Kinetic Mechanism for Predicting Secondary Organic Aerosol

For d-limonene a value of 85:15 for ozone attack on the endo- and ..... and in computing kon,which had units of cm3molecule-1sec-1 from Kp and koff, t...
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Environ. Sci. Technol. 2005, 39, 9583-9594

Kinetic Mechanism for Predicting Secondary Organic Aerosol Formation from the Reaction of d-Limonene with Ozone SIRAKARN LEUNGSAKUL, MOHAMMED JAOUI,† AND RICHARD M. KAMENS* Department of Environmental Sciences and Engineering, CB# 7431 Rosenau Hall, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-7431

A semi-explicit mechanism of d-limonene was developed and tested against experimental results obtained from large outdoor Teflon film chambers at the University of North Carolina (UNC) smog chamber facility. The model couples gasphase reactions with partitioning processes and possible particle-phase reactions. The model not only tracks the gasphase ozonolysis reaction of d-limonene, but also provides a reasonable prediction of the secondary aerosol mass production under different conditions. Limononaldehyde was the major identified product, followed by limona-ketone, referred to here as keto-limonene, keto-limononaldehyde, limononic acid, and keto-limononic acid. Identified particlephase products accounted for about 60% of the observed particle mass in the initial stages of the reaction. Model sensitivity was tested and discussed with respect to effects of temperature, humidity, water uptake, and reactant concentrations.

Introduction Globally, monoterpenes are estimated to account for 11% of the 1150 Tg C annual natural volatile organic emissions (1). In the United States, more than half of the monoterpene emissions which come from conifers and crops are composed of R- and β-pinene and d-limonene (2, 3). Andersson-Sko¨ld and Simpson (4) estimated that terpene reactions may contribute up to 50% of the total organic aerosol in Scandinavia, and this can be greater than the organic aerosol emissions from anthropogenic sources depending on location and season. In some places, while d-limonene may only represent ∼5% of the overall terpene emissions on a mass basis, it may account for more than 20% of the terpene secondary aerosol material depending on the vegetative species distribution (4, 5). This reactivity makes d-limonene the second most important monoterpene from the perspective of aerosol formation potential. On a microscale, the indoor aerosol potential of terpene reactions and the resulting fine particles pose possible health risks (6-9). Terpenes, particularly d-limonene and R-pinene, are major components of essential oils from many types of vegetation. These essential oils have been used as components of household products such as wood surface finishing, liquid cleaning agents, insect repellents, and air fresheners, which can react with ozone and other radicals to form indoor aerosols (10-14). In contrast to R- and β-pinene, there are few papers identifying and describing the mechanism of d-limonene * Corresponding author phone: 919-966-5452; fax: 919-966-7911; e-mail: [email protected]. † Now at Alion Science and Technology, Durham NC 27709. 10.1021/es0492687 CCC: $30.25 Published on Web 11/08/2005

 2005 American Chemical Society

reaction products (15-19). This paper presents a kinetic mechanism for the prediction of SOA from the reaction of d-limonene with ozone based on outdoor smog chamber experiments.

Experimental Section Smog Chambers. Experiments were carried out either in a single 190-m3 or dual 270-m3 outdoor Teflon film chamber located in Pittsboro, North Carolina (19-21). Chamber descriptions were outlined elsewhere (for the single 190-m3 chamber see refs 19 and 20 and for the dual 270-m3 chamber see ref 21). Prior to experiments, the chambers were flushed with rural background air for 12-24 h and dehumidified if necessary (23). d-Limonene + O3 experiments were performed under darkness to preclude photochemical reactions. A known amount of d-limonene (97%, Aldrich, Milwaukee, WI) was first injected into the chamber to establish a desired concentration, then ozone was injected while the internal chamber mixing fans were running. Instrumentation. Ozone was measured with a Bendix chemiluminescent ozone meter (model 8002, Bendix Corp., Roncerverte, WV) or a UV photometric ozone analyzer (model 49P/S, Thermo-Environmental Instruments, Indianapolis, IN). The instruments were calibrated by gas-phase titration with a NIST traceable nitric oxide (NO) tank. Gas-phase concentrations of d-limonene were measured by gas chromatography with flame ionization detection, GC-FID (Shimadzu, Japan) equipped with a 1.5 m × 3.2 mm i.d. Supelco 5% Bentanone 34 packed column. The GC-FID was calibrated against a U.S. National Institute of Science and Technology (NIST) traceable hydrocarbon standard tank. Particle size distribution data (14 to 690 nm) were taken every 5-10 minutes using a diffusion mobility analyzer and a condensation nuclei counter (model SMPS 3936, TSI, St. Paul, MN). Chamber temperature was measured with a temperature probe mounted inside a white radiation shield (R. M. Young Company, Traverse City, MI) positioned inside the chamber. Dew point was measured either with a dew point hygrometer (model 800, EG&G, Waltham, MA) or with a relative humidity analyzer (model RH-100, Sable Systems, Las Vegas, NV). Chamber dilution rates were measured by monitoring the disappearance of SF6, an inert gas, using a 6 mm o.d. × 20 mm stainless steel molecular sieve column (50-80 mesh) and an electron capture detector (PDD D-5, Valco Instruments Co. Inc., Houston, TX). Sample Collection and Workup Procedure. Gas- and particle-phase products were collected using a sampling train consisting of a two-stage 47-mm Teflon-impregnated glass fiber filter (type T60A20, Pallflex Product Corp., Putnam, CT), followed by a 40-cm 5-channel denuder (University Research Glassware, Chapel Hill, NC) coated with XAD-4 (24). The sample was drawn from the chamber through the sampling train with a flow rate of 20 L/min. A backup filter was used to correct the amount of gas adsorbed onto the first filter (25). Filters were weighed before and after sampling for aerosol mass, stored in an amber glass jar at 4 °C, spiked with an internal standard mix, and then transported back to the UNC lab for extraction. Denuders were spiked with an internal standard, rinsed three times with methylene chloride (Optima grade, Fisher Scientific, Fair Lawn, NJ), and dried under a nitrogen stream before the next sampling (24). The extracts were stored in a capped round-bottom flask, covered with aluminum foil to prevent light exposure, and also transported back to the UNC lab for analysis. Three internal VOL. 39, NO. 24, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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standard (IS) compounds were selected: bornyl acetate (97%), methyl perillate (97%), and R-asarone (98%). Initially, trans-p-menth-6-ene-2,8-diol (trans-sobrerol) was also used as an IS, however, it presented an interference with dlimonene products because of its fragmentation similarity when analyzed with GC-EIMS. Bornyl acetate was used as IS for compounds eluting before 9 min, methyl perillate was used for compounds eluting between 9 and 13 min, and R-asarone was used for compounds eluting after 13 min. At the UNC lab, denuder extracts were concentrated within 24 h to ∼1 mL using rotary evaporation. Filter samples were extracted with methylene chloride by Soxhlet extraction for 3 h. Filter extracts were concentrated to ∼1 mL under a gentle nitrogen stream. Extracted samples were analyzed with a Hewlett-Packard (HP) 5890 Series II gas chromatograph (GC) (30 m × 0.25 mm i.d. DB-5 capillary column, Supelco) equipped with a HP5971A mass selective detector (MS). The injector temperature was 300 °C. The oven temperature started at 60 °C, was held for 1 min, then ramped to 280 °C at the rate of 10 °C/min, and maintained at 280 °C for 20 min. The MS transfer line temperature was 310 °C. Mechanistic Details. The d-limonene mechanism was constructed from gas-phase reactions to describe the initial reactivity of d-limonene with atmospheric oxidants. These reactions produce low vapor pressure compounds, which partition between the gas and condensed phases. A kinetic partitioning approach suggested by Kamens (26, 27) was used to describe time-dependent phase distribution during the reaction. Mechanistic details were developed from literature and experimental data. Where no experimental data were available, gas-phase kinetic parameters were calculated using a structure-reactivity relationship (28), the master chemical mechanism (MCM) development protocol (29), or data from compounds of similar structure (30, 31). The d-limonene reactions were associated with the latest generation of the inorganic chemistry represented in the Carbon Bond 4 photochemical smog mechanism, CB4_2003 (32). A kinetic solver called MORPHO developed by Jeffries was used to perform the model simulations (33). Ozone Chemistry. Only a few studies (15, 17) are available in the literature that can be used for mechanism development of the d-limonene and ozone reaction system. Very little product information is presented in these studies and, therefore, product analysis of similar compounds, e.g., cyclohexene, 1-methyl cyclohexene, and 1,2-dimethyl cyclohexene, was used as a guide. Initial O3 attack on d-limonene can take place at either the internal cyclo double-bonded carbon or at the external “iso-propylene” bond. The resulting ozonides produce four excited Criegee intermediates (CIs) as shown in Scheme 1. The two CIs which result from O3 attack on the cyclo double bond of d-limonene are called CI-CH3-OO* and CI-OO* and still retain the external carbon double bond. The third CI, CIx-OO*, retains the methyl cyclohexene structure from O3 attack in the external carbon bond, and a fourth CI from this attack gives the same one-carbon CI formed from O3 attack on ethylene, CH2OO*. This sequence is given below in reaction 1.

Limonene + O3 f 0.85(0.65CI-CH3-OO* + 0.35CIOO*) + 0.15(0.68(CIx-OO* + HCHO)+ 0.32 (keto-limonene + CH2OO*)) (rxn 1) where keto-limonene stands for limona ketone or 4-acetyl1-methylcyclohexene. Subsequent reactions of the CIs in Scheme 1 involve multiple reaction steps and these are often lumped to give final products. For example, CIx-OO* can decompose through the ester channel (44) and rearrange to form 1,4-dimethyl9584

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SCHEME 1 . d-Limonene + O3 Reaction Pathway for Model Products and Product Yields

cyclohexene or MCH-CH3 and carbon monoxide (CO2). CIxOO* can also experience a bond breakage at the radical site of the tertiary carbon and form, after O2 addition, a 1-methyl cyclohexene peroxy radical (MCH-OO) plus the peroxyacetyl radical, CH3-CO-OO. Limonalic acid and methyl alcohol (CH3-OH) can form from the decomposition of CI-CH3OO*, which can first lose a methyl radical through unimolecular decomposition. The resulting product then reacts with water. To provide initial guidance on the relative O3 attack ratio on the endo- vs the exo-carbon bonds of d-limonene, the rates of O3 attack on cyclohexene vs 2-hexene were compared from their respective O3 rate constants (31) and their observed products (16). For d-limonene a value of 85:15 for ozone attack on the endo- and exo-double bonded carbons gave the best fit to our experimental data. For the CIs formed from the endo cyclic O3 attack, a 65:35 split for internal CIs was assigned as per Atkinson’s (31) recommendation for the R-pinene product distribution (34, 35). A branching of external CIs was based on 2,3-dimethyl-1-butene, 2-methyl-1penetene, and β-pinene product distributions (30, 36). The branching or split ratios were also adjusted slightly to give the best fit to overall experimental results. Currently two literature reaction rate coefficients for d-limonene ozonolysis are available: one recommended by Shu and Atkinson (37) is 2.01 × 10-16 and another recommended by Khamaganov and Hites (38) is 2.95 × 10-15exp(-783/T). Both rates agree within 10% at 298 K, but the temperature-dependent rate coefficient expression was used to better cover the temperature range of our data. The Criegee reactions are illustrated

in the rxn 2-4 sequences. In this sequence XO2 is a generalized radical that implicitly accounts for the oxidation of NO to NO2 and RO2 to RO, and is common to Carbon Bond 4 mechanisms. HO2 is the hydroperoxy radical.

CI-CH3-OO* f 0.2(limonalic acid + CH3-OH) + 0.2 (stabCI-CH3-OO) + 0.1(7OH-lim) + 0.5(0.5(oxylim + XO2 + HO2) + 0.5(limonic acid-CO-OO + H-COH)) + 0.9(OH) + 0.01(O3P) (rxn 2) CI-OO* f 0.2(CdC7-OO + H-CO-CO-H + XO2) + 0.3(limononic acid) + 0.5(stabCI-OO) + 0.9(OH) + 0.01(O3P) (rxn 3) CIx-OO* f 0.15(MCH-OO + CH3-CO-OO) + 0.45 (MCH-CH3 + CO2) + 0.35(stabCIx-OO) + 0.05 (keto-limonene + O3P) + 0.9(OH) + 0.01(O3P) (rxn 4) where 7OH-lim stands for 7-hydroxy-limononaldehyde or 3-isopropenyl-6-oxo-7-hydroxy-hetanal, oxylim represents oxy-substituted limononaldehydes, CdC7-OO stands for a C8 peroxy radical containing one double bond, H-COCO-H stands for glyoxal, limonic acid-CO-OO stands for an acylperoxy radical (a precursor of limononic acid), and O3P stands for ground-state singlet oxygen. Currently, it is still uncertain how these CIs decompose. Studies suggest that these Criegee intermediates quickly rearrange, decompose, or thermally stabilize and then react with other atmospheric constituents to form carbonyls, ketones, hydroxyl ketones, and carboxylic acids (29, 39-43). The three major pathways represented in this mechanism are the hydroperoxy channel, the ester channel, and the stabilized CI channel (44). The stabilized form of the Criegee from the excited state for CI-CH3-OO* is represented for example as stabCI-CH3-OO. An overall hydroxyl radical (OH) molar yield of 0.86 with an error factor of 1.5 from ozonolysis of d-limonene has been reported (45, 46). This yield is slightly higher than the OH yield from the R-pinene system, which ranges from 0.75 to 0.85 (47, 48). In our simulations, an OH yield of 0.90 gave the best fit to our d-limonene- O3 decay data. To simulate a high OH yield without altering product distributions, OH was treated as being generated directly from all of the excited CI biradical decompositions (rxns 2-4). Stabilized Criegee intermediates can react with a number of compounds such as water (H2O), NO, nitrogen dioxide (NO2), carbon monoxide (CO), formaldehyde (HCHO), formic acid (HCOOH), and other aldehydes and carboxylic acids (48-50). In the H2O + stabilized Criegee reaction, we varied the yields of the carboxylic acid over a range of 0.1 to 0.6, with the remainder going to carbonyl products. Little change in the overall behavior of the model was observed.

stabCI-OO + H2O f 0.1 limononic acid + H2O + 0.9 limononaldehyde + H2O2 (rxn 5) stabCI-OO + NO f limononaldehyde + NO2 (rxn 6) stabCI-OO + NO2 f limononaldehyde + NO3 (rxn 7) stabCI-OO + CO f limononaldehyde + CO2 (rxn 8) stabCI-OO + H-CO-H f limononic acid + H-COH (rxn 9) Evidence from direct mass spectrometry analysis of the condensed phase SOA (44, 51-53) suggests that hydroperoxides, peroxyhemiacetals, and secondary ozonides were formed from reactions of stabilized CI in the presence of alcohols, carboxylic acids, aldehydes, and water vapor. These

products were represented as “seed1” in the mechanism to allow lesser volatile species to partition onto “nucleating” particles.

stabCI-OO + limononaldehyde f seed1

(rxn 10)

stabCI-OO + ketolim f seed1

(rxn 11)

stabCI-OO + keto-limononic acid f seed1

(rxn 12)

where ketolim stands for keto-limononaldehyde or 3-acetyl6-oxo-heptanal and keto-limononic acid stands for 3-acetyl6-oxo-hetonoic acid. A reaction rate coefficient for the stabilized Criegee, stabCH2OO with formaldehyde, has been reported to be in a range of 2 × 10-12 to 2 × 10-17 cm3molecule-1sec-1 by Fenske et al. (41). A lower rate constant, 1.2 × 10-14, for reaction of stabilized Criegee with carbonyls was used by Kamens and co-workers (26, 27) as recommended by Atkinson (31). This rate seems to give more reasonable results than the faster rates of Fenske et al. (41). In our mechanism, 99% of stabilized CIs react with water and less than 1% proceed by reactions 10-12 to produce seed aerosol, which is sufficient to initiate particle growth. Recently, Ziemann (39) proposed that reaction of excited CIs generates low-volatility diacyl peroxides, which are responsible for seed nuclei formation. We have found, however, in our model that the multiple reactions steps that involve a sequence of HO2 and RO2 oxidations, kinetically, do not generate seed nuclei fast enough to be consistent with our observed experimental data. Particle Formation Mechanisms. Gas-Particle Partitioning. In our chambers, before the addition of either d-limonene or O3, the concentration of background aerosols was called “seed” in the mechanism. After the injection of d-limonene and O3 a burst of particles was immediately observed when the ozone was introduced. To simulate these instantaneous particle-phase products, the Criegee biradicals (rxns 2-4) were rearranged and quickly decomposed to form low-vapor pressure products, i.e., limononic acid and limonalic acid, and stabilized Criegee bi-radicals, stabCI-OO. These serve as seed precursor products (rxns 10-12). This burst of seed aerosol, called “seed1” in the model, and the background seed, provided surfaces for more compounds to partition onto. The partitioning process employed kinetic rate constants for rates of absorption (kon) from the gas to the particle phase and for rates of desorption (koff) from the particle phase to the gas phase (54). An example of the limononaldehyde partitioning sequence is illustrated below in rxns 13 and 14.

limononaldehyde + seed1 f part14 kon part14 f limononaldehyde koff

(rxn 13) (rxn 14)

where part14 represents limononaldehyde in the particle phase. koff was calculated from an inverse of the molecular vibrational frequency, kbT/h, and its activation energy, Ea (55-59). Ea was estimated from vapor pressure as per Kamens et al. (26)

koff )

( )

kbT -Ea exp h RT

(eq 1)

where kb is Boltzmann’s constant (1.381 × 10-23 Joules K-1) and h is Planck’s constant (6.626 × 10-34 Joules sec). At equilibrium, the ratio of kon over koff equaled the gas-particle partitioning equilibrium constant, KP. With the assumption of equilibrium, theoretical kon could be calculated from KP and koff. For liquid absorption, KP can be calculated from theoretical considerations (60) from the fraction of organic VOL. 39, NO. 24, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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particle mass (fom), the liquid vapor pressure (p0L), the temperature (T), the gas constant (R), and the mean molecular weight of the particle (MWom).

KP )

7.501RTf 109MWomp0Lγom

(eq 2)

This permitted an estimate of kon.

kon ) KP × koff

(eq 3)

Because there is no actual vapor pressure information for most d-limonene products, the standard method of Mackay et al. (61) (eq 4) was used to estimate product vapor pressures (p0L) as a function of the boiling point (Tb) and entropy of vaporization (∆S).

ln p0L )

[ (

)

( )]

Tb Tb ∆S 1.803 (atm) (eq 4) - 1 - 0.803 ln R T T

The normal boiling point was estimated using the Joback and Reid (62) extended group contribution method to cover more groups, and corrected for a temperature-dependent bias (63). The entropy of vaporization was computed based on the group contribution approach of Zhao et al. (64). Values for ∆S, Tb, and Ea for each of the partitioning species are given as Supporting Information in Table S1. Particle-Phase Reactions. Jang et al. (65) and Tolocka et al. (66) recently provided evidence of acid-catalyzed heterogeneous reactions of particle-phase carbonyl compounds. These reactions led to enhanced particle formation. Aldehydes in the particle phase can undergo polymerization to form dimers and trimers, e.g., C20 to C30 compounds, with estimated vapor pressures of 10-6 to 10-9 Torr. This is considerably lower than the estimated vapor pressure of the parent C10 aldehydes, (2.3-7.5) × 10-2 Torr. To represent this potential process, particle phase aldehydes were allowed to react to form more stable products that reside entirely in the particle phase. Here, as an example, limonaldehyde in the particle phase (part14) is permitted to react with the carbonyl portion associated with Criegee-secondary ozonides (seed1) to form a low vapor pressure compound composed of part14s + seed1. Since, however, seed1 has additional carbonyl groups, it could undergo further aldol condensation or acetal reactions. These made it available for additional polymer growth. In the mechanism, it therefore appears on the right side of the reaction. As will be discussed, Fourier transform infrared analysis suggests polymer formation in the particle phase.

part14 + seed1 f part14s + seed1

(rxn 15)

part14 + part14 f part14s

(rxn 16)

Since the rate coefficients for these reactions were far from being quantified, they were set so the mass through this channel was low compared to the total SOA mass produced by the model. When these processes were not included in the model, the model tended to underpredict observed particle concentrations by ∼10%. A reverse particlephase reaction rate coefficient of 0.0001 s-1 was used and a forward reaction rate constant of 8 × 10-15 cm3 molecule-1 sec-1 was used. The heterogeneous rates used in the mechanism were assumed to be non-temperature-dependent and the rates were extrapolated from particle-phase to gasphase rates. This tended to give an average increase in simulated particle mass of about 12%. Inorganic Mechanism. Two explicit inorganic mechanisms extracted from the 1999 version of the Carbon Bond 9586

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mechanism (67), CB4-99, and from the 2003 version (32), CB4-2003, were added to the d-limonene mechanism. Two major reactions were updated in the inorganic mechanism of CB4-2003 compared to CB4-99; these included an increased reaction rate constant of NO2 + OH to form HNO3, and an added reaction of N2O5 with H2O. Other reaction rates were also updated with new literature rate coefficients (32). Simulation results showed that the nighttime d-limonene mechanism was insensitive to the changes made to inorganic mechanism from CB4-99 to CB4-2003. However, to keep the mechanism current, the new inorganic mechanism from CB2003 was integrated into the d-limonene mechanism. Chamber Wall Mechanism. To simulate chamber experiments, a UNC auxiliary wall mechanism was included. It accounted for physical properties and chemical reactivity based on the dual 300 m3 UNC outdoor chamber and for sampling line reactions. Details of the auxiliary mechanism were described by Jeffries (68) and updated by Voicu (37). Since the wall model mainly affected radical sources and sinks involving oxides of nitrogen, it did not affect nighttime ozonolysis processes. Therefore, the main concern during the nighttime experiment was the ozone wall loss rate. The new 270-m3 dual aerosol chamber was characterized for chamber reactivity. A number of characterized experiments were performed, which included NOx/O3 decay, matched propylene with NOx, matched particle, and particle wall loss experiments. Losses in the chamber due to exchange with the outside air were less than one percent per hour. The average dark O3 loss rate corrected for dilution was 7.53 × 10-7 s-1 for each half of the 270- m3 chamber. The particle deposition was 1.16 × 10-5 s-1 on a total mass basis. The residence time of the gas phase in the sample line was 1 s, which was used to correct a difference between observed and measured ozone and oxides of nitrogen concentrations due to reaction in the sample line.

Results and Discussion Experimental Conditions. Nine experiments were conducted inside the Teflon chambers and used for model simulation. Experimental conditions are shown in Table 1. The initial concentration ratio of d-limonene to ozone ranges from 0.8 to 4.2. This represents ozone excess and ozone deficit conditions. Aerosol Yield. Since the experimental aerosol yield, Y, has been widely used as an indicator of aerosol forming potential (69-72), a maximum aerosol yield is calculated and reported in Table 2. The aerosol yield is defined as a ratio of the amount of SOA formed, ∆M0, to the amount of reacted d-limonene, ∆HC. This aerosol yield has been linked to partitioning parameters using a thermodynamic approach (69), which allows an estimation of aerosol mass contribution from each hydrocarbon. An interesting comparison between aerosol yields from d-limonene (from this study, Table 2) and R-pinene ozonolysis systems (from other studies) shows d-limonene particle yields of 43-94% and R-pinene particle yields of 18-67% (26, 69, 70). This difference illustrates the tremendous particle formation potential of d-limonene in the indoor and outdoor environments (4, 5, 9-11, 73, 74). Product Analysis and Identification. A typical gas chromatogram of d-limonene products is shown in Figure 1. d-Limonene and limonene-oxide were identified with an authentic standard. Limononaldehyde was synthesized (75) in our laboratory and confirmed by interpretation of Fourier transform infrared (FTIR), nuclear magnetic resonance (NMR), and GC-MS analyses, to have more than a 95% purity. Keto-limononaldehyde, limononic acid, keto-limononic acid, 7-hydroxy-limononaldehyde, 7-hydroxy-keto-limononaldehyde, 7-hydroxy-limononic acid, and 7-hydroxy-keto-limononic acid were tentatively identified by their retention times and the interpretation of their electron impact (EI)

TABLE 1. Experimental Conditions of d-Limonene + O3 System exp. date August 18, 1999a,c September 10, 1999a August 1, 2001a August 13, 2003Sb,c August 13, 2003Nb,c January 22, 2004Sb January 22, 2004Nb January 23, 2004Sb January 23, 2004Nb

initial concentration (ppm) d-limonene O3 0.63 0.18 0.65 0.48 0.45 0.09 0.09 0.30 0.14

0.30 0.12 0.81 0.18 0.35 0.02 0.08 0.25 0.24

NOx

seedbgdd µg/m3

temp (K)

DP (K)

Nd Nd 0.005 0.002 0.002 0.000 0.000 0.001 0.001

120 (9) 58 (21) 17 (1) 147 (3) 78 (3) 6 (4) 1 (2) 0 (0) 37 (7)

297-302 292-298 288-292 295-302 295-302 277-274 277-274 269-270 269-270

293-296 287-290 288-289 295 295 269 269 264 264

a Performed in 190-m3 chamber. b Performed in 270-m3 dual chamber; N is North chamber and S is South chamber. c d-Limonene was first injected followed by ozone. Reactions of d-limonene with background ozone were simulated to get initial conditions for particle mass, d-limonene, limononaldehyde, keto-limononaldehyde, and limononic acid. d Background particle concentration (seed1) after injecting ozone (or after injecting d-limonene, if d-limonene was first injected) measured with SMPS assuming particle density is 1 g/m3. Values in parentheses are initial particle concentrations (seed) before the addition of O3 or d-limonene; these were used as initial seed concentrations in model simulation assuming a seed molecular weight 120.

FIGURE 1. Chromatogram of d-limonene + O3 products in (I) particle phase and (II) gas phase on August 13, 2003. Mass spectra of tentatively identified products are shown. mass spectra (23, 27). Some examples of product mass spectra are presented in Figure 2. Limononaldehyde, the most significant primary product, was detected in both the gas and particle phases immediately after d-limonene was injected into the chamber. Ketolimononaldehyde, a secondary product, appeared slightly later than limononaldehyde and was observed mostly in the particle phase; it forms from O3 attack on the exterior carbon double bond of limononaldehyde. Limononaldehyde was the most abundant product in the gas phase and ketolimononaldehyde was the most abundant product in the particle phase. During the initial stages of the reaction, identified products accounted for about 60% of the total aerosol mass, but as the reaction continued, identified products accounted for less of the measured mass. Overall product analyses had (30% uncertainties from sample collection, extraction losses, injection losses, and GC-MS response, when standards or surrogate standards could be reasonably used. When this was not possible, the uncertainties in product analysis were assumed to be ( a factor of 3.

Evidence for Particle-Phase Reactions. The products of d-limonene contained carbonyl, hydroxy, and carboxylic acid functional groups. A five-compound model UNIFAC calculation (76) gave an average γom of the products in a range of 1.2-1.3 (see discussion in Model Sensitivity section). From d-limonene product analysis, the MWom was estimated to range between 120 and 180 g/mol depending on humidity in the chamber (ref 77 and later discussion). With these assumptions, KP calculated at 298 K ranges from 1.3 × 10-3 to 1.7 × 10-3 m3mg-1 and 5.9 × 10-3 to 7.7 × 10-3 m3 mg-1 for limononaldehyde and keto-limononaldehyde, respectively. As shown in Table 3, experimental KP values were as much as 2 orders of magnitude higher than calculated KP values. This finding was consistent with observed or apparent vapor pressures of product aldehydes from R-pinene oxidation (32), which were 1-2 orders of magnitude lower than calculated theoretical values. These observations were further corroborated by the IR spectrum of d-limonene particle phase products. Here, the FTIR spectrum (Figure 3) of particles collected on ZnSe FTIR windows from a 5 ppm d-limonene and 1 ppm O3 system is shown. After 2.5 h of reaction in the dark, the appearance of a C-O-C stretching band in the 1000 cm-1 region suggested possible polymerization functionality (65, 78, 79). An explanation for all of the above observations, that is consistent with other studies conducted by our group, is the reaction of carbonyl compounds in particle phase to form larger molecules (65, 66, 79). This process creates an balance between that gas and particle phases and “drives” gas-phase carbonyl products to partition more into the particle phase. Analytically, some fraction of these oligomers may decompose back to original compounds during our workup procedures and were detected as their parent carbonyl compounds; hence the large deviation between observed KP and predicted KP values for aldehydes. This is consistent with our work on R-pinene by Kamens et al. (27). Simulation of d-Limonene and Ozone and Aerosol Mass. Simulations of the experimental data developed for this study were based on the selection of rate constants that were available in the literature, and on estimates of rate constants when these were not available. A complete listing of the mechanism is given in the Supporting Information as Table S2. We attempted in the development of the model to first fit ozone and d-limonene gas-phase data. Simulations of product limononaldehdye were then used as indicators of model performance, as was the overall simulation of the particle phase mass concentration. Fits that were within 5% of the data were called very good fits, fits that were within 15% were called good, and fits within ( 30% were called reasonable. Simulation results (Figure 4) of d-limonene VOL. 39, NO. 24, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Mass spectrum of particle phase products from d-limonene and ozone reactions; relative abundance is on the y axis. chemistry were very reasonable. The model predictions in some cases corresponded exactly with measured concentrations of d-limonene and ozone. Simulations of experiments on August 13, 2003 illustrated the model’s ability to perform when either d-limonene or ozone was in excess. Although the model simulation results were very good when ozone was in excess over d-limonene, the fits for excess d-limonene were not as good, but still very reasonable. 9588

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Filter masses corrected by backup filter subtraction were used for model comparisons. SMPS data were a factor 1.3 to a factor of 3 lower than filter mass data. When SMPS data were very low (factor of 3) compared to particle mass concentrations derived from filters, the particle volume size distributions tended to grow beyond the DMA range of 690 nm. The observation that a factor of 1.3 is often needed to correct the SMPS data when the size distributions stay within

FIGURE 3. FTIR spectra of d-limonene reaction products with ozone compared with spectra from limonene diol and limononaldehyde standards.

TABLE 2. Maximum Aerosol Yields from d-Limonene + O3 Chamber Experiments initial concn (ppm) exp. date

d-limonene

O3

August 18, 1999 September 10, 1999a August 1, 2001 August 13, 2003S August 13, 2003N January 22, 2004S January 22, 2004N January 23, 2004S January 23, 2004N

0.63 0.18 0.65 0.48 0.45 0.09 0.09 0.30 0.14

0.30 0.12 0.81 0.18 0.35 0.02 0.08 0.25 0.24

aerosol ∆HC ∆M0,maxa yield, Yb mg/m3 mg/m3 (∆M0/∆HC) 2.17 0.59 3.59 2.35 2.37 0.51 0.30 1.64 0.85

1.38 0.40 3.39 1.81 1.86 0.13 0.23 1.19 0.65

0.63 0.68 0.94 0.77 0.78 0.43 0.45 0.72 0.77

a ∆M 0,max ) maximum aerosol mass - background aerosol mass, (b) aerosol yield was not corrected for dilution or losses.

TABLE 3. Average Values of Experimental KP in m3/mg for Limononaldehyde and Keto-Limononaldehydea nighttime exp. ID Au0101 Au1303R Au1303B

experimental KP (m3/mg) limononaldehyde keto-limononaldehyde 0.0689 0.0607 0.0458

1.7564 1.6685 1.4052

a Experimental K ) C P part/Cgas[TSP], predicted KP ) 7.501RTfom/ 106MWomp0Lγom. Predicted KP of limononaldehyde ) 0.0016 m3/mg, predicted KP of keto-limononaldehyde ) 0.0071 m3/mg.

the DMA range suggests that the density of these SOA aerosol systems may be on the order of 1.3. The model simulations predicted aerosol mass formation that was within 20% or better of measured gravimetric mass that ranged between 0.4 and 3.4 mg/m3. Gas-Particle Product Distributions. The model simulated limononaldehyde and keto-limononaldehyde in both gas and particle phases, are shown in Figure 5. The differences between measured data and simulation may have occurred due to (i) extraction and measurement procedures, which are harsh techniques and may destroy or decompose longchain products to some extent; or (ii) often the “lumped” products in the model may represent 3 of 4 similar products, and hence it would not be expected that a single product could be fit by the “lumped product” simulation.

Model Sensitivity Analysis. Temperature. When particle production was dominated by seed production and heterogeneous reactions, the model was less sensitive to temperature despite a temperature-dependent partitioning process. A pronounced temperature effect has been shown in the R-pinene system (31, 32). It should be noted that the Kamens R-pinene mechanism used a constant kon rate coefficient, which left only koff as temperature dependent. This resulted in 25-30% total particle mass change over 10 K. In contrast, the d-limonene mechanism employed a direct relationship between KP and kon. However, this only made a slight difference because kon varied by the square root of the reciprocal of temperature. A more important parameter that had a direct effect on KP value was vapor pressure, which was calculated in the model and was updated every time step to account for the temperature change. Our experimental results (Table 1) show that aerosol yields from the d-limonene and ozone system tended to be lower as temperature decreased from 300 to 270 K (average temperature). To simulate this behavior, the particle mass production from seed1/SOZ was set to be lower than the temperature-dependent partitioning mass. The temperature-sensitivity test simulations were run at 0.4 ppm d-limonene, and at a dew point (DP) temperature of 283 K; temperature varied in the model from 283 to 313 K. Results showed that the lower ozone system (0.1 ppm O3) was slightly more sensitive to changes in temperature than the higher concentrations (0.2-0.3 ppm). For every 10 °C decrease, total aerosol mass increased about 16% and 10% for lower-ozone and higher-ozone systems, respectively. Product distribution for the lower-ozone system was dominated by mainly limononic acid, limonalic acid, oxylimononaldehyde, and limononaldehyde, which will be called first generation products. The relative amounts of higher volatility products in the particle phase (limonaldehyde, and oxy-limonaldehyde) were greater in higher-ozone system (which generated more particle mass) than in the lower particle mass, low-ozone particle system. These higher volatility products have lower absolute Ea values compared carboxylic acid products in eq 1, and thus make the higher O3 system less temperature sensitive than the lower O3 system with respect to particle formation. Humidity and Water Uptake. The model showed sensitivity to humidity change. At an initial condition of 0.4 ppm d-limonene and 0.2 ppm O3 at a temperature of 298 K, the model-predicted particle mass decreased by 10% from 0.8 VOL. 39, NO. 24, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Model simulations (lines) vs. experimental data (symbols) for d-limonene with ozone experiments. LDT is local daylight-saving time. Thick lines (-) are simulated ozone, dash lines (- -) are simulated d-limonene, solid lines (-) are simulated aerosol, diamonds ()) are measured ozone, cross marks (×) are measured d-limonene, and solid squares (9) are measured aerosol. to 0.72 mg/m3 as the dew point increased from 268 to 298 K (13-100% RH). This result was supported by dry and humid experiments of stabilized Criegee radical reactions (40). Here, 9590

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under normal atmospheric conditions, a major pathway of stabilized CI was reaction with water to form carbonyls. Once water became limited, stabilized CI reacted with other

FIGURE 5. Simulations (lines) vs data (symbols) of limononaldehyde and keto-limononaldehyde in gas and particle phases from August 01, 2001 experiment. constituents; in this case, aldehydes, to form less volatile compounds, i.e., secondary ozonides, which increased the total mass. As discussed earlier, humidity also had a reverse effect on particle formation by reacting with stabilized CIs; however, to assess the effect of water uptake by particles and eliminate the influence of water reactions, a base case simulation was run at 298 K, 288 K dew point temperature, 0.4 ppm d-limonene, and 0.2 ppm O3. The base case was simulated to obtain a product distribution to estimate an average molecular mass, that did not include water uptake. The resulting activity coefficients for different product classes in particles with different particle water contents were then computed. Average activity coefficients (Table 4) varied in a narrow range between 1.2 and 1.32; however, the average particle molecular weight of the particles varied greatly from 179 to 126. This had more influence on KP calculations (eq 3). KP has units of m3mg-1, and in computing kon,which had units of cm3molecule-1sec-1 from Kp and koff, the average MWom canceled and left only the activity coefficient parameter in kon. The UNIFAC calculation suggested that as the water content in the particle increased, the activity coefficient of carboxylic acids decreased, but the activity coefficients of keto-aldehydes and Criegee type seeds slightly increased from one. Since this change was very small, however, an activity coefficient at a constant value of 1.27 at 3% water uptake by particle mass was assumed, and this value was embedded in the kon value. Impact of Background Air on Model Simulations. Typically, in the chamber experiments of this study, the background NOx in the chamber was on the order of 3-6 ppb. At these levels, our modeling results showed that the reaction with “dark” OH from O3 attack on the carbon double bonds of d-limonene was almost as competitive as that of O3 with d-limonene. In addition at these low NOx levels, model predicted alkyl nitrate, and acylperoxy nitrate species were found to contribute minimally to the overall aerosol yields.

When the initial background NOx as NO2 was increased from measured levels of 3-6 ppb to concentrations as high as 20 ppb, no perceptible change in the d-limonene, O3, or particle formation behavior was observed in the model predicted behavior. This was even true for the experiment (Table 1) which started with 0.09 ppm of d-limonene and 0.02 ppm of O3. Inputs to the mechanism in this study included background or outside air concentrations of O3 and hydrocarbons, and these were exchanged into the chamber as it diluted at an exchange rate of 1%/hour. For ozone exchange, varying outdoor O3 between 20 and 40 ppb did not impact model results, and a default value of 40 ppb was used as an outdoor exchange concentration. Measured background total hydrocarbon levels were always less than 100 ppb C. Outdoor nighttime R-pinene and d-limonene concentrations were both 5 ppb C or less. In the model total outdoor nonmethane hydrocarbons were represented by a species called background volatile organic carbon (BVOC). Under sunlight conditions, Jeffries et al. (33) have shown that the reactivity of outdoor exchanged background hydrocarbons with the chamber air could be represented by

OH + BVOC f 0.667 CH3-OO + 0.167 CH3-CO-OO (rxn 16) A rate constant of 3.0 × 10-12 cm3 molecule-1 s-1 is suggested (33) with a BVOC concentration of 85 ppb where CH3-OO is a generic peroxy radical and CH3-CO-OO is a peroxyacetyl radical. Under darkness, whether this reaction was included or not at the concentrations of d-limonene and O3 in Table 1, it did not have any impact on the observed model predictions. Background particle masses in the chamber (called seed in the mechanism) at the beginning of an experiment were generally in the range of 0.3-9 µg/m3. In one cases the background aerosol was 22 µg/m3. On the basis of the VOL. 39, NO. 24, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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observation that extractable mass of these starting aerosols is very low and seemed to be dominated by sulfate, we have assumed these to be largely inorganic aerosols. Particle nucleation, however, generally took place in the presence of these background aerosols as soon as both d-limonene and O3 were simultaneously present in the chamber. Hence, varying these initial seed concentrations by (50% tended not to greatly influence the timing of the simulations or the amount of aerosol generated. If d-limonene was added first to background O3 in the chamber, (background O3 was generally in the range of 0.01-0.04 ppm) there was an initial burst of aerosol, and this was called seed1 in Table 1. When d-limonene was added after O3 was in the chamber, there was also an initial burst, but this seemed to be dramatically influenced by the rate of addition and mixing in the chamber as d-limonene was added. Model Simulation of Other Studies. Three sets of published experimental results were simulated with this d-limonene mechanism. Li and co-workers (74) performed experiments in an office setting. d-Limonene and ozone were emitted in a closed-door office with low and high air exchange rates. Sarwar and co-workers (10) experimented with different kinds of scented household products in a stainless steel (SS) chamber. These scented products, which contained dlimonene in different fractions, were applied or released inside the chamber which had a certain level of ozone. Rohr and co-workers (12) used a flow reactor tube with a small plexiglass chamber attached to the end where samples were taken. A constant flow of d-limonene and ozone was introduced continuously. Reactor characteristic parameters were adjusted to suit each setting. Particle wall loss rate constants in the office, SS chamber, and flow reactor were approximated as 1.2 × 10-4, 1.2 × 10-4, and 2 × 10-3 sec-1, respectively. Entrained gas concentrations were assumed to be at background levels of 10 ppb for the office and 45 ppb for the SS chamber. Because the office setting was not well-characterized, average values of 15 and 2 hr-1 were used for high and low air exchange rates (AER). The d-limonene concentration of 270 ppb reported on the February 11, 2000 experiment without an ozone generator was used to get an average emission rate of d-limonene from the diffusion vessel. This rate was used for all low AER experiments except for the experiment on January 4, 2000, when a higher rate was used to get a reported residual d-limonene of 360 ppb. For high AER experiments, emission rates of ozone and d-limonene were adjusted to get the reported net ozone and d-limonene concentrations. For the SS chamber, only the d-limonene fraction emitted from tested products was used to simulate the particle formation; all experiments were simulated as a puff injection. In the flow reactor the reactants were injected continuously with a certain flow rate. Different AER affected the time required to reach steady state, which varied from 2 to 20 min. Experimental conditions of these three sets of experiments are summarized in Table 5. One feature of the model developed in this study was that it could be easily extended and applied to a flow reactor system. As demonstrated, the model was capable of predicting particle mass generated from different systems despite approximated chamber parameters for particle and ozone deposition rates.

Acknowledgments This work was supported by U.S. EPA STAR grants (R828176, R 831084) and an NSF grant (ATM 0097462) to the University of North Carolina at Chapel Hill. Sirakarn Leungsakul received a full scholarship from the Royal Thai Government for her study at the UNC-CH. We thank Bharadwaj Chandramouli, Nadine Czoschke, Di Hu, and Sangdon Lee for assisting with chamber experiments, Myoseon Jang for assisting with the FTIR sample and interpretation, and Dr. Ramiah Sangaiah 9592

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TABLE 4. Average Molecular Weight of Particles and Activity Coefficient as a Function of Water Uptake by Particles water uptakea % by mass

average MWomb

average γomc

MWom × γom

0 1 2 3 4 5

179 164 152 142 133 126

1.2 1.23 1.24 1.27 1.29 1.32

215 202 188 180 172 166

a Reported from R-pinene + O dark experiment as wC 3 om (g/g) ) 5.2 × 10-4 × %RH by Jang and Kamens (1998). b Estimated from simulation result after reaching a maximum mass, conditions are described in text. c Using 5-compound UNIFAC model: keto-limononaldehyde 35%, keto-limononic acid 35%, limonalic acid 24%, SOZ 6%.

TABLE 5. Simulation Conditions Used in the d-Limonene Model to Simulate Particle Mass (PM) from Different Studies exp. ID

dexp. measuredb predicted PM O3 limonene temp RH AERa PM (ppb) (°C) (%) (hr-1) (µg/m3) (ppb) (µg/m3)j

Office (ref 74) 12/16/1999 12/29/1999 1/13/2000 1/19/2000 1/27/2000 2/4/2000 2/11/2000 2/15/2000 2/16/2000

125c 100c 100c 100c 80c 80c 350g 4 >350g >350g

68 0.01 77 78 364 456 5.2 479 396

SS Chamber (ref 10) 24 24 24 24 24

20i 20i 20i 20i 20i

0.66 0.62 0.79 0.71 0.83

203 110 16 70 1.9

247 141 7 20 2

Flow Tube (ref 12) 20 21 34 23 27

19 8.6 15 8.7 33 36.0 54 45.0 51 8.4

51 100 9179 8649 11213

153 230 10232 12319 7446

a AER is air exchange rate. b Reported particle mass converted from particle volume. Sarwar et al. (2004) used particle density of 1.2 g/cm3, Rohr et al. (2003) used particle density of 1 g/cm3. c Net ozone measured during experiments. d Average value measured with passive sample devices. e Assumed constant room temperature at 21°C. f Average AER reported as high rate (12-18 hr-1) and low rate (0.5-4 hr-1). g Exceeded instrument limit. h Estimated d-limonene concentration calculated from maximum terpene concentration × d-limonene content. i Assumed relative humidity at 20%. j Reported PM and predicted PM from the model are compared in the last two columns

(whose recent unexpected death is a loss to us all) for assisting with limononaldehyde synthesis and analysis.

Supporting Information Available Tables of data used for calculations and references. This material is available free of charge via the Internet at http:// pubs.acs.org.

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Received for review May 17, 2004. Revised manuscript received August 5, 2005. Accepted August 23, 2005. ES0492687