Environ. Sci. Technol. 2001, 35, 2656-2664
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H NMR Application for Characterizing Water-Soluble Organic Compounds in Urban Atmospheric Particles
Y U K I O S U Z U K I , * ,† MASARU KAWAKAMI,‡ AND KAZUYUKI AKASAKA‡ Kobe Institute of Health, 4-6 Minatojimo-nakamachi, Chuo-ku, Kobe-shi, Hyogo 650m, Japan, The Graduate School of Science and Technology, Kobe University, 1-1, Rokkodaicho, Nada-ku, Kobe-shi, Hyogo 657m, Japan
Water-soluble organic compounds (WSOC) in urban atmospheric particles separated by particle size were analyzed by 1H NMR. This is the first utilization of 1H NMR for characterizing WSOC in atmospheric particles. The WSOC dissolved in D2O were analyzed without a separation procedure. Twelve low molecular weight WSOC could be identified and their atmospheric concentrations determined. One of these, monomethyl hydrogen sulfate (MHS), was first detected in an urban area where no oil or coal power plant existed. Methanesulfonic acid (MSA) and hydroxymethanesulfonic acid (HMSA) were detected as major organosulfur compounds. Relatively high concentrations of these low molecular weight WSOC were observed in the particle diameter range of 0.43-1.1 µm. Many complex signals at 3-4 ppm in the NMR spectrum were seen only for the coarse particle samples (1.1 µm < particle diameter). Mannitol was believed to exist in the coarse particles as a major polyol corresponding to these signals. On the other hand, a large broad signal, observed at 2.5-3 ppm, was mostly present in the fine particles. Finally, it was believed that a major part of the WSOC in urban atmospheric fine particles is attributed to ketocarboxylic acids, ketodicarboxylic acids, and dicarboxylic acids.
Introduction Water-soluble compounds in atmospheric particles have been confirmed to act as cloud condensation nuclei due to their hygroscopic properties. The hygroscopic particles readily absorb water vapor in the atmosphere at high relative humidities (1). Their molecular composition, therefore, is important from the standpoint of understanding their sources, their impact on human health upon inhalation, the wet scavenging of atmospheric particles, and their role in the formation of hazes and the modulation of the earth’s radiative balance (2). The dominant hygroscopic components in the atmospheric particles are inorganic ions such as sulfate, nitrate, chloride, sodium, and ammonium. In addition, some investigators have estimated that organics typically account for 20-50% of the total fine particles and the proportion of the water-soluble organic carbon to total carbon in the * Corresponding author telephone: +81-78-302-4321; fax: +8178-302-0894; e-mail:
[email protected]. † Kobe Institute of Health. ‡ Kobe University. 2656
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 13, 2001
atmospheric particles is ∼20-67% (2). Therefore, the watersoluble organic compounds (WSOC) cannot be considered to be negligible in discussions of the composition of the watersoluble compounds in atmospheric particles. Hydrocarbons such as alkanes, alkenes, and polyaromatic hydrocarbons in the atmospheric particles are usually determined by GC-MS. The determination of WSOC by GCMS, however, requires conversion of the polar compounds to nonpolar ones prior to the GC injection. Derivatization aimed at the dicarboxylic acids, ketoacids, and dicarbonyls was successful by reacting them with reagents (3, 4). Although the monocarboxlyic, dicarboxylic, and aromatic carboxylic acids and carbonyls were identified by GC-MS, the composition of multifunctional or more ionic compounds remains poorly characterized. Fourier transform infrared spectroscopy (FTIR) has often been used for specifying the functional groups, and the presence of aliphatic, carbonyl, organosulfur, and organonitrate groups has already been elucidated in atmospheric particles (5-7). FTIR may support the limitation of GC-MS. On the other hand, ionic compounds can be identified by ion chromatography (IC). However, IC is rarely used for analyzing the organic ions in atmospheric particles because it may be difficult to analyze the organic ions due to the interference of the relatively large amount of inorganic ions coexisting in the particles. 1H NMR is an effective technique for analyzing the structure of macromolecular compounds such as proteins. In recent environmental studies, the structures of humic and fulvic acids in river water samples have been analyzed (8). There may be two reasons the NMR method has not been frequently applied to environmental samples. One is the necessary preparation of a pure and single compound for assigning the structure. The other is that the NMR sensitivity is too low to quantify ultratrace amounts in environmental samples. The NMR method, however, has several merits. First, the 1H NMR spectrum is not susceptible to inorganic ions. Second, 1H NMR can measure the spectrum using a small amount (
