Environ. Sci. Technol. 2000, 34, 3023-3030
Determination of Chemical Species in Individual Aerosol Particles Using Ultrathin Window EPMA C H U L - U N R O , * , † J AÄ N O S O S AÄ N , ‡ IMRE SZALO Ä K I , §,# K E U N - Y O U N G O H , † HYEKYEONG KIM,† AND R E N EÄ V A N G R I E K E N § Department of Chemistry, University of Antwerp (UIA), Universiteitsplein 1, B-2610 Antwerpen, Belgium, KFKI Atomic Energy Research Institute, H-1525 Budapest, Hungary, and Department of Chemistry, Hallym University, ChunCheon, KangWonDo, 200-702, Korea
The determination of low-Z elements such as carbon, nitrogen, and oxygen in individual atmospheric aerosol particles is of interest in studying environmental pollution. By the application of a newly developed EPMA technique, which employs either windowless or thin-window EDX detector, chemical compositions, including the low-Z components, of individual particles can quantitatively be elucidated. The determination of low-Z elements in individual environmental particles allows to improve the applicability of the single particle analysis; many environmentally important atmospheric particles, e.g. sulfates, nitrates, ammonium, and carbonaceous particles, contain low-Z elements, which cannot be characterized using conventional energy dispersive-EPMA (ED-EPMA). Furthermore, the diversity and the complicated heterogeneity of atmospheric particles in chemical compositions can be investigated in details, using the new EPMA technique. This work demonstrates that the quantitative determination of chemical species in individual particles is possible using ultrathin window EPMA coupled with Monte Carlo based quantification. Using the new EPMA method, molar concentrations of major chemical species in individual environmental particles can be determined. For example, the molecular concentrations of ammonium sulfate and nitrate in single particle were analyzed for particles internally mixed with ammonium sulfate and nitrate species. When particles are composed of several chemical species so that the number of equations is smaller than the number of chemical species to be determined, the quantitative analysis of each chemical species can be ambiguous; however, many particles are composed of one or two major chemical species, and thus this technique could provide direct observation of atmospheric chemistry for airborne particles in more detail.
Introduction Characterization of airborne particles deepens our understanding about the source, reaction, transport, and removal * Corresponding author fax: +82 361 256 3421; e-mail: curo@ sun.hallym.ac.kr. † Hallym University. ‡ KFKI Atomic Energy Research Institute. § University of Antwerp. # On leave from Institute of Experimental Physics, KLTE, Bem te ´r 18/a, H-4026, Debrecen, Hungary. 10.1021/es9910661 CCC: $19.00 Published on Web 06/09/2000
2000 American Chemical Society
of atmospheric chemical species. Since atmospheric particles are chemically and morphologically heterogeneous, and the average composition and the average aerodynamic diameter do not describe well the population of the particles, microanalytical methods have proven to be useful for studying atmospheric particles. Electron probe X-ray microanalysis (EPMA) is capable of simultaneously detecting the chemical composition and morphology of a microscopic volume as a single atmospheric particle (1, 2). The recently developed EPMA technique (3, 4) allows to determine the concentration of low-Z elements such as carbon, nitrogen, and oxygen as well as the elements which are observed using conventional ED-EPMA. Conventional energy-dispersive X-ray (EDX) detectors are not suitable for low-Z element analysis mainly because their Be window, used for protecting semiconductor detector surface from contamination, absorbs low-energy X-rays and thus hinders the detection of the low-Z element X-rays. By the application of the newly developed EPMA technique, which employs either windowless or thin-window EDX detector, chemical compositions, including the low-Z components, of individual particles can be at least semiquantitatively elucidated. The determination of low-Z elements in individual environmental particles allows to improve the applicability of the single particle analysis; many environmentally important atmospheric particles, e.g. sulfates, nitrates, ammonium, and carbonaceous particles, contain low-Z elements, which have not been characterized using the conventional EPMA. Furthermore, the diversity and the complicated heterogeneity of atmospheric particles in chemical compositions can be investigated in details, using the new EPMA technique. There have been approaches to specify environmentally important chemical species in individual particles, e.g. nitrate and sulfate, using EPMA (5-8). These techniques employ chemical reactions on individual particles of interest; for example, barium chloride and Nitron are used as reaction agents for sulfate and nitrate species, respectively. The particles with nitrate or sulfate react with those agents to produce characteristic morphologies if reaction occurs, so that those particles can be identified using either SEM or TEM. These techniques have proven to be useful to study the atmospheric chemistry of nitrate or sulfate species in the reaction with other airborne particles, such as sea-salt, mineral, and carbonaceous particles. However, these techniques only allow to analyze nitrate and sulfate species; furthermore, this analysis is purely qualitative since only the presence or absence of those chemical species can be determined. Another single particle analysis technique allows to identify chemical species of individual particles qualitatively, using laser microprobe mass spectrometry, so-called ATOFMS (9-11). This technique can analyze the aerodynamic sizes and chemical compositions of individual particles in real time and even the instrument can be portable (11), so that it can be used in the field. By the application of the ATOFMS technique, the complex nature of airborne particles has been directly revealed (12, 13), and it was demonstrated that the technique can clearly elucidate the atmospheric chemistry between sea-salt particles and gas-phase nitric acid, as it occurs in the atmosphere (14). However, due to its poor reproducibility, the technique can now only provide qualitative determination of chemical species in individual particles. In our previous study, it was found that excitation interactions between electrons and the atoms of the matrix and the matrix and geometric effects of fluorescence signals for light elements in individual atmospheric microparticles can be described by Monte Carlo simulation (3). By the VOL. 34, NO. 14, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3023
TABLE 1. Comparison of Nominal (Cn) and Calculated (Cc) Concentrations in Atomic Fraction for Standard Particlesa CaCO3
Cn (in %)
Cc (in %)
σc
∆ (in %)
KNO3
Cn (in %)
Cc (in %)
σc
∆ (in %)
C O Ca
20.0 60.0 20.0
20.3 61.5 18.1
1.3 2.6 3.0
1.5 2.5 9.5
K N O
20.0 20.0 60.0
22.1 21.6 56.3
2.2 2.2 2.6
10.5 8.0 6.2
SiO2
Cn (in %)
Cc (in %)
σc
∆ (in %)
NaCl
Cn (in %)
Cc (in %)
σc
∆ (in %)
O Si
66.7 33.3
65.1 34.9
2.2 2.2
2.4 4.8
Na Cl
50.0 50.0
54.7 45.3
1.5 1.5
9.4 9.4
CaSO4‚2H2O
Cn (in %)
Cc (in %)
σc
∆ (in %)
BaSO4
Cn (in %)
Cc (in %)
σc
∆ (in %)
O S Ca
75.0 12.5 12.5
74.5 13.0 12.5
3.9 1.8 2.4
0.7 4.0 0.0
O S Ba
66.7 16.7 16.7
72.0 12.5 15.5
3.2 0.8 2.6
7.9 25.1 7.2
Fe2O3
Cn (in %)
Cc (in %)
σc
∆ (in %)
O Fe
60.0 40.0
61.9 38.1
4.0 4.0
3.2 4.8
(NH4)2SO4
Cn (in %)
Cc (in %)
σc
∆ (in %)
NH4NO3
Cn (in %)
Cc (in %)
σc
∆ (in %)
N O S
28.6 57.2 14.3
25.2 62.0 12.8
1.7 3.0 1.9
11.9 7.7 10.5
N O
40.0 60.0
42.0 58.0
3.9 3.9
5.0 3.3
a The standard deviation of C is σ . The relative differences between calculated and nominal concentrations (in atomic fraction) are assigned c c as ∆ in %.
application of a quantification method, which employs Monte Carlo simulation combined with successive approximation, it was shown that at least semiquantitative specification of the chemical compositions can be done (4). In this work, our major objective is to demonstrate that the chemical species, in addition to chemical composition, in individual urban particles collected in Antwerp, Belgium, can be determined quantitatively by the application of the new EPMA.
Experimental Section Samples. Chemical compounds, generally present in atmospheric aerosol particles such as CaCO3, CaSO4‚2H2O, NaCl, SiO2, Fe2O3, BaSO4, KNO3, (NH4)2SO4, and NH4NO3, were used to evaluate the concentration calculation method using iterative simulations. Those particulate standard samples were prepared from pro analysis grade solid chemical compounds. The grains were ground to microscopic size using an agate mortar. To avoid the additional absorption and the spectral overlap possibly caused by the conductive coating on the sample, aluminum foil was used as substrate material for the collection of the standard particles. The particles were suspended in 0.01 M n-hexane, and a volume of 10 µL of the suspension was dropped by micro-pipet onto the surface of the Al foil and dried in air. Atmospheric particles were collected in a suburban area on the campus of the University of Antwerp, Belgium, on August 2, 1998, using a nine-stage Berner impactor. Aluminum foil was used as a collecting substrate. The sampling time varied between 2 (for stage 3) and 120 (for stage 7) min, to obtain the best loading of particles in the impacted spots. Samples on stage 3, 5, and 7 in the Berner impactor were analyzed using ultrathin window EPMA, without applying a conductive coating. The aerodynamic cutoff diameters for the stage 3, 5, and 7 for the Berner impactor are 0.25, 1, and 4 µm, respectively. Some 300 particles for each stage sample were analyzed, in total, 900 particles. EPMA Measurements. The measurements for the samples collected on the Al foil were carried out on a JEOL 733 electron probe microanalyzer equipped with an OXFORD energydispersive X-ray detector. For optimizing the detection of low-Z element X-ray lines, and to avoid the change of detector efficiency as a function of time due to ice condensation to windowless detectors, the Si(Li) detector is equipped with 3024
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 14, 2000
an atmospheric ultrathin window. The resolution of the detector is 150 eV for Mn-KR X-rays. The spectra were recorded by a CANBERRA S100 multichannel analyzer under control of homemade software. Measurements on individual particles were carried out manually in the point analysis mode. To achieve optimal experimental conditions, such as low background level, in the spectra and high sensitivity for light element analysis, a 10 kV accelerating voltage was chosen (3). The beam current was 0.5 nA for all the measurements. To obtain statistically enough counts in the X-ray spectra and to minimize the beam damage effect on the sensitive particles, a typical measuring time of 20 s was used. Also, to analyze volatile particles under electron beam and vacuum, e.g. ammonium nitrate, all the measurements were carried out at around -193 °C stage temperature, using the cold stage of the electron microprobe cooled by liquid nitrogen. The vacuum system of the EPMA instrument consists of oil rotary (1 unit) and oil diffusion pumps (2 units). The samples were introduced to the EPMA chamber after cooling the sample stage at liquid nitrogen temperature, so that the organic residues and moistures condense on the sample stage rather than on the sample itself. The size and shape of each individual particle was measured and estimated from a highmagnification secondary electron image (M > 10000×). These estimated geometrical data were set as input parameters for the quantification procedure. The net X-ray intensities for the elements were obtained by nonlinear least-squares fitting of the collected spectra using the AXIL program (15). A number of well-developed and rigorously tested quantification procedures are available in EPMA [e.g. ZAF and φ(Fz) methods] especially for the analysis of bulk samples. However, these procedures are limited for light element analysis of individual atmospheric microparticles (considering the size and shape of the particles and matrix effect for light elements); therefore a new quantification method, which employs a Monte Carlo simulation in the combination of successive approximation, is used in this work. Details on the quantification procedure are elsewhere (4).
Results and Discussion Evaluation of the Concentration Calculation Procedure. The quantification method based on Monte Carlo simulation combined with successive approximation was evaluated by
FIGURE 1. X-ray spectra of (A) NaNO3 particle and (B) a particle internally mixed with ammonium nitrate and sulfate. comparing the atomic concentrations obtained from EPMA measurements with their nominal atomic concentrations. At least 10 independent analyses were performed for each type of standard particles with their diameters varying between 0.5 and 5 µm. Table 1 shows the comparison of the nominal (Cn) and calculated (Cc) average concentrations for CaCO3, KNO3, SiO2, NaCl, CaSO4‚2H2O, BaSO4, Fe2O3, (NH4)2SO4, and NH4NO3. The second and third columns contain the nominal and the calculated atomic fractions of the composite elements, respectively. The sources of the standard deviation of the average calculated concentration (σc) are the fluctuations in the shape and size parameters of the particles. The relative differences in averages between the nominal and calculated concentrations, ∆ ) |Cc - Cn|/Cc, are shown in the last column of the table. Even though diverse chemicals with different chemical and physical properties are used, the relative differences in averages between the nominal and calculated concentrations are within 12%, except S in BaSO4. The reason the calculated sulfur concentration in BaSO4 is much off from the nominal concentration seems to be the presence of water. Since the calculated
oxygen concentrations for (NH4)2SO4 and BaSO4 are larger than the nominal values, it implies that standard sulfates contain some amount of water. It is not possible to directly confirm the amount of water content in the sulfates due to the lack of detection for hydrogen in EPMA. However, as seen in CaSO4‚2H2O particles, if the accurate amount of water is known, this method provides quite a good agreement. For CaSO4‚2H2O, the nominal and calculated values of each element are different within 4%. Also, the relatively large deviation between the nominal and calculated concentrations for NaCl seems to be due to the water attached to NaCl particles because the particles are hygroscopic and a small amount of oxygen is always seen in X-ray spectra of the NaCl particles. Based on the analysis of the various standard samples, the method provides rather quantitative concentration values even for light elements, which can allow to determine the chemical species in individual particles. Determination of Chemical Species in Individual Atmospheric Particles. The determination of chemical species in individual particles was done in the way to fully utilize the information contained in the data. The chemical composition VOL. 34, NO. 14, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3025
TABLE 2. Measured Intensities for and Sizes of the Particles Discussed in the Text and Calculated Atomic Concentrations from Iterative Monte Carlo Procedure measd calcd element intensity intensity Na N O C Mg S Cl K Ca
rel difference between measd and calcd calcd atomic intensities (in %) concn (in %)
(A) NaNO3 Particle (Shape: Spherical, Diameter: 2.1 µm) 8655 8673 0.2 18.0 3197 3200 0.1 19.3 15 602 15 589 0.1 58.7 629 623 0.9 2.6 331 331 0.1 0.7 166 166 0.3 0.3 82 82 0.3 0.2 52 52 0.2 0.1 103 103 0.1 0.3 (B) Internally Mixed Particle (Shape: Hemispherical, Diameter: 0.7 µm) 4912 4909 0.1 29.8 13 801 13 967 1.2 61.7 3159 3149 0.3 4.8 720 717 0.4 3.2 239 238 0.4 0.5
N O S C K
of each particle is never exactly the same as that of others; it is very rare to see particles with only one pure chemical species, and particles with two or more chemical species have different compositions. Especially carbonaceous species are ubiquitous in almost every fine particle. Our finding is not the first; there are hundreds of different particle types, which can be classified based on their different chemical compositions (10, 12, 13, 16, 17). Among 900 particles analyzed, many distinctive particle types, according to their chemical compositions, were identified. First of all, particles with one major chemical species are classified as “pure” particles; in this case, the “pure” particles were defined in a somewhat arbitrary way if one chemical species is present for more than 90% in atomic concentration. For example, an X-ray spectrum of a particle composed of mainly NaNO3 and also minor C, Mg, S, Cl, K, and Ca species is shown in Figure 1(A). In Table 2(A), the atomic fraction of each element calculated by the iterative Monte Carlo method is given together with the input data. In the calculation, aluminum intensity is not considered because Al X-rays can originate from the Al foil used as collecting substrate. The Na, N, and O concentrations in atomic fraction are 18.0%, 19.3%, and 58.7%, respectively, which are very close to the stoichiometry of NaNO3, e.g. Na:N:O ) 1:1:3. In this particle, NaNO3 species is present for more than 96% in atomic fraction. Internally mixed particles were also analyzed, and the relative abundance of two or more chemical species in individual particles is determined using the new EPMA technique. For example, the X-ray spectrum of a particle internally mixed with ammonium nitrate and sulfate is shown in Figure 1(B). The atomic concentration of each element calculated by the iterative Monte Carlo method is shown in Table 2(B) together with the input data. Nitrogen, oxygen, and sulfur excluding aluminum are the major elements in this particle, and the sum of the concentration of the major elements is 96.3% in atomic fraction. Reaction between gasphase NH3 and H2SO4 species in the atmosphere can produce (NH4)2SO4, NH4HSO4, and (NH4)3H(SO4)2; their relative abundances depend on the concentration of the gas-phase species as well as the relative humidity and temperature (18). However, it was found that (NH4)2SO4 is the main chemical species in urban particles (19). Since sulfur can be present in this particle only as sulfate and sulfate mostly exists as (NH4)2SO4, the atomic sulfur concentration is used for the calculation of the (NH4)2SO4 content. For this particle, the 3026
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 14, 2000
content of (NH4)2SO4 is 33.6% in atomic fraction, i.e., 4.8% for sulfur, 9.6% for nitrogen, and 19.2% for oxygen. The remaining atomic concentration of nitrogen is now 20.2%. By the use of an EDX detector, chemical states of nitrogen in nitrate and ammonium cannot be differentiated, due to the poor energy resolution of the detector. Since the remaining nitrogen atoms make ammonium nitrate, the content of NH4NO3 was calculated as 50.5% in atomic fraction, i.e., 20.2% for nitrogen and 30.3% for oxygen. The particle is composed of mainly ammonium nitrate and sulfate; the sum of the contents of these two major species is 84.1% of the total atomic concentration. The other species in the particle are carbonaceous, water, and potassium-containing species, which take the remaining 15.9%. In EPMA, hydrogen is not detected, so that we cannot identify the carbonaceous species as either elemental or organic carbon. If it is assumed that the carbonaceous species, which takes 3.2% in atomic fraction, is elemental carbon, the water content in the particle can be calculated as 12.2% in atomic fraction, e.g. 61.7% of atomic oxygen content in total minus 19.2% for ammonium sulfate and 30.3% for ammonium nitrate. In the analysis, the potassium species of the particle was not considered because of its low concentration. If the potassium concentration is higher, then a more systematic approach for the molecular speciation is needed. Still the particle internally mixed with ammonium nitrate and sulfate can be used as an example for the more generalized approach. When the potassium species in this particle is considered in the molecular speciation, the probable species would be either K2SO4 or KNO3 because nitrate and sulfate are feasible anions which can combine with K+; K2O, K2CO3, and KOH species are highly unreasonable to consider for this particle, because the size of the particle is small (0.7 µm). This particle is assumed to be composed of ammonium nitrate, ammonium sulfate, carbonaceous species, potassium sulfate, potassium nitrate, and water. In this case, five equations (eqs 1-5) can be used to obtain the concentration of each molecular species
Cap ) CC
(1)
Oap ) 4CAS + 3CAN + 4CPS + 3CPN + CH2O
(2)
Nap ) 2CAS + 2CAN + CPN
(3)
Sap ) CAS + CPS
(4)
Kap ) 2CPS + CPN
(5)
where Cap, Oap, Nap, Sap, and Kap are the atomic concentrations for carbon, nitrogen, oxygen, sulfur, and potassium, respectively, and CC, CAS, CAN, CPS, CPN, and CH2O are the molecular concentrations of carbonaceous, ammonium sulfate, ammonium nitrate, potassium sulfate, potassium nitrate, and water, respectively. The number of unknowns is six, whereas the number of equations is five; there exists no exact solution for this system. If the boundary conditions of each molecular concentration, e.g. 0 e CPS e 0.25 and 0 e CPN e 0.5, are considered, the molecular contents for the particle can be estimated; here, the content of ammonium nitrate is in the range of 46.8% and 49.3% in atomic fraction, ammonium sulfate is of 31.9% and 33.6%, potassium sulfate is of 0% and 1.7%, potassium nitrate is of 0% and 2.5%, water is 11.5%, and carbonaceous species is 3.2%. Considering that the accuracy of this new EPMA method is within 12% as shown from the analysis of standard particles, the uncertainty of the values given above was estimated by error propagation analysis using eqs 1-5. The estimated uncertainty values for the species mentioned above are 2.0%, 2.0%, 0.1%, 0.1%, 7.5%, and 0.4%, respectively, in atomic fraction. The water
TABLE 3. Particle Types and the Numbers of Particles Found in Stage 3, 5, and 7 Samplesa number of particles particle type AlOx AlOx/CaSO4/C AlSib AlSi/Cc AlSi/C/Fe AlSi/C/(NH4)2SO4 AlSi/C/O AlSi/CaCO3/CaO AlSi/Ca(NO3)2 AlSi/Ca(NO3)2/C AlSi/Ca(NO3)2/FeOx AlSi/Ca(NO3)2/Ti AlSi/CaO/C AlSi/CaSO4/C AlSi/K AlSi/(Mg,Ca)(PO4) AlSi/MgSO4/C AlSi/N AlSi/N/C biogenic Carbon-rich Organic CaCO3 CaCO3/C CaCO3/C/O CaCO3/CaO Ca(CO3,NO3) Ca(CO3,NO3)/AlSi CaCO3/SiO2 (Ca,Mg)CO3 Ca(NO3)2 Ca(NO3)2/C Ca(NO3,SO4) (Ca,NH4)NO3 CaSO4 CaSO4/C Ca(SO4,NO3)/C CaSO4/C/O CaSO4/Na Ca(SO4,NO3)
number of particles
stage 3
stage 5
stage 7
particle type
1
14
1 1 25 26
CuOx/C F Fe/C Fe/C/Mg/Si Fe/Cr Fe/Cu/Si/C Fe/Si/O/C FeOx FeOx/AlSi/C FeOx/C FeOx/C/Si FeOx/Ca FeOx/Na FeOx/SO4 FeS2/C KNO3/C Mg(NO3,CO3) MgSO4/C/O MnOx NaNO3 NaNO3/C NaNO3/Cu/Fe Na(NO3,SO4) Na2SO4 NH4NO3/C NH4(NO3,SO4)/C NH4(NO3,SO4) (NH4)2SO4 (NH4)2SO4/C (NH4)2SO4/C/Mg (NH4)2SO4/C/Mn (NH4)2SO4/Cu Pb SiO2 SiO2/C SiO2/C/O SO4/C SO4/C/O Ti
1 2 1
29 4 2 49 2 12 18
1 1 1 1 1 1
29 47
1 1 2 7 15 29
1 1
2
27 1 5 11 3 1 2 3 2 1 2 22 4 1 2
6 10 4 2 5
2 1
sum a
The aerodynamic cut-off diameters for stage 3, 5, and 7 are 0.25, 1, and 4 µm, respectively.
content has the largest relative error (around 65%), which is a result of the uncertainty of the determination of the oxygen concentration. The estimated relative uncertainty of the other species is below 12%, allowing the semiquantitative determination of the chemical species. The technique has still a limitation for heterogeneous particle systems, e.g. surfaceenriched particle with different chemical species, because the homogeneity of the particle in the excitation volume has to be assumed for the calculations. Based on this analytical procedure, about 80 types of particles were identified. Some particles have several chemical species internally mixed, so that it becomes difficult to quantitatively determine all the chemical species present in the particles. In this case, the major chemical species and elements are specified, e.g. notation by FeOx/Ca in Table 3 represents a particle type which is composed of iron oxide and Ca species of which the chemical formula cannot be determined. The most abundant particles are carbonaceous, soil derived, nitrates, sulfates, and iron oxide particles. 1. Carbonaceous Particles. Carbonaceous particles in urban atmospheric environment form one of the dominating particle types, especially for fine particles (20, 21). From the new EPMA technique, three different carbon particles could be distinguished; carbon-rich, organic, and biogenic particles.
stage 3
stage 5 1 1
1 2 2 3 8 22
1 64 12
13 1 3
9 1 3 1
1 1 3 2
4 1 11 1
1
1 19 2 2 2
2 7 10 3
8 41 21
2
16 2 1 2 1 1 6
6
3 10 2 6
3 18 4 11 283
b
stage 7
8 3 1
1
275
286
c
AlSi: aluminosilicates. C: carbonaceous species.
In the studied size range, we could not see particles only with carbon but always together with oxygen, even for “pure” carbon-rich particles. When soot or elemental carbon particles are in the air, some of the carbon will be oxidized. Considering that EPMA can analyze particles larger than 0.3 µm in diameter and this kind of particle is in accumulation mode, no elemental carbon particles without having experienced oxidation in the air can be seen. Also, organic particles, which are originally various types of hydrocarbons when emitted from combustion engines, become particles after experiencing atmospheric reactions. Since EPMA cannot detect hydrogen, it is difficult to clearly distinguish organic particles from elemental carbon particles, based only on carbon and oxygen contents in individual particles. However, carbon-rich particles are defined and differentiated from organic particles, when the carbon content in atomic fraction is much larger than the oxygen content, e.g. 3 times larger. For coarse particles, biogenic particles are similar to organic particles in atomic concentrations; carbon and oxygen atomic concentrations are the major and similar to each other. Biogenic particles were differentiated from anthropogenic organic particles, due to the presence of N, P, K, and S elements in them (22). VOL. 34, NO. 14, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3027
FIGURE 2. X-ray spectra of a particle with calcium nitrate, calcium carbonate, and aluminosilicate. The calculated content ratio of calcium nitrate and calcium carbonate is approximately 1.7 in molar concentration. Among the 900 particles analyzed, the total numbers of “pure” carbon-rich and organic particles are 45 (5% of the total number of particles)) and 81 (9% of the total), respectively, as shown in Table 3. This type of particle is usually found in fine fraction. “Pure” biogenic particles were observed mostly in stage 7 sample (cutoff diameter; 4 µm), and no such particle is detected in stage 3 sample (cutoff diameter; 0.25 µm). Although the pure carbonaceous particles are just 17.7% of the analyzed particles, carbon species were found in the particles internally mixed with other chemical species. For example, it is common to see carbonaceous particles internally mixed with soil derived species, iron oxide, nitrates, or/and sulfates. When particles with carbon atomic concentration, above 10% are counted, the number of particles is 639, and those account for 71% of the total number of particles. The internally mixed carbonaceous particles are discussed later. 2. Soil Derived Particles. Four abundant “pure” soil derived particle types are aluminosilicates, silicon oxide, calcium carbonate, and iron oxides. The most common mineral particles are aluminosilicates. Even though we could not exactly determine the Al concentration due to the possible interference from the Al substrate used, aluminosilicates have Na, Mg, K, Ca, and Fe as well as Al and Si. Especially when the particle is big enough to minimize the aluminum influence from the collecting substrate, the atomic concentrations of aluminosilicate particles can be determined. The aluminosilicate particles originate from soil, and thus pure aluminosilicate particles were observed mainly in stage 5 and 7 samples (see Table 3). With less abundance than aluminosilicates, pure silicon oxide particles were also observed mostly in stage 5 and 7 samples. Pure calcium carbonate particles were found only in stage 7 sample. In addition to calcium carbonate, a small number of calcium carbonates with MgCO3 and CaO were also observed in stage 7. As shown in Table 3, the total number of these particles is 85 (9% of the total number of particles), and no particles were found in stage 3 sample. Many aluminosilicates and silicon oxide particles are internally mixed mostly with carbonaceous particles; 17 for stage 3, 28 for stage 5, and 53 for stage 7 sample. The total number of calcium carbonates, either pure or containing carbon, calcium oxide, silicon, or magnesium carbonate species, was 23, only observed in stage 7; this implicates that calcium carbonates are soil derived ones. 3028
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 14, 2000
3. Nitrate Particles. Nitrate is also one of the main components in urban particles. From our analysis, two types of nitrates with one major chemical species were found: sodium nitrate and calcium nitrate. Those particles were observed only in the coarse fraction collected on stage 7 (cutoff diameter; 4 µm) of Berner impactor. The number of pure NaNO3 and Ca(NO3)2 particles was 21 and 9, respectively, among the total of 300 particles analyzed in the stage 7 sample. Since NaNO3 particles are the products of atmospheric reactions between NaCl and nitric acid (as in eq 6) and pure NaCl particles are not observed at all for this sample, marine particles were introduced into the Antwerp atmosphere quite a long time before our sampling and stayed long enough to react completely with nitric acid. It was reported that NaCl particles in marine air mass reacted with anthropogenic HNO3 to completely deplete chloride when the air mass stagnated over the Long Beach area for 11 h (12). Also our assumption is supported by the facts that the pure NaNO3 particles have a small amount of Cl, which is a fingerprint of the marine origin of those particles.
NaCl + HNO3 f NaNO3 + HCl
(6)
The formation of calcium nitrate particles in the atmosphere is not investigated as much as NaNO3. It was proposed that Ca(NO3)2 is formed by the reaction between CaCO3 and HNO3 (eq 7) (23). Also one indirect evidence that the reaction occurs in the atmosphere was reported (24). In this study, it was observed that there are several particles which are internal mixtures of CaCO3 and Ca(NO3)2; it seems that the reaction has not completely occurred for these particles. For example, the particle shown in Figure 2 contains more Ca(NO3)2 than CaCO3 according to our analysis, but there exists still significant amount of CaCO3. Since this type of particle also contains small amounst of aluminosilicates in them, Ca(NO3)2 is most probably the product of a reaction between anthropogenic nitric acid and soil derived CaCO3 species. This argument would be conclusive if this EPMA technique could distinguish surface species, which would be Ca(NO3)2, from the core CaCO3 species of the particle. The recently developed grazing exit EPMA technique (25) can be applied to address this question.
CaCO3 + 2HNO3 f Ca(NO3)2 + H2O + CO2
(7)
Pure ammonium nitrate particles were not observed; however, we found a significant number of ammonium
FIGURE 3. X-ray spectra of (A) a particle with calcium sulfate species (Due to its high concentration of oxygen, this particle contains significant amounts of moisture in it.) and (B) a particle with sodium sulfate species (Sodium sulfates are mostly with other chemical species. For this particle, a small amount of calcium and magnesium sulfates are also present.) nitrate particles internally mixed with ammonium sulfate (62 particles in stage 3 sample, 3 particles in stage 5 sample). A typical X-ray spectrum is shown in Figure 1(B). Ammonium nitrate particles with carbon contents are also found (8 particles in stage 3, 10 in stage 5 and 2 in stage 7). The reason pure ammonium nitrate particles were not observed in this measurement will be discussed later with the discussion on ammonium nitrate and ammonium sulfate mixture particles. 4. Sulfate Particles. Sulfate is one of the most abundant chemical species in urban particles, especially in fine urban particles, mainly as ammonium sulfate species. However, pure ammonium sulfates were not observed in stage 3 sample, 16 pure ammonium sulfate particles only for stage 5. For this sample, the sulfate species are almost ubiquitous in the particles of stage 3; the number of particles which have sulfur species is 250 among 300 particles analyzed in the stage 3 sample (83%). However, they are always with other chemical species. For example, in stage 3 sample, ammonium sulfate was found internally mixed with ammonium nitrate (21 particles), carbon (6 particles), and carbon and ammonium
nitrate (41 particles). Considering that most of ammoniumsulfate-nitrate particles were observed in stage 3, of which the cutoff aerodynamic diameter is 0.25 µm, their sizes are surprisingly big, ranging from 0.9 to 10.3 µm (the number of those particles is 21 and average of sizes is 4.4 µm). When we analyze the contents of ammonium sulfate and nitrate in this type of particles, the ratio of ammonium nitrate and sulfate molar contents is 1.5 with a relative standard deviation of 16.4% for 21 particles in total. Those particles are believed to be formed on the Al foil after collection of aerosols. In the Berner impactor, particles pass through regularly spaced holes, impact on small areas, especially for the stage 3, and thus were overloaded in stage 3. Usually ammonium sulfates and nitrates contain moisture, and after collection they can easily agglomerate on the Al foil surface due to the high surface tension of water. The sulfate and nitrate can be mixed after collection, and then water vaporizes to form big ammonium-sulfate-nitrate particles. That is why we have particles with constant relative molar concentration between ammonium sulfate and nitrate. This assumption is supported VOL. 34, NO. 14, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3029
by the observations that we observed these big particles less frequently in stage 4 and never saw in stage 5, where the impacted spots were not overloaded. This argument also explains why the pure ammonium sulfate particles were observed only in stage 5. Calcium and sodium sulfates were also found in stage 5 sample, and in many cases the other chemical species are present in the particles (see Figure 3A,B). 5. Iron Oxide Particles. For iron oxide, pure particles were usually detected in stage 5 and 7 samples. Also, fine iron oxide particles were present mostly mixed with carbonaceous species. This observation implies that iron oxide particles have two sources: soil derived and anthropogenic particles. Soil derived iron oxide particles were observed without having carbonaceous species and also their size is rather big. It was already known that there are many iron oxide particles in Antwerp. Particulate matters collected at the same place in Antwerp, Belgium, were analyzed using particle-induced X-ray emission (PIXE) (26) and conventional EPMA (22). However, they could not clearly differentiate anthropogenic iron oxides from soil-derived ones. In the present study, many of them were found to have anthropogenic origin; iron oxide particles were abundant in stage 3 and 5 samples, and yet they were mostly together with carbonaceous species (76 particles in stage 3, 9 in stage 5, and 3 in stage 7). Iron oxide particles either pure or containing soil-like elements such as Na, Mg, Si, and Ca are mostly observed in stages 5 and 7 (1 particle for stage 3, 39 particles for stage 5, and 15 for stage 7), and these are believed to be soil derived particles.
Acknowledgments This work was partially financed by the Belgian State Office for Scientific, Technical and Cultural Affairs through a research project (in the framework of the program “Sustainable Management of the North Sea”; contract MN/DD/10) and through a research grant to one of us (I.Sz.) and partially by the European Union through project ENV4-CT95-0088. The support of the Hallym Academy of Sciences, Hallym University, Korea, is also appreciated.
Literature Cited (1) Osa´n, J.; To¨ro¨k, Sz.; To¨ro¨k, K.; Ne´meth, L.; La´ba´r, J. L. X-ray Spectrom. 1996, 25, 167-172.
3030
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 14, 2000
(2) Jambers, W.; Van Grieken, R. Environ. Sci. Technol. 1997, 31, 1525-1533. (3) Ro, C.-U.; Osa´n, J.; Van Grieken, R. Anal. Chem. 1999, 71, 15211528. (4) Osa´n, J.; Szaloki, I.; Ro, C.-U.; Van Grieken, R. Mikrochim. Acta 2000, 132, 349-355. (5) Roth, B.; Okada, K. Atmos. Environ. 1998, 32, 1555-1569. (6) Mamane, Y.; Gottlieb, J. Atmos. Environ. 1992, 26A, 1763-1769. (7) Mamane, Y.; Gottlieb, J. J. Aerosol Sci. 1989, 20, 575-584. (8) Mamane, Y.; Gottlieb, J. J. Aerosol Sci. 1989, 20, 303-311. (9) Carson, P. G.; Johnston, M. V.; Wexler, A. S. Aerosol Sci. Technol. 1997, 26, 291-300. (10) Noble, C. A.; Prather, K. A. Environ. Sci. Technol. 1996, 30, 26672680. (11) Gard, E.; Mayer, J. E.; Morrical, B. D.; Dienes, T.; Fergenson, D. P.; Prather, K. A. Anal. Chem. 1997, 69, 4083-4091. (12) Murphy, D. M.; Thomson, D. S. J. Geophys. Res. 1997, 102, 63416352. (13) Murphy, D. M.; Thomson, D. S. J. Geophys. Res. 1997, 102, 63536368. (14) Gard, E. E.; Kleeman, M. J.; Gross, D. S.; Hughes, L. S.; Allen, J. O.; Morrical, B. D.; Fergenson, D. P.; Dienes, T.; Galli, M. E.; Johnson, R. J.; Cass G. R., Prather, K. A. Science 1998, 279, 11841187. (15) Vekemans, B.; Janssens, K.; Vincze, L.; Adams, F.; Van Espen, P. X-ray Spectrom. 1994, 23, 278-285. (16) Kim, D.; Hopke, P. K. Aerosol Sci. Technol. 1988, 9, 133. (17) Kim, D.; Hopke, P. K. Aerosol Sci. Technol. 1988, 9, 221. (18) Pilinis, C.; Seinfeld, J. H. Atmos. Environ. 1987, 21, 2453-2466. (19) John, W.; Wall, S. M.; Ondo, J. L.; Winklmayr, W. Atmos. Environ. 1990, 24A, 2349-2359. (20) Rogge, W. F.; Mazurek, M. A.; Hildemann, L. M.; Cass, G. R. Atmos. Environ. 1993, 27A, 1309-1330. (21) Harrison, R. M.; Jones, M. Sci. Total Environ. 1995, 168, 195214. (22) Van Borm, W.; Adams, F. C.; Maenhaut, W. Atmos. Environ. 1990, 23, 1139-1151. (23) Dentener, F. J.; Carmichael, G. R.; Zhang, Y.; Lelieveld, J.; Crutzen, P. J. J. Geophys. Res. 1996, D17, 22869-22889. (24) Carmichael, G. R.; Hong, M.; Ueda, H.; Chen, L.; Murano, K.; Park, J. K.; Lee, H.; Kim, Y.; Shim, S. J. Geophys. Res. 1997, D5, 6047-6061. (25) Tsuji, K.; Wagatsuma, K.; Nullens, R.; Van Grieken, R. Anal. Chem. 1999, 71, 2497-2501. (26) Van Borm, W.; Adams, F. C.; Maenhaut, W. Atmos. Environ. 1990, 24B, 419-435.
Received for review September 15, 1999. Revised manuscript received April 12, 2000. Accepted April 20, 2000. ES9910661
