Performance Characteristics of PM10 Samplers under Calm Air Conditions

ABSTRACT

The size range of airborne particles that is closely related to specific deposition regions in the human respiratory tract and excess lung burden of these deposited particles is associated with disease. Size-selective sampling, therefore, needs to be performed to assess the related health risks. Performance criteria applied to these samplers must be well characterized in order to provide accurate and reliable results. The PM₁₀ samplers that have been used in place of the total suspended particulate samplers for the collection of ambient air particles are more relevant to potential inhalation hazards. In order to be certified, a PM₁₀ sampler must meet reliable performance specifications, primarily the aerosol penetration test with liquid and solid particles in a wind tunnel (wind speeds of 2, 8, and 24 km/hr). This testing is intended to assure reasonable accuracy in aerosol measurements. However, the sampler performance under calm air conditions has not been well studied.

In the present study, the sampling heads of three devices—the Harvard impactor, the Personal Environmental Monitor (PEM), and the Sierra Andersen model 241 dichotomous sampler PM₁₀ inlet head—were tested for aerosol separation efficiency. With the consideration of bias and imprecision of the measurements, five specimens of each type of sampler were chosen for performance testing, repeating the tests 5 times for each specimen. An ultrasonic atomizing nozzle was used to nebulize potassium sodium tartrate tetrahydrate and dioctyl phthalate particles as the solid and liquid challenge aerosols, respectively. The aerosol number concentrations and size distributions upstream and downstream of the samplers were measured by using an aerosizer calibrated against a settling velocity chamber. The results showed that among the samplers tested, the dichotomous sampler PM₁₀ inlet head had the best fit to the PM₁₀ convention, while the other two samplers not only appeared to have a steeper separation-curve slope but also had significant particle bounce when challenged with solid particles. Analysis of variance also confirmed the superiority of the dichotomous samplers. Surface-coating with oil or grease greatly reduced the problem of particle bounce.     

Performance characteristics of PM10 samplers under calm air conditions

The size range of airborne particles that is closely related to specific deposition regions in the human respiratory tract and excess lung burden of these deposited particles is associated with disease. Size-selective sampling, therefore, needs to be performed to assess the related health risks. Performance criteria applied to these samplers must be well characterized in order to provide accurate and reliable results. The PM10 samplers that have been used in place of the total suspended particulate samplers for the collection of ambient air particles are more relevant to potential inhalation hazards. In order to be certified, a PM10 sampler must meet reliable performance specifications, primarily the aerosol penetration test with liquid and solid particles in a wind tunnel (wind speeds of 2, 8, and 24 km/hr). This testing is intended to assure reasonable accuracy in aerosol measurements. However, the sampler performance under calm air conditions has not been well studied. In the present study, the sampling heads of three devices--the Harvard impactor, the Personal Environmental Monitor (PEM), and the Sierra Andersen model 241 dichotomous sampler PM10 inlet head--were tested for aerosol separation efficiency. With the consideration of bias and imprecision of the measurements, five specimens of each type of sampler were chosen for performance testing, repeating the tests 5 times for each specimen. An ultrasonic atomizing nozzle was used to nebulize potassium sodium tartrate tetrahydrate and dioctyl phthalate particles as the solid and liquid challenge aerosols, respectively. The aerosol number concentrations and size distributions upstream and downstream of the samplers were measured by using an aerosizer calibrated against a settling velocity chamber. The results showed that among the samplers tested, the dichotomous sampler PM10 inlet head had the best fit to the PM10 convention, while the other two samplers not only appeared to have a steeper separation-curve slope but also had significant particle bounce when challenged with solid particles. Analysis of variance also confirmed the superiority of the dichotomous samplers. Surface-coating with oil or grease greatly reduced the problem of particle bounce.     

The Mass Distribution of Large Atmospheric Particles

This paper describes the results of a study to determine the total mass and the mass distribution of atmospheric aerosols, especially that mass associated with particles greater than 10 μm diameter. This study also determined what fraction of the total aerosol mass a standard high-volume air sampler collects and what fraction and size interval settle out on a dust fall plate. A special aerosol sampling system was designed for this study to obtain representative samples of large airborne particles. A suburban sampling site was selected because no local point sources of aerosols existed nearby. Samples were collected under various conditions of wind velocity and direction to obtain measurements on different types of aerosols.

Study measurements show that atmospheric particulate matter has a bimodal mass distribution. Mass associated with large particles mainly ranged from 5 to 100 μm in size, while mass associated with small particles ranged from an estimated 0.03 to 5 μm in size. Combined, these two distributions produced a bimodal mass distribution with a minimum around 5 μm diameter. The high-volume air sampler was found to collect most of the total aerosol mass, not just that fraction normally considered suspended particulate. Dust fall plates did not provide a good or very useful measure of total aerosol mass. The two fundamental processes of aerosol formation, condensation and dispersion appear to account for the formation of a bimodal mass distribution in both natural and anthropogenic aerosols. Particle size distribution measurements frequently are in error because representative samples of large airborne particles are not obtained. Considering this descrepancy, air pollution regulations should specify or be based upon an upper particle size limit.     

Design and Testing of a New Size Classifying Isokinetic Sequential Aerosol Sampler

Two versions of a size-classifying isokinetic sequential aerosol sampler (SCISAS) have been designed, built, tested, and deployed in a field program in the southwestern United States. The SCISAS units can operate at unattended sites, exposing four or more filter types simultaneously, in two size ranges, for six sampling time Intervals. Design considerations included theoretical estimates of aerosol particle losses in the 0-15 μm size range.

SCISAS prototypes have been tested to evaluate their sampling efficiency as a function of flow rate, the sensitivity of the sampling efficiency to isokinetic matching within the SCISAS sampling stack, the equivalency of their sampling ports, and their passive deposition characteristics. The prototypes were also compared to several other types of aerosol filtration samplers already in common use. These tests show that particle loss mechanisms within the SCISAS usually cause no more than 5 percent losses, and that the SCISAS units agree, within one to two measurement uncertainty intervals, with other types of aerosol samplers.     

A Field Intercomparison and Fundamental Characterization of Various Dust Samplers with a Reference Sampler

The European Economic Community Council Directive 80/779/EEC¹ describes air quality limit values and guide values for sulfur dioxide and suspended particulates. However, article 10, paragraph 1 does not define well enough the reference method required, amongst others, for the gravimetric measurement of suspended particulates. It is explicity stated in article 10, paragraph 5 of the directive, that “the commission shall, in selected locations in the member states and in cooperation with the latter, carry out studies on the sampling…of suspended particulates. These studies shall be designed in particular to promote the harmonization of methods of sampling and analysis of these pollutants.” Therefore, in a joint research program the Umweltbundesamt (grant #104 02263), the commission of European Communities (grant #84-B-6642-11-017-11-N) and the U.S. EPA (grant #2-43211-3580) funded the development of a reference dust sampler by the Fraunhofer-lnstitute of Toxicology and Aerosol Research (FhITA, in Hannover, Federal Republic of Germany) in order: ? to study the particle size distribution of ambient air aerosol at a number of selected sites;

? to compare the results obtained with the reference dust sampler with those of particle samplers operated in Europe and the United States;

? to verify the applicability of wind tunnel results to sampling behaviour in the free atmosphere.

     

A directional passive air sampler for monitoring polycyclic aromatic hydrocarbons (PAHs) in air mass

A passive air sampler was developed for collecting polycyclic aromatic hydrocarbons (PAHs) in air mass from various directions. The airflow velocity within the sampler was assessed for its responses to ambient wind speed and direction. The sampler was examined for trapped particles, evaluated quantitatively for influence of airflow velocity and temperature on PAH uptake, examined for PAH uptake kinetics, calibrated against active sampling, and finally tested in the field. The airflow volume passing the sampler was linearly proportional to ambient wind speed and sensitive to wind direction. The uptake rate for an individual PAH was a function of airflow velocity, temperature and the octanol-air partitioning coefficient of the PAH. For all PAHs with more than two rings, the passive sampler operated in a linear uptake phase for three weeks. Different PAH concentrations were obtained in air masses from different directions in the field test.     

A Wind Tunnel Study to Design Large,Open-top Chambers for Whole-tree Pollutant Exposure Experiments

A wind tunnel study was conducted to determine the optimal design features of a large, open-top chamber, as needed for pollution exposure studies on mature trees. An optimally-designed, open-top chamber must minimize the incursion of ambient air through its opening and maintain a uniform treatment concentration throughout the chamber. The design features of interest are the diameter and height of the chamber and the deflection angle and opening size of any frustum that may be mounted on top of a model chamber.

Design specifications depend on the turbulence regime about the chamber, which is influenced by the nature of the surrounding vegetation. Consequently, our investigation was performed on scale-model, open-top chambers in a wind tunnel populated with a model coniferous forest. Turbulence measurements demonstrated the similarity between the turbulence regime of the model and a natural forest. A hydrocarbon tracer was injected into the wind tunnel flow to characterize chamber performance.

The main design features of open-top chambers are the velocity of air exiting through the top and the relationship between the length scale of the turbulence and the diameter of the chamber opening. As exit velocities increase, the proportion of eddies with sufficient force to penetrate into the chamber decrease. Therefore, for equal volumetric air flows, smaller opening sizes increase the exit velocities and reduce the number and extent of ambient air incursions. Almost total exclusion of ambient air is achieved as the exit velocity of the air exceeds the magnitude of one standard deviation of the vertical wind velocity measured at the chamber top. The incursion of ambient air is also reduced when the diameter of the chamber opening is smaller than the characteristic length scale of the turbulence, a measure of mean eddy size.

Frusta deflect air flow over the chamber. Three prototypes, with 30?, 45? and 60-degree angles were tested. A 30-degree frustum slightly improves the performance of the chamber and is more effective in preventing ambient air from entraining into the chamber opening than frusta with either a 45? or 60-degree angle. A flatter frustum allows for a smoother transition in the wind velocity streamline and is less apt to cause wake turbulence, as is the case with steeper frusta.

Knowledge of the turbulence characteristics of plant canopies are readily available in the literature and can aid scientists and engineers in designing the optimal chamber and frusta dimensions for their particular application. Therefore, the empirical approach to chamber design can be avoided, and substantial savings can be realized.     

Design of a compact dilution sampler for stationary combustion sources

The dilution sampling method simulates the rapid cooling and dilution processes after hot flue gas have left the stack. This allows gases or vapors to nucleate both homogeneously and heterogeneously, and to condense on preexisting particles in processes analogous to those that occur in the ambient environment. Using this method the authors can collect filterable particulate matter (PM) and condensible PM, that is, primary PM, simultaneously. In order to make this method more suitable for field investigation, a compact dilution sampler was developed. The sampler enhances mixing of dilution air with the stack gas, and thus shortens the length of the mixing section. The design decreases the nominal flow rate through the aging section, and accordingly reduces the size of the residence chamber. The decreased size of the sampler is suitable for field test. Sampling gas is pressured into the residence chamber, and air pressure in the chamber is micro-positive. Uncollected redundant gas is automatically discharged through unused sampling ports, which keeps the unit stable. Performance evaluation tests demonstrate that the design is reasonable. The sampler has been applied to characterize PM emissions from various combustion sources in China.     

Performance of the Button Personal Inhalable Sampler for the measurement of outdoor aeroallergens

No personal aerosol sampler has been evaluated for monitoring aeroallergens in outdoor field conditions and compared to conventional stationary aerobiological samplers. Recently developed Button Personal Inhalable Aerosol Sampler has demonstrated high sampling efficiency for non-biological particles and low sensitivity to the wind direction and velocity. The aim of the present study was to evaluate the Button Sampler for the measurement of outdoor pollen grains and fungal spores side-by-side with the widely used Rotorod Sampler. The sampling was performed for 8 months (spring, summer and fall) at a monitoring station on the roof of a two-storied office building located in the center of the city of Cincinnati. Two identical Button Samplers, one oriented towards the most prevalent wind and the other towards the opposite wind and a Rotorod Sampler were placed side-by-side. The total fungal spore concentration ranged from 129 to 12,980 spores m(-3) (number per cubic meter of air) and the total pollen concentration from 4 to 4536 pollen m(-3). The fungal spore concentrations obtained with the two Button Samplers correlated well (r = 0.95; p<0.0001). The pollen data also showed positive correlation. These findings strongly support the results of earlier studies conducted with non-biological aerosol particles, which demonstrated a low wind dependence of the performance of the Button Sampler compared to other samplers. The Button Sampler's inlet efficiency was found to be more dependent on wind direction when sampling larger sized Pinaceae pollen grains (aerodynamic diameter approximately 65 mum). Compared to Rotorod, both Button Samplers measured significantly higher total fungal spore concentrations. For total pollen count, the Button Sampler facing the prevalent wind showed concentrations levels comparable to that of the Rotorod, but the Button Sampler oriented opposite to the prevalent wind demonstrated lower concentration levels. Overall, it was concluded that the Button Sampler is efficient for the personal sampling of outdoor aeroallergens, and is especially beneficial for aeroallergens of small particle size.     

A New Approach to Roof Monitor Particulate Sampling

A new method for sampling of emissions from building monitors using high volume samplers while continuously recording the exit gas velocity is presented. A technique is also outlined for automatic isokinetic sampling using a high volume sampler.     

A New Real-Time Isokinetic Dust Mass Monitoring System

A new real-time dust mass monitor has been developed by combining an automatic isokinetic sampling probe with a tapered element oscillating microbalance (TEOM). Fly ash from a room temperature wind tunnel is sampled through the isokinetic sampler and collected on an astroquartz mat filter in the TEOM detector. The filter is first excited and oscillated at low frequency (about 200 Hz). As the particles deposit on the filter, the mass increase of the filter is reflected in a frequency reduction which yields the collected particle mass directly in real time. The TEOM detector normally has a high mass resolution (10^?9 g) and wide dynamic range (105⁵–106⁶). It is desensitized for high particle loading applications. Good agreement has been obtained between the mass collected through the isokinetic sampling system and the weight loss of the dust feeder, in real time. The body of information presented in this paper is directed to those concerned with particle emission and control in fossil fuel combustion systems.     

A Technique for Respirable Sampling

The purpose of this paper is to describe instrumentation to aerodynamically size suspended particulates found in ambient air and to summarize results of field testing utilizing the new technique.

A four-stage, multiorifice high-volume fractionating impactor with backup filter, which can be operated as a component of the standard high-volume sampler, collects particulate matter in five separate aerodynamic size ranges: 7 micrometer (μm) or larger, 3.3 to 7 μm, 2.0 to 3.3 μm, 1.1 to 2.0 μm, and 0.01 to 1.1 μm.

Comparative field tests utilizing duplicate sampling techniques were conducted to determine the feasibility of using the size fractionator on a routine basis in field operations. Verification of the actual particle size separation was not undertaken; however, earlier tests utilizing laboratory-generated aerosols have been performed with satisfactory results.

The results of field tests indicate that the fractionator can be used to determine the aerodynamic size distribution of particulate matter. A glass fiber surface with a pH of 11.0 was found to adsorb atmospheric acid gases during sampling and thus gave erroneous mass concentration results when compared to the standard high-volume sampler. Glass fiber filters with a pH of 6.5 eliminated the acid gas adsorption.     

A wind tunnel test of newly developed personal bioaerosol samplers

In this study the performance of two newly developed personal bioaerosol samplers was evaluated. The two test samplers are cyclone-based personal samplers that incorporate a recirculating liquid film. The performance evaluations focused on the physical efficiencies that a personal bioaerosol sampler could provide, including aspiration, collection, and capture efficiencies. The evaluation tests were carried out in a wind tunnel, and the test personal samplers were mounted on the chest of a full-size manikin placed in the test chamber of the wind tunnel. Monodisperse fluorescent aerosols ranging from 0.5 to 20 microm were used to challenge the samplers. Two wind speeds of 0.5 and 2.0 m/sec were employed as the test wind speeds in this study. The test results indicated that the aspiration efficiency of the two test samplers closely agreed with the ACGIH inhalable convention within the size range of the test aerosols. The aspiration efficiency was found to be independent of the sampling orientation. The collection efficiency acquired from these two samplers showed that the 50% cutoff diameters were both around 0.6 microm. However the wall loss of these two test samplers increased as the aerosol size increased, and the wall loss of PAS-4 was considerably higher than that of PAS-5, especially in the aerosol size larger than 5 microm, which resulted in PAS-4 having a relatively lower capture efficiency than PAS-5. Overall, the PAS-5 is considered a better personal bioaerosol sampler than the PAS-4.     

Polyurethane foam (PUF) disks passive air samplers: wind effect on sampling rates

Different passive sampler housings were evaluated for their wind dampening ability and how this might translate to variability in sampler uptake rates. Polyurethane foam (PUF) disk samplers were used as the sampling medium and were exposed to a PCB-contaminated atmosphere in a wind tunnel. The effect of outside wind speed on PUF disk sampling rates was evaluated by exposing polyurethane foam (PUF) disks to a PCB-contaminated air stream in a wind tunnel over air velocities in the range 0 to 1.75 m s-1. PUF disk sampling rates increased gradually over the range 0-0.9 m s-1 at approximately 4.5-14.6 m3 d-1 and then increased sharply to approximately 42 m3 d-1 at approximately 1.75 m s-1 (sum of PCBs). The results indicate that for most field deployments the conventional 'flying saucer' housing adequately dampens the wind effect and will yield approximately time-weighted air concentrations.     

Evaluation of the Use of Diffusive Air Samplers for Determining Temporal and Spatial Variation of Volatile Organic Compounds in the Ambient Air of Urban Communities

Abstract

The Houston-Galveston metropolitan area has a relatively high density of point and mobile sources of air toxics, and determining and understanding the relationship between emissions and ambient air concentrations of air toxics is important for evaluating potential impacts on public health and formulating effective regulatory policies to control this impact, both in this region and elsewhere. However, conventional ambient air monitoring approaches are limited with regard to expense, siting limitations, and representative sampling necessary for adequate exposure assessment. The overall goal of this multiphase study is to evaluate the use of simple passive air samplers to determine temporal and spatial variability of the ambient air concentrations of selected volatile organic compounds (VOCs) in urban areas. Phase 1 of this study, reported here, was a field evaluation of 3M organic vapor monitors (OVMs) involving limited comparisons with commonly used active sampling methods, an assessment of sampler precision, a determination of optimal sampling duration, and an investigation of the utility of a simple modification of the commercial sampler. The results indicated that a sampling duration of 72 hr exhibited generally low bias relative to automated continuous gas chromatography measurements, good overall precision, and an acceptable number of measurements above detection limits. The modified sampler showed good correlation with the commercial sampler, with higher sampling rates, although lower than expected.     

Sampling Submicrometer Particles Suspended in Near Sonic and Supersonic Free Jets

This paper is concerned with sampling submicrometer particles in perisonic flows. The study employed a high volume (30-45 L/min) condensation aerosol generator to produce stearic acid particles having a mean diameter of 0.8μm and a geometric standard deviation of 1.28. The aerosol was diluted with dry air and accelerated to Mach 0.6, 0.8, 1.26, or 1.47 through a flow nozzle. Aerosol mass concentrations were determined using a small bore probe in the jet and by a large bore probe sampling isokinetically upstream of the jet nozzle. The results of both samples were compared to compute the sampling error associated with I ho high spood jot sample. The mass of stearic acid colloctod on polycarbonate mombrono flltors was determined by gravimoirlc and chromalogrophlc mothods. Studies at Mach 0.8 with four sampling probes having Inlet wall to bore area ratios ranging from 3.8 to 0.28 (a knife edge) demonstrated that probe wall thickness effects are not significant when the sample is extracted isokinetically. Subisokinetic experiments using the knife edged probe showed relative errors of 124 ± 12% when sampling at 2 0% of the isokinetic condition. The subisokinetic results are compared favorably with the extended empirical results of other authors. For the supersonic cases it is shown that the subsonic velocity downstream of the sampling probe bow shock can be used in estimating the sampling error.     

Chemical Composition and Source Apportionment of PM25 Particles in the Sihwa Area,Korea

ABSTRACT

To investigate the chemical characteristics of fine particles in the Sihwa area, Korea, atmospheric aerosol samples were collected using a dichotomous PM₁₀ sampler and two URG PM_2.5 cyclone samplers during five intensive sampling periods between February 1998 and February 1999. The Inductively Coupled Plasma (ICP)-Atomic Emission Spectrometry (AES)/ICP-Mass Spectrometry (MS), ion chromatograph (IC), and thermal manganese dioxide oxidation (TMO) methods were used to analyze the trace elements, ionic species, and carbonaceous species, respectively. Backward trajectory analysis, factor analysis, and a chemical mass balance (CMB) model were used to estimate quantitatively source contributions to PM_{2 5} particles collected in the Sihwa area.

The results of PM_2.5 source apportionment using the CMB7 receptor model showed that (NH₄)₂SO₄ was, on average, the major contributor to PM_2.5 particles, followed by nontraffic organic carbon (OC) emission, NH₄NO₃, agricultural waste burning, motor vehicle emission, road dust, waste incineration, marine aerosol, and others. Here, the nontraffic OC sources include primary anthropogenic OC emitted from the industrial complex zone, secondary OC, and organic species from distant sources. The source impact of waste incineration emission became significant when the dominant wind directions were from southwest and west sectors during the sampling periods. It was found that PM_2.5 particles in the Sihwa area were influenced mainly by both anthropogenic local sources and long-range transport and transformation of air pollutants.     

A Passive Flux Denuder for Evaluating Emissions of Ammonia at a Dairy Farm

Abstract

Passive samplers have been shown to be an inexpensive alternative to direct sampling. Diffusion denuders have been developed to measure the concentration of species such as ammonia (NH₃), which is in equilibrium with particulate ammonium nitrate. Conventional denuder sampling that inherently requires air pumps and, therefore, electrical power. To estimate emissions of NH₃ from a fugitive source would require an array of active samplers and meteorological measurements to estimate the flux. A recently developed fabric denuder was configured in an open tube to passively sample NH₃ flux. Passive and active samplers were collocated at a dairy farm at the California State University, Fresno, Agricultural Research Facility. During this comparison study, NH₃ flux measurements were made at the dairy farm lagoon before and after the lagoon underwent acidification. Comparisons were made of the flux measurements obtained directly from the passive flux denuder and those calculated from an active filter pack sampler and wind velocity. The results show significant correlation between the two methods, although a correction factor needed to be applied to directly compare the two techniques. This passive sampling approach significantly reduces the cost and complexity of sampling and has the potential to economically develop a larger inventory base for ambient NH₃ emissions.     

A Critical Assessment of Atmospheric Visibility and Aerosol Measurements in the Eastern United States

As part of a study examining the technical basis for a secondary national ambient air quality standard for fine particulate matter to protect visibility, we reviewed available data on atmospheric aerosol and visibility in the eastern U.S. This paper presents the results of that visibility and aerosol characterization.

Analysis of airport visibility data indicates that the annual median visual ranges in the East are in the 16-25 km range. In the absence of a "reference method," limited measurements of visibility using various types of instruments provide data generally in agreement with the airport visibility estimates when a contrast threshold of 0.05 is assumed in calculating visual range from the instrumental measurements.

Both long- and short-term aerosol measurements have yielded consistent results; however, because of the differences in instrumentation and laboratory analytical techniques among various studies, data often are not directly comparable. The measured annual average fine particulate matter mass concentration is about 18 μg/m³ in the rural East; during summer it increases to about 23 μg/m³. If all the sulfur in the fine mass is assumed to exist as ammonium sulfate, it would constitute 46 percent of the annual mean and about 60 percent of the summer mean fine mass concentrations. Carbon and volatiles, including water, are believed to constitute significant fractions of the fine mass; however, there are little data quantifying their contributions to fine mass and visibility impairment. Additional long-term measurements of visibility and fine aerosol and its various components are necessary to completely characterize visibility and aerosol in the East.