Development of a Portable Aerosol Collector and Spectrometer (PACS)
Changjie Caia, Geb W. Thomasb , Tianbao Yangc, Jae Hong Parkd , Sivaram P. Goginenie, and
Thomas M. Petersf
aDepartment of Occupational and Environmental Health, University of Oklahoma Health Sciences Center, University of Oklahoma, Oklahoma City, Oklahoma, USA; bDepartment of Mechanical and Industrial Engineering, University of Iowa, Iowa City, Iowa, USA; cDepartment of Computer Science, The University of Iowa, Iowa City, Iowa, USA; dSchool of Health Sciences, Purdue University, West Lafayette, Indiana, USA; eSpectral Energies, LLC, Beavercreek, Ohio, USA; fDepartment of Occupational and Environmental Health, University of Iowa, Iowa City, Iowa, USA
ABSTRACT
This article presents the development of a Portable Aerosol Collector and Spectrometer (PACS), an instrument designed to measure particle number, surface area, and mass concentrations continuously and time-weighted mass concentration by composition from 10 nm to 10 mm. The PACS consists of a six-stage particle size selector, a valve system, a water condensation particle counter to detect number concentrations, and a photometer to detect mass concentrations. The stages of the selector include three impactor and two diffusion stages, which resolve particles by size and collect particles for later chemical analysis.
Particle penetration by size was measured through each stage to determine actual collection performance and account for particle losses. The data inversion algorithm uses an adaptive grid-search process with a constrained linear least-square solver to fit a tri-modal (ultrafine, fine, and coarse), log-normal distribution to the input data (number and mass concentration exiting each stage). The measured 50% cutoff diameter of each stage was similar to the design. The pressure drop of each stage was sufficiently low to permit its operation with portable air pumps. Sensitivity studies were conducted to explore the influence of unknown particle density (range from 500 to 3,000 kg/m3) and shape factor (range from 1.0 to 3.0) on algorithm output. Assuming standard density spheres, the aerosol size distributions fit well with a normalized mean bias of 4.9% to 3.5%, normalized mean error of 3.3% to 27.6%, and R2 values of 0.90 to 1.00. The fitted number and mass concentration biases were within ±10% regardless of uncertainties in density and shape. However, fitted surface area concentrations were more likely to be underestimated/overestimated due to the variation in particle density and shape. The PACS represents a novel way to simultaneously assess airborne aerosol composition and concentration by number, surface area, and mass over a wide size range.
Scanning DMA Data Analysis I. Classification Transfer Function
Huajun Maia and Richard C. Flagana,b
aDivision of Engineering and Applied Science, California Institute of Technology, Pasadena, California, USA; bDivision of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California, USA
ABSTRACT
The scanning electrical mobility spectrometer (SEMS; also known as the scanning mobility particle sizer, SMPS) enables rapid particle size distribution measurements with a differential mobility analyzer (DMA)/condensation particle counter (CPC) combination by ramping the classifier voltage, and continuously counting particles into time bins throughout the scan. Inversion of scanning measurements poses a challenge due to the finite time response of the CPC; the distorted data can be deconvoluted to improve the fidelity of size distributions obtained with the SEMS/SMPS. Idealized models of the classification region have shown that, for rapid voltage scans that approach the particle residence time in the DMA, the nondiffusive transfer function deviates from the symmetric one seen at constant voltage. Nonetheless, most SEMS/SMPS data analyses employ the constant voltage transfer function, a result that is valid only for plug flow in the classification region. This article develops the
scanning-mode transfer function for the actual geometry of the TSI Model 3081 DMA. Finite element calculations are used to determine the flow and electric fields through the entire DMA. The instantaneous scanning-DMA transfer function for diffusive particles is determined using Brownian dynamics simulations. Comparisons of the results from this simulation of a real instrument to those from the idealized models reveal the shortcomings of prior models in describing the instantaneous scanningDMA transfer function. A companion paper (Part II) combines this scanning-mode transfer function with response functions for the other components of a SEMS/SMPS measurement system in order to derive the response function for the integrated measurement system.
Scanning DMA data analysis II. Integrated DMA-CPC instrument response
and data inversion
Huajun Maia , Weimeng Kongb , John H. Seinfelda,b , and Richard C. Flagana,b
aDivision of Engineering and Applied Science, California Institute of Technology, Pasadena, California, USA; bDivision of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California, USA
ABSTRACT
Analysis of scanning electrical mobility spectrometer (SEMS) or SMPS data requires coupling the scanning differential mobility analyzer (DMA) transfer function with the response functions for the instrument plumbing and the detector. In the limit of plug flow (uniform velocity) within the DMA, the scanning DMA transfer function has the same form as that for constant voltage. Most SEMS/SMPS data analysis uses this model, though previous studies have shown that boundary layers distort the transfer function during scanning DMA measurements. Part I determined the instantaneous transfer function during scanning of the TSI Model 3081 A long column DMA by modeling the flows, fields, and particle trajectories within the actual DMA geometry. This study (Part II) combines that transfer function with empirical data on the efficiencies and delay time distributions of the plumbing and detector of the SEMS/SMPS to determine the instantaneous rate at which particles are counted, and integrates the count rate over the finite counting time interval to obtain the integrated SEMS/SMPS response function. Simulations using this geometrical model are compared with those obtained using traditional, idealized DMA models for scan rates ranging from slow (240 s) to very fast (10 s), and with measurements of monodisperse calibration aerosols. Data inversion studies show that both increasing and decreasing voltage scans can be used to determine the particle size distribution, even with fast scans.
Influence of gas velocity on the particle collection and reentrainment in an
air-cleaning electrostatic precipitator
Rafael Sh. Islamova,b
aInstitute on Laser and Information Technologies - Branch of the Federal Scientific Research Centre “Crystallography and Photonics” of Russian Academy of Sciences, Shatura, Moscow Region, Russia; bDepartment of Optics, Moscow State University of Geodesy and Cartography (MIIGAIK), Moscow, Russia
ABSTRACT
Particle deposition and reentrainment experiments were performed in a two-stage electrostatic precipitator (ESP), consisting of positive corona precharger and collecting electrode sections. Attention was focused on studying the indoor air pollution deposition and reentrainment into six size ranges from 0.3 to > 10 lm. Tests were performed in an office room (200 m3) for airflow velocities from 1.4 to 8 m/s. The effect of airflow velocity on the collection efficiency of the ESP was investigated both experimentally and analytically to study reentrainment phenomena in a turbulent flow. A stationary two-dimensional analytical
model was carried out by modeling the particle transport. The boundary conditions for charged particles on collecting and repelling electrodes were determined by physical considerations, including chaotic and drift motions, the reflection of charged particles from a surface, and the reentrainment of charged particles. A decrease in the experimental collection efficiency for large particle diameters ( 0.5 lm), as compared to the theoretical prediction, was interpreted as the reentrainment of particles. The size-resolved dust reentrainment fluxes from the collecting electrode were evaluated in two limiting cases, considering that either the reentrained particles are not charged or that they are charged as the particles in the deposition flux. Dimensional analysis is applied to these results, introducing the wall friction velocity as a universal parameter that determines the flow character. In general, the particles with diameters < 5 lm and > 5 lm exhibit different reentrainment behavior.
A refractive-index and position-independent single-particle detector for
large, nonabsorbing, spherical particles
Mir Seliman Waeza, Steve J. Eckelsa, and Christopher M. Sorensenb
aDepartment of Mechanical and Nuclear Engineering, Institute for Environmental Research (IER), Kansas State University (KSU), Manhattan, Kansas, USA; bDepartment of Physics, Kansas State University, Manhattan, Kansas, USA
ABSTRACT
We show that for spherical particles with real refractive index and diameters greater than ca. 10 microns, the differential scattering cross-section is only independent of the refractive index at angles near 37 ± 5 . We built a device with a modified Gaussian incident beam profile so that the beam transit time of a particle passing through the beam can determine the true incident intensity for the scattering of the particle. By combining the modified Gaussian incident beam profile with detection of scattered light near 37 ± 5 , we demonstrate a refractive-index independent measurement of single spherical particles as they pass
through the beam.
