Size-selective sampling performance of six low-volume “total” suspended particulate (TSP) inlets

Several low-volume inlets (flow rates ≤ 16.7 liters per minute (Lpm)) are commercially available as components of low-cost, portable ambient particulate matter samplers. Because the inlets themselves do not contain internal fractionators, they are often assumed to representatively sample “total” mass concentrations from the ambient air, independent of aerodynamic particle size and wind speed. To date, none of these so-called “TSP” inlets have been rigorously tested under controlled conditions. To determine their actual size-selective performance under conditions of expected use, wind tunnel tests of six commonly used omnidirectional, low-volume inlets were conducted using solid, polydisperse aerosols at wind speeds of 2, 8, and 24 km/h. With the exception of axially-oriented, isokinetic sharp-edge nozzles operating at 5 and 10 Lpm, all low-volume inlets showed some degree of nonideal sampling performance as a function of aerodynamic particle size and wind speed. Depending upon wind speed and assumed ambient particle size distribution, total mass concentration measurements were estimated to be negatively biased by as much as 66%. As expected from particle inertial considerations, inlet efficiency tended to degrade with increasing wind speed and particle size, although some exceptions were noted. The implications of each inlet's non-ideal behavior are discussed with regards to expected total mass concentration measurement during ambient sampling and the ability to obtain representative sampling for size ranges of interest, such as PM_2.5 and PM₁₀. Overall test results will aid in low-volume inlet selection and with proper interpretation of results obtained with their ambient field use.

Copyright © 2018 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     

Sampling efficiency of low-volume PM10 inlets with different impaction substrates

The louvered 16.7?L min^?1 PM₁₀ inlet is commonly used in PM₁₀ and PM_2.5 FRM samplers or FEM monitors. Its sampling efficiency is influenced by particle bounce, re-entrainment, and overloading since the PM₁₀ inlet contains a PM₁₀ impactor with an uncoated impaction surface. In this study, a modified PM₁₀ (M-PM₁₀) inlet with an oil-soaked glass fiber filter substrate supported by an oil-soaked porous metal disc was developed to eliminate the particle bounce and overloading effects. The oiled M-PM₁₀ inlet and the traditional PM₁₀ inlets with and without grease coating were collocated at the field for long-term comparison tests. The results show that the traditional uncoated PM₁₀ inlet which is cleaned initially but not cleaned daily afterwards oversamples PM₁₀ concentration after one 24-h sampling day and has the high positive average sampling bias during 14 sampling days due to particle bounce and re-entrainment. The grease-coated PM₁₀ inlet without daily cleaning shows a better performance with a smaller sampling bias, but it still oversamples PM₁₀ concentrations after the first three 24-h sampling days and then undersamples after 10 sampling days due to particle bounce and overloading effects, respectively. In comparison, the M-PM₁₀ inlet shows a good performance with a small average sampling bias during 35 sampling days since vacuum oil wicks up through the deposit to eliminate particle bounce and overloading. It is suggested that the oiled M-PM₁₀ inlet can be used to replace the traditional EPA PM₁₀ inlet and for long-term sampling of over 1 month without the frequent maintenance need.

Copyright © 2019 American Association for Aerosol Research     

Numerical Study of the RespiCon Sampler Performance in the Calm Air

Results of a numerical study of the RespiCon sampler performance in the calm air are presented. The air flow is described by the Navier–Stokes equations of axisymmetric stationary viscous flow of incompressible fluid that are numerically integrated by the computational fluid dynamics (CFD) software FLUENT. The collection efficiencies of RespiCon impactor stages agree quite well with experimental data and curves of the European standards for the thoracic and respirable dust fractions. The aspiration efficiencies derived from the numerical model overestimate the experimental data in the range of particle sizes of 10 μm < d_p < 40 μm; however, they correctly predict the value of maximal size of aspirated particles. A new design of the RespiCon sampler with a higher volume flow rate was developed.

Copyright 2014 American Association for Aerosol Research     

Thermal/optical reflectance equivalent organic and elemental carbon determined from federal reference and equivalent method fine particulate matter samples using Fourier transform infrared spectrometry

A fine particulate matter (PM_2.5) monitoring network of filter-based federal reference methods and federal equivalent methods (FRM/FEMs) is used to assess local ambient air quality by comparison to National Ambient Air Quality Standards (NAAQS) at about 750 sites across the continental United States. Currently, FRM samplers utilize polytetrafluoroethylene (PTFE) filters to gravimetrically determine PM_2.5 mass concentrations. At most of these sites, sample composition is unavailable. In this study, we present the proof-of-principle estimation of the carbonaceous fraction of fine aerosols on FRM filters using a nondestructive Fourier transform infrared (FT-IR) method. Previously, a quantitative FT-IR method accurately determined thermal/optical reflectance equivalent organic and elemental carbon (a.k.a., FT-IR organic carbon [OC] and elemental carbon [EC]) on filters collected from the chemical speciation network (CSN). Given the similar configuration of FRM and CSN aerosol samplers, OC and EC were directly determined on FRM filters on a mass-per-filter-area basis using CSN calibrations developed from nine sites during 2013 that have collocated CSN and FRM samplers. FRM OC and EC predictions were found to be comparable to those of the CSN on most figures of merit (e.g., R²) when the type of PTFE filter used for aerosol collection was the same in both networks. Although prediction accuracy remained unaffected, FT-IR OC and EC determined on filters produced by a different manufacturer show marginally increased prediction errors suggesting that PTFE filter type influences extending CSN calibrations to FRM samples. Overall, these findings suggest that quantifying FT-IR OC and EC on FRM samples appears feasible.

© 2018 American Association for Aerosol Research     

Comparison of two size-differentiating air samplers for detecting airborne swine viruses under experimental conditions

Detection and quantification of dilute viral aerosols, as encountered outside animal housing facilities, requires methods that are able to detect small numbers of viruses in large volumes of air. This study compared the performance of two size-differentiating cascade impactors; an Andersen 8-stage (ACI; 28.3 L/min) and a high volume Tisch (TCI; 1,133 L/min) to assess sampling efficiency for detecting porcine reproductive and respiratory syndrome virus (PRRSV) and influenza A virus (IAV). Samples of particles sorted by aerodynamic diameter were analyzed by quantitative polymerase chain reaction (qPCR) and collection efficiency was assessed by particle size. Collection media (minimum essential medium [MEM] and beef extract [BE]), elution technique (active versus passive), and sampling times (10, 20, and 30 min) were variables assessed for the TCI sampler. Extraction efficiency was 35% higher with BE as compared to that of MEM (p = 0.0007); active extraction technique was 19% more efficient than the passive technique (p = 0.03); time of sampling did not significantly affect the amount of virus recovered. The ACI sampler was more efficient in detecting both viruses from small and medium sized airborne particles (≤3 μm) as compared to the TCI sampler (p < 0.001). The latter sampler, however, was more efficient at IAV detection from large airborne particles (>3 μm) (p = 0.0025) indicating the potential of this sampler in detecting the presence of small amounts of viruses in aerosols under field conditions.

© 2017 American Association for Aerosol Research     

Design and Performance of a Low-Cost Micro-Channel Aerosol Collector

Aerosol sampling and identification is vital for assessment and control of particulate matter pollution, airborne pathogens, allergens and toxins, and their effect on air quality, human health, and climate change. Assays capable of accurate identification and quantification of chemical and biological airborne components of aerosol provide very limited sampling time resolution and relatively dilute samples. A low-cost micro-channel collector (μCC) which offers fine temporal and spatial resolution, high collection efficiency, and delivers highly concentrated samples in very small liquid volumes was developed and tested. The design and optimization of this μCC was guided by computational fluid dynamics (CFD) modeling. Collection efficiency tests of the sampler were performed in a well-mixed aerosol chamber using aerosolized fluorescent microspheres in the 0.5–6 μm diameter range. Samples were collected in the μCC and eluted into 100 μL liquid aliquots; bulk fluorescence measurements were used to determine the performance of the collector. Typical collection efficiencies were above 50% for 0.5 μm particles and 90% for particles larger than 1 μm. The experimental results agreed with the CFD modeling for particles larger than 2 μm, but smaller particles were captured more efficiently than predicted by the CFD modeling. Nondimensional analysis of capture efficiencies showed good agreement for a specific geometry but suggested that the effect of channel curvature needs to be further investigated.

Copyright 2014 American Association for Aerosol Research     

Development of a High-Pressure Aerodynamic Lens for Focusing Large Particles (4–10 μm) into the Aerosol Time-of-Flight Mass Spectrometer

A new aerodynamic lens system for an online aerosol time-of-flight mass spectrometer (ATOFMS) has been designed and constructed to transmit and allow the analysis of individual particles in the 4–10-μm-size range. Modeling was used to help design the lens within the bounds of ATOFMS instrumental constraints. The aerodynamic lens operates at a high inlet pressure, 3066 Pa (23 Torr), with a unique tapered relaxation region to improve large particle transmission. Every stage of the lens was tested empirically using a combination of particle deposition and light scattering experiments. The critical orifice was found to significantly impact large particle transmission, with orifices <200 μm in diameter completely suppressing large particle transmission. The addition of a virtual impactor allowed for the use of large orifices without any loss of functionality in the ATOFMS. The detection efficiency of the ATOFMS was >10% for particles from 4–10 μm with a peak efficiency of 74 ± 9% for 6-μm particles. With the extended size range provided by this inlet, the ATOFMS can now be extended to investigate single cell metabolomics.

Copyright 2014 American Association for Aerosol Research     

Investigation of airflow and particle behavior around a subway train running in the underground tunnel

Airflow around an eight-passenger-car subway train running in the underground tunnel at a cruise speed of 70 km/h was numerically simulated, and the trajectories of the particles that were assumed to be re-suspended from the ground or generated at the contact points between the wheels and rails were predicted. In addition, field experiments were conducted to measure airflow velocity and PM₁₀ mass concentration under a T-car (trailer car without a driving cab) during the running of a subway train in straight sections of the underground tunnel of the Seoul Subway Line 5. The numerically predicted airflow velocities agreed well with the experimental data with the error of less than 30%, and the predicted particle distribution showed a similar tendency to the experimental results. The airflow under the T-car was predicted to be relatively uniform compared to the airflow under other passenger cars. Both numerical results and experimental data signified that a lot of particles could drift under the T-car by showing a higher particle concentration in the central area of the space under the T-car than in the edge area. As a result, the space underneath the T-car is anticipated to be a good place for installing a dust-removal system.

© 2016 American Association for Aerosol Research     

Development and evaluation of a palm-sized optical PM2.5 sensor

A new palm-sized optical PM_2.5 sensor has been developed and its performance evaluated. The PM_2.5 mass concentration was calculated from the distribution of light scattering intensity by considering the relationship between scattering intensity and particle size. The results of laboratory tests suggested that the sensor can detect particles with diameters as small as ～0.3 µm and can measure PM_2.5mass concentrations as high as ～600 µg/m³. Year-round ambient observations were conducted at four urban and suburban sites in Fukuoka, Kadoma, Kasugai, and Tokyo, Japan. Daily averaged PM_2.5 mass concentration data from our sensors were in good agreement with corresponding data from the collocated standard instrument at the Kadoma site, with slopes of 1.07–1.16 and correlation coefficients (R) of 0.90–0.91, and with those of the nearest observatories of the Ministry of the Environment of Japan, at 1.7–4.1 km away from our observation sites, with slopes of 0.97–1.23 and R of 0.89–0.95. Slightly greater slopes were observed in winter than in summer, except at Tokyo, which was possibly due to the photochemical formation of relatively small secondary particles. Under high relative humidity conditions (>70%), the sensor has a tendency to overestimate the PM_2.5 mass concentrations compared to those measured by the standard instruments, except at Fukuoka, which is probably due to the hygroscopic growth of particles. This study demonstrates that the sensor can provide reasonable PM_2.5 mass concentration data in urban and suburban environments and is applicable to studies on the environmental and health effects of PM_2.5.

Copyright © 2018 American Association for Aerosol Research     

Assessment of indoor bioaerosols using a lab-made virtual impactor

To assess indoor bioaerosols, a virtual impactor having 1 µm cutoff diameter was designed, fabricated, and evaluated with computational fluid dynamics simulation and also with laboratory test using polystyrene latex particles. Two other cutoff diameters of 635 nm and 1.5 µm were obtained by changing the inlet flow rate and the ratio of minor channel-to-inlet flow rates. In field test, the virtual impactor was operated with varying cutoff diameter and field-emission scanning electron microscope (FE-SEM) analysis was performed for each cutoff diameter to observe morphologies of indoor aerosol particles sampled at the major and minor outlet channels. Particles were sampled at both outlet channels using the SKC Button Aerosol sampler and subsequently cultured. By colony counting, it was found that 56% of cultured fungal particles and 63% of cultured bacterial particles had aerodynamic sizes smaller than 1 µm. MALDI-TOF analysis and visual inspection of culture samples were used to identify indoor bacterial and fungal species, respectively. Nearly same species of bacteria and fungi were detected both in the major and minor flow channels.

© 2017 American Association for Aerosol Research     

Characteristics of secondhand electronic cigarette aerosols from active human use

The electronic cigarette (EC) is a new source of indoor airborne particles. To better understand the impacts of secondhand vaping (SHV) emissions on indoor air quality, real-time measurements of particle size distribution, particle number concentration (PNC), fine particulate matter (PM_2.5), CO₂, CO, and formaldehyde were conducted before, during, and after 10 min EC-use among 13 experienced users in an 80 m³ room. To assess particle transport in the room, multiple sampling locations were set up at 0.8, 1.5, 2.0, and 2.5 m away from the subjects. The arithmetic mean (standard deviation) of background PNC and PM_2.5 concentrations in the room were 6.39 × 10³ (1.58 × 10²) particles/cm³ and 8 (1) μg/m³, respectively. At 0.8 m away from EC users, right after initiation of puffing, the PNC and PM_2.5 concentrations can reach a peak of ～10⁵ particles/cm³ and ～3 × 10³ µg/m³, respectively, and then dropped quickly to background levels within 20 s due to dilution and evaporation. At the 0.8 m sampling location, the mean PNC and PM_2.5 concentrations during puffing were 2.48 × 10⁴ (2.14 × 10⁴) particles/cm³ and 188 (433) µg/m³, respectively. In addition, two modes of SHV particles were observed at about 15 and 85 nm. Moreover, concentrations of SHV particles were negatively correlated with the distances to EC users. At the 1.5 m location, PNC and PM_2.5 levels were 9.91 × 10³ (1.76 × 10³) particles/cm³ and 19 (14) µg/m³, respectively. Large variations of mean PNC levels exhaled per puff were observed both within and between EC users. Data presented in this study can be used for SHV particle exposure assessment.

Copyright © 2017 American Association for Aerosol Research     

Particulate matter characteristics,dynamics, and sources in an underground mine

Particulate matter (PM) from mining operations, engines, and ore processing may have adverse effects on health and well-being of workers and population living nearby. In this study, the characteristics of PM in an underground chrome mine were investigated in Kemi, Northern Finland. The concentrations and chemical composition of PM in size ranges from 2.5 nm to 10 µm were explored in order to identify sources, formation mechanisms, and post-emission processes of particles in the mine air. This was done by using several online instruments with high time-resolution and offline particulate sampling followed by elemental and ionic analyses. A majority of sub-micrometer particles (<1 µm in diameter, PM₁) originated from diesel engine emissions that were responsible for a rather stable composition of PM₁ in the mine air. Another sub-micrometer particle type originated from the combustion products of explosives (e.g., nitrate and ammonium). On average, PM₁ in the mine was composed of 62%, 30%, and 8% of organic matter, black carbon, and major inorganic species, respectively. Regarding the analyzed elements (e.g., Al, Si, Fe, Ca), many of them peaked at >1 µm indicating mineral dust origin. The average particle number concentration in the mine was (2.3 ± 1.4)*10⁴ #/cm³. The maximum of particle number size distribution was between 30 and 200 nm for most of the time but there was frequently a distinct mode <30 nm. The potential origin of nano-size particles remained as challenge for future studies.

Copyright © 2018 The Authors. Published with license by Taylor & Francis     

Effect of convergence angle on impactor performance

Two nozzles, modified and original, were tested in a sampler that was placed in a wind tunnel and penetration efficiencies, √Stk50, and slope of the performance curve were determined by challenging the sampler with fluorescent-tagged monodisperse test aerosol particles having known concentration. It was shown that a change in convergence angle of the modified nozzle can affect impactor performance. The √Stk50 for original and modified nozzles were 0.57 and 0.49, respectively. The slope of the efficiency curve for original and modified nozzles was 1.52 and 1.36, respectively.

© 2017 American Association for Aerosol Research     

Numerical,wind-tunnel,and atmospheric evaluation of a turbulent ground-based inlet sampling system

The use of inlets for transferring aerosols from the environment to instrumentation can introduce uncertainty in the measurement of aerosol properties. Aerosol loss during this process is a non-negligible issue that may bias the subsequent measurements. These loss mechanisms include aspiration at the inlet head and deposition/evaporation/condensation during transport through the sampling lines. Coarse-mode aerosol is significantly impacted by the aspiration and inertial loss mechanisms within an inlet system. This work uses wind tunnel experiments to investigate aerosol losses through the Storm Peak Laboratory’s (SPL) new aerosol inlet system. The inlet is used extensively for both intensive field campaigns and long-term aerosol monitoring. The results of numerical simulations of the SPL aerosol inlet sampling efficiency are provided at several wind speeds, and experimental results demonstrate the system has a 50% cut off for the coarse-mode at an aerodynamic diameter of approximately 13?μm and wind speed of 0.5?m s^?1. This investigation will lead to improved accuracy of in situ aerosol measurements at SPL and this system can be replicated at other atmospheric stations.

Copyright © 2019 American Association for Aerosol Research     

Mixing State of Black Carbon Aerosol in a Heavily Polluted Urban Area of China: Implications for Light Absorption Enhancement

Black carbon (BC) is important for climate forcing, and its effects on the Earth's radiative balance remain a major uncertainty in climate models. In this study, we investigated the mixing state of refractory black carbon (rBC) and aerosol optical properties in a polluted atmosphere at Xi’an, western China. The average rBC mass concentration was 9.9 μg m^?3 during polluted periods, 7.6 times higher than that in clean periods. About 48.6% of the rBC was internally-mixed or coated with nonrefractory materials during polluted periods; this was 27% higher than in clean periods. Correlation analysis between the number fraction of thickly-coated rBC particles (f_BC) and the major particulate species indicate that organics may be the primary contributor to rBC coatings during polluted periods. The average mass absorption cross section of rBC (MAC_BC) particles at λ = 870 nm was 7.6 ± 0.02 m² g^?1 for the entire campaign. The MAC_BC showed a positive correlation with f_BC, and the enhancement of MAC_BC due to internal mixing was 1.8 times. These observations suggest that an enhancement of BC absorption by a factor of ～2 could be appropriate for climate models associated with high PM_2.5 levels.

Copyright 2014 American Association for Aerosol Research     

New PM10 Inlet Design and Evaluation

ABSTRACT

A new PM₁₀ inlet for a beta-gauge sampler was designed based on the particle cup impactor concept. The performance of the inlet was evaluated at near-zero wind velocity in the test chamber and at the wind velocities of 2 and 8 km/hr in the wind tunnel. The performance indicated that particles with aerodynamic diameters of 10 μm or larger were collected in the particle cup and the inlet proved to meet the basic requirement of PM₁₀ sampling.     

Laboratory characterization of an aerosol chemical speciation monitor with PM2.5 measurement capability

The Aerodyne Aerosol Chemical Speciation Monitor (ACSM) is well suited for measuring non-refractory particulate matter up to approximately 1.0 µm in aerodynamic diameter (NR-sub-PM₁). However, for larger particles the detection efficiency is limited by losses in the sampling inlet system and through the standard aerodynamic focusing lens. In addition, larger particles have reduced collection efficiency due to particle bounce at the vaporizer. These factors have limited the NR-sub-PM₁ ACSM from meeting PM_2.5 (particulate matter with aerodynamic diameter smaller than 2.5 µm) monitoring standards. To overcome these limitations, we have redesigned the sampling inlet, the aerodynamic lens, and particle vaporizer. Both the new lens and vaporizer are tested in the lab using a quadruple aerosol mass spectrometer (QAMS) system equipped with light scattering module. Our results show that the capture vaporizer introduces additional thermal decomposition of both inorganic and organic compounds, requiring modifications to the standard AMS fragmentation table, which is used to partition ion fragments to chemical classes. Experiments with mixed NH₄NO₃ and (NH₄)₂SO₄ particles demonstrated linearity in the NH₄⁺ ion balance, suggesting that there is no apparent matrix effect in the thermal vaporization-electron impact ionization detection scheme for mixed inorganic particles. Considering a typical ambient PM_2.5 size distribution, we found that 89% of the non-refractory mass is detected with the new system, while only 65% with the old system. The NR-PM_2.5 system described here can be adapted to existing Aerodyne Aerosol Mass Spectrometer (AMS) and ACSM systems.

Copyright © 2017 American Association for Aerosol Research     

Electrostatic capillary collector for in-situ spectroscopic analysis of aerosols

Identification of particulate matter is important in assessing an individual’s exposure to potentially harmful particles, such as aeroallergens, toxins, and emissions from combustion sources, which can contribute to cardio-pulmonary diseases. Efficient collection of aerosols is essential for aerosol exposure studies such as analysis of chemical and biological components. We present the design and evaluation of a capillary collector that collects PM_2.5 onto the outer surface of a capillary for in-situ spectroscopic analysis. The capillary collector uses a needle-to-ring corona generator to charge particles; the electric field between a cylinder and a wire inserted into the bore of a capillary is used to collect the charged particles. Corona and repelling voltages are optimized for maximum collection of ambient PM_2.5 particles and fluorescent polystyrene latex microspheres in the PM_2.5 size range, on the capillary. The capillary collection efficiency of ambient PM_2.5 at 3 slpm operating flow rate and optimal operating voltages is 63%. Fluorescence spectroscopy is used to quantify the collection of polystyrene latex microspheres. The fluorescence-based capillary collection efficiency is in close agreement with the capillary collection efficiency of ambient PM_2.5. The collection and analysis methodology can be used to develop a compact, low-cost sensor for in-situ spectroscopic analysis of aerosols to determine their chemical composition for source apportionment.

Copyright © 2019 American Association for Aerosol Research