Evidence for High Molecular Weight Nitrogen-Containing Organic

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Environ. Sci. Technol. 2010, 44, 4441–4446

Evidence for High Molecular Weight Nitrogen-Containing Organic Salts in Urban Aerosols XIAOFEI WANG,† SONG GAO,‡ X I N Y A N G , * ,† H O N G C H E N , † JIANMIN CHEN,† GUOSHUN ZHUANG,† JASON D. SURRATT,§ MAN NIN CHAN,| A N D J O H N H . S E I N F E L D * ,§,| Department of Environmental Science and Engineering, Fudan University, 220 Handan Road, Shanghai, 200433, China, Division of Math, Science and Technology, Nova Southeastern University, Fort Lauderdale, Florida 33314, and Division of Chemistry and Chemical Engineering and Division of Engineering and Applied Science, California Institute of Technology, Pasadena, California 91125

Received January 12, 2010. Revised manuscript received April 27, 2010. Accepted May 4, 2010.

High molecular weight (Mw) species were observed at substantial intensities in the positive-ion mass spectra in urban Shanghai aerosols collected from a single-particle timeof-flight mass spectrometer (in the m/z range 250-500) during three separate periods over 2007-2009. These species correlate well with the CN- mass signal, suggesting that C-N bonds are prevalent and that the observed high-Mw species are potentially nitrogen-containing organic salts. Anticorrelation with the ambient O3 concentration suggests that photochemical oxidants are not involved directly in the formation of these species. The Mannich reaction, among amines (or ammonia), formaldehyde, and carbonyls with an adjacent, acidic proton, is proposed as a plausible pathway leading to these organic salts. Although the high-Mw species observed in the singleparticle mass spectra appear to be nitrogen-containing organics, further chemical confirmation is desired to verify if the proposed Mannich reaction can explain the formation of these high-Mw species in regions where ammonia, amines, and carbonyls are prevalent.

Introduction Atmospheric fine particulate matter comprises a large fraction of organic material (1). It is well established that oxidation of volatile organic compounds (VOCs) produces low-volatility compounds that partition into the particle phase to yield secondary organic aerosol (SOA) (2). Particle-phase accretion reactions, which are neither fully understood nor currently incorporated in atmospheric models, lead to low-volatility products and subsequently increase the particulate mass fraction of SOA (2, 3). High-Mw species identified in SOA by a variety of mass spectrometric techniques have suggested * Address correspondence to either author. E-mail: yangxin@ fudan.edu.cn (X.Y.); [email protected] (J.H.S.). † Fudan University. ‡ Nova Southeastern University. § Division of Chemistry and Chemical Engineering, California Institute of Technology. | Division of Engineering and Applied Science, California Institute of Technology. 10.1021/es1001117

 2010 American Chemical Society

Published on Web 05/17/2010

the occurrence of particle-phase accretion reactions (4-8). Mass spectra of SOA generated in laboratory studies frequently show a series of high-Mw ion peaks separated by ∆m/z 12, 14, and 16, indicating the likely presence of oligomeric species (9, 10). In addition, gas-phase glyoxal and methyl glyoxal can be absorbed into the aqueous particle phase and undergo accretion reactions (11-14), which may be acid-catalyzed (15). While an increase of the seed particle acidity is found to enhance oligomer formation (7), oligomeric species are present even when particles are not acidic (4, 6, 7, 16). Although high-Mw species have been clearly observed in laboratory-generated organic aerosols, the ambient significance of particle-phase reactions responsible for these species is not well established. High-Mw organosulfates (Mw up to 374) have been identified in ambient aerosol from the Southeastern U.S. and forested regions of Europe as well as in laboratory-generated SOA produced from the photooxidation of biogenic VOCs (such as isoprene and monoterpenes) in the presence of acidified sulfate seed aerosol (17-19). This is currently one of the only laboratoryobserved accretion reactions to be found atmospherically significant. Despite recent observations of high-Mw organosulfates, information on the exact nature and formation mechanisms of other possible classes of high-Mw species in ambient aerosols is lacking. By deploying an aerosol single-particle time-of-flight mass spectrometer (ATOFMS), we report for the first time high-Mw nitrogen-containing organic salts that are widely present in aerosols in an urban atmosphere. Our results also demonstrate a relationship with particle acidity in the formation of this class of aerosol components.

Experimental Section 1. Real-Time Single Particle Analysis by ATOFMS. The size and chemical composition of individual particles were characterized in real time by an ATOFMS (TSI-3800), the design and operation of which have been described previously (20). Briefly, aerosols are introduced into a vacuum region through an aerodynamic lens that is designed to effectively transmit particles in the size range of 100 nm to 3 µm. Each particle is accelerated to a size-dependent terminal velocity. The velocity and thus the aerodynamic diameter of each particle are determined by the time-offlight between two orthogonal, continuous diode-pumped green lasers. The firing of the desorption/ionization laser (Nd:YAG laser, 266 nm) is triggered based on the velocity measurement when the particle enters the center of the source region of the mass spectrometer. Both positive and negative ions generated from laser ablation are analyzed simultaneously. The power density of the desorption/ ionization laser was kept at about 1.0 × 108 W cm-2 in the measurements reported here. Monodisperse polystyrene latex spheres (Nanosphere Size Standards, Duke Scientific Corp., Palo Alto) of 0.22-2.00 µm diameter were suspended using an atomizer (TSI-3076) for size calibration. The ATOFMS was located on the campus of Fudan University in Shanghai at an urban site close to residential, traffic, and construction emissions. Ambient aerosols were sampled into the ATOFMS through a 4 m copper tube of 1.27 cm diameter. A cyclone was used on the inlet and pulled at 10 L min-1 to minimize particle residence time in the sampling line. The inlet of the sampling tube was about 5 m above the ground and 0.5 m above the roof of the building on which the instrument was located. The ATOFMS was operated around the clock in separate campaigns during August 2007, December 2008, and March, May, and June VOL. 44, NO. 12, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Average mass spectrum of particles containing high-Mw species identified by ATOFMS. 2009. Local meteorological data, including temperature, relative humidity (RH), atmospheric pressure, wind speed, and direction, and hourly average O3 concentration were provided by the Shanghai Meteorological Bureau. 2. Bulk Aerosol Sampling and LC-MS Analysis. PM2.5 aerosol samples were collected daily in 24 h intervals by a medium-volume sampler (Model: (TSP/PM10/PM2.5)-2, flow rate: 76.67 L min-1) in August 2007. An 8-stage impactor (Anderson 8-Stage Non-Viable Cascade Impactor, Series 20-800, Thermo Fisher Scientific) was used to collect sizesegregated aerosol samples during May and June in 2009. All the samples were collected on Whatman 41 polycarbonate filters (Whatman Inc., Maidstone, U.K., diameter: 90 mm). Samples were placed in polyethylene plastic bags immediately after sampling and stored in a freezer (-18 °C). All the filters were weighed before and after sampling with an analytical balance (Sartorius 2004 MP, precision 10 µg) after stabilizing under constant temperature (20 ( 1 °C) and humidity (40 ( 2%). All procedures were strictly qualitycontrolled to avoid possible contamination of the samples. PM2.5 samples collected on August 1 and 2, 2007 and fine particles collected in stages 5 (3.3-2.1 µm) to 8 (0.7-0.4 µm) from the 8-stage impactor during May 24-25, June 11-12, and June 13-15, 2009 were chemically characterized offline by ultra performance liquid chromatography (UPLC) interfaced to an electrospray ionization time-of-flight mass spectrometer (ESI-TOFMS). Coarse-mode aerosols (i.e., stages 1 to 4) were not chemically characterized in the present study, since they were not measured by the ATOFMS. To provide sufficient mass for detection, each film was extracted individually in 5 mL of high-purity methanol (LC-MS CHROMASOLV-Grade, Sigma-Aldrich) via 45 min of sonication, combined into a single 20 mL methanol extract, and then dried under a gentle ultrahigh purity N2 stream at room temperature. The dried sample was then reconstituted with 100 µL of 1:1 (v/v) solvent mixture of 0.1% acetic acid in water and 0.1% acetic acid in methanol. Five microliters of each reconstituted sample extract was then injected into a Waters ACQUITY UPLC system, coupled with a Waters LCT Premier TOFMS equipped with an ESI source, allowing for accurate mass measurement for each observed ion. Operation protocols, including column information and chromatographic method, for the UPLC/ESI-TOFMS technique have been previously described (17). All sample extracts were analyzed in both the positive and negative ion modes; for the present study only the positive ion mode data are considered. Blank films were also extracted and prepared in 4442

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the same manner as the aerosol samples; the chemical analyses of these blank films, as well as the solvent mixture used to reconstitute the dried samples, by the UPLC/ESITOFMS technique served as the controls and were carefully compared to the ambient samples to disregard any background signals. Using the MassLynx Version 4.1 software, molecular formula searches for each measured ion employed the following elemental limits: C (up to 1000 atoms), H (up to 1000 atoms), N (up to 10 atoms), O (up to 1000 atoms), S (up to 5 atoms), Na (up to 1 atom), K (up to 1 atom), Ca (up to 1 atom), and Fe (up to 1 atom). In addition to using these elemental limits, we considered only those molecular formulas that were within 5 mDa and had an i-Fit (i.e., a measure of how well the isotopic distributions match, closer to zero is best) within 100.

Results and Discussion Ambient ATOFMS measurements were carried out over 40 days in separate campaigns during August 2007, December 2008, and March, May, and June 2009. Here we focus on the measurements during August 1-5 and 8-9, 2007, which are characteristic of all samples (Supporting Information, Figure S1). During the 7 days of sampling in August 2007, a total of 97,716 particles were identified as containing ion peaks in the m/z range 250 to 500 (high-Mw species), which account for 25.7% of the total collected particles for which both positive and negative ion mode mass spectra were available. Figure 1 shows the average mass spectra of particles containing high-Mw species. These spectra are unique in that most of the high-Mw ion peaks appear in the positive ion mode, whereas relatively few appear in the negative ion spectra. In a study of SOA generated in a laboratory chamber, Gross et al. (21) found that high-Mw ion peaks appear in both positive and negative ion spectra, but predominantly in the negative ion spectra. Ambient ATOFMS measurements identified oxygenated high-Mw ion peaks, also present mostly in the negative ion spectra (10), attributed largely to the negative charge centers on oxygen. Ion peaks of neutral organic compounds would be present in both positive and negative spectra; organic ion peaks that appear mostly in the positive spectrum suggest strongly the presence of other classes of compounds, such as organic salts (22, 23). Cations of organic salts are likely responsible for the numerous high-Mw ion peaks found in the positive ion spectrum in Figure 1; some of the respective counterions, inorganic in nature, can be seen in the negative ion mass

FIGURE 2. Comparison of hourly averaged relative peak areas of high-Mw signals with (a) those of the CN- signal in the mass spectra and (b) ambient O3. spectrum. An organic salt cation requires a stable positive charge center other than carbon, such as nitrogen. Indeed, the strong signal at m/z 26 in the negative ion mode data can be attributed to CN-, which is a definitive marker for species with carbon-nitrogen (C-N) bonds. The CN- m/z of 26 can also be assigned to C2H2-; however, the ion at m/z 26 correlates well with that of C3N- at m/z 50 (as shown in Supporting Information, Figure S2), which confirms the CNassignment. Figure 2a shows the comparison of hourly averaged relative peak areas of CN- and high-Mw signals. The degree of correlation strongly suggests that C-N bonds are prevalent in these high-Mw species. The possibility that some of the signals represent fragments without N, generated in laser ablation, cannot be totally ruled out, but such signals would tend to be equally distributed in both positive and negative ion spectra, not exclusively in the positive spectrum (considering that C and H have lower electronegativity than N). The mass signal for (C2H5)2NCH2+ (m/z 86) in Figure 1, one of the marker ions for alkylamines (24), indicates that amines were probably involved in the formation of at least some of the high-Mw species observed. The m/z 86 ion could be an aerosol component or a fragment from larger compounds. In either case, amines were likely involved in its formation. Amines are major nitrogen-containing atmospheric organic compounds, which are emitted by a variety of sources including industry, motor vehicles, animal husbandry, sewage treatment plants, and waste incinerators. Amines can easily form amine salts with acids (25) or react with O3, OH, or NO3 and partition to the particle phase (26-28). Amine salts produced by the direct reaction with inorganic acids usually are of relatively low molecular weight (Mw < 150 Da), while products from ozonolysis or other oxidation reactions would not give the mass spectral patterns in Figure 1 (predominantly present in the positive ion mode).

Particulate C-N compounds have been recently identified in laboratory studies via reactions of glyoxal with ammonium sulfate, amino acids, and amines (26-32). For example, imidazoles have been suggested as a major form of C-N compounds, which can be protonated to form organic salts. With sufficient ambient abundance of glyoxal, ammonia, sulfate, and amines, imidazole salts could be the source of some of the ion peaks in Figure 1. However, the elevated baseline with no simple, repetitive mass patterns as shown in Figure 1 suggests that the high-Mw species result from a variety of precursors. Even though ATOFMS data (with relatively low mass resolution at around 700) do not allow one to pinpoint the exact structures of these compounds, it is unlikely that imidazole salts alone can explain all of the complex mass spectral patterns observed. Taking into consideration the elevated concentrations of formaldehyde in Shanghai (33), a general mechanism for the formation of C-N containing products based on the Mannich reaction (34) can be proposed. As shown in Figure 3, in this mechanism primary or secondary amines react with formaldehyde (or other aldehydes) to form iminium cations. Note that tertiary amines lack an N-H proton to form the intermediate iminium cations. Under acidic conditions, compounds with a carbonyl functional group can tautomerize to the enol form, which can attack the iminium ion via nucleophilic addition to form Mannich bases. The bases can be converted to organic cations in the presence of H+. In the particle phase, aldehydes or ketones can undergo aldol condensation and/or carboxyl chemistry forming high-Mw species. Mannich bases can condense further with formaldehyde and amines through similar pathways, giving rise to a network of condensation reactions and eventually forming a number of high-Mw species. Ammonia, which is more abundant than amines in the atmosphere, can also undergo Mannich reactions. The overall Mannich reaction among formaldehyde, a primary or secondary amine (or ammonia), and a carbonyl that has an adjacent, acidic proton, can be expressed as follows:

Altogether, this proposed class of organic salts is relatively non-volatile and chemically stable (as opposed to accretion reaction products that are reversible in formation). Like organosulfates, they may represent a substantial fraction of previously unidentified species in ambient organic aerosols, especially in the presence of abundant formaldehyde and amines (or ammonia). The extent to which these species contribute to the total organic aerosol mass cannot be accurately known until some of these compounds can be quantified, or at least reasonable surrogate standard compounds can be identified for quantification. For this work, we used soft-ionization MS techniques to further examine the presence of these species and corresponding structures. The control filter blanks show multiple peaks in the positive ion mode mass spectra, indicating the presence of background species either in the filter media, in the solvents, or because of the relatively long storage duration. Nevertheless, after disregarding ion signals observed in the base peak ion chromatograms (BPCs) of both the blank films (or filters) and solvent only samples, there are noticeable peaks in the BPCs of the Shanghai aerosol filter samples (see Supporting Information, Figure S3). Peaks with very similar retention times are seen in samples from different sampling periods, indicating the presence of common species in these samples. By comparing each chromatographic peak in the controls versus the samples (see Supporting Information, Figure S3 as an example), a number of molecular ions (i.e., [M+H]+ under the positive ion mode) are identified, as listed VOL. 44, NO. 12, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Proposed formation mechanism of organic salts: Amines react with aldehydes and/or ketones in the presence of an acid.

TABLE 1. Nitrogen-Containing Organic Constituents Observed by UPLC/(+)ESI-TOFMS TOFMS suggested ion formula [M+H]+ retention time (RT) (min)

measured mass

actual mass of the TOFMS suggested ion formula

C

H

N

0.95 1.57 1.81 3.91 4.66 5.66 6.96 7.09 8.33 8.73 9.58

102.1279 102.1276 102.1279 163.1240 128.1432 130.1592 182.1903 226.1417 240.1587 244.2086 282.2048

102.1283 102.1283 102.1283 163.1235 128.1439 130.1596 182.1909 226.1443 240.1600 244.2065 282.2045

6 6 6 10 8 8 12 12 13 17 12

16 16 16 15 18 20 24 20 22 26 24

1 1 1 2 1 1 1 1 1 1 1

in Table 1. The accurate mass measurements for these species are also listed, along with the elemental composition suggested by the ESI-TOFMS software (with a maximum tolerance of (5 mDa error between the measured and theoretical mass). The m/z 102 [M+H]+ ion corresponds most likely to triethylamine or its isomeric alkylamines (26). While it is difficult to pinpoint the structures of other species from these data alone, it is likely that many of these species are nitrogen-containing (e.g., amines) owing to odd-number molecular masses. For those species with nitrogen but no oxygen, amines are likely structures, analogous to triethylamine. They can exist either in the original aerosol samples as alkylaminium salts formed from acid-base reactions (35) or be derived from the Mannich-type products through degradation during analysis. On the other hand, species that contain both nitrogen and oxygen are also present. Based on the C:H ratio, it can be seen that some of these species (e.g., C13H22NO3 and C12H20NO3) have unsaturated bonds such as CdO. Since the observed high-Mw species are anticorrelated with O3 (see below), direct photooxidation is unlikely the dominant mechanism leading to formation of these positiveion mode species. Overall, the ESI-TOFMS data provide some further, albeit indirect, evidence that the Mannich reaction is a plausible pathway to form some of the observed highMw species in the positive ion mode ATOFMS spectra. The relationship between particle acidity and the abundance of high-Mw species can be explored. The relative acidity 4444

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O

S

Na

error (mDa)

i-Fit

1

-0.4 -0.7 -0.4 -0.5 -0.7 -0.4 -0.6 -2.6 -1.3 2.1 0.3

1.5 25.9 36.1 4.1 9.3 0.9 23.8 21.8 12.0 53.1 2.3

3 3 3

FIGURE 4. Relative peak area of high-Mw species versus hourly averaged relative acidity. of particles is defined as the ratio of the sum of the absolute average peak areas of NO3- (m/z 62) and HSO4- (m/z 97) to that of NH4+ (m/z 18) (10). Figure 4 compares hourly averaged relative peak area of high-Mw compounds and the relative acidity. The data suggest that formation of organic salts is associated with elevated particle acidity, which is consistent

with the fact that the proposed reactions are generally acidcatalyzed. The leveling-off for high-Mw species exceeding about 1500 in the relative acidity is consistent with proton saturation of amines, which react with carbonyl groups less readily at high acidity (36). To rule out the possibility of SOA formation by ozonolysis or other oxidation reactions as the main source of the mass signals, the correlation of the high-Mw species with O3 levels is examined. Figure 2b shows the temporal profiles for the relative peak areas of high-Mw signals in the mass spectra and for ambient O3. The anti-correlation between high-Mw species and O3 suggests a pathway of SOA formation involving nitrogen-containing species that occurs under relatively low oxidant levels. High concentrations of oxidants may either oxidize the carbonyls, ammonia, and amines or degrade the Mannich-type products. Considering the presence of ammonia, amines, and carbonyls in the urban atmosphere, this pathway could account for a substantial fraction of organic aerosol mass.

Acknowledgments The ATOFMS work here was supported by The National Natural Science Foundation of China (20937001, 40875074 and 40875073). Support of J.D.S. and J.H.S. by the Electric Power Research Institute is acknowledged. The authors thank Douglas Worsnop for helpful discussions.

Supporting Information Available Mass spectra of particles containing high-Mw species obtained in other sampling periods and the correlation between the signal intensities of CN- and C3N-. This material is available free of charge via the Internet at http://pubs.acs.org.

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