ARTICLE pubs.acs.org/est
Characterization of Solvent-Extractable Organics in Urban Aerosols Based on Mass Spectrum Analysis and Hygroscopic Growth Measurement Toshiyuki Mihara† and Michihiro Mochida*,‡,§ †
Department of Earth and Environmental Science, Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan ‡ Institute for Advanced Research, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan
bS Supporting Information ABSTRACT: To characterize atmospheric particulate organics with respect to polarity, aerosol samples collected on filters in the urban area of Nagoya, Japan, in 2009 were extracted using water, methanol, and ethyl acetate. The extracts were atomized and analyzed using a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) and a hygroscopicity tandem differential mobility analyzer. The atmospheric concentrations of the extracted organics were determined using phthalic acid as a reference material. Comparison of the organic carbon concentrations measured using a carbon analyzer and the HR-ToF-AMS suggests that organics extracted with water (WSOM) and ethyl acetate (EASOM) or those extracted with methanol (MSOM) comprise the greater part of total organics. The oxygencarbon ratios (O/C) of the extracted organics varied: 0.510.75 (WSOM), 0.370.48 (MSOM), and 0.270.33 (EASOM). In the ion-group analysis, WSOM, MSOM, and EASOM were clearly characterized by the different fractions of the CH and CO2 groups. On the basis of the hygroscopic growth measurements of the extracts, k of organics at 90% relative humidity (korg) were estimated. Positive correlation of korg with O/C (r 0.70) was found for MSOM and EASOM, but no clear correlation was found for WSOM.
1. INTRODUCTION Atmospheric particulate organics are a major aerosol component, accounting for 1070% of the total aerosol mass.1 For this reason, their characterization is important to elucidate the role of aerosols in visibility, climate, and human health.1,2 Particulate organics comprise myriad species, among which only about 1020% in a bulk sample are identifiable on a molecular level.1,3 Chemical characterization of a wider fraction of organics therefore remains as an important task to elucidate the key roles of atmospheric aerosol particles, such as their role as cloud condensation nuclei (CCN). Evidence of the influence of organic aerosol aging on changes in chemical and physical properties has been reported.47 Therefore, characterization of unidentified and identified organics in view of aging is important.6,7 Chemical characterization of a broader range of particulate organics has been undertaken using several approaches with assessments of functional groups, degree of oxidation, volatility, and solubility in solvents.2,411 Russell et al. investigated the functional groups of organics using FTIR spectroscopy, reporting that the CCN activity correlated with the oxygencarbon ratios (O/C).8 Polidori et al. used FTIR spectroscopy to determine the organic mass (OM) to organic carbon (OC) ratios of organics fractionated by extraction with solvents having different polarities.2 Zhang et al. reported the characterization of organics based on results of factor analysis using an aerosol mass spectrometer (AMS).11 These recent studies have provided information that is useful to clarify the behavior of particulate organics in the atmosphere. More detailed characterization is r 2011 American Chemical Society
necessary to clarify the characteristics of organics further and to elucidate how those characteristics affect the particle properties related to atmospheric processes. For this study, using multiple solvents with different polarities, we extracted organics from ambient particles collected on filters in the urban area of Nagoya, Japan. We investigated them using a high-resolution time-of-flight aerosol mass spectrometer (HRToF-AMS) and a hygroscopicity tandem differential mobility analyzer (HTDMA). The chemical compositions of the extracts were determined. Furthermore, elemental analysis of organics and ion-series analysis combined with ion-group analysis using high-resolution (HR) mass spectra of organics were conducted to assess the characteristics of the chemical structures. Although AMS is commonly used in online aerosol measurements,12 results of this study demonstrate its high applicability to off-line analysis of atmospheric organic aerosol components. In addition, we assessed the hygroscopicity of solvent-extractable organics in light of AMS-derived characteristics.
2. EXPERIMENTAL SECTION 2.1. Aerosol Sampling. Eight aerosol samples were collected on quartz fiber filters (25 20 cm) for 7296 h using a Received: April 14, 2011 Accepted: August 30, 2011 Revised: August 9, 2011 Published: August 30, 2011 9168
dx.doi.org/10.1021/es201271w | Environ. Sci. Technol. 2011, 45, 9168–9174
Environmental Science & Technology
ARTICLE
Figure 1. Schematic representation of different organic fractions. WSOM, MSOM, and EASOM are the organic fractions extracted from a filter sample. WS-EASOM is the organic fraction extracted from EASOM with water. Blue boxes show the organic fractions measured directly using HR-ToF-AMS. White boxes show the organic fractions calculated from the differences of two measured fractions. Gray box shows EOM, which is similar to MSOM (see section S8 in the Supporting Information).
high-volume air sampler (ca. 1100 L min1, model 120B; Kimoto Electric Co. Ltd.) equipped with a cascade impactor (50% cutoff diameter, 0.95 μm, TE-230; Tisch Environmental, Inc.) on the balcony (10 m agl) of a building at Nagoya University in Nagoya, Japan (35°090 N and 136°580 E) during spring and summer of 2009. Filter punches (9.1 cm2 3) were ultrasonicated with 3 g of a solvent three times (10 min each) to prepare the extract. The solvents used were water (Fluka/Sigma-Aldrich Corp.), methanol (Wako Pure Chemical Inds. Ltd.), and ethyl acetate (Wako), whose solvent polarity indices are, respectively, 9.0, 6.6, and 4.3.2 We also prepared a water-soluble (WS) fraction in ethyl acetate extract (WS-EASM) by concentrating an ethyl acetate extract (EASM) using a rotary evaporator, drying it under a N2 flow, and dissolving the WS fraction in the residue with 2.5 g of water. Definitions of different organic fractions are presented in Figure 1. All definitions and abbreviations of organic fractions are summarized in section S1 in the Supporting Information. Aerosols were generated from each extract (3 mL) using an atomizer (see section S2, Supporting Information). The aerosol was passed through a prehumidifier to allow for possible compaction of the particles because some atomized aerosol particles might be aspherical or porous in a dry condition. The solvents in the aerosols were removed using diffusion-dryer-type scrubbers filled with (1) activated carbon (Wako) mixed with silica gel (only used for HTDMA measurements of aerosols from extracts with organic solvents), (2) silica gel (Wako), and (3) molecular sieves (13X/4A; Supelco and Sigma-Aldrich). We expected the removal of the vapor of organic solvents by activated carbon and of water by silica gel and molecular sieves. The dry aerosol (
