East versus West in the US: Chemical Characteristics of PM 2.5 during the Winter of 1999

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1 Aerosol Science and Technology ISSN: (Print) (Online) Journal homepage: East versus West in the US: Chemical Characteristics of PM 2.5 during the Winter of 1999 Michael P. Tolocka, Paul A. Solomon, William Mitchell, Gary A. Norris, David B. Gemmill, Russell W. Wiener, Robert W. Vanderpool, James B. Homolya & Joann Rice To cite this article: Michael P. Tolocka, Paul A. Solomon, William Mitchell, Gary A. Norris, David B. Gemmill, Russell W. Wiener, Robert W. Vanderpool, James B. Homolya & Joann Rice (2001) East versus West in the US: Chemical Characteristics of PM 2.5 during the Winter of 1999, Aerosol Science and Technology, 34:1, 88-96, DOI: / To link to this article: Published online: 30 Nov Submit your article to this journal Article views: 312 View related articles Citing articles: 43 View citing articles Full Terms & Conditions of access and use can be found at Download by: [ ] Date: 05 December 2017, At: 02:35

2 Aerosol Science and Technology 34: (2001) c 2001 American Association for Aerosol Research Published by Taylor and Francis =01=$12.00 C.00 East versus West in the US: Chemical Characteristics of PM 2.5 during the Winter of 1999 Michael P. Tolocka, 1 Paul A. Solomon, 1 William Mitchell, 1 Gary A. Norris, 1 David B. Gemmill, 1 Russell W. Wiener, 1 Robert W. Vanderpool, 2 James B. Homolya, 3 and Joann Rice 3 1 U.S. Environmental Protection Agency, NERL, Research Triangle Park, North Carolina 2 Research Triangle Institute, Research Triangle Park, North Carolina 3 U.S. Environmental Protection Agency, OAQPS, Research Triangle Park, North Carolina The chemical composition of PM 2:5 was investigated at four sites (Rubidoux, CA, Phoenix, AZ, Philadelphia, PA, and Research Triangle Park, NC) in January and February of Three samplers were used to determine both the overall mass and the chemical composition of the aerosol. Te on lters were weighed for total mass. Ions were analyzed using ion chromatography. Elements were determined using X-ray uorescence. Organic and elemental carbon were measured using a thermo-optical method. At all of the sites, reconstructed mass was observed to be greater than or equal to the measured mass. Good ionic balance was found for ammonium, nitrate, and sulfate at each of the sites. Overall, the chemical composition of the aerosol for each site was in good agreement with the expected composition based upon previous studies, with the exception of relatively high nitrate contribution to the total mass at Philadelphia. Good agreement was found between the predicted amount of sulfate by XRF analysis of sulfur and the sulfate measured by ion chromatography. As expected, sulfate was a more important contributor to the total mass at the East Coast sites. Nitrate contributed more to the total mass at the West Coast sites and was an important factor in the highest observed mass concentration at Rubidoux. Te on lters appear to lose nitrate to a greater extent than heat-treated quartz ber lters. Organic carbon was also found to be the largest part of the aerosol mass on minimum days for all sites and a signi cant portion of the mass on other days with 25 50% of the total mass at all of the sites. At three of the sites, organic carbon (OC) collected on denuded lters was less than that found on nondenuded samples, indicating an absorptive artifact on the quartz ber lters. It was also found that the crustal component to PM 2:5 was highest at Phoenix. PM 2:5 was also found to contribute signi cantly to the PM 10 particle mass at all the sites. INTRODUCTION In January and February, 1999, the U.S. EPA conducted a study to evaluate the operational and species-speci c collec- Received 7 February 2000; accepted 9 June Address correspondence to Michael P. Tolocka, U.S. EPA, NERL, Research Triangle Park, NC tion performance of three newly developed and commercially available chemical speciation samplers. Due to potential artifacts associated with using lters and potential differences in the collection characteristics of the samplers, the chemical speciation samplers were tested at 4 locations to provide a variety of atmospheric chemical and environmental conditions (Pace 1998; Wongphatarakul et al. 1998; U.S. EPA 1999). The locations were: Rubidoux, CA (high nitrate and carbon and low sulfate; daily temperatures ranged from 10 to 18 ± C), Phoenix, AZ (high crustal material and moderate carbon and nitrate daily temperatures ranged from 9 to 18 ± C), Philadelphia, PA (high sulfate, moderate carbon, and low nitrate; daily temperatures ranged from 4 to 12 ± C), and Research Triangle Park (RTP), NC (low PM 2:5 concentrations, daily temperatures ranged from 1 to 20 ± C). Samples were analyzed for mass, major components, and eight trace elements. This study provides a unique database for comparing the PM 2:5 chemical composition among speci c regions of the U.S. We also examine the magnitude of potential biases or sampling artifacts associated with use of the Federal Reference Method (FRM PM 2:5 Sampler) with Te on and heat-treated quartz ber lters across the different locations under wintertime conditions. Finally, differences due to potential variations in the composition of each climate and its effect on sampling artifacts for organic carbon (OC) and particulate nitrate at the 4 locations is examined. METHODS The Versatile Air Pollution Sampler (VAPS), two PM 2:5 FRM samplers, and the newly developed Andersen RAAS chemical speciation sampler were collocated at the each site. The experimental apparati are shown in Figures 1 3. The speciation sampler (Figure 1) was designed to minimize sampling artifacts for nitrate via MgO coated diffusion denuder followed by a Nylon lter (Koutrakis et al. 1988; Brauer et al. 1989; Solomon et al. 1992; Hering and Cass 1999). The VAPS (Figure 2) was 88

3 CHEMICAL CHARACTERISTICS OF AEROSOLS 89 Figure 1. Diagram of the Andersen chemical speciation sampler. For each channel, the analytes are identi ed. used to determine organic carbon using a XAD-4 coated annular denuder to remove semi-volatile gas phase species, followed by a single heat treated quartz- ber lter (Stevens et al. 1993; Gundel et al. 1995). The two collocated PM 2:5 FRM samplers, one with a Te on lter and one with a quartz- ber lter, allowed for comparable chemical analyses between the FRM sampler and the speciation samplers (Figure 3). PM 10 was determined by the sum of the ne and coarse particle mass collected by the VAPS. For all samplers, mass was determined by gravimetric analysis on the Te on lter as speci ed in the Code of Federal Regulations 40 Part 50 Appendix L. Filters were cold shipped at 4 ± C. Trace elements (Si, K, Ca, Fe, Cu, Zn, Pb, and As) were determined on Te on lters using X-ray uorescence (Dzubay and Stevens 1975; Chow and Watson 1999a). Sulfate, nitrate, and ammonium were determined on Te on, heat-treated quartz- ber or Nylon lters by ion chromatography (Mulik et al. 1976; Figure 2. Setup for the VAPS sampler. The analytes for each leg of the sampler are classi ed accordingly.

4 90 M. P. TOLOCKA ET AL. Figure 3. Setup for the FRM samplers. The identi cation of the species that were quanti ed is shown below each sampler. Tejeda et al. 1978; Chow and Watson 1999b). OC and elemental carbon (EC) were determined on heat-treated quartz- ber lters (Hering et al. 1990; Chow et al. 1996). The XAD coated diffusion denuder was followed by single heat-treated quartz- ber lter. RESULTS AND DISCUSSION For the average during each respective sampling period, PM 2:5 mass concentrations were highest at Rubidoux and Phoenix (41 and 34 ¹g/m 3, respectively) and lowest at Philadelphia and RTP (about 24 and 19 ¹g/m 3, respectively). Again, the highest values for PM 10 were observed at Rubidoux (82 ¹g/m 3 ), while at Phoenix and RTP, the maximum measured values were identical (67 ¹g/m 3 ). Philadelphia had a lower maximum value of 55 ¹g/m 3. Some insight to these values can be gleaned from the PM 2:5 fraction to the total mass at each site. During the winter, ne particles are, on average, 46% of the PM 10 mass at both western sites and greater than 70% at both of the two eastern sites. This data set reveals a signi cant relationship (r D 0:73) between ne and PM 10 particle mass for all sites. Ideally, the gravimetrically measured PM 2:5 mass should be equal to the reconstructed mass from the measured chemical species taken from the FRM. To correct for unmeasured oxygen and hydrogen from the organic carbon, these values were multiplied by 1.4 (Chow et al. 1994, 1996; Solomon et al. 1989). To estimate the oxygen content of the crustal material, silicon concentrations were multiplied by 2.14, calcium by 1.4, and iron by 1.43 (Chow et al. 1996). Aluminum was not measured in this study. The reconstructed mass includes these components; in addition to the measured components, carefully avoiding double counting OC and the aforementioned elemental species. The relationship between measured mass by the FRM and the sum of species is shown in Figure 4. For the most part, the reconstructed mass agrees well with the measured mass, although at these concentrations, there appears to be a positive bias (Solomon et al. 1989; Chow et al. 1996), which will be discussed below. Figure 4. Reconstructed mass from the individual species measurements plotted against the measured mass using the FRM. Shown in Figures 5a d is the predicted ammonium ion concentration in the aerosol for the four sites compared to the measured ammonium ion concentration. The predicted amount assumes that NH C 4 is associated with nitrate, sulfate, and/or bisulfate. For Rubidoux, (Figure 5A) the predicted amount of ammonium is in good agreement with the measured concentration, regardless of whether sulfate or bisulfate is used as the counter ion. This is due to the fact that most of the ammonium is tied up with the nitrate ion as sulfate concentrations are lower in this area. For Phoenix (Figure 5B), the predicted amount is higher than the measured quantity no matter what counter-ion is used. This indicates that the anion is coordinated with other cations such as sodium and potassium rather than ammonium (Solomon and Moyers 1986). At Philadelphia (Figure 5C), the good correlation with sulfate indicates that, at this site and for these conditions, most of the ammonium is in the form of ammonium sulfate and ammonium nitrate (Suh et al. 1995). At RTP, using sulfate and bisulfate over-predicts and under-predicts the amount of ammonium ion concentrations, respectively. This may indicate a mixture of sulfate and bisulfate in this aerosol. Table 1 and Figures 6 8 provide species concentration data and percentages of the total calculated mass (to ensure the charts total 100%) for the average of the sampling study, as well as the maximum and minimum day, as measured by the FRM for each of the four locations studied. Rubidoux, CA, had the highest maximum and average mass observed for the four sites over the sampling moment. The next highest average and maximum masses were measured at Philadelphia, followed by Phoenix and RTP, respectively. The only exception was the minimum day where Rubidoux had the lowest value. As mentioned above, sulfate, nitrate, ammonium, OC, and EC comprise the majority of the mass, accounting for approximately 100% of the measured mass, even before hydrogen and oxygen are included for OC

5 CHEMICAL CHARACTERISTICS OF AEROSOLS 91 Table 1 Summary of concentrations for every site for the average, maximum, and minimum day Average day Maximum day Minimum day Species RUB PHX PHI RTP RUB PHX PHI RTP RUB PHX PHI RTP y PM2: PM Temperature :8 6:1 Sulfate Nitrate Ammonium OC EC S z Si K Ca Fe Cu Zn Pb As RUB D Rubidoux, CA, PHX D Phoenix, AZ, PHI D Philadelphia, PA, and RTP D Research Triangle Park. Concentrations are given in ¹g/m 3. z Concentrations are given in ng/m 3. Temperature data in Celsius are given as the average, maximum, and minimum not necessarily correlated with the concentration data. Figure 5. Ionic balance as calculated using nitrate and either sulfate or bisulfate as the other counter-ion for each of the sites: (a) Rubidoux, CA, (b) Phoenix, AZ, (c) Philadelphia, PA, (d) Research Triangle Park, NC. Solid lines indicate a 1:1 ratio.

6 92 M. P. TOLOCKA ET AL. Figure 6. Pie chart illustrating the composition of the ambient aerosol mass for each of the sites for the average during the study. Labels indicate the reconstructed mass from the individual chemical species (see text) divided by the aerosol mass measured by the FRM. Figure 7. Pie chart illustrating the composition of the ambient aerosol mass for each of the sites for the maximum day. Labels indicate the reconstructed mass from the individual chemical species (see text) divided by the aerosol mass measured by the FRM.

7 CHEMICAL CHARACTERISTICS OF AEROSOLS 93 Figure 8. Pie chart illustrating the composition of the ambient aerosol mass for each of the sites for the minimum day. Labels indicate the reconstructed mass from the individual chemical species (see text) divided by the aerosol mass measured by the FRM. (for total organic mass) and the trace elements (for total crustal mass) for all of the sites. The prediction of higher calculated mass relative to the measured mass may be due to a positive sampling artifact for nitrate and OC, as discussed below. After calculating organic material from OC and crustal material from Fe, Ca, and Si (Solomon et al. 1989), on average, sulfate is much higher at Philadelphia and RTP than at Phoenix and Rubidoux in both percentage of the total mass and absolute concentration. This trend is also observed for the maximum and minimum day. As summarized in Table 1, it also is important to note that sulfur ( 3) measured from the FRM samples by XRF was in good agreement with sulfate measured by ion chromatography (Stevens and Dzubay 1978). In addition, the determination of sulfur by XRF was accomplished using samples collected on Te on lters, while sulfate by IC was resolved using quartz- ber lters. The ratio (S 3/SO D 4 ) ranged from 1.06 at Rubidoux to 1.01 at RTP. This also indicates that Te on or quartz- ber lters are suitable for the measurement of SO D 4 in PM 2:5 samples under the conditions of this experiment (Chow 1995). The Rubidoux site typically had twice as much or more of the total mass consist of nitrate compared to the rest of the sites. A similar pattern is observed for the peak mass concentration days at each site, with the percentages only slightly changing from the average values, except at the Rubidoux site where nitrate comprised almost 50% of the mass. Surprisingly, Philadelphia had the next highest nitrate values, followed closely by Phoenix, while the concentrations at RTP were negligible in both total amount and as a contributor to the total mass. Nitrate was quanti ed on heat-treated quartz lters from the FRM because of potential sample losses during XRF for elemental analysis on the Te on lter (Chow 1995). Nitrate measured on the Te on lter was investigated relative to the particle nitrate concentration measured on a Nylon lter preceded by a sodium carbonate denuder (Appel et al. 1981; John et al. 1988; Solomon et al. 1988). Additionally, the nitrate concentrations measured on heat-treated quartz ber lters were compared to both the particle nitrate and the nitrate collected on the Te on lters. This comparison is shown in Figure 9. Volatilized nitrate, de ned here as the nitrate collected on the Nylon lter that follows the Te on lter in the VAPS as a function of the total nitrate is <15% of the total nitrate at Rubidoux and Philadelphia, 37% at Phoenix, and 51% at RTP. Rubidoux and Phoenix experienced similar temperature pro les; however, much less nitrate vaporized from the Rubidoux lters, likely due to the high concentrations of ammonia that are emitted from upwind sources in that air basin (Solomon et al. 1989). As mentioned above, for Philadelphia the nitrate concentrations were unexpectedly high in both amount and percentage of the mass on the average and maximum days (U.S. EPA 1999). In contrast to RTP, the temperature pro les were generally lower at Philadelphia, shifting the gas/particle phase equilibrium to the condensed phase (Hering and Cass 1999). For

8 94 M. P. TOLOCKA ET AL. Figure 9. Nitrate measured from both Te on and heat-treated quartz ber lters compared to the true particle nitrate quanti ed from the Andersen chemical speciation sampler. warmer temperatures in the east at RTP, the trend is reversed, assuming similar concentration ratios of nitric acid and ammonia. In general, the amount of vaporized nitrate comprised only a small fraction of the measured PM 2:5. It is interesting to note that at every site the nitrate collected on the heat-treated quartz lter was higher than that measured on the Te on lter. This indicates that heat treating a quartz ber lter may activate it, causing it to take up gaseous nitrogeneous species like nitric acid. This was unexpected, as quartz ber lters were found to be suitable for the collection and measurement of aerosol nitrate (Chow 1995). Average OC concentrations were found highest at Phoenix, followed by Rubidoux, Philadelphia, and RTP. A similar trend is observed for the maximum (mass concentration) day. For the minimum (mass concentration) day, Philadelphia has marginally more organic mass than Phoenix. Relative to the total PM 2:5 mass, Phoenix has a much higher fraction of organic constituents (54%) than the other sites, which are all in the expected range of 25 50% of the measured mass (U.S. EPA 1999). Also of note is the dominance of organic material to the total mass observed on the minimum day for all sites (>70% at the western sites and 44 and 55% at Philadelphia and RTP, respectively). This indicates that the PM 2:5 mass concentrations may be near the lower limit de ned by the regional background on dynamic days with high mixing layers. The OC to EC ratios >2:0 are indicative of the presence of secondary organic aerosols (Gray et al. 1996; Turpin et al. 1990; Hildemann et al. 1991). For the average of all the data during the study (see Table 1), both Phoenix and RTP appear to have some impact from secondary organic aerosol processes, while the other sites do not appear to have a signi cant contribution. The same trend is observed for the maximum day. For the minimum day, all appear to have ratios of about 3, characteristic of the presence of some secondary organic aerosols. Figure 10 shows that the OC concentrations, with the exception of Rubidoux, reported by the FRM are consistently higher than the OC values obtained by the VAPS, which had an XAD-4 coated diffusion denuder in front of the quartz- ber lter. Upon further examination, the linear regression results from the FRM data appear to have a concordantly higher intercept of about 2 ¹g/m 3 than the VAPS data and the slopes are nearly parallel. This, and the fact that the EC concentrations were in good agreement for both samplers at all the sites, suggests a positive artifact (McDow and Huntzicker 1990; Hart et al. 1992; Hart and Pankow 1994) for OC, with the exception of Rubidoux, where the chemical composition of the gas phase semi-volatile organic material and aerosol or the distribution processes may be different than the other sites (Gustavson and Dickhut 1997; Jang et al. 1997; Goss and Schwartzenbach 1998; Feilberg et al. 1999). These issues could affect the observed partitioning behavior of those compounds (Yamasaki et al. 1982; Ligocki and Pankow 1989; Foreman and Bidleman 1990; Kamens et al. 1995; Kamens and Coe 1997; Strommen and Kamens 1997, 1999).

9 CHEMICAL CHARACTERISTICS OF AEROSOLS 95 Figure 10. OC concentrations obtained from both the FRM and VAPS compared to the total mass, measured by the FRM. CONCLUSIONS Measurements of the chemical composition of PM 2:5 were made at four sites during the winter of The reconstructed mass was observed to be approximately equal to the measured mass within the experimental error. For all of the sites, there appears to be good ionic balance for the major ions. The measured chemical composition of the aerosol for each site was in good agreement with the composition measured during previous studies, with the exception of relatively high nitrate contribution to the total mass at the Philadelphia, PA, site. For sulfate, good agreement was found between the sulfate measured by ion chromatography and that predicted by XRF analysis of sulfur. Sulfate was an important contributor to the total mass at eastern sites. Nitrate contributed more to the total mass at the western sites and was an important factor in the maximum concentration day at Rubidoux, CA. Te on lters collected less nitrate than heat treated quartz ber lters. For all the sites, organic material averaged 25 50% of the total mass. OC was also found to be the largest part of the aerosol mass on minimum concentration days for all sites. With the exception of Rubidoux, OC collected on denuded lters was less than that found on nondenuded samples, indicating an absorptive artifact on the heat treated quartz ber lters. It was also found that the crustal component to PM 2:5 was highest at Phoenix. During this sampling study PM 2:5 signi cantly contributed to the PM 10 particle mass at all the sites. Finally, this study met a wide variety of chemical composition conditions needed to evaluate the newly developed chemical speciation samplers. DISCLAIMER This work has been funded wholly or in part by the United States Environmental Protection Agency. Portions of the work were performed under contract no. 68-D by Research Triangle Institute. It has been subjected to Agency review and approved for publication. Mention of trade names or commercial products does not constitute endorsement or recommendation for use. REFERENCES Appel, B. R., Tokiwa, Y., and Halik, M. (1981). Sampling Nitrates in Ambient Air, Atmos. Environ. 15: Brauer, M., Koutrakis, P., Wolfson, J. M., and Spengler, J. D. (1989). Evaluation of the Gas Collection of an Annular Denuder System Under Simulated Atmospheric Conditions, Atmos. Environ. 23: Chow, J. C., and Watson, J. G. (1999a). X-Ray Fluorescence Analysis of Ambient Air Samples. In Elemental Analysis of Airborne Particles, edited by S. Landsberger and M. Creatchman. Gordon and Breach Science Publishers, Amsterdam, The Netherlands, pp Chow, J. C., and Watson, J. G. (1999b). Ion Chromatography. In Elemental Analysis of Airborne Particles in Elemental Analysis of Airborne Particles, edited by S. Landsberger and M. Creatchman. Gordon and Breach Science Publishers, Amsterdam, The Netherlands, pp Chow, J. C., Watson, J. G., Lu, Z., Lowenthal, D. H., Frazier, C. A., Solomon, P. A., Thuilier, R. H., and Magliano, K. (1996). Descriptive Analysis

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