Development of a Sample Equilibration System for the TEOM Continuous PM Monitor

ABSTRACT

In recent years, scientific discussion has included the influence of thermodynamic conditions (e.g., temperature, relative humidity, and filter face velocity) on PM retention efficiency of filter-based samplers and monitors. Method-associated thermodynamic conditions can, in some instances, dramatically influence the presence of particle-bound water and other light-molecular-weight chemical components such as particulate nitrates and certain organic compounds. The measurement of fine particle mass presents a new challenge for all PM measurement methods, since a relatively greater fraction of the mass is semi-volatile.

The tapered element oscillating microbalance (TEOM) continuous PM monitor is a U.S. Environmental Protection Agency (EPA) PM₁₀ equivalent method (EQPM-1090-079). Several hundred of these monitors are deployed throughout the United States. The TEOM monitor has the unique characteristic of providing direct PM mass measurement without the calibration uncertainty inherent in mass surrogate methods. In addition, it provides high-precision, near-real-time continuous data automatically. Much attention has been given to semi-volatile species retention of the TEOM method.

While using this monitor, it is desirable to maintain as low an operating temperature as practical and to remove unwanted particle-bound water. A new sample equilibration system (SES) has been developed to allow conditioning of the PM sample stream to a lower humidity and temperature level. The SES incorporates a special low-particle-loss Nafion dryer. This paper discusses the configuration and theory of the SES. Performance results include high time-resolved PM_2.5 data comparison between a 30 °C sample stream TEOM monitor with SES and a standard 50 °C TEOM monitor. In addition, 24-hr integrated data are compared with data collected using an EPA PM_2.5 Federal Reference Method (FRM)-type sampler. The SES is a significant development because it can be applied easily to existing TEOM monitors.     

Evaluation and comparison of continuous fine particulate matter monitors for measurement of ambient aerosols

To provide a scientific basis for the selection and use of continuous monitors for exposure and/or health effects studies, and for compliance and episode measurements at strategic locations in the State of New Jersey, we evaluated the performance of seven continuous fine particulate matter (PM2.5) monitors in the present study. Gravimetric samplers, as reference methods, were collocated with realtime instruments in both laboratory and field tests. The results of intercomparison of real-time monitors showed that the two nephelometers used in this study correlated extremely well (r2 approximately 0.97), and two tapered element oscillating monitors (TEOM 1400 and TEOM filter dynamics measurement system [FDMS]) correlated well (r2 > 0.85), whereas two beta gauges displayed a weaker correlation (r2 < 0.6). During a summertime controlled (laboratory) evaluation, the measurements made with the gravimetric method correlated well with the 24-hr integrated measurements made with the real-time monitors. The SidePak nephelometer overestimated the particle concentration by a factor of approximately 3.4 compared with the gravimetric method. During a summertime field evaluation, the TEOM FDMS monitor reported approximately 30% higher mass concentration than the Federal Reference Method (FRM); and the difference could be explained by the loss of semi-volatile materials from the FRM sampler. Results also demonstrated that 24-hr average PM2.5 mass concentrations measured by beta gauges and TEOM (50 degrees C) in winter correlated well with the integrated gravimetric method. Seasonal differences were observed in the performance of the TEOM (50 degrees C) monitor in measuring the particle mass attributed to the higher semi-volatile material loss in the winter weather. In applying the realtime particulate matter monitoring data into Air Quality Index (AQI) reporting, the Conroy method and the 8-hr end-hour average method were both found to be suitable.     

Laboratory characterization of modified tapered element oscillating microbalance samplers

Laboratory tests with generated aerosols were conducted to test the efficacy of two recent design modifications to the well-established tapered element oscillating microbalance (TEOM) continuous particulate matter (PM) mass monitor. The two systems tested were the sample equilibration system-equipped TEOM monitor operating at 30 degrees C, which uses a Nafion dryer as part of the sample inlet, and the differential TEOM monitor, which adds a switched electrostatic precipitator and uses a self-referencing algorithm to determine "true PM mass." Test aerosols included ammonium sulfate, ammonium nitrate, sodium chloride, copper (II) sulfate, and mixed aerosols. Aerosols were generated with an atomizer or a vibrating orifice generator and were equilibrated in a 450-L slow flow chamber before being sampled. Relative humidity in the chamber was varied between 10 and 90%, and step changes in humidity were executed while generating aerosol to test the response of the instruments. The sample equilibration system-equipped TEOM monitor does reduce, but not totally eliminate, the sensitivity of the TEOM mass monitor to changes in humidity. The differential TEOM monitor gives every indication of being a very robust technique for the continuous real-time measurement of ambient aerosol mass, even in the presence of semi-volatile particles and condensable gases.     

Laboratory Characterization of Modified Tapered Element Oscillating Microbalance Samplers

Abstract

Laboratory tests with generated aerosols were conducted to test the efficacy of two recent design modifications to the well-established tapered element oscillating microbalance (TEOM) continuous particulate matter (PM) mass monitor. The two systems tested were the sample equilibration system-equipped TEOM monitor operating at 30 °C, which uses a Nafion dryer as part of the sample inlet, and the differential TEOM monitor, which adds a switched electrostatic precipitator and uses a self-referencing algorithm to determine “true PM mass.” Test aerosols included ammonium sulfate, ammonium nitrate, sodium chloride, copper (II) sulfate, and mixed aerosols. Aerosols were generated with an atomizer or a vibrating orifice generator and were equilibrated in a 450-L slow flow chamber before being sampled. Relative humidity in the chamber was varied between 10 and 90%, and step changes in humidity were executed while generating aerosol to test the response of the instruments. The sample equilibration system-equipped TEOM monitor does reduce, but not totally eliminate, the sensitivity of the TEOM mass monitor to changes in humidity. The differential TEOM monitor gives every indication of being a very robust technique for the continuous real-time measurement of ambient aerosol mass, even in the presence of semi-volatile particles and condensable gases.     

Long-term field characterization of tapered element oscillating microbalance and modified tapered element oscillating microbalance samplers in urban and rural New York State locations

Long-term field comparisons of continuous and integrated filter measurements of mass concentrations of particulate matter (PM) with an aerodynamic diameter less than or equal to 2.5 microm (PM2.5) were performed at rural and urban sites in New York State. Two versions of the continuous tapered element oscillating microbalance (TEOM) mass monitor are deployed at each site, in addition to Federal Reference Method filter samplers. Data are grouped into monthly averages to retain and demonstrate seasonal differences. Strong seasonal dependence is observed-the TEOM monitors with the heated sensors are biased systematically low with respect to the Federal Reference Method measurements during the cold season. For the rural site, the average bias for the sample equilibration system (SES)-equipped and standard TEOM monitors is 14 and 24%, respectively. At this location, the TEOM monitor measurements were biased low for all 34 months. For the urban site, the average bias for the SES and standard TEOM monitors is 8 and 18%, respectively. At this location, the TEOM monitor measurements are as likely to be biased high as low during the warm-season months. The hour averaged data from the two versions of the TEOM monitor are also compared, and also indicate that the SES-equipped version of the TEOM monitor captures 7-11% more PM2.5 mass at these locations.     

Evaluation of the RAMS continuous monitor for determination of PM2.5 mass including semi-volatile material in Philadelphia,PA

The real-time ambient mass sampler (RAMS) is a continuous monitor based on particle concentrator, denuder, drier, and tapered element oscillating microbalance (TEOM) monitor technology. It is designed to measure PM2.5 mass, including the semi-volatile species NH4NO3 and semi-volatile organic material, but not to measure PM2.5 water content. The performance of the RAMS in an urban environment with high humidity was evaluated during the July 1999 NARSTO-Northeast Oxidant and Particles Study (NEOPS) intensive study at the Baxter water treatment plant in Philadelphia, PA. The results obtained with the RAMS were compared to mass measurements made with a TEOM monitor and to constructed mass obtained with a Particle Concentrator-Brigham Young University Organic Sampling System (PC-BOSS) sampler designed to determine the chemical composition of fine particles, including the semi-volatile species. An average of 28% of the fine particulate material present during the study was semi-volatile organic material lost from a filter during particle collection, and 1% was NH4NO3 that was also lost from the particles during sampling. The remaining mass was dominantly nonvolatile (NH4)2SO4 (31%) and organic material (37%), with minor amounts of soot, crustal material, and nonvolatile NH4NO3. Comparison of the RAMS and PC-BOSS results indicated that the RAMS correctly monitored for fine particulate mass, including the semivolatile material. In contrast, the heated filter of the TEOM monitor did not measure the semi-volatile material. The comparison of the RAMS and PC-BOSS data had a precision of +/-4.1 microg/m3 (+/-9.6%). The precision of the RAMS data was limited by the uncertainty in the blank correction for the reversible adsorption of water by the charcoal-impregnated cellulose sorbent filter of the RAMS monitor. The precision of the measurement of fine particulate components by the PC-BOSS was +/-6-8%.     

Continuous determination of fine particulate matter mass in the Salt Lake City Environmental Monitoring project: a comparison of real-time and conventional TEOM monitor results

Fine particulate matter (PM2.5) mass was determined on a continuous basis at the Salt Lake City Environmental Protection Agency Environmental Monitoring for Public Awareness and Community Tracking monitoring site in Salt Lake City, UT, using three different monitoring techniques. Hourly averaged PM2.5 mass data were collected during two sampling periods (summer 2000 and winter 2002) using a real-time total ambient mass sampler (RAMS), sample equilibration system (SES)-tapered element oscillating microbalance (TEOM), and conventional TEOM monitor. This paper compares the results obtained from the various monitoring systems, which differ in their treatment of semivolatile material (SVM; particle-bound water, semivolatile ammonium nitrate, and semivolatile organic compounds). PM2.5 mass results obtained by the RAMS were consistently higher than those obtained by the SES-TEOM and conventional TEOM monitors because of the RAMS ability to measure semivolatile ammonium nitrate and semivolatile organic material but not particle-bound water. The SES-TEOM monitoring system was able to account for an average of 28% of the SVM, whereas the conventional TEOM monitor loses essentially all of the SVM from the single filter during sampling. Occasional mass readings by the various TEOM monitors that are higher than RAMS results may reflect particle-bound water, which, under some conditions, is measured by the TEOM but not the RAMS.     

Semi-volatile species in PM2.5: comparison of integrated and continuous samplers for PM2.5 research or monitoring

Fine particles in urban atmospheres contain substantial quantities of semi-volatile material [e.g., NH4NO3 and semi-volatile organic compounds (SVOCs)] that are lost from particles during collection on a filter. Several diffusion denuder samplers have been developed for the determination of both NO3- and organic semi-volatile fine particulate components. The combination of technology used in the BOSS diffusion denuder sampler and the Harvard particle concentrator has resulted in the Particle Concentrator-Brigham Young University Organic Sampling System (PC-BOSS) for the 24-hr (or less) integrated collection of PM2.5, including NH4NO3 and semi-volatile organic material. Modification of the BOSS sampler allows for the weekly determination of these same species. Combination of BOSS denuder and tapered element oscillating microbalance (TEOM) monitor technology has resulted in the real-time ambient mass sampler (RAMS) for the continuous measurement of PM2.5, including the semi-volatile components. Comparison of the results obtained with the BOSS and with each of the newly developed modifications of the BOSS indicates that the modified versions can be used for the continuous, daily, or weekly monitoring of PM2.5, including semi-volatile species, as appropriate to the design of each sampler.     

Long-Term Field Characterization of Tapered Element Oscillating Microbalance and Modified Tapered Element Oscillating Microbalance Samplers in Urban and Rural New York State Locations

Abstract

Long-term field comparisons of continuous and integrated filter measurements of mass concentrations of par-ticulate matter (PM) with an aerodynamic diameter less than or equal to 2.5 μm (PM_2.5) were performed at rural and urban sites in New York State. Two versions of the continuous tapered element oscillating microbalance (TEOM) mass monitor are deployed at each site, in addition to Federal Reference Method filter samplers. Data are grouped into monthly averages to retain and demonstrate seasonal differences. Strong seasonal dependence is observed—the TEOM monitors with the heated sensors are biased systematically low with respect to the Federal Reference Method measurements during the cold season. For the rural site, the average bias for the sample equilibration system (SES)-equipped and standard TEOM monitors is 14 and 24%, respectively. At this location, the TEOM monitor measurements were biased low for all 34 months. For the urban site, the average bias for the SES and standard TEOM monitors is 8 and 18%, respectively. At this location, the TEOM monitor measurements are as likely to be biased high as low during the warm-season months. The hour averaged data from the two versions of the TEOM monitor are also compared, and also indicate that the SES-equipped version of the TEOM monitor captures 7-11% more PM_2.5 mass at these locations.     

New York State urban and rural measurements of continuous PM2.5 mass by FDMS, TEOM, and BAM

Field evaluations and comparisons of continuous fine particulate matter (PM2,5) mass measurement technologies at an urban and a rural site in New York state are performed. The continuous measurement technologies include the filter dynamics measurement system (FDMS) tapered element oscillating microbalance (TEOM) monitor, the stand-alone TEOM monitor (without the FDMS), and the beta attenuation monitor (BAM). These continuous measurement methods are also compared with 24-hr integrated filters collected and analyzed under the Federal Reference Method (FRM) protocol. The measurement sites are New York City (the borough of Queens) and Addison, a rural area of southwestern New York state. New York City data comparisons between the FDMS TEOM, BAM, and FRM are examined for bias and seasonality during a 2-yr period. Data comparisons for the FDMS TEOM and FRM from the Addison location are examined for the same 2-yr period. The BAM and FDMS measurements at Queens are highly correlated with each other and the FRM. The BAM and FDMS are very similar to each other in magnitude, and both are approximately 25% higher than the FRM filter measurements at this site. The FDMS at Addison measures approximately 9% more mass than the FRM. Mass reconstructions using the speciation trends network filter data are examined to provide insight as to the contribution of volatile species of PM2.5 in the FDMS mass measurement and the fraction that is likely lost in the FRM mass measurement. The reconstructed mass at Queens is systematically lower than the FDMS by approximately 10%.     

The measurement of fine particulate semivolatile material in urban aerosols

Ammonium nitrate and semivolatile organic material (SVOM) are significant components of fine particles in urban atmospheres. These components, however, are not properly determined with methods such as the fine particulate matter (PM2.5) Federal Reference Method (FRM) or other single filter samplers because of significant losses of semivolatile material (SVM) from particles collected on the filter during sampling. The R&P tapered element oscillating microbalance (TEOM) monitor also does not measure SVM, because this method heats the sample to remove particle bound water, which also results in evaporation of SVM. Recent advances in monitoring techniques have resulted in samplers for both integrated and continuous measurement of total PM2.5, including the particle concentrator-Brigham Young University organic sampling system (PC-BOSS), the real-time total ambient mass sampler (RAMS), and the R&P filter dynamics measurement system (FDMS) TEOM monitor. Results obtained using these samplers have been compared with those obtained with either a PM2.5 FRM sampler or a TEOM monitor in studies conducted during the past five years. These studies have shown the following: (1) the PC-BOSS, RAMS, and FDMS TEOM are all comparable. Each instrument measures both the nonvolatile material and the SVM. (2) The SVM is not retained on the heated filter of a regular TEOM monitor and is not measured by this sampling technique. (3) Much of the SVM is also lost during sampling from single filter samplers such as the PM2.5 FRM sampler. (4) The amount of SVM lost from single filter samplers can vary from less than one-third of that lost from heated TEOM filters during cold winter conditions to essentially all during warm summer conditions. (5) SVOM can only be reliably collected using an appropriate denuder sampler. (6) A PM2.5 speciation sampler can be easily modified to a denuder sampler with filters that can be analyzed for semivolatile organic carbon (OC), nonvolatile OC, and elemental carbon using existing OC/elemental carbon analytical techniques. The research upon which these statements are based for various urban studies are summarized in this paper.     

Field evaluation of particulate matter measurements using tapered element oscillating microbalance in a layer house

The tapered element oscillating microbalance (TEOM) is one type of continuous ambient particulate matter (PM) monitor. Adsorption and desorption of moisture and semivolatile species may cause positive or negative artifacts in TEOM PM mass measurement. The objective of this field study was to investigate possible uncertainties associated with TEOM measurements in the poultry operation environment. For comparisons of TEOM with filter-based gravimetric method, four instruments (TEOM-PM10, low-volume PM10 sampler TEOM-PM2.5, and PM2.5 speciation sampler) were collocated and tested inside a poultry house for PM2.5 and PM10 (PM with aerodynamic equivalent diameter < or =2.5 and < or =10 microm, respectively) measurements. Fifteen sets of 24-hr PM10 concentrations and 13 sets of 24-hr PM2.5 measurements were obtained. Results indicate that compared with filter-based gravimetric method, TEOM gave significantly lower values of both PM10 and PM2.5 mass concentrations. For PM10, the average ratio of TEOM to the gravimetric method was 0.936. For PM2.5, the average ratio of TEOM to the gravimetric method was 0.738. Particulate matter in the poultry houses possibly contains semivolatile compounds and moisture due to high levels of relative humidity (RH) and gas pollutants. The internal heating mechanism of the TEOM may cause losses in mass through volatilization. To investigate the effects of TEOM settings on concentration measurements, the heaters of two identical TEOMs were set at 50 degrees C, 30 degrees C, or no heating at all. They were collocated and tested for total suspended particle (TSP), PM10, and PM25 measurements in layer house for 6 weeks. For all TSR PM10, and PM2.5 measurements, the internal TEOM temperature setting had a significant effect (P < 0.05). Significantly higher PM mass concentrations were measured at lower temperature settings. The effects of environmental (i.e., temperature, RH, NH3 and CO2 concentrations) and instrumental (i.e., filter loading and noise) parameters on PM measurements were also assessed using regression analysis.     

Comparison of real-time instruments used to monitor airborne particulate matter

Measurements collected using five real-time continuous airborne particle monitors were compared to measurements made using reference filter-based samplers at Bakersfield, CA, between December 2, 1998, and January 31, 1999. The purpose of this analysis was to evaluate the suitability of each instrument for use in a real-time continuous monitoring network designed to measure the mass of airborne particles with an aerodynamic diam less than 2.5 microns (PM2.5) under wintertime conditions in the southern San Joaquin Valley. Measurements of airborne particulate mass made with a beta attenuation monitor (BAM), an integrating nephelometer, and a continuous aerosol mass monitor (CAMM) were found to correlate well with reference measurements made with a filter-based sampler. A Dusttrak aerosol sampler overestimated airborne particle concentrations by a factor of approximately 3 throughout the study. Measurements of airborne particulate matter made with a tapered element oscillating microbalance (TEOM) were found to be lower than the reference filter-based measurements by an amount approximately equal to the concentration of NH4NO3 observed to be present in the airborne particles. The performance of the Dusttrak sampler and the integrating nephelometer was affected by the size distribution of airborne particulate matter. The performance of the BAM, the integrating nephelometer, the CAMM, the Dusttrak sampler, and the TEOM was not strongly affected by temperature, relative humidity, wind speed, or wind direction within the range of conditions encountered in the current study. Based on instrument performance, the BAM, the integrating nephelometer, and the CAMM appear to be suitable candidates for deployment in a real-time continuous PM2.5 monitoring network in central California for the range of winter conditions and aerosol composition encountered during the study.     

Aerosol Measurement: The Use of Optical Light Scattering for the Determination of Particulate Size Distribution,and Particulate Mass,Including the Semi-Volatile Fraction

Abstract

The GRIMM model 1.107 monitor is designed to measure particle size distribution and particulate mass based on a light scattering measurement of individual particles in the sampled air. The design and operation of the instrument are described. Protocols used to convert the measured size number distribution to a mass concentration consistent with U.S. Environmental Protection Agency protocols for measuring particulate matter (PM) less than 10 μm (PM₁₀) and less than 2.5 μm (PM_2.5) in aerodynamic diameter are described. The performance of the resulting continuous monitor has been evaluated by comparing GRIMM monitor PM_2.5 measurements with results obtained by the Rupprecht and Patashnick Co. (R&P) filter dynamic measurement system (FDMS). Data were obtained during month-long studies in Rubidoux, CA, in July 2003 and in Fresno, CA, in December 2003. The results indicate that the GRIMM monitor does respond to total PM_2.5 mass, including the semi-volatile components, giving results comparable to the FDMS. The data also indicate that the monitor can be used to estimate water content of the fine particles. However, if the inlet to the monitor is heated, then the instrument measures only the nonvolatile material, more comparable to results obtained with a conventional heated filter tapered element oscillating microbalance (TEOM) monitor. A recent modification of the model 180, with a Nafion dryer at the inlet, measures total PM_2.5 including the nonvolatile and semi-volatile components, but excluding fine particulate water. Model 180 was in agreement with FDMS data obtained in Lindon, UT, during January through February 2007     

Evaluation of the RAMS Continuous Monitor for Determination of PM2.5 Mass Including Semi-Volatile Material in Philadelphia,PA

Abstract

The real-time ambient mass sampler (RAMS) is a continuous monitor based on particle concentrator, denuder, drier, and tapered element oscillating microbalance (TEOM) monitor technology. It is designed to measure PM_2.5 mass, including the semi-volatile species NH₄NO₃ and semi-volatile organic material, but not to measure PM_2.5 water content. The performance of the RAMS in an urban environment with high humidity was evaluated during the July 1999 NARSTO-Northeast Oxidant and Particles Study (NEOPS) intensive study at the Baxter water treatment plant in Philadelphia, PA. The results obtained with the RAMS were compared to mass measurements made with a TEOM monitor and to constructed mass obtained with a Particle Concentrator-Brigham Young University Organic Sampling System (PC-BOSS) sampler designed to determine the chemical composition of fine particles, including the semi-volatile species. An average of 28% of the fine particulate material present during the study was semi-volatile organic material lost from a filter during particle collection, and 1% was NH₄NO₃ that was also lost from the particles during sampling. The remaining mass was dominantly nonvolatile (NH₄)₂SO₄ (31%) and organic material (37%), with minor amounts of soot, crustal material, and nonvolatile NH₄NO₃. Comparison of the RAMS and PC-BOSS results indicated that the RAMS correctly monitored for fine particulate mass, including the semi-volatile material. In contrast, the heated filter of the TEOM monitor did not measure the semi-volatile material. The comparison of the RAMS and PC-BOSS data had a precision of ±4.1 μg/m³ (±9.6%). The precision of the RAMS data was limited by the uncertainty in the blank correction for the reversible adsorption of water by the charcoal-impregnated cellulose sorbent filter of the RAMS monitor. The precision of the measurement of fine par-ticulate components by the PC-BOSS was ±6-8%.     

Air quality measurements from the Fresno Supersite

The Fresno Supersite intends to 1) evaluate non-routine monitoring methods, establishing their comparability with existing methods and their applicability to air quality planning, exposure assessment, and health effects studies; 2) provide a better understanding of aerosol characteristics, behavior, and sources to assist regulatory agencies in developing standards and strategies that protect public health; and 3) support studies that evaluate relationships between aerosol properties, co-factors, and observed health end-points. Supersite observables include in-situ, continuous, short-duration measurements of 1) PM2.5, PM10, and coarse (PM10 minus PM2.5) mass; 2) PM2.5 SO4(-2), NO3-, carbon, light absorption, and light extinction; 3) numbers of particles in discrete size bins ranging from 0.01 to approximately 10 microns; 4) criteria pollutant gases (O3, CO, NOx); 5) reactive gases (NO2, NOy, HNO3, peroxyacetyl nitrate [PAN], NH3); and 6) single particle characterization by time-of-flight mass spectrometry. Field sampling and laboratory analysis are applied for gaseous and particulate organic compounds (light hydrocarbons, heavy hydrocarbons, carbonyls, polycyclic aromatic hydrocarbons [PAH], and other semi-volatiles), and PM2.5 mass, elements, ions, and carbon. Observables common to other Supersites are 1) daily PM2.5 24-hr average mass with Federal Reference Method (FRM) samplers; 2) continuous hourly and 5-min average PM2.5 and PM10 mass with beta attenuation monitors (BAM) and tapered element oscillating microbalances (TEOM); 3) PM2.5 chemical speciation with a U.S. Environmental Protection Agency (EPA) speciation monitor and protocol; 4) coarse particle mass by dichotomous sampler and difference between PM10 and PM2.5 BAM and TEOM measurements; 5) coarse particle chemical composition; and 6) high sensitivity and time resolution scalar and vector wind speed, wind direction, temperature, relative humidity, barometric pressure, and solar radiation. The Fresno Supersite is coordinated with health and toxicological studies that will use these data in establishing relationships with asthma, other respiratory disease, and cardiovascular changes in human and animal subjects.     

Development and evaluation of a continuous coarse (PM10-PM2.5) particle monitor

In this paper, we describe the development and laboratory and field evaluation of a continuous coarse (2.5-10 microm) particle mass (PM) monitor that can provide reliable measurements of the coarse mass (CM) concentrations in time intervals as short as 5-10 min. The operating principle of the monitor is based on enriching CM concentrations by a factor of approximately 25 by means of a 2.5-microm cut point round nozzle virtual impactor while maintaining fine mass (FM)--that is, the mass of PM2.5 at ambient concentrations. The aerosol mixture is subsequently drawn through a standard tapered element oscillating microbalance (TEOM), the response of which is dominated by the contributions of the CM, due to concentration enrichment. Findings from the field study ascertain that a TEOM coupled with a PM10 inlet followed by a 2.5-microm cut point round nozzle virtual impactor can be used successfully for continuous CM concentration measurements. The average concentration-enriched CM concentrations measured by the TEOM were 26-27 times higher than those measured by the time-integrated PM10 samplers [the micro-orifice uniform deposit impactor (MOUDI) and the Partisol] and were highly correlated. CM concentrations measured by the concentration-enriched TEOM were independent of the ambient FM-to-CM concentration ratio, due to the decrease in ambient coarse particle mass median diameter with an increasing FM-to-CM concentration ratio. Finally, our results illustrate one of the main problems associated with the use of real impactors to sample particles at relative humidity (RH) values less than 40%. While PM10 concentrations obtained by means of the MOUDI and Partisol were in excellent agreement, CM concentrations measured by the MOUDI were low by 20%, and FM concentrations were high by a factor of 5, together suggesting particle bounce at low RH.     

Direct field observation of the relative humidity effect on the beta-gauge readings

The effect of ambient relative humidity (RH) on hourly particulate matter (PM10) readings of beta-gauge monitors has been studied using two collocated monitors in the field. The inlet air of monitor 1 was conditioned with water vapor to increase its RH, whereas monitor 2 operated normally in ambient conditions. Experimental data showed that PM10 readings of monitor 1 were nearly the same as monitor 2, as long as the RH of its conditioned incoming air did not exceed approximately 80-85%. However, when the RH exceeded approximately 80-85%, PM10 readings of monitor 1 became higher than monitor 2, and the difference increased with increasing RH. The measurement of pressure drop across the filter was also conducted, and the data revealed that the increase of pressure drop per unit of PM10 concentration decreased when RH was higher than approximately 80-85%, as compared with the case when RH was lower than 80-85%. This is perhaps because of more porous structure of deposited particles in the beta-gauge monitor when RH is greater than approximately 80-85%. The theoretical calculation using an evaporation model and a thermodynamic model has been conducted to simulate the beta-gauge readings. The results show that the theoretical PM10 concentrations using the evaporation model are in better agreement with the actual beta-gauge readings than those using the thermodynamic equilibrium model.     

The regional nature of PM2.5 episodes in the upper Ohio River Valley

From October 1999 through September 2000, particulate matter (PM) with aerodynamic diameter > or =2.5 microm (PM2.5) mass and composition were measured at the National Energy Technology Laboratory Pittsburgh site, with a particle concentrator Brigham Young University-organic sampling system and a tapered element oscillating microbalance (TEOM) monitor. PM2.5 measurements had also been obtained with TEOM monitors located in the Pittsburgh, PA, area, and at sites in Ohio, including Steubenville, Columbus, and Athens. The PM data from all these sites were analyzed on high PM days; PM2.5 TEOM particulate mass at all sites was generally associated with transitions from locally high barometric pressure to lower pressure. Elevated concentrations occurred with transport of PM from outside the local region in advance of frontal passages as the local pressure decreased. During high-pressure periods, concentrations at the study sites were generally low throughout the study region. Further details related to this transport were obtained from surface weather maps and estimated back-trajectories using the hybrid single-particle Lagrangian integrated trajectory model associated with these time periods. These analyses indicated that transport of pollutants to the Pittsburgh site was generally from the west to the southwest. These results suggest that the Ohio River Valley and possible regions beyond act as a significant source of PM and its precursors in the Pittsburgh area and at the other regional sites included in this study.     

Human Exposure to Sulfates from Coal-Fired Power Plants

Abstract

Increased interest in the health effects of ambient par–ticulate mass (PM) has focused attention on the evaluation of existing mass measurement methodologies and the definition of PM in ambient air. The Rupprecht and Patashnick Tapered Element Oscillating MicroBalance (TEOM^®) method for PM is compared with time–integrated gravimetric (manual) PM methods in large urban areas during different seasons. Comparisons are conducted for both PM₁₀ and PM_2.5 concentrations.

In urban areas, a substantial fraction of ambient PM can be semi–volatile material. A larger fraction of this component of PM₁₀ may be lost from the TEOM–heated filter than the Federal Reference Method (FRM). The observed relationship between TEOM and FRM methods varied widely among sites and seasons. In East Coast urban areas during the summer, the methods were highly correlated with good agreement. In the winter, correlation was somewhat lower, with TEOM PM concentrations generally lower than the FRM. Rubidoux, CA, and two Mexican sites (Tlalnepantla and Merced) had the highest levels of PM₁₀ and the largest difference between TEOM and manual methods.

PM_2.5 data from collocation of 24–hour manual samples with the TEOM are also presented. As most of the semi–volatile PM is in the fine fraction, differences between these methods are larger for PM_2.5 than for PM₁₀.