Particle-Phase Chemistry of Secondary Organic Material - American

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Particle-Phase Chemistry of Secondary Organic Material: Modeled Compared to Measured O:C and H:C Elemental Ratios Provide Constraints Qi Chen,† Yingjun Liu,† Neil M. Donahue,‡ John E. Shilling,§ and Scot T. Martin*,†,|| †

School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, United States Center for Atmospheric Particle Studies, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States § Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory, Richland, Washington, United States Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, United States

)

‡

bS Supporting Information ABSTRACT: Chemical mechanisms for the production of secondary organic material (SOM) are developed in focused laboratory studies but widely used in the complex modeling context of the atmosphere. Given this extrapolation, a stringent testing of the mechanisms is important. In addition to particle mass yield as a typical standard for model-measurement comparison, particle composition expressed as O:C and H:C elemental ratios can serve as a higher dimensional constraint. A paradigm for doing so is developed herein for SOM production from a C5
C10
C15 terpene sequence, namely isoprene, R-pinene, and β-caryopyhllene. The model MCM-SIMPOL is introduced based on the Master Chemical Mechanism (MCM v3.2) and a group contribution method for vapor pressures (SIMPOL). The O:C and H:C ratios of the SOM are measured using an Aerosol Mass Spectrometer (AMS). Detailed SOM-specific AMS calibrations for the organic contribution to the H2Oþ and COþ ions indicate that published O:C and H:C ratios for SOM are systematically too low. Overall, the measurement-model gap was small for particle mass yield but significant for particle-average elemental composition. The implication is that a key chemical pathway is missing from the chemical mechanism. The data can be explained by the particle-phase homolytic decomposition of organic hydroperoxides and subsequent alkyl-radical-promoted oligomerization.

’ INTRODUCTION Secondary organic material (SOM) produced by the oxidation of biogenic volatile organic compounds (VOCs) is a major global contributor to the mass concentration of organic particles. The chemical and physical processes associated with SOM formation are, however, complex and not fully understood or quantified.1 Environmental chambers have been used as an approach for simulating the complexity of atmospheric SOM in a laboratory setting, allowing for the systematic control of variables and replicate measurements of the observations. The results from chamber experiments have contributed greatly to the scientific community’s knowledge of important end points relevant to climate and air quality, such as the particle mass yield, the molecular composition, and the chemical mechanisms of SOM production.2
4 Chemical models, with their results juxtaposed to measurements, are valuable for assessing the accuracy and completeness of our current knowledge of SOM production. Detailed gasphase reaction mechanisms and absorptive partitioning thermodynamics have been employed to predict SOM production.5
7 Model skill typically has been judged by comparison between observed and modeled particle mass yields over a range of r 2011 American Chemical Society

experimental conditions. For example, Ceulemans et al.8 showed that the particle mass yields of R-pinene oxidation were predicted well for a large set of experiments. More recently, molecular measurements have imposed additional constraints on the models. Analysis by electrospray ionization mass spectrometry (ESI-MS), for example, of organic material collected on filters provided qualitative, if not quantitative, information on some specific molecules present in the particle-phase SOM.9
11 Camredon et al.12 showed that modeled products of R-pinene oxidation in large part explained the monomer region of the ESI-MS observations. This comparison was an important qualitative test of the chemical model at a molecular level. A recent development is the capability for in situ quantitative measurements of oxygen-to-carbon (O:C) and hydrogen-to-carbon (H:C) elemental ratios of particle-phase organic material.13,14 Specifically, the Aerodyne high-resolution time-of-flight Aerosol Mass Received: December 31, 2010 Accepted: April 28, 2011 Revised: April 20, 2011 Published: May 11, 2011 4763

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Environmental Science & Technology Spectrometer (HR-ToF-AMS) provides real-time electron-impact ionization mass spectra with sufficient mass-resolving power to determine elemental ratios. The in situ capability is important because the semivolatile nature of SOM implies that, for ex situ methods, shifts in the particle-phase composition can occur during sample collection, storage, and analysis.15 Herein, HR-ToF-AMS data sets are reported for particle-phase SOM produced by (i) the photooxidation of isoprene (C5H8),16 (ii) the dark ozonolysis of the monoterpene R-pinene (C10H16),17 and (iii) the dark ozonolysis of the sesquiterpene β-caryophyllene (C15H24).18 The data sets were collected over a range of organic particle mass concentrations Morg, as is important because of the shifting composition of SOM with Morg.19 The data are used in an analysis paradigm that compares measured particle mass yields and elemental ratios to those predicted by a chemically detailed model. The model couples the gas-phase oxidation scheme from the Master Chemical Mechanism (MCM v3.2)20 to an online gas-toparticle absorptive partitioning model using a group contribution method (SIMPOL)21 to predict vapor pressures. For all three precursor VOCs, the comparisons constrain the plausibility of potential particle-phase SOM formation pathways.

’ EXPERIMENTAL SECTION Secondary organic material was produced by VOC oxidation in the Harvard Environmental Chamber (HEC). The chamber was operated as a steady-state continuous-flow mixed reactor under low-NOx conditions and with a residence time of 3.5 h at 25 ( 1 °C and 40 ( 2% relative humidity (RH). Both photo-oxidation and ozonolysis experiments were conducted. For the ozonolysis experiments, the O3 concentration was maintained by feedback control, and OH scavengers were 1-butanol and cyclohexane for the Rpinene and β-caryophyllene ozonolysis experiments, respectively. For the photo-oxidation experiments, H2O2 photolysis was used as the initiator of the OH radical cycle. The minimum detection limit (MDL) of the NOx analyzer (Teledyne 200E) was 1 ppbv, and most experiments corresponded to conditions of NOx < MDL. The SOM particles exiting the chamber were continuously characterized by an HR-ToF-AMS.22 Data were averaged for 4
12 h to represent each steady-state condition. A collection efficiency of 1.0 was used on the basis of the agreement of the AMS mass concentration with the density-compensated volume concentration measured by a scanning mobility particle sizer (TSI 3936).17 The O:C and H:C elemental ratios for the particle-phase SOM were determined using the general method outlined by Aiken et al.13,14 but with specific calibration for (COþ)org and (H2Oþ)org. The chemically detailed MCM-SIMPOL model was used to predict the production and the elemental composition of the particle-phase SOM. The kinetic schemes for the gas-phase oxidation of the precursor VOCs were extracted from MCM v3.2.20 The vapor pressure of each species in the mechanism was estimated using SIMPOL.21 Dynamic particle-phase SOM production was governed by a net rate of absorptive condensation.21 Model runs were initialized using the conditions of the experiments.16
18 Further information concerning the experiments, the data analysis, and the modeling is provided in the Supporting Information (SI) as well as in refs 16
18. ’ RESULTS AND DISCUSSION Model-Measurement Comparisons. The results for each type of particle-phase SOM are represented in the columns of

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Figure 1. The four rows respectively represent the particle mass yield, the grouping of products by volatility, the particle-average O:C and H:C elemental ratios for increasing Morg, and the O:C and H:C ratios of specific molecules compared to the AMSmeasured particle-average values. The detailed reaction conditions, the produced organic particle mass concentrations Morg, and the measured O:C and H:C elemental ratios are listed in Table S1 of the SI. Particle mass yield (row a) for 1 < Morg < 15 μg m
3 ranges from 1 to 5%, 5 to 40%, and 10 to 40% for isoprene photooxidation, R-pinene dark ozonolysis, and β-caryophyllene dark ozonolysis, respectively, all at low NOx. The studied range of Morg is representative of atmospheric conditions, which typically vary from less than 1 up to 15 μg m
3 outside urban regions.23,24 For isoprene photooxidation, yield depends on many factors, including Morg, the NOx concentration, the temperature, and the extent of reaction (i.e., oxidation). Compared to these factors, the effect of relative humidity is smaller, at least for a range of 10
40% RH with solid seed particles.25
27 In a metastudy covering a range of laboratory conditions, Carlton et al.25 formulated a general parametrization for isoprene photooxidation. The measured yields of this study are in agreement with that parametrization (panel a1). The MCM-SIMPOL modeled yields also reproduce the measurements within a difference of less than 1% yield. Given that our low-NOx conditions are defined as 103 μg m
3. There are few products having C*i < 10 μg m
3, as is consistent with the 4764

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Figure 1. HR-ToF-AMS measurements and MCM-SIMPOL predictions. Results are shown in columns 1, 2, and 3 for the photooxidation of isoprene, the dark ozonolysis of R-pinene, and the dark ozonolysis of β-caryophyllene, respectively. Row a shows particle mass yields at 298 K. Particle mass yield is the measured organic particle mass concentration divided by the reacted VOC mass concentration.17 Curves are shown for the domains of the original experimental data. The data from Carlton et al.25 and Pathak et al.28 were normalized to a density of 1400 kg m
3. The data of King et al.16 and Shilling et al.17 were re-analyzed by the methodology of the present study. Model conditions are listed in SI Table S2. Row b shows the modeled mass concentrations of gas- and particle-phase molecules grouped into bins of decadal volatility. Row c shows the modeled and measured particle-average O:C (purple) and H:C (green) ratios for increasing particle mass concentrations Morg. Error bars represent the measurement precision (1 σ). Row d shows the O:C and H:C ratios of the AMS-measured particle-average values for different Morg (gray squares; SI Table S1) as well as specific molecules reported in the literature (cross symbols; SI Table S3). The gas- and particle-phase molecules predicted by MCM-SIMPOL are shown by circles of different sizes. The area of each circle represents the species concentrations on a carbon-number basis. In cases that have several different species that have the identical O:C and H:C ratios, the area represents the sum of the concentrations. In panels b2, d2, b3, and d3, results are shown for model runs that control OH at 0 molecules cm
3, thereby avoiding the confounding products of OH scavenger oxidation.

Figure 2. Proposed dehydration and oligomerization reaction sequence. The decomposition of organic hydroperoxides (R
OOH) produces radicals that initiate the sequence. Abbreviations in the figure include VOC, volatile organic compounds (e.g., isoprene, R-pinene, β-caryophyllene); SVOC, semivolatile organic compounds (produced by the oxidation of VOCs); R•, alkyl radical; RO•, alkoxy radical; R
OOH and RH are SVOC molecules.

observed low particle mass yields.25 In contrast, the oxidation products of R-pinene ozonolysis are widely distributed across low-volatility bins (0.1
10 μg m
3). As an alternative presentation to row b, the volatility distribution can be reformatted using the volatility-basis-set (VBS) approach. The transformation equations are presented in section D of the SI. Figure S2 shows that the reformatted distribution of the MCM-SIMPOL results is in agreement with the distribution inferred from laboratory measurements of particle yield during R-pinene ozonolysis17,30 (cf. Figure 2, Presto et al.30). The panels in row c show the measured particle-average O:C and H:C ratios. For the range of investigated Morg, the measured ratios are 0.75
0.88 (O:C) and 1.78
1.89 (H:C) for isoprene photooxidation, 0.33
0.52 and 1.47
1.56 for R-pinene ozonolysis, and 0.33
0.54 and 1.47
1.57 for β-caryophyllene ozonolysis (SI Table S1). The respective average carbon oxidation states are
0.3 to 0.1,
0.9 to
0.4, and
0.8 to
0.5 for the three different SOMs.15 The ranges correspond to the addition of 4
5, 3
5, and 5
8 oxygen atoms to C5, C10, and C15 structures, respectively. Isoprene SOM is the most oxygenated among the three types of studied SOMs. For fixed Morg, the O:C ratio of β-caryophyllene SOM is greater than that of R-pinene SOM, which can be explained by the two double bonds in the former compared to one in the latter. Row c further shows that the

measured O:C ratios increase for decreasing Morg for all three SOMs. The explanation is that the least-volatile products condense to the particle phase at low Morg31 and that O:C serves as predictor of volatility for many molecular classes. The measured particle-average O:C and H:C ratios are in agreement with other reports using the HR-ToF-AMS to study SOM provided that the same default values for the (COþ)org: þ þ (COþ 2 )org and (H2O )org:(CO2 )org ratios are used in the AMS 14,26 However, in place of these data analysis (SI Figure S3). default values, SOM-specific calibrations for the (COþ)org and (H2Oþ)org signals were carried out as part of the present study, as þ described in the SI. The measured (COþ):(COþ 2 ) and (H2O ): þ (CO2 ) ratios of the studied SOMs were greater than the default values of the AMS analysis software. A possible explanation is the fragmentation of multifunctional organic hydroperoxides, which can have high concentrations in SOM.32,33 Compared to the conventional analysis based on default values, the SOMspecific calibrations increased O:C by 0.05 to 0.20 and H:C by 0.05 to 0.30, depending on the SOM studied and the value of Morg (SI Table S1). As a check on accuracy, the O:C ratios we report for particlephase SOM produced by R-pinene ozonolysis are consistent with those obtained using high-resolution ESI-MS and nano laserablated mass spectrometry, specifically 0.34 to 0.53 for Morg of 3.5 to 400 μg m
3.34,35 Our measured H:C ratios are also consistent with the main composition domain reported for the ESI-MS analysis of filter samples (i.e., 1.4
1.8).10,11 For isoprene, our measured ratios for SOM produced by photooxidation are greater than the O:C of 0.6 ( 0.2 and H:C of 1.5 ( 0.3 at Morg of 15
40 μg m
3 measured by ESI-MS for SOM produced by isoprene dark ozonolysis.36 The explanation can be a combination of the lower values of Morg of the present study (i.e., 1 < Morg < 15 μg m
3) as well as the greater extent of oxidation that can be expected from the OH-driven pathways of photooxidation compared to the O3-driven ones of ozonolysis. Specific particle-phase molecules produced by isoprene, Rpinene, and β-caryophyllene oxidation have been identified in the literature (SI Table S3), principally ex situ by chromatography, and in some cases molar yields have been reported.18,37 In these cases particle-average O:C and H:C ratios can be estimated to further check the accuracy of the AMS-measured elemental ratios. Results are shown in panels c2 and c3. For high Morg, the molecularly derived estimates agree well with the in situ measurements of the HR-ToF-AMS. The implication is that the products identified ex situ are indeed the major compounds in the particles when suspended as an aerosol, at least for high Morg. By comparison, in the case that data are available for low Morg (e.g., panel c3 for β-caryophyllene SOM), the measured O:C ratios are greater than those calculated from the known products, 4766

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Environmental Science & Technology suggesting that unidentified products having low volatility and high O:C compose a significant fraction of the particle mass. Such unidentified products are also important for R-pinene ozonolysis at low Morg, as suggested by Shilling et al.19 and Gao et al.35 These unidentified products are possibly formed by an oligomerization mechanism within the particle phase1 and appear to include unspeciated hydroperoxides.32,33 The panels of row c also show the comparison between the measurements (points) and the MCM-SIMPOL predictions (dashed lines). The modeled ratios differ from the measured ones by up to þ0.25 (O:C) and þ0.20 (H:C) for isoprene SOM, þ0.14 (O:C) and þ0.25 (H:C) for R-pinene SOM, and
0.25 (O:C) and þ0.13 for β-caryophyllene SOM. These differences exceed the uncertainty of the AMS measurement accuracy, stated as 30% for O:C and 10% for H:C by Aiken et al.14 based on a data set of 59 molecular standards. These uncertainties arguably represent upper limits for particle-phase SOM because of compensating molecular effects in a mixed organic material that has a wide range of functional groups.14 For isoprene photooxidation, SI Figure S1 shows that the stated model-measurement differences are lower values because there is a dependence of the modeled ratios on the NOx concentration and the actual concentrations of this study vary from less than 1 up to 2 ppbv (SI Table S1). In addition to the quantitative offset between measurement and model for any single Morg, the directions of change for increasing Morg are also not fully consistent between model and measurements. For isoprene photooxidation, the modeled O:C and H:C ratios have little dependence on Morg whereas the measured values decrease for increasing Morg. For R-pinene ozonolysis, although the modeled and measured O:C ratios have similar trends with Morg, the H:C ratios have opposite trends. For β-caryophyllene ozonolysis, however, both the modeled and measured O:C and H:C ratios have similar directionality for increasing Morg. The overall conclusion of these model-measurement comparisons for elemental ratios is that there are important gaps in the accuracy of the underlying chemical mechanism and thermodynamics represented in MCM-SIMPOL. The panels in row d of Figure 1 show Van Krevelen plots (i.e., H:C and O:C axes) for the gas- and particle-phase molecules predicted by MCM-SIMPOL (circles), the known molecular products reported in the literature (crosses), and the particleaverage values (gray squares: measurements; red squares: predictions). Bias is apparent for MCM-SIMPOL to high O:C and H:C for isoprene and R-pinene SOM and to high H:C and low O:C for β-caryophyllene SOM. Particle-Phase Peroxide Chemistry. The particle-phase products predicted by MCM-SIMPOL are largely organic hydroperoxides (SI Table S4). Organic hydroperoxides have been suggested several times in the literature as important components of particle-phase SOM, though molecular characterization is difficult. For ozonolysis, Reinnig et al.38 using mass spectrometry and Maksymiuk et al.39 using NMR both provided evidence of organic hydroperoxides. Docherty et al.32 studied organic peroxides present in SOM produced by R-pinene oxidation. Surratt et al.33 found that organic peroxides were an important component of particle mass for SOM produced by isoprene photooxidation under low-NOx conditions. Although some of these studies were carried for experiments of under 5% RH, compared to our experiments at 40% RH, Docherty et al.32 found no significant difference in organic peroxide production for R-pinene ozonolysis at 105 ppm (carbon number basis) of reactive functional groups is present. For a multiple of >104 in the relative concentrations of reactive functional groups and molecular oxygen, we propose that reaction of the alkyl radical with molecular oxygen is a minor pathway and that instead the alkyl radical has a variety of different fates in the particle phase. Many of these reactions can lead to the formation of particle-phase oligomers.43 As shown in Figure 2, example pathways include the addition to the double bond of an enol as a radical propagation step or recombination with another alkyl radical as a termination step.44 Meanwhile, possible fates of the alkoxy radical of Reaction 1 are decomposition (e.g., RO• f R• þ HCHO) and isomerization (e.g., RO• f (OH)R•), both producing an additional alkyl radical.42 The net result of the proposed chemistry is (i) a shift in O:C and H:C ratios in a manner that reduces the measurementmodel gap, (ii) a decrease in the overestimated peroxide concentration but no significant change in the total particle mass, and (iii) the production of high-molecular-weight organic molecules of diverse multigeneration structures that are representative of literature reports for oligomeric species. For β-caryophyllene SOM, some additional measurementmodel gaps are apparent in Figure 1. Most notably, MCMSIMPOL overpredicts the particle mass yield (panel a3). About 80% of the predicted particle mass consists of secondary ozonides (SOZs) (SI Table S4). This high fraction, however, is not congruent with our concurrent molecular product analysis reported in ref 18. That study substantially accounted for particle mass without any detection of secondary ozonides in the particlephase products, as studied by ESI-MS. Rather than the MCM chemistry that produces SOZs, the stabilized Criegee intermediate seems not to rearrange in SOZs at 40% RH but rather to react with gas-phase water.1 Another factor is that SIMPOL possibly underestimates the vapor pressure of the SOZs,45 thereby including them as particle-phase species when instead they remain in the gas phase, if they form. The MCM-SIMPOL model was run with a modification that prevented the gas-to-particle partitioning of SOZs (SI Table S2), and the result (SI Figure S6) shows that the measurement-model gap for particle mass yield is significantly reduced. In summary, the analysis paradigm of this study was to compare modeled and measured particle mass yields and elemental ratios in a multidimensional framework to constrain complex chemical mechanisms. With just information on particle mass yields (i.e., panels a1 and a2), a conclusion might have been reached that measurement and model were in good agreement, yet a higher

dimensional comparison between the in situ O:C and H:C measurements and the values calculated from known products and their yields (i.e., row c) revealed important disagreement. The model had to be evolved by including chemical pathways that moved the SOM composition in the direction of Δ(H:C) < 0 and Δ(O:C) < 0. In this light, the present study concludes that the model-measurement gap can be significantly reduced by a newly proposed reaction pathway: the particle-phase decomposition of organic hydroperoxides and subsequent oligomerization involving free radicals. Such free-radical oligomerization, though well-known in other fields of research,44,46 has not been widely considered in previous thinking of SOM production pathways. The products might in many ways resemble the humic-like substances (HULIS) that figure prominently in atmospheric organic material.47 The presented analysis paradigm of multidimensional data constraints for testing the accuracy of chemical mechanisms is important because climate-relevant chemical transport models use these mechanisms for much wider atmospheric conditions than can be tested in laboratory experiments. The accuracy of the SOM production mechanisms significantly affects the predictions of climate-related end points of aerosol particles. MCMSIMPOL, as with any other model, omits many important topics, each of which has been a study in itself in recent years 48
50 (see SI Figure S7 for the comparison of MCM v3.1 and v3.2). In particular, reaction pathways in the particle phase, which have been a qualitative focus of much recent laboratory work but with limited kinetic data,33,40 are not included in MCM-SIMPOL. As more kinetic data become available in the future, these additional reaction pathways may also prove important for further development of the analysis presented herein.

’ ASSOCIATED CONTENT

bS

Supporting Information. Additional information concerning the AMS elemental analysis, the MCM-SIMPOL model, the volatility basis set approach, and additional tables and figures referenced in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This material is based upon work supported by the Office of Science (BES), U.S. Department of Energy, Grant No. DEFG02-08ER64529 (STM). N.M.D. acknowledges support from U.S. EPA STAR Grant R833746. J.E.S. acknowledges support from the Pacific Northwest National Laboratory Aerosol Climate Initiative. PNNL is operated for the DOE by Battelle Memorial Institute under contract DE-AC06-76RLO 1830. Q.C. acknowledges support from the NASA Earth and Space Science Fellowship. We thank Yongjie Li, Mackenzie Smith, Amanda Mifflin, Mikinori Kuwata, and Soeren Zorn for their assistance with the experiments. We also thank Jose Jimenez, Douglas Worsnop, and Paul Ziemann for helpful discussions. ’ REFERENCES (1) Hallquist, M.; Wenger, J. C.; Baltensperger, U.; Rudich, Y.; Simpson, D.; Claeys, M.; Dommen, J.; Donahue, N. M.; George, C.; 4768

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