Calibration of Sharp Cut Impactors for Indoor and Outdoor Particle Sampling

ABSTRACT

A low-flow rate, sharp cut point inertial impaction sampler was developed in 1986 that has been widely used in PM exposure studies in the United States and several other countries. Although sold commercially as the MS&T Area Sampler, this sampler is widely referred to as the Harvard Impactor, since the initial use was at the Harvard School of Public Health. Impactor nozzles for this sampler have been designed and characterized for flows of 4,10, 20, and 23 L/min and cut points of 1, 2, 5, and 10 |im. An improved method for determining the actual collecting efficiency curve was developed and used for the recent impactor calibrations reported here. It consists of placing a multiplet reduction impactor inline just downstream of the vibrating orifice aerosol generator to remove the multiplets, thus allowing only the singlet particle s to penetrate through to the impactor being calibrated.

This paper documents the techniques and results of recent nozzle calibrations for this sampler and compares it with other size-selective inertial impactors. In general, the impactors were found to have sharp cutoff characteristics. Particle interstage losses for all of the impactors were very low, with the exception of the 10-|im cut size 20 L/ min impactor, which had greater losses due to the higher flow rate. All of the cut nozzle laboratory calibrations compare favorably to the U.S. Environmental Protection Agency (EPA) WINS-96 fine particle mass (PM_{2 5}) impactor calibration data.     

On the sensitivity of particle size to relative humidity for Los Angeles aerosols

A TDMA system (Tandem Differential Mobility Analyzer; Rader D.J. and McMurry P.H. J. Aerosol Sci. 17, 771–787, 1986) was used to measure the sensitivity of particle size to relative humidity for monodisperse Los Angeles aerosols. Measurements were made at Claremont, CA on 13 days between 19 June and 3 September 1987, in conjunction with the Southern California Air Quality Study (SCAQS). The particle sizes that were studied ranged from 0.05 μm to 0.5 μm diameter at ambient relative humidity (typically 45–65%).The data provide clear evidence that these atmospheric aerosols were externally mixed. Monodisperse ambient aerosols were often found to split into nonhygroscopic (no water uptake) and hygroscopic portions when humidified. An average of 30% of the particles in the 0.2–0.5 μm range were nonhygroscopic. However, the proportion of the particles that was nonhygroscopic varied considerably from day to day and was, on occasion, as high as 70–80% of the particles. There was no clear evidence for nonhygroscopic 0.05 μm particles, but the data are not definitive on this point.The data also show that for the hydrophilic aerosol fraction, the larger particles (0.4–0.5 μm) grew more when humidified than did smaller particles (0.05–0.2 μm). As relative humidities were increased from 50% to 90%, particle diameters grew by average factors of 1.46 ±0.02 (for 0.5 μm particles), 1.49 ± 0.08 (0.4 μm), 1.19 ± 0.08 (0.2 μm) and 1.12 ± 0.05 (0.05 μm). Similarly, when particles were dried from 50% RH to 6–10% RH, particle diameters changed by factors ranging from 0.94 ± 0.03 (0.5 μm) to 0.98 ± 0.01 (0.05 μm).     

Size-resolved trace metal characterization of aerosols emitted by four important source types in Switzerland

In central Switzerland five types of emission sources are mainly responsible for airborne trace metals: traffic, industrial plants burning heavy oil, resuspension of soil particles, residential heatings and refuse incineration plants. The particulate emissions of each of these source types except refuse incineration were sampled using Berner impactors and the mass and elemental size distributions of Cd, Cu, Mn, Pb, Zn, As and Na determined.Cd, Na and Zn are not characteristic for any of these source types. As and Cu, occurring in the fine particle fractions are characteristic for heavy oil combustion, Mn for soil dust and sometimes for heavy and fuel oil combustion and Pb for traffic aerosols. The mass size distributions of aerosols originating from erosion and abrasion processes show a maximum mass fraction in the coarse particle range larger than about 1 μm aerodynamic equivalent diameters (A.E.D.). Aerosols originating from combustion processes show a second maximum mass fraction in the fine particle range below about 0.5μm A.E.D.Scanning electron microscopy combined with an EDS analyzer was used for the morphological characterization of emission and ambient aerosols.     

Calibration of sharp cut impactors for indoor and outdoor particle sampling

A low-flow rate, sharp cut point inertial impaction sampler was developed in 1986 that has been widely used in PM exposure studies in the United States and several other countries. Although sold commercially as the MS&T Area Sampler, this sampler is widely referred to as the Harvard Impactor, since the initial use was at the Harvard School of Public Health. Impactor nozzles for this sampler have been designed and characterized for flows of 4, 10, 20, and 23 L/min and cut points of 1, 2, 5, and 10 microns. An improved method for determining the actual collecting efficiency curve was developed and used for the recent impactor calibrations reported here. It consists of placing a multiplet reduction impactor inline just downstream of the vibrating orifice aerosol generator to remove the multiplets, thus allowing only the singlet particle s to penetrate through to the impactor being calibrated This paper documents the techniques and results of recent nozzle calibrations for this sampler and compares it with other size-selective inertial impactors. In general, the impactors were found to have sharp cutoff characteristics. Particle interstage losses for all of the impactors were very low, with the exception of the 10-micron cut size 20 L/min impactor, which had greater losses due to the higher flow rate. All of the 2.5-micron cut nozzle laboratory calibrations compare favorably to the U.S. Environmental Protection Agency (EPA) WINS-96 fine particle mass (PM2.5) impactor calibration data.     

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.     

Indoor-outdoor relationship of suspended particulate matter and its chemical composition at different sizes

The indoor-outdoor concentration relationship of particulate matter PM_9.0 (aerodynamic diameter 9 μm or smaller) and its chemical composition (sulfate, nitrate, chloride and ammonium) has been studied. Samples were collected using four identical Anderson impactors, each one collecting nine size ranges by eight impactor stages (9, 5.8, 4.7, 3.3, 2.1, 1.1, 0.65 and 0.43 μm) plus a back-up filter representing particles finer than 0.45 μm. Concentrations of sulfate, nitrate and chloride were determined by ion chromatography, and an ammonium-selective ion electrode plus a Corning pH ion meter were used to determine ammonium ion. The results revealed that sulfate was the predominant component and chloride the least abundant. The size distribution of sulfate, nitrate and ammonium very strongly peaked near 0.65 μm and with very little at the larger sizes. The chloride concentration was depleted in the fine particles and enhanced in the relatively coarser particles, with the peak at 3.3 μm. All these concentrations had a significant linear relationship with mass concentrations in outdoor samples. In indoor samples, the same relation was observed only for sulfate and ammonium, which were also significantly correlated with each other. Furthermore, indoor sulfate, chloride and ammonium concentrations were higher towards the finest particle sizes, indicating a higher potential inhalation health hazard. The study also confirmed that indoor air quality depends on outdoor atmospheric pollution level, indoor activities and virtually on the particle size. Finally, the study would assist in selecting the type of collector required to reduce the level of particulates to an acceptable level for indoor ambient air.     

冲击采样器设计参数分析

在颗粒物研究中 ,分级采样是一种常用的监测方法 ,而冲击采样器是颗粒物分级采样的重要仪器。从颗粒物的受力分析着手 ,得到了冲击采样器重要参数———斯托克斯数的表达式 ,并对TSP、PM1 0 和PM2 5冲击采样器设计参数进行了详细分析 ,得到了采样器设计参数的关系式及相关曲线     

Sampling and analysis of biological aerosols

The extreme particle size range and enormous heterogeneity of airborne biological particles make sampling a significant challenge. Three major sampler types available include gravity devices, impactors and suction samplers. Gravity methods, while most commonly used, are neither qualitatively or quantitatively accurate and of very limited use. Impaction samplers (rotating, centrifugal) accelerate air by rotating the collecting surface or with a fan. Particles are collected from measured volumes of air but these devices preferentially sample particles larger than 10 μm. Suction samplers, which efficiently collect particles of a wide size range from measured volumes of air, include slit samplers, cascade impactors, filtration devices and liquid impingers. Suction samplers can retrieve viable particles by direct impaction on culture media, or by subsequent culture of impinger fluid or filter eluates. Nonviable particles can often be identified by microscopic examination of slides, filters or filtrates of impinger fluids. Immunoassays and biochemical assays can be used with impinger fluid and filter eluates to assess antigen and toxin levels in measured air samples.     

Utilisation de techniques spectroscopiques pour l'etude de particules atmospheriques produites par des sources ponctuelles

Secondary Ion Mass Spectrometry (SIMS) is used to study atmospheric particle production by an industrial source responsible for the acid precipitation phenomenon. A sampling network has been put in place at Rouyn-Noranda to collect local aerosols with cascade impactors. Aerosols have also been collected in the plume of the most important source of SO₂ in Quebec. Instruments have been developed and constructed to collect these samples. A multi-analytical technique approach permits physico-chemical analysis of individual aerosol: SIMS using a 1 μm ion probe and Scanning Electron Microscopy (SEM) coupled with an X-ray analyzer (EDAX) perform micro-analysis, whereas bulk analysis is provided by a SIMS using a 200 μm Argon ion probe. Devices used for localization of microparticles permit manipulation of samples without damaging them and allow analysis of individual aerosol particles with a SIMS after analysis with SEM-EDAX. Laser Microprobe Mass Analysis (LAMMA) is also possible. Negative and positive mass spectra, obtained with a SIMS-Ar⁺, have been analyzed with a multivariate statistical technique. Relations between these mass spectra and specific conditions existing during sampling have been determined.     

The Hamilton Study: Estimating Exposure to Ambient Suspended Particles

In the industrial city of Hamilton, Ontario, Canada, we recently carried out an epidemiological study of the effect of environmental factors on respiratory health in 3500 elementary, schoolchildren. The level and size distribution of suspended particles in ambient air was measured from 24-h samples taken at 6-day intervals from a network of 29 hivol TSP samplers, and nine Andersen 2000 4-stage cascade impactors. Exposure was computed by generating a 3-dimensional response surface (in TSP and in easting and northing geographical coordinates) using a linear regression model of the form: TSP = (1 + E + N)², based on monthly geometrical mean data for all sites. From the response surface generated for a given month, TSP levels were predicted by the model for all schools by specifying their geographical coordinates. The yearly exposure for a given child was determined from the arithmetic mean of the predicted values for 12 monthly TSP levels. A similar procedure was employed for calculation of the exposure to the "fine" (≤3.3 μm) and "coarse" (>3.3 μm) size fraction, as well as the aerodynamic mass median diameter of particles from the network of cascade impactors. Results of the measurements showed that gradients for TSP up to approximately 10 μg/m³/km exist over the city covering distances from 5 to 10 km. The range of 1 yr mean exposure values calculated for each child was from 30.5 μg/m³ to 74.5 μg/m³. Comparable figures for particle size were up to 0.3 μm AMMD (aerodynamic mass median diameter)/km and annual mean particle size exposure from 2.69 to 3.53 μm AMMD.     

Aerosol Aerodynamic Size Conventions For Inertia! Sampler Calibration

The body of information presented in this paper is directed to investigators using inertial samplers for precise and accurate studies of respirable aerosols. The conventions commonly used for aerodynamic size for aerosol particles are discussed including the definition popularized by the "Task Group on Lung Dynamics" of the ICRP, and the "Lovelace" definition. To emphasize the distinction, the Task Group definition (unit density sphere equivalent) is called the aerodynamic equivalent diameter, D_ae, and the Lovelace definition (characteristic expression based upon viscous resistance) is called the aerodynamic resistance diameter, D_ar. The implications and efficacy of these conventions are related to procedures for calibration of cascade impactors, cyclones, and spiral centrifuges. The calibration of a spiral centrifuge at different altitudes is used as an example of the potential problems associated with the use of the different conventions for describing aerodynamic size. The aerodynamic resistance diameter is recommended for calibration of inertial samplers to be used to collect aerosols in the respirable size range.     

Growth of ultrafine particles by brownian coagulation

Current atmospheric observations tend to support the view that continental tropospheric aerosols (particularly urban aerosols) show multimodal mass distributions in the size range of 0.01–100 μm. The origin of these aerosols is both natural and anthropogenic. Recently, trimodal sub-μm size distributions from combustion measurements at 0.008, 0.035 and 0.15 μm were also observed. Our interest in the present study is the secondary process of growth of sub-μm size aerosols by the coagulation process alone. Using the ‘J-space’ (integer-space) distribution method of Salk (Suck) and Brock (1979, J. Aerosol Sci.10, 58–590), we report an accurate numerical simulation study of the evolution of ultrafine to fine particle size distributions. Comparision with the analytic solution of Scott (1968, J. atmos. Sci.25, 54–64) was made to test the accuracy of our J-space or integer-space distribution method. Our multimodal sub-μ particle size distribution study encompassed the particle size range of 0.001–0.20 μm. Details of particle growth in each mode and interaction between different modes in the multimodal distribution were qualitatively analyzed.     

Variations in different sized water insoluble particulate matter in rain water

Temporal variations in different sized water insoluble particles in rain water during five rainfall events were investigated in a suburban area. The mass of particles ranged: 45–88 % (particle size > 8 μm), 12–43 % (1–8 μm) and 0–13 % (0.4–1 μm). A high concentration and following sharp decrease of the particles larger than 8 μm were observed at the initial stage of rainfall; the rate of the decrease was higher in the heavier rain. In many cases, the mass concentration of particles in rain water showed an inverse correlation with rainfall intensity. The variation of the smaller size particles was less significant compared to the larger ones.     

Proinflammatory effect of fine and ultrafine particulate matter using size-resolved urban aerosols from Paris

Epidemiological and experimental studies have underlined that exposure to particulate matter (PM) leads mainly to airway inflammation, but the roles of particle size and chemical composition associated to such adverse health outcomes need to be better investigated. This study was performed to validate novel strategies of particle sampling, recovery and cell exposure in order to evaluate the pro-inflammatory potential of fine and ultrafine particles from a fractionated aerosol. Samplings of Paris background aerosols using 13-stage low pressure impactors (0.03-10 microm) gave bimodal mass distributions with an accumulation mode centered on a median diameter of 0.42 microm and a coarse one on 3.25 microm. PM 1 accounted for 70% and PM 0.1 for 12% of PM 10. The latter mainly comprised carbon-chained aggregates. The development of an efficient and reproducible method to recover fine (PM 1-0.1) and ultrafine (PM 0.1-0.03) particulate matter has permitted experimental comparison of the impact of such particles on human bronchial epithelial cells (HBECs). In this study we have compared the relative effects of fine and ultrafine particles at non-cytotoxic concentrations over 24h on the production of the pro-inflammatory cytokine GM-CSF by HBECs. Combining two cell exposure strategies to the size-fraction particles according to either their proportion (isovolume exposure) or their quantity in the aerosol (isomass exposure), we showed that both ultrafine and fine particles induced a concentration-dependent GM-CSF release by HBECs which is significant from 1 microg cm(-2). In conclusion, short duration samplings using 13-stage impactors enable to obtain size-resolved PM in sufficient quantities to carry out toxicological investigations. These findings are promising in view to conduct a more intensive study joining chemical and toxicological assays.     

Calculated Droplet Size Distributions and Opacities of Condensed Sulfuric Acid Aerosols

The properties of condensed polydisperse sulfuric acid aerosols in industrial flue gas were calculated. The condensed aqueous acid volume concentration, composition, droplet size distributions and condensed plume opacity were calculated for typical flue gas compositions, condensation nucleus size distributions and flue gas dilution ratios. The assumed initial flue gas at 170°C contained 0.035 g/acm fly ash particles, 1-20% water vapor, and 1-50 ppmv sulfuric acid vapor. The assumed gas cooling mechanism was by adiabatlc dilution with cool ambient air. Polydisperse droplet growth was calculated by assuming equal surface area increase for each particle. The calculations show that sulfuric acid condensation should have minimal effect on particles larger than 1 μm, but will form a high concentration of particles in the narrow size range of 0.05-0.5 μm diameter. Depending on the initial sulfuric acid and water vapor concentrations in the hot flue gas, the calculated maximum plume opacity following gas dilution ranged from 5% to 25%, compared to 4% for the dry condensation nucleus aerosol.     

Mass and Elemental Composition of Fine and Coarse Particles in Six U.S. Cities

Inhalable particulate matter (IP) samples have been collected in six U.S. cities in conjunction with an air pollution health study. The IP were collected using dichotomous virtual impactors in two size ranges: fine particles (FP) having aerodynamic diameter (d_a) <2.5 μm, and coarse particles (CP) with 2.5 μm < da < 15 μm. The mass measurements were determined by beta-gauge attenuation. The elemental composition of the FP and CP were determined by X-ray fluorescence. The means and distributions for FP and CP and selected elemental data highlight the similarities and differences that exist among these cities in the health study. Examining the temporal variations gives additional information on the meteorology and sources influencing the FP and CP fractions of Inhalable particle mass.

Differences in the concentration (and ratios) of selected elements have indicated the varying presence of crustal, steel industry, automotive, oceanic and fuel combustion sources in these cities. The noted variation in the concentrations and character of ambient aerosols in these cities are pertinent to interpreting differences in population exposures.     

Size distribution of polycyclic aromatic hydrocarbons in a roadway tunnel in Lisbon, Portugal

Atmospheric aerosols of four aerodynamic size ranges were collected using high volume cascade impactors in an extremely busy roadway tunnel in Lisbon (Portugal). Dust deposited on the tunnel walls and guardrails was also collected. Average particle mass concentrations in the tunnel atmosphere were more than 30 times higher than in the outside urban background air, revealing its origins almost exclusively from fresh vehicle emissions. Most of the aerosol mass was concentrated in submicrometer fractions (65%), and polycyclic aromatic hydrocarbons (PAH) were even more concentrated in the finer particles with an average of 84% of total PAH present in sizes smaller than 0.49 μm. The most abundant PAH were methylated phenanthrenes, fluoranthene and pyrene. About 46% of the total PAH mass was attributed to lower molecular weight compounds (two and three rings), suggesting a strong influence of diesel vehicle emissions on the production of local particulate PAH. The application of diagnostic ratios confirmed the relevance of this source of PAH in the tunnel ambient air. Deposited dust presented PAH profiles similar to the coarser aerosol size range, in agreement with the predominant origin of coarser aerosol particles from soil dust resuspension and vehicle wear products.     

Predicting cutoff aerodynamic diameter and sharpness of single round-nozzle impactors with a finite impaction plate diameter

This investigation numerically examined the cutoff aerodynamic diameter (da50) and the sharpness (GSD) of the particle collection efficiency curve of impactors with a finite impaction plate diameter. Results revealed that the inertial impactors have a limited cutoff aerodynamic diameter at different air velocities. The extreme value of the cutoff aerodynamic diameter increases with the nozzle diameter (W)/the plate diameter (Dc). The computed da50/Dc values of the impactors increase with W/Dc at various Reynolds numbers (Re) and with the nozzle-toplate distance (S)/Dc when Re is 100. The value of GSD slightly increases with W/Dc for Re of 10 and 100, although the effect of S/Dc on GSD is not evident at various Res. The particle collection efficiency curve of the impactor with a lower Re is less sharp than that with a high Re at various W/Dc and S/Dc values. Statistical equations closely fitted the obtained numerical results for Res of 10-3000. The equations are useful for directly calculating the cutoff aerodynamic diameter and the sharpness of the particle collection efficiency curve for single round-nozzle impactors with a finite impaction Dc.     

Classification of multiple types of organic carbon composition in atmospheric particles by scanning transmission X-ray microscopy analysis

A scanning transmission X-ray microscope at the Lawrence Berkeley National Laboratory is used to measure organic functional group abundance and morphology of atmospheric aerosols. We present a summary of spectra, sizes, and shapes observed in 595 particles that were collected and analyzed between 2000 and 2006. These particles ranged between 0.1 and 12 μm and represent aerosols found in a large range of geographical areas, altitudes, and times. They include samples from seven different field campaigns: PELTI, ACE-ASIA, DYCOMS II, Princeton, MILAGRO (urban), MILAGRO (C-130), and INTEX-B. At least 14 different classes of organic particles show different types of spectroscopic signatures. Different particle types are found within the same region while the same particle types are also found in different geographical domains. Particles chemically resembling black carbon, humic-like aerosols, pine ultisol, and secondary or processed aerosol have been identified from functional group abundance and comparison of spectra with those published in the literature.     

The effects of aging processes on critical supersaturation ratios of ultrafine carbon aerosols

The condensation properties of polydisperse aged ultrafine carbon aerosols (particle diameter<1 μm) have been investigated by means of a variable supersaturation condensation nucleus counter. The critical supersaturation (S_c), as the point, where 50% of all particles have been activated and grew to droplets was compared to the median dry particle diameter for pure carbon aerosols, benzo[a]pyrene-tagged carbon aerosols and external mixtures of the carbon particles with sodium chloride and sulphuric acid aerosols. Additionally, ozone as oxidising gaseous compound was added in some of the experiments. Simple coagulation of pure and benzo[a]pyrene-tagged carbon particles resulted in only slightly lower values for S_c due to the increased median particle diameter. The formation of soluble functionalities on the particle surface, i.e. the coagulation with the soluble sodium chloride and sulphuric acid aerosols or the chemical decomposition of benzo[a]pyrene into polar, hydrophilic products due to the reaction with ozone resulted in significant lower values for S_c for the modified carbon aerosol. The necessary supersaturations for the increased hydrophilic particles dropped to atmospherically relevant values of 3% after 5 h reaction time (benzo[a]pyrene decomposition) and 15 h (coagulation with soluble particles), respectively.