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McMurry, P. H.; Rader, D. J. Aerosol Sei. Technol. 1985, 4 , 249-268.
Yin, F. D.; Grosjean, D.; Flagan, R. C.; Seinfeld, J. H. J. Atmos. Chem. 1990, 2 2 , 365-399. Reischl, G. 1988, private communication. Wang, S. C.; Flagan, R. C. Scanning Electrical Mobility Spectrometer. Aerosol Sei. Technol. 1990, 13, 230-240. Badger, E. H. M.; Dryden, I. G. C. Trans. Faraday Soc. 1939, 35, 607. Scholtz, F. Die Herstellung eines Testaerosols durch Photouse von H,S. Ph.D. Dissertation,University of Vienna, 1970. McGraw, R.; Saunders, J. H. Aerosol Sei. Technol. 1984, 3, 367-380. Pratsinis, S. E.; Friedlander,S. K.; Perlstein, A. J. AIChE J . 1986, 32, 177-185.
(20) Stoltzenberg, M. R.; McMurry, P. H. In Aerosols. Liu, B. Y. H., Pui, D. Y. H., Fissan, H., Eds.; Elsevier: New York, 1984; pp 59-62. (21) Romay-Novas, F. J.; Pui, D. Y. H. Aerosol Sei. Technol. 1988, 9, 123-132. (22) Gupta, A.; McMurry, P. H. Aerosol Sei. Technol. 1989, 10, 451-462. (23) Agarwal, J. K.; Remiarz, R. J.; Quant, R. J.;Sem, G. J. J . Aerosol Sei. 1982, 13, 122.
Receiued for reuiew July 23,1990. Accepted December 16,1990. This research was supported through grants from the National Science Foundation, the Coordinating Research Council, and the Austrian Fonds zur Forderung der uissenschaftlichen Forschung.
Multimodal Size Spectra of Submicrometer Particles Bearing Various Elements in Rural Air Jeffrey A. Dodd,+John M. Ondov,” and Gurdal Tuncel’
Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742 Thomas G. Dzubay and Robert K. Stevens
Atmospheric and Exposure Assessment Research Laboratory, U S . Environmental Protection Agency, Research Triangle Park, North Carolina 277 11 Samples of size-segregated submicrometer aerosol particles were collected with a microorifice impactor near Deep Creek Lake, a recreational area in rural western Maryland, and analyzed for up to 44 elements by instrumental neutron activation and X-ray fluorescence analyses. Differential concentration vs particle size spectra revealed as many as four distinct submicrometer aerosol modes with diameters between 0.09 and 1.0 pm in a single sample. The spectra of many elements, including Al, Zn, Na, K, Br, Ca, Ga, Fe, La, Sb, Ce, Ti, and I contained two or more modes. Modal diameters and S/Se ratios for samples influenced by northeasterly winds were smaller than those influenced by westerly winds, suggesting a greater age for the latter, despite the close proximity of several large coal-fired power plants to the west of the site. Large peaks in the spectra of S, As, Se, Sb, and V occurred a t mass median aerodynamic diameters ranging from 0.3 to 0.6 pm in both day and nighttime samples. Simple dispersion estimates suggest that large peaks observed in daytime samples must include material from multiple sources, whereas mass in those observed in nighttime samples could be accounted for by single sources.
Introduction Natural and anthropogenic high-temperature combustion sources (HTCSs) such as forest fires, active volcanoes, coal- and oil-fired power plants (CFPPs and OFPPs), smelters, incinerators, cement kilns, home furnaces and fireplaces, and moter vehicles, i.e., virtually all of the major sources of anthropogenic particulate pollutants, discharge an abundance of atmospheric fine particles, often with major fractions of their particulate mass in the size range below 1 pm (1-13). These particles are especially important as they have long atmospheric residence times, are ‘Present address: S-CUBED, 1800 Diagonal Rd., Alexandria, VA 22314. Present address: Environmental Engineering Department, Middle East Technical University, Ankara 06531, Turkey.
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efficiently deposited in the human respiratory system, and are frequently enriched in toxic species as well as species useful for apportioning sources and tracking air masses (14). Knowledge of their composition, size distribution, and atmospheric behavior is important as these parameters govern respirability, toxicity, deposition, nucleation of water vapor, and light scattering and their utility as tracer species. In HTCSs, both particle composition and size are primarily governed by fuel composition, time-temperature history of the process, and the type and efficiency of control devices (15-17). Most types of HTCSs are controlled and emit aerosol in the accumulation region of the aerosol spectrum, where diameters range from >0.05 to 3 pm (29) and an external after filter was used to collect particles too small to be collected by impaction. Particles with diameters of 0.14 0.54
Based on X-ray fluorescence data.
stage of the impactor is defined as
0.02
0.1
1 .o
Particle diameter, ,urn Figure 3. Calibration efficiency curves for the horizontally configured prototype microorifice impactor, adapted from ref 29.
these estimates is limited because the true distributions may not be log-normal and because the ambiguity in the bin widths imposed by nonideal efficiency curves influences estimates of both the mmad and the rg. Geometric standard deviations estimated in this manner are often overestimated, especially for narrow peaks, because of cross-sensitivity. Nevertheless, the parameters obtained are useful in comparing the size distributions for different time periods or different elements collected in the same impactor. Better estimates of the distribution parameters, made with the technique of Dzubay and Hasan (31),will be presented in a later communication. Their technique uses the individual efficiency curves to compute the mass on each stage resulting from collection of an aerosol described by the sum of up to three LNDFs and thus accounts for cross-sensitivity between stages. To facilitate interpretation of chemical information contained in these data we computed enrichment factors (EF) as well as various other elemental ratios described below. The E F of an element X in particles on the ith 894
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where the numerator is the ratio of the measured concentrations of X and A1 in particles and the denominator is the ratio of these elements in average crustal material as compiled by Wedepohl (33). The value of this normalization is that the E F of an element is near 1 for particles derived from crustal materials (termed lithophiles) or sources in which the elements are near the crustal composition, whereas continental particles with EFs much greater than 1 are most often indicative of hightemperature combustion sources. Enrichment factors for samples 21-24 (chosen for their completeness) are listed in Table IV. Those for several sources are listed in Table
v.
Discussion General Features. Inspection of Table IV shows that the elements may be conveniently classified on the basis of their E F relative to Al. Group 1 elements have small EFs (typically 1ym tend to have near-crustal abundances. Thus, material collected on the first MOI stage might represent a mixture of soil and coal-derived particles. At DCL, Mamane and Dzubay (34) identified coal fly ash as accounting for 11%of particles larger than 2.5 Wm in one dichotomous filter sample. However, EFs for most of the group 1 elements typically differed significantly from unity on later stages, most often on or near the peak stage in the size spectra (see Figures 4 and 5). This was especially evident for La and Ce, for which quite well resolved peaks were clearly observable
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in the size distribution spectra, and for which EFs ranged from 7 to 20 and nearly 4 to 6, respectively. The EFs for Ga, Gd, Hf, Ta, Fe, Mn, K, and Na were similar, ranging from < I to -30, while those for Sm and Sc tended to be more nearly 1 throughout the entire size range. Samarium and Sc might therefore, represent the upper limit for the L
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coal/crustal background of larger particles that tend to contaminate impactor stages designed to collect small particles. The sources of particles bearing these moderate enrichments in group 1 elements are difficult to ascertain. However, significant enrichment combined with fineparticle residence suggests anthropogenic, rather than
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Figure 5 . Concentration (ng/m3.d log d) vs aerodynamic particle diameter spectra for MOI samples influenced by northerly and northeasterly winds. S,Pb, and Br were determined by XRF; all others were determined by INA.
soil-derived sources for these elements (35). The EFs for elements in grloup 2 are typically >>50. Most of these are termed chalcophiles, which tend to concentrate in discrete sulfide phases in the crust and invariably become highly enriched in particles relative to crustal elements in particulate emissions from combustion sources. Figures 4 and 5 reveal a wealth of fine structure in the submicrometer aerosol. The spectra for many of the elements, includiing As, Sb, In, Na, K, Ti, Fe, Mn, Ga, Al, Ce, and La, contain more than one clearly identifiable mode in at least one (of the samples. Overall, at least four discrete aerosol populations are evident in most of the samples. In sample 21, for example, Na, Fe, Ti, Mn, and A1 peak on stage 2 with mmads of approximately 1pm; As, Sb, Se, V, La, and Ce have mmads ranging from ~ 0 . to 5 0.6 pm and peak on stage 3; S, with an mmad of 0.28 pm, peaks on stage 4; and the spectra for Na, Ga, Ti, Al, La, Ce, and Fe show a narrow peak a t ~ 0 . pm 1 (stage 5). In sample 23, Na and K show peaks on the backup filter, stage 6, displaying yet a fifth mode. This suggests that the ambient submicrometer aerosol is far more complex than indicated by the simple trimodal model developed by Whitby et al. (36) to describe ambient sulfate aerosol, in which submicrometer aerosol is bimodal. Although often difficult to fit, peaks with 0.1-pm mmads clearly tended to be quite narrow. For example, most of the mass of Ce and La in samples 22 and 23 is contained on stage 5 and estimates of the a 8 for these peaks averaged 1.27 f 0.05. These surely represent primary particulate emissions from high-temperature combustion sources that have undergone little if any growth during transport. Most
often, the majority of the mass for most elements, was contained in somewhat broader peaks (i.e., spread over more than one MOI stage) with modal diameters of >0.3 pm. For elements such as As, Sb, Se, S, V, and Pb, which are largely associated with fine-particle emissions from fossil-fuel combustion and refuse incineration, mmads ranged from ~ 0 . to 3 ~ 0 . pm 6 and u s ranged from ~ 1 . to 3 ~ 2 . 5 These . elements often peakec! on the same stage of the sample and often had similar ugs, whereas mmads for lithophiles, such as Al, Sc, Na, Fe, and Ti, were usually >0.5 pm. Their ugs were similar to those of the chalcophiles, ranging from ~ 1 . to 3 ~ 2 . although, 6 as illustrated by Sc in sample 22 (Figure 51, the lithophile peaks sometimes lay beyond the range of the MOI and could not always be fit. Influence of Wind Direction. The mmads for As, Sb, Se, S, and V tended to be quite similar within a sample, especially for sample 23, where their mmads ranged only from 0.40 to 0.43 pm. With the exception of S , their mmads ranged from ~ 0 . 4 to 4 0.6 pm when winds blew from westerly, northwesterly, and northerly directions, but were generally significantly smaller, Le., ~ 0 . and 4 ~ 0 . pm 3 for samples influenced by northeasterly air trajectories. As indicated in Figure la, several coal-fired power plants lie in the southwest/northwest quadrant of DCL, including Albright (270 MW), Fort Martin (1152 MW), and Rivesville (108 MW), all located nearly due west of DCL a t distances of 32, 46, and 108 km, respectively. McMurray 5 in the plume et al. (22) observed mmads for S of ~ 0 . 2 pm of the Cumberland CFPP at distances ranging from 16 to 32 km. Given the proximity of CFPPs to DCL, we exEnviron. Sci. Technol., Vol. 25, No. 5, 1991 897
Table IV. Enrichment Factors for Elements Determined in Microorifice Impactor Samples 21-24, Relative to A1 in Crustal Material element
sample/stage
AI Ca
21-24
Ce
23 24 21
22 23 24
Ga
21 22 23 24
Hf
21 22 23 24
La
21 22
23 24
Sm
sc Ta
Fe
22
23 24 22 23 24 21 22 23 24 21
22 23 24 Mn
K
Na
As
In
Mo
21
22 23 24 21 22 23 24 21 22 (23) 24 21 22 23 24 21 22 23 24
21 22
Sb
23 24 21 22 23 24
Sb
21
22 23 24 898
2
1 1.0 f 0.1
1.0 f 0.1 1.7 f 0.2 1.6 f 0.2 1.7 f 0.4
3.1 f 0.3 1.7 f 0.2 3.1 f 1.5 1.5 f 0.4 0.7 f 0.6 1.3 f 0.3
