Impact of Humidity on the OzoneInitiated Oxidation of Limonene,∆3-Carene, and r-PineneÅ S A M . J O N S S O N , *M A T T I A S H A L L Q U I S T , A N DE V E R T L J U N G S T R O M

Department of Chemistry, Atmospheric Science, GoteborgUniversity, SE-412 96 Goteborg, Sweden

The effect of water on the initial secondary organicaerosol (SOA) formation from gas-phase ozonolysis oflimonene, ∆3-carene, and R-pinene (∼3 and ∼1.5 × 1011

molecule cm-3 reacted) has been investigated in a flow reactorat controlled relative humidity (RH), temperature (298 (0.4 K), and reaction time (270 ( 2 s). Low amounts of terpeneconverted minimize the impact of secondary reactions. Acomparison of the SOA formation from the three terpeneswas made for initial rate of reactions being around 7.5× 108 and 15 × 108 molecule cm-3 s-1. The most efficientspecies in producing SOA was limonene, while R-pinenewas the least efficient. The results showed that anenhancement in water vapor concentration (<2-85% RH)caused an increase in both integrated mass (M10-300nm)and total number (N10-300nm). The effect on number and masswere a factor of 2-3 and 4-8, respectively. Physicalwater up-take can partly explain the increase in mass,but not the observed increase in number. Therefore it wasconcluded that the increase in water concentrationmust, by a gas-phase reaction, produce more low volatilityproduct(s).

IntroductionWhen organic compounds are oxidized in the atmosphere,low volatility organic products may be formed. The fate ofthese products is dependent on a variety of properties, suchas vapor pressure, water solubility, and atmospheric reactiv-ity. It has long been acknowledged that a significant fractionof these products condenses, i.e., forms secondary organicaerosol (SOA).

One set of reactions contributing to SOA formation is theoxidation of monoterpenes (1) and numerous laboratorystudies have been conducted on such reactions (reviewed,e.g., by Calvert et al. (2) and Atkinson and Arey (3)).Monoterpenes are unsaturated natural hydrocarbons of thegeneral formula C10H16, and originate mainly from coniferousforests (4). In the atmosphere, monoterpenes react eitherwith OH radicals, NO3 radicals, or O3 (5, 6). The NO3 and OHreactions have similar mechanisms. Either there is abstractionof a hydrogen atom or addition to a double bond, subse-quently forming a radical species. Ozone will always add tothe double bond, producing a primary ozonide. This ozonidedecomposes, whereupon noncyclic alkenes give two frag-ments; one carbonyl and one excited biradical, the excitedCriegee Intermediate (CI*). For endocyclic alkenes, thecarbonyl functional group is included in the CI molecule.

Reactions I-III show the production of the excited CI* fromlimonene, ∆3-carene, and R-pinene.

Limonene contains a double bond both outside and insidethe ring structure but the ozone attack is predominantlytaking place at the ring double bond. The formation andyield of the stable carbonyl compound have been establishedfor a number of alkenes (2). The possible reaction pathwaysof the CI* and the subsequent products are numerous, whichgives a large number of possible end products. Even for smallalkenes, the observed product yields vary considerably amongstudies. The current understanding and the main features ofthe subsequent reactions of the CI* have been summarized,e.g., in ref 2 and are illustrated by one of the CI*s formedfrom the ozonolysis of R-pinene, by reactions IV-VII.

* Corresponding author e-mail: asajon@chem.gu.se.

Environ. Sci. Technol. 2006, 40, 188-194

188 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 1, 2006 10.1021/es051163w CCC: $33.50 2006 American Chemical SocietyPublished on Web 11/24/2005

In brief, the produced Criegee Intermediate (CI) is in anexcited state (CI*) and can either be stabilized (CIS), undergounimolecular rearrangement (e.g., VII), and/or undergofragmentation (e.g., V-VI). After stabilization, the CI can react,e.g., with water.

Presence of water vapor can influence the productdistribution from ozone degradation of unsaturated com-pounds, either by reactions with the stabilized CI or by other,so-far unknown pathways (1, 2, 7, 8). Water can thereforeeither directly or indirectly influence the formation of lowvolatility products and hence the aerosol yield (9-11). Aerosolparticles can in turn affect the concentration of compoundsin the gas phase. An additional effect of water can be provisionof hydrophilic surfaces that may attract water solubleproducts.

In this work, the effect of water on the secondary organicaerosol formation process from the ozonolysis of limonene,∆3-carene, and R-pinene has been investigated. A newlyconstructed aerosol flow reactor setup was used for theexperiments.

Experimental SectionSetup. The experimental setup (Figure 1) is a combinationof a laminar flow aerosol tube/reactor and a scanning mobilityparticle sizer (SMPS) system. The reactor is a 140-cm longcylinder with a volume of 9.5 dm3 and is made of Pyrex glass.To reduce wall losses of reactive gas species, the inner wallof the reactor was coated with a thin film of halocarbon wax.To suppress the influence of OH radicals produced from theO3-alkene reaction, 2-butanol was used as a scavenger. Thescavenger was introduced to the reactor by passing a flowof N2 through the liquid, while kept at constant temperature.The monoterpenes were delivered to the flow reactor byletting a flow of N2 pass the open end of a diffusion tube(kept in an in-line wash bottle) containing the liquid reactant.By using diffusion tubes, studies of terpenes at concentrationsapproaching ambient levels were enabled. The concentrationwas varied by changing the temperature of the organic liquid.Ozone was generated by letting a mixture of N2 and O2 pass

through an Hg-Pen Ray UV lamp (UVP, Stable OzoneGenerator, SOG-2) and entered into the flow reactor,separated from the terpenes, through a movable glass injector.The flow of N2 and O2, as well as all other flows in the system,were set by using mass flow controllers (Sierra 820 seriesTop-Trak mass flow meters). The concentration of ozonewas measured by using an UV photometric O3 analyzer(model 49C, Thermo Environmental Instruments Inc.).

Water vapor was added by letting the air flow pass througha Gore-Tex tube (50 cm long, 3 mm diameter, permeability5 × 10-2 cm3 cm-2 s-1), submerged in deionized water. Theunit was kept in a thermostated water bath, where the desiredrelative humidity of the air flow was set by selecting thetemperature of the bath. The humidity in the system wasmonitored by a dew point meter (Dewmet, cooled mirrordewpoint meter, Michell Instruments) and a relative humiditymeter (Vaisala).

An SMPS-system (TSI, model 3936) was used to determineparticle number size distribution (10-300 nm) from whichintegrated number and mass were calculated. The entiresystem, including the laminar flow reactor and the SMPS-system, was placed in a temperature-controlled housing toenable temperature control.

Procedure. The formation of secondary organic aerosol(SOA) particles was investigated for selected ozone/terpenereaction systems, limonene, ∆3-carene, and R-pinene, as afunction of relative humidity (RH). The experimental condi-tions are summarized in Table 1. Ozone was in excess andthe concentration was set to give the same initial rate ofreaction in all experiments, enabling comparison of the SOAformation efficiency. Two sets of experiments were con-ducted: a high- and a low-terpene concentration set with∼3 × 1011 and ∼1.5 × 1011 molecule cm-3 reacted, respectively.In the text, tables, and figures, the high- and low-concentra-tion sets are denoted with a subscript H and L. The initialrate of reaction for the two sets were ∼7.5 × 108 and ∼15 ×108 molecule cm-3 s-1, respectively. The rate coefficients usedfor the rate calculation were 200 × 10-18, 37 × 10-18, 86.6 ×10-18 cm3 molecule-1 s-1 for limonene, ∆3-carene, andR-pinene respectively (12). The reaction time was keptconstant (268-272 s) and the total flow in the system was1.65-1.68 Lpm. In both sets of experiments, the relativehumidity was varied between <2 and 85%. 2-Butanol wasused as an OH-scavenger and more than 98% of the OHproduced reacted with the scavenger.

The initial organic concentration was determined by usinggas chromatography (Finnigan/Tremetrics 9001, GC) con-figured with a flame ionization detector (FID). Samples wereobtained by using adsorption tubes packed with beds ofTenax TA and Carbopack B. The adsorbed material wasanalyzed by two-step thermal desorption (Unity Thermaldesorber, Markes International Ltd.). Two samples weretaken before each set of experiments and two were takenafter.

After each change in the relative humidity, the systemwas allowed to equilibrate for at least 60 min before theaerosol was measured. The measured size distributions wereintegrated averaging 5 consecutive distributions over the 10-300 nm particle size range to derive the total particle number(N10-300nm), and total particle mass (M10-300nm). When cal-culating the integrated mass, spherical particles with a unitdensity of 1 g cm-3 were assumed. The time needed forobtaining one size distribution comprised an up-scanningtime of 300 s and a down-scanning time of 120 s, i.e., a totalof 7 min. This long scanning time ensured a representativesize distribution since short scanning times may, dependingon the aerosol, distort the shape of the size distribution.

FIGURE 1. Schematic of the experimental setup. The equipmentwithin the dotted area is placed in a temperature controlled housing.The SMPS system consists of a differential mobility analyzer (DMA,TSI model 3081) and a condensation particle counter (CPC, TSImodel 3010).

VOL. 40, NO. 1, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 189

ResultsA majority of previous studies on SOA formation has beenconducted in large smog chambers where conditions suchas temperature and relative humidity, which are cruciallyimportant in the formation process, can be difficult to managein a precise way. This study shows that a laminar flow systemis well suited for analyzing SOA, as both temperature andrelative humidity can be carefully controlled. As shown inTable 1, the temperature and RH were constant over theduration of a typical experiment. Furthermore, temperaturedifferences between extremes at any given time within thehousing were negligible (less than 0.7 K). In Figure 2 theconsecutive size distributions measured over 112 min canbe seen. As fluctuations in the size distribution over time issmall, sampling takes place from a “frozen” aerosol, contraryto static chamber studies where the aerosol is changing withtime.

The difference in size distribution between the differentozone/terpene reaction systems under the same conditions(∼3 × 1011 molecule cm-3 converted, 60% RH, and 298 K) isshown in Figure 3. The most efficient species in producingSOA during the initial phase of the ozonolysis is limonene

while R-pinene is the least efficient, which is in accordancewith previous studies, e.g., Koch et al. (13). The comparisonbetween the terpenes is possible since the ozone concentra-tion is chosen to keep the initial rate of reaction and henceamount reacted similar for all experiments shown in Figure3. However, the initial rate for R-pinene is slightly highercompared to the limonene and ∆3-carene rates in bothexperiments, but this does not affect the outcome of R-pinene

TABLE 1. Experimental Resultsa

[Org]0 1011

(molecule cm-3)∆org. 1011

(molecule cm-3)[O3]0 1013

(molecule cm-3)initial rate 108

(molecule cm-3 s-1)temp

(K)RH(%)

N10-300nm(no. cm-3)

M10-300nm 10-2

(µg m-3)

Limonene/LH6.75 ( 0.21 2.97 1.06 ( 0.01 14.3 298.1 ( 0.1 1.56 ( 0.04 4831 ( 67 62 ( 26.75 ( 0.21 2.97 1.06 ( 0.01 14.3 298.3 ( 0.1 40.61 ( 0.04 9886 ( 195 141 ( 66.75 ( 0.21 2.97 1.06 ( 0.01 14.3 298.3 ( 0.1 60.59 ( 0.05 10101 ( 600 160 ( 96.75 ( 0.21 2.97 1.06 ( 0.01 14.3 298.3 ( 0.1 81.01 ( 0.12 12986 ( 578 226 ( 106.75 ( 0.21 2.97 1.06 ( 0.01 14.3 298.2 ( 0.1 84.88 ( 0.06 12047 ( 297 229 ( 56.75 ( 0.21 2.97 1.06 ( 0.01 14.3 298.2 ( 0.1 41.14 ( 0.08 9065 ( 204 149 ( 5

Limonene/LL3.48 ( 0.14 1.52 1.07 ( 0.01 7.5 298.0 ( 0.1 1.71 ( 0.01 1115 ( 26 2.7 ( 0.13.48 ( 0.14 1.52 1.07 ( 0.01 7.5 298.3 ( 0.1 19.90 ( 0.02 1788 ( 53 5.6 ( 0.23.48 ( 0.14 1.52 1.07 ( 0.01 7.5 298.3 ( 0.1 40.59 ( 0.01 1764 ( 29 6.1 ( 0.33.48 ( 0.14 1.52 1.07 ( 0.01 7.5 298.3 ( 0.1 59.58 ( 0.02 1969 ( 110 7.2 ( 0.63.48 ( 0.14 1.52 1.07 ( 0.01 7.5 298.3 ( 0.1 79.1 ( 0.1 2473 ( 112 10.5 ( 0.33.48 ( 0.14 1.52 1.07 ( 0.01 7.5 298.3 ( 0.1 40.52 ( 0.13 1988 ( 94 8.4 ( 0.8

∆3-Carene/CH7.24 ( 0.28 3.12 5.72 ( 0.01 15.3 298.2 ( 0.1 1.54 ( 0.04 2371 ( 59 15.3 ( 0.77.24 ( 0.28 3.12 5.72 ( 0.01 15.3 298.4 ( 0.1 20.69 ( 0.01 3543 ( 164 22.2 ( 0.67.24 ( 0.28 3.12 5.72 ( 0.01 15.3 298.3 ( 0.1 41.45 ( 0.01 4238 ( 51 30.6 ( 0.77.24 ( 0.28 3.12 5.72 ( 0.01 15.3 298.3 ( 0.1 60.61 ( 0.04 5064 ( 94 50 ( 27.24 ( 0.28 3.12 5.72 ( 0.01 15.3 298.4 ( 0.1 85.87 ( 0.1 6375 ( 272 94 ( 17.24 ( 0.28 3.12 5.72 ( 0.01 15.3 298.3 ( 0.1 42.20 ( 0.03 3917 ( 91 38 ( 2

∆3-Carene/CL3.50 ( 0.19 1.52 5.72 ( 0.01 7.4 298.1 ( 0.1 1.48 ( 0.02 516 ( 14 0.78 ( 0.033.50 ( 0.19 1.52 5.72 ( 0.01 7.4 298.3 ( 0.1 21.02 ( 0.02 591 ( 25 0.76 ( 0.043.50 ( 0.19 1.52 5.72 ( 0.01 7.4 298.3 ( 0.1 44.09 ( 0.11 683 ( 73 1.0 ( 0.23.50 ( 0.19 1.52 5.72 ( 0.01 7.4 298.3 ( 0.1 60.82 ( 0.02 937 ( 63 1.61 ( 0.083.50 ( 0.19 1.52 5.72 ( 0.01 7.4 298.4 ( 0.1 87.40 ( 0.04 1513 ( 81 3.8 ( 0.23.50 ( 0.19 1.52 5.72 ( 0.01 7.4 298.3 ( 0.1 42.70 ( 0.02 797 ( 15 1.8 ( 0.8

r-Pinene/APH8.53 ( 0.15 3.89 2.58 ( 0.01 19.1 297.8 ( 0.1 1.39 ( 0.03 1186 ( 91 9.1 ( 0.68.53 ( 0.15 3.89 2.58 ( 0.01 19.1 298.0 ( 0.1 21.79 ( 0.01 1288 ( 47 9.5 ( 0.38.53 ( 0.15 3.89 2.58 ( 0.01 19.1 298.0 ( 0.1 42.80 ( 0.02 1416 ( 33 13.0 ( 0.78.53 ( 0.15 3.89 2.58 ( 0.01 19.1 298.0 ( 0.1 59.70 ( 0.07 1668 ( 60 18.3 ( 0.98.53 ( 0.15 3.89 2.58 ( 0.01 19.1 298.0 ( 0.1 84.57 ( 0.04 2282 ( 63 40 ( 1

r-Pinene/APL4.77 ( 0.21 2.09 2.48 ( 0.01 10.2 297.8 ( 0.1 1.32 ( 0.01 167 ( 24 0.39 ( 0.084.77 ( 0.21 2.09 2.48 ( 0.01 10.2 297.9 ( 0.1 43.14 ( 0.04 210 ( 29 0.43 ( 0.054.77 ( 0.21 2.09 2.48 ( 0.01 10.2 297.9 ( 0.1 61.19 ( 0.05 292 ( 11 0.66 ( 0.044.77 ( 0.21 2.09 2.48 ( 0.01 10.2 298.0 ( 0.1 86.11 ( 0.06 446 ( 30 3.0 ( 0.64.77 ( 0.21 2.09 2.48 ( 0.01 10.2 298.1 ( 0.1 60.62 ( 0.03 268 ( 15 0.92 ( 0.09a N10-300nm is the integrated number and M10-300nm is the integrated mass of formed SOA. These entries are the average of five consecutive scans

by the SMPS-system. [Org]0 and [O3]0 are start concentrations and ∆org.is [Org]0 - [Org]t and calculated by using [Org]t ) [Org]0 × e-kt[O3]. Initialrate is calculated from initial concentrations and corresponding rate coefficient. Stated errors are at the statistical 95% confidence level

FIGURE 2. Number size distributions for 16 consecutive scans ()112 min), for the oxidation of ∆3-carene, CH (RH ) 41% and 3.12 ×1011 molecule cm-3 reacted).

190 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 1, 2006

being least efficient in producing SOA. The size distributionsare comparable in shape, all species giving a maximumnumber concentration at around 50 nm for these conditions.When comparing the integrated mass and numbers, i.e.,M10-300nm and N10-300nm, at 60% RH for limonene and R-pinenefactors of 9 and 6 are obtained, respectively. The same patternwas observed for both the high- and low-concentration set.

As shown in Table 1, as more of the organic reacted themass (M10-300nm) and number concentration (N10-300nm) oforganic particles increased, as expected. The ratio betweenhigh/low M10-300nm experiments varied between 13 and 30and for N10-300 nm between 4.3 and 7.1, while the ratio ofamount reacted was two. Considering the differences incompounds and relative humidities, these intervals are quitenarrow. It is noteworthy that particles were detected in allcases, also for the low concentration experiment where theamount reacted was about 1.5 × 1011 molecule cm-3.

Number size distributions from ozonolysis of ∆3-careneat various relative humidities are presented in Figure 4. Forall ozone/terpene reaction systems, an increase in the watervapor concentration gives an increase in number of particlesformed, as well as in size. Figure 5 is a summary of the effectof relative humidity on total mass (M10-300nm) and totalnumber (N10-300nm) of the produced particles. For ozonolysisof R-pinene and ∆3-carene, there is a small increase withhumidity up to at least 60% RH. This effect is further increasedwhen going to 85% RH, except for integrated number(N10-300nm) from the CH and APH experiments (Figure 5c). Forlimonene there is also a general increase, but in theintermediate RH region (20-60%) the influence of enhancedwater vapor is uncertain (Figure 5b and d).

Figures 2, 3, and 4 suggest there may be a nucleatingmode around 10 nm. However, by using a nano DMA (TSI,3085) that measures in the range 4-79 nm this could be

ruled out. This apparent peak around 10 nm gives anoverestimation of integrated number of particles of amaximum of 15%. The reproducibility was tested by repeatingan experiment at an intermediate relative humidity (40-60% RH). The results are shown in Table 1 and illustrated bya ∆3-carene experiment shown in Figure 4, where the 40%relative humidity experiment was made twice. A relativelygood agreement between experiments was seen, even thoughthe 95% confidence intervals from the two sets of fiveconsecutive scans did not always overlap.

FIGURE 3. Number size distributions of the formed SOA fromozonolysis of limonene (top), ∆3-carene (middle), and r-pinene(bottom), at RH ) 60% and high terpene conversion (LH ) 2.97 ×1011, CH ) 3.12 × 1011, APH ) 3.89 × 1011 molecule cm-3 reacted).Solid lines give averages and dotted lines enclose the 95%confidence intervals.

FIGURE 4. Impact of RH on the number size distribution for theozone initiated oxidation of ∆3-carene, CH (3.12 × 1011 moleculecm-3 converted).

FIGURE 5. Effect of RH on the SOA formation: (a) integrated mass(M10-300nm) for high-concentration experiments, (b) (M10-300nm) forlow-concentration experiments, (c) integrated number (N10-300nm)for high-concentration experiments, and (d) (N10-300nm) for low-concentration experiments. L, C, and AP are abbreviations forlimonene, ∆3-carene, and r-pinene. The sub indexes H and L standfor high- and low-concentration experiments, respectively.

VOL. 40, NO. 1, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 191

DiscussionWater and SOA Formation. An increase in both integratednumber (N10-300nm) and integrated mass (M10-300nm) withincreased relative humidity was observed in the present work.The increase in mass, as shown in Table 1, can partly beexplained by water up-take. The water up-take by organicparticles from terpene oxidation has previously been mea-sured by hygroscopic-tandem-DMA systems (11, 14). In suchsystems, physical up-take of water is measured as the changein diameter between wet and dry conditions. Growth factors(D85/Ddry) due to water (85% RH) of 1.07 (14) and 1.09 (11)have been reported. This increase in diameter can onlyexplain, at a maximum, 30% of the increase in the SOAvolume. The effect of water on the partitioning of the organicproducts seems to be small/negligible, especially for anorganic aerosol formed in experiments without inorganicseed particles (15). The significant growth measured for wetconditions thus supports the view that there is a mechanisticeffect present where water affects the product distribution,giving more low volatility compound(s). The fact that N10-300nm

increases with water vapor concentration further emphasizesa mechanistic effect of water. The increase in N10-300nm caneither be due to the rate of production or the character ofthe nucleating species being affected by the presence of water.In addition to changing the product distribution, one canspeculate that water could change the properties of theinitially produced clusters thereby producing a larger fractionof clusters which are able to grow beyond their critical sizefor particle nucleation.

The literature on the water dependence of SOA formationfrom O3 initiated oxidation of the three investigated mono-terpenes is inconclusive and Table 2 summarizes previousstudies (9-11, 16-18). Regarding the mass of SOA, moststudies found an increase of SOA due to water (9, 11, 19).

Regarding number of particles, the results are even morecontradictory. Our study showed an increase in number ofparticles, while most of the previous studies showed adecrease or no effect of increased humidity (9, 10, 17, 18).The major difference between the present work and thestudies with reduced number of particles is the use of differentOH-scavengers and that the present study extended the RHrange to 85%. Temperature is an important parameter forSOA, both for the partitioning and the nucleation (7, 20, 21).For a large smog chamber, it is a challange to maintain aconstant and gradient-free temperature over the time of anexperiment. The mixing of the reactants is of great concernwhen it comes to particle nucleation. Ideally, the mixingshould promptly give a homogeneous mixture of thereactants. The mixing time for smog chambers will usually

be greater than those obtained in flow systems. Consideringthe dependence of number distribution, the effect of watermay be obscured due to coagulation if the system has a largeconversion rate over long time scales. Bonn et al. (9) showedthat the SOA formation had different water dependence inhigh- and low-concentration experiments (Table 2). Com-pared to most other studies, our experiments were conductedat significantly lower amount reacted. Thus there are severalreasons why the water dependence may vary between studies.

Water Influence on Gas-Phase Reactions. The mostfrequently discussed influence of water is its direct reactionwith the stabilized CI. From various studies the followingreactions have been suggested:

The product from the reactions of CI with water can be acarbonyl or an acid. Even from the simple reaction betweenozone and ethene, the actual ratios of the reaction pathwaysvaries between studies (2, 22). The R-hydroxy-hydroperoxideintermediate formed in this reaction is unstable and itsheterogeneous chemistry makes sampling and evaluation ofexperimental data difficult (22). Pathway IX will, for theinvestigated monoterpenes, produce a ten-carbon multi-functional acid which would contribute to SOA and exhibitpositive dependence on water concentration.

Recently, Ryzhkov and Ariya (23) presented theoreticalcalculations on the reaction of CI with water and with thewater-dimer. They concluded that water-dimer reactionscould enhance the effect of water concentration. Both waterand water-dimer reactions give the R-hydroxy-hydroperoxide.The rate of the dimer reaction is faster than the monomerreaction and the concentration of the dimer increases withthe square of the water concentration. Therefore, theimportance of the dimer reaction will increase rapidly withrelative humidity and thereby the rate of the CI conversion.This could then be the link to the observed increased numberof particles for the high RH experiments.

Water is competing with other available trace compounds,e.g. carbonyls, for the stabilized CI. When a large amount ofthe organic reactant has been converted, the possibility forsuch reactions increases and the fate of the stabilized CIs is

TABLE 2. Comparison of Literature on the Impact of Relative Humidity on Integrated Number and Massa

org. precursor(ppb)b

type ofstudy

OHscavenger

temp(K)

RH(%) Mtot

c Ntotc source

L (1000) static reactor cyclohexane 295 ( 2 0.01 & 31 no effect - Bonn et al. (9)L (19.5 ( 1.2) flow reactor n.s 295 ( 2 15, 30 & 43 no effect no effect Fick et al. (18)L (15 & 30) flow reactor 2-butanol 298 ( 0.4 < 2-85 + + this study

C (1000) static reactor cyclohexane 295 ( 2 0.01 & 31 no effect - Bonn et al. (9)C (18.5 ( 1.2) flow reactor n.s 295 ( 2 15, 30 & 43 no effect no effect Fick et al. (18)C (15 & 30) flow reactor 2-butanol 298 ( 0.4 < 2-85 + + this study

AP (1000) static reactor cyclohexane 295 ( 2 0.01 & 31 no effect no effect Bonn et al. (9)AP (50) static reactor cyclohexane 295 ( 2 0.01 & 31 + - Bonn et al. (9)AP (41-124 reacted) static reactor 2-butanol 301-303 < 2-58 + n.s Cocker et al. (11)AP (49-713) flow reactor cyclohexane 295 ( 0.5 0.2 & 40 n.s - (small) Berndt et al. (10)AP (20.1 ( 1.3) flow reactor n.s 295 ( 2 15, 30 & 43 no effect no effect Fick et al. (18)AP (56000 - 266000) flow reactor n.s 293-302 13-41 no effect no effect Rohr et al. (17)AP (15 & 30) flow reactor 2-butanol 298 ( 0.4 < 2-85 + + this study

a n.s ) Not stated. L ) Limonene. C ) ∆3-Carene. AP ) R-pinene. b Concentrations are given as start concentrations in ppb. c A positive sign(+) means an increase with relative humidity and a negative sign (-) means a decrease with relative humidity.

RHC•OO• + H2O f RHC(OH)OOH f RCH(O) + H2O2

(VIII)

RHC•OO• + H2O f RHC(OH)OOH f RC(O)OH + H2O(IX)

RR′C•OO• + H2O f RR′C(OH)OOH f RR′C(O) + H2O2

(X)

192 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 1, 2006

dependent on the ratios in the reaction rates. For comparison,the reaction between water and the simplest biradical(H2C•OO•) is about 14 000 times slower than the reactionwith HCOOH (22). For most experiments the water con-centration is several orders of magnitude higher than any ofthe products. It has been suggested that the CI reactionswith larger aldehydes can give secondary ozonides that cancontribute to SOA (24, 25). For the R-pinene/ozone system,pinonaldehyde is one of the major carbonyl products.Warschield and Hoffman (26) reported an increase inpinonaldehyde yield with increased relative humidity. Thisis in line with increased production of R-hydroxy-hydro-peroxide from the CI + H2O reaction X that subsequentlydecomposes to pinonaldehyde and H2O2. On the other hand,Berndt et al. (10) presented data where a decrease inpinonaldehyde yield with relative humidity took place. Onthe basis of this observation and the measured OH and H2O2

yield at different RH, they questioned whether the water +CI reaction was responsible for the production of pinon-aldehyde and H2O2. A third study by Baker et al. (27) observedno water effect on the pinonaldehyde yield.

Water Influence on HOx Chemistry. Production of OHradicals from O3 degradation of terpenes has been shown inprevious studies (2, 28). By using an OH scavenger, theradicals may be removed from the system without affectingthe reaction to be investigated. The most frequent OHscavenger used is cyclohexane. However, Chew and Atkinson(29) suggested 2-butanol as an alternative and showed thatthere was no difference between 2-butanol and cyclohexanein their ability to scavenge OH radicals. However, oneimportant aspect as well as the scavenger ability is theadditional chemistry taking place due to the scavenger. Anissue of particular importance is the production of hydro-peroxy (HO2) and alkylperoxy (RO2) radicals, where, e.g.,2-butanol gives more HO2 than cyclohexane (30). Keywoodet al. (30) have proven that the presence of HO2 influencesthe SOA formation. A suggested pathway affecting the SOAproduction is the HO2 reaction with acylperoxy radicals,leading to acid and peracid products. Production of acyl-peroxy radicals from the investigated terpenes seems likely.The requirement is that the parent compound contains avinylic hydrogen atom, something that all of the investigatedterpenes do. It is noteworthy that Bonn et al. (9) report astrong negative water dependence on SOA mass fromozonolysis of â-pinene, which lacks a vinylic hydrogen. Thisis contrary to the absent or positive water effect for theinvestigated endocyclic terpenes.

The acids produced from the reaction between acylperoxy-HO2 have very low volatility and can contribute to SOAformation (30). Our study, where a clear RH dependence canbe seen, is in contrast to that of Bonn et al. (9), where theeffect on particle number was negative. The effect of watermay depend on the use of different scavengers, i.e., differencein HO2 abundance. It is well-known that the HO2 self-reactionis affected by the water activity (31, 32). There is also asuggestion that other HO2 reaction systems are dependenton water vapor concentrations (33), where increased relativehumidity causes an increase in the apparent rate coefficientdue to formation of HO2-nH2O complexes. Indirectly, theobserved water effect on SOA formation in the present studycould be due to a water dependence on the correspondingHO2-RO2 or HO2-acylperoxy radical reaction. However, nofirm kinetic data are at hand for substantiating such astatement.

Water Influence on Partitioning and Condensed PhaseChemistry. Seinfeld et al. (34) pointed out that the waterdependence may change with the amount of terpeneconverted, where high initial organic concentrations willmake the particles less hydrophilic. When a large amount ofterpene is converted, more aerosol will be available for

coagulation/condensation and gas-to-particle partitioning.In a similar way, experiments conducted with preexistingseed particles will affect the amount of SOA produced, bothnumber and mass (35-38). It has been proven that presenceof acid aerosol particles will enhance the SOA mass yield(35-37, 39). It is possible that semivolatile compounds willpartition into the particle phase and undergo liquid phasepolymerization (40-42). If this process is nonreversible itwill drive the gas-to-particle equilibrium toward the con-densed phase. The extent of such polymerization and itspossible dependence on water availability is unclear. How-ever, it has been concluded that the acidity of the seed aerosolaffects the polymerization by contributing to higher yield oflarger oligomers (35) through a catalytic effect. In the presentstudy no seed aerosol was used, but results presented byGao et al. (35) indicate that the oxidation products, i.e.,organic acids, can provide the necessary acidity to enhanceSOA relative to alkaline or neutral seed particles. If waterincreases the rate of reaction IX it will lead to more acidproduction that could enhance the acidity of the resultingaerosol, thus producing more oligomers. Furthermore, thepresence of water is necessary for the organic acids todissociate. However, the importance of acid catalysis in thereal atmosphere may be small and is still a subject of ongoinginvestigation.

Atmospheric Implications. Oxidation of monoterpenesin the atmosphere takes place predominantly within theboundary layer where significant water concentrations areencountered. The observed water dependence will, for allthree monterpenes, lead to an increase in number and massof particles in humid environments. There are several possibleexplanations for the water dependence, where the increasein SOA, e.g., could be attributed to the water reaction withthe stabilized CI, producing carboxylic acids. At atmosphericconditions, this water effect could be even more importantif the reaction proceeds via a water-dimer, as the rate of thedimer reaction is faster than the monomer reaction and theconcentration of the dimer increases with the square of thewater concentration. The competing reactions of the sta-bilized CI with carbonyl products should be slow at the lowconcentrations encountered in the atmosphere and in linewith experiments conducted at low amount of precursorconverted. The suggested HO2-RO2 reactions would be moresignificant at atmospheric conditions than RO2-R′O2 reac-tions. However, it is not clear which of these two processescontribute the most to SOA formation and both may havea water concentration dependence. The possibilities forliquid-phase reactions of semivolatile products are dependenton water availability, even if the extent of this process in theatmosphere is not yet established.

Both the observed mass and number dependence on waterconcentration are useful for understanding atmosphericaerosol observations. For future model evaluation of SOAfrom ozonolysis of terpenes, both water and possible HO2-RO2 chemistry should be included. Hopefully, specificproduct analysis at selected humidities can give furtherinformation on the detailed mechanism for ozonolysis ofmonoterpenes (8, 26).

AcknowledgmentsThis work was supported by MISTRA, the Swedish Foundationfor Strategic Environmental Research, the Swedish ResearchCouncil, and the graduate school “Climate and Mobility”.Benny Lonn, Senior Research Engineer, is acknowledged forskillful technical support.

Literature Cited(1) Kanakidou, M.; Seinfeld, J. H.; Pandis, S. N.; Barnes, I.; Dentener,

F. J.; Facchini, M. C.; Van Dingenen, R.; Ervens, B.; Nenes, A.;Nielsen, C. J.; Swietlicki, E.; Putaud, J. P.; Balkanski, Y.; Fuzzi,

VOL. 40, NO. 1, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 193

S.; Horth, J.; Moortgat, G. K.; Winterhalter, R.; Myhre, C. E. L.;Tsigaridis, K.; Vignati, E.; Stephanou, E. G.; Wilson, J. Organicaerosol and global climate modelling: a review. Atmos. Chem.Phys. 2005, 5, 1053-1123.

(2) Calvert, J. G.; Atkinson, R.; Kerr, J. A.; Madronich, S.; Moortgat,G. K.; Wallington, T. J.; Yarwood, G. The Mechanism ofAtmospheric Oxidation of the Alkenes; Oxford Univeristy Press:New York, 2000.

(3) Atkinson, R.; Arey, J. Atmospheric chemistry of biogenic organiccompounds. Acc. Chem. Res. 1998, 31, 574-583.

(4) Fuentes, J. D.; Lerdau, M.; Atkinson, R.; Baldocchi, D.; Botten-heim, J. W.; Ciccioli, P.; Lamb, B.; Geron, C.; Gu, L.; Guenther,A.; Sharkey, T. D.; Stockwell, W. Biogenic hydrocarbons in theatmospheric boundary layer: A review. Bull. Am. Meteorol. Soc.2000, 81, 1537-1575.

(5) Atkinson, R.; Arey, J. Gas-phase tropospheric chemistry ofbiogenic volatile organic compounds: a review. Atmos. Environ.2003, 37, S197-S219.

(6) Calogirou, A.; Larsen, B. R.; Kotzias, D. Gas-phase terpeneoxidation products: a review. Atmos. Environ. 1999, 33, 1423-1439.

(7) Jenkin, M. E. Modelling the formation and composition ofsecondary organic aerosol from alpha- and beta-pineneozonolysis using MCM v3. Atmos. Chem. Phys. 2004, 4, 1741-1757.

(8) Ziemann, P. J. Evidence for low-volatility diacyl peroxides as anucleating agent and major component of aerosol formed fromreactions of O3 with cyclohexene and homologous compounds.J. Phys. Chem. A 2002, 106, 4390-4402.

(9) Bonn, B.; Schuster, G.; Moortgat, G. K. Influence of water vaporon the process of new particle formation during monoterpeneozonolysis. J. Phys. Chem. A 2002, 106, 2869-2881.

(10) Berndt, T.; Boge, O.; Stratmann, F. Gas-phase ozonolysis ofalpha-pinene: gaseous products and particle formation. Atmos.Environ. 2003, 37, 3933-3945.

(11) Cocker, D. R.; Clegg, S. L.; Flagan, R. C.; Seinfeld, J. H. The effectof water on gas-particle partitioning of secondary organicaerosol. Part I: alpha-pinene/ozone system. Atmos. Environ.2001, 35, 6049-6072.

(12) Atkinson, R. Gas-Phase Tropospheric Chemistry Of Organic-Compounds, Monograph 2. J. Phys. Chem. Ref. Data 1994, R1-R216.

(13) Koch, S.; Winterhalter, R.; Uherek, E.; Kolloff, A.; Neeb, P.;Moortgat, G. K. Formation of new particles in the gas-phaseozonolysis of monoterpenes. Atmos. Environ. 2000, 34, 4031-4042.

(14) Virkkula, A.; Van Dingenen, R.; Raes, F.; Hjorth, J. Hygroscopicproperties of aerosol formed by oxidation of limonene, alpha-pinene, and beta-pinene. J. Geophys. Res., [Atmos.] 1999, 104,3569-3579.

(15) Griffin, R. J.; Dabdub, D.; Seinfeld, J. H. Development and initialevaluation of a dynamic species-resolved model for gas-phasechemistry and size-resolved gas/particle partitioning associatedwith secondary organic aerosol formation. J. Geophys. Res.,[Atmos.] 2005, 110, D05304. doi:10.1029/2004JD005219.

(16) Bonn, B.; Moortgat, G. K. New particle formation during alpha-and beta-pinene oxidation by O3, OH and NO3, and the influenceof water vapour: particle size distribution studies. Atmos. Chem.Phys. 2002, 2, 183-196.

(17) Rohr, A. C.; Weschler, C. J.; Koutrakis, P.; Spengler, J. D.Generation and quantification of ultrafine particles throughterpene/ozone reaction in a chamber setting. Aerosol Sci.Technol. 2003, 37, 65-78.

(18) Fick, J.; Pommer, L.; Nilsson, C.; Andersson, B. Effect of OHradicals, relative humidity, and time on the composition of theproducts formed in the ozonolysis of alpha-pinene. Atmos.Environ. 2003, 37, 4087-4096.

(19) Bonn, B.; von Kuhlmann, R.; Lawrence, M. G. High contributionof biogenic hydroperoxides to secondary organic aerosolformation. Geophys. Res. Lett. 2004, 31, L10108. doi:10.1029/2003GL019172.

(20) Takekawa, H.; Minoura, H.; Yamazaki, S. Temperature depen-dence of secondary organic aerosol formation by photooxidationof hydrocarbons. Atmos. Environ. 2003, 37, 3413-3424.

(21) Tsigaridis, K.; Kanakidou, M. Global modelling of secondaryorganic aerosol in the troposphere: a sensitivity analysis. Atmos.Chem. Phys. 2003, 3, 1849-1869.

(22) Neeb, P.; Sauer, F.; Horie, O.; Moortgat, G. K. Formation ofhydroxymethyl hydroperoxide and formic acid in alkeneozonolysis in the presence of water vapour. Atmos. Environ.1997, 31, 1417-1423.

(23) Ryzhkov, A. B.; Ariya, P. A. A theoretical study of the reactionsof parent and substituted Criegee intermediates with water andthe water dimer. Phys. Chem. Chem. Phys. 2004, 6, 5042-5050.

(24) Kamens, R.; Jang, M.; Chien, C. J.; Leach, K. Aerosol formationfrom the reaction of alpha-pinene and ozone using a gas-phasekinetics aerosol partitioning model. Environ. Sci. Technol. 1999,33, 1430-1438.

(25) Neeb, P.; Horie, O.; Moortgat, G. K. Formation of secondaryozonides in the gas-phase ozonolysis of simple alkenes.Tetrahedron Lett. 1996, 37, 9297-9300.

(26) Warscheid, B.; Hoffmann, T. On-line measurements of R-pineneozonolysis products using an atmospheric pressure chemicalionisation ion-trap mass spectrometer. Atmos. Environ. 2001,35, 2927-2940.

(27) Baker, J.; Aschmann, S. M.; Arey, J.; Atkinson, R. Reactions ofstabilized Criegee intermediates from the gas-phase reactionsof O3 with selected alkenes. Int. J. Chem. Kinet. 2001, 34, 73-85.

(28) Aschmann, S. M.; Arey, J.; Atkinson, R. OH radical formationfrom the gas-phase reactions of O3 with a series of terpenes.Atmos. Environ. 2002, 36, 4347-4355.

(29) Chew, A. A.; Atkinson, R. OH radical formation yields from thegas-phase reactions of O3 with alkenes and monoterpenes. J.Geophys. Res., [Atmos.] 1996, 101, 28649-28653.

(30) Keywood, M. D.; Kroll, J. H.; Varutbangkul, V.; Bahreini, R.;Flagan, R. C.; Seinfeld, J. H. Secondary organic aerosol formationfrom cyclohexene ozonolysis: Effect of OH scavenger and therole of radical chemistry. Environ. Sci. Technol. 2004, 38, 3343-3350.

(31) Kanno, N.; Tonokura, K.; Tezaki, A.; Koshi, M. Water dependenceof the HO2 self-reaction: Kinetics of the HO2-H2O complex. J.Phys. Chem. A 2005, 109, 3153-3158.

(32) Kircher, C. C.; Sander, S. P. Kinetics and Mechanism of HO2 andDO2 Disproportionations. J. Phys. Chem. 1984, 88, 2082-2091.

(33) Reichert, L.; Hernandez, M. D. A.; Stobener, D.; Burkert, J.;Burrows, J. P. Investigation of the effect of water complexes inthe determination of peroxy radical ambient concentrations:Implications for the atmosphere. J. Geophys. Res., [Atmos.] 2003,108 (D1), 4017. doi:10.1029/2002JD002152, 2003.

(34) Seinfeld, J. H.; Erdakos, G. B.; Asher, W. E.; Pankow, J. F. Modelingthe formation of secondary organic aerosol (SOA). 2. Thepredicted effects of relative humidity on aerosol formation inthe alpha-pinene-, beta-pinene-, sabinene-, Delta(3)-carene-,and cyclohexene-ozone systems. Environ. Sci. Technol. 2001,35, 3272-3272.

(35) Gao, S.; Ng, N. L.; Keywood, M.; Varutbangkul, V.; Bahreini, R.;Nenes, A.; He, J. W.; Yoo, K. Y.; Beauchamp, J. L.; Hodyss, R. P.;Flagan, R. C.; Seinfeld, J. H. Particle phase acidity and oligomerformation in secondary organic aerosol. Environ. Sci. Technol.2004, 38, 6582-6589.

(36) Czoschke, N. M.; Jang, M.; Kamens, R. M. Effect of acidic seedon biogenic secondary organic aerosol growth. Atmos. Environ.2003, 37, 4287-4299.

(37) Iinuma, Y.; Boge, O.; Gnauk, T.; Herrmann, H. Aerosol-chamberstudy of the alpha-pinene/O3 reaction: influence of particleacidity on aerosol yields and products. Atmos. Environ. 2004,38, 761-773.

(38) Jang, M. S.; Czoschke, N. M.; Lee, S.; Kamens, R. M. Hetero-geneous atmospheric aerosol production by acid-catalyzedparticle-phase reactions. Science 2002, 298, 814-817.

(39) Zhang, R. Y.; Suh, I.; Zhao, J.; Zhang, D.; Fortner, E. C.; Tie, X.X.; Molina, L. T.; Molina, M. J. Atmospheric new particleformation enhanced by organic acids. Science 2004, 304, 1487-1490.

(40) Kalberer, M.; Paulsen, D.; Sax, M.; Steinbacher, M.; Dommen,J.; Prevot, A. S. H.; Fisseha, R.; Weingartner, E.; Frankevich, V.;Zenobi, R.; Baltensperger, U. Identification of polymers as majorcomponents of atmospheric organic aerosols. Science 2004, 303,1659-1662.

(41) Jang, M.; Kamens, R. M. Newly characterized products andcomposition of secondary aerosols from the reaction of alpha-pinene with ozone. Atmos. Environ. 1999, 33, 459-474.

(42) Tolocka, M. P.; Jang, M.; Ginter, J. M.; Cox, F. J.; Kamens, R. M.;Johnston, M. V. Formation of oligomers in secondary organicaerosol. Environ. Sci. Technol. 2004, 38, 1428-1434.

Received for review June 20, 2005. Revised manuscript re-ceived October 14, 2005. Accepted October 23, 2005.

ES051163W

194 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 1, 2006