Identification of Fulvic Acids and Sulfated and Nitrated Analogues in

Anal. Chem. 2006, 78, 8299-8304

Identification of Fulvic Acids and Sulfated and Nitrated Analogues in Atmospheric Aerosol by Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Thorsten Reemtsma,*,† Anja These,† Prasanna Venkatachari,‡ Xiaoyan Xia,‡ Phillip K. Hopke,‡ Andreas Springer,§ and Michael Linscheid§

Department of Water Quality Control, Technical University of Berlin, Sekr KF 4, Strasse des 17 Juni 135, 10623 Berlin, Germany, Center for Air Resources Engineering and Science, Clarkson University, Box 5708, Potsdam, New York 13699-5708, and Department of Chemistry, Humboldt-Universita¨t zu Berlin, Brook-Taylor-Strasse 2, 12489 Berlin, Germany

The water-soluble organic fractions of aerosol samples collected in Riverside, CA, in summer 2005 were analyzed by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). Elemental compositions of about 1000 molecular species were determined in the range m/z 220-420, and four series of organic compounds were identified, fulvic acids, and S-containing, N-containing, and S- and N-containing molecules. Low-resolution product ion spectra proved the presence of organosulfates, organonitrates, and mixed organosulfates and -nitrates that appear to be structurally closely related to each other and to the fulvic acids. This is the first unambiguous detection of fulvic acid molecules and sulfated components in atmospheric aerosol and the first detection even of nitrated analogues. These species provide new clues to the nature of particulate organic matter in atmospheric aerosol. Secondary organic aerosol (SOA), i.e., organic aerosol material that is formed in the atmosphere from low molecular weight components, is an important constituent of atmospheric aerosol. In addition to the extra particulate mass, SOA may function as a sink for those volatile compounds from which it is formed, and upon formation, SOA may significantly change the properties of an aerosol and, thus, indirectly affect the weather and climate.1 Therefore, the formation of SOA has been investigated intensively using field samples and laboratory experiments with photochemical chambers.2-7 * To whom correspondence should be addressed. Phone: +49-30-31426429. Fax: +49-30-31423850. E-mail: [email protected]. † Technical University of Berlin. ‡ Clarkson University. § Humboldt-Universita¨t zu Berlin. (1) Fuzzi, S.; Andreae, M. O.; Huebert, B. J.; Kumala, M.; Bond, T. C.; Boy, M.; Doherty, S. J.; Guenther, A.; Knakidou, M.; Kawamura, K.; Kerminen, V.M.; Lohmann, U.; Russell, L. M.; Po¨schl, U. Atmos. Chem. Phys. 2006, 6, 2017-2038. (2) Limbeck, A.; Kulmala, M.; Puxbaum, H. Geophys. Res. Lett. 2003, 30, ASC6-1 - 6-4. (3) Koehler, C. A.; Fillo, J. D.; Ries, K. A.; Sanchez, J. T.; de Haan, D. O. Environ. Sci. Technol. 2004, 38, 5064-5072. 10.1021/ac061320p CCC: $33.50 Published on Web 11/16/2006

© 2006 American Chemical Society

A portion of the water-soluble fraction of organic aerosol has been termed “humic-like substances” (HULISs) due to its similarity to humic substances from soil and water in many respects (reviewed in ref 8). Using liquid chromatography-mass spectrometry (LC-MS), products of SOA formation in laboratory experiments have also been characterized7,9,10 Some oligomeric patterns were identified from different precursor components4,9,10 and MS/MS experiments showed decarboxylations and losses of water as important fragmentation processes of SOA,9 similar to the behavior of fulvic acids. Electrospray ionization (ESI) high-resolution mass spectrometry using either time-of-flight (TOF-MS) with a mass resolution above 1000011 or Fourier transform ion cyclotron resonance (FTICR-MS) with a mass resolution of 100000 and above has been sucessfully used to determine elemental compositions of fulvic acid molecules12-14 and other natural organic matter (NOM)15 isolated from aquatic compartments and recently in an arctic ice core.16 In this paper we report the results of the first application of FTICR-MS to the water-soluble fraction of organic aerosol. On (4) Kalberer, M.; Paulsen, D.; Sax, M.; Steinbacher, M.; Dommen, J.; Prevot, A. S. H.; Fisseha, F.; Weingartner, E.; Frankevich, V.; Zenobi, R.; Baltensperger, U. Science 2004, 303, 1659-1662. (5) Joutsensaari, J.; Toivonen, T.; Vaattovaara, P.; Vesterinen, M.; Vepsa¨la¨inen, J.; Laaksonen, A. Aerosol Sci. 2004, 35, 851-867. (6) Liggio, J.; Li, S.-M.; McLaren, R. Environ. Sci. Technol. 2005, 39, 15321541. (7) Docherty, K. S.; Ziemann, P. J. J. Phys. Chem. A 2006, 110, 3567-3577. (8) Graber, E. R.; Rudich, Y. Atmos. Chem. Phys. 2006, 6, 729-753. (9) Iinuma, Y.; Bo ¨ge, O.; Gnauk, T.; Herrmann, H. Atmos. Environ. 2044, 38, 761-773. (10) Tolocka, M. P.; Jang, M.; Ginter, J. M.; Cox, F. J.; Kamens, R. M.; Johnston, M. V. Environ. Sci. Technol. 2004, 38, 1428-1434. (11) These, A.; Winkler, M.; Thomas, C.; Reemtsma, T. Rapid Commun. Mass Spectrom. 2004, 18, 1777-1786. (12) Kujawinski, E. B.; Hatcher, P. G.; Freitas, M. A. Anal. Chem. 2002, 74, 413-419. (13) Stenson, A. C.; Marshall, A. G.; Cooper, W. T. Anal. Chem. 2003, 75, 12751284. (14) Reemtsma, T.; These, A.; Springer, A.; Linscheid, M. Environ. Sci. Technol. 2006, 40, 5839-5845. (15) Koch, B. P.; Witt, M.; Engbrodt, R.; Dittmar, T.; Kattner, G. Geochim. Cosmochim. Acta 2005, 69, 3299-3308. (16) Grannas, A. M.; Hockaday, W. C.; Hatcher, P. G.; Thompson, L. G.; MosleyThompson, E. J. Geophys. Res. 2006, 111, D04304.

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the basis of the mass spectral data, four classes of organic material could be identified in the aerosol that appear to be part of the SOA formed in the atmosphere. MATERIALS AND METHODS Sampling. Samples were collected for 24 h in Riverside, CA, during July and early August 2005 using a TISCH 1000 highvolume sampler with a 2.5 µm cut-point inlet at a flow rate of 220 L min-1. The samples were collected on a 90 mm baked quartz fiber filter (Pall Tissuequartz, Pall Corp., East Hills, NY). All of the filters were baked at 550 °C for 12 h. After sampling, the samples were stored in the freezer at -10 °C. Preparation of Extracts. The water-soluble organic compounds (WSOCs) of four aerosol samples were obtained as follows: The quartz filter was placed in a glass vial and eluted with 50 mL of ultrapure water by ultrasonification for 60 min. The eluates were filtered through 0.45 µm membrane filters (cellulose nitrate) and acidified with HCl to pH 2. The acidified eluates were then extracted by sequential solid-phase extraction using two preconditioned SPE cartridges [(a) a C18 SPE (500 mg, LiChrolut RP-18, Merck, Darmstadt, Germany) and (b) a 60 mg OASIS HLB (Waters)]. SPE cartridge a was eluted with 6 mL of MeOH and b with 6 mL of MeOH/acetone, 6/4. The eluates were reduced in volume to about 1 mL. The eluates were (a) yellow-brown and (b) almost colorless. FTICR-MS Analysis. All extracts and a Suwannee River fulvic acid (SRFA) standard of the International Humic Substances Society (IHSS) were subjected to electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry using a Finnigan LTQ FTICR-MS (Thermo Electron Co., Bremen, Germany) linear ion trap FTICR hybrid mass spectrometer with a 6 T superconducting magnet. Instrumental settings were as follows: spray voltage, -4.3 kV; shealth gas, 15 arbitrary units; transfer capillary temperature, 230 °C; transfer capillary voltage, -47 V; tube lens voltage, -190 V; number of injected ions (ICR cell), (1-3) × 106. In all experiments negative ions were aquired with a mass resolution of 100000 (m/ ∆m at fwhm for m/z 400). The instrument was externally calibrated each day by the standard procedure using ascorbic acid and Ultramark 1620 as calibrants. MS data were analyzed with Xcalibur version 1.4 SR1. Of the aerosol eluate only the C18 extract yielded measurable ions, while no useful signals were detected in the second extract (OASIS HLB). Therefore, only the C18 extract is considered. Product and Precursor Ion Spectra. Low-resolution product ion spectra were recorded by infusion of the methanolic solutions into a triple-quadrupole mass spectrometer (Quattro LC, Micromass, Manchester, U.K.) operated in the negative ion mode and using electrospray ionization. The capillary voltage was 2.9 kV, cone voltage 25 V, source temperature 120 °C, desolvation temperature 100 °C, nebulizer gas flow 100 L/h, and drying gas flow 600 L/h. Argon pressure in the collision cell was kept at 1.0 × 10-3 bar, and the collision energy was varied between 12 and 25 eV. Precursor ions of m/z 63 and m/z 97 for the selective detection of nitrated and sulfated compounds were recorded at similar conditions with a collision energy of 18 eV. To check whether the sulfuric acid and nitric acid esters detected by FTICR-MS as well as by triple-quadrupole MS were real molecules or only adducts generated from the inorganic 8300

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anions and fulvic acids during the electrospray process, equimolar concentrations of either sulfuric acid or nitric acid or both were added to a 1 g/L solution of SRFA. After storage for 1 day these samples were analyzed by a triple-quadrupole mass spectrometer in the scan mode, and no changes were recognized in comparison to the pure SRFA solution. Additionally, neither sulfated nor nitrated derivatives/adducts were detected in the precursor ion mode. Thus, the possibility that those S- and N-containing molecules detected by mass spectrometry were adducts rather than molecules can be excluded. Calculation of Elemental Composition. Molecular formula calculations were performed from the FTICR-MS data in a twostep process from a composite spectrum obtained by combining signals with 0.05-5% relative signal intensity over a recording time of 3 min (approximately 100 spectra). Initially, only C, H, and O were allowed and the formulas of the fulvic acids determined over the mass range of m/z 220 to m/z 420. In this composite spectrum the difference between the measured and the calculated masses increased linearily from -0.3 mmu for the lower end up to -1.5 mmu at the higher end of the mass range. The fulvic acid signals occurring at most odd m/z values were then used as internal calibrants to ascribe elemental compositions to many other signals of odd and even mass anions. For these calculations C, H, O, N, and S were allowed. Of the formulas proposed by the Xcalibur software, the one with the lowest mass deviation relative to the calibrant signals was selected. In these internally calibrated spectra the deviation of most ions remained below (0.5 mmu. Identification was further supported by the regular 0.0364 amu distance found for most molecular series at one nominal mass. RESULTS AND DISCUSSION To eludicate the chemical nature of water-soluble organic compounds in the PM2.5 aerosol samples collected in Riverside, CA, the C18 extracts of the aqueous eluates of four samples were analyzed by FTICR-MS in the mass range m/z 200 to m/z 600. These mass spectra were dominated by a few strong signals, but we focused on the vast majority of “background” signals occupying each nominal mass in a mass range m/z 200-420. The mass spectral data for all four samples largely agreed, and therefore, only one of them will be discussed in more detail in this paper. Using the high resolving power of FTICR-MS, it became obvious that each of the nominal masses is occupied by several signals (Figure 1). The characteristic signal pattern known from fulvic acids that solely consist of C, H, and O was utilized for internal calibration of the mass spectra. The mass deviation between calculated and detected masses after internal calibration ranged from 0.0 to -0.2 mmu for all ions shown in Figure 1, while the mass deviation for the C-, H-, and O-containing fulvic acids before internal calibration ranged from -0.6 mmu (m/z 293, Figure 1a) to -0.9 mmu (m/z 341, closest odd anion mass for Figure 1c). Following internal calibration, molecular formulas for about 1000 signals in the selected mass range were calculated. Besides the fulvic acids that consist solely of C, H, and O, the high-resolution mass spectra in the low mass range clearly indicated the presence of S-containing compounds, with a mass difference from the closest non-sulfur-containing anions of only +0.0034 amu (-3C + 4H + S) (Figure 1a). Nitrogenous molecules were detected, predominantly at the even anion masses (Figure 1b,c).

Figure 1. Extension of an internally calibrated ESI-FTICR mass spectrum of the aerosol sample showing examples of molecular anions ([M - H]-) of (a) fulvic acids and S compounds at odd anion masses, (b) N compounds at even anion masses, and (c) S + N compounds at even anion masses.

Figure 2. Reconstructed mass spectra showing the signal intensity of the organic molecules identified in the aerosol sample as (a) fulvic acids and (b) S1-containing, (c) N1-containing, and (d) S1N1-containing molecules. Most lines represent several ions of the same integer m/z value.

These data were arranged in four series: (a) approximately 460 molecules containing only C, H, and O and no heteroatoms, (b) 210 molecules additionally containing sulfur, (c) 230 molecules containing C, H, O, and nitrogen, and (d) 110 molecules containing sulfur and nitrogen. The reconstructed mass spectra show the signal intensity of all members of each group (Figure 2). All of them exhibit the same pattern of increasing and decreasing intensities having an average period of 14 mass units. Fulvic Acids. The 460 molecular formulas that consist solely of C, H, and O are identical to molecular series found in aquatic fulvic acids. The formulas agree, as well as the relative intensity patterns at the level of the integer masses (average periodicity of 14 amu, Figure 2a) and within the ions of the same integer mass (mass distance of 0.0364 amu, Figure 1a).13-15 Patterns similar to the regular intensity fluctuation in the mass spectrum in Figure 2a have been called “oligomeric patterns” in other mass spectrometric investigations of organic aerosol components. In this case,

however, it is not an oligomeric pattern in the sense that each period (approximately 14 amu) would reflect one of the monomeric subunits from which the oligomeric organic aerosol was formed. Rather this periodicity is based on the stepwise increase in mass due to the inclusion of one additional carbon (alkyl homologues) or oxygen in the fulvic acid molecules. This result can be deduced from Figure 3a, in which the carbon number of each molecule is plotted against its mass. In this diagram all of the molecules for which the sum of carbon and oxygen atoms is constant are arranged in an islandlike pattern. Within such an island horizontal series represent hydrogen homologues (+2.0157 amu). Simultaneously, the vertical direction shows the exchange of one oxygen against one carbon atom and four hydrogens, resulting in the characteristic mass increase of 0.0364 amu.13 The distance between the intensity maxima of two subsequent islands is mostly 14 amu (Figure 2a) and originates Analytical Chemistry, Vol. 78, No. 24, December 15, 2006

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Figure 3. Number of carbon atoms per molecule versus their mass for (a) fulvic acids, (b) S1 molecules, (c) N1 molecules, and (d) S1N1 molecules identified in the aerosol sample. Molecules with the same total number of C and O atoms are marked by the same symbol and denoted as one “island” (legend in (d)). The enlarged points in island 19 correspond to molecules in Table 1.

from the addition of either CH2 (14.016 amu) or O - 2H (13.979 amu). The same pattern has been detected in fulvic acid isolates of different origins14 (Figure 3a). Additional support for the thesis that these molecules are, indeed, fulvic acid molecules comes from low-resolution product ion mass spectra (Figure 4a) that provide evidence for the structure of these molecules. The same fragmentation patterns known for fulvic acids, namely, the series of decarboxylations (-44) with a parallel loss of water (-18)11,13 are visible. To date this scarcity of fragments was observed exclusively for fulvic acids and was interpreted as an indication of the uniformity of their molecular structure.11,14 On this basis we conclude that not only HULISs but also real fulvic acids occur in the atmospheric aerosol. A high degree of similarity between HULISs and fulvic acids has been shown in previous mass spectrometric investigations (reviewed in ref 8). However, only the elemental formulas derived here from highresolution mass spectra together with the structure information in the product ion spectra can prove the identity between certain HULIS components of aerosol and aquatic fulvic acids. This is the first identification of fulvic acids outside the soil/water environment. This identification of fulvic acids by a combination of mass spectrometric methods and comparison with reference material does not follow the traditional definition of fulvic acids that is based on a defined enrichment and fractionation procedure.17 Using that traditional approach, however, it is impossible to verify the identify of fulvic acid molecules in the various isolates. Sulfur Compounds. The sulfur-containing molecules show an integer mass intensity pattern similar to that of the fulvic acids (Figure 2b) and the same pattern at the level of isobaric masses, i.e., series of components with a mass distance of 0.0364 amu (17) IHSS Home Page. http://www.ihss.gatech.edu/. (Accessed Nov 10, 2006).

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(Figure 1a). Correspondingly, they also arrange in islands like the fulvic acids (Figure 3b). Compared to the aerosol fulvic acids

Figure 4. Product ion spectra obtained by ESI-MS/MS from the aerosol sample. Each product ion spectrum shows the fragment ions of all isobaric precursor ions: (a) m/z 269, almost pure fulvic acid molecules; (b) m/z 293 with S1 compounds of highest intensity; and (c) m/z 296 with S1N1 compounds of highest intensity.

Table 1. Most Intensive Ions of Island 19 (∑O+C ) 19)a formula C13H20O6 C12H22O7S1 C10H15O9N1 C10H19O9N1S1

detected mass diffb theor massc O/C H/C mass (m/z) (mmu) (amu) ratio ratio DBEd 271.1182 309.1012e 292.0674e 328.0707e

-0.51 -0.1e 0.0e -0.1e

272.1260 310.1068 293.0747 329.0781

0.46 0.58 0.90 0.90

1.54 1.83 1.50 1.90

4 2 4 2

a These maxima are highlighted in Figure 3. b Mass difference between detected and calculated m/z values of the anion [M - H]-. c Theoretical mass of the molecule. d DBEs ) double bond equivalents of the molecule. e After internal calibration of the spectrum with the closest fulvic acid anion.

(Figure 3a), sulfur-containing molecules with the same sum of carbon and oxygen per molecule (same island number) occur at higher masses, due to the inclusion of S (+32 amu) and the higher hydrogen content of these molecules. This mass shift indicates that sulfur occurs in an oxidized form: the inclusion of a reduced bivalent sulfur (thiol or thioether) into a non-sulfur-containing molecule would leave the H number unaltered, whereas with oxidized hexavalent sulfur species less oxygen remains available for the carbon skeleton (at a given sum of O and C) and relatively more hydrogen must be included. For the same reason the hydrogen homologue series are far less extended toward lower H numbers (low m/z values, Figure 3b). The number of double bond equivalents (DBEs) calculated from these molecular formulas indicates that most intense S1 compounds (Figure 3b) are all aliphatic (DBE < 4). Product ion spectra were recorded for more than 15 odd mass anions and looked basically similar. They provide evidence that some of the sulfur occurs as a sulfate moiety by the losses of 80 amu (-SO3 from m/z 293) and by the occurrence of m/z 97 (HSO4-) (Figure 4b). These fragmentations are in full agreement with recently published data of PM10 material from Switzerland.18 In some cases the m/z 80 ion is more intense than the m/z 97 ion, and thus, the presence of sulfonates moieties cannot be ruled out. However, the elemental composition of the S1 molecules (O/S ratio from 4 to 12 with a median of 6, Table 1) would allow the sulfur to occur as a sulfate moiety in all these 200 S1 molecules. A fragment ion of m/z 97 could also indicate organophosphates (H2PO4-),19 but the lack of m/z 79 that used to be much more prominent in organophosphates allows their occurrence to be excluded here.19 Organosulfates have been previously determined in natural aerosol by FT-IR spectroscopy20 as well as in SOA generated in laboratory experiments indirectly as sulfate by aerosol mass spectrometry6 and by a tandem differential mobility analyzer,5 but the nature of these compounds remains unknown. In a recent FTICR-MS study of NOM of an arctic ice core, 20% of the ions determined in a layer dated to the 1950s contained sulfur,16 and the authors assumed these compounds to be deposited after a long-range atmospheric transport. Esterification of alcohol moieties has been proposed as the likely process of organosulfate formation (18) Romero, F.; Oehme, M. J. Atmos. Chem. 2005, 52, 283-294. (19) Lamouroux, C.; Virelizier, H.; Moulin, C.; Tabet, J. C.; Jankowski, C. K. Anal. Chem. 2002, 72, 1186-1191. (20) Maria, S. F.; Russell, L. M.; Turpin, B. J.; Porcja, R. J.; Campos, T. L.; Weber, R. J.; Huebert, B. J. J. Geophys. Res., [Atmos.] 2003, 108, 8637.

Figure 5. Precursor ion spectra generated in a triple-quadrupole MS instrument from the aerosol sample: (a) precursors of m/z 97 showing sulfated molecules and (b) precursors of m/z 62 showing nitrated molecules.

in aerosol,5,6 but an electrophilic addition of H2SO4 to an aliphatic double bond or some radical process involving SO3 could also be possible. The mass spectrometric data suggest a close relationship between the S1 compounds and the fulvic acids determined in these aerosol samples, on the basis of the similarity of their elemental compositions, their intensity patterns (Figures 2a,b and 3a,b), and their structures. In addition to the fragments related to the sulfate-specific fragmentations, these molecules show no other fragmentation than the fulvic acids (Figure 4a,b). The triplequadrupole mass spectrometer was used to determine precursor ions of m/z 97. The precursor ion spectrum (Figure 5a) largely resembles the reconstructed mass spectrum of the S-containing anions (Figure 2b). In this way low-resolution MS data confirm the elemental calculations made on the basis of FTICR-MS data. Nitrogenous Compounds. Nitrogenous molecules were also detected over the whole mass range (Figure 2c), and molecular formulas for more than 200 of these molecules could be derived, about 10% of which contained 3 and more nitrogen atoms. The N1 compounds are highly oxygenated with an O/N ratio between 5 and 14 and a median of 8. Correspondingly, the islands occur at much lower C numbers when compared to those of the unsubstituted fulvic acids (Figure 3c). The hydrogen content per molecule is comparable to that of the fulvic acids (Table 1), and the major N components exhibit DBE value of 3-4, allowing some of them to be aromatic. The product ion mass spectra of these nitrogenous compounds show decarboxylations as for fulvic acids but no neutral loss of Analytical Chemistry, Vol. 78, No. 24, December 15, 2006

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NO (-30) and NO2 (-46) that would indicate a nitro group.21 Instead the loss of 63 mass units (-HNO3) and the occurrence of m/z 62 (NO3-) suggest that organonitrates, esters of nitric acid, are present. This assignment agrees with a very recent laboratory study in which oleic acid was used as a model SOA precursor and reacted with NO3 radicals. By ESI-MS/MS in positive mode the authors identified nitric acid esters rather than nitro components on the basis of the expulsion of HNO3 from the molecular cation.7 A radical formation process involving NO3 and O2 was proposed from that laboratory study. Precursor ions of m/z 62 (NO3-) were recorded here by a triple-quadrupole mass spectrometer in negative mode to confirm the identification. Indeed, the precursor ion spectrum of the nitric acid esters (Figure 5b) is comparable to the reconstructed mass spectrum of the N1 compounds (Figure 2c). Contrary to these nitric acid esters, nitro (aromatic) compounds, in which the carbon is directly bound to the nitrogen, are well documented in aerosol material. We cannot rule out that a certain number of nitrosated molecules are also present in these samples, but the precursor ion spectrum of m/z 62 (Figure 5b) indicates that the N1 fraction is dominated by organonitrate components, which are reported here for the first time. Sulfur- and Nitrogen-Containing Molecules. The fourth series of molecules that were detected in the WSOC of the aerosol contain both sulfate and nitrate moieties. These compounds preferentially occur at higher masses than the fulvic acids (Figure 2d). Of the 110 detected elemental compositions about 10% contained more than one N per molecule. All of these S1N1 molecules carry sufficient oxygen to allow the S to occur as sulfate and N as nitrate (seven or more oxygens). This result is consistent with the product ion mass spectra that reflect the presence of the sulfate group (m/z 97, m/z 80) as well as of the nitrate group (M - H - 63, m/z 62] (Figure 4c). As the fragmentation of the S1N1 molecules is governed by the strongly acidic moieties, other fragments are weak. However, they still correspond to the known fragments observed in pure fulvic acid spectra (Figure 4a). The number of N1S1 molecules was comparatively low, and vertical and horizontal series are short (Figure 3d). With the inclusion of a sulfate and a nitrate moiety into a molecule, significantly less variability in its elemental composition remains as compared to that of a nonsubstituted fulvic acid molecule or those of molecules bearing only one of these moieties, sulfate or nitrate. CONCLUSIONS The identification of fulvic acids together with three series of sulfated, nitrated, and mixed sulfated and nitrated molecules in atmospheric aerosol raises questions regarding their origins. Generally, organic matter in aerosol may be either directly emitted material (primary organic aerosol) or lower vapor pressure materials that have formed in the atmosphere from compounds that were initially emitted in the gas phase (secondary organic aerosol).1 Considering the low particle size of the aerosol investigated here (