Detailed Microphysical Modeling of the Formation of Organic and Sulfuric Acid Coatings on Aircraft Emitted Soot Particles in the Near Field

The development of a detailed microphysical model that describes the complex multicomponent interactions between organic vapors and soot particles emitted from aircraft gas turbine engines is presented. Our model formulation includes both soot surface activation by organic vapors and organic vapor condensation on the activated part of the soot surfaces. To enable this formulation, approaches to estimate chemical and physical properties of aerosols containing complex mixtures of sulfuric acid, water, and organic molecules were developed. Relevant distributions of a list of organic surrogates at the engine exit plane were used to represent complex organic emissions from aircraft engines. A parametric study was performed using this new formulation to understand the effects of ambient conditions, organic emissions levels, and mass accommodation coefficient values on the evolution of near field volatile particulate matter emissions from aircraft engines at ground level.

Copyright 2014 American Association for Aerosol Research     

Cloud condensation nuclei from the activation with ozone of soot particles sampled from a kerosene diffusion flame

Due to the exponential increase in aircraft traffic in recent decades, the role of soot particles emitted by aircraft engines on the radiative forcing needs to be addressed, and especially their interaction with clouds has to be better understood and quantified. In this work, we investigate the hygroscopic properties of fresh and aged soot sampled on line in a kerosene flame. The activated fraction (F_a) of size selected soot is measured by means of a variable supersaturation condensation nucleus counter at several heights above the burner (HAB), thereby probing soot particles with different residence times in the flame, i.e., different degrees of maturity. In order to simulate atmospheric aging, the activity of soot as cloud condensation nuclei is measured as a function of ozone exposure. We show that fresh soot is hydrophobic (F_a～0), while F_a increases when soot is exposed to ozone. The measurements depend on the HAB at which soot particles are sampled showing that activation of soot particles is related to their chemical composition. This study brings new results on the link between atmospheric aging of soot and its hygroscopic properties, which is of great interest for understanding the role of soot in the cloud formation.

Copyright © 2018 American Association for Aerosol Research     

Response of real-time black carbon mass instruments to mini-CAST soot

Soot is a climate forcer and a dangerous air pollutant that has been increasingly regulated. In aviation, regulatory measurements of soot mass concentration in the exhaust of aircraft turbine engines are to be based on measurements of black carbon (BC) calibrated to elemental carbon (EC) content of diffusion flame soot. The calibration soot must currently meet only one criterion: minimum EC to total carbon (TC) ratio of 0.8. However, not including soot properties other than the EC/TC ratio may potentially lead to discrepancies between different BC measurements. We studied the response of two instruments, the AVL Micro-Soot Sensor (MSS) and the Artium Laser-Induced Incandescence 300 (LII), to soot from two miniature combustion aerosol standard (mini-CAST) burners. By changing the air-fuel ratio, premixing nitrogen into the fuel, and using a catalytic stripper to remove volatile compounds, we produced a wide range of particle morphologies and EC contents. As the EC content decreased, both the instruments underreported the EC mass, but the LII diverged more severely. Upon closer investigation of eight conditions with EC/TC > 0.8, the LII underreporting was found independent of primary particle size, but increased with decreasing geometric mean diameter of the soot agglomerates. As the geometric mean diameter decreased from 160 nm to 50 nm, the differences between the LII and MSS increased from 15% to 50%. The results suggest that in addition to EC content, calibration procedures for the regulatory BC measurements may need to take particle size distributions into account.

© 2016 American Association for Aerosol Research     

A novel miniature inverted-flame burner for the generation of soot nanoparticles

Lab-scale soot nanoparticle generators are used by the aerosol research community to study the properties of soot over a broad range of particle size distributions, and number and mass concentrations. In this study, a novel miniature inverted-flame burner is presented and its emitted soot particles were characterized. The burner consisted of two co-annular tubes for fuel and co-flow air and the flame was enclosed by the latter. The fuel used was ethylene. A scanning mobility particle sizer (SMPS) and an aerodynamic aerosol classifier (AAC) were used to measure mobility and aerodynamic size distribution of soot particles, respectively. Particle morphology was studied using transmission electron microscopy (TEM). The elemental carbon (EC) and organic carbon (OC) content of the soot were measured using thermal-optical analysis (TOA). The burner produced soot particles with mobility diameter range of 66–270?nm, aerodynamic diameter range of 56–140?nm, and total concentration range of 2?×?10⁵–1?×?10⁷?cm^?3. TEM images showed that most soot particles were sub-micron soot aggregates. Some soot superaggregates, typically larger than 2?µm in length, were observed and their abundance increased with ethylene flow rate. TOA showed that the concentration of EC in the generated soot increased with ethylene flow rate, and the soot was observed to have high EC fraction at high ethylene flow rates. The miniature inverted-flame burner was demonstrated to produce soot nanoparticles over a range of concentrations and sizes with high EC content, making it a practical device to study soot nanoparticle properties in different applications.

Copyright © 2019 American Association for Aerosol Research     

How well can aerosol instruments measure particulate mass and solid particle number in engine exhaust?

Aerosol instruments provide more informative engine exhaust particulate matter (PM) data than the gravimetric filter and solid particle number methods prescribed by regulations. Yet their lack of conformity to the regulatory methods can limit their acceptance for vehicle development. This article examines the ability of the Dekati Mass Monitor (DMM), Engine Exhaust Particle Sizer (EEPS), and Micro Soot Sensor (MSS) to measure PM_2.5 mass and solid particle number relative to current motor vehicle PM emissions standards. Simultaneous PM measurements are made by these three instruments and the two regulatory methods for 50 tests of six gasoline direct injection and two port fuel injection vehicles over the U.S. Federal Test Procedure. DMM and EEPS determinations of PM mass correlate well to gravimetric values (regression slopes of 1.06 ± 0.04 and 0.98 ± 0.08) down to a few mg/mile, below which filter weighing variability becomes problematic. The MSS exhibits a lower slope of 0.79 ± 0.03 consistent with it measuring the soot fraction, rather than total PM. At emissions rates above ～10¹³ particles/mile, solid particle number determined from DMM and EEPS data correlates respectably with, but overestimates the regulatory method (regression slopes are 1.7 ± 0.1 and 1.4 ± 0.15, respectively). Below this emissions rate, the correlation degrades. EEPS estimates of PM mass are significantly improved with the recent soot optimized inversion algorithm (slope improves from 0.45 to 0.98). While they cannot replace filters and solid particle counting, the present study suggests that these instruments can be used as more informative surrogates during motor vehicle development.

© 2016 Ford Motor Company     

Observations of a Correlation Between Primary Particle and Aggregate Size for Soot Particles

For decades, soot has been modeled as fractal-like aggregates of nearly equiaxed spherules. Cluster–cluster aggregation simulations, starting from a population of primary particles, give rise to structures that closely match real aerosols of solid particles produced in flames. In such simulations, primary particle size is uncorrelated with aggregate size, as all aggregates contain primary particles drawn from the same population. Aerosol measurements have been interpreted with this geometric model. Examination of transmission electron micrographs of soot samples from various sources shows that primary particle sizes are not well mixed within an aerosol population. Larger aggregates tend to contain larger primary particles and the variation in size is much larger between aggregates than within aggregates. The soot sources considered here are all substantially not well-mixed (aircraft jet engine, inverted diffusion flame, gasoline direct injection engine, heavy-duty compression ignition engine). The observed variations in primary particle size can be explained if soot aggregates are formed and grew by coagulation in small zones of the combustion chamber, prior to dilution and transport (with minimal coagulation) to the sampling system.

Copyright 2014 American Association for Aerosol Research     

Distinguishing fuel and lubricating oil combustion products in diesel engine exhaust particles

The main sources of particulate emissions from engines are fuel and lubricating oil. In this study, particles emitted by a medium speed diesel engine for locomotive use were characterized chemically by using a soot particle aerosol mass spectrometer (SP-AMS). Additionally, positive matrix factorization (PMF) was applied to the SP-AMS data for the separation of fuel from lubricating oil and/or oil additives in diesel engine emissions. The mass spectra of refractory species, i.e., metals and rBC, were included in the PMF input matrix in addition to organics in order to utilize the benefit of the SP-AMS to measure non-refractory and refractory species. In general, particulate matter emitted by the diesel engine was dominated by organics (51%) followed by refractory black carbon (rBC; 48%), trace metals and inorganic species (1%). Regarding the sources of particles, PMF indicated four factors for particle mass of which two were related to lubricating oil-like aerosol (LOA1, 29% and LOA2, 24%) and two others to diesel-like fuel aerosol (DFA1, 35% and DFA2, 12%). The main difference between LOA1 and LOA2 was the presence of soot in LOA1 and metals in LOA2 factors. DFA factors represented burned (DFA1) and unburned fuel (DFA2). The results from the PMF analysis were completed with particle size distributions, volatility measurements and particle morphology analyses.

Copyright © 2019 American Association for Aerosol Research     

A practical set of miniaturized instruments for vertical profiling of aerosol physical properties

In situ atmospheric aerosol measurements have been performed from a Manta unmanned aircraft system (UAS) using recently developed miniaturized aerosol instruments. Flights were conducted up to an altitude of 3000 m (AMSL) during spring 2015 in Ny-Ålesund, Svalbard, Norway. We use these flights to demonstrate a practical set of miniaturized instruments that can be deployed onboard small UASs and can provide valuable information on ambient aerosol. Measured properties include size-resolved particle number concentrations, aerosol absorption coefficient, relative humidity, and direct sun intensity. From these parameters, it is possible to derive a comprehensive set of aerosol optical properties: aerosol optical depth, single scattering albedo, and asymmetry parameter. The combination of instruments also allows us to determine the aerosol hygroscopicity.

Copyright © 2017 American Association for Aerosol Research     

Characteristics of traffic-induced fugitive dust from unpaved roads

The characteristics of fugitive dust emitted from vehicles traveling on unpaved dirt roads were measured using a suite of instruments including a real-time fugitive dust sampler. The fugitive dust sampler is formed from a combination of a large particle inlet and an optical particle spectrometer that reports particle sizes from 6 to 75 µm. The large particle inlet permits the sampling of particles up to 75 µm with only a moderate dependence of sampling efficiency on wind-speed. Measurements made with the sampler showed that particles as large as ～50 µm were suspended from vehicular movement on the dirt roads, with the mode of the fugitive dust particle number size distribution ～2 µm, while the mass distribution mode was ～7 µm. A comparison of the fugitive dust sampler measurements with those made using standard PM instruments showed that the conventional instruments have a wind-direction bias that can result in under-sampling of large particles. The current measurements suggest that particles suspended from dirt roadways are of importance for local air quality within the near-road environment.

Copyright © 2017 American Association for Aerosol Research     

Effective density and volatility of particles sampled from a helicopter gas turbine engine

The effective density and size-resolved volatility of particles emitted from a Rolls-Royce Gnome helicopter turboshaft engine are measured at two engine speed settings (13,000 and 22,000 RPM). The effective density of denuded and undenuded particles was measured. The denuded effective densities are similar to the effective densities of particles from a gas turbine with a double annular combustor as well as a wide variety of internal combustion engines. The denuded effective density measurements were also used to estimate the size and number of primary particles in the soot aggregates. The primary particle size estimates show that the primary particle size was smaller at lower engine speed (in agreement with transmission electron microscopy analysis). As a demonstration, the size-resolved volatility of particles emitted from the engine is measured with a system consisting of a differential mobility analyzer, centrifugal particle mass analyzer, condensation particle counter, and catalytic stripper. This system determines the number distributions of particles that contain or do not contain non-volatile material, and the mass distributions of non-volatile material, volatile material condensed onto the surface of non-volatile particles, and volatile material forming independent particles (e.g., nucleated volatile material). It was found that the particulate at 13,000 RPM contained a measurable fraction of purely volatile material with diameters below ～25 nm and had a higher mass fraction of volatile material condensed on the surface of the soot (6%–12%) compared to the 22,000 RPM condition (1%–5%). This study demonstrates the potential to quantify the distribution of volatile particulate matter and gives additional information to characterize sampling effects with regulatory measurement procedures.

Copyright © 2017 American Association for Aerosol Research     

Investigation of the size of the incandescent incipient soot particles in premixed sooting and nucleation flames of n-butane using LII,HIM, and 1 nm-SMPS

Laser-induced incandescence (LII) measurements were conducted to explore the ability of LII to detect small soot particles of less than 10 nm in two sooting flat premixed flames of n-butane: a so-called nucleation flame obtained at a threshold equivalence ratio Φ = 1.75, in which the incipient soot particles undergo only minor soot surface growth along the flame, and a more sooting flame at Φ = 1.95. Size measurements were obtained by modeling the time-resolved LII signals detected using 1064 nm laser excitation. Spectrally-resolved LII signals collected in the nucleation flame display a similar blackbody-like behavior as mature soot. Soot particle temperature was determined from spectrally-resolved detection. LII modeling was conducted using parameters either relevant to those of mature soot or derived from fitting the modeled results to the experimental LII data. Particle size measurements were also carried out using (1) ex situ analysis by helium-ion microscopy (HIM) of particles sampled thermophoretically and (2) online size distribution analysis of microprobe-sampled particles using a 1 nm-SMPS. The size distributions of the incipient soot particles, found in the nucleation flame and in the early soot region of the Φ = 1.95 flame, derived from time-resolved LII signals are in good agreement with HIM and 1 nm-SMPS measurements and are in the range of 2–4 nm. The thermal and optical properties of incipient soot were found to be not radically different from those of mature soot commonly used in LII modeling. This explains the ability of incipient soot particles to produce continuous thermal emissions in the visible spectrum. This study demonstrates that LII is a promising in situ optical particle sizing technique that is capable of detecting incipient soot as small as about 2.5 nm and potentially 2 nm and resolving small changes in soot sizes below 10 nm.

© 2017 American Association for Aerosol Research     

Investigation of the absorption Ångström exponent and its relation to physicochemical properties for mini-CAST soot

In this work, a mini-CAST soot generator was used to produce soot with different optical and physicochemical characteristics. Absorption Ångström exponents (AAE) expressing the absorption wavelength dependence were assessed by multiwavelength in-situ and filter-based (aethalometer) laser extinction. The two optical techniques showed good agreement. For the chosen mini-CAST operating conditions, AAEs between 1 and 3.5 were found. Soot with high mass-fractions of organic carbon (OC) and pyrolytic carbon (PC) determined with thermal optical analysis were associated with AAEs significantly higher than 1. Heating to 250 and 500°C removed the majority of polycyclic aromatic hydrocarbons. However, the thermal-optical analysis revealed that OC and PC were abundant in the soot with AAE > 2 also after heating the aerosol. Analysis of mass absorption cross section ratios for elemental carbon and OC indicated that elevated AAEs also after heating to 500°C could be related to persistent OC and PC components and/or the refractory soot. By comparing the mini-CAST soot optical properties with soot properties derived from in-situ extinction measurements in a premixed flame, mini-CAST soot with a higher AAE could be identified as less mature soot.

Copyright © 2018 The Authors     

Characterization of a new miniCAST with diffusion flame and premixed flame options: Generation of particles with high EC content in the size range 30 nm to 200 nm

Flame-generated soot from miniCAST burners is increasingly being used in academia and industry as engine exhaust soot surrogate for atmospheric studies and instrument calibration. Previous studies have found that elemental carbon (EC) content of miniCAST soot is proportional to the mean particle size. Here, the characterization of a prototype miniCAST generator (5201 Type BC), which was designed to decouple the soot composition from the particle size and produce soot particles with high EC and BC content in a large size range, is reported. This prototype may operate either in a diffusion-flame or a partially premixed-flame mode, an option that was not available in former models. It was confirmed that soot properties, such as EC content and Ångström absorption exponent (AAE), were linked to the overall flame composition. In particular, combustion under fuel-rich conditions provided particles with size coupled to the EC fraction and AAE, i.e. smaller particles exhibited a lower EC fraction and higher AAE. In contrast, with fuel-lean diffusion flames and especially with premixed flames under near overall stoichiometric conditions small particles (down to 30?nm) with high EC/TC ratios (>60%) and low AAE (≈1.4) could be generated even without any thermal after-treatment. This new source might thus serve in the future as a useful surrogate for engine exhaust emissions and help to improve calibration procedures of common aerosol instruments.

Copyright © 2018 The Author(s). Published with license by Taylor & Francis Group, LLC     

Influence of waste cooking oil biodiesel on the nanostructure and volatility of particles emitted by a direct-injection diesel engine

To reduce air pollution and the reliance on fossil fuel, biodiesel has been widely investigated as an alternative fuel for diesel engines. The purpose of this study is to investigate the influence of waste cooking oil (WCO) biodiesel on the physical properties and the oxidation reactivity of the particles emitted by a diesel engine operating on WCO biodiesel as the main fuel. Experiments were conducted on a direct-injection diesel engine fueled with biodiesel, B75 (75% biodiesel and 25% diesel on volume basis, v/v), B50, B20, and diesel fuel, at five engine loads and at an engine speed of 1920 rev/min. Particulate samples were collected to analyze the particulate nanostructure, volatility, and oxidation characteristics. Biodiesel or low-load operation leads to smaller primary particles and more disordered nanostructures having shorter and more curved graphene layers. It can be found that particles from biodiesel, blended fuels, or low-load operation have higher volatile mass fractions and faster oxidation reaction rates than particles from diesel or heavy-load operation. The higher oxidation reaction rates are due mainly to the smaller particle size, the more disordered nanostructure, and the higher volatile mass fraction. It is also found that changes in primary particle size and particulate nanostructure are not directly proportional to the biodiesel content, while changes in particulate volatility and particulate oxidation reactivity are proportional to the biodiesel content. The use of biodiesel can enhance particulate oxidation reactivity and the regeneration of soot particles in an after-treatment device.

Copyright © 2016 American Association for Aerosol Research     

Ns-Soot: A Material-Based Term for Strongly Light-Absorbing Carbonaceous Particles

The climate-change and environmental literature, including that on aerosols, is replete with mention of black carbon (BC) and soot. The terms are used interchangeably in much of the literature, although BC and soot commonly have operational and source-based definitions, respectively, and reliable reference samples and aerosol standards do not exist for either one. The uncertainty about their exact chemical nature and properties can be decreased by materials-based measurement techniques and terminology. Here, we discuss ambiguities in common uses of BC and soot and propose the term ns-soot, where “ns” refers to carbon nanospheres, for a characteristic constituent of BC and soot. Based on its composition, morphology, and structure, we define ns-soot as particles that consist of nanospheres, typically with diameters <100 nm, that possess distinct structures of concentrically wrapped, graphene-like layers of carbon and with grape-like (aciniform) morphologies. We additionally propose that, because of their importance for climate modeling and health issues, distinctions are made among bare, coated, and embedded ns-soot particles.

Copyright 2014 American Association for Aerosol Research     

Intercomparison of a portable and two stationary mobility particle sizers for nanoscale aerosol measurements

During occupational exposure studies, the use of conventional scanning mobility particle sizers (SMPS) provides high quality data but may convey transport and application limitations. New instruments aiming to overcome these limitations are being currently developed. The purpose of the present study was to compare the performance of the novel portable NanoScan SMPS TSI 3910 with that of two stationary SMPS instruments and one ultrafine condensation particle counter (UCPC) in a controlled atmosphere and for different particle types and concentrations.

The results show that NanoScan tends to overestimate particle number concentrations with regard to the UCPC, particularly for agglomerated particles (ZnO, spark generated soot and diesel soot particles) with relative differences >20%. The best agreements between the internal reference values and measured number concentrations were obtained when measuring compact and spherical particles (NaCl and DEHS particles). With regard to particle diameter (modal size), results from NanoScan were comparable < [± 20%] to those measured by SMPSs for most of the aerosols measured.

The findings of this study show that mobility particle sizers using unipolar and bipolar charging may be affected differently by particle size, morphologies, particle composition and concentration. While the sizing accuracy of the NanoScan SMPS was mostly within ±25%, it may miscount total particle number concentration by more than 50% (especially for agglomerated particles), thus making it unsuitable for occupational exposure assessments where high degree of accuracy is required (e.g., in tier 3). However, can be a useful instrument to obtain an estimate of the aerosol size distribution in indoor and workplace air, e.g., in tier 2.     

Understanding particles emitted from spray and wall-guided gasoline direct injection and flex fuel vehicles operating on ethanol and iso-butanol gasoline blends

Traffic-related pollutants are an ever-growing concern. However, the composition of particle emissions from new vehicle technologies using relevant current and prospective fuel blends is not known. This study tested four current and up-and-coming vehicle technologies with nine fuel blends with various concentrations of ethanol and iso-butanol. Vehicles were driven on both the federal test procedure (FTP) and the unified cycle (UC). Additional tests were conducted under steady-state speed conditions. The vehicle technologies include spray-guided gasoline direct injection (SG-GDI), wall-guided gasoline direct injection (WG-GDI), port-fuel injection flex fuel vehicle (PFI-FFV), and a wall-guided GDI-FFV. The fuels consisted of 10–83% ethanol and 16–55% iso-butanol in gasoline. The composition of soot, water-insoluble mass (WIM), water-soluble organic mass, and water-insoluble organic mass (WIOM), and OM was measured. The majority of emissions over FTP and UC were water-insoluble (>70%), and WIOM contributes mostly to OM. PFIs have lower soot and particulate matter (PM) emissions in comparison to the WG-GDI technology even while increasing the renewable fuel content. SG-GDI technology, which has not penetrated the market, show promise as soot and PM emissions are comparable to PFI vehicles while preserving the GDI fuel economy benefits. The WIM fraction in GDI-FFV consistently increased with increasing ethanol concentration. Lastly, the impact of the future vehicle emissions and traffic pollutants is discussed. SG-GDI technology is found to be a promising sustainable technology to enhance fuel economy and also reduce PM, soot, and WIM emissions.

Copyright © 2017 American Association for Aerosol Research     

Filtration efficiency analysis of fibrous filters: Experimental and theoretical study on the sampling of agglomerate particles emitted from a GDI engine

Fibrous filters are commonly used for aerosol purification and sampling. The filtration efficiency has been extensively studied using standard aerosol generators, yet the literature on experimental data and theoretical study concerning the filtration of agglomerates from real engines remains scarce. A filtration efficiency test system was developed to determine the filtration efficiency of two types of filters (uncoated and fluorocarbon coated) loaded by particulate matter (PM) emissions from a gasoline direct injection (GDI) engine. The experimental results showed that the filtration efficiency in terms of PM mass and number increased over time for both types of filters. The fractional efficiency (penetration efficiency) curves for the test fibrous filters rendered a U-shaped curve for particle sizes from 70 to 500 nm, and the most penetrating particulate size (MPPS) decreased over time. A small fraction of accumulation mode particles with the size between 70 nm to 500 nm penetrated the filters while almost all nucleation mode particles with the size below 50 nm were captured by the filters. The filtration efficiency derived from an empirical model based on classical single-fiber theory for laden filters generally agreed with the experimental data for the first 500 s, but suffered a significant deviation by approximately one order of magnitude at 948 s. A better estimate of the filtration efficiency trend with the maximum deviation of about 20% (except for large particles at the high end of the measurement spectra) was obtained by using a revised model which incorporates the effects of the increase in filter solidity, local velocity, dynamic shape factor and effective total length of fibers during the filtration process.

© 2017 American Association for Aerosol Research     

Electron tomography of soot for validation of 2D image processing and observation of new structural features

Transmission electron microscopy of soot aggregates is performed using a tilting-stage with view angles from –60 to +50 degrees. The resulting series of images was used to produce videos and solid models to aid in the visualization of the structure. The images from 31 angles for each aggregate were analyzed using various 2D image processing techniques. It was found that the measurements of primary particle size varied by less than 16% as angles changed. Projected area measurements only varied by 6% as viewing angle changed. These variations are comparable to the errors expected from the image processing. Practically, 2D images are adequate for determining primary particle size and projected area with little noise or bias introduced by random projection angles. In addition, it was found that most of the aggregates contained closed ring structures, implying that during aggregation, a small amount of rotation at contact points occurs. This structural feature is not apparent in 2D images and suggests that tomography may be useful in studying soot formation and aging processes.

Copyright © 2019 American Association for Aerosol Research