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Keywords:

  • FDMS;
  • Differential TEOM;
  • RAMS;
  • PC-BOSS;
  • PM2.5 mass measurement;
  • semivolatile particulate material

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Experimental Sampling Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Summary
  8. Acknowledgments
  9. References
  10. Supporting Information

[1] Field studies have been performed in Lindon, Utah (February 2003) and Rubidoux, California (July 2003) to determine if the Rupprecht and Patashnick (R&P) Filter Dynamic Measurement System (FDMS) determines total fine particulate mass, including the semivolatile ammonium nitrate and organic material. Collocated measurements were made with the FDMS, a conventional tapered element oscillating microbalance (TEOM) monitor with a heated filter, an R&P differential TEOM monitor, the Brigham Young University (BYU) Real-Time Total Ambient Mass Sampler (RAMS), the BYU particle concentrator-organic sampling system (PC-BOSS), a PM2.5 Federal Reference Method (FRM), a PM2.5 speciation sampler, an R&P continuous nitrate monitor, and two Sunset continuous carbon monitors (one to measure quartz filter-retained particulate carbon and one to measure particulate semivolatile carbonaceous material lost from the particles on a filter during sampling). The RAMS and PC-BOSS samplers have been shown to determine fine particulate material, including both the semivolatile and the nonvolatile components. Linear regression analysis at the Lindon site between the FDMS (X) and the PC-BOSS (Y), and the FDMS (X) and the RAMS (Y), resulted in zero-intercept slopes of 1.01 ± 0.06 (r2 = 0.63) and 1.00 ± 0.01 (r2 = 0.69), respectively. At the Rubidoux sampling site, linear regression analysis between the PC-BOSS (X) and the FDMS (Y) gave a zero-intercept slope of 0.96 ± 0.02 (r2 = 0.90). Linear regression analysis between the FDMS (X) and the RAMS (Y) resulted in a zero-intercept slope of 0.99 ± 0.01 (r2 = 0.80). Measurements made at the two sites indicate that the FDMS and the R&P differential TEOM monitors do measure total fine particulate mass, including the semivolatile ammonium nitrate and organic material. Both the heated TEOM monitor and PM2.5 FRM did not measure the semivolatile material. The difference between the FDMS and a heated TEOM monitor was explained by the semivolatile ammonium nitrate and organic material measured by the various chemical composition monitors.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Experimental Sampling Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Summary
  8. Acknowledgments
  9. References
  10. Supporting Information

[2] It is desirable to monitor fine particulate mass on a continuous basis. Such data would allow for the better understanding of atmospheric processes and sources which contribute to fine particulate pollution and for timely public reporting and forecasting of air pollution exposure. An instrument commonly used for this purpose is the Tapered Element Oscillating Microbalance (TEOM) monitor [Patashnick and Rupprecht, 1991]. However, semivolatile nitrate and organic material associated with fine particles is not accurately measured with a conventional TEOM monitor because the filter is heated to avoid collection of particle-bound water [Long et al., 2003; Mignacca and Stubbs, 1999]. The real time total ambient mass sampler (RAMS) [Eatough et al., 2000, 2003] is a modified TEOM monitor with a combination of a particle collection filter (TX40) and a charcoal-impregnated, glass-fiber filter on the oscillating tapered element of a TEOM monitor to retain the semivolatile species and allow determination of total fine particulate material mass, including the semivolatile species. The RAMS uses diffusion denuders and Nafion dryers to remove interfering gas phase material, including water, from the aerosol prior to the collection of particles. While the RAMS does measure total fine particulate mass, including the semivolatile ammonium nitrate and semivolatile organic components, the sampler is not suitable for routine use in field sampling because it requires regular observation and maintenance.

[3] R&P recently developed the differential TEOM monitor as a reference standard for particulate matter mass as described by Patashnick et al. [2001], and subsequently developed the Filter Dynamics Measurement System (FDMS) [Meyer et al., 2002], both of which attempt to correct for loss of semivolatile species from the TEOM filter by alternately making measurements with particle-containing and particle-free air passing through the filters on the tapered element oscillating microbalance of a TEOM monitor. In this study, the new FDMS monitor is being evaluated by comparison of measurements with a RAMS and a differential TEOM system similar to that described by Meyer et al. [2002]. In addition, integrated average particulate mass and composition data were obtained using a particle concentrator-Brigham Young University organic sampling system (PC-BOSS) [Lewtas et al., 2001], which provides an alternate method for measurement of nonvolatile and semivolatile species, to allow interpretation of any differences which may be seen between the FDMS and RAMS monitors. Measurements have been made during field studies in January–February 2003 in Lindon, UT and July 2003 in Rubidoux, CA. This paper compares the various results from these studies related to the measurement of semivolatile fine particulate material with the FDMS monitor.

2. Experimental Sampling Methods

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Experimental Sampling Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Summary
  8. Acknowledgments
  9. References
  10. Supporting Information

2.1. R&P TEOM Monitor

[4] One-hour averaged, nonvolatile PM2.5 mass concentrations were determined using an R&P TEOM monitor heated above ambient temperature to avoid water condensation [Patashnick and Rupprecht, 1991]. As stated above, semivolatile PM will evaporate at the standard operating temperature of the instrument (50°C, 30°C during winter months), which is required to remove particle-bound water [Eatough et al., 2003; Long et al., 2003; Mignacca and Stubbs, 1999]. This technique measures nonvolatile PM. In this study we used the TEOM to obtain a direct measurement of the mass without the corrections normally used to give better agreement with the Federal Reference Method (FRM) samplers.

2.2. R&P FDMS

[5] The Rupprecht and Patashnick Filter Dynamics Measurement System (FDMS, Series 8500) is designed to account for both the semivolatile and nonvolatile components of particulate matter, reporting the combination as a mass concentration result. This result is accomplished by measuring the semivolatile portion of the sample independently from the total incoming sample, and using this fraction in calculating the PM2.5 mass concentration. To accomplish this, the FDMS unit constantly samples ambient air and uses a switching valve to change the path of the main flow every 6 min. The sampling process consists of an alternate sample and purge (filtered) air stream passing through the exchangeable filter in the TEOM mass sensor. The purge filter in the FDMS removes aerosols at 4°C prior to passage of the sampled air to the TEOM monitor. The sample and purge air flows alternately pass through the exchangeable filter in the TEOM microbalance, which generates a direct measurement of the collected mass. The system automatically adjusts the mass concentrations from the particle-laden air stream by correcting the measurement for the mass change that may occur during purging. For example, if the FDMS unit measures a decrease of filter mass during the 6-min purging period prior to or after collection of particle-laden air, this mass decrease is added back to the mass measurement obtained with the particle-laden air.

2.3. R&P Differential TEOM Monitor

[6] The differential TEOM monitor is an R&P research instrument which incorporates a modification of the technology used in the FDMS. Instead of removing the particles with a cold filter in the purge step of the measurement, the differential TEOM removes the particles with an electrostatic precipitator [Meyer et al., 2002; S. Yi et al., Evaluation of a prototype electrostatic precipitator (ESP) for a differential TEOM system, submitted to Aerosol Science and Technology, 2004]. Calculation of the PM2.5 concentration is then based on the measurement of mass with a TEOM microbalance during the particle-laden air cycle, corrected for any mass loss measured from the TEOM monitor filter when the particles are removed with the electrostatic precipitator before or after the particle-laden air measurement period.

2.4. RAMS

[7] The Real-Time Total Ambient Mass Sampler (RAMS), based on diffusion denuder, Nafion dryer and TEOM monitor technology, was used for the real-time determination of total PM2.5 mass, including semivolatile species [Eatough et al., 2000]. The RAMS measures total PM2.5 mass with a TEOM monitor using a sandwich filter to retain semivolatile ammonium nitrate and organic material which may be lost from particles in a conventional TEOM monitor. The sandwich filter consists of a Teflon-coated particle collection filter (R&P TX40) followed by a charcoal-impregnated glass fiber filter (CIG, Schleicher and Schuell, Dassell, Germany) to collect any semivolatile compounds lost from the particles during sampling.

[8] Care must be taken to remove from the sample stream all gas phase species that can be absorbed by the CIG filter in order to prevent over-determination of PM2.5 mass. Gas removal is accomplished with a series of denuders to remove gas phase organic compounds, O3, NO2, SO2 and HNO3 and two Nafion dryers to remove gas phase water. The configuration and operation of the RAMS as used in this study has been previously described [Eatough et al., 2000, 2003; Long et al., 2003]. The configuration includes an active blank sampler to monitor and correct for gas phase compounds not removed before the sandwich filter which can be sampled with the CIG. RAMS data were averaged over 1-hour periods for each of the two studies for comparison with 1-hour averaged FDMS, differential TEOM and conventional heated filter TEOM data. The RAMS data were also averaged as needed for comparison with results obtained with the PC-BOSS sampler.

2.5. PC-BOSS

[9] The combination of technology used in the High-Volume Brigham Young University Organic Sampling System (BIG BOSS) [Tang et al., 1994] and the Harvard particle concentrator [Sioutas et al., 1994] has resulted in the Particle Concentrator-Brigham Young University Organic Sampling System (PC-BOSS) [Ding et al., 2001; Lewtas et al., 2001]. The configuration and operation of the PC-BOSS has been previously described [Eatough et al., 2003; Lewtas et al., 2001].

[10] The PC-BOSS was used for sample collection to determine fine particulate mass, sulfate, carbonaceous material (elemental and organic), nitrate, semivolatile organic material, and semivolatile nitrate. Samples for the chemical characterization of PM2.5 in the minor flow following a particle concentrator and a BOSS diffusion denuder were collected in a filter pack containing a prefired 47 mm quartz filter (Pallflex) followed by 47 mm charcoal impregnated glass fiber filter (CIG, Schliecher and Schuell, Dassel, Germany) to determine fine particulate carbonaceous material and nitrate, including semivolatile species lost from the particles during sampling. A second parallel filter pack containing a 47 mm Teflon (Whatman) filter followed by a 47 mm Nylon (Gelman, Nylasorb) filter was used to determine PM2.5 filter retained (nonvolatile) mass, sulfate and nitrate, and any nitrate lost from the particles during sample collection. A side flow filter pack, prior to the particle concentrator, contained a 47 mm polycarbonate (Whatman, Nuclepore, 0.4 m pore size) filter followed by a 47 mm CIG to collect particles (excluding semivolatile species lost during sampling) and gas phase organic material after the 2.5 m outlet cut. The various quartz and CIG filter collection areas were reduced to 4 cm2 with a stainless steel mask to improve measurement sensitivity. The side-flow data were compared to data from the minor flow filters to determine the particle concentrator efficiency [Ding et al., 2001; Lewtas et al., 2001]. Multiple 3-hour samples were collected at selected times periods at each sampling site for comparison with 3- or 6-hour averaged FDMS TEOM, differential TEOM and RAMS results.

[11] Temperature Programmed Volatilization [Tang et al., 1994; Ellis and Novakov, 1982] was used in the analysis of collected samples for total carbonaceous material. In this method, a 2 cm2 portion of each filter is heated from ambient to a final temperature at a known ramp rate. The ramp rate and termination temperatures are dependent on the type of filter being analyzed. Quartz filters are heated to 800°C in an N2/O2 atmosphere. Charcoal impregnated filters are heated to 450°C in an N2 atmosphere. Carbon in compounds desorbed from the filters during the heating process is catalytically converted to CO2 and detected by nondispersive infrared absorption. Sulfate and nitrate concentrations were determined by ion chromatography.

2.6. R&P Continuous Nitrate Monitor

[12] Hourly average fine particulate nitrate were determined using an R&P Model 8400N nitrate monitor [Long and McClenny, 2004].

2.7. Sunset Laboratory Carbon Aerosol Field Instruments

[13] The Sunset instrument is a semicontinuous, real-time carbon aerosol analysis monitor. The inlet is a 2.5 μm sharp cut cyclone (R&P) with a total flow of 16 L/m. Eight L/m of the flow goes to the carbon monitor and the remaining flow is directed to a modified Sunset instrument described below. After the flow split, the sampled air passes through a parallel plate charcoal impregnated filter denuder similar to that described for the BYU RAMS [Eatough et al., 2000] and supplied by Sunset Laboratory with the instrument. This denuder is intended to remove gas phase organic compounds which can be absorbed by a quartz filter, thus eliminating any positive quartz filter artifact for the data obtained with the monitor [Eatough et al., 2003; Turpin and Huntzicker, 1994]. The particles in the sampled air stream are then collected on an 12.3 mm diameter quartz filter for a controlled time period (45 min in the study reported here). Sample collection is then interrupted and the sample analyzed, using a volatilization method comparable to the NIOSH Method 5040. Instruments were used which had either an FID (Lindon site) or an NDIR (Rubidoux site) detector. The data analysis step is followed with a calibration step for each analysis.

[14] A second Sunset Monitor was modified to allow for the determination of semivolatile organic carbonaceous material, SVOC, lost from particles during the 45 min sample collection period. The modified instrument sampled the second of the two split flow lines after the sharp cut cyclone inlet. A diffusion denuder, identical to that used in the unmodified instrument, removed gas phase material with an expected efficiency of better than 99%. After the removal of the gas phase material, the particles were removed from the sampled air stream immediately before the entrance to the Sunset Monitor using a prefired (800°C) 47-mm quartz filter in a MACE in-line Teflon filter holder. The particle-free air (with any SVOC lost from the particles during sample collection) passed into the filter collection region of the Sunset Monitor. The quartz filter normally used in the unmodified instrument was preceded by a charcoal impregnated glass fiber filter (CIG, Schleicher and Schuell, Dassell, Germany). The quartz filter was kept after the CIG to provide additional support for the CIG filter. Any SVOC lost from particles collected on the inlet quartz filter were collected with high efficiency by this CIG filter. At the end of the 45 min sample period, the SVOC collected on the CIG were analyzed by thermal evolution. This analysis was done in a three-step temperature program in a He atmosphere to separate any gas phase VOC not removed by the denuder from fine particulate SVOC. Details of the measurements with the two Sunset instruments have been published [Grover et al., 2004].

2.8. Sample Collection

[15] Initial studies were conducted in February 2003 in Lindon, UT. The Lindon sampling site has been previously described [Long et al., 2003]. In these experiments, results obtained with the FDMS were compared to 1-hour averaged fine particulate mass determined with a conventional TEOM monitor operated at 30°C, to results obtained with a RAMS, and also to fine particulate mass determined in 3-hour integrated samples with the PC-BOSS. More extensive studies were conducted during July 2003 at the SCAQMD sampling site in Rubidoux, CA. In these studies, both the FDMS and differential TEOM monitors were used. Results were compared with 1-hour averaged RAMS and TEOM monitor measurements, 1-hour average R&P particulate nitrate measurements, 1-hour average Sunset Laboratory nonvolatile and semivolatile C measurements, and 3-hour integrated PC-BOSS sampler results. Particle separation for all continuous mass measurements was done with an R&P PM10 16.67 L/min inlet followed by an R&P 2.5 μm Sharp Cut Cyclone.

3. Results

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Experimental Sampling Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Summary
  8. Acknowledgments
  9. References
  10. Supporting Information

3.1. Lindon Study FDMS and RAMS Results

[16] One-hour average PM2.5 mass measured in Lindon, UT during a two week period in January–February 2003 using the RAMS, and the FDMS are given in Figure 1. As indicated, there was good agreement between the RAMS and the FDMS results as shown in Figure 2 and Table 1. Linear regression analysis results and the bias corrected precision of the comparison are given in Table 1. The precision of the comparison is limited by the expected ±2 to 3 μg/m3 uncertainty in the RAMS data [Eatough et al., 2003; Long et al., 2003]. The uncertainty in the comparison is σ = ±2.8 μg/m3 (±21%), consistent with the expected precision of the RAMS results and therefore, within the precision of the RAMS measurement the RAMS and FDMS results agreed.

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Figure 1. Data from the Lindon 2003 study. One-hour average PM2.5 mass determined with the RAMS and FDMS in Lindon, UT, during January and February 2003.

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Figure 2. Data from the Rubidoux July 2003 study. (a) Comparison of 1-hour average FDMS and differential TEOM PM2.5 mass measurements. (b) Comparison of 1-hour average FDMS and RAMS PM2.5 mass measurements. (c) Comparison of 1-hour average FDMS and 50°C TEOM monitor PM2.5 mass measurements.

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Table 1. Results of the Statistical Analysis of PM2.5 Measurements During the Lindon and Rubidoux Studies
X versus Ynr2SlopeaIntercept, μg/m3X Average, μg/m3X-Y Bias, μg/m3σ, μg/m3σ, %
  • a

    The 1-hour average results.

  • b

    The 3-hour average results.

  • c

    The 24-hour average results.

  • d

    NA, σ could not be calculated because of the large bias.

  • e

    Here 38 peak concentrations with bias greater than 15 μg/m3 (FDMS > Diff TEOM) not included in statistical analysis.

Lindon
FDMS TEOM versus RAMS PM2.5a3320.691.00 ± 0.010 ± 4.013.50.22.820.9
  0.690.92 ± 0.031.3 ± 3.9    
FDMS TEOM versus PC-BOSS PM2.5b110.631.01 ± 0.060 ± 2.713.00.31.813.9
  0.660.89 ± 0.211.8 ± 2.8    
 
Rubidoux
FDMS versus Diff TEOM PM2.5a,e4260.850.97 ± 0.010 ± 5.334.61.23.811.2
  0.850.98 ± 0.02−0.6 ± 5.3    
FDMS TEOM versus RAMS PM2.5a3370.800.99 ± 0.010 ± 8.234.60.45.916.8
  0.810.93 ± 0.022.4 ± 8.2    
PC-BOSS versus FDMS TEOM PM2.5b330.900.96 ± 0.020 ± 3.939.41.83.07.7
  0.900.96 ± 0.06−0.3 ± 3.9    
FDMS TEOM versus FRM PM2.5c290.870.70 ± 0.020 ± 3.335.811.3NAd 
  0.900.96 ± 0.06−9.3 ± 3.9    
PC-BOSS versus R&P nitrateb310.610.89 ± 0.040 ± 2.910.80.52.221.0
  0.730.65 ± 0.073.3 ± 2.4    
PC-BOSS versus Sunset Ctotalb210.910.99 ± 0.020 ± 2.218.8−0.11.89.6
  0.930.90 ± 0.062.0 ± 2.1    

3.2. Rubidoux Study FDMS, Differential TEOM, and RAMS Results

[17] Both an FDMS and a differential TEOM monitor were used at Rubidoux. The results obtained with these two instruments when both instruments were operational are given in Figure 2a. The two measurements are generally in good agreement. Significant exceptions occurred only at peak concentrations. Of the total 474 data points, the 38 peak values have FDMS concentrations which are biased 21 μg/m3 higher than the differential TEOM concentrations. The reasons for this bias are not currently understood. Linear regression analysis of the FDMS and differential TEOM data, (excluding the 38 peak concentrations; see remaining data in Figure 3) are shown in Table 1. The average concentration is 34.6 μg/m3 and the FDMS concentrations are biased only 1.2 μg/m3 higher than the differential TEOM concentrations. The bias corrected uncertainty in the comparison is σ = ±3.8 μg/m3 (±11%).

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Figure 3. Comparison of 1-hour average PM2.5 mass determined with the FDMS and differential TEOM in Rubidoux, CA. The solid line is the regression slope with a zero intercept, and the dashed line is the regression line with a calculated intercept.

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[18] Comparison between the FDMS and RAMS measurements are shown in Figure 2b. Note that the data points in Figures 2a and 2b are not completely the same because of some differences in when the various instruments were producing valid data. The very high concentrations in Figure 2b are associated with fireworks and a local fire near the Rubidoux site on the night of 4 July. Differential TEOM data were not obtained during the 4 July time period. The FDMS and RAMS measurements are generally in agreement. The uncertainty in the RAMS data during the 1 to 9 July period is about three times larger than that of the RAMS data during the latter part of the study. The poor precision was due to incomplete control of humidity in the RAMS measurements during the first part of the study. The humidity control in the RAMS was improved after 9 July. This same problem was evident in only a small fraction of the samples after 18 July. Linear regression analysis statistic of the FDMS and RAMS data (for the valid data during both the 1–9 and 18–31 July time periods) are given in Table 1. The uncertainty in the comparison is σ = ±5.8 μg/m3 (±17%) and is again limited by the precision of the RAMS data. The average concentration for these samples was 30.6 μg/m3 and the bias between the RAMS and FDMS data sets was only 0.4 μg/m3. The RAMS and FDMS PM2.5 data were in agreement.

[19] The FDMS and 50°C TEOM data are given in Figure 2c. As expected, high concentrations of ammonium nitrate and semivolatile organic material, as detailed below, result in the concentrations measured with the heated filter of the TEOM monitor being substantially lower than those obtained with the FDMS, differential TEOM or RAMS monitors.

4. Discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Experimental Sampling Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Summary
  8. Acknowledgments
  9. References
  10. Supporting Information

[20] A second check on the accuracy of the FDMS data for each sampling site was made by comparison with the constructed mass obtained from the PC-BOSS integrated samples. Sulfate and nitrate were assumed to be present as the ammonium salts. Both nonvolatile (NVOM) and semivolatile (SVOM) organic material were assumed to be 62% C [Turpin and Lim, 2001]. For the Lindon site, the PC-BOSS data were obtained on a 3-hour average basis (about 3 samples/day) on 29 January and 7–8 and 10–12 February (see Figure 1). Linear regression analysis statistics of the FDMS and PC-BOSS data are given in Table 1. The intercept calculated slope is lower, but uncertainty in the slope includes unity and the uncertainty in the intercept includes zero. The uncertainty in the comparison of the FDMS and PC-BOSS PM2.5 results is σ = ±1.8 μg/m3 (±14%), consistent with the expected precision of the PC-BOSS results. In contrast, the measurements obtained with the 30°C TEOM monitor and the FDMS unit were very different. The 30°C TEOM monitor gave an average PM2.5 concentration (N = 265) of 6.1 μg/m3. For the same data points, the FDMS unit averaged 14.8 μg/m3 The mass measured with the 30°C TEOM monitor was consistently lower than that measured with the FDMS unit. The difference between the two measurement is consistent with the concentrations of semivolatile organic material and ammonium nitrate measured with the PC-BOSS sampler.

[21] The precision of two of the components included in the calculation of the PC-BOSS calculated mass for the Rubidoux study can be estimated by comparison with an independent measurement of that component by a second sampler. Either 4 or 8 three-hour PC-BOSS samples were collected on 2, 8, 11, 16, 17, 23, 26, and 30 July. In addition, a single 24-hour PC-BOSS sample was collected on 5, 14, 26 and 29 July. These 24-hour samples were collected on days when speciation sampler results were available. The fine particulate nitrate concentrations were determined in both 1-hour averaged measurements with the R&P nitrate monitor and 3-hour average measurements with the PC-BOSS. The R&P nitrate data were obtained over the time period from 9 through 20 July. The R&P nitrate data were averaged over the PC-BOSS 3-hour sampling time periods. These data are compared to the PC-BOSS fine particulate nitrate concentrations in Table 1. There is a definite bias between the two measurements at the higher concentrations, with the PC-BOSS data being higher in concentration. It has been suggested that this difference is due to incomplete volatilization of the sampled ammonium nitrate at higher concentrations (and higher relative humidity) for the R&P monitor [Long and McClenny, 2004].

[22] If only the concentrations below 20 μg/m3 are included in the regression analysis, the zero intercept slope (n = 31, R2 = 0.44) is 0.96 ± 0.06 and the precision of the comparison is σ = ±2.0 μg/m3 (±20%). This result is taken as an estimate of the uncertainty in the PC-BOSS nitrate results. The corresponding uncertainty in ammonium nitrate is ±2.6 μg/m3. The uncertainty in ammonium sulfate can be estimated by comparison with 24-hour SCAQMD speciation sampler results (N = 10) to be ±1.9 μg/m3. The uncertainty in the PC-BOSS ammonium nitrate results obtained by comparison with the SCAQMD speciation sampler results (N = 9) is ±2.1 μg/m3.

[23] The second 1-hour average PM2.5 component which can be compared to the PC-BOSS results is total fine particulate carbonaceous material. Both nonvolatile and semivolatile fine particulate carbonaceous material were determined using the PC-BOSS and the two Sunset monitors [Grover et al., 2004]. The Sunset measurements were available for the time period from 13 through 26 July. Comparison between these two measurements is given in Table 1. As indicated, the two measurements are in good agreement. Assuming that the organic material is 62% carbon [Turpin and Lim, 2001] the uncertainty in the comparison of these two measurements is ±2.9 μg/m3. The combination of the ammonium nitrate, ammonium sulfate and carbonaceous material precision estimates leads to an expected uncertainty in the calculated PC-BOSS PM2.5 mass of ±2.9 μg/m3.

[24] The FDMS and PC-BOSS determined PM2.5 mass results are compared in Table 1 and Figure 4. Included in Figure 4 are four data points (given as open squares) for which the differences between the two measurements were different from the rest of the data set by greater than 3σ. These four data points are not included in the statistical analysis summarized in Table 1. The four data points were obtained in four 3-hour sequential samples, beginning at midnight on 23 July. As indicated by the data in Figure 2a, the FDMS and differential TEOM data were in good agreement during this time period. However, the carbon measurements made with the PC-BOSS and the Sunset monitors were also in good agreement. R&P nitrate measurements were not made during this time period. However, the eight 3-hour nitrate concentrations determined with the PC-BOSS for the entire day and the results of the SCAQMD 24-hour results from the speciation sampler this day are in agreement. Thus there is nothing in the data set which allows one to ascertain which of the two measurements (PC-BOSS versus the FDMS) is the more accurate.

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Figure 4. Comparison of 3-hour average PM2.5 mass determined with the PC-BOSS and FDMS in Rubidoux, CA. The solid line is the regression slope with a zero intercept, and the dashed line is the regression line with a calculated intercept. The four data points where the difference between the two measurements is greater than 3σ compared to the remaining data are indicated with a square data point; see text.

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[25] For the data points included in the statistical analysis, the PC-BOSS and FDMS data are in good agreement with a regression slope (N = 33, R2 = 0.90) of 0.96 and σ = ±3.0 μg/m3 (±7.7%)., with a bias between the two measurements of only 1.8 μg/m3. Included in the analysis are four time periods (1200–1500 and 1500–1800 on 11 and 23 July) when PC-BOSS, FDMS and differential TEOM data were all available and there was a significant bias between the FDMS and differential TEOM during peak concentrations, with the FDMS averaging 9.0 μg/m3 higher than the differential TEOM monitor. These time periods include five of the 38 peak concentrations not included in the FDMS versus differential TEOM comparison as previously discussed. The PC-BOSS and FDMS measurements agreed during this time periods, with a bias of only 0.3 μg/m3, providing an indication that the occasional significant difference seen between the FDMS and the differential TEOM monitor cannot be attributed to systematically inaccurate measurements by the FDMS. This point deserves further study.

[26] The measurements made with the FDMS can also be compared with the 24-hour average mass measurements obtained with the PM2.5 single filter FRM monitor. The comparison between these two measurements is given in Table 1 and Figure 5. As indicated, there is a consistent bias between the two measurements (N = 29), with the FRM averaging 11.3 μg/m3 (32%) lower than the FDMS measurements. While data coverage for the study period was not complete for the speciation data, for the time periods associated with these comparisons, the average concentration of ammonium nitrate was 12 μg/m3 and for SVOM was 13 μg/m3. Thus some combination of partial loss of ammonium nitrate and SVOM during sampling with the single filter FRM can account for the under-measurement of PM2.5 mass with this sampler.

image

Figure 5. Comparison of 24-hour average PM2.5 mass determined with the FDMS and the single filter PM2.5 FRM sampler in Rubidoux, CA. The solid line is the slope equals 1.

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[27] The average difference between the FDMS PM2.5 mass measurement and that of a conventional TEOM monitor operating with the filter at 50°C is shown in Figure 2c. The conventional TEOM monitor measured mass is consistently lower than the FDMS measured mass. Where both measurements were made, the FDMS data averaged 36.8 μg/m3 and the heated TEOM monitored averaged 18.4 μg/m3. The bias between the two measurements averaged 18.5 μg/m3 (50% of the FDMS PM2.5 mass). The difference between the two measurements can be directly compared with the hourly average measurements of ammonium nitrate by the R&P nitrate monitor and SVOM by the modified Sunset C monitor. The comparison with the ammonium nitrate data is shown in Figure 6 for the time period when all three measurements (FDMS, heated TEOM and R&P nitrate, expressed as ammonium nitrate) were monitored. As indicated, the difference between the FDMS and heated TEOM monitors is usually greater than the measured ammonium nitrate concentration. Part, but not all of the difference between the FDMS and heated TEOM monitor PM2.5 mass measurements can be attributed to the loss of ammonium nitrate from the 50°C filter of the heated TEOM monitor. During part of the time period given in Figure 7, both ammonium nitrate and SVOM measurements were made. The sum of these two measurements is referred to as semivolatile material, SVM, and is compared to the difference in the FDMS and heated TEOM monitor PM2.5 mass in Figure 7. With few exceptions, the total SVM material is either equal to or somewhat greater than the difference between the FDMS and heated TEOM monitors. It appears that most of the SVM is generally lost from the heated filter of the TEOM monitor on an hourly average measurement basis.

image

Figure 6. Data from the Rubidoux July 2003 study. The circles indicate 1-hour average FDMS PM2.5 mass, the squares the difference between 1-hour average FDMS and 50°C TEOM monitor PM2.5 mass, and the bars 1-hour average R&P PM2.5 ammonium nitrate mass measurements.

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image

Figure 7. Data from the Rubidoux July 2003 study. The circles indicate 1-hour average FDMS PM2.5 mass, the squares the difference between 1-hour average FDMS and 50°C TEOM monitor PM2.5 mass, and the bars the sum of 1-hour average R&P PM2.5 ammonium nitrate mass measurements and the modified Sunset monitor semivolatile organic material (SVOM) mass measurements. This sum is indicated as SVM, semivolatile material.

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5. Summary

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Experimental Sampling Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Summary
  8. Acknowledgments
  9. References
  10. Supporting Information

[28] The data obtained in the Lindon and Rubidoux studies indicate that the FDMS and differential TEOM monitors both measure total PM2.5, including the semivolatile particulate matter. In contrast, neither the conventional heated TEOM monitor nor the PM2.5 FRM single filter sampler measured the semivolatile material. Semivolatile particulate matter includes both the ammonium nitrate and semivolatile organic material. None of the continuous samplers used in the studies reported here measure fine particulate water content because of the use of Nafion dryers. Precision of the comparison of the R&P FDMS and differential TEOM monitor sampler PM2.5 is ±11% (±3.8 μg/m3). Precision of the comparison of the FDMS and PC-BOSS PM2.5 mass is ±7.7% (±3.0 μg/m3). There is a suggestion in the data that the results obtained with the FDMS unit may be biased about 1 to 2 μg/m3 higher than the differential TEOM monitor for the urban environments studied here. Agreement with the RAMS and PC-BOSS monitor may be slightly better for the FDMS than the differential TEOM monitor, however, the comparison are all generally within the uncertainty of the RAMS and PC-BOSS data. The precision of both the FDMS and differential TEOM monitors was a factor of 2 to 4 better than that for the RAMS. Both the FDMS and differential TEOM monitors proved to be rugged units which needed little attention from site operators during the studies reported here.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Experimental Sampling Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Summary
  8. Acknowledgments
  9. References
  10. Supporting Information

[29] Appreciation is expressed to the State of Utah Air Quality Monitoring Division, and the South Coast Air Quality Management District for assistance in sample collection at Lindon and Rubidoux, respectively. The U.S. Environmental Protection Agency (EPA) through its Office of Research and Development funded the research described in this paper under contract 3C-R044-NAEX with Brigham Young University. This paper has been reviewed in accordance with EPA policy and approved for publication. However, the views expressed in this paper are those of the authors and do not necessarily reflect the views or policies of the EPA. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Experimental Sampling Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Summary
  8. Acknowledgments
  9. References
  10. Supporting Information
  • Ding, Y., Y. Pang, and D. J. Eatough (2001), High volume diffusion denuder sampler for the routine monitoring of fine particulate matter: I. Design and optimization of the PC-BOSS, J. Aerosol Sci. Technol., 36, 369382.
  • Eatough, D. J., N. L. Eatough, F. Obeidi, Y. Pang, W. Modey, and R. Long (2000), Continuous determination of PM2.5 mass, including semi-volatile species, Aerosol Sci. Technol., 34, 18.
  • Eatough, D. J., R. W. Long, W. K. Modey, and N. L. Eatough (2003), Semi-volatile secondary organic aerosol in urban atmospheres: Meeting a measurement challenge, Atmos. Environ., 37, 12771292.
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Supporting Information

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Experimental Sampling Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Summary
  8. Acknowledgments
  9. References
  10. Supporting Information
FilenameFormatSizeDescription
jgrd11587-sup-0001-t01.txtplain text document1KTab-delimited Table 1.

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