Article pubs.acs.org/est
Chemical Structural Characteristics of HULIS and Other Fractionated Organic Matter in Urban Aerosols: Results from Mass Spectral and FT-IR Analysis Qingcai Chen,† Fumikazu Ikemori,†,‡ Hayato Higo,§ Daichi Asakawa,∥ and Michihiro Mochida*,† †
Graduate School of Environmental Studies, Nagoya University, Nagoya, Japan Nagoya City Institute for Environmental Sciences, Nagoya, Japan § Fukuoka City HAKATA Health & Welfare Center, Fukuoka, Japan ∥ Osaka City Institute of Public Health and Environmental Sciences, Osaka, Japan ‡
S Supporting Information *
ABSTRACT: The chemical characteristics of complex organic matter in atmospheric aerosols remain poorly understood. Waterinsoluble organic matter (WISOM) and water-soluble organic matter (WSOM) in the total suspended particulates collected in the city of Nagoya in summer/early autumn and winter were extracted using multiple solvents. Two fractions of humic-like substances, showing neutral and acidic behavior (HULIS-n and HULIS-a, respectively), and the remaining highly polar part (HPWSOM) were fractionated from WSOM using solid phase extraction. The chemical structural characteristics and concentrations of the organic matter were investigated using mass spectrometry and Fourier transform infrared (FT-IR) spectroscopy. WISOM and HULIS-n had low O/C ratios (0.1 and 0.4, respectively) and accounted for a large fraction of the organics in aerosols (70%). HULIS-a and HP-WSOM had higher O/C ratios (0.7 and 1.0, respectively), and their concentrations in summer and early autumn were on average ∼2 times higher than those in winter. The mass spectrum and FT-IR analyses suggest the following: (1) WISOM were high-molecular-weight aliphatics (primarily C27−C32) with small proportions of −CH3, −OH, and CO groups; (2) HULIS-n was abundant in aliphatic structures and hydroxyl groups (primarily C9−C18) and by branched structures; (3) HULIS-a and HP-WSOM contained relatively large amounts of low-molecular-weight carboxylic acids and alcohols (primarily C4−C10); and (4) WISOM and HULIS-n were relatively abundant in amines and organic nitrates.
■
organic matter (WSOM).8,14−21 Furthermore, solid-phase extraction (SPE) has been used in some cases to isolate humic-like substances (HULIS) from WSOM based on the polarity and/or acidity of these substances.9,22−28 Among organic fractions, WSOM and HULIS have been subjected to chemical characterization, and their properties have been investigated in a number of studies.23−28 However, their entire structural characteristics are not well understood. Furthermore, water-insoluble organic matter (WISOM) and the fraction of WSOM other than HULIS remain poorly characterized, despite the fact that they constitute a large fraction of organics in aerosols.11,12 The non-HULIS WSOM is polar and may be closely linked to aerosol hygroscopicity.9,26 In the case of WISOM, the significant light absorptivity and toxicity have been addressed.14−16 The characterization of organic fractions
INTRODUCTION Organic compounds constitute a large fraction of the aerosol particles in the atmosphere (20−90%).1,2 Their presence influences the properties of aerosols (e.g., hygroscopicity, light-absorptivity, and toxicity) thereby affects the role of aerosols in climate and human health.3 Characterization of the chemical structures of organic mixtures is important for understanding the sources of these organics and their contribution to aerosol properties. However, these organics remain poorly characterized because of their highly complex composition.4−6 An effective means of understanding the chemical structures of highly complex organic compounds is the fractionation of organic aerosol components according to their physicochemical characteristics (e.g., volatility, solubility, and polarity).7−13 Solvent extraction and solid-phase extraction (SPE) are methods used to extract the organic component of aerosols.8−28 Among the solvent-based methods used to fractionate organics, methods based on differences in water solubility have been widely utilized to fractionate water-soluble © XXXX American Chemical Society
Received: October 27, 2015 Revised: January 8, 2016 Accepted: January 15, 2016
A
DOI: 10.1021/acs.est.5b05277 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology
high polarity (HP-) WSOM were fractionated from the WSOM fraction using an Oasis HLB (6 cc, 200 mg; Waters) in a manner analogous to that described in Varga et al. and Lin et al. (SI).9,24 Briefly, we employed a two-step method to separate WSOM into the three groups. The HULIS-n and HULIS-a were eluted from Oasis HLB cartridges in the first and second steps with 2 mL of MeOH containing 2 wt % of ammonia (Fluka, for trace analysis) and 6 mL of MeOH, respectively. The extract of HULIS-n was dried under N2 and then redissolved in 6 g of MeOH. HP-WSOM consisted of organics in the flow-through, which passed through the cartridges without adsorption owing to their high polarity. With the adjustment of pH, as described above, the organic acids in HULIS-a, if present, should primarily have acid ionization constants (pKa) of 2−7. HR-AMS Measurements. Each extract was atomized with pure air (CO, CO2 and THC: 1, CHON, and CS. The elemental analyses to determine the molar elemental ratios (O/C, H/C, and N/C) and the mass ratio of organic matter (OM) to organic carbon (OC) were performed as described in Aiken et al.38 (for N/C) and Canagaratna et al.39 (for O/C, H/C, and OM/OC). Using the O/C and H/C ratios, the densities of the extracted organic fractions were estimated.40 Determination of Molecular-Weight Distribution. A high-performance quadrupole time-of-flight mass spectrometer (micrOTOF-QII, Bruker) equipped with either of two soft ionization sources, electrospray (ESI) or atmospheric pressure chemical ionization (APCI), was employed to investigate the MWD of fractionated organics. The extracts were directly injected from a syringe pump into an ionization source. The spray voltage was set to −3.5 and 4.5 kV for the ionization of organics in negative and positive modes, respectively. The signals of ions with mass-to-charge ratios (m/z) in the range of 50−2000 were recorded for ∼8 min and preprocessed to eliminate some peaks with a signal-to-noise ratio (S/N) ≤ 10 or those present in blank samples. The m/z axis was calibrated using ESI Taning mix and APCI-Low concentration Taning mix (Agilent Technologies). The accuracy of the mass was
