Environ. Sci. Technol. 2010, 44, 2299–2304
Hosted and Free-Floating Metal-Bearing Atmospheric Nanoparticles in Mexico City KOUJI ADACHI AND PETER R. BUSECK* School of Earth and Space Exploration & Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona, 85287
Received August 15, 2009. Revised manuscript received December 3, 2009. Accepted January 28, 2010.
Nanoparticles (NPs) are ubiquitous in the atmosphere. Because of their small sizes, they can travel deeply into the lungs and other parts of the body. Many are highly reactive which, combined with their large surface areas, means they can seriously affect human health. Their occurrences in the atmosphere and their biological effects are not well-understood. We focus on NPs that were either free-floating or hosted within large aerosol particles (aerodynamic diameter 50-300 nm) and consist of or contain transition or post-transition metals (m-NPs). The samples were collected from ambient air above Mexico City (MC). We used transmission electron microscopy to measure their sizes and compositions. More than half of the 572 m-NPs that we analyzed contain two or more metals, and Fe, Pb, or Zn occurs in more than 60%. Hg occurs in 21% and is especially abundant in free-floating m-NPs. We find that m-NPs are common in polluted air such as in the MC area and, by inference, presumably other megacities. The range and variety of compositions of m-NPs that we encountered, whether free-floating or hosted within larger aerosol particles, indicate the complicated occurrences that should be considered when evaluating the health effects of m-NPs in complex urban areas.
Introduction Atmospheric nanoparticles (NPs) are ubiquitous and have important effects on human health. We define NPs as particles smaller than 50 nm in at least one dimension (ref 1 and Supporting Information). In this study we focus on the ambient NPs that consist of or contain transition metal or post-transition elements (hereafter m-NPs) and that are hosted within larger aerosol particles (aerodynamic diameter 50-300 nm). m-NPs are of interest because they are widespread and highly reactive within the human body (2). The major sources of atmospheric m-NPs are anthropogenic, although volcanoes and meteor smoke (3) can also be important. Many anthropogenic m-NPs form through condensation of hot vapors from incinerators, industries, and transportation. The toxicity of NPs is of increasing interest because of the rapid development and growth of nanotechnology (4). m-NPs can cause adverse effects in humans because of their small sizes, high surface-to-volume ratios, and dominance in number even though their total mass is relatively small (2, 4-6). Because NPs are so tiny, they travel throughout the * Corresponding author phone: 480-965-3945; fax: 480-965-8102; e-mail:
[email protected]. 10.1021/es902505b
2010 American Chemical Society
Published on Web 03/01/2010
body, deposit in organs, penetrate cell membranes (4), and appear in unexpected places in the body (5). Once they deposit on or within biological tissues, they can cause negative health effects such as oxidation stress, pulmonary inflammation, and cardiovascular events like heart attacks and cardiac-rhythm disturbances (2, 4, 7). The biological activity and biokinetics depend on their specific compositions, sizes, solubilities, shapes, and surface and crystal structures (2, 4, 8). Aerosol particles that contain metals are commonly detected using aerosol particle mass spectrometers (9) or chemical analyses of filtered samples (10, 11). Lee et al. (9) found that various metals in single aerosol particles 0.35-2.5 µm in aerodynamic diameter also contain organic matter, sulfate, nitrate, soot, or mineral dust, suggesting that relatively large aerosol particles could host small metal-bearing particles or soluble metals. However, from those studies, the shapes, sizes, occurrences, and compositions of hosted m-NPs, all of which are important for understanding health effects, were unknown. Thus, the current study focuses on those properties of hosted m-NPs as well as those of freefloating ones. Transmission electron microscopy (TEM) is a powerful way to analyze internally mixed small particles (12-18). Fine and ultrafine metal particles in ambient air from Detroit, MI, were studied by Utsunomiya et al. (15), who detected and mapped the distributions of Cr, Fe, Zn, Y, Zr, Ag, Sn, La, Au, and Pb in various particles. In the current study, we investigated m-NPs from aerosol samples using TEM together with energy-dispersive X-ray spectrometry (EDS). Given their widespread occurrence and potential toxicity, our goal is to show the occurrences, associations, and compositions of a significant number of m-NPs that were either hosted or freefloating in the atmosphere from a major urban environment.
Experimental Section Mexico City Atmospheric Research Projects. Aerosol particles were collected during the MILAGRO (Megacity Initiative: Local and Global Research Observations) campaign conducted in March 2006. The goal was to determine the evolution of trace gases and particles from anthropogenic emissions in and around Mexico City (MC) as an example of a tropical megacity (19). Adachi and Buseck (13) reported on the aerosol mixing states, sizes, volumes, and compositions of the aerosol particles used in the current study and found that most are complex mixtures of soot, organic matter, sulfate, minerals, and metal-bearing particles. Metal concentrations in aerosol particles collected on filters in MC were measured as bulk samples during MILAGRO and earlier campaigns (20-22). For example, Querol et al. (20) determined the compositions of PM2.5 aerosol samples collected from MC ground site T0 for Fe, Zn, Pb, Ti, Sn, Mn, V, Ni, Cr, and Hg, with respective mean concentrations of 400, 224, 62, 36, 29, 16, 13, 3, 2.1, and 0.13 ng/m3. Our sampling parameters are the same as those described by Adachi and Buseck (13). Samples were collected using impactor samplers (MPS-3, California Measurements, Inc.) that have a 50% cutoff aerodynamic diameter of 50 nm (see the “m-NP Details” section of the Supporting Information). Units were placed in both the NCAR/NSF C130 and the U.S. Forest Service Twin Otter (23, 24) aircraft. In this study, we used aerosol samples collected from ambient air above MC (Table S1 of the Supporting Information). TEM Analysis. Aerosol samples were collected on lacycarbon TEM grids. Such grids, which have carbon substrates that resemble the fibers of spider webs, are designed to be used for chemical analyses and high-resolution imaging. The subVOL. 44, NO. 7, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Examples of m-NPs within various host particles. These images are from Sample #2 in Table S1 of the Supporting Information. The host particles are on lacy-carbon substrates. The bottom-right schematic image indicates the positions of the respective host particles. The numbers in the bottom-right TEM image refer to the respective enlargements. Bold numbers in parentheses in the various TEM image refer to the EDS spectra at the bottom of the figure. Areas in the spectra surrounded by dotted lines on the left side are enlarged in the images shown on the right sides of the spectral images. The Cu* peaks in the spectra are from the TEM grids. strate causes minimal overlap with samples and yields clear images where the particles extend over holes in the substrate. 2300
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We used a CM 200 (FEI Corp.) transmission electron microscope operated at an accelerating voltage of 200 kV for
TABLE 1. Numbers of Host Particles, Percent of Host Particles that Contain m-NPs, and Numbers of Measured m-NPs
sample #
number of measured host particles
percent of host particles that contain m-NPs
number of measured m-NP
1 2 3 4 5 6 7 8 9 10
30 67 76 63 30 30 33 33 57 44
47 76 38 60 13 10 42 45 7 82
31 139 61 165 6 4 9 23 8 126
EDS analyses. Average compositions of m-NPs were measured using an electron beam ∼12 nm in diameter (Figure S1 of the Supporting Information). Relatively short exposures (30 s) were used for the EDS analyses in order to minimize beam damage (see the “m-NP Details” section of the Supporting Information). Cu was excluded from the analyses because the TEM grids are made of Cu. The diameters of aerosol particles were determined by fitting ellipses to the particle outlines and taking the geometric mean of the minor and major axes (13, 25). Analysis of m-NPs. m-NPs were defined on the basis of their compositions and sizes as measured from TEM images. We first chose one to four areas (∼10-60 µm2) on TEM grids from each of 10 samples (Table S1 of the Supporting Information) and selected all aerosol particles larger than 50 nm within these areas (Figure S1A of the Supporting Information). Hereafter, we call such aerosol particles “host particles,” and these can be either potential or actual hosts of m-NPs. Although most host particles are considerably larger (99% are >100 nm), we chose 50 nm as their lower cutoff size because it approaches the geometric cutoff limit for hosting. TEM images of each host particle were then obtained at magnifications ranging from ∼10000 to 50000 to find m-NPs on and around the host particles (Figure S1B of the Supporting Information). m-NPs typically appear dark in TEM images because of their greater interferences with the electron beam than that of the host materials (Figure 1 and Figure S2 of the Supporting Information). We measured the sizes and compositions of m-NPs that were evident in the TEM images (Figure S1 of the Supporting Information). Most m-NPs we collected were embedded within host particles. However, we also analyzed unattached m-NPs if they were collected on the grids. The metals were identified from EDS spectra. In most cases they displayed two or more diagnostic peaks (Figure 1). Although Hg and Sn produce relatively weak signals in EDS spectra, both produce multiple peaks that thereby permit unambiguous detection and identification.
Results and Discussion Occurrences of m-NPs. We examined 458 host particles (see the “Host Particles” section and Table S2 of the Supporting Information) and measured 572 m-NPs (Table 1). Their sizes peak between 10 and 20 nm (Figure 2). Thirty-seven percent of the host particles either contain one or more m-NPs or have them attached to their surfaces (Figure 1). We located and analyzed 83 m-NPs that occurred on the substrates adjacent but unattached to the host particles and that were presumably free-floating in the atmosphere (images 8-10 in Figure 1b), although there is no way of knowing whether some of these might have been attached to volatile host
particles that evaporated in the vacuum of the TEM chamber. The average compositions of free-floating m-NPs differ from those attached to or embedded within host particles, i.e., Hg is abundant in free-floating m-NPs, and Mn, Fe, Zn, and Pb are abundant in the hosted m-NPs (Table S3 of the Supporting Information). More than 70% of the measured m-NPs were attached to or embedded in particles that also contain soot, 13% occurred within particles consisting of organic matter or sulfate but free of soot, and less than 1% were associated with uncoated soot particles. A possible reason for these m-NP occurrences is that coating and embedding of soot by organic matter and sulfate occurred through condensation, which proceeds in polluted air (13, 26, 27) where anthropogenic m-NPs can be abundant. However, there is no obvious correlation between the compositions of m-NPs and the occurrence of soot, which suggests that many m-NPs were incorporated into the host particles during atmospheric transport. There is a wide range of m-NPs contained within the host particles in the various samples. The host particles in samples 2, 4, and 10 are relatively large (Table S2 of the Supporting Information), and more than 60% contain m-NPs (Table 1). These samples also contain abundant host particles that embed soot (Table S2 of the Supporting Information). They were collected within 32 km of the center of MC between 1 and 3 pm local time when the MC pollution plume was most concentrated. Compositions of m-NPs. Fe is the most abundant metal by number, occurring in 44% of analyzed m-NPs (Table 2). Zn, Mn, Pb, Hg, and Sn were detected in >10% of m-NPs, and Cr, Ni, Ti, V, and Ag were found in fewer than 10%. Se, Ba, Sb, Br, and I were also detected within m-NPs. Fifty-six percent of our measured m-NPs contain two or more metals. The m-NPs in samples 2, 4, and 10, i.e., those that contain the greatest number of m-NPs, have their most abundant elements (Mn, Fe, Zn, and Pb) in roughly the same proportions (Table S3 of the Supporting Information). A possible explanation is that these samples were collected from wellmixed air plumes. Varieties of m-NPs. Fe was detected in more than 90% of the m-NPs that contain Ti, Cr, or Mn and 84% of the Nibearing ones (Table 2). Most m-NPs containing both Ti and Fe also contain Si. Although Si is a constituent of many rockforming minerals, this particular combination is unusual and suggests that the source is more likely fly ash (28). When occurring with Ni, V is taken as an indicator of fuel-oil combustion (20, 21). Twenty-one percent of the V-bearing m-NPs in our samples also contain Ni. Querol et al. (20) proposed that V and Ni came from industrial sources located north of MC. Zn and Pb occurred in 35% and 28% of m-NPs, respectively. About half of each also contained the other element. Moffett et al. (29) also found high abundances of Zn- and Pb-bearing particles ranging in size from 0.2 to 2.0 µm in MC aerosol and concluded that they mainly came from industrial incineration. We detected between one and five Ag-bearing m-NPs in half of our TEM samples. More than 70% of those occurred with Br, I, or Se. Although Ag is a relatively rare element, it has been detected in aerosol particles collected from MC and other areas (3, 9, 22). Chow et al. (22) measured the Ag concentration of PM2.5 samples in MC at 0.03 µg/m3 in 1997. In the United States and Mexico, Ag has been detected in aerosol particles larger than 0.2 µm using aerosol mass spectrometry (3, 9). Nanoproducts that contain Ag are widely used in medical and related applications (30) and are possible sources of the MC m-NPs. Sn is common as a minor component in our m-NPs and host materials. In comparison, Querol et al. (20) and Johnson VOL. 44, NO. 7, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Size distribution of m-NPs.
TABLE 2. Metals in MC m-NPs Based on EDS Measurements metals
total Na
multiple metalsb (%) on substratec (%)
coexisting metalsd
other elementse
Fe
251 (44)
93
6
Mn(59), Zn(52), Pb(27), Cr(19), Ni(17), Sn(16), Ti (10), V(4)
Sb(2), Ba(2), Se(1)
Zn
201 (35)
88
8
Fe(65), Mn(47), Pb(46), Sn(21), Ti(4), Cr(4), Ni(4), V(3), Ag(1), Hg(1)
Sb(5), Se(1)
Pb
159 (28)
67
4
Zn(58), Fe(43), Mn(31), Sn(22), V(4), Cr(1), Ni(1)
Sb(4), Ba(1)
Mn
155 (27)
99
5
Fe(95), Zn(61), Pb(32), Cr(19), Ni(17), Sn(14), Ti(11), V(4)
Sb(5), Se(1)
Hg
120 (21)
9
44
Sn(7), Zn(3)
Br(2)
Sn
110 (19)
67
9
Zn(38), Fe(35), Pb(32), Mn(20), V(7), Hg(7), Cr(4), Ni(2), Ag(1), Ti(1)
Sb(14), Br(5), Se(1)
Ni
51 (9)
98
0
Fe(84), Cr(75), Mn(53), Zn(16), V(12), Sn(4), Pb(4)
Se(2), Sb(2)
Cr
50 (9)
94
0
Fe(94), Ni(76), Mn(60), Zn(16), Sn(8), Pb(4)
Se(4)
Ti
28 (5)
94
21
V
28 (5)
86
0
Ag
14 (2)
79
21
Fe(93), Mn(61), Zn(32), V(7), Sn(4) Fe(39), Sn(29), Mn(21), Ni(21), Zn(21), Pb(21), Ti(7) Ba(4) Zn(14), Sn(7)
Se(47), I(21), Br(7)
a
Total number of particles that contain the metal in column 1. The values in parentheses indicate percentages of m-NPs relative to the total (N ) 572). m-NPs that contain more than one metal are counted multiple times, i.e., with each of their constituent metals. b Percentage of particles that contain other metals in addition to that in column 1. c Percentage of particles that occur on the substrate, free of host particles, and were presumably free-floating in the atmosphere. d Metals coexisting with those listed in column 1. Numbers in parentheses indicate the percentage containing the indicated metal. e Other coexisting elements with atomic number g32.
et al. (21) reported relatively high amounts of Sn (29 and 13.3 ng/m3, respectively) from PM2.5 in MC. Hg is a constituent of a surprisingly large number of m-NPs. Their median diameter is the smallest among the m-NPs we measured (14 ( 8 nm). Some Hg-bearing NPs occur in aggregates, and others are attached as individual particles to the surfaces of host particles or the substrate, suggesting they occurred in the atmosphere as free-floating m-NPs. They are relatively sensitive to the electron beam, and consequently, some could have been lost during EDS analysis. Some m-NPs contain Br, but because the EDS spectral peak of Hg Lβ (11.8 keV) overlaps that of Br KR (11.9 keV), our analytical method is insensitive to detecting coexisting Br and Hg. Several sources have been proposed for the Hg in MC samples. These include industrial point sources (31), “widespread multi-source urban/industrial emissions” (32), and sources external to MC (20). In Mexico, 2302
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gold mining operations contributed to two-thirds of the Hg emissions (33), although fossil-fuel combustion, including coal-fired power plants, is the dominant source of global anthropogenic Hg (34). Murphy et al. (35) detected Hg particles smaller than 20 nm in diameter from the lower stratosphere and found that in many cases the Hg is associated with I and Br. Because of their small sizes, Murphy et al. concluded that the Hg-bearing NPs condensed from a vapor. The sizes and shapes of those particles are consistent with those of our study, suggesting that our Hgbearing m-NPs also formed through condensation. The Hg mass concentrations determined by bulk analyses are relatively low compared to those of other elements. For example, Rutter et al. (31) reported 187 ( 300 pg/m3 for particulate Hg at the MC urban ground site. However, we detected Hg as a major component in a large number of m-NPs, suggesting that much particulate Hg occurs as m-NPs.
Implications of m-NPs for Human Health. When inhaled, free-floating m-NPs in the range of 20 nm have the greatest deposition efficiency in the alveolar region (2) and can cause adverse health effects (4, 36). We found that many such m-NPs are embedded within or attached to host particles, and as a result of their larger sizes, these will deposit into shallower parts of the human respiratory tract (2). The host materials are mainly organic matter and sulfate, both of which are more soluble than most m-NPs (37), and will dissolve, leaving the m-NPs behind. Thus, our findings imply that m-NPs carried by host particles could deposit in different physiological areas and cause different effects on the body than free-floating m-NPs. Our study shows that a wide range of m-NPs are common in ambient air above MC. Most health effect studies have been done using pure m-NPs (2, 38). However, the true health effects are presumably more complicated because we found that many m-NPs contain more than two metals and also that many are carried by or within larger aerosol particles such as those consisting largely of organic matter. In vivo and in vitro studies using m-NPs having the characteristics that we found in this study, as opposed to the idealized materials used in some laboratory studies, would be important for understanding the health effects of m-NPs in the actual environment.
Acknowledgments We thank T. Karl, F. Flocke, and S. Madronich for their help with collection of samples from the C130 aircraft and R. Yokelson, T. Christian, S. Urbanski, C. Wold, D. Toohey, M. Fisher, E. Thompson, G. Moore, J. Stright, A. Knobloch, and K. Bailey for their help with collection of samples from the Twin Otter aircraft. C. Wilson and J. Cagle helped greatly with the aircraft inlets. We are grateful to E. J. Freney, P. Westerhoff, and anonymous reviewers for their helpful comments on the manuscript. We acknowledge the use of TEMs within the LeRoy Eyring Center for Solid State Science at Arizona State University. This study was supported by NSF Grant ATM-0531926.
Supporting Information Available Discussions of size definitions, m-NP details, and host particles; Table S1, samples analyzed; Table S2, number fractions and sizes of host particles; Table S3, distributions of detected metals within m-NPs; Figure S1, representative host particles and m-NPs; and Figure S2, example of m-NPs. This information is available free of charge via the Internet at http://pubs.acs.org/.
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