Comparison of FTIR and Particle Mass Spectrometry for the

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Environ. Sci. Technol. 2010, 44, 1056–1061

Comparison of FTIR and Particle Mass Spectrometry for the Measurement of Particulate Organic Nitrates EMILY A. BRUNS,† ´ RONIQUE PERRAUD,† VE ALLA ZELENYUK,‡ MICHAEL J. EZELL,† STANLEY N. JOHNSON,† YONG YU,§ DAN IMRE,| B A R B A R A J . F I N L A Y S O N - P I T T S , * ,† A N D M . L I Z A B E T H A L E X A N D E R * ,‡ Department of Chemistry, University of CaliforniasIrvine, Irvine, California 92697-2025; Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99354; and Imre Consulting, Richland, Washington 99352

Received September 30, 2009. Revised manuscript received December 9, 2009. Accepted December 18, 2009.

While multifunctional organic nitrates are formed during the atmospheric oxidation of volatile organic compounds, relatively little is known about their signatures in particle mass spectrometers. High resolution time-of-flight aerosol mass spectrometry (HR-ToF-AMS) and FTIR spectroscopy on particles impacted on ZnSe windows were applied to NH4NO3, NaNO3, and isosorbide 5-mononitrate (IMN) particles, and to secondary organic aerosol (SOA) from NO3 radical reactions at 22 °C and 1 atm in air with R- and β-pinene, 3-carene, limonene, and isoprene. For comparison, single particle laser ablation mass spectra (SPLAT II) were also obtained for IMN and SOA from the R-pinene reaction. The mass spectra of all particles exhibit significant intensity at m/z 30, and for the SOA, weak peaks corresponding to various organic fragments containing nitrogen [CxHyNzOa]+ were identified using HR-ToF-AMS. The NO+/NO2+ ratios from HR-ToF-AMS were 10-15 for IMN and the SOA from the R- and β-pinene, 3-carene, and limonene reactions, ∼5 for the isoprene reaction, 2.4 for NH4NO3 and 80 for NaNO3. The N/H ratios from HR-ToF-AMS for the SOA were smaller by a factor of 2 to 4 than the -ONO2/ C-H ratios measured using FTIR. FTIR has the advantage that it provides identification and quantification of functional groups. The NO+/NO2+ ratio from HR-ToF-AMS can indicate organic nitrates if they are present at more than 15-60% of the inorganic nitrate, depending on whether the latter is NH4NO3 or NaNO3. However, unique identification of specific organic nitrates is not possible with either method.

Introduction Atmospheric aerosols are known to impact climate, visibility, light scattering, and human health (1-3). Secondary organic aerosols (SOA) are a significant constituent of atmospheric aerosols. There is a variety of evidence from both laboratory and air sampling studies that organic nitrates, RONO2, are present in SOA (4-34). Organic nitrates are formed in the atmosphere during the day through photooxidation of organic compounds in the presence of NOx and at night, by NO3 radical reactions (4-9, 35, 36). Smaller molecular weight organic nitrate products are in the gas phase, while larger molecular weight and multifunctional products have lower vapor pressures and increasingly partition into the particle phase (37, 38). Traditional analytical approaches involve the collection of particles on filters and subsequent extraction for analysis using chromatographic, spectroscopic, and mass spectrometric techniques. This provides specific composition and quantification, but can be subject to positive or negative artifacts (e.g., adsorption of gas-phase products or evaporation of semivolatile organics) (39-41). Also, the nature of observed compounds (e.g., polar or nonpolar) can be sensitive to the choice of solvents used for extraction. Finally, this approach does not provide data in real-time. Particle mass spectrometry techniques have been developed to provide real-time information on aerosol composition and, in some cases, mass spectra of single particles (42-46). Particle mass spectrometers currently fall into two main categories based on the methods for vaporization and ionization: single particle instruments in which vaporization and ionization are accomplished using pulsed lasers (47-49), and instruments based on vaporization by thermal desorption followed by electron beam ionization (33, 34, 44, 46, 50-54). While particle mass spectrometry overcomes artifacts associated with traditional particle analysis and provides data in real-time, the spectra typically show significant fragmentation which can preclude molecular speciation of organics. Of particular interest to the present study is the qualitative and quantitative response of particle mass spectrometers to organic nitrates in particles and their ability to differentiate organic from inorganic nitrates such as NaNO3 and NH4NO3 that are common in ambient aerosol samples (35). We report here a comparison of results from a high resolution time-of-flight aerosol mass spectrometer (HRToF-AMS) and FTIR applied to the detection of organic nitrates in aerosols. Particles formed during the NO3 oxidation of R- and β-pinene, 3-carene, limonene, and isoprene were studied, along with a standard, isosorbide 5-mononitrate (IMN). For comparison, spectra of NH4NO3 and NaNO3 were also obtained. Spectra of IMN and of particles from the NO3R-pinene reaction were compared to spectra from a single particle mass spectrometer (SPLAT II). Implications for using particle mass spectrometry for real-time monitoring of organic nitrates are discussed.

Methods * Address correspondence to either author. Phone: (949) 824-7670 (B.J.F.-P.); (509) 371-6229 (M.L.A.). Fax: (949) 824-2420 (B.J.F.-P.); (509) 376-5106 (M.L.A.). E-mail: [email protected] (B.J.F.-P.); [email protected] (M.L.A.). † Department of Chemistry, University of CaliforniasIrvine. ‡ Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory. § Present address: California Air Resources Board, 9528 Telstar Avenue, El Monte, California 91731. | Imre Consulting. 1056

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The reactions of NO3, generated from the thermal decomposition of N2O5, with a series of biogenic terpenes were studied in 300 L Teflon chambers. The biogenics included (+)-R-pinene, (-)-β-pinene, (+)-∆3-carene, (+)-limonene, and isoprene. Details of the chemical sources and preparation, N2O5 synthesis and chamber preparation can be found in Supporting Information (SI). The biogenic concentrations were ∼2.2 ppm and the N2O5 concentration was ∼1 ppm for all biogenics, except isoprene, for which N2O5 was ∼6 ppm 10.1021/es9029864

 2010 American Chemical Society

Published on Web 01/08/2010

FIGURE 1. FTIR spectra of the organic nitrate reference compounds (a) isosorbide 5-mononitrate and (b) 2-ethyl-1-hexyl nitrate. as isoprene does not readily form SOA (55-57). Two experiments were performed for each biogenic with data collected by an aerosol mass spectrometer (AMS) and FTIR. For isoprene experiments, seed aerosol ((NH4)2SO4) was used to facilitate particle growth. Details of aerosol preparation for (NH4)2SO4, as well as NaNO3, NaNO2 and NH4NO3 are in the SI. All experiments were carried out at room temperature and under dry conditions, i.e., relative humidity e5%. FTIR. After particles had formed and grown to at least 150-200 nm in mean aerodynamic diameter as determined by AMS, particles were collected by impaction on polished ZnSe windows (25 × 2 mm, Reflex Analytical Corporation) using a modified Sioutas cascade impactor (SKC, Model 225-370) and then analyzed using transmission FTIR (see SI). Calibration for quantification purposes was conducted using isosorbide 5-mononitrate (LKT Laboratories, Inc., >98%) as a standard. A solution of IMN in acetonitrile was placed directly on a ZnSe window and analyzed after the solvent evaporated. Neat 2-ethyl-1-hexyl nitrate (2-EHN) (Sigma Aldrich, 97%) was analyzed using the same method. HR-ToF-AMS. Real-time particle sampling was performed using an Aerodyne HR-ToF-AMS (Aerodyne Research Inc.). Details of this instrument have been described elsewhere (44, 46) and its application here is described in the SI. Continuous monitoring of the chamber by HR-ToF-AMS began after the addition of ultra zero grade air and the biogenic precursor and continued during the addition of N2O5 and subsequent particle formation and growth. SPLAT II. A description of SPLAT II (49, 58) can be found in the SI. SPLAT II data were collected for aerosolized IMN, as well as from an additional experiment performed in a 100 L Teflon chamber containing R-pinene (∼1 ppm) and N2O5 (∼1.1 ppm).

Results and Discussion FTIR. FTIR spectra of the organic nitrate references IMN and 2-EHN are shown in Figure 1. IMN was selected as the standard due to its multifunctional character, which is also expected of the biogenic oxidation products, and its commercial availability. In contrast, 2-EHN contains only the nitrate group. Despite these structural differences, the reference compounds show few differences in the characteristic organic nitrate peaks at 860 cm-1 (NO symmetric stretch), 1280 cm-1 (NO2 symmetric stretch) and 1630-1640 cm-1 (NO2 asymmetric stretch) (59). Peaks in the 2800-3050 cm-1 region correspond to C-H stretching vibrations. Figure 2 shows the infrared spectra of particles from the NO3 oxidations of R- and β- pinene, limonene, 3-carene, and

FIGURE 2. FTIR spectra of products from oxidation of biogenics by NO3. isoprene. The spectra are similar to those of IMN and 2-EHN, with strong organic nitrate peaks, and weaker peaks in the C-H stretching region. Small peaks in the 1700-1740 cm-1 region corresponding to CdO are also seen in these particles. While the formation of such groups is expected (4, 5, 8, 9, 14, 16, 18, 24, 29, 35, 60), the relative intensity of the CdO peaks are perhaps surprisingly small. Pinonaldehyde is the major carbonyl-containing product of the R-pinene-NO3 reaction (5, 8, 9), and although its absorption cross section is similar to that of the 1280 cm-1 nitrate peak (32), its vapor pressure is at least an order of magnitude greater than the expected organic nitrate products (29). Hence, pinonaldehyde should partition primarily to the gas phase and not contribute as much to the particles for which the spectrum is shown in Figure 2. The limonene product spectrum shows a small peak at 3080 cm-1 due to a vinyl CdC-H group, suggesting that under these reaction conditions, one of the double bonds in the molecule has not completely reacted. Particle Mass Spectrometry. Figure 3 shows low resolution (V-mode) AMS mass spectra, as well as SPLAT II mass spectra for IMN and for particles formed in the NO3-oxidation of R-pinene. There are some differences between the AMS and SPLAT II mass spectra, reflecting the different vaporization/ionization processes. In SPLAT II, photoionization at 193 nm to give NO+ (m/z 30) is extremely efficient, whereas with AMS, volatilization occurs when the particle strikes a 600 °C metal plate and ionization occurs by electron impact of the volatilized species. Electron impact mass spectra are available for organic nitrates (61, 62). The molecular cation formed by electron impact undergoes extensive fragmentation, with four common pathways for terminal alkyl nitrates (61, 62): {RCH2ONO2}+ f RCH2O + NO+ 2

(1)

f RCH+ 2 + ONO2

(2)

f R′CH2ONO+ 2 + H

(3)

f R + CH2ONO+ 2

(4)

Further fragmentation of NO2+ (m/z 46) from reaction 1 generates NO+ (m/z 30). The alpha cleavage in reaction 2 produces an organic cation fragment that will be detected by mass spectrometry while the neutral -ONO2 fragment will be “silent” because it does not carry a charge. For VOL. 44, NO. 3, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. AMS and SPLAT II mass spectra of atomized isosorbide 5-mononitrate and particles from the NO3 (1 ppm N2O5) reaction with r-pinene (1 ppm).

TABLE 1. Organic Nitrate Fragments Seen in Biogenic Oxidation Experiments from HR-ToF-AMS

TABLE 2. Quantification of AMS and FTIR Resultsa,b

precursor

CH2ONO2+ (m/z 76)

additional organic nitrate fragments (m/z)

precursor

HR-ToF-AMS NO+/NO2+((2s)

R-pinene

no

β-pinene limonene 3-carene isoprene

yes no no yes

CH3NO+ (45), C3H7NO+ (73), C3H5NO2+ (87) CH3NO+ (45) CH3NO+ (45) CH3NO+ (45) CH3NO+ (45), CH2NO2+ (60)

R-pinene β-pinene limonene 3-carene isoprene NH4NO3 NaNO3

11 ( 8c 10 ( 2 15 ( 8 14 ( 12 5.0 ( 0.7 2.4 ( 1.6d 80 ( 40d

multifunctional compounds, e.g., containing -OH, loss of neutral NO2 or HNO3 to generate an organic cation and its fragments is common, sometimes with further loss of H2O (22, 61, 62). Consistent with the known fragmentation of organic nitrates (61, 62), high resolution AMS data for IMN, as well as oxidation product mass spectra, show that the m/z 46 is due to NO2, the m/z 30 peak is primarily due to NO+, and there are a number of organic fragments. [CxHyNzOa]+ Fragments. Peaks at NO+ and NO2+ are also observed for inorganic nitrate that is common in airborne particles, and hence these fragments are not specific to organic nitrates. Therefore, a search of the AMS high resolution spectra was carried out for [CxHyNzOa ]+ fragments that could in principle be used for specific identification of organic nitrates. Table 1 summarizes the organic nitrate fragments observed at m/z < 101. As expected from reaction 4, the CH2ONO2+ fragment at m/z 76 is observed in the case of the terminal alkenes β-pinene and isoprene, but not in the products from R-pinene or 3-carene chemistry. Limonene contains two double bonds, one of which is terminal, yet CH2ONO2+ was not observed. As seen in the FTIR spectra in Figure 2, the remaining CdC-H peak at 3080 cm-1 shows that the products still contain a double bond. The lack of a CH2ONO2+ fragment in the mass spectrum is consistent with earlier studies that report addition of NO3 at the ring double bond is favored (5, 8). While these fragments are unique to organic nitrates, their intensities are weak relative to other organic peaks in the spectrum, including peaks at the same nominal mass, and under most atmospheric conditions, may be undetectable in the complex mixtures of organics and inorganics found in airborne particles. For example, in the β-pinene and isoprene product spectra, the CH2ONO2+ peak is roughly a factor of 100 smaller than the NO+ peak. 1058

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HR-ToF-AMS N/H ((absolute uncertainty)

FTIR n(-ONO2)/ n(C-H) ((2s)

0.03 ( 0.01e 0.03 ( 0.01 0.04 ( 0.01 0.04 ( 0.01 0.10 ( 0.03

0.10 ( 0.06c 0.10 ( 0.01 0.14 ( 0.03 0.12 ( 0.08 0.39 ( 0.03

a For the IMN standard, NO+/NO2+ is 15. b For an R-pinene-NO3 oxidation experiment, there was no intensity at m/z 46 attributed to NO2+ using SPLAT II. c Errors (2 × sample standard deviations) for the Teflon chamber experiments were calculated from the values in the two sets of experiments. d Errors for the inorganic nitrate values were calculated from data of several atomizer experiments (11 for NaNO3 and 3 for NH4NO3). e HRToF-AMS N/H errors are from error propagation of absolute uncertainties for N/C and H/C (69, 71).

Ratio of NO+/NO2+. As an alternative approach, the ratio of NO+/NO2+ was examined to determine if it could be used to differentiate between inorganic and organic nitrate. Ammonium nitrate is particularly common in airborne particles, but sodium nitrate is also important in coastal urban areas where chloride in sea salt can be replaced by reactions with various oxides of nitrogen (35, 63). The low resolution AMS spectra give high sensitivity, but because of the complex nature of the organic products, organic fragments may also contribute to m/z 30 and m/z 46. An example is given in the SI (Figure S1), where both NO+ and CH2O+ contribute to the peak at m/z 30 from particles formed in the β-pinene-NO3 reaction. High resolution AMS data were therefore used to unequivocally determine NO+/NO2+. Table 2 summarizes the NO+/NO2+ ratio for the particles formed in the NO3-biogenic oxidation experiments. For the products of R-pinene, β-pinene, limonene, and 3-carene, the average ratio is 13 ( 5 (2s) while the ratio for isoprene products is 5.0 ( 0.7 (2s), much different than 80 for NaNO3 and 2.4 for NH4NO3. Fry et al. (16) reported a similar NO+/ NO2+ ratio of ∼10 for the oxidation of β-pinene by NO3 using a HR-ToF-AMS. If the contribution of organic nitrate to total nitrate is greater than 15%, then the NO+/NO2+ ratio will be statistically different than for NH4NO3 alone. For NaNO3, the contribution of organic nitrates to total nitrate must be greater than 60%.

The ratio for m/z 30/46 from literature EI spectra is 0.3-0.5 (61, 62), much smaller than measured here for NO+/NO2+ (Table 2). Alkyl nitrates are known to undergo thermal decomposition with lifetimes of the order of 10-7 s at 600 °C (13, 64): ∆

RONO2 98 RO + NO2

(5)

Although the contact time with the heated plate in AMS is short, some fraction of the organic nitrate will decompose during vaporization of the particle which occurs over ∼10-4 to 10-5 s (54). Support for reaction 5 is found in the decrease in the ratio of the intensity of CH2ONO2+ (m/z 76) to that of total organic signal as the vaporizer temperature was increased from 150 to 300 °C (Figure S2 of the SI). After vaporization, the electron beam will ionize RO, NO2, and undissociated RONO2. The relatively strong signal at m/z 43 (C2H3O+) seen in the AMS spectra (Figure 3) compared to SPLAT II spectra may result from efficient ionization and fragmentation of the RO formed in (1). A NIST database electron impact spectrum for NO2 shows an m/z 30/46 ratio of ∼3 (65). If NO2 is generated in reaction 5, and subsequently ionized, the NO+/NO2+ ratio would be larger than the 0.3-0.5 reported for organic nitrates (61, 62), but not above a value of 3. However, increased fragmentation could result if the NO2 has excess energy. There is some indirect support for the possible contribution of excess internal energy to the decomposition of NO2 in terms of the quantum yields for its photolysis in the region around 400 nm. The thermodynamic limit for photodissociation of NO2 to O(3P) at room temperature is 398 nm, but significant O(3P) production continues out to 420 nm (35), which has been attributed to contributions from internal (vibration-rotation) energy and collisions (66). The NO+/NO2+ ratio of 2.4 for NH4NO3 is in good agreement with a value of 2.9 reported by Fry et al. (16). Ammonium nitrate is known to decompose to HNO3 + NH3 and to N2O + 2H2O (67). The NO+/NO2+ ratio is ∼0.5 for an electron impact mass spectrum of pure HNO3 (68), much smaller than the 2.4 measured here for NH4NO3. However, N2O has a strong peak at m/z 30, but no peak at m/z 46 and a contribution from small amounts of N2O would therefore increase the measured NO+/NO2+ ratio. Sodium nitrate is known to decompose at ∼320 °C to form NaNO2 + 1/2O2 and presumably the NaNO2 can also undergo some thermal decomposition in the vaporizer region (67). Atomized NaNO2 was measured here to give a NO+/NO2+ of ∼700. Even a relatively small contribution from NaNO2 could account for the higher NO+/NO2+ for NaNO3 compared to NH4NO3. Comparison of the Nitrogen Content of Particles by AMS and FTIR. The N/H ratio can be measured from AMS data using the approach of Aiken et al. (69) which is incorporated into APES, the AMS software for the determination of atomic ratios of C, H, N, and O (see SI). Analysis of IMN gave an N/H ratio of 0.10 ( 0.02 (absolute uncertainty), in good agreement with the true value of 0.11. The FTIR data independently provides a ratio of -ONO2 groups to C-H bonds as described elsewhere (32) and in the SI. This should be the same as the N/H ratio from AMS if it is assumed that each N represents one organonitrate functionality and that each hydrogen is bonded to a carbon (after removal of contributions due to H2O). The last two columns of Table 2 compare the N/H ratio measured using AMS and the -ONO2/ C-H ratio from FTIR for each of the biogenic reactions. The AMS ratio is smaller by a factor of ∼3 to 4 compared to that from FTIR. There are a number of possible reasons for this discrepancy. A likely factor is that the fragmentation of the initially formed molecular cation of the multifunctional organic

nitrate products expected for these reactions preferentially generates an organic cation and neutral NOx fragments such as NO2, HNO3, or ONO2 which would be silent in the mass spectrum (22, 61, 62). To test this, the approach of Aiken et al. (69) was applied to obtain the N/H ratio from the published EI mass spectrum of HOCH2CH2ONO2 (61, 62). The N/H ratio obtained without taking into account an N+ fragment (data not reported for m/z 14) was 0.08, a factor of 2.5 smaller than the true value of 0.20. While addition of a contribution from N+ would increase the N/H ratio from the mass spectrum, the correction is unlikely to be as large as the factor of 2.5. The relatively good agreement for IMN likely reflects less fragmentation to give neutral NOx species, which may be due to the larger separation of the -OH and -ONO2 groups and to its rigid ring structure compared to the multifunctional compounds expected from the NO3-biogenic reactions where these groups will be adjacent. Organic nitrate products could also be lost in the vacuum of the HR-ToF-AMS. Using the group contribution method of Pankow and Asher (38) to calculate the typical vapor pressures of expected major organic nitrate products yields vapor pressures in the range of 10-6 to 10-8 atm (29). Given these relatively small vapor pressures, it is not clear that losses of organic nitrates of the order of 70% from the particles could occur in the milliseconds that the particles are in vacuum before reaching the vaporizer. Both positive and negative artifacts can occur during impactor collection used for FTIR analysis. A higher ratio for -ONO2/C-H from FTIR compared to the AMS data could occur if uptake of gas phase organic nitrate products was enhanced by the presence of the organic thin film on the impactor, i.e., a shift in the partitioning of RONO2 occurs. Alternatively, volatilization of non-nitrate organic products from the impactor plate would also cause a higher -ONO2/ C-H ratio. A time-series study comparing the organic nitrate signal relative to the carbonyl peak from 15 min to 2 h showed no change within experimental error in the relative peak strengths, even though there is a 20-fold increase in the amount of organic on the impactor, suggesting such an artifact is not important. In summary, FTIR has the advantage that it provides identification of functional groups such as organic nitrates, and if care is taken to rule out interfering peaks, quantification of the contribution of functional groups is also possible. Indeed, this approach has been successfully used for ambient air samples (11, 17, 25). Bae et al. (70) used AMS measurements of the m/z 30/46 ratio to infer a contribution of organic nitrates in particles at a rural site, although these studies were carried out using a lower resolution instrument so there may have been contribution of organic fragments at these masses. Our data show that organic nitrates must be present at concentrations equal to at least 15-60% of inorganic nitrate (depending on whether the inorganic is NH4NO3 or NaNO3 respectively) to be statistically observable as a change in the NO+/NO2+ ratio. While fragments of the type [CxHyNzOa]+ are indicative of the presence of organic nitrates, their small intensities are unlikely to be useful in atmospheric samples. There is clearly a need for the development of new techniques that will provide real-time organic molecular speciation as well as measurements of the inorganic components.

Acknowledgments We are grateful to the U.S. Department of Energy (Grant No. DE-FG02-05ER64000) for support of this work. This research was in part performed in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the U.S. Department of Energy’s Office of Biological and Environmental Research at Pacific Northwest National Laboratory (PNNL) and supported by the U.S. Department of Energy Office of Basic Energy Sciences, Chemical Sciences VOL. 44, NO. 3, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Division. PNNL is operated by the U.S. Department of Energy by Battelle Memorial Institute under Contract No. DE-AC0676RL0 1830. E.A.B. would like to thank the National Science Foundation for a Graduate Research Fellowship. Additional support was provided by the AirUCI Environmental Molecular Science Institute (Grant No. CHE-0431312) funded by the National Science Foundation. We thank P. Ziemann and J. Pankow for helpful discussions.

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Supporting Information Available Experimental methods, as well as two figures mentioned in the text. This information is available free of charge via the Internet at http://pubs.acs.org/.

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