Molecular Characterization of Nitrogen-Containing Organic

Apr 17, 2009 - High-resolution electrospray ionization mass spectrometry ... Environmental Science & Technology 2017 51 (11), 5951-5961 ... MS/MS Digi...
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Environ. Sci. Technol. 2009, 43, 3764–3771

Molecular Characterization of Nitrogen-Containing Organic Compounds in Biomass Burning Aerosols Using High-Resolution Mass Spectrometry A L E X A N D E R L A S K I N , * ,† J E F F R E Y S . S M I T H , ‡,§ A N D J U L I A L A S K I N * ,‡ William R. Wiley Environmental Molecular Sciences Laboratory and Chemical and Materials Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, MSIN K8-88, Richland, Washington 99352

Received December 4, 2008. Revised manuscript received February 17, 2009. Accepted February 18, 2009.

Although nitrogen-containing organic compounds (NOC) are important components of atmospheric aerosols, little is known about their chemical composition. Here we present detailed characterization of the NOC constituents of biomass burning aerosol (BBA) samples using high-resolution electrospray ionization mass spectrometry (ESI/MS). Accurate mass measurements combined with MS/MS fragmentation experiments of selected ions were used to assign molecular structures to individual NOC species. Our results indicate that N-heterocyclic alkaloid compounds (species naturally produced by plants and living organisms) comprise a substantial fraction of NOC in BBA samples collected from test burns of five biomass fuels. High abundance of alkaloids in test burns of ponderosa pine (a widespread tree in the western U.S. areas frequently affected by large scale fires) suggests that N-heterocyclic alkaloids in BBA may play a significant role in dry and wet deposition of fixed nitrogen in this region.

Introduction Nitrogen is an important nutrient for the growth of all organisms. Formation, transformation, deposition, and transport of both inorganic and organic fixed nitrogen species are essential parts of the total nitrogen cycle on Earth (1). It is accepted that deposition of nitrogen-containing organic compounds (NOC) from atmospheric particles has a profound effect on the nitrogen concentration in aquatic and terrestrial ecosystems (2-4). NOC typically account for a significant fraction of the total atmospheric particulate nitrogen. Cornell et al. showed that more than 30% of the total nitrogen in clean marine aerosol and rainwater collected in Oahu, Hawaii is stored as NOC (5). In that study most of the NOC was found in fine particles and attributed to gasto-particle conversions and long-range transport in the * Address correspondence to either author. J.L. e-mail: [email protected]; phone: 1-509-371-6136; fax: 1-509-371-6139. A.L. e-mail: [email protected]; phone: 1-509-371-6129; fax: 1-509-371-6139. † William R. Wiley Environmental Molecular Sciences Laboratory. ‡ Chemical and Materials Sciences Division. § Undergraduate student from the University of Washington, Seattle, WA. 3764

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atmosphere. Wedyan et al. showed that NOC contributed up to 80% of the total nitrogen in aerosol over the Gulf of Aqaba (6). Nakamura et al. reported that long-range transport of NOC-containing particles from East Asia is an important source of atmospheric nitrogen in the coastal areas of the Pacific Ocean (7). Zhang et al. demonstrated that particulate NOC collected in Davis, California accounted for ∼20% of the total fine particle mass and suggested that NOC was a significant component of particulate nitrogen in that region (8). In all studies cited above, the presence of NOC was inferred from the difference between quantitative measurements of the total and inorganic particulate nitrogen, and its molecular composition was not characterized in detail. To date, only few studies have reported molecular characterization of NOC in atmospheric aerosol samples. Amino acids and alkyl amines have been observed and quantified in samples of atmospheric aerosol and fogwater collected in Davis, CA (9, 10). NOC accounted for an average of 13% of the dissolved OC in fog waters and 10% of water-soluble organic carbon in aerosol samples. It has been suggested that the presence of NOC could affect chemical and physical properties of water droplets and aerosol particles by altering their buffering capacity and basicity. Aliphatic and cyclic amides, aryl amides, and several N-heterocyclic compounds have been identified in fine aerosol particles collected in the Lower Fraser Valley, British Columbia (11). Spectroscopic signatures of pyridino and amino NOC associated with airborne particulates were reported for urban smog samples collected in Pasadena, CA (12). Most recently, nitrated organic molecules have been reported in water-soluble organic fractions of aerosols collected in Riverside, CA (13). Biomass burning is one of the largest sources of organic aerosol in the atmosphere (14-16). Mace et al. reported a detailed study of water-soluble organic nitrogen in Amazon Basin aerosols (17). They found that NOC accounted for more than 40% of the total particulate nitrogen. Substantially higher concentrations of NOC were observed in aerosol samples collected during the dry season that are dominated by particles produced by biomass burning. They suggested that biomass burning aerosol (BBA) is an important source of NOC released into the atmosphere. Similar results were reported for aircraft measurements conducted in the Lake Tahoe Basin (18). Urea, amines, and amino acids were identified in BBA collected in the Amazon Basin (17), in the Mediterranean atmosphere (19), and in Tasmania (20). Alkyl amides and alkyl nitriles were observed in particulate matter from Kuala Lumpur (21) and Santiago de Chile (22). In addition, heterocyclic aromatic compounds with single N atom were recently identified in BBA samples collected from test burns of rice straw (23). The presence of alkyl amides and nitriles was attributed to a series of reactions of alkanoic acids (fatty acids) with ammonia during biomass burning (21, 22), while amines and N-heterocyclics are generally ascribed to volatilization from plant material and pyrolysis of biopolymers (9, 23). In addition, resuspension of plant debris and soil organic matter could contribute to deposition of N-heterocycles into BBA. Previous studies demonstrated that electrospray ionization mass spectrometry (ESI/MS) is a valuable tool for characterization of polar organic compounds in aerosol samples (13, 24-30). Because nitrogen-containing compounds are readily ionized using ESI, this technique is ideally suited for detection and characterization of NOC in atmospheric aerosols. Here we report first detailed mass spectrometric characterization of NOC in BBA samples collected in test burns of five biomass fuels. We utilized high-resolution 10.1021/es803456n CCC: $40.75

 2009 American Chemical Society

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ESI mass spectrometry (28, 30) combined with MS/MS fragmentation experiments for detection and structural characterization of molecular structures of individual nitrogen-containing species in BBA. In this study we focused on qualitative characterization of NOC because of the very limited information on the identity of this class of compounds in BBA available in the literature. Future studies will assess the utility of ESI/MS for quantitative analysis of NOC in atmospheric aerosols. Our results indicate that alkaloid compounds are abundant constituents of NOC in some of the BBA samples. Alkaloids, naturally occurring compounds containing basic nitrogen atoms, are mutagens produced by plants and living organisms. It has been suggested (23) that these compounds can be emitted as a result of biomass pyrolysis in smoldering forest fires that typically involve combustion of vegetation material, i.e., foliage stems and roots. Alkaloids in aerosols may affect the environment in a variety of ways. Depending on their composition they can either be toxic or essential nutrients for different organisms thereby affecting ecosystems and human health. In addition, basic alkaloid compounds can modulate the acidity of particles, and thereby have an impact on the heterogeneous chemistry and the composition of aged particles, which in turn affect their hygroscopic and optical properties. To the best of our knowledge chemical composition of alkaloids in BBA has never been investigated.

Experimental Section BBA samples were collected during the FLAME experiment conducted at the U.S. Forest Service Fire Science Laboratory (FSL, Missoula, MT) where a series of laboratory measurements and aerosol sampling of biomass burning emissions was performed in June 2006 (31). Particles produced from biomass burning were collected onto Teflon and aluminum substrates using a ten-stage Micro-Orifice Uniform Deposit Impactor (MOUDI) model 110R (MSP, Inc.). This manuscript provides results of high-resolution mass spectrometric analysis of BBA samples for five different biofuels: (1) Alaskan duff (AD; Duff is organic matter in various stages of decomposition on the floor of the forest. The AD sample was composed of predominantly feather mosses.), (2) dried ponderosa pine needles and sticks (PPNS), (3) ponderosa pine duff (PPD), (4) dried southern pine needles (SPN), and (5) southern California ceanothus (SCC). Size distribution measurements performed at the sampling site indicated that particle mass distributions were typically centered around 0.2-0.3 µm for all tested fuels (32). Therefore samples of size-fractionated particles of 0.18-0.32 µm aerodynamic diameter collected on the eighth impactor stage were selected for this study. Solvent extracts were prepared by ultrasonic washing of the Teflon and aluminum substrates in methanol. Detailed analysis demonstrated that less than 3% of the total number of the observed NOC in high-resolution MS spectra were accompanied by possible methanol adducts resulting from analyte reaction with the solvent (33). Our results suggest that ESI/MS analysis of NOC is much less affected by solvent-analyte reactions than the analysis of oxygencontaining organic compounds, for which a substantially larger number of modified peaks has been previously reported (30). The extracts were filtered using Whatman 0.45 µm Teflon (PTFE) membrane disposable filters and analyzed using a high-resolution LTQ-Orbitrap mass spectrometer (Thermo Electron, Inc., Bremen, Germany) with modified electrospray ionization (ESI) source. Samples were injected through a pulled fused silica capillary (50 µm ID) at a flow rate of 0.3-1.0 µL/min using a spray voltage of 3.5-4 kV. The system was operated in a positive ion mode with a resolving power of 60,000 at m/z 400. The instrument was calibrated using a

standard ESI calibration mixture containing caffeine, MRFA, and Ultramark 1621 (a commercially available mixture of fluorinated phosphazenes). Background spectra were obtained by analyzing solvent extracts of blank Teflon and aluminum substrates prepared using the same procedure. Several experiments were performed to assess the reproducibility of ESI/MS spectra. Similar spectra were obtained following solvent extraction of different portions of the same filter or extraction from different filters. For example, ESI/MS analysis of solvent extracts from both Teflon and aluminum substrates of the BBA obtained from burning of the PPNS sample resulted in qualitatively similar mass spectra. In addition, similar spectra were obtained following extraction of the AD samples collected during two different FLAME experiments in 2006 and 2007. Mass spectral features with the signal-to-noise ratio of 3 and higher were extracted from raw spectra using the Decon2LS program developed at PNNL (http://ncrr.pnl.gov/ software/). Background subtraction was performed using an Excel macro with a tolerance of 0.001 amu. All peaks appearing in both blank and aerosol extract spectra and differing in abundance by less than a factor of 3 were eliminated. It should be noted that none of the NOC compounds discussed in this work were present in the background spectra. The remaining peaks were assigned likely empirical formulas using Formula Calculator v. 1.1 developed at the National High Magnetic Field Laboratory (http://magnet.fsu.edu/∼midas/download.html) and through the use of Kendrick diagrams (34).

Results and Discussion In our previous study we reported results of high-resolution ESI/MS analysis of several BBA samples (30) with the major emphasis on identification and characterization of oxygencontaining organic compounds. Very different mass spectra were obtained for aerosol samples collected for different types of biofuels. Unambiguous assignment of the elemental composition of hundreds of individual compounds in BBA was performed (30) based on accurate mass measurements combined with Kendrick analysis (34). We demonstrated that 10-20% of all assigned peaks in ESI-MS spectra of solvent extracts of BBA samples correspond to nitrogen-containing organic compounds (NOC). Here we present detailed characterization of NOC species detected in solvent extracts of BBA samples. Figure 1 shows the distribution of NOC extracted from high-resolution mass spectra of the five BBA samples, for which full ESI/MS spectra have been presented in our previous manuscript (30). The number of NOC peaks observed in each ESI/MS spectrum is as follows: AD - 12; PPNS - 70; PPD - 54; SPN - 26; SCC - 64, corresponding to 5, 18, 22, 20, and 31%, respectively, of all assigned peaks. The absolute abundance of NOC species also varies among samples with the highest intensity observed for the PPNS extract. ESI/MS spectra of the AD and the SPN samples contain the smallest number of low-abundance NOC species. It should be mentioned that the abundance of these species could be correlated with the total amount of organic aerosol collected for each sample. Indeed, the absolute intensity of the peak corresponding to sodiated levoglucosan (LG), a common biomass burning marker, varies significantly for different biomass fuels. The signal-to-noise ratio for the LG peak (C6H10O5Na+, m/z 185.042) is 1000 for the AD, 4300 for the PPNS, 530 for the PPD, 740 for the SPN, and 140 for the SCC sample. While ESI/MS signal of LG could be affected by the matrix effects, we found a remarkable correlation (see Figure S1 of the Supporting Information) between the measured signal in our ESI/MS spectra and volume concentrations of LG in sampled air reported by Sullivan et al. VOL. 43, NO. 10, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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C25H39NO2, C25H45NO3, C25H51NO4, C27H41NO). Accurate mass measurement is crucial for assigning reliable elemental composition for such species. The major homologous [C4H6N2(CH2)n + H]+ and [C6H8N2(CH2)n + H]+ series are shown in Figure 2. The first series is clearly dominated by the [C8H14N2 + H]+ ion (n ) 4) while a rather uniform distribution as a function of the number of CH2 units, n, is observed for the [C6H8N2(CH2)n + H]+ series. Broad distribution of peaks is observed in all spectra except for the SPN sample. It is possible that burn conditions have a significant effect on the formation and deposition of nitrogen-containing species onto the particles. The SPN sample was produced by flaming burning of dry pine needles, while all other samples were obtained from slower, smoldering burns (35). It is likely that slow smoldering burning that leads to incomplete combustion of pyrolysis products is responsible for the presence of a large number of NOC species in the BBA samples. Physical and chemical properties of organic aerosols are affected by the molecular composition of their constituents (36). For example, light absorption properties of organic molecules vary dramatically with the number of double bonds and rings in the molecule (37); commonly expressed as the double bond equivalent (DBE). In addition, UV/vis spectra of organic molecules and their hygroscopic properties are strongly affected by the presence of oxygen and nitrogen atoms. Finally, we note that atmospheric oxidation and aging of organic aerosols is commonly described using the oxygento-carbon (O:C) and hydrogen-to-carbon (H:C) ratios (38). Kendrick and van Krevelen diagrams have been extensively used as graphical tools for improved classification of organic compounds in complex mixtures (39). Figure 3a shows the van Krevelen diagram obtained by plotting the H:C vs N:C ratio for NOC peaks in the PPNS spectrum. The DBE values shown in Figure 3b were calculated from the elemental formula using eq 1: DBE ) 1 - h ⁄ 2 + n ⁄ 2 + c FIGURE 1. Distribution of NOC peaks in high-resolution ESI/MS spectra of solvent extracts of the five biomass burning samples: AD - Alaskan duff, PPNS - dried ponderosa pine needles and sticks, PPD - ponderosa pine duff, SPN - dried southern pine needles, SCC - southern California ceanothus. Exact masses and elemental assignments of all peaks are listed in Table 1. (32). This comparison suggests that ESI/MS signal of LG could be used as a normalization factor. The intensity of the LG peak follows the same trend as the intensity of NOC species in ESI/MS of biomass burning samples (Figure 1) of all biofuels except for the SCC extract, for which the dramatic decrease in the intensity of the LG peak is not associated with any substantial decrease in the abundance of NOC species. Kendrick analysis of high-resolution mass spectra described in detail in our previous study showed the presence of a number of CH2-based homologous series of nitrogencontaining peaks (30). The elemental composition of the observed individual peaks and homologous series is summarized in Table 1. Homologous series are presented as a core molecule plus (CH2)n, where n is greater than or equal to 0. Most of the peaks correspond to protonated molecules. Several molecules cationized on Na+ most likely contain carboxylic acid groups that readily form [M + Na]+ ions in the positive mode electrospray ionization (28, 30). The most abundant series of peaks contains two nitrogen atoms. For example, the [C4H6N2(CH2)n + H]+ series dominates spectra of all five samples. In addition, a variety of nitrogencontaining peaks with several oxygen atoms are observed. It is interesting to note that mass spectra of the PPD, SPN, and SCC samples contain a number of fairly abundant highMW (m/z > 300) NOC (e.g., C16H31NO10S2, C24H33NO6, 3766

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(1)

where h, n, and c correspond to the number of hydrogen, nitrogen, and carbon atoms in the assigned peaks, respectively. Both plots shown in Figure 3 clearly separate different series and individual peaks according to the number of nitrogen and oxygen atoms and the degree of unsaturation. Similar plots obtained for PPD and SCC samples are shown in Figures S2 and S3. C7H15NO2 is an abundant peak in the PPNS spectrum with DBE of 1. This peak observed only in the PPNS extract most likely corresponds to a methylated derivative of amino acids leucine or isoleucine. It is interesting to note that leucine is one of the most thermally stable amino acids (40) suggesting that this amino acid is likely to survive high temperatures during biomass burning. The [C4H6N2(CH2)n + H]+ series is characterized by the lowest degree of unsaturation among the homologous series with DBE of 3. The next abundant [C6H8N2(CH2)n + H]+ series has the DBE of 4 corresponding to a linear molecule with four double bonds or a ring with three double bonds. The [C8H8N2(CH2)n + H]+ series has the DBE of 6 characteristic of molecules with fused five- and six-membered rings. At higher degrees of unsaturation there is an overlap between series containing two nitrogen atoms and series containing both nitrogen and oxygen. For example, three series have the DBE of 7: [C10H10N2(CH2)n + H]+, [C10H9NO(CH2)n + H]+, and [C11H12N2O(CH2)n + H]+. Finally, the [C11H8N2(CH2)n + H]+ and [C13H11NO(CH2)n + H]+ series have the highest degree of unsaturation with the DBE of 9. High DBE values of 7, 8, and 9 in these series can be explained only by the presence of N-heterocyclic compounds with fused aromatic rings. Molecular identification based on the elemental composition alone is challenging because most complex mol-

TABLE 1. Elemental Composition of NOC Peaks and Their Homologous Series Detected in Five BBA Samples core molecule

charge carrier

DBE

C3H7NO3 C4H6N2 C4H11N C4H11NO2 C5H5NO C5H5N2O2 C5H9NO C5H9NO2 C5H10N2 C5H10N2O C6H5N2 C6H6N2O2 C6H9NO C6H11NO C6H13N C6H13NO2 C6H15NO3 C7H7N3 C7H15NO2 C8H8N2 C8H13NO5 C8H16N2 C8H17N3OS C8H19N C9H17NO3 C9H18N2O2 C9H21NO3 C9H21NO4 C10H9NO C10H10N2 C10H21NO3 C11H5N2 C11H12N2O C11H16N2 C12H11NO C12H11NO2 C13H11NO C13H19NO8 C13H21NO6 C14H16N2 C14H27NO7 C14H27NO8 C15H23NO5 C16H31NO10S2 C16H33NO4 C18H35NO4 C24H33NO6 C25H39NO2 C25H45NO3 C25H51NO4 C27H41NO

Na+ H+ H+ H+ H+ Na+ H+ Na+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ Na+ H+ H+ H+ Na+ H+ Na+ Na+ H+ H+ Na+ H+ H+ H+ H+ H+ H+ H+ Na+ H+ H+ H+ Na+ H+ H+ Na+ H+ H+ H+ H+ H+

1 3 0 0 4 3 2 2 2 2 4 5 3 2 1 1 0 6 1 6 3 2 2 0 2 2 0 0 7 7 1 9 7 5 8 8 9 5 4 8 2 2 5 2 1 2 9 7 4 1 8

number of CH2 units added to the core molecule (n g 0) AD

a

2-7

PPNSa 1-9 0

PPDa 0, 1, 7, 11, 13 1-8

SPNa 4, 5 0 0

0-3,11 0 0-7

2-7 0

0-8 0 0, 1 0

0, 1, 4, 13 0, 1 0, 5, 6

2, 4-6

SCCa

0

0-6 0, 4, 5 0-6 0

0 0, 3 0

0 0, 3 0

0, 3 0-2

0 0-6

0-5 0 0, 2, 3 0

0

0

0 0

0, 4, 10

0 0

0 0 0-2 0-5 0-3 0-3 0-1 0-2 0-2 0, 1

0-1

0 0-3

0, 1

0, 1 0

0, 2 0-2 0 0 0 0 0 0 0

0 0

0, 2 0, 1 0, 2 0

a

AD - Alaskan duff, PPNS - dried ponderosa pine needles and sticks, PPD - ponderosa pine duff, SPN - dried southern pine needles, SCC - southern California ceanothus.

ecules have a number of stable isomeric forms. For example, abundant CxHyN2 species observed in BBA extracts could be potentially assigned to alkyl dinitriles formed during biomass burning from dialkanoic acids and ammonia according to the mechanism suggested by Simoneit et al. (22). However, dinitrile compounds have never been previously detected in BBA samples. In addition, the even-to-odd carbon preference, the characteristic feature of alkyl nitrile compounds (21, 22), was not observed for NOC detected in our samples (Figure 2). Alternatively, the CxHyN2 species can be assigned to a variety of N-heterocyclic alkaloids having two nitrogen atoms embedded into five-membered (pyrazole, imidazole, and their derivatives) or six-membered rings (e.g., pyrazine, pyrimidine, pyridazine, and their derivatives) and molecules with single nitrogen atom heterocyclic rings fused together.

Possible structures of NOC species in several series were examined using MS/MS experiments. It should be noted that while structural characterization of odd-electron organic ions produced by electron impact ionization performed through comparison with large fragmentation databases is relatively straightforward, identification of even-electron ions produced by ESI is difficult because of the lack of appropriate databases. Fragmentation behavior of both protonated, [M + H]+, and deprotonated, [M - H]-, molecules has been extensively discussed in the literature more than two decades ago (41-44). Recently, Levsen et al. (45) reported a detailed account of the fragmentation behavior of 121 model compounds composed of an aromatic core with different functional groups. The following discussion of the fragmentation behavior of NOC is mainly based on the general trends derived in that study. VOL. 43, NO. 10, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Distribution of species in the (a) C4H6N2(CH2)n series, and (b) C6H8N2(CH2)n series. Collision-induced dissociation (CID) was performed in the linear quadrupole (LTQ) followed by high-resolution analysis of the resulting fragments in the Orbitrap. Conducting MS/MS experiments for all NOC, for which the chemical composition has been determined in this study, is impossible and impractical. Here we present selected results that provided interesting structural information for some of the abundant species. Figure 4 shows representative MS/MS spectra obtained for different homologous series of the observed NOC. Fragment ions were unambiguously identified using accurate mass assignments. CID spectrum of the [C9H16N2 + H]+ ion from the C5H8N2(CH2)n series (Figure 4a) shows losses of CH3, C2H5, C3H6, C3H7, and C4H8. Loss of alkyl radicals from nitrogen-containing evenelectron ions is characteristic of aromatic N-alkylamines (45). The smallest fragment observed in the spectrum, [C5H8N2 + H]+ ion, most likely corresponds to the protonated dimethylimidazole. The [C9H16N2 + H]+ precursor ion is, probably, present in a mixture of isomers with alkyl chains of different length attached to either of the nitrogen atoms. Similar MS/MS spectrum was obtained for the [C12H20N2 + H]+ ion of the C6H8N2(CH2)n series (Figure 4b) suggesting that this progression of peaks is derived from the tetramethylpyrazine core or from its pyrimidine analog. Loss of CH3 is the only fragmentation pathway observed for the [C12H10N2 + H]+ ion (Figure 4c) from the C11H8N2(CH2)n series. The resulting fragment, C11H8N2+ ion, is very stable and does not undergo subsequent fragmentation. The observed fragmentation behavior indicates that the CH3 group of the [C12H10N2 + H]+ ion is attached to the nitrogen atom of a stable heterocyclic compound with molecular formula C11H8N2. We propose that N-methylated β-carboline, an alkaloid commonly found in plants, is a reasonable candidate for the C12H10N2 species. An interesting fragmentation pattern was obtained for the [C11H12N2 + H]+ ion (Figure 4d) from the C10H10N2(CH2)n series showing losses of NH, N2, CH2N2, and C6H6 characteristic of substituted phenylpyrazoles (46). Loss of N2 and CH2N2 but not C6H6 was observed for species from the 3768

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[C8H8N2(CH2)n + H]+ series (data not shown) suggesting that these species correspond to methylindazole derivatives. Fragmentation pattern of the [C6H11NO + H]+ species shown in Figure 4e is remarkably similar to the fragmentation pattern reported for the protonated ε-caprolactam (47). However, other cyclic isomers of C6H11NO containing an amino group and a carbonyl or a hydroxyl group could yield a similar MS/MS spectrum. The results presented in this study demonstrate the presence of a significant number of N-heterocyclic alkaloid compounds in BBA extracts. This observation is not surprising because these compounds are naturally produced by a variety of plants, bacteria, fungi, and other living organisms and therefore are plentiful in the biomass material. The thermal stability of N-heterocyclic cores of these compounds is comparable to the thermal stability of polycyclic aromatic hydrocarbons (PAHs) (48) commonly observed in BBA extracts (14). At relatively low combustion temperatures typical for smoldering fires, substituted N-heterocyclic compounds typically undergo minor pyrolytic and oxidative fragmentations that do not affect their aromatic cores. The characteristic property of alkaloids is the presence of lone pairs of electrons on their nitrogen atoms that determine the basicity (alkalinity) of these compounds. In addition to ammonia, alkaloids may present a considerable source of basic compounds in BBA. Buffering capacity of alkaloids may impact pH of particles thereby affecting the heterogeneous chemistry of BBA particles with atmospheric gases. Basic alkaloids can form salts with mineral and organic acids present in the aerosol thereby affecting the hygroscopic properties and CCN activity of BBA particles. For example, Dinar et al. (49) showed that acid-base reactions of ammonia with model organic acids results in substantial changes in hygroscopic growth and CCN activity of atmospherically relevant particles. In summary, we presented the first characterization of NOC in smoke particles using high-resolution mass spectrometry combined with MS/MS structural analysis

FIGURE 4. Representative MS/MS fragmentation spectra of NOC species detected in BBA samples. Elemental formulas in squared parentheses indicate precursor ions in each of the MS/MS experiments. Elemental formulas above fragment ion peaks denote the corresponding loss of neutral molecules in MS/MS experiments. Elemental formulas of fragments corresponding to the stable molecular core of the two major CH2 series are labeled in panels (a) and (b) and the corresponding structures are shown on the right; the proposed structures of neutral precursor molecules for spectra shown in panels (c)-(e) are displayed on the right. FIGURE 3. (a) van Krevelen plot obtained for NOC species detected in the PPNS sample, and (b) double bond equivalent calculated for the same data set as a function of m/z. The size of the data points is proportional to the logarithm of the peak intensity. of representative compounds. We demonstrated that in all samples NOC were dominated by heterocyclic compounds containing two nitrogen atoms (CxHyN2). The amount of NOC is higher in BBA produced under smoldering conditions. In addition, most samples contained unique high-MW alkaloid compounds that could potentially be used as markers of different types of biofuels. Information presented in this work can be utilized in targeted future studies focused both on the quantitation and more detailed structural characterization of NOC in atmospheric aerosols. Observation of elevated amounts of both oxygenated organic compounds and alkaloids in samples obtained from burning of ponderosa pine is particularly important because of the possible impact of forest fires on the environment and human health. Ponderosa pine trees are widespread in the western United States and Canada (50), often growing in droughty areas and forests that are subject to large-scale outbreaks of forest fires. High levels of alkaloids in ponderosa pine foliage (51, 52) combined with thermal stability of these compounds may be responsible for efficient transfer of toxic NOC into BBA and subsequent long-range transport of alkaloids in the atmosphere. The results obtained in this study considered together with very frequent large scale forest fires in the western U.S. suggest that alkaloids may account for a significant fraction of NOC in regional aerosol.

Acknowledgments The research described in this manuscript was performed at the W. R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the U.S. Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is operated by Battelle for the U.S. Department of Energy. J. L. acknowledges support from the Chemical Sciences Division, Office of Basic Energy Sciences of the U.S. DOE. A.L. acknowledges support from the Atmospheric Science Program, Office of Biological and Environmental Research of the U.S. DOE. J.S.S. acknowledges support from the DOE Science Undergraduate Laboratory Internship (SULI) program at Pacific Northwest National Laboratory (PNNL). The authors gratefully acknowledge Drs. William C. Malm, Wei-Min Hao, Jeffery L. Collett Jr., and Sonia Kreidenweiss for organizing the FLAME project and the staff at the USDA/USFS Fire Sciences Laboratory for technical help. Additionally, the authors acknowledge the support of Dr. Yuri Desyaterik at the sampling site.

Supporting Information Available Figures S1-S3. This material is available free of charge via the Internet at http://pubs.acs.org.

Literature Cited (1) Chapin, F. S., III.; Matson, P. A.; Mooney, H. A. Principles of Terrestrial Ecosystem Ecology; Springer Verlag: New York, NY, 2002. VOL. 43, NO. 10, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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