Ionization Aerosol Mass

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Anal. Chem. 2010, 82, 7915–7923

Near-Infrared Laser Desorption/Ionization Aerosol Mass Spectrometry for Investigating Primary and Secondary Organic Aerosols under Low Loading Conditions† Scott Geddes, Brian Nichols, Stevenson Flemer, Jr., Jessica Eisenhauer, James Zahardis, and Giuseppe A. Petrucci* Department of Chemistry, University of Vermont, Burlington, Vermont 05452 A new method, near-infrared laser desorption/ionization aerosol mass spectrometry (NIR-LDI-AMS), is described for the real time analysis of organic aerosols at atmospherically relevant mass loadings. Use of a single NIR laser pulse to vaporize and ionize particle components deposited on an aluminum probe results in minimal fragmentation to produce exclusively intact pseudomolecular anions at [M - H]-. Limits of detection (total particulate mass sampled) for oxidized compounds of relevance to atmospheric primary and secondary organic aerosol range from 89 fg for pinic acid to 8.8 pg for cholesterol. NIR-LDI-AMS was used in conjunction with the University of Vermont Environmental Chamber to study secondary organic aerosol (SOA) formation from ozonolysis of limonene at total aerosol mass loadings ranging from 3.2 to 25.0 µg m-3 and with a time resolution of several minutes. NIR-LDI-AMS permitted direct delineation between gas-phase, homogeneous SOA formation and subsequent heterogeneous aerosol processing by ozone. Aerosols are of central importance in the atmosphere where they can influence climate, visibility, trace gas levels, and health.1-3 Organic aerosols (OA) make a significant contribution to the total mass of fine aerosols (i.e., PM2.5), contributing ∼20-50% at continental midlatitudes and up to 90% in tropical forested areas.1 OA may be broadly categorized as primary or secondary (i.e., POA and SOA, respectively). POA is directly emitted into the atmosphere from a source, whereas SOA is formed in the atmosphere via oxidation of volatile organic compounds (VOCs). At present, there exists a disparity between modeled SOA fluxes and concentrations and those measured in the field. Traditional “bottom-up” estimates that combine VOC fluxes with † Part of the special issue “Atmospheric Analysis as Related to Climate Change”. * Corresponding author. E-mail: [email protected]. (1) Kanakidou, M.; Seinfeld, J. H.; Pandis, S. N.; Dentener, F. J.; Facchini, M. C.; Van Dingenen, R.; Ervens, B.; Nenes, A.; Nielson, C. J.; Swietlicki, E.; Putaud, J. P.; Balkanski, Y.; Fuzzi, S.; Horth, J.; Moortgat, G. K.; Winterhalter, R.; Myhre, C. E. L.; Tsigaridis, K.; Vignati, E.; Stephanou, E. G.; Wilson, J. Atmos. Chem. Phys. 2005, 5, 1053–1123. (2) Po ¨schl, U. Angew. Chem., Int. Ed. 2005, 44, 7520–7540. (3) Seinfeld, J. H.; Pankow, J. F. Annu. Rev. Phys. Chem. 2003, 54, 121–140.

10.1021/ac1013354  2010 American Chemical Society Published on Web 08/26/2010

chamber-derived data estimate total biogenic and anthropogenic SOA fluxes of 12-70 and 2-12 Tg year-1, respectively.4 These bottom-up estimates of SOA production are 1-2 orders of magnitude lower than recent “top-down” (inverse) estimates, whereby SOA production is estimated by constraining the atmospheric fate of VOC precursors to SOA, thereby abnegating the need for chamber-derived data of SOA production.5 As noted recently, the large difference in the range of these two approaches “clearly suggests that chamber oxidation experiments substantially underestimate total SOA production during the full course of the VOC oxidation process and is an issue that needs to be addressed.”4 Continental concentrations of organic aerosol (COA) are typically less than 10 µg m-3; however, most current chamberbased studies employ organic mass loadings of greater than 15 µg m-3.4,6 Consequently, bottom-up estimates of SOA fluxes are based on parametrization of chamber-based SOA formation experiments typically done at COA levels that are not representative of ambient concentrations. The advantages of conducting SOA formation experiments at atmospherically relevant COA levels were demonstrated recently by Presto and Donahue in a study which showed that extrapolations of current models to atmospheric COA levels significantly underestimate SOA under dark, low NOx conditions, while conversely overestimating SOA production under illuminated, higher NOx conditions typical of polluted air masses.7 It is also clear that the chemical composition and associated properties, such as density of SOA formed in chambers under high COA levels differ from SOA particles in the atmosphere.4,6 Chamber-derived SOA typically has a significantly lower O/C atomic ratio compared to ambient oxygenated organic aerosol (OOA).8,9 Recently, aerosol mass spectrometry (AMS) was coupled to a continuous-flow chamber to investigate SOA under a wide range of organic particle loadings (4) Hallquist, M.; Wenger, J. C.; Baltensperger, U.; Rudich, Y.; Simpson, D.; Claeys, M.; Dommen, J.; Donahue, N. M.; George, C.; Goldstein, A. H.; Hamilton, J. F.; Herrmann, H.; Hoffmann, T.; Iinuma, Y.; Jang, M.; Jenkin, M.; Jimenez, J. L.; Kiendler-Scharr, A.; Maenhaut, W.; McFiggans, G.; Mentel, T. F.; Monod, A.; Pre´voˆt, A. S. H.; Seinfeld, J. H.; Surratt, J. D.; Szmigielski, R.; Wildt, J. Atmos. Chem. Phys. 2009, 9, 5155–5236. (5) Goldstein, A. H.; Galbally, I. E. Environ. Sci. Technol. 2007, 41, 1514– 1521. (6) Kroll, J. H.; Seinfeld, J. H. Atmos. Environ. 2008, 42, 3593–3624. (7) Presto, A. A.; Donahue, N. M. Environ. Sci. Technol. 2006, 40, 3536–3543.

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(∼0.5 to >140 µg m-3),10 showing that SOA at lower COA had greater compositional variability, higher density, and a higher O/C atomic ratio compared to SOA at elevated COA levels. The results of all of the aforementioned studies suggest that extrapolation of results derived from chamber-based studies obtained under elevated levels of COA (i.e., COA > 15 µg m-3) may be inappropriate for modeling the atmospheric production of SOA formation in lower organic loadings (i.e., COA < 10 µg m-3). Aerosol mass spectrometry has become an essential tool in monitoring aerosols both in field and in laboratory-based studies. Comprehensive descriptions of AMS methods and their application to atmospheric science have been given in several recent reviews.11-13 Fine aerosols in the troposphere are typically multicomponent particles and, as noted, have a significant organic fraction that typically undergoes extensive fragmentation with electron impact (EI) ionization;14 therefore, soft ionization methods, which minimize molecular fragmentation, are advantageous for AMS instrumentation applied to organic aerosols. Broadly, soft ionization methods (as applied to AMS to date) can be classified into two categories: chemical ionization (CI) and pulsed-laser methods. In CI, ions are produced through the collision of the analyte with reagent gas ions generally resulting in the formation of a single mass ion. Several variations of CI have been applied to AMS by different research groups including aerosol CIMS,15 thermal desorption CIMS (TDCIMS),16 and proton-transferreaction mass spectrometry (PTR-MS).17,18 Additionally, a number of different laser-based, pulsed ionization methods have also been developed and applied to AMS to softly ionize OA, including single (8) Jimenez, J. L.; Canagaratna, M. R.; Donahue, N. M.; Prevot, A. S. H.; Zhang, Q.; Kroll, J. H.; DeCarlo, P. F.; Allan, J. D.; Coe, H.; Ng, N. L.; Aiken, A. C.; Docherty, K. S.; Ulbrich, I. M.; Grieshop, A. P.; Robinson, A. L.; Duplissy, J.; Smith, J. D.; Wilson, K. R.; Lanz, V. A.; Hueglin, C.; Sun, Y. L.; Tian, J.; Laaksonen, A.; Raatikainen, T.; Rautiainen, J.; Vaattovaara, P.; Ehn, M.; Kulmala, M.; Tomlinson, J. M.; Collins, D. R.; Cubison, M. J.; Dunlea, E. J.; Huffman, J. A.; Onasch, T. B.; Alfarra, M. R.; Williams, P. I.; Bower, K.; Kondo, Y.; Schneider, J.; Drewnick, F.; Borrmann, S.; Weimer, S.; Demerjian, K.; Salcedo, D.; Cottrell, L.; Griffin, R.; Takami, A.; Miyoshi, T.; Hatakeyama, S.; Shimono, A.; Sun, J. Y.; Zhang, Y. M.; Dzepina, K.; Kimmel, J. R.; Sueper, D.; Jayne, J. T.; Herndon, S. C.; Trimborn, A. M.; Williams, L. R.; Wood, E. C.; Middlebrook, A. M.; Kolb, C. E.; Baltensperger, U.; Worsnop, D. R. Science 2009, 326, 1525–1529. (9) Aiken, A. C.; DeCarlo, P. F.; Kroll, J. H.; Worsnop, D. R.; Huffman, J. A.; Docherty, K. S.; Ulbrich, I.; Mohr, C.; Kimmel, J. R.; Sueper, D.; Sun, Y.; Zhang, Q.; Trimborn, A.; Northway, M.; Ziemann, P. J.; Canagaratna, M. R.; Onasch, T. B.; Alfarra, M. R.; Prevot, A. S. H.; Dommen, J.; Duplissy, J.; Metzger, A.; Baltensperger, U.; Jimenez, J. L. Environ. Sci. Technol. 2008, 42, 4478–4485. (10) Shilling, J. E.; Chen, Q.; King, S. M.; Rosenoern, T.; Kroll, J. H.; Worsnop, D. R.; DeCarlo, P. F.; Aiken, A. C.; Sueper, D.; Jimenez, J. L.; Martin, S. T. Atmos. Chem. Phys. 2009, 9, 771–782. (11) Nash, D. G.; Baer, T.; Johnston, M. V. Int. J. Mass Spectrom. 2006, 258, 2–12. (12) Sullivan, R. C.; Prather, K. A. Anal. Chem. 2005, 77, 3861–3886. (13) Canagaratna, M. R.; Jayne, J. T.; Jimenez, J. L.; Allan, J. D.; Alfarra, M. R.; Zhang, Q.; Onasch, T. B.; Drewnick, F.; Coe, H.; Middlebrook, A.; Delia, A.; Williams, L. R.; Trimborn, A. M.; Northway, M. J.; DeCarlo, P. F.; Kolb, C. E.; Davidovits, P.; Worsnop, D. R. Mass Spectrom. Rev. 2007, 26, 185– 222. (14) Mirsaleh-Kohan, N.; Robertson, W. D.; Compton, R. N. Mass Spectrom. Rev. 2008, 27, 237–285. (15) Hearn, J. D.; Smith, G. D. Anal. Chem. 2004, 76, 2820–2826. (16) Voisin, D.; Smith, J. N.; Sakurai, H.; McMurry, P. H.; Eisele, F. L. Aerosol Sci. Technol. 2003, 37, 471–475. (17) Helle´n, H.; Metzger, A.; Gascho, A.; Duplissy, J.; Tritscher, T.; Prevot, A. S. H.; Baltensperger, U. Environ. Sci. Technol. 2008, 42, 7347–7353. (18) Holzinger, R.; Williams, J.; Herrmann, F.; Lelieveld, J.; Donahue, N. M.; Ro ¨ckmann, T. Atmos. Chem. Phys. 2010, 10, 2257–2267.

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photon ionization (SPI),19-26 resonance enhanced multiphoton ionization (REMPI),19,27-29 and photoelectron resonance capture ionization (PERCI).30 To date, these soft ionization methods have been used successfully in a variety of laboratory SOA studies; however, their sensitivity precludes measurements at low COA or they require extensive sampling times, negatively impacting their ability to monitor SOA formation in the early stages of particle genesis and growth. Laser desorption/ionization (LDI) is a highly sensitive ionization source that has been coupled to time-of-flight (TOF) mass spectrometry since the early 1990s.31-33 As has been discussed in several reviews, UV LDI has typically been coupled to AMS;11,12,34 however, this often results in significant fragmentation of organic molecules in aerosol particles. Two-step laser desorption (vaporization)/ionization methods employ an IR pulse for desorption (or vaporization) of the organic constituents, followed by ionization of the resulting vapor by UV or VUV radiation.19-22 Most recently, this approach has led to development of the photoionization AMS (PIAMS)20 and IR-VUV-ion trap mass spectrometry (IR-VUV-ITMS)19 instruments, both of which have reported enhancements in sensitivity and reduced molecular fragmentation. Nonetheless, the sensitivity of PIAMS is somewhat lower than needed for the SOA measurements discussed above; whereas, the IR-VUV-ITMS instrument still exhibits some fragmentation for more fragile analytes, such as carboxylic acids and others of atmospheric interest. Herein we describe a new analytical instrument recently developed in our laboratory to measure and monitor the composition of OA at low total organic mass concentrations. This new approach, near-infrared laser desorption/ionization aerosol mass spectrometry (NIR-LDI-AMS), employs a single-wavelength of near-infrared radiation for both desorption of the organic component and the generation of pseudomolecular anions with the reproducible loss only of a hydrogen atom. As we will show, utilization of a single laser pulse, in conjunction with the (19) Hanna, S. J.; Campuzano-Jost, P.; Simpson, E. A.; Burak, I.; Blades, M. W.; Hepburn, J. W.; Bertram, A. K. Phys. Chem. Chem. Phys. 2009, 11, 7963– 7975. ¨ ktem, B.; Tolocka, M. P.; Johnston, M. V. Anal. Chem. 2004, 76, 253– (20) O 261. (21) Woods, E., III; Smith, G. D.; Dessiaterik, Y.; Baer, T.; Miller, R. E. Anal. Chem. 2001, 73, 2317–2322. (22) Nash, D. G.; Liu, F.; Mysak, E. R.; Baer, T. Int. J. Mass Spectrom. 2005, 241, 89–97. (23) Northway, M. J.; Jayne, J. T.; Toohey, D. W.; Canagaratna, M. R.; Trimborn, A. M.; Akiyama, K.-I.; Shimono, A.; Jimenez, J. L.; DeCarlo, P. F.; Wilson, K. R.; Worsnop, D. R. Aerosol Sci. Technol. 2007, 41, 828–839. (24) Mysak, E. R.; Wilson, K. R.; Jimenez-Cruz, M.; Ahmed, M.; Baer, T. Anal. Chem. 2005, 77, 5953–5960. (25) Shu, J.; Gao, S.; Li, Y. Aerosol Sci. Technol. 2008, 42, 110–113. (26) Gloaguen, E.; Mysak, E. R.; Leone, S. R.; Ahmed, M.; Wilson, K. R. Int. J. Mass Spectrom. 2006, 258, 74–85. (27) Lazar, A.; Reilly, P. T. A.; Whitten, W. B.; Ramse, J. M. Environ. Sci. Technol. 1999, 33, 3993–4001. (28) Bente, M.; Adam, T.; Ferge, T.; Gallavardin, S.; Sklorz, M.; Streibel, T.; Zimmermann, R. Int. J. Mass Spectrom. 2006, 258, 86–94. (29) Zelenyuk, A.; Cabalo, J.; Baer, T.; Miller, R. E. Anal. Chem. 1999, 71, 1802– 1808. (30) LaFranchi, B. W.; Petrucci, G. A. Int. J. Mass Spectrom. 2006, 285, 120– 133. (31) McKeown, P. J.; Johnston, M. V.; Murphy, D. M. Anal. Chem. 1991, 63, 2069–2073. (32) Mansoori, B. A.; Johnston, M. V.; Wexler, A. S. Anal. Chem. 1994, 66, 3681–3687. (33) Prather, K. A.; Nordmeyer, T.; Salt, K. Anal. Chem. 1994, 66, 1403–1407. (34) Hunt, A. L.; Petrucci, G. A. Trends Anal. Chem. 2002, 21, 74–81.

Figure 1. Schematic of (a) NIR-LDI-AMS instrument and (b) the University of Vermont Environmental Chamber. SM ) sampling manifold for particulate filter samples and sorbent cartridges for thermal desorption-gas chromatography/mass spectrometry (TD-GC/MS) analysis.

exceptionally soft ionization provided, results in improved analytical sensitivities that are sufficient to measure organic particle mass loadings below 1 µg m-3 with time resolutions of several minutes. EXPERIMENTAL SECTION NIR-LDI-AMS. A schematic diagram of the NIR-LDI-AMS instrument is shown in Figure 1a. The unit has been constructed upon a portable frame and is designed to operate with two 120 V-16 A power outlets allowing for transport to and use at external laboratories and field sites. The system consists of a Liu-type aerodynamic lens35,36 mounted on a modified high-vacuum flange (35) Liu, P.; Ziemann, P. J.; Kittelson, D. B.; McMurry, P. H. Aerosol Sci. Technol. 1995, 22, 293–313.

to permit X-Y adjustments and supported by a Z-manipulator inside the first vacuum stage pumped by a 500 L min-1 turbomolecular pump (TMH 521, Pfeiffer Vacuum, Nashua, NH). A ball valve is used to sample aerosol into the instrument through a 100 µm diameter critical orifice (O’Keefe Controls, Monroe, CT) at the entrance of the inlet, which sets the aerosol sampling rate to 83 cm3 min-1. The particle beam is directed through a 5 mm orifice (separating stages 1 and 2) into the high vacuum ionization region (pumped with a second 500 L min-1 turbomolecular pump) where it impinges upon the concave surface of a 1 mm diameter aluminum wire probe (99.9% purity, Alfa-Aesar Chemicals, Ward (36) Liu, P.; Ziemann, P. J.; Kittelson, D. B.; McMurry, P. H. Aerosol Sci. Technol. 1995, 22, 314–324.

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Hill, MA). This probe, mounted centrally in the ion extraction region of a reflectron time-of-flight mass spectrometer (R.M. Jordan Inc., Grass Valley, CA), is readily accessible via a high vacuum load lock (Nor-Cal Products Inc., CA). Alignment of the particle beam to the Al probe is aided by the use of a 150 mW continuous wave (cw) 532 nm laser (Viasho VA-I-N-532, Beijing, China) focused to a spot in front of the probe, allowing visualization of high particle density beams; however this is turned off during mass spectral measurements. Following particle deposition, laser desorption and ionization are performed with a 1064 nm laser pulse (5 ns) from a Nd:YAG laser (model Brio, Quantel, Big Sky, CO) incident on the surface of the Al probe at approximately 30° from normal. The laser passes through a high energy beam attenuator (model 935-5-OPT, Newport, CA), offering fine control of the pulse energies, and is steered onto the Al probe with no additional focusing. Laser energy densities on the probe are ∼4 mJ mm-2 (80 MW cm-2). The laser spot size is larger than the Al probe surface ensuring small variations in the beam direction have a minimal effect on the desorption/ionization process. Typical delay times of 5-8 µs are employed between the laser Q-switch trigger and generation of a time-zero pulse (DG535, Stanford Research Instruments, Sunnyvale, CA), which is synchronized to the pulsed power supply (model PVM-4210, Directed Energy, Inc., Fort Collins, CO) used for ion extraction. Mass spectral data is acquired at 1 GS/s with a digital oscilloscope (WavePro 7000, LeCroy, Chestnut Ridge, NY), which currently limits data acquisition to one mass spectrum (i.e., laser firing) per second. Chemicals. All chemicals were used as purchased without further purification (with the exception of pinic acid): oleic acid (99%, Mallinckrodt Chemicals), (R)-(+)-limonene (97%, Aldrich Chemicals), stearic acid (95%, Sigma-Aldrich), nonanal (95%, Aldrich Chemicals), cholesterol (95%, Aldrich Chemicals), azelaic acid (98%, Aldrich Chemicals). Pinic acid was synthesized in-house from a procedure adapted from Moglioni et al.37 All chemicals used in the synthesis were purchased from Aldrich Chemicals and used without further purification. Initially, 4.0 g (21.7 mmol) of cis-pinonic acid was dissolved in 71 mL of dioxane and cooled to 0 °C. In a separate flask, a solution of NaOBr was prepared in situ by dropwise addition of 3.4 mL (65.9 mmol) of Br2 to 11.6 g (290 mmol) of NaOH dissolved in 275 mL of water at 0 °C. The NaOBr solution was then added slowly to the cis-pinonic acid solution over a period of 1 h, maintaining the reaction temperature at 0 °C throughout the addition. Following this step, the reaction mixture was allowed to warm to room temperature and was then stirred for an additional 5 h. Workup of the pinic acid was carried out by first extracting the aqueous solution with 200 mL of CH2Cl2, followed by acidification of the aqueous phase with concentrated HCl. The resulting acidified solution was extracted with diethyl ether (2 × 150 mL). The ether extracts were combined, dried over MgSO4, and concentrated to yield the pinic acid as a colorless dense solid (>95% purity by NMR analysis).

POA Measurements. Initial alignment and testing of the NIRLDI-AMS was performed using oleic acid aerosol as a proxy for POA, as described in a prior report on this method.38 Particles were generated by homogeneous nucleation of oleic acid vapor generated in a small flask held at 110 °C and flushed through a condenser (T ) 4 °C) by a flow of zero air (USP Medical Air, Airgas East, Williston, VT) into a 0.5 m3 Teflon chamber. As this process results in a time dependent particle mass loading, aerosol particle number and mass size distributions, as well as total aerosol mass loadings, were measured continuously with a scanning mobility particle sizer (SMPS, model SMPS 3080, TSI Inc., Shoreview, MN). Aerosol (number) geometric mean diameter and standard deviation were typically on the order of 180 nm and 1.3, respectively (see inset a of Figure S-1 in the Supporting Information for a typical distribution). Particle sampling was performed by opening the sampling valve for a determined length of time, then manually firing the laser at the maximum rate dictated by the data processing steps until all observable analyte signals were gone. Following sample data collection, the laser was allowed to fire at 20 Hz for approximately 1 min to remove any residual organic compound mass on the Al probe. Occasional background checks were performed by sampling laboratory air for 10 min through a HEPA filter to ensure no persistent ion signals existed. Relative Sensitivity Measurements. Oleic acid was used as an internal standard to measure the instrumental response for different analytes of environmental interest. Internally mixed particles of the analyte(s) with oleic acid were generated by nebulization of ethanolic solutions. Solutions were typically prepared to be 50 ppmv oleic acid in ethanol and the concentration of the second analyte adjusted accordingly to yield ion signals of similar amplitude. The solution was then nebulized using a glass, concentric pneumatic nebulizer (J.E. Meinhard Associates, Santa Ana, CA) and the particles desolvated by passage through a diffusion drier. Deposition times were typically on the order of a few seconds. SOA Experiments. University of Vermont Environmental Chamber (UVMEC). An 8 m3 environmental chamber (Figure 1b) has recently been constructed in our laboratory for investigations of SOA formation under atmospherically relevant VOC concentrations and aerosol mass loadings. The chamber is constructed of 5.0 mil Teflon FEP (Welch Fluorocarbon, Dover, NH) and is equipped with ambient O3 (Serinus O3 model E020010, American Ecotech, Cincinnati, OH) and NOx analyzers (EC9041A NOx Analyzer, American Ecotech, Cincinnati, OH) as well as a combined ambient temperature and RH probe (Vaisala HMT331, Woburn, MA). The chamber is designed to be flushed with outside air for cleaning, although a zero-air generation system can be used to dilute background levels further. Particle size and concentration measurements are performed using the SMPS as noted above. NaCl aerosols (with geometric means of 65-220 nm and typical geometric standard deviations of 1.3) were used to measure a particle half-life of 3.3 h, although no corrections for wall loss have been performed for the experiments reported herein. A filter

(37) Moglioni, A. G.; Garcı´a-Expso´ito, E.; Aguado, G. P.; Parella, T.; Branchadell, V.; Moltrasio, G. Y.; Ortun ˜o, R. M. J. Org. Chem. 2000, 65, 3934–3940.

(38) Geddes, S.; Nichols, B.; Todd, K.; Zahardis, J.; Petrucci, G. A. Atmos. Meas. Tech. Discuss. 2010, 3, 2013–2033.

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Figure 2. COA evolution and ozone concentration (a) for SOA formation from limonene, along with correlated NIR-LDI-AMS mass spectra (b-d) of SOA taken at three different time points: 1 (COA ) 3.2 µg m-3), 2 (COA ) 10.6 µg m-3), and 3 (COA ) 22.1 µg m-3), respectively. Ion signals are normalized to the 187 m/z ion.

sampling manifold (“SM” in Figure 1b) and thermal desorptionGC/MS (Clarus 600T with 300TD TurboMatrix Desorber, PerkinElmer Inc., Shelton, CT) are currently being integrated for offline particulate analysis as well as direct determination of organic gas phase species. Limonene Ozonolysis SOA. Preliminary investigations into SOA formation from the ozonolysis of limonene were carried out to demonstrate the feasibility of NIR-LDI-AMS for chamber studies of oxygenated organic aerosol formation at atmospherically relevant mass loadings. Ozone was added to the UVMEC before the addition of limonene, with the mixing fan on, to rapidly reach a steady state concentration of 620 ppbv. Subsequently, 2 µL of limonene (corresponding to a chamber mixing ratio of approximately 38 ppb) was introduced to the chamber by evaporation over a warm water bath into a carrier flow of zero air. The mixing fan was kept off during VOC introduction to increase the particle formation induction period and decrease the rate of SOA growth, facilitating initial particle inception measurements. NIR-LDI-AMS measurements were performed over the course of 4 h following particle inception, growth and aging to a maximum mass loading of 25.0 µg m-3 (Figure 2a). A typical limonene SOA number size distribution is shown in Figure S-2 in the Supporting Information.

RESULTS AND DISCUSSION Analytical Figures of Merit for Organic Aerosol. The monounsaturated fatty acid, oleic acid (282 u, C18:1 cis-9) served as a proxy for POA because it is an ubiquitous compound observed in urban aerosols at reported ambient concentrations on the order of 1-5 ng m-3 and is a molecular marker for meat-cooking aerosol.39-41 Preliminary results have been presented in a recent report on NIR-LDI-AMS;38 however, because the figures of merit for other organic compounds common to atmospheric organic aerosol are referred to oleic acid, a brief summary of the key results of that work are presented herein (Figure S-1 in the Supporting Information). The signal response of NIR-LDI-AMS for pure oleic acid POA was monitored for the 281 m/z pseudomolecular ion, [OL - H]-, which was the base peak in the mass spectra (inset Figure S-1b in the Supporting Information). A linear response was measured for the ion signal as a function of aerosol mass sampled (MOL). For these measurements, oleic acid mass loading in the UVMEC ranged from 0.15 to 4.4 µg m-3 and particles were deposited on the LDI probe for periods of 5-30 s to achieve total mass depositions from (39) Fraser, M. P.; Yue, Z. W.; Tropp, R. J.; Kohl, S. D.; Chow, J. C. Atmos. Environ. 2002, 36, 5751–5758. (40) Robinson, A. L.; Subramanian, R.; Donahue, N. M.; Bernardo-Bricker, A.; Rogge, W. F. Environ. Sci. Technol. 2006, 40, 7820–7827. (41) Zheng, M.; Cass, G. R.; Schauer, J. J.; Edgerton, E. S. Environ. Sci. Technol. 2002, 36, 2361–2371.

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7 to 97 pg. The analytical sensitivity was 0.144 V pg-1, yielding an instrumental limit of detection (LOD, 3σ) for oleic acid of 140 fg. Measurement of relative sensitivities for other analytes of interest to SOA is not straightforward due to difficulties with fractionation effects in the homogeneous nucleation process; therefore, the signal response of NIR-LDI-AMS for other organic analytes was determined by studies featuring internally mixed particles (generated by solution nebulization), where oleic acid served as an internal standard for comparison. With the exception of cholesterol and cis-pinonic acid, for which the mole ratios relative to oleic acid were 12:1 and 54:1, respectively, all other mixtures used mole ratios ranging from 3:1 to 1:1. All analytes were measured as the corresponding pseudomolecular anion, [M - H]-. The C9 dicarboxylic acid azelaic acid (188 u), a cooking emission42 and an oxidation product of fatty acids,43 was determined to have a sensitivity relative to oleic acid, SA/OL, of 0.15, corresponding to an instrumental LOD of 933 fg. The relative sensitivity of pinic acid, another C9 dicarboxylic acid (186 u) commonly observed in biogenic SOA,1,4 was 1.58, corresponding to an LOD of 89 fg. Cholesterol (387 u), a molecular marker for meat cooking aerosol40 had a relative sensitivity and LOD of 0.016 and 8.8 pg, respectively. Finally, cis-pinonic acid, a C10 keto-carboxylic acid proposed as a marker for monoterpenederived SOA,4 had a relative sensitivity and LOD of 0.072 and 2.0 pg, respectively. A similar assessment was made of the sensitivity for the saturated fatty acid stearic acid (SA, 284 u, C18:0) that was monitored as the 283 m/z pseudomolecular ion, [SA - H]-, again, the base peak in the mass spectrum. Stearic acid presents a logical choice in analytes for these studies being a saturated analogue to oleic acid, solid phase at room temperature and a ubiquitous component of ambient aerosols.39-41 The LOD for stearic acid was calculated at 470 fg. This difference in LOD may arise in part from bounce-related collection efficiency (Eb)44 on the LDI probe that would likely be lower for SA, due to the formation of solid, nonspherical particles with a high aspect ratio.45 It should be noted that the calculated figures of merit are for particles sampled into the AMS and do not account for any sampling or collection losses. Future studies will assess experimental factors, including particle bounce and beam divergence, that may lower particle collection efficiency and how that translates to the instrumental capacity of NIR-LDI-AMS. Despite the relative instrumental simplicity of NIR-LDI-AMS, the LOD for most organics assayed is improved compared to other AMS methods that employ soft ionization. For example, using continuous sampling and ionization, Hearn and Smith15 using their aerosol CIMS technique reported a LOD (2σ) for oleic acid of 1.0 × 1010 molecules cm-3, corresponding to an atmospheric mass loading of approximately 5 µg m-3 for a 5 s sampling time. From their reported experimental sampling rate of 1500 sccm, a detection limit (in terms of mass sampled) of approximately (42) Huang, X.-F.; He, L.-Y.; Hu, M.; Zhang, Y.-H. Atmos. Environ. 2006, 40, 2449–2458. (43) Zahardis, J.; Petrucci, G. A. Atmos. Chem. Phys. 2007, 7, 1237–1274. (44) Salcedo, D.; Onasch, T. B.; Canagaratna, M. R.; Dzepina, K.; Huffman, J. A.; Jayne, J. T.; Worsnop, D. R.; Kolb, C. E.; Weimer, S.; Drewnick, F.; Allan, J. D.; Delia, A. E.; Jimenez, J. L. Atmos. Chem. Phys. 2007, 7, 549–556. (45) Katrib, Y.; Biskos, G.; Buseck, P. R.; Davidovits, P.; Jayne, J. T.; Mochida, M.; Wise, M. E.; Worsnop, D. R.; Martin, S. T. J. Phys. Chem. A 2005, 109, 10910–10919.

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625 pg is estimated. Extrapolating to a 1 min sampling time (for comparison with the present work), a 250 ng m-3 aerosol mass loading is estimated, as compared to 1.7 ng m-3 for NIRLDI-AMS. The calculated LOD for oleic acid and cholesterol by NIR-LDIAMS are also an improvement over photoionization AMS (PIAMS), where the LOD has been reported at 35 and 150 pg, respectively.20 The LOD for oleic acid reported herein also is a significant improvement over other pulsed laser methods of AMS, including photoelectron resonance capture ionization AMS (30 ng)30 and laser desorption-single photon ionization time-of-flight mass spectrometry (LDSPI-TOFMS, 5 pg).46 The LOD for cholesterol (8.6 pg) is comparable to that of 15 pg reported for LDSPI-TOFMS.46 IR-VUV-ITMS, recently reported by Hanna et al.,19,47 shows great potential to reach previously unattainable detection limits for particle bound organics. To date only a few analytes have been demonstrated, including caffeine (0.3 fg),47 2,4-dihydroxybenzoic acid (DHB, 0.2 fg),19 and oleic acid (12 fg).19 For fragile molecules such as oleic acid however, no significant ion signal was measured at the molecular ion. The base peak for all IR vaporization energies used (except the highest of 40 mJ) was the dehydrated fragment at 264 m/z. It is important to keep in mind that the fragmentation pattern (minimal as it may be) depended on the specific analyte under investigation, as well as experimental factors such as particle matrix, IR coupling into the particle, IR energy, and VUV wavelength. These unknown and variable factors significantly complicate the analysis of chemically complex particles, such as encountered in SOA experiments and real atmospheric particles. Furthermore, the presence of fragile components such as oleic acid leads to a complex structure of carbon clusters, which, again, would make chemical identification below about 150 m/z difficult, if not impossible. In contrast, NIR-LDI-AMS reproducibly generates pseudomolecular anions by loss of a hydrogen atom for all analytes studied to date, greatly simplifying the resulting mass spectra and chemical deconvolution of complex, multicomponent particles. Secondary Organic Aerosol. In a previous report, the utility of NIR-LDI-AMS to monitor chamber-based SOA at ambient levels of total organic particulate mass was demonstrated.38 In that study, particle-phase oxidation products of R-pinene were measured directly with NIR-LDI-AMS with high time resolution under atmospherically relevant aerosol mass loadings, COA, in the range of 1.5-8.7 µg m-3. In the present report, application of the NIRLDI-AMS has been extended to the monoterpene limonene (136 u, 4-isopropenyl-1-methyl-cyclohexene), which, as a prevalent biogenic emission from vegetation48 as well as being a commonly added agent to household cleaners,49 is of importance to both atmospheric and indoor chemistry. Secondary organic aerosol formation resulting from the ozonolysis of limonene has been the focus of a number of recent (46) Ferge, T.; Mu ¨ hlberger, F.; Zimmermann, R. Anal. Chem. 2005, 77, 4528– 4538. (47) Hanna, S. J.; Campuzano-Jost, P.; Simpson, E. A.; Robb, D. B.; Burak, I.; Blades, M. W.; Hepburn, J. W.; Bertram, A. K. Int. J. Mass Spectrom. 2009, 279, 134–146. (48) Geron, C.; Rasmussen, R.; Arnts, R. R.; Guenther, A. Atmos. Environ. 2000, 34, 1761–1781. (49) Nazaroff, W. W.; Weschler, C. J. Atmos. Environ. 2004, 38, 2841–2865.

chamber-based studies.50-58 Limonene presents an interesting SOA precursor in that it has two double bonds: an endocyclic, trialkyl substituted double bond and a terminal exocyclic double bond. As will be discussed below, the ozonolysis of limonene has high conversion to SOA due to minimal carbon loss in the oxidative cleavage of either or both double bonds from ozone, forming oxidation products with a significant decrease in vapor pressure compared to the parent VOC limonene. Moreover, as was determined by Zhang et al.,54 the endocyclic double bond is oxidized approximately 30 times faster than the exocyclic one, with rate constants of 2 × 10-16 and 7 × 10-18 cm3 molecule-1 s-1, respectively. As recently noted by Maksymiuk et al.,51 the significant difference in the time scale of reactivity of these two double bonds may afford interesting chemistry for this system in that the endocyclic double bond undergoes homogeneous, gas-phase ozonolysis, while under excess ozone conditions, the exocyclic double bond that is retained in the low-volatility, particle-phase products subsequently undergoes heterogeneous ozonolysis in the particle-phase. In the following section, we show the analytical utility of NIR-LDI-AMS for monitoring products that may arise from both homogeneous and heterogeneous chemistry in SOA formation and demonstrate clearly the ability to measure trends in product formation during SOA growth, with special emphasis on its early stages. Figure 2b-d shows representative NIR-LDI mass spectra of SOA particles from three time points in the SOA forming experiment (COA ) 3.2, 10.6, and 22.1 µg m-3), with the predominant product ions for the subsequent discussion labeled (i.e., 115, 185, 187, 199, and 201 m/z). These product assignments are based on established ozone-initiated oxidation of alkenes.59 Oxidation of limonene (Figure 3) is initiated in the gas phase by addition of ozone to the endocyclic double bond forming a primary ozonide (POZ1), which undergoes cycloreversion to two different excited Criegee intermediates (ECI1, ECI2). There are several major channels that are commonly evoked to describe the fate of the short-lived ECIs, including the stabilized Criegee intermediate (SCI) and the hydroperoxide channels, with the latter appearing to be the most important channel to the interpretation of the measured mass spectra. Briefly, the first step of the SCI channel involves stabilization of the ECIs by collision with N2 or O2 in air to yield SCIs, which can undergo intramolecular rearrangement to form a secondary (50) Iinuma, Y.; Mu ¨ ller, C.; Bo ¨ge, O.; Gnauk, T.; Herrmann, H. Atmos. Environ. 2007, 41, 5571–5583. (51) Maksymiuk, C. S.; Gayahtri, C.; Gil, R. R.; Donahue, N. M. Phys. Chem. Chem. Phys. 2009, 11, 7810–7818. (52) Pan, X.; Underwood, J. S.; Xing, J.-H.; Mang, S. A.; Nizkorodov, S. A. Atmos. Chem. Phys. 2009, 9, 3851–3865. (53) Walser, M. L.; Park, J.; Gomez, A. L.; Russell, A. R.; Nizkorodov, S. A. J. Phys. Chem. A 2007, 111, 1907–1913. (54) Zhang, J.; Huff Hartz, K. E.; Pandis, S. N.; Donahue, N. M. J. Phys. Chem. A 2006, 110, 11053–11063. (55) Leungsakul, S.; Jeffries, H. E.; Kamens, R. M. Atmos. Environ. 2005, 39, 7063–7082. (56) Jaoui, M.; Corse, E.; Kleindienst, T. E.; Offenberg, J. H.; Lewandowski, M.; Edney, E. O. Environ. Sci. Technol. 2006, 40, 3819–3828. (57) Leungsakul, S.; Jaoui, M.; Kamens, R. M. Environ. Sci. Technol. 2005, 39, 9583–9594. (58) Walser, M. L.; Desyaterik, Y.; Laskin, J.; Laskin, A.; Nizkorodov, S. A. Phys. Chem. Chem. Phys. 2008, 10, 1009–1022. (59) Calvert, J. G. Atkinson, R. Kerr, J. A. Madronich, S. Moortgat, G. K. Wallington, T. J. Yarwood, G., The Mechanism of Atmospheric Oxidation of Alkenes; Oxford University Press: Oxford, U.K., 2000.

ozonide (SOZ), with the secondary limonene endo-ozonide (184 u, SOZ1) having been determined by Nørgaard et al.60 to be a major gas-phase product. The presence of a medium intensity 183 m/z ion in all NIR-LDI mass spectra suggests that SOZ1 may be a minor product in these SOA particles, including in the early stages of SOA formation. However, besides the aforementioned channels, both the carbonyl oxide moieties of ECI1 and ECI2 may undergo isomerization, respectively, forming 7-hydroxy-limonaldehyde (I) and limononic acid (II) (structures given in Table S-1 in the Supporting Information). In light of the high degree of reactivity of SCI1 and SCI2, the 183 m/z ion is assigned generically to a 184 u compound (C10H16O3), with I, II, and SOZ1 being logical candidates. All three compounds have been assigned as possible structures for 184 u compounds in previous studies of SOA particles by Walser et al.58 employing electrospray ionization-mass spectrometry (ESI-MS), and compounds I and II have also been assigned in other studies.55-57 It should be noted that O-atom elimination of ECI1 and ECI2 can lead to the formation of limonaldehyde (168 u, III), which has been determined in prior studies56-58 and has generally been shown to be in low to moderate abundance in SOA, most likely due to its relatively high volatility. On the basis of the ionization trends we observed for the NIR-LDI of other aldehydes (e.g., nonanal), namely, the loss of hydrogen, it is expected that III would be present at 167 m/z in the mass spectrum. In fact, this ion is present in all of the mass spectra collected for COA > 2 µg m-3, albeit at significantly lower intensity than the 183 m/z ion. The reduced signal strength for aldehyde III may be due, in part, to its relatively high volatility resulting in significant partitioning to the gas phase, wherein it likely undergoes further oxidation to form products with reduced volatility. In accord with the established ionization mechanism of NIRLDI-AMS (i.e., loss of H to yield the [M - H]- pseudomolecular anion), the strong ion signals at 185, 199, and 201 m/z are assigned to products with masses 186, 200, and 202 u as follows (Table S-1 in the Supporting Information): The 185 m/z ion is assigned to two parent products with the molecular formula C9H14O4 (i.e., limonic acid (IV), keto-limononic acid (V)). Limonic acid has been reported as a major product in SOA in prior studies55,56 and as shown on Figure 3, may arise through the hydroperoxide channel of ECI1. Briefly, in the hydroperoxide channel, the ECI rearranges to a vinyl hydroperoxide, which rapidly decomposes to the hydroxyl radical and an alkyl radical (R). The alkyl radical then proceeds to undergo rapid reaction with molecular oxygen forming an alkylperoxy radical, RO2, which plays an important role in the production of lower-volatility products.61 This mechanism was recently used to describe the formation of ozone-initiated oxidation products of limonene by Walser et al.58 As shown in Figure 3, the 199 u alkylperoxy radical can undergo two distinct reactions: reaction with the HO2 radical forming a hydroperoxide or reaction with other alkylperoxy radicals forming more reactive alkoxy radicals (RO). The former pathway leads to a 200 u product that may isomerize to 7-hydroxy-limononic acid (199 m/z, VI), discussed (60) Nørgaard, A. W.; Nøjgaard, J. K.; Larsen, K.; Sporring, S.; Wilkins, C. K.; Clausen, P. A.; Wolkoff, P. Atmos. Environ. 2006, 40, 3460–3466. (61) Hatakeyama, S.; Izumi, K.; Fukuyama, T.; Akimoto, H.; Washida, N. J. Geophys. Res. 1991, 96, 947–958.

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Figure 3. Partial proposed mechanism describing SOA formation from limonene.

below. The latter pathway may lead to the formation of limonic acid (IV). Keto-limononic acid (V), which has been reported in SOA in other studies,56-58 has the same molecular formula as limonic acid; however, formation of this product most likely involves successive oxidation of the endo- and exocyclic double bonds. A logical mechanism of formation (not shown) of this C9 acid is the oxidative cleavage of the exocyclic double bond of II, resulting in the formation of V along with the Criegee intermediate, H2COO. Other structures with the same molecular formula C9H14O4, such as keto-7-hydroxy-limonaldehye, have been assigned to this structural composition in prior studies.57,58 A suggested pathway to this product from (I) is shown in Figure 3. As stated above, the 199 m/z ion is assigned to 7-hydroxylimononic acid (200 u, VI). Other structurally similar products have been assigned in other studies, each bearing two carbonyl 7922

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and hydroxyl equivalents and retaining the exocyclic double bond.56,58 Conversely, the 201 m/z ion is assigned to a 202 u product(s) with a molecular formula C9H14O5 that likely arises from gas-phase oxidation of the endocyclic double bond followed by subsequent heterogeneous ozonolysis of the exocyclic double bond; however, a contribution to this ion signal may arise from the addition of water to SCI1 and/or SCI2. Two possible and related structures for C9H14O5 are 7and 5-hydroxy-keto-limononic acid (VII, VIII) that have been reported in prior studies.50,56,57 Compound VII may arise from the ozonolysis of the exocyclic double bond of VI. These products may also be derived, in part, from the oxidative cleavage of the exocyclic double bond of limononic acid (II). Formation of the 201 m/z ion suggests successive ozonolysis of the endocyclic and exocyclic double bonds. Similar successive

ozone.51,57,58 However, as shown in Figure 2b-d, there is a strong ion signal at 115 m/z, which is the base peak in the mass spectra of SOA sampled early in the experiment (Figure 2b). This ion may arise from maleic acid (X) and/or levulinic acid (XI), which were high and moderate yield compounds identified by Jaoui et al.56 in SOA arising from the photooxidation of limonene. A better mechanistic understanding of the formation of these tentative C4 and C5 acids in SOA derived from limonene and the possible role of these low volatility species in the early stages of SOA growth merits further consideration.

Figure 4. Limonene oxidation product evolution as a function of time and processing. Signals are plotted as ion ratios for (black 9) 201/ 199, (red b) 187/199, and (blue O) 201/187. All ratios are normalized to the first measurement time point with COA ) 3.2 µg m-3. Dashed lines represent linear fits to the data. Error bars represent 1 standard deviation.

ozonolysis likely accounts for the formation of the 187 m/z ion, which arises from a 188 u product with formula of C8H12O5. This ion signal is assigned to keto-limonic acid (IX), which has been reported in other studies50,57 and shown by Jaoui et al.56 to be a high yield product in SOA derived from the photooxidation of limonene and may form through ozonolysis of the exocyclic double bond of limonic acid (IV). Figure 4 gives supporting evidence that reaction products arising from oxidation of the endocyclic double bond are important in the early stages of SOA formation and that these products are subsequently consumed in the later stages of SOA formation and growth via oxidation of the remaining (exocyclic) double bond. The 199 m/z ion signal that we assign to 7-hydroxy-limononic acid (VI), as well as slightly different structures proposed in other studies,56,58 all retain the exocyclic double bond, arising predominantly from endocyclic oxidation pathways; therefore the 199 m/z ion (i.e., 200 u product) is a proxy to endo-only double bond oxidation products. Conversely, the ion signals at 187 and 201 m/z have been assigned to particlephase compounds that arise from subsequent exocyclic ozonolysis (see above) and thus can be used as proxies to second generation heterogeneous oxidative chemistry. As would be expected from a stepwise process, there is a linear increase of both the 187 m/z and 201 m/z products relative to the first generation proxy ion signal (199 m/z) over the course of the chamber-based SOA forming experiment (and increasing COA) as shown by the ion signal intensity ratios: I201/I199 and I187/I199. Conversely, the relative ion intensities corresponding to the two successively ozonized proxy signals are relatively constant over the course of the experiment as shown by I201/I187. Most of the features of the NIR-LDI mass spectra can be rationalized in terms of the formation of low volatility C9 and C10 compounds that are in accord with the recent general mechanistic description of SOA formation from the oxidation of limonene by (62) Cabada, J. C.; Rees, S.; Takahama, S.; Khlystov, A.; Pandis, S. N.; Davidson, C. I.; Robinson, A. L. Atmos. Environ. 2004, 38, 3127–3141.

CONCLUSIONS This report has presented important results laying the groundwork for further analysis of organic aerosols under experimental conditions more closely simulating the real atmosphere utilizing a new near-infrared laser desorption/ionization aerosol mass spectrometer. In studies featuring pure as well as mixed particles, NIRLDI-AMS was shown to have single-compound limits of detection in the mid to low femtogram range, corresponding to total analysis times on the order of minutes for SOA mass loadings at low micrograms per meter cubed. Therefore, this method will be of direct utility for laboratory-based studies addressing the atmospheric processing of POA and further refinement may allow for highly time-resolved field detection of important components of POA such as oleic acid that are often on the nanogram per meter cubed scale in continental ambient aerosols. The utility of NIR-LDI-AMS for chamber-based SOA formation studies was clearly demonstrated with studies of secondary organic particles formed by ozonolysis of limonene. The direct, online monitoring afforded by NIR-LDI-AMS of important particlephase products, including limonic and keto-limononic acid (186 u, 185 m/z), keto-limonic acid (188 u, 187 m/z), 7-hydroxylimononic acid (200 u, 199 m/z), and isomers of hydroxy-ketolimononic acid (202 u, 201 m/z), at COA values from 3.2 to 25.0 µg m-3 are within the reported range of the total mass concentration of organic components of continental submicrometer aerosols.8,62 The development and advancement of new, high-sensitivity, analytical instrumentation, such as NIR-LDI-AMS, will help the atmospheric research community simulate and measure SOA formation and growth under conditions more representative of the actual atmosphere and enable more accurate parametrizations of chamber-based SOA formation. This data will allow further exploration of the discrepancy between modeled and observed atmospheric total SOA production rates and concentrations. ACKNOWLEDGMENT Financial support for this work was provided by the National Science Foundation (Grant ATM-0925052). SUPPORTING INFORMATION AVAILABLE Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

Received for review May 21, 2010. Accepted August 10, 2010. AC1013354 Analytical Chemistry, Vol. 82, No. 19, October 1, 2010

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