Multielement Size Characterization of Urban Aerosols John J. Paciga' and Robert E. Jervis* Institute for Environmental Studies and Department of Chemical Engineering, University of Toronto, Toronto, Ont., Canada M5S 1A4 ~~~
~
I The concentrations of 25 elements were determined in-
strumentally by 'neutron and photon activation analysis in aerosol samples collected at urban and industrial sites in Toronto. Size distributions for most of these elements were obtained with a five-stage, high-volume Andersen sampler. Many elements associated with larger aerosols (Al, Ca, Fe, La, Mg, Sm, Sc, Na, and Ti) appeared to be mainly soil derived, whereas some elements associated with the smaller size fractions (Sb, As, Br, C1, Pb, V, and Zn) were abnormally enriched in the atmosphere and appeared to have significant anthropogenic sources. In several cases, competing sources of a single element could be distinguished quantitatively on the basis of size distributions and interelement ratios. Seasonal changes in size patterns for Na and C1 were related to source variations, and marked changes in the size patterns for Pb, As, and Sb were observed near secondary lead refineries.
In those studies in which it has been employed, multielement analysis of airborne particulate matter has greatly benefited attempts to determine source coefficients for urban aerosols, primarily because emissions from several major sources can be characterized by their associated elemental composition ( I , 2 ) . Aerosol size sampling followed by multielement analysis can provide more discriminating information ( 3 ) ,since aerosol size is often related to the mode of production. For example, well-controlled combustion sources often emit very small particles, whereas particles generated by such mechanisms as wind action on soil usually have larger mean diameters. Although detailed information on the composition of source material is desirable, the health significance of emissions can only be evaluated by ambient sampling. For example, trace element fractionation during coal combustion results in higher relative concentrations of several toxic elements on the smallest particles which are the most difficult to control ( 4 ) . The multiplicity of primary sources and complexity of secondary processes in the atmosphere also radically alter the composition of ambient aerosols. The present study was undertaken to assess the usefulness of a high-volume Andersen sampler in conjunction with multielement analysis for determining the sources, behavior, and fate of airborne trace elements in a complex urban environment. In particular, it was hoped to identify those elements which were abnormally enriched in the atmosphere as well as to differentiate sources of these elements and examine seasonal or local variations in composition and size patterns.
Experimental Methods Size-fractionated aerosol samples were collected a t a number of urban and industrial Sites in Toronto by use of a Model 65-000 high-volume (0.57 m"min) Andersen sampler manufactured by Andersen 2000 Inc. ( 5 ) .The sampler consisted of four polyethylene-covered stages followed by a Whatman-41 after-filter. Aerosols were fractionated into size classes 7.0pm equivalent aerodynamic diameter, corresponding to the after-filter and stages 4 to 1,respectively. Standard high-volume samplers (1.5 Present address, New Brunswick Electric Power Commission, Fredericton, N.B., Canada E3B 4x1. 1124
Environmental Science & Technology
m3/min) were also used to collect samples on Whatman-41 filters for the determination of total concentrations. Data in this study indicated that concentrations measured with the Andersen sampler were generally within f 1 0 % of the highvolume concentrations, although A1 and Ti concentrations were approximately 20% lower in Andersen samples because of wall losses from particle bounce-off. Bounce-off also contributes to "cross-talk'' between stages, a phenomenon which is common to impaction devices and can best be evaluated by comparing a variety of samples (6).Although analytical precision and accuracy can be held to f15%,sampling errors associated with "bounce-off" are difficult to assess directly and may be significant. Whatman-41 filters were used because their low blank concentrations make them ideal for multielement microanalysis. The major concern over the use of Whatman-41 has been the possible loss of submicrometer particles because of a lower collection efficiency. At face velocities characteristic of the Andersen sampler (11 m/min), however, the data of Lindeken et al. ( 7 )indicate an efficiency of approximately 80% for 0.26-bm aerosols, with a fairly rapid increase of efficiency with time. Less detailed information is available for trace element retention by Whatman-41. Dams et al. (8)compared the efficiency of Whatman-41 with a polystyrene filter and found that 25 of 30 elements agreed to within one standard deviation with all relative efficiencies greater than 70%. In the present study a high efficiency Delbag filter was placed behind a Whatman-41 filter to assess losses. For P b and V, both typically associated with submicrometer aerosols, Whatman-41 retained 85 and 90%, respectively, of the total filterable quantities of these elements. Measurements reported in this paper were obtained at four different locations in Toronto, Canada. One station was located at an urban site on the University of Toronto campus within 100 m of a major intersection having a traffic volume of about 60 000 vehicledday. Another station was located approximately 100 m from an elevated expressway which carried approximately 120 000 vehicledday. The remaining stations were in mixed residential/light industry areas of Toronto. These areas are subsequently referred to as industrial sites A and B, and in each case samplers were located within 100-120 m of secondary lead refineries. All samples were collected about 1 m above ground level for 24-h periods. Meteorological data, including hourly wind measurements, were available from a meteorological station located 1.5-6 km from the sampling sites. During the summer of 1974, simultaneous high-volume samples were collected a t all four sites for 30 sampling days. A total of 40 Andersen samples were collected at the various sites during 1973-74. The composition of each sample was determined using instrumental nuclear methods of analysis. One quarter of each polyethylene collection surface from the Andersen sampler and one eighth of each 20 X 25 cm Whatman-41 filter were heat-sealed in individual polyethylene envelopes before neutron irradiation with appropriate standards in the University of Toronto's SLOWPOKE research reactor. The neutron activation procedure allowed the instrumental determination of Al, Sb, As, Br, Ca, C1, Cr, Co, Cu, I, Fe, La, Mg, Mn, Hg, K, Sm, Sc, Na, Ti, V, and Zn. The elements Pb, Ni, and Zr were determined by instrumental photon activation analysis in which samples were irradiated in a beam of 35-MeV photons produced in an electron linear accelerator. Lead was
also determined by conventional atomic absorption spectrophotometry for intercomparison purposes. Details of the instrumental neutron and photon activation analysis procedures are described elsewhere (9).
Results Composition and Size Distributions. The elemental composition of high-volume samples collected during the summer of 1974 is presented in Table I. Mechanical problems resulted in 2-5 rejected samples a t each site, although an attempt was made to operate the four samplers every second day for a total of 30 sampling periods. With the possible exceptions of Sb, As, Cr, Cu, Pb, and Zn, atmospheric concentrations did not differ substantially between sites. The increase in Sb, As, and P b a t industrial site A resulted from emissions from a battery recycling plant within 100 m of the sampler. Copper contamination from brush wear on the copper commutator of high-volume sampler motors is significant ( I O ) and was reflected in this study by poor correlations of copper with other elements and with meteorological parameters such as wind direction. From Table I, this contamination is variable, although elevated copper levels a t industrial site B were possibly a reflection of the manufacture of bronze castings by a local industry. Since the measured concentration of an element in the atmosphere provides limited information and can vary considerably with changes in meteorological conditions, Gordon and coworkers ( 1 1 )have proposed the use of an enrichment factor, EF, to indicate the enrichment of an element in the atmosphere with respect to its natural abundance in the earth's crust. In general,
EF
=
( X I C Iatrnos
(1)
( X I C ),rust
where x is any element of interest, and c is an appropriate normalizing element which is predominantly of natural origin. Various normalizing elements including A1 ( I I ) , Fe (12),Sc (13),and Na (14) have been proposed. Silicon would also be
an excellent choice because of its high natural abundance, but it is not readily determined by conventional nuclear techniques. Using A1 as a normalizing element and taking average elemental abundances for the upper continental crust ( 1 5 ) , EF values were calculated for Toronto aerosols (Table 11).T o obtain representative figures for the average composition of Toronto aerosols in Table 11, the data for all high-volume samples and Andersen samples were combined, excluding the values of As, Sb, and P b a t the industrial sites because of the known presence of significant local sources of these elements. On the basis of the Andersen sampler data, mass median diameters (MMD) for each element were estimated by plotting the logarithms of the effective cut-off diameters for each stage vs. the cumulative percent mass I each cut-off diameter on log-probability paper (16).Again, known source anomalies were excluded to ensure representative results. Although there is evidence that aerosol size distributions are multimodal, impaction devices with a limited number of stages do not have sufficient resolution to distinguish these modes ( 17, 18). Consequently, log-normal plots often exhibit slight deviations from linearity (Figure 2 ) , but nevertheless provide a convenient means of ordering the elements according to size (Table 11).For comparison purposes, the percent mass < 1.1km in diameter has also been included. An examination of the data in Table I1 indicates that the elements Al, Ca, Co, La, Mg, Fe, Sm, Sc, Na, and Ti were concentrated on larger particles according to their mass median diameters and had relatively low E F values. These elements were also well correlated with each other and can be assumed to have a major soil component. However, the presence of local sources cannot be discounted, especially in the case of Ca (EF = 6.8) which is a major constituent of cement. As indicated below, other sources such as coal combustion may also contribute to the airborne concentrations of these elements. An enhancement of elemental concentrations associated with smaller particles may indicate long-distance transport,
Table I. Elemental Composition of High-Volume Samples Collected a t Four Sites in Toronto (Summer, 1974)a (ng/m3) Industrial Element
Aluminum
Antimony Arsenic Bromine Calcium Chlorine Chromium Copperb Iron Lanthanum Lead Magnesium Manganese Nickel Potassium Samarium Scandium Sodium Titanium Vanadium
Zinc Zirconium
U r b a n site, N = 28
2100 (930-4100) 8 (0.9-36) 15 (2-50) 280 (90-590) 5400 (2400-9600) 1000 (400-1900) 20 270 (100-700) 1900 2.9 (0.8-6.5) 970 (280-1800) 1300 (500-2500) 75 (40-170) 16 930 (390-1600) 0.38 (0.16-1.0) 0.22 420 (140-880) 130 (80-200) 14 (4-32) 70 16
Expressway site, N =
26
2000 (690-4600) 6 (0.6-24) 10 (2-43) 340 (100-650) 5500 (1200-14000) 1000 (590-1900) 17 88 (50-180) 1900 1.9 (0.3-4) 1000 (360-1800) 1600 (200-4500) 74 (15-180) 19 870 (300-1300) 0.27 (0.05-0.5) 0.20 430 (120-1100) 150 (50-360) 11 (3-21) 200 13
S i t e A, N =
28
1800 (560-3300) 45 (5-170) 77 (13-370) 290 (100-720) 4500 (1200-9200) 1200 (270-3300) 19 340 (60-1800) 1700 2.2 (0.5-4) 2300 (560-4900) 1100 (100-2400) 71 (20-180) 23 770 (300-1200) 0.31 (0.06-0.8) 0.19 380 (70-770) 160 (40-390) 9 (2-18) 250 16
S i t e B, N =
25
2500 (740-4700) 17 (2-70) 18 (3-80) 240 (90-580) 6000 /2000-1300) 900 (200-1700) 53 820 (200-6400) 2400 2.3 (0.6-4) 1300 (370-2500) 1400 (350-2500) 72 (20-170) 27 890 (500-1400) 0.35 (0.08-0.9) 0.28 600 (160-1300) 200 (40-840) 10 (2-19) 110 23
a A range i s q u o t e d i f t h e n u m b e r o f d e t e r m i n a t i o n s at each site is >15. F o r o t h e r e l e m e n t s t h e n u m b e r o f d e t e r m i n a t i o n s at each site i s < 6 . "v represents t h e n u m b e r o f samples c o l l e c t e d at each site. b C o n c e n t r a t i o n s elevated because o f c o n t a m i n a t i o n f r o m sampler m o t o r .
Volume 10, Number 12, November 1976
1125
well-controlled combustion sources, or absorption of volatile components by the cellulose after-filter of the sampler itself. For several elements the enrichments on the smaller size fractions are directly related to pollution sources. Both P b and Br arise primarily from automobile emissions. The lower mass median diameter for Br may be partly attributable to the absorption of gaseous Br by the after-filter, and similar absorption may occur for C1, I, and Hg. Source Differentiation. Size information can be combined with interelement ratios as an effective means of source differentiation. For example, Martens et al. (19) have used V/A1 ratios in ponjunction with aerosol size measurements to distinguish between soil-derived and combustion-source V in the atmosphere. Although the enrichment factor for V in the Toronto atmosphere is only 5.7 (Table 11),a plot of V/A1 ratios vs. impactor stage indicated that the latter ratio was high on submicrometer particles but approached the measured ratio in Toronto soils for the larger size fractions (Figure l a ) . Manganese exhibited a similar enhancement with respect to A1 for the smallest particles (Figure l b ) , whereas soil-derived elements such as Sc did not (Figure IC). These observations suggest the use of A1 as a tracer of soilderived material in the atmosphere. Assuming no change in elemental ratios upon entrainment by wind action, contributions to the atmosphere from entrained material can be estimated from the relation (Allatmos (X/Al)soil
Xatmos
from entrainment
(2)
where X is the element being considered. This calculation will clearly overestimate entrainment contributions if other
sources of A1 are present. For example, coal combustion is a potentially significant source of airborne Al, but relative contributions are difficult to estimate because of the similar trace element composition of soil and coal (2,20). The present study suggested that soil was the major source of airborne A1 in Toronto since A1 concentrations were not correlated with winds from the direction of a major oil/coal-fired generating station near industrial site B. In general, airborne A1 was poorly correlated with wind direction and speed a t all sites, unlike elements such as Pb, Br, and C1 which have significant pollution sources. The latter group of elements exhibited a significant negative correlation with increasing wind speed. Consequently, the above equation could be used to strip out the soil contributions in Figure 1, with the net result that approximately 80% of the V and 40% of the Mn in Toronto aerosols appeared to be derived from pollution sources rather than wind-blown soil. Sodium and Chlorine. In marine environments sea spray is the major source of airborne Na and C1, and the size distributions of these elements are shifted toward larger particles ( 3 ) .A t sites far removed from the ocean, sea spray represents only a minor source, and Na and C1 exhibit more complex interrelationships. In Toronto the size distribution of Na was significantly different from that of C1 during the summer, with Na (MMD = 4 pm) corresponding to a soil-derived element and C1 (MMD = 0.6 pm) corresponding to a combustion source element (Figure 2). Although automobiles are a source of C1, as evidenced by strong correlations with Br and Pb, the C l P b ratio was more than twice the ethyl fluid ratio of 0.34, indicating additional sources of C1 in the urban atmosphere. Concentrations of this element exhibited positive correlations 0.05)
I
i
I
I
2
1
I
I
0
f
0.04
lo1
V/AI
Table II. Classification of Airborne Trace Elements According to Size
Element
Aluminum
Magnesium Calcium Samarium Iron Scandium Titanium Lanthanum Sodium Cobalt Potassium Manganese Copperd Arsenic Chromium Iodine Me rc u ry. Nickel
Zinc Antimony Va n a d i um
Lead Chlorine Bromine
A v concn in Toronto,a ng/m3
Enrichment fact0r.b EF
2100 1400 5300 0.33 2200 0.27 170 2.4 650 1.0 870 74
