Physical and Chemical Characteristics and Volatility of PM in the Proximity of a Light-Duty Vehicle Freeway

Abstract

Volatility properties of ultrafine particles were analyzed next to State Route 110 (Pasadena freeway CA), a light-duty vehicle freeway where heavy-duty traffic is prohibited. In addition, mass concentration and chemical composition of particulate matter (PM) were measured in coarse, accumulation, and ultrafine modes. On weekdays from 17 May to 4 June 2004, measurements were performed in two locations, one very close to the freeway (within 2.5 m from the curb) and one at a distance of about 50 m from the freeway. For measurement of mass and chemical composition, the study employed in each location a micro-orifice uniform deposit impactor (MOUDI) and a modified high-volume sampler. Both instruments sampled with the same size cutpoints: a coarse mode from 2.5 to 10 μm, an accumulation mode from 0.18 to 2.5 μm, and an ultrafine mode of particles less than 0.18 μm in aerodynamic diameter. Alternately, a tandem differential mobility analyzer (TDMA) was used at the two sites. A heater between the two DMAs evaporated volatile material from the monodisperse aerosol, size selected by the first DMA. The second DMA analyzed the losses of volatile components. The ultrafine number concentrations next to the freeway were 46,000 cm^?3 on average during the sampling period. The MOUDI ultrafine mass concentration, nitrate, and EC were higher next to the freeway than at the background site farther from the freeway. The other components analyzed in the ultrafine mode had similar concentrations next to the freeway and at the background site. Volatility ranged from about 65% volume losses of 120 nm particles heated to 110°C to 95% of 20 nm particles. The 20 nm aerosol was only internally mixed, whereas increasing nonvolatile fractions were found for 40 nm (6% next to the freeway), 80 nm (20%), and 120 nm (28%) aerosols.     

DEVELOPMENT OF A DICHOTOMOUS SLIT NOZZLE VIRTUAL IMPACTOR

A high-volume slit nozzle virtual impactor has been developed to collect fine and coarse particles. The size cut-off and particle loss characteristics of the developed slit virtual impactor agree well with those of the 16.7 lmin⁻¹ commercially available dichotomous sampler. The effects of various flow and physical design parameters on the collection of both fine and coarse particles have been investigated. The results of these tests indicated that many of the theoretical principles established for round nozzle virtual impactors can be successfully applied to slit nozzle virtual impactors. However, the effects of the flow volume and Reynolds number (Re) on the cut-off behavior and particle losses are more pronounced for slit virtual impactors. The impactor's particle size cutpoint decreased as the total inlet flow and Re increased. For Re<about 7000, particle losses increased with particle size. For Re of about 7000, particle losses exhibited a maximum near the 50% cutpoint, which is typical in round nozzle virtual impactors. For Re>7000, losses of fine particles were significant, while coarse particle losses were low. Changes in the minor-to-total flow ratio and collection slit width also affected particle losses.     

Calibration,Intersampler Comparison,and Field Application of a New PM-10 Personal Air-Sampling Impactor

The MS&T personal impactor is designed to provide a particle size cut (d _50%) of 10 μm in aerodynamic diameter (PM-10) at a flow rate of 4 L/min. Data are presented that verify the designed particulate mass cut specifications of this impactor for personal sampling. These data are derived from three different analyses', laboratory calibration, intersampler comparison, and field application. Laboratory calibration using monodispersed liquid aerosol shows a sharp 10-μm particle cut size.

The performance of the personal impactor was tested using ambient and combustion-generated aerosols. Established PM-10 samplers (the Sierra/Andersen dichotomous and the MS&T indoor air sampler impactor) were run side by side with the personal impactor. The intra- and intersampler vari-abilities in PM-10 measurements were evaluated. Results showed good precision among personal impactors (CV = 3.2%). The PM-10 sampled by the personal impactor was found to be highly correlated with measurements made with the indoor air sampler impactor (r ² = 0.99) and the dichotomous sampler (r¹ = 0.97).

The impactor was subsequently employed for personal air sampling in the Total Human Environmental Exposure Study (THEES). The THEES sampling protocol entailed 24-hour sampling during a 14-day study interval. THEES field measurements included indoor, outdoor, and personal PM-10 samples. The personal impactor measurements for 13 participants were predicted by a time-weighted exposure model using indoor and outdoor PM-10 and specific activity variables (p < 0.01).     

Source Identification of Size-Segregated Aerosol in Münster,Germany, by Factor Analysis

Sources of size-segregated PM ₁₀ , as collected with a five stage Berner type impactor during six intensive measurement campaigns between January 2006 and August 2007 in Münster, NW Germany, were studied by applying factor analysis. PM samples were collected twice a day, each with a sampling duration of 5 to 7.5 hours. Samples were analyzed for water soluble ions, Ca²⁺, Cl^?, Mg²⁺, Na⁺, NH⁺ ₄ , NO^? ₃ , SO^2? ₄, and elemental and organic carbon. Positive matrix factorization (PMF) was used for source apportionment where each size class accounted for a single variable. Five factors were identified of which at least four could be found during each season. Traffic, ammonium nitrate, and long distance transport showed a weekly cycle, whereas the factor representing power generation and industrial products seemed to be relatively stable through the week. Sea salt particles were independent of the day of week. The five factors do not only have a similar composition, but also a similar size distribution, throughout all seasons of the year. Particles from long-distance transport were identified in the accumulation mode while PM originating from power generation were characterized by a larger diameter. Sea salt aerosol consisted mainly of coarse particles, ammonium nitrate is mainly found in the largest size fraction. Even without a detailed chemical trace elements analysis, precise PM source apportionment was accomplished.     

Design and calibration of a 5-stage cascade impactor (K-JIST cascade impactor)

Particulate matter including PM₁₀, PM_2.5, and PM₁ are one of the major concerns in the atmospheric environment. Due to the simplicity of their construction and use, and their relatively sharp cutoff characteristics, cascade impactors have been widely used for the size classification of particulate matter. In this study, a 5-stage cascade impactor (K-JIST cascade impactor) was designed and tested. Each stage of the impactor was calibrated using a gravimetric or counting method and the fully assembled impactor was evaluated by using a fluorometric method. Experimental data were fitted by non-linear regression using a newly suggested sigmoidal function, and stage response functions were obtained from the collection efficiency curves. Calibrated cut-point diameters were well matched with designed values with sufficient stiffness. Total wall losses, inlet sampling efficiency and the distribution of wall losses for each impactor stage were also evaluated.     

The Influence of Relative Humidity on Nanoparticle Concentration and Particle Mass Distribution Measurements by the MOUDI

A humidity control system was operated upstream of two collocated MOUDIs (micro-orifice uniform deposit impactors) for sampling ambient aerosol particles. One MOUDI used silicone-grease-coated aluminum foils (ALs) as the impaction substrates and was considered as the reference impactor, while the other used uncoated ALs or uncoated Teflon filters (TFs) as the impaction substrates for quantifying the effect of different relative humidities (RHs) and impaction substrates on the PM_0.1 concentrations and mass distributions of ambient PMs. Test results showed that decreasing RH in general increased particle bounce from uncoated substrates with the bounce from uncoated ALs being more severe than that from uncoated TFs. Particle bounce did not influence the overall mass distribution of ambient fine particles when RH ranged between 40% and 80%, whereas it led to undersampling of particles greater than 2.5 μm in aerodynamic diameter severely. Oversampling of PM_0.1 occurred by as much as 95%–180% or 25%–55% when the MOUDI used uncoated ALs or TFs, respectively, as RH was reduced from 50% to 25%. Particle bounce was found to be negligible, and PM_0.1 and PM_2.5 could be sampled accurately with less than 5% error at the RH of 75%–80% or 65%–80% when uncoated ALs or TFs were used, respectively.     

Design of a Glass Impactor for an Annular Denuder/Filter Pack System

A glass impactor for an annular denuder/filter pack system was developed, to further the application of denuder technology in sampling atmospheric gases and particles. The glass impactor consists of an entrance section containing the inlet tube, the acceleration jet, and the impaction plate, which is mounted at the entrance to the annular denuder. The impaction plate is a removable porous glass disk which can be impregnated with mineral oil to minimize bounce-off of the collected particles during sampling. Calibration tests showed that the impactor has a 50% aerodynamic particle cutoff size of 2.1 μm, at a flow of 10 L min^?1. Particle loss experiments were conducted. Total losses on surfaces inside the impactor, annular denuder, and filter pack, determined for particle sizes ranging between 1.50 and 2.77 μm, were lower than 3%. Co-located air sampling was conducted using the glass impactor and the Harvard impactor. Mass concentrations determined using the Harvard impactor were about 10% higher than for the glass impactor because the glass impactor has a slightly lower aerodynamic particle cutoff point, while sulfate concentrations obtained from the two systems were in excellent agreement.     

Inertial Separation of Ultrafine Particles Using a Condensational Growth/Virtual Impaction System

ABSTRACT

A system for the separation of ultrafine particles (i.e., particles smaller than 0.1 μm) has been developed and evaluated. Ultrafine particles are first grown by means of supersaturation to a size that can be easily separated in a virtual impactor. Thus, inertial separation of ultrafine particles occurs without subjecting them to a high vacuum. The condensational growth/virtual impaction system has been evaluated using monodisperse 0.05 and 0.1 μm fluorescent PSL particles, as well as polydisperse ultrafine ammonium sulfate and potassium nitrate aerosols. The generated aerosols were first drawn over a pool of warm water (50°C) where they became saturated. Subsequently, the saturated aerosol was drawn through a cooling tube (8°C) where particles grew due to supersaturation to sizes in the range 1.0–4.0 μm. By placing a virtual impactor with a theoretical 50% cutpoint of 1.4 μm downstream of the condenser, ultrafine particles were separated from the majority (i.e., 90%) of the surrounding gas. The sampling flow rate of the virtual impactor was 8 L/min and its minor-to-total flow ratio was 0.1. For these operating conditions, the particle collection efficiency of the virtual impactor averaged to about 0.9 for particle concentrations in the range 7 × 10⁴-5 × 10⁵ particles/cm³. Particle losses through the system were found less than 5%. Increasing the particle concentration to levels in the range 106–107 particles/cm³ resulted in a decrease in the collection efficiency of the virtual impactor to about 50–70%, presumably due to the smaller final droplet size to which the ultrafine particles grew for the available supersaturation.     

Investigation of Cut-Off Sizes and Collection Efficiencies of Portable Microbial Samplers

This research investigated the physical collection efficiencies and cut-off sizes of SMA MicroPortable, BioCulture, Microflow, Microbiological Air Sampler (MAS-100), Millipore Air Tester (MAT), SAS Super 180, and RCS High Flow portable microbial samplers when collecting Polystyrene Latex particles ranging from 0.5 to 9.8 μm in aerodynamic size. Traditional collection efficiency measurements often directly compare particle concentrations upstream and downstream of the sampler without considering the particle losses. Here, we developed a new approach which tests collection efficiencies of the sampler with and without agar collection plate loaded. This method thus allows estimating the effective collection efficiency, i.e., the fraction of incoming particles deposited onto the agar collection medium only. The experimental cut-off sizes, or d ₅₀, of the investigated samplers ranged from 1.2 μm for the RCS High Flow, 1.7 μm for the MAS-100, 2.1 μm for SAS Super 180, to 2.3 μm for MAT; for other three samplers they were close to or above 5 μm. In most cases the theoretical d ₅₀ was lower than the experimental value, which was likely due to the dissipation of impactor jets and the influence of cross-flow in the multi-nozzle impactors. For most samplers, we observed a notable difference between the collection efficiency obtained by the traditional measurement method and the effective collection efficiency. In general, all samplers collected 10% or less of 0.5 μm particles onto the agar medium. This study indicates that the use of most of the tested bioaerosol samplers may result in a substantial underestimation of bacterial concentrations, especially of single bacterial cells with diameter 0.5–1.0 μm. On the other hand, most of the investigated samplers would be more efficient when collecting larger fungal spores.     

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.     

Evaluation of a Denuder-MOUDI-PUF Sampling System to Measure the Size Distribution of Semi-Volatile Polycyclic Aromatic Hydrocarbons in the Atmosphere

This study describes a field comparison conducted between 2 methods employing different MOUDI impactor configurations to evaluate their performance in sampling and measurement of the size distribution of 15 priority pollutant polycyclic aromatic hydrocarbons (PAHs). Samples were collected during 24 h periods approximately every 7th day, beginning at 8:00 AM, in 2 different sites of the Los Angeles Basin. One site was near Central Los Angeles in an area impacted by high vehicular traffic, whereas the other site was located about 60 km downwind of central Los Angeles (receptor site). Particle samples from about 43 m 3 of air were collected using collocated MOUDI impactors and classified in 3 aerodynamic diameter size intervals: 0-0.18 w m (ultrafine mode I), 0.18-2.5 w m (accumulation mode II), and 2.5-10 w m (coarse mode III). One MOUDI operated in the conventional mode, the other with a vapor trapping system that included an XAD-4 coated annular denuder placed upstream of the impactor and a polyurethane foam plug (PUF) placed in series behind the impactor. PAHs were separated and quantified by HPLC with fluorescence detection optimized for the highest sensitivity. The results showed that for both sites, using either sampling system, the size distributions obtained are similar for the less volatile PAHs (log [ p ° L ] h m 3.2), but different for the more volatile PAHs (log [ p ° L ] S m 2.06). In the central Los Angeles site, the largest PAH fraction was found in the 0-0.18 w m (mode I) size range, typical of primary emissions. At the downwind location, the largest fraction was in the 0.18-2.5 w m (mode II) size range, consistent with an "aged" aerosol. At both sites, albeit not statistically significant, the mean regular to denuded MOUDI mass ratios were 33-36% and 11-19% higher, respectively, for the more volatile and the less volatile PAH groups. Sampling with the regular MOUDI configuration is simpler and thus recommended for measurement of the size distribution of PAHs in either group.     

Influence of Air Pollutants in the 7Be Size Distribution of Atmospheric Aerosols

ABSTRACT

The atmospheric behavior of ⁷Be aerosols was studied by using 1-ACFM cascade impactors. The activity distribution of Be measured by gamma spectrometry (E _γ=477 keV), was largely associated with submicron aerosols in the accumulation mode (0.4–2.0 μm). The activity median aerodynamic diameter, AM AD ranged from 0.62 to 1.00 yam (average 0.80 μm). The geometric standard deviation, σ_g ranged from 1.87 to 2.50 (average 2.22). Low AMADs of ⁷Be aerosols have been observed at locations characterized with relatively low pollution. Some dependency of AMADs on height has been also observed. In near marine environment the ⁷Be activity size distribution was observed in higher size range of aerosol particles (AMAD 0.82 μm).     

Development and Evaluation of a Compact,Highly Efficient Coarse Particle Concentrator for Toxicological Studies

A high-efficiency coarse-mode particle concentrator (CPC) has been developed and evaluated in the laboratory as well as validated in the field experiments at the University of Southern California, in Los Angeles, CA, and in Bilthoven, the Netherlands. The CPC operates with a total intake flow of 1000 LPM. The minor flow rate, containing the concentrated coarse-mode particles (2.5-10 w m), can be adjusted from 33 to 120 LPM in order to enrich ambient coarse PM concentrations by a factor of 8-30, depending on the desirable exposure level and flow rate needed. The laboratory evaluation of the virtual impactors at 3 minor flow rates (3.3, 7, and 10 LPM, respectively) indicated that extremely efficient concentration enrichment was obtained for 2.5-10 w m particles. In the field tests, the CPC operated at a minor flow rate of 33 LPM and the mass obtained was compared to the mass collected by a reference sampler, a (rotating) micro-orifice uniform deposit impactor (MOUDI), which sampled at 30 LPM. Concentration enrichment factors in the range of 26 to 30 were achieved based on particle mass, sulfate, and nitrate as well as selected trace element and metal concentrations (Al, Si, Ca, Fe, K, Mn, Cu, Zn, Ti). CPC and MOUDI concentrations were highly correlated for all species, with R 2 in the range of 0.74 to 0.89. The use of round (compared to rectangular geometry) nozzle virtual impactors in the CPC results in a high concentration efficiency, which reduces the CPC size as well as the power requirement that is required for its operation. The compact size of the CPC makes it readily transportable to desired locations for exposure to coarse-mode particles derived from different sources and thus of a varying chemical composition.     

Size Spectra and Atmospheric Growth of V-Containing Aerosol in Washington,DC

ABSTRACT

Size-segregated submicrometer aerosol particles were collected with micro-orifice impactors (MOI) at three sites in the heavily urban, but nonindustrial-ized Washington, DC metropolitan area during a 40-day period in August and September of 1990, when atmospheric V (an important marker of oil combustion emissions) was principally derived from commercial and utility oil combustion. Results for 34 MOI samples, analyzed for V by instrumental neutron activation analysis, were fit with a least-squares technique which used impactor calibration data to determine log-normal distribution parameters, i.e., mass median aerodynamic diameter (mmad) and geometric standard deviation σ_g; for fine particles bearing V. Geometric size distribution parameters were also determined. The median mmad for 19 College Park (CP) samples was 0.361 ± 0.006 μm. At this site, mmads for samples collected in the absence of rain and with V concentrations > 0.61 ng/m³ increased continuously with increasing relative humidity (RH) from 56% to 79% according to the equation d_p ³= ?0.0222 ± 0.0033/ln(a_w) ? 0.013 ± 0.009. Mmads for samples collected at Andrews AFB were characteristically smaller than those determined at CP at comparable RH, probably due to the influence of a nearby oil-fired boiler.     

Development and Evaluation of a High Loading PM2.5 Speciation Sampler

A high loading sampler for the chemical characterization of fine particles (PM _2.5 ) was developed and validated through laboratory and field experiments. This speciation sampler consists of two identical serially connected impaction stages to remove particles larger than 2.5 μm, following by a chamber to allow use of one or two all-glass honeycomb diffusion denuders, and a holder for a 47 mm filter. Two configurations of the sampler allow sampling at flows of 10 lpm and 16.7 lpm. System performance was evaluated in laboratory experiments using artificially generated polydisperse aerosols. This novel sampler provides a much larger mass loading capacity than previous impactors that use flat, rigid substrate surfaces. The polyurethane foam (PUF) substrate maintains adequate performance characteristics (retention of size cut-off, sharpness of cut-off curve, and minimal particle bounce and re-entrainments) at loadings of at least 35 mg. This is equivalent to 728 μg/m³ for a 48 h sampling period (or 500 h of sampling at 70 μg/m³). System performance was also evaluated in a series of field intercomparison experiments for both flow configurations (10 and 16.7 lpm). Measurements of PM _2.5 mass and sulfate concentrations showed excellent agreement between the US EPA Federal Reference Method (FRM) Sampler and the speciation sampler.     

Hydrodynamic characteristics of cold-bed circulating fluidized beds for the methanol to olefins process

The effects of the riser inlet velocity (2.2–3.9 m/s), seal-pot inlet velocity (2.4–7.1 U _mf ), aeration flow rate (2.5×10^?7–3.7×10^?6 m³/s) in seal-pot, and solid inventory (0.15–0.2 kg) on the hydrodynamic characteristics of a 9 mm-ID×1.9 m-high cold-bed circulating fluidized bed for methanol to olefins (MTO) process were investigated. FCC (Engelhard; 82.4 μm) particles were used as bed particles. Most of the experimental flow regimes were observed in fast fluidization and pneumatic transport regimes. The axial solid holdup in a riser increased with increasing solid mass flux and solid inventory. Solid mass flux increased proportionally until reaching a maximum value and then decreased with increasing seal-pot inlet velocity. The obtained hydrodynamic characteristics in the cold-bed circulating fluidized beds were compared with previous results.     

Stage Response Calibration of the Mark III and Marple Personal Cascade Impactors

Experimental and correlated stage responses (the fraction of particles entering an impactor that are collected on a stage) are presented for the Andersen Mark III and Marple personal cascade impactors. The impactors were operated upright and fully assembled so that interstage interference and wall losses could be properly studied. The observed stage responses showed maxima that fell significantly short of unity, meaning that a monodisperse aerosol is never collected exclusively on one stage, but is distributed among several stages and internal losses. Correlations for the stage responses are presented so that the experimental results can be used to determine size distributions with available data-inversion algorithms. Simulations with log-normal distributions show significant differences between d_pa50 histograms and the more accurate distributions that result by taking the response functions into account.     

A novel quartz crystal cascade impactor for real-time aerosol mass distribution measurement

A quartz crystal microbalance (QCM) based instrument has been developed for real-time aerosol mass distribution measurement. It includes two key components: a six-stage QCM micro-orifice cascade impactor and a novel relative humidity (RH) conditioner. This instrument operates at a flow rate of 10 L·min^?1 and measures the mass of the collected particles in six aerodynamic diameter channels between 45 nm and 2.5 μm. The RH conditioner ensures that the aerosol particles are collected at an RH between 40% and 65%, which is critical for eliminating particle bounce and for ensuring optimal particle coupling with the QCM. The nozzles of the impactors are clustered in the center of the nozzle plates. Therefore, particles are deposited on the central electrode of the QCM, where the mass calculated from first principles (i.e., Sauerbrey equation) agrees with the actual collected mass. The QCM response is linear up to around 130 μg for solid particles and up to around 2 μg for liquid particles. The collection efficiency curves of the QCM impactor stages were measured experimentally with monodisperse aerosols, and the results agree with the predictions of established impactor theory. This QCM-based instrument has also been tested with ambient aerosols with varying temperature and relative humidity. The aerosol distributions measured by this new instrument are in good agreement with simultaneous independent measurements carried out with a wide-range particle spectrometer (MSP Model 1000XP WPS).

Copyright © 2016 American Association for Aerosol Research     

Ion Exchange in Selected Low Rank Coals. Part II: Kinetics

ABSTRACT

The kinetics of ion exchange in a subbituminous coal have been investigated. The rate determining step in the exchange processes was determined to be intraparticle diffusion for particle sizes of 9,500 × 4,000 μm, 425 × 250 μm and 180 × 125 μm. The kinetics of exchange for the 180 × 125 μm particles was very rapid and approached film diffusion control. A relatively simple mathematical model developed by Helfferich (1) describe adequately ion exchange kinetics. The model was effective in predicting the extent of exchange as a function of time and the time required to reach complete exchange. Interdiffusion coefficients at 298 K obtained from the model for exchange of H for Na ⁺, Mg²⁺and Ca²⁺ranged from 2.0 × 10^10?6to 2.6 × 10^?8cm²/s with the values decreasing in the order:

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Design Modification of Virtual Impactor for Enhancing Particle Concentration Performance

Experimental efforts were made to improve the performance of virtual impactors by introducing new design modifications. A flow guide to the minor flow inlet was conceived, designed, and implemented onto the conventional virtual impactor design. Several versions of the flow guides were designed. Subsequently, the effects of the modifications were experimentally evaluated. It was found that if designed properly, a flow guide can improve substantially the particle concentration performance of virtual impactors. The guide angle and distance between the guide and the exit of the aerosol inlet tube ahead of the guide were varied to obtain the optimum design. Subsequently, the performance of one conventional design of virtual impactor and of three different modification designs were compared systematically and the best design among the four was identified. It is expected that this new finding can help improve design of future virtual impactors.