Environ. Sci. Techno/. 1995, 29, 321-329
Individual Particle Types in the Aerosol of Phoenix, Arizona KAREN A. KATRINAK,**tb* J A M E S R . ANDERSON,§ A N D PETER R . BUSECKtsS Departments of Geology and Chemistry, Arizona State University, Tempe, Arizona 85287
~~
Aerosol samples were collected in Phoenix, AZ, from July 1989 through April 1990. Elemental compositions were determined for individual particles using an electron microprobe with an energydispersive X-ray spectrometer. Multivariate statistical techniques were used to identify particle types. "Zerocount" particles, consisting entirely of elements lighter than Na, dominate the fine fraction ( 5 2 ,urn). Si- and Ca-rich mineral particles dominate the coarse fraction but are also abundant in the fine fraction. S-bearing particles are common in the fine fraction as well. Lower Pb concentrations in the 1989-1990 samples relative to samples collected in the early 1980s probably reflect decreased usage of leaded gasoline. A difference in the number of S-bearing particles was also noted. S-bearing particles comprised 30% of the fine fraction reported for samples collected in the early 1980s and 17% of the fine fraction in the 1989-1990 samples.
Introduction Phoenix is located in the Salt River Valley of central Arizona in the Sonoran Desert (Figure 1). The metropolitan area had a population of 2.1 million in 1990 ( I ) . The Phoenix aerosol is dominated by soil-derived materials and by carbonaceous particles. Soil-derived particles are most important in the coarse fraction of the aerosol ('2 pm particle diameter), where they comprise approximately half the total mass (2) (Table 1). Soil dust is a lesser contributor to the fine aerosol mass (2). Major sources for soil-derived particles are paved and unpaved roads, agriculture, and construction (2, 3). The fine fraction (52pmparticle diameter) is dominated by carbon, which comprises over half of the total mass; nitrates; and sulfates (Table 1). These materials also occur in coarse particles in the Phoenix aerosol but in lower concentrations (4). Industrial sources produce fine particles, but chemicalmass balancing has shown that motor vehicles are the major emitters, contributing much of the fine aerosol mass in the form of carbonaceousparticles (2). Visibility is inversely proportional to the bulk aerosol concentration of carbon (21,indicating carbonaceous particles as the primary cause of the urban haze that forms almost daily in Phoenix during the winter months when early morning temperature inversions concentrate pollutants in the surface mixinglayer. The winter haze is a matter of widespread concern in Phoenix and other areas because it causes visible degradation of the atmosphere, perturbs the atmospheric radiative balance, and is also associated with health problems (2, 5-7). Individual-Particle Analysis. The use of individualparticle analysis in the study of tropospheric aerosols is well-established (3, 8-13]. Our method of individualparticle analysis requires the use of an electron-beam instrument, typically an electron microprobe or scanning electron microscope (SEMI equipped for energy-dispersive X-ray spectrometry(EDS),to measure sizes and elemental compositions of single particles. Using computer automation, thousands of particles can be characterized in a reasonable time. Multivariatestatisticaltechniques are then applied to the data to aid in interpretation of particle formation processes and in source apportionment. Goal of This Work. The major goal of this work was to identify individual-particle types in the Phoenix aerosol. These results were compared with those of similar studies of the Phoenix aerosol conducted over the past decade.
Methods Sample Collection and Preparation. Aerosol samples were collected on the roof of the two-story Maricopa County Air Pollution Control building at 1845East Roosevelt,Phoenix, during 2-10-h collection periods between July 1989 and April 1990. For each samplingperiod,approximately1m3/h ambient air was pumped through two stacked Nuclepore polycarbonate filters mounted in a 47-mm holder. The upper and lower filters in each holder have 8-pm and 0.2+ Department
of Geology. Energy and Environmental Research Center, Fax (701) Universityof North Dakota, Grand Forks, ND 58202-9018;
* Present address:
777-5181. §
0013-936)(/95/0929-0321$09.00/0
0 1995 American Chemical Society
Department of Chemistry.
VOL. 29, NO. 2, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
321
CHANDLER
pm pores, respectively. The 50% cutoff sue ( D d for the assemblage is between 1and 2pm (14). The two filtersizes were used for convenience of analysis in the scanning electronmicroscoperatherthan for absolutedetermination of particle loadings in a given size range. Samples were stored under vacuum in a desiccator following collection. Vacuum storage was used to avoid hydration of sulfate-bearing particles. This storage technique may also have resulted in sublimation of some panicles containing volatile materials such as ammonium nitrate and organic compounds. Eleven fine-fraction and four coarse-fraction samples were analvzed. A 1-cm sauare was cut from the center of each sample filter. Colloidal graphite was used to mount each filter square on a 1.3-cm pin-type carbon-foil stub. The mounted samples were coated with approximately20 nm ofcarbon to improve conductivityandpreventthermal damage. AU sample preparation was performed on a positive airtlow bench to minimize contamination. 322 m ENVIRONMENTAL SCIENCE &TECHNOLOGY I VOL.
29. NO. 2.1995
Microprobe Analysis and Data Reduction. Samples were analyzed using an automated JEOL Model W-8600 microprobe equippedwithaTracor-Northem(nowNORAN Instruments) energy-dispersive X-ray detector with a Be window. The microprobehasanattachedTracor-Northern TN-5500 Series I1 multichannel analyzer and a TracorNorthern TN-5600 automation package. The instrument can run unattended for 48 h or longer. Approximately 1000particles/samplewasanalyzed using a modified version of the Tracor-Northern particle characterization program VISTA (13). Magnificationsused for fine and coarse sampleswere 2000 x and 8OOx, respectively, resultine in frame sizes of 43 x 43 um for the fine samdes and 108 x 108 pm for the coarse filters. The system was configuredtodetectparticleswithprojecteddiameten 20.1 pmviatheir backscatteredelectronsignal. For each particle, an X-ray spectrum was acquired for 100 s at energies of 0-20 keV, using a beam current of 500 PA and an accelerating voltage of 20 kV,with relative dead times of
-
TABLE 1
Average Chemical Composition of Bulk Aerosol Samples Collected in Downtown Phoenix from September 1989 through January 1990, Adapted from Data Obtained from the Phoenix Urban Haze stw
@I8
element or ion organic C elemental C nitrate, NOSsulfate, SO.+ ammonium, NH4+ Si
S Ca Fe AI CI K Zn Ti
Ba minor elements ( e0.4 eachIb
% of fine fraction
YO of coarse
32.6 30.3 15.0 6.4 4.5 2.6 2.6 1.1 1.1 0.8 0.8 0.8 0.4 e 0.4 e 0.4 0.8
not analyzed not analyzed not analyzed not analyzed not analyzed 49.6 1.7 14.8 8.1 17.0 1.7 4.2 0.4
fraction
0.9 0.4 1.3
The fine fraction includes particleswith aerodynamicdiameter ~ 2 . 5 pm; the coarse fraction includes particles with diameters of 2.5-10pm. bAg, As, Au, Br, Cd, Co, Cr, Cu, Ga, Hg, In, La, Mn, Mo, Ni, P, Pb, Pd, Rb, Sb, Se, Sn, Sr, TI, U, V, Y, and Zr.
less than 30%. Spectra were stored on diskette, as were size and shape data for each particle. The Tracor-Northern program MICROQ was used to reduce each spectrum by comparing it with regions of interest from standard spectra for each of 24 elements: Na, Mg, Al, Si, P, S, C1, K, Ca, Ti, Cr, Mn, Fe, Ni, Cu, Zn, Ga, Ge, As, Se, Br, Cd, Sn, and Pb. Filtered least-squares fittingwas applied to the X-ray spectra to compensate for potential interferences, e.g., among Pb and As, Pb and S, Zn and Na, and Br and Al. The interferences between Br and Al, and Zn and Na, proved to be particularly problematic for our data set and did not appear to be adequately corrected. K ratios, the values produced by MICROQ, represent uncorrected elemental concentrations and suffice to establish compositional similarities among particles. The datareduction procedure is described in greater detail in Anderson et al. (13). Kratios and size and shape data were transferred to a DEC VAX 11/750 microcomputer for further processing. StatisticalAnalysis. The chemical data were processed using the statistical analysis Fortran program EXPLOR, developed at Arizona State University (13-16). Cluster analysis was used to identlfy distinct particle types representative of the data set. “Seed points” representing approximate average cluster compositions were first selected. Each particle was considered a potential seed point and was tested by comparison with every other particle through the calculation of an angle-based similarity measure, s4, defined by Saucy et d. ( 1 4 ) . A value of s4 correspondingto 20”was the maximum separation allowed to classify a particle and a given seed point as similar. A minimum of 1%of all particleswas required to be classified as similar to a particular seed point in order for that seed point to be retained. The seed point set served as a starting point for cluster analysis of each sample using the Forgy k-means algorithm (17, 18).
Each cluster was described by the composition of its seed point, defined as the average K ratios of the particles in the cluster (16‘). After attempts were made to classify every particle, average cluster compositions were recalculated to better represent the assigned particles. Particles were then reassigned to different clusters if doing so would produce a better match. The cycle of clustering, recalculation of compositions, and particle reassignment was repeated until clusters were stable. Average cluster compositions were converted from relative intensities to atomic fractions using TracorNorthern’sZAF correctionprogram. The correctionfactors assume flat samples. This assumption introduced some systematic error but did not affect the identification of particle types. The atomic fractions in the ZAF-corrected compositions of the particle types have relative errors of 10-20%. Concentrations of particle types were determined for each sample date. Filter loading is uneven, and thus calculated concentrationshave errors of approximately 1020% (10,12). The relative numbers of particles assigned to different clusters for each sample were normalized and used to calculate a correlation matrix (16). Particle types comprising 5 1%of the total number of particles in every sample were removed from the matrix prior to correlation analysis. Principal components analysis was used to clarify relationshipsamong particle types. In our samples,particle types are classified into the same principal component if they occur in similar patterns of concentrations over the different sampling dates (12, 15). Particle types were assigned to principal components based on the calculated eigenvectors of the correlation coefficients. Principal components with eigenvectors 1.0 did not account for a significant portion of the variation in the matrix and so were removed. The remaining principal components were subjectedto varimaxrotation,an orthogonaltransformation (19,ZO). To investigate temporal trends, absolute principal component scores were calculated from the rotated principal components and normalized particle type concentrations, followingthe method of Thurston and Spengler (21). FilterBlanks. No air was drawn throughthe blank filters, but otherwise they were handled in the same manner as the samples. The loading of the fine filter blank is 0.01 particle/frame, excluding Cr-rich particles. The Cr-rich particles are considered contaminants, a possible byproduct of the etching process used in the manufacture of polycarbonate filters. The average loading on the fine-fraction sample filters is 14.9 particledframe. Ti-rich particles (particle type 16) were found in the fine filter blank in concentrations equivalentto 8-100% of the concentrations of type 16 particles in the samples. On the average, the concentration of type 16 particles in the fine filter blank is equivalent to 23% of the sample concentrations. Other reports of this problem have not been found in the literature: thus, it is likely that the Ti contamination originates from some aspect of sample handling in the present study. Spray paint is a source for Ti-rich particles (9)and could possibly be a contaminant in the fine-fraction samples. The coarse filter blank contained an average of 0.1 particlelframe, excluding the zero-count particles likely to represent image artifacts formed by the filter hole edges. Cu-richparticles (particletype 24) occur in the coarseblank at 2-7% of the concentrations of particles of that type in the samples (averageof 4%of sample concentrations). The VOL. 29, NO. 2, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 1323
~~~
TABLE 2
Sampling Dates and Conesponding Haze Episodes in Phoenix from Late 1989 thm@ Early 1 9 9 9 sampling date J u l 6 , 1989 Aug 3,1989 Sep 14,1989 Oct 12,1989 Nov 16,1989 Dec 14,1989 Jan 11,1990 Jan 25, 1990 Feb 15, 1990 Mar 22,1990 Apr 19, 1990
sampling times start finish 6:45 AM 6:53 AM 7:02 AM 6:40 AM 7:12 AM 7:25 AM 7:42 AM 7:42 AM 7:20 AM 7:26 AM 7:25 AM
4:53 PM 4:45 PM 4:42 PM 2:48 PM 11:29~~ 9:40 AM 9:56 AM 10:06 AM 1o:Io AM 9:33 AM 9:40 AM
visibility (km)
*
PMU (PslmJ)
* *
* * *
26 13 17 28
15 42 28 15
*
*
* *
* * * *
haze dates (if any)
* * * no haze NOV15-18 Dec 12-16 Jan & I O b
* * * *
upper-level direction (500 mbar)
E
sw WNW
sw
NNW NNW
sw
NNW NNW W
sw
surface temp high/low (“C) 46/33 42/32 38/24 38/23 28112 1713 29/12 2216 1211 33118 28117
a Visibility and PM2.5data are taken from the Phoenix Urban Haze Study (23. These data were only available for the four dates shown. Wind directions and surface temperatures are from daily synoptic charts (41, 423. There was no precipitation on or immediately preceding any of the sampling dates. An asterisk ( * ) indicates haze conditions not monitored by the Phoenix Urban Haze Study. The Jan 11 sample was collected immediately following a haze episode.
Cu-rich particles may thus originate, at least in part, as a contaminant. Aluminosilicate (type 1)and Ca-rich particles (type 10)also occurred in the coarse blank, but at negligible levels (an average of 0.1%of the sample concentrations for each of those two types).
Haze Episodes Meteorology. Atmospheric haze in Phoenix occurs repeatedly during the winter months in episodes lasting 1-5 days. The haze generally develops between 0700 and 0800, Mountain Standard Time. Its intensity is determined by the thicknessof the surfacemixing layer. Pollutantsemitted following the establishment of the mixing layer are retained in the air mass, forming the urban haze (2). 1989-1990 Haze Season. We collected samples from July 1989 through April 1990, bracketing the 1989-1990 haze season,which extended from October 1989to January 1990. The state of Arizona sponsored a separate project, the Phoenix Urban Haze Study, to characterize haze episodes in the metropolitan area. As part of the Phoenix Urban Haze Study,haze conditions including visibility and P M z , ~(the concentration of particles with aerodynamic diameters Na and were placed in type 0. Particles listed as “unclassified” have unique compositions and could not be grouped into types under the clustering conditions used. Mineral Particles. Mineral particles are abundant in the Phoenix aerosol samples. The 16 mineral types were divided into four categories: aluminosilicates(Si-rich),Carich without S, Ca-rich with S, and Ti-rich, together comprising36%of fine particlesand 77% of coarse particles. The elemental compositions of particlesin the first category are suggestive of typical rock-forming aluminosilicate minerals. Si is the major detected element, present as 0.440.65 atomic fraction. Other major elements, ranging up to 0.25 atomic fraction, include Al, Mg, Ca, and Fe. Br, identified in particle types 2 and 5, may originate from spectral interferences with Al. Si-richparticles are abundant in the submicron fraction of our samples and are probably largely soil-derived. Sirich particle types in the Phoenix aerosol have previously been reported mainly in the coarse fraction (3, 4, 10). Sirich particles have also been reported as important constituents of the fine fractions of aerosols collected in other desert locations, including southern Utah (22),the northern Sahara (231, and Cairo, Egypt (24). In Phoenix, airborne soil particles derived primarily from igneous and metamorphic rocks are transported into the metropolitan area by strong winds from southern and eastern Arizona and northern Mexico (25). There are also many local sources for soil particles within the metropolitan area. Ca-rich particle types in the Phoenix aerosol samples are characterized by the presence of Si and Mg; some Carich types also contain Al, s, P, or Fe. Three of the Ca-rich types (nos. 10, 12, and 13), contain Si, Al, Mg, and Fe, all common crustal elements. These particles may be produced by cement manufacture or by traffic on concrete roads (3). Ca-rich particle type 11 contains 0.30 atomic fraction P and may represent apatite, a common mineral.
1000
El
Fine Coarse
100 July 6 Aug.3 Scpf.14 OCt.12 Nov.16*Dec.14* Jan.11
Jan.25 Fcb.15 Mar.22 Apr.19
Sample collection date (* indicates haze) FIGURE 2. Total panicle concentredionsfor each sampling date. Note the logarithmic scale. Cnane panicles were only measured for the four indicated dates.
Alternatively,particle type 11 maybe Ca5(P04)30H usedas fertilizer. The presence of S in the Ca-rich particle types 14 and 15 is suggestive of gypsum (CaS042H20)or anhydrite (CaSOa). Gypsum and anhydrite particles can originate from the weathering of natural materials. In addition, gypsum is widely used as a soil additive and so may occur with crustal particles. CaSOl can also form as a secondary aerosol when CaC03 panicles are convened through reaction with SO2, either in utility stacks as part of an emissions control process or in the troposphere (26). We suggest a crustal origin for type 15, because it has 0.27 atomic fractionSi, acommon crustalelement, and because it is most abundant in the coarse fraction. Type 14 occurs mainlyinthefinefractionandcontainsnoadditionalmajor elements; origin as a secondary aerosol is thus likely. Particles in type 16 are rich in Ti. It is likely that at least Some type 16 particles are contaminants, as discussed above,possiblyoriginatingfrom spray paint (9). However, those type 16 panicles that are authentic constituents of the sample may originate from crustal sourcesasthemineral rutile (TiOz). Metal-RichParticles. Metal-rich particlesinthe Phoenix aerosol were divided into six S-bearing and five non-Sbearing types. Ofthe five non-S-bearing types, three (nos. 17, 18,and 20) have similar compositions, containing Fe, Si, Mg, and fd (Table 3). Particles in these types may originate as metal oxides from foundry emissions or other industrial sources. Iron oxide panicles are common in foundry emissions (3,27, 28). The scrap metals used by secondary foundriesvary considerablyin composition, and thus particles in Al-rich type 21 and Mg-rich type 22 may have asimilarsowce. Giventhe fluctuationsin the quantity and composition of materials handled by secondary foundries,it is not possibleto accuratelypredict their likely impact on the Phoenix aerosol at any particular time.
Foundry sources are also suggested for types 18 and 25 based on their elemental compositions. Na was detected in these two types, but its peaks in the X-ray spectra are likely to be at least partidy caused by interference from Zn. The presence of Zn in both particle types is suggestive of foundry emissions (29). Panicles in type 18 occur only in the finefraction,corroboratingahigh-temperaturesource (30).
Smelter sources are suggested for panicle types 23 and 24. Type 23 contains As and Pb (0.37 and 0.29 atomic fractions, respectively), both of which occur as minor constituents in sulfide ores commonly used in nonferrous smelting (31). Particle type 24 contains Cu (0.87 atomic fraction)andminor S (0.06 atomic fraction). Particle types 26 and 27 are also S-bearing,containing predominantly Na and Mn, respectively. Sulfur-Rich Particles. Particles in types 28-34 contain S (0.16-0.97atomic fraction) plus Mg, Ca, Na, or K (Table 3). These types may represent mixtures of two or more sulfates or sulfates and non-sulfates (3). S-rich particles are likely to originate from smelters or other industrial sources, possiblyas secondaryaerosol. Particlesintype 31 are rich in Mg and Ca and may represent S-coated mineral particles. Particle type 33 (containingNa, S,and Mg) may represent salt particles from which CI was displaced by reaction with sulfuric acid. SaltParticles. Particletype35 istheonlyone to contain bothNaandCl(O.47andO.37atomicfractions.respectively). Particles in this type have compositions similarto aged sea salt (9). Based on their compositions, particles in type 35 may also represent salts from playas in southeastern Arizona, as identified in aerosol samples collected in rural eastern Arizona (32). As mentioned above, type 33 might represent altered salt particles. Zero-CountParticles. The zero-count group, type 0, comprises 3544 fine particleswith no detectable elements. This number is equal to 32% of the total fine-fraction VOL. 29. NO. 2.1995 I ENVIRONMENTAL SCIENCE &TECHNOLOGY. 325
TABLE 3
Particle Types for Phoenix Aerosol Samples, Determined Using Combined Cluster Analysis of Fine and Coarse Samplesa no. of particles (%)* av diameter bm) tYP0
fine
particle-type compositions(atomic fractions of elements with Z =- Ne)
coarse
~
fine
coarse
1.1 1.2 1.1 1.O 0.9 1.o 0.9 1.1 1.o
3.7 3.5 3.3 3.8 2.5 3.6 3.5 3.4 3.3
1.1 1.2 0.9 0.8
4.0 3.0 2.9 3.1
1.o 1.2
3.4 3.9
0.7
5.2
0.9 0.8 0.5 1.1 1.3 0.9
3.1 3.0
0.8 1.o 0.7 0.9 0.5
1.o 3.2 2.7 0.6
0.6 0.7 0.9 1.1 1.1 1.1 0.7
2.1 4.5 3.0
1.1
4.3
Section I: Mineral Particle Types I* 2* 3 4* 5 6* 7* 8* 9*
14.0 3.2 0.5 2.6 0.3 2.3 0.6 1.7 1.2
29.7 2.1 0.6 12.4 0.5 4.6 1.5 6.6 1.6
Si(0.65) Si(0.65) Si(0.57) Si(0.55) Si(0.54) Si(0.50) Si(0.48) Si(0.46) Si(0.44)
Al(0.18) AI(0.10) Mg(0.24) Al(0.18) Ca(0.20) AI(O.18) Al(0.12) Ca(0.25) Fe(0.25)
Aluminosilicates(No Sulfur) K(0.06) Na(0.04) Fe(0.03) Br(0.07) Fe(0.05) K(0.05) AI(0.11I Fe(0.03) Ca(O.11) MgI0.06) Fe(0.05) Br(0.07) Mg(0.07) AI(0.05) Mg(0.13) Fe(0.12) Ca(0.03) Ca(0.12) Fe(0.12) Mg(O.11) AI(0.14) Mg(0.07) Fe(0.04) AI(0.13) Mg(0.08) Na(0.06)
IO* 11 12 13
6.1 0.3 0.5 0.2
13.7 0.4 0.7 0.8
Ca(0.59) Ca(0.51) Ca(0.43) Ca(0.41)
Si(0.25) P(0.30) Si(0.23) Mg(0.36)
Calcium-Rich (No Sulfur) AU0.06) Mg(0.05) Si(O.11) Al(0.03) Mg(0.03) Fe(0.15) AI(0.11) Mg(0.05) Si(0.17) Al(0.04)
14* 15
1.3 0.4
0.8 0.5
Ca(0.43) Ca(0.33)
S(0.36) Si(0.27)
Calcium-Rich with Sulfur Mg(0.08) Si(0.08) Na(0.03) S(0.26) Al(0.06) K(0.03)
16
0.3
0.4
Ti(0.70)
Si(O.15)
Titanium-Rich (No Sulfur) Al(0.05) Fe(0.05) Mg(0.03)
Mg(0.03) Mg(0.05) K(0.03) Fe(0.04) K(0.03) Ca(.03)
Mg(0.03)
Ca(0.03) Na(0.03) K(0.03)
Section II: Metal-Rich Particle Types 17* 18* 19 20 21 22*
7.6 0.6 0.1 0.5 0.3 0.4
23 24* 25" 26" 27
0.4 0.6 0.9 0.5 0.2
4.5 0.1
0.0 0.2 0.1
0.0 0.1 0.4 0.4 0.1
0.0
Non-Sulfur-Bearing Mg(0.09) Al(0.05) Si(0.18) Mg(0.05) Fe(0.17) Zn(0.17) Fe(0.17) AI(0.03)
Fe(0.57) Fe(0.44) Na(0.43) Mg(0.40) Al(0.94) Mg(0.86)
Si(0.24) Ca(0.23) Si(0.22) Si(0.36) Na(0.03) Al(0.12)
Ca(0.03)
As(0.37) Cu(0.87) Na(0.65) Na(0.74) Mn(0.76)
Pb(0.29) S(0.06) Zn(0.23) Mg(0.14) Ca(0.07)
Sulfur-Bearing S(0.15) Cl(0.13) Si(0.03) Mg(0.04) S(0.04) S(0.08) Si(0.03) P(0.05) Fe(0.04)
Ca(0.03) Al(0.04)
P(0.03)
S(0.03)
2.4 1.5
Section 111: Sulfur-Rich Particle Types 28* 29 30 31 32" 33* 34
5.3 0.1 0.2 0.3 0.3 0.9 0.3
0.5 0.3 0.1 0.0
0.0 0.1 0.0
S(0.97) S(0.57) S(0.54) S(0.34) S(0.32) Na(0.56) K(0.84)
K(0.03) Si(0.37) K(0.44) Mg(0.24) Mg(0.27) S(0.30) S(0.16)
K(0.04) Ca(0.22) Na(0.26) Mg(0.12)
Na(0.15) Ca(0.14)
Si(0.04)
2.5
Section IV Salt Particle Type 35
0.2
0.3
Na(0.47)
Cl(0.37)
Ca(0.05)
Mg(0.05)
S(0.03)
Si(0.03)
Section V Zero-Count Particles 31.6
0
no detectable elements
0.9
An asterisk (*) indicates this particle type was used in the principal components analysis. Total number of particles analyzed = 14 720. 11 209 particles in 11fine-fraction samples;3511 particles in fourcoarse-fraction samples. 1489fine particles (13.3% of total) and 562coarse particles (16.0% of total) were not assigned to particle types because of their unique compositions. Number of particles expressed as percentage of total particles in each size fraction. a
particles. On the basis of the predominance of carbonaceous particles reported in studies of bulk samples from Phoenix and other urban aerosols (2,33,341, it is possible that many of these zero-count particles are rich in carbon. Nitrates are also likelyto be present, based on bulk analysis results reported for the fine fraction of the Phoenix aerosol (33).
The methods used here do not lend themselves well to analysisof the zero-count particles. Many of the low atomic number particles in the aerosol may not be represented in 326 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 2, 1995
the analysis,due to the difficultyof detectingthese particles in the electron microscope. In addition, as previously discussed, some volatile low atomic number particles may be lost from the samples during storage under vacuum. Although substantial uncertainty is involved in interpreting these results, it is reasonable to suggest that, based on their compositionsand small mean size, the zero-count particles are probably anthropogenic (34). Motor vehicles are the major source of carbonaceous aerosol in Phoenix (2). Motor vehicle sources emit many
TABLE 4
Eigenvectors Determined for Phoenix Aerosol Samplesa palticle tVPe
1 0.940 0.720 0.854 0.854 0.894 0.872
1
2 4 6 7
a
9 10 14 17 18 22 24 25 26 28 32 33 variance
0.344 0.767 0.505 0.495
0.311 -0.047 0.528
0.319 -0.138 -0.035 0.327 -0.162 53.0%
principal component number 2 3
4
-0.080 -0.029 -0.184 -0.080 -0.034 -0.220 -0.007 -0.139 0.211 -0.046 -0.041 -0.031 0.358 0.041
0.I a3 0.175 0.266 0.359 0.203 0.332
0.966 0.979
0.039 0.044 0.088 -0.085 11.1%
-0.069 0.964
18.3%
0.077 0.485 0.1 1 1 0.173 0.110 0.044 0.073 0.125 0.202 0.040 0.139 0.010 -0.365 -0.146 -0.091 -0.098
0.819 0.576 0.785 0.820 0.844 0.915 0.426 0.830
0.844
0.098 5.5%
detectable elements (> 0.03 atomic fraction)
+ + + + + K + Mg + + + + Fe + + + Mg + + + + Fe + + + Na + + + + + + + + + + + + AI + + + + +
Si + A I K + Na Si AI Br Fe Si AI Ca Mg Si + A I Mg Fe Fe+ Si +AI + C a Si Ca AI Mg Si Fe + A I Mg Ca Si AI Mg Ca S Mg + S i Fe Si Mg AI Fe Ca Si Mg Mg AI cu s Na Zn Mg S Na Mg + S S S Mg Na Ca Na S Mg
+
+ + + + +
Significant eigenvectors are in boldface; these were used to define each principal component. The four principal components together account for 87.9% of the variance in the matrix.
particles with diameters 10.1pm, and as this size range is excluded from the analyses in this study, it is possible that motor vehicle particles are underrepresented in the data set. Particulate emissions from jet aircraft are also rich in carbon (35,36). Although jet exhaust particles have not explicitly been identified as a major component of the Phoenix aerosol, our sampling site was located less than 2 mi from a large commercial airport, and we consider jet exhaust a likely contributor to the aerosol mass.
Principal Components Principal components analysis was used to identify temporal patterns in the Phoenix aerosol samples. Although a substantially greater number of samples are required for a strictly acceptable principal components analysis, treatment of this data set appears to yield some useful results. Considering the errors inherent in principal components analysis of such a small data set, however, the results presented here should be interpreted cautiously. Eighteen particle types, indicated by asterisks in Table 3, were used for principal components analysis. Seventeen of the remaining 18 particle types were excluded from principal components analysis because they were only minor contributors to the total aerosol volume on each sampling date. The zero-countparticles were also excluded because their concentrationscould not be measured reliably using our methods. We obtained four significant principal components, accounting for 88% of the temporal variations in concentrations of the 18 particle types used. Only coefficients with absolute values ’0.400 were used. From 2 to 10 particle types were assigned to each principal component (PC). PC1 consists of 10particle types with a wide range of compositions: nos. 1, 2, 4, 6, 7, and 8, all Si-rich mineral types; nos. 10 and 14, both Ca-rich and nos. 17 and 24, both metal-rich. Types 14 and 24 are the only S-bearing particles in PCI. PC2 contains three particle types, nos. 26, 28, and 33, all S-bearing. Particles in type 28 contain no detectable
elements other than S, whereas particles in types 26 and 33 both also contain Na and Mg. PC3 contains eight particle types: no. 9, an Si-rich type; nos. 10 and 14, both Ca-rich and nos. 17,18,22,24, and 25, all metal-rich. Of these, types 14, 24, and 25 are S-bearing. Four of the particle types in PC3, nos. 10, 14, 17, and 24, are shared with PC1 (Table 4). PC4 includes only two particle types: no. 2, a Si-rich mineral type; and no. 32, a S-rich type. Type 2 is shared with PC1. Particle types contained within a single PC are likely to be produced from the same source. The abundance of Si-rich types in PCl suggests a crustal source for these particles. The presence of probable nonmineral (metalrich) types in this PC is not incongruousfor a crustal source in a sprawling desert city such as Phoenix. The ubiquitous unpaved roads, construction sites, and tracts of farmland interspersed among the urban developmentwould tend to produce particles of widely varied composition, all with sources that could justifiably be termed “crustal”. The composition of particles in PC2 is much more narrowly defined than in PC1. In PC2, all three types are S-bearing, and two of the types (nos. 26 and 33) contain the same three detectable elements (Na, Mg, and S) in varied proportions. This composition is suggestive of sulfate particles, likely to be emitted from a power plant (37) or other industrial source such as a foundry (27). The varied compositions of particles in PC3 suggest an anthropogenicorigin, such as a foundryor other industrial source. The sharing of particle types with PC1, which is interpreted as representing a crustal source, is not surprising considering that many small industrial and other local sources of metal-rich particles, such as those that characterize PC3, are in close proximity to sources of crustal particles. The two particle types in PC4 are distinctly different: an aluminosilicate (type 2) and a S-rich, metal-bearing type (no. 32). The composition of particles in type 32 (S+ Mg Na Ca) suggests they may have originated through the
+ +
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TABLE 5
Absolute Principal Component Scores for Phoenix Aerosol Samplesa PC1 date Jul 6 Aug 3 Sep 14 Oct 12 Nov 16b Dec 14b Jan 11 Jan 25 Feb 15 Mar 22 Apr 19 a
fine 2.16 1.73 4.03 4.25 12.55 6.14 4.89 6.17 2.82 2.53 4.90
PC2 coarse NA NA NA 1.09 1.86 2.31 1.16 NA NA NA NA
fine 0.33 6.87 0.03 0.86 -0.82 -0.38 0.75 0.28 0.73 0.75 -0.04
PC3
PC4
coarse
fine
coarse
fine
coarse
NA NA NA -0.12 -0.32 -0.41 -0.16 NA NA NA NA
1.oo 2.35 3.75 4.29 9.47 4.23 4.37 9.17 2.97 2.25 4.33
NA NA NA 0.72 0.96 1.23 0.64 NA NA NA NA
3.72 0.05 1.92 0.51 5.67 2.65 1.37 2.13 2.06 0.13 2.78
NA NA NA 0.30 0.56 0.70 0.32 NA NA NA NA
Negative values represent noise in the statistical analysis; NA = not applicable. Dates associated with episodes of urban haze.
interaction of SO, emissions with metal oxide particles, whereas the aluminosilicate particles in type 32 probably have a crustal origin.
Discussion Haze-RelatedEffects. Absolute scores for PC1, representing crustal particles, are highest for samples collected on the two dates (November 16and December 14,1989)associated with episodes of urban haze (Table 5). PC2, which has a possible power plant or other industrial source, is absent from the coarse-fraction samples on the two haze dates. Absolute scores for PC3, with asuggested industrial source, are highest for the coarse-fractionsamples on the two haze dates. Absolute scoresfor PC4, representing a combination of crustal and secondary aerosol particles, do not correspond with the occurrence of haze episodes. Possible mechanisms for increased particle concentrations during haze episodes include local emission into a stagnant air mass; advection of polluted air masses over long distances into the local area; and acceleration of atmospheric reactions leading to sulfate or nitrate particle production. Given the limited data available,it is difficult to select likely mechanisms to explain the temporal trends in the present study. One exception is particle type 32, which was assigned to PC4. Its loading is greatest on July 6, 1989, the only day the wind was from the east (Tables 2 and 3). These particles may thus have originated from the copper smelters located approximately 150 km east of Phoenix. In the December 14, 1989, sample, the number concentration of particles in the fine fraction is lower than that found on several days without haze, as shown in Figure 2. However, the December 14th sample also had the highest concentration of coarse particles, thus yielding a high total particle concentration on that date. The temporal concentration trends of S-bearingparticles in our samples do not correlate well with haze dates, indicating S is not an important aerosol component during these periods. This trend is similar to results reported in previous studies of the Phoenix and Denver aerosols. Haze episodes in these two cities occur in the winter and are associated with carbon- and nitrate-rich aerosols, in contrast to the carbon- and sulfate-rich aerosols found in areas with summer hazes (33). In samples collectedduring haze episodes in Phoenix in January 1983,an average nitrate concentration up to twice that of the average sulfate concentration was reported (33). By comparison, during 328 1 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 2. 1995
summer haze episodes in other locations, the average [NOs-]/[S042-l ratio is much less than 1 (33, 38, 39). Comparisonwith Earlier PhoenixAerosol Studies. The Phoenix aerosolhas been the subject of anumber of studies. Analysis of bulk samples has shown that the fine fraction is dominated by carbonaceous particles (2, 33). Previous individual-particleanalysis studies noted a predominance of S- and metal-bearing particles in the fine fraction (3,10, 12, 27). The coarse fraction of the Phoenix aerosol is dominated by mineral particles, mostly Si-rich, as shown by both bulk and individual-particleanalysis methods (24 ) . Metal-bearing particles have also been reported in the coarse fraction (12). Our results suggest a number of differencesfrom earlier studies. In samples collectedin Phoenix in February 1980, S-bearing particles comprised 60-80% of the submicron particles. Additionally, many of the clay mineral particles contained minor S (3). In samples collected in February and March 1982 in nearby Chandler, AZ, 37.4% of all assigned particles in the 0.4-2 pm diameter size range containeds (10). Itisalessimportant element'inour 19891990 Phoenix samples, found in 12%of fine particles and 3.6% of coarse particles (Table 3). Differences in the abundance of S-bearing particles in the 1980 and 1989-1990 samples may be explained in part bywind directionsduring samplingperiods. Wind direction varied for the two different studies and undoubtedly affected the relative amounts of S-bearing particles collected. During our sampling periods, the upper-level winds were never from the southeast, and only once from the east, the optimal directions for sampling copper smelter emissions, which are characteristicallyrich in S-bearing particles. In comparison, wind directions and sampling locations were ideal for detecting smelter emissions when the Chandler aerosol was sampled in 1982 (10). A second possible shift in the composition of the Phoenix aerosol is shown by the metal-bearing particle types. In samples collected in February 1980,lead halide compounds a-2PbBrC1.NH4C1, PbBrC1, (PbO)2-PbBrC1,and 3Pb3(P0&. PbBrClwere inferred through the use of individual-particle analysis (40). Up to 90%of Pb-bearing particles (0.2-2,um in diameter) in the 1980 samples appeared to contain lead halides, evidently emitted by automobiles using leaded fuel (40). In samples collectedin 1982in Chandler, Pb-bearing particles comprise 10.4%of all fine-fractionparticles. Only 1.5%of fine particles in the 1982 samples contains one or more of the elements Pb, Br, and C1, which are suggestive
of amotor vehicle source. The remainder of the Pb-bearing particles were attributed to industrial sources, including smelters (10). In the present work, most of the metalbearing particles were classified into Si- and Fe-bearing types (nos. 17-20). We identified only one Pb-bearing particle type (no. 231, containingAs, Pb, S, C1, and P. This composition suggests an industrial source. The absence of lead halide particle types in our 1989-1990 Phoenix samples suggests a decline in the influence of leaded fuels on the composition of the atmospheric aerosol.
Conclusions Samples of the Phoenix aerosol collected in 1989 and 1990 consist mainly of mineral-rich and low atomic numberrich particles in the fine fraction and mineral-rich particles in the coarse fraction. The mineral-rich particles are likely to have a crustal source. The low atomic number-rich particles are likely to be carbonaceous and probably originated from anthropogenic sources such as motor vehicle emissions. Comparisons with earlier individual-particle analysis results indicate variations in the composition of the Phoenix aerosol. Pb-bearing particles were the predominant metalbearing particles in samples collected 7-10 years ago, whereas Fe-bearing particles are more common in the samplesdiscussedhere. The decrease in Pb emissions was probably caused by reductions in the use of leaded gasoline. Sulfur-bearing particles constituted approximately 30% of the fine fraction in the early 1980%but only 12%of the fine fraction in samples collected in 1989 and 1990.
Acknowledgments We thank R. Haddow at the Maricopa County Air Pollution Control office for graciously cooperating in sample collection. We acknowledge P. A. Kosecki, T. Patterson, P. Rez, and T. Shattuckfor helpful discussions. The electron microprobe data were obtained with the assistance of J. Clark, on equipment purchased with the aid of National Science Foundation Grant EAR-8408163. This work was supported in part by NSF Grants ATM-9007796 and ATM8 707070.
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Received for review March 15, 1994. Revised manuscript received October 21, 1994. Accepted November 4, 1994."
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Abstract published in Advance ACSAbstracts, December 1,1994.
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