence

The emphasis of this work is on the development of an intrusive particle sampling system to track the evolution of soot nanoparticles in ethylene counterflow diffusion flames. The overarching objective is to determine the mobility size distributions in a spatially resolved manner using the developed probe system coupled with differential mobility analysis, i.e., a scanning mobility particle sizer (SMPS). The probe system involves a tailor-made quartz probe, gas supply, pressure control periphery, and a traverse system enabling a precise positioning along the flame axis. In preliminary experiments, the dilution ratio of the quartz probe as function of boundary conditions as well as particle losses during intrusive particle sampling are studied. To demonstrate the capability of the developed particle sampling system, results from ethylene counterflow diffusion flames with different fuel mass fractions and strain rates are presented and compared with results derived by non-intrusive laser-based diagnostics, i.e., two-colour time-resolved laser induced incandescence (2C-TiRe-LII). Results of these experiments indicate that the particle sampling system is capable of tracking the development of particle size distributions – independent of the distribution function, i.e., mono-, bi- or multimodal shape – in counterflow flames. Likewise, the agreement between soot volume fractions and particle size distributions measured via intrusive particle sampling coupled with differential mobility analysis and non-intrusive laser-based 2C-TiRe-LII is excellent at varying the fuel mass fractions and strain rates of the ethylene counterflow flames.

A two-dimensional nanoparticle characterization method combining differential mobility analyzer and single-particle inductively coupled plasma-mass spectrometry with an atomizer-enabled sample introduction (ATM-DMA-spICP-MS): Toward the analysis of heteroaggregated nanoparticles in wastewater

Characterizing engineered nanoparticles (ENPs) in complex environmental matrices remains a challenging task. This work presents a two-dimensional size analysis method by combining differential mobility analyzer (DMA) and single-particle inductively coupled plasma-mass spectrometry (spICP-MS) with a new atomizer (ATM)-enabled sample introduction that is relatively easy to operate. The tailing of electrical mobility size distributions was solved by heating the aerosol flow, where water-shelled gold nanoparticles (AuNPs) were dehydrated, effectively eliminating the tailing. The improved method has a good sizing performance and can resolve the size fractions of mixed 30 nm and 50 nm AuNPs. It can reliably analyze 7.8 × 105 to 1.9 × 107 # of 50 nm AuNPs (or 4.1 × 105 to 107 # NPs/mL, equivalent to 0.6 to 14.3 μg Au/L) with a linear response and a limit of detection of 7.8 × 105 # AuNPs (equivalent to 4.1 × 105 # AuNPs/mL) that is relevant to NP concentrations in surface water and wastewater samples. The potential of this method to analyze environmental samples was demonstrated by characterizing AuNPs and silver nanoparticles (AgNPs) spiked in wastewater, where both NPs were revealed to form heteroaggregates with colloids existing in wastewater. The method can even directly analyze nanosized Ag particles inherent in the wastewater before adding external AgNPs. The result indicates that ATM-DMA-spICP-MS is a relatively simple two-dimensional size analysis method that has a great potential to characterize heteroaggregated NPs in aqueous environmental samples.

Understanding Solvothermal Growth of Metal-Organic Framework Colloids for CO2 Capture Applications.

A quantitative study of the synthesis of metal-organic framework (MOF) colloids via a solvothermal growth process was demonstrated using electrospray-differential mobility analysis (ES-DMA), a gas-phase electrophoresis approach. HKUST-1, a copper-based MOF (Cu-MOF), was selected as the representative MOF of the study. The effects of the synthetic parameters, including ligand concentration (CBTC), synthetic temperature (Ts), and synthetic time (ts) versus material properties of the Cu-MOF, were successfully characterized based on the mobility size distributions measured by ES-DMA. The results show that the mobility size of Cu-MOF was proportional to Ts, ts, and CBTC during the solvothermal growth. X-ray diffraction and Brunauer-Emmett-Teller analyses were employed complementarily to the ES-DMA, confirming that the increase in mobility size of Cu-MOF was correlated to the increase in crystallinity (i.e., larger specific surface area and crystallite size). The results of CO2 pulse adsorption show that the synthesized Cu-MOF possessed a good CO2 adsorption ability under 1 atm, 35 °C, and the cumulative amount of CO2 uptake was proportional to the measured mobility size of Cu-MOF. The work provides a proof of concept for the controlled synthesis of MOF colloids with the support of gas-phase electrophoretic analysis, and the quantitative methodology is useful for the development of MOF-based applications in CO2 capture and utilization.

Promotion of particle formation by resonance-stabilized radicals during hydrocarbon pyrolysis

We measured aerosol mass spectra and mobility size distributions of particles formed during the pyrolysis of ethylene, indene, and a mixture of ethylene with a small amount of indene. Measurements were recorded with a scanning mobility particle sizer and an aerosol mass spectrometer employing vacuum ultraviolet photoionization using tunable radiation from the Advanced Light Source at Lawrence Berkeley National Laboratory. The results demonstrate that particle formation occurs at a temperature of ∼1123±50 K for ethylene alone, at ∼923±50 K for indene alone, and at a comparable temperature to that of indene for ethylene seeded with a small amount of indene. Our results demonstrate that indene and indenyl, a resonance-stabilized radical (RSR) formed from indene pyrolysis, promote particle formation at lower temperatures and in the absence of acetylene, supporting the hypothesis that RSRs promote soot-particle inception. Even a small amount of indene added to ethylene promotes particle formation while circumventing the traditional pathways for mass growth by acetylene-addition reactions. Further support is provided by the appearance in the aerosol mass spectra of prominent peaks for masses corresponding to other RSRs from particles formed at lower pyrolysis temperatures. In addition, odd-numbered carbon species, associated with RSRs, appear in higher concentrations at low temperatures and are closely tied to particle inception. In indene pyrolysis, mass signals indicative of covalently bound indenyl dimers and trimers are prominent at pyrolysis temperatures near soot onset. Photoionization efficiency (PIE) curves demonstrate that, while the isomeric composition for some of these peaks may differ between ethylene and indene, the addition of indene to ethylene does not notably alter the isomers formed at these peaks between ethylene and the ethylene-indene mixture. PIE curves also show that m/z 202, which is commonly assumed to be pyrene, is composed principally of fluoranthene under these conditions.

Fracturing and healing of basaltic magmas during explosive volcanic eruptions

The eruption of basaltic magmas dominates explosive volcanism on Earth and other planets within the Solar System. The mechanism through which continuous magma fragments into volcanic particles is central in governing eruption dynamics and the ensuing hazards. However, the mechanism of fragmentation of basaltic magmas is still disputed, with both viscous and brittle mechanisms having been proposed. Here we carry out textural analysis of the products of ten eruptions from seven volcanoes by scanning electron microscopy. We find broken crystals surrounded by intact glass that testify to the brittle fragmentation of basaltic magmas during explosive activity worldwide. We then replicated the natural textures of broken crystals in laboratory experiments where variably crystallized basaltic melt was fragmented by rapid deformation. The experiments reveal that crystals are broken by the propagation of a network of fractures through magma, and that afterwards the fractures heal by viscous flow of the melt. Fracturing and healing affect gas mobility, stress distribution, and bubble and crystal size distributions in magma. Our results challenge the idea that the grain size distribution of basaltic eruption products reflects the density of fractures that initially fragmented the magma and ultimately indicate that brittle fracturing and viscous healing of magma may underlie basaltic explosive eruptions globally. In explosive basaltic eruptions, brittle fragmentation and subsequent healing by viscous melt are documented by textural analysis of products from ten disparate eruptions, suggesting that grain size may not reflect the initial fracture density of magma.

Determination of the optimum amount of iodine in electrophoretic deposition of hydroxyapatite (HA) nanoparticles

In this research, the hydroxyapatite (HA) nanoparticles were deposited on sandblasted titanium by electrophoretic deposition method. In this case, isopropanol-acetone suspension with 50/50 ratio was used by using iodine as a dispersant. The suspensions were prepared with various concentrations of iodine (0, 0.2, 0.4, 0.6, and 0.8 g/L). Furthermore, the pH of suspension, the electrical conductivity of the suspension mediums, and the zeta potential, mobility, and particle size distribution of HA nanoparticles were measured in suspensions. In the next step, the optimum content of iodine was determined to provide a sustainable suspension. Then, HA coating was deposited on sandblasted titanium surface. The current density during electrophoretic deposition (EPD) and deposition rate in different voltages were investigated. The microstructure and morphology of the sediments were examined by using FE-SEM and AFM. Finally, the structure and phase composition of the coatings were analyzed by using XRD before and after sintering. Furthermore, the adhesion strength of the coating to the substrate was measured. The results show that 0.6-g/L iodine prepared a stable suspension and the effect of current density and potential on the deposition weight is determined. In additional, the results show a finer and narrower particle size distribution can be observed. Also, adhesion strength of the coatings to the sandblasted titanium surface is about 11 MPa.

Estimating the internal and surface oxidation of soot agglomerates

Oxidation of soot takes place inside and on the surface of its constituent primary particles at a rate that depends on temperature, T, and O2 concentration. Even though accurate oxidation kinetics are essential in both industrial uses and environmental impact of soot, they are often derived neglecting internal particle oxidation and the structure of such soot agglomerates. Here, the detailed evolution of the fractal-like agglomerate soot mass, m, and mobility diameter, dm, during both internal and surface oxidation is determined by a moving sectional model. The model predictions are in excellent agreement with oxidation data of mature ethylene soot dm for T = 900–1200 K. The oxidation mode index, a, given by the ratio of the characteristic O2 reaction and diffusion times is used to quantify the contributions of internal and surface oxidation of soot. At low Τ (e.g., < 1100 K), O2 diffuses into the primary particles and reacts with bulk soot, hardly altering the dm and yielding a > 3. As T increases, surface oxidation becomes dominant, decreasing both dm and a. The common assumption that soot agglomerates are spheres underestimates their dm up to 50 % during oxidation. Coupling this detailed moving sectional model with soot mobility size distributions can yield realistic soot oxidation rates. Accounting for soot morphology and internal oxidation shows that the classic NSC rate increasingly underestimates (by 3–7 times) the oxidation rate of soot (from ethylene and toluene flames) with decreasing temperature (900–1800 K) and/or oxygen concentration (0.2–21 vol %).

A facile quantification of hyaluronic acid and its crosslinking using gas-phase electrophoresis.

We report a facile, high-resolution approach to quantitatively characterize hyaluronic acid (HA) and study its crosslinking reaction using electrospray-differential mobility analysis (ES-DMA). Mobility size distributions, number concentrations, molecular mass distributions, and polydispersity index of HAs were obtained successfully via a rapid analysis by ES-DMA (< 30min). The limit of detection, the limit of quantification, and the precision of the mobility size measurement achieve 2.5nm, 4.0nm, and 0.3nm, respectively. Size exclusion chromatography (SEC) was employed as an orthogonal approach, showing that the averaged molecular mass and polydispersity index of HA measured by ES-DMA were close to the results of SEC on a semi-quantitative basis. The 1,4-butanediol diglycidyl ether (BDDE)-induced crosslinking of HA was also able to be successfully characterized through a time-dependent study using ES-DMA, which has shown the promise of direct analysis of solution-based reactions. Both the extent and the rate of HA crosslinking (induced by BDDE) were proportional to reaction temperature and concentration ratio of HA to BDDE. The activation energy of the reaction-limited BDDE-induced crosslinking of HA was found to be ≈ 21kJ/mol. The prototype study demonstrates ES-DMA as a new method for a rapid quantitative characterization of HA and its derivative product and providing a capability of real-time monitoring of the HA crosslinking during formulation process. Graphical abstract.

On the Effective Density and Fractal–Like Dimension of Diesel Soot Aggregates as a Function of Mobility Diameter

A new technique is proposed for the assessment of aggregate morphology based on combined information of aerodynamic and mobility size distributions. Instead of formulating a complex inverse problem having aggregate morphology as unknown, the actual problem is separated to two stages. The aerodynamic distribution is determined directly by the electric low pressure impactor (ELPI) data whereas the morphology is independently assessed by matching the distribution arises from ELPI and from scanning mobility particle sizer (SMPS). The advantage of this approach is that it can estimate different fractal dimension for different aggregate size. The approach is applied to soot aggregates from three different diesel engines. In all cases, a non-monotonic behavior of fractal dimension versus aggregate size is observed. In particular, the fractal dimension initially decreases and then increases (passing through a minimum) as the mobility diameter increases.

Evolution of grain size distributions and bed mobility during hydrographs in gravel‐bed braided rivers

AbstractEvolution of bed material mobility and bedload grain size distributions under a range of discharges is rarely observed in braiding gravel‐bed rivers. Yet, the changing of bedload grain size distributions with discharge is expected to be different from laterally‐stable, threshold, channels on which most gravel bedload theory and observation are based. Here, simultaneous observations of flow, bedload transport rate, and morphological change were made in a physical model of a gravel‐bed braided river to document the evolution of grain size distributions and bed mobility over three experimental event hydrographs. Bedload transport rate and grain size distributions were measured from bedload samples collected in sediment baskets. Morphological change was mapped with high‐resolution (~1 mm precision) digital elevation models generated from close‐range digital photogrammetry. Bedload transport rates were extremely low below a discharge equivalent to ~50% of the channel‐forming discharge (dimensionless stream power ~70). Fractional transport rates and plots of grain size distributions indicate that the bed experienced partial mobility at low discharge when the coarsest grains on the bed were immobile, weak selective mobility at higher discharge, and occasionally near‐equal mobility at peak channel‐forming discharge. The transition to selective mobility and increased bedload transport rates coincided with the lower threshold for morphological change measured by the morphological active depth and active width. Below this threshold discharge, active depths were of the order of D90 and active widths were narrow (&lt; 3% of wetted width). Above this discharge, both increased so that at channel‐forming discharge, the active depth had a local maximum of 9D90 while active width was up to 20% of wetted width. The modelled rivers approached equal mobility when rates of morphological change were greatest. Therefore, changes in the morphological active layer with discharge are directly connected to the conditions of bed mobility, and strongly correlated with bedload transport rate. © 2018 John Wiley &amp; Sons, Ltd.

Effective density of airborne particles in a railway tunnel from field measurements of mobility and aerodynamic size distributions

The objective of this study is to investigate the particle effective density of aerosol measurements in a railway tunnel environment. Effective density can serve as a parameter when comparing and c...

Impact of Humidity on Silica Nanoparticle Agglomerate Morphology and Size Distribution.

The effect of humidity on flame-made metal oxide agglomerate morphology and size distribution is investigated, for the first time to our knowledge, and compared to that on soot, which has been widely studied. Understanding the impact of humidity on such characteristics is essential for storage, handling, processing, and eventual performance of nanomaterials. More specifically, broadly used agglomerates of flame-made silica nanoparticles are humidified at various saturation ratios, S = 0.2-1.5, and dried before characterization with a differential mobility analyzer (DMA), an aerosol particle mass (APM) analyzer, and transmission electron microscopy. At high humidity, the constituent single and/or aggregated (chemically bonded) primary particles (PPs) rearrange to balance the capillary forces induced by condensation-evaporation of liquid bridges between PPs. Larger agglomerates restructure more than smaller ones, narrowing their mobility size distribution. After humidification at S = 1.5 and drying, agglomerates collapse into compact structures that follow a fractal scaling law with mass-mobility exponent Dfm = 3.02 ± 0.11 and prefactor km = 0.27 ± 0.07. This critical S = 1.5 for silica agglomerates is larger than the 1.26 obtained for soot because of the hydrophilic surface of silica that delays water evaporation. The relative effective density, ρeff/ρ, of collapsed agglomerates becomes invariant of mobility diameter, dm, similar to that of fluidized and spray-dried granules. The average silica ρeff/ρ = 0.28 ± 0.02 is smaller than the 0.36 ± 0.04 measured for the humidified-dried soot because of the larger size of silica aggregates, dm/ dp, and number of constituent primary particles, np, of diameter dp. This is verified by tandem-DMA (TDMA) measurements, yielding maximum dm = 3 dpor 5 dp and np = 13 or 36 for the soot or silica aggregates studied here, in good agreement with those reported from microscopy and high-pressure agglomerate dispersion. A scaling law relating the initial dm,o to dm, Dfm, and km after condensation-drying is developed. The mass-mobility relationship of collapsed silica and soot agglomerates obtained by combining this law with fast TDMA measurements is in excellent agreement with that measured by the direct, but tedious, DMA-APM analysis.