Measuring aerosol size distributions with the aerodynamic aerosol classifier

The Aerodynamic Aerosol Classifier (AAC) is a novel instrument that selects aerosol particles based on their relaxation time or aerodynamic diameter. Additional theory and characterization is required to allow the AAC to accurately measure an aerosol's aerodynamic size distribution by stepping whilst connected to a particle counter (such as a Condensation Particle Counter, CPC). To achieve this goal, this study characterized the AAC transfer function (from 32 nm to 3 μm) using tandem AACs and comparing the experimental results to the theoretical tandem deconvolution. These results show that the AAC transmission efficiency is 2.6 to 5.1 times higher than a combined Krypton-85 radioactive neutralizer and Differential Mobility Analyzer (DMA), as the AAC classifies particles independent of their charge state. However, the AAC transfer function is 1.3 to 1.9 times broader than predicted by theory. Using this characterized transfer function, the theory to measure an aerosol's aerodynamic size distribution using an AAC and particle counter was developed. The transfer function characterization and stepping deconvolution were validated by comparing the size distribution measured with an AAC-CPC system against parallel measurements taken with a Scanning Mobility Particle Sizer (SMPS), CPC and Electrical Low Pressure Impactor (ELPI). The effects of changing AAC classifier conditions on the particle selected were also investigated and found to be small (<1.5%) within its operating range

Effects of multiple injection strategies on gaseous emissions and particle size distribution in a two-stroke compression-ignition engine operating with the gasoline partially premixed combustion concept

[EN] In order to improve performance of internal combustion engines and meet the requirements of the new pollutant emission regulations, advanced combustion strategies have been investigated. The newly designed partially premixed combustion concept has demonstrated its potential for reducing NOx and particulate matter emissions combined with high indicated efficiencies while still retaining proper control over combustion process by using different injection strategies. In this study, parametric variations of injection pressure, second injection and third injection timings were experimentally performed to analyze the effect of the injection strategy over the air/fuel mixture process and its consequent impact on gaseous compound emissions and particulate matter emissions including its size distribution. Tests were carried out on a newly designed two-stroke high-speed direct injection compression-ignition engine operating with the partially premixed combustion concept using 95 research octane number gasoline fuel. A scanning particle sizer was used to measure the particles size distribution and the HORIBA 7100DEGR gas analyzer system to determine gaseous emissions. Three different steady-state operation modes in terms of indicated mean effective pressure and engine speed were investigated: 3.5 bar indicated mean effective pressure and 2000 r/min, 5.5 bar indicated mean effective pressure and 2000 r/min, and 5.5 bar indicated mean effective pressure and 2500 r/min. The experimental results confirm how the use of an adequate injection strategy is indispensable to obtain low exhaust emissions values and a balance between the different pollutants. With the increase in the injection pressure and delay in the second injection, it was possible to obtain a trade-off between NOx and particulate matter emission reduction, while there was an increase in hydrocarbon and carbon monoxide emissions under these conditions. In addition, the experiments showed an increase in particle number emissions and a progressive shift in the particles size distribution toward larger sizes, increasing the accumulation-mode particles and reducing the nucleation-mode particles with the decrease in the injection pressure and delay in the third injection.The authors kindly recognize the technical support provided by Mr Pascal Tribotte from RENAULT SAS in the frame of the DREAM-DELTA-68530-13-3205 Project.Bermúdez, V.; Ruiz-Rosales, S.; Novella Rosa, R.; Soto-Izquierdo, L. (2018). Effects of multiple injection strategies on gaseous emissions and particle size distribution in a two-stroke compression-ignition engine operating with the gasoline partially premixed combustion concept. Proceedings of the Institution of Mechanical Engineers Part D Journal of Automobile Engineering. 233(10):1-19. https://doi.org/10.1177/0954407018802960S1192331

A condensation particle counter insensitive to volatile particles

A High Temperature Condensation Particle Counter (HT-CPC) is described that operates at an elevated temperature of up to ca. 300. °C such that volatile particles from typical combustion sources are not counted. The HT-CPC is functionally identical to a conventional CPC, the main challenge being to find suitable non-hazardous working fluids, with good stability, and an appropriate vapour pressure. Some key design features are described, and results of modelling which predict the HT-CPC counting efficiency. Experimental results are presented for several candidate fluids when the HT-CPC was challenged with ambient, NaCl and diesel soot particles, and the results show good agreement with modelled predictions, and confirm that counting of particles of diameters down to at least 10. nm was achievable. Possible applications are presented, including measurement of particles from a diesel car engine and comparison with a near PMP system. © 2014 Elsevier Ltd

Mass-mobility measurements using a centrifugal particle mass analyzer and differential mobility spectrometer

Mass-mobility measurements using a centrifugal particle mass analyzer (CPMA) and differential mobility spectrometer (DMS) are demonstrated. The CPMA, which classifies an aerosol by massto- charge ratio, is used upstream of aDMS,whichmeasures the mobility size distribution of the mass-classified particles in real-time. This system allows formass-mobilitymeasurements to bemade on transient sources at one particle mass or an entire effective density distribution for steady state sources in minutes. Since the CPMA classifies particles by mass-to-charge ratio and multiply charged particles are present, particles of several different masses will be measured by the DMS. Therefore, a correction scheme is required to make accurate measurements. To validate this measurement scheme, two different CPMA-DMS systems were used to measure the known density of di(2ethylhexyl) sebacate (DEHS). The first system consisted of a CPMA and standard DMS500 (Cambustion). This system measured an average effective density of 1027 kg/m3 or within 12.6% of the accepted value with an estimated uncertainty of 30.1% (with 95% confidence). The second system consisted of a CPMA and modified DMS. The modified DMS was a DMS500 with the corona charger disabled and sample and sheath flow rates lowered, decreasing the uncertainty in the mobility measurement. This system measured an average effective density of 964 kg/m3 or within 5.7% of the accepted value with an uncertainty of 9.5-10.4% depending on particle mobility size. Finally, it was determined that multiple-charge correction and size calibration were required, with each correction causing a maximum change in measured effective density greater than 10%. Copyright © American Association for Aerosol Research

Accelerated measurements of aerosol size distributions by continuously scanning the aerodynamic aerosol classifier

Using an Aerodynamic Aerosol Classifier (AAC) upstream of a particle detector is a relatively new method for measuring the aerodynamic size distribution of an aerosol. This approach overcomes limitations of previous methodologies by leveraging the high transmission efficiency, independence from particle charging, and adjustable classification range and resolution of the AAC. However, the AAC setpoint must be stepped and stabilized before each measurement, which forces tradeoffs between measurement time and step resolution. This study is the first to develop and validate theory which allows the speed of the AAC classifier to be continuously varied (following an exponential function), rather than stepped. This approach reduces measurement time, while increasing the resolution of the measured distribution. Assuming uniform axial flow, the transfer function of the scanning AAC and its inversion are determined. Limited trajectory theory is used to derive the idealized transfer function of the scanning AAC, while parameterized, particle streamline theory is used to develop the non-idealized transfer function, which accounts for non-idealized particle and flow behaviors within the classifier. This theory and the practical implementation of the scanning AAC are validated by the high agreement of its measurements of polystyrene latex (PSL) particles (within 8.7% for six sizes between 100 nm to 2.02 μm), and of size distributions of three aerosol sources (Bis(2-Ethylhexyl) sebacate, NaCl and soot) to those measured by the stepping AAC (within 2% or better if the source stability is considered). The validity of assuming uniform axial flow in the classifier and downstream plumbing/detector are also discussed. Copyright © 2020 American Association for Aerosol Research

Measuring aerosol size distributions with the aerodynamic aerosol classifier

The Aerodynamic Aerosol Classifier (AAC) is a novel instrument that selects aerosol particles based on their relaxation time or aerodynamic diameter. Additional theory and characterization is required to allow the AAC to accurately measure an aerosol’s aerodynamic size distribution by stepping while connected to a particle counter (such as a Condensation Particle Counter, CPC). To achieve this goal, this study characterized the AAC transfer function (from 32 nm to 3 μm) using tandem AACs and comparing the experimental results to the theoretical tandem deconvolution. These results show that the AAC transmission efficiency is 2.6–5.1 times higher than a combined Krypton-85 radioactive neutralizer and Differential Mobility Analyzer (DMA), as the AAC classifies particles independent of their charge state. However, the AAC transfer function is 1.3–1.9 times broader than predicted by theory. Using this characterized transfer function, the theory to measure an aerosol’s aerodynamic size distribution using an AAC and particle counter was developed. The transfer function characterization and stepping deconvolution were validated by comparing the size distribution measured with an AAC-CPC system against parallel measurements taken with a Scanning Mobility Particle Sizer (SMPS), CPC, and Electrical Low Pressure Impactor (ELPI). The effects of changing AAC classifier conditions on the particle selected were also investigated and found to be small (<1.5%) within its operating range. Copyright © 2018 American Association for Aerosol Research

Measuring the bipolar charge distribution of nanoparticles: Review of methodologies and development using the Aerodynamic Aerosol Classifier

A review of methodologies to measure the bipolar charge distribution of nanoparticles is completed, including their advantages/disadvantages and sequential development. This summary also provides context for a new development, which uses an Aerodynamic Aerosol Classifier (AAC) and Differential Mobility Analyzer (DMA) in tandem for a similar purpose. It is demonstrated that the tandem AAC-DMA system overcomes some significant limitations of the previous methodologies, such as multiply-charged particle artefacts and low measurement signals. The tandem AAC-DMA methodology also has the sensitivity to detect other charging phenomena, such as the effects of different sample flow rates through the charger, free-ions downstream of the charger, the inlet insert on the 85Kr charger and different particle chargers (x-ray, old 85Kr and new 85Kr). The charge fractions of the particles at low-flow (0.6 L/min) through the new $^{85}$Kr charger agreed well (average absolute difference of 0.007) with widely-used charging theory. However, significant deviations from theory (up to a 0.044 difference in charge fraction) were found with a higher-sample flow rate (1.2 L/min), with different exposure times to free-ions downstream of the charger, or with the inlet insert on the new $^{85}$Kr charger. It was found that regardless of flow rate, a soft x-ray charger resulted in charge fractions which deviated significantly from theory (up to a 0.084 difference in charge fraction), producing higher fractions of positively charged particles and lower fractions of negatively charged particles relative to theory. All of these deviations are likely due to the simplifying assumptions made by the charging theory. Therefore, rigorous measurement of particle charge distributions are necessary for accurate aerosol characterization, such as standard SMPS measurements