Environ. Sci. Technol. 1996, 30, 385-391
Characterization of Antarctic Aerosol Particles Using Laser Microprobe Mass Spectrometry K E I I C H I R O H A R A , * ,† TADASHI KIKUCHI,† KEIICHI FURUYA,† MASAHIKO HAYASHI,‡ AND YOSHIYUKI FUJII§ Department of Applied Chemistry, Faculty of Science, Science University of Tokyo, 1-3 Kagurazaka, Shinjukuku, Tokyo 162, Japan, Solar-Terrestrial Environment Laboratory, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464, Japan, and National Institute of Polar Research, 1-9-10, Kaga, Kitaku, Tokyo 114, Japan
Antarctic aerosol particles were collected on aluminum foils by means of a multistage high-volume air sampler from February until December 1991 at the Syowa station in Antarctica. Particles on the third stage (5.4-1.6 µm) were analyzed by means of a reflection-type laser microprobe mass analyzer (LAMMA1000) for their constituents and their seasonal variations. As a result, the seasonal behavior of methanesulfonate, nitrate, sulfates, and metallic elements including Ba, Cu, and Pb was clarified.
Introduction The Antarctic region is the best site for monitoring global environmental changes because it is remote from human activities on the other continents. The Antarctic atmosphere is very sensitive to changes of climate and air quality including that of aerosol particles, which scatter solar light and contribute to cloud formation. For the evaluation of behaviors of aerosol particles, it is specifically important to clarify the background behaviors at the region without an effect of local human emission sources. Previous studies on background aerosol have been carried out at polar regions (1-3) and mid-ocean regions (4). At Antarctica, a large number of sulfate and/or methanesulfonate aerosol particles have been observed in the summer season (5-7), and their generation mechanisms and their behaviors have been intensively studied along with their transport mechanisms and chemistry. For analysis of aerosol particles, most papers have been focused on their size distribution and their elemental compositions, for which atomic absorption spectrometry (AAS) (8-13) and ion chromatography (IC) (9-14) for bulk analysis and scanning electron microscopy-energy dispersive X-ray analysis (SEM-EDX) (6, 7, 15-17) for microprobe analysis have been applied. Bulk analysis offers mean concentrations and total amounts but not information on †
Science University of Tokyo. Nagoya University. § National Institute of Polar Research. ‡
0013-936X/96/0930-0385$12.00/0
1996 American Chemical Society
individual particles. It requires a certain amount of samples, for which a fairly long collection time is needed. On the other hand, microprobe analyses have often been utilized for the analysis of individual particles. SEM-EDX provides information of elemental composition of individual particles but hardly gives information of light elements, ones of the major constituents of aerosol particles, and their compositional states. Laser microprobe mass spectrometry (LAMMS) provides elemental and compositional information including light elements and is very suitable for characterization of aerosol particles. LAMMS has been applied for the analysis of Antarctic aerosol particles by Wouters et al. (18), of North Sea aerosol particles by Dierck et al. (19) and Bruynseels et al. (20), of aerosol particles in the Amazon Basin by Bruynseels et al. (21) and Wouters et al. (22), and of sea salt particles by Bruynseels et al. (23). All of these researchers used a transmission-type apparatus (LAMMA-500), by which Bruynseels et al. (24), Dennemont et al. (25), Kolaitis et al. (26), and Otten et al. (27) showed spectra of nitrogen compounds, such as ammonium and nitrate salts, as their reference materials. Meanwhile, a reflection-type apparatus (LAMMA-1000), which was utilized in this paper, has been used for the analysis of troposphere aerosol particles by Furuya et al. (28) and of urban aerosol particles by Nadahara et al. (29). All through the reports, no researchers studied the seasonal variation of elements in aerosol particles by LAMMS. Wouters et al. only reported the results on particles collected in the summer season (18) in Antarctica. The analytical problem of LAMMS is its poor quantitative reproducibility of spectrum intensity, and therefore, most studies remained unqualitative. For elemental analysis of Antarctic aerosol particles, Parungo et al. (3) and Harvey et al. (17) studied them by means of SEM-EDX, and Pereira et al. (30), Koide et al. (31), and Meanhaut et al. (8) studied them by neutron activation analysis (NAA) and AAS. Most of these papers are on comparative studies of summer and winter aerosol particles. Methanesulfonate, sulfate, and elemental carbon have been studied for their seasonal variations. Prospero et al. reported their observation of sulfates (5), and Wagenbach et al. reported seasonal variations in sea salt and sulfate by means of IC and AAS (12). Hansen et al. (32) dealt with the behavior of elemental carbon. However, there are no comprehensive reports on the seasonal variation of each constituent throughout the year. The quantitative estimation by means of LAMMS is possible when an abundance of characteristic peaks detected from spectra for more than 100 individual particles is obtained. This paper intends to clarify the seasonal behavior of constituents of Antarctic aerosol particles by LAMMS collected with a high-volume air sampler at the Syowa station for every season throughout 1 year.
Experimental Section Instruments. A reflection-type laser microprobe mass spectrometer, LAMMA-1000 (Leybold-Heraeus GmbH), was used for the measurement. Individual aerosol particles collected on an aluminum stage were ionized by a laser pulse beam (6 ns) of Q-switched Nd-YAG laser (266 nm)
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FIGURE 1. Location of Syowa station and sampling site.
focused about 2 µm in diameter, and the generated ions were perpendicularly introduced into a time-of-flight type mass spectrometer, where the ions were separated according to their m/z ratios. The method offers individual particle analysis (>2 µm) with very high sensitivity for all elements for elemental, fragmental, and molecular information with regulated laser power density. For measurements of the samples, the laser energy density was maintained at slightly over the threshold of ionization in order to obtain fragmental and molecular information. Because of poor reproducibility of LAMMS spectra, the relative abundance was used as the denominator in this study. A fixed number of individual particles visually focused were shot, and the ratios of the frequency of detected characteristic peaks for a specific component (e.g., SOn, Pb...) to the total analyzed particle number was defined as relative abundance of the species; hence, no matrix effect of poor reproducibility was encountered. Thus, the relative abundance means the abundance of particles that contains the species of concern at more than their detection limit vs the total analyzed particles on the stage. However, attention should be paid to the fact that the relative abundance does not necessarily mean the absolute concentration of the species in the atmosphere, because the total amount of particles collected was different for each sampling and the amounts of particles on each stage were too small to be weighed. The discussion in this paper used the relative abundance as the denominator but not as their absolute concentration in the atmosphere. The relative error of the abundance in this paper was less than 2% against 160 shots. Interpretation of LAMMS spectra is manually done by comparison with reference spectra. Sample. Because the ionization mechanism is still unknown, it is difficult to identify unknown samples directly. For interpretation of sample spectra, pure reagents were used as the reference materials. Preparation of Reference Samples. An aliquot of a saturated solution of a reference reagent prepared with Milli-Q water was placed on an aluminum foil substrate and dried in vacuum set on a sample stage of the LAMMS apparatus. Soil samples (20-50 µm) as references were fixed with double adhesive tape on aluminum foil after rinsing with water.
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TABLE 1
Detailed Information about Aerosol Particle Collection date Feb 17-18 Feb 20 Mar 19 Apr 16-17 Apr 28-29 May 13-14 June 8-9 July 28-29 Aug 29-30 Sep 26-27 Oct 28-29 Nov 28-29 Dec 29-31
time (min)
vol (m3)
1076 586 855 513 637 380 1165 696 794 474 1430 858 481 282 712 425 1600 968 1410 853 1395 1435 861 2479 1360
av wind velocity (m/s) 1.9 4.7 6.4 3.9 7.5 9.2 10.5 12.7 5.9 3.0 5.4 8.2 5.7
comments
blizzard drifting snow
partly open ground
Collection of Aerosol Particles. A five-stage high-volume air sampler (Kimoto Elec. Ind. Inc.; CPS-105) with commercial cooking aluminum foils as a collection medium was used for sample collection. The aerodynamic diameters of particles on each stage were >10.9, 10.9-5.4, 5.4-1.6, 1.6-0.7, and
