Influence of OH Scavenger on theWater Effect on Secondary OrganicAerosol Formation from Ozonolysisof Limonene, ∆3-Carene, andr-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 D E V E R T L J U N G S T R O M

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

Received October 3, 2007. Revised manuscript receivedFebruary 22, 2008. Accepted June 12, 2008.

The effect of OH scavengers on how water vapor influencesthe formation of secondary organic aerosol (SOA) in ozonolysisof limonene, ∆3-carene, and R-pinene at low concentrationshasbeeninvestigatedbyusingalaminarflowreactor.Cyclohexaneand 2-butanol (3-40 × 1013 molecules cm-3) were used asscavengers and compared to experiments without any scavenger.The reactions were conducted at 298 K and at relativehumidities between <10 and 80%. The yield of SOA decreasedin the order “no scavenger” > 2-butanol > cyclohexane.The effect of water vapor was similar for 2-butanol and withouta scavenger, with an increase in particle number and massconcentration with increasing relative humidity. The water effectfor cyclohexane was more complex, depending on theterpene, scavenger concentration, and SOA concentration.The water effect seems to be influenced by the HO2/RO2 ratio.The results are discussed in relation to the currently suggestedmechanism for alkene ozonolysis and to atmospheric importance.The results imply that the ozone-initiated oxidation of terpenesneeds revision in order to fully account for the role ofwater in the chemical mechanism.

IntroductionA central issue in atmospheric science is to elucidate theeffect of organic aerosols on, for example, climate and health.Especially the secondary aerosol, that is, the fraction of theorganic aerosol that is produced by gas-to-particle conversionin the atmosphere, has been difficult to quantify due to itscomplexity (1–3). One important source of compounds takingpart in secondary organic aerosol (SOA) formation is thegas-phase oxidation of terpenes. Here, the monoterpenes(C10H16) have attracted great interest due to their hugeemission and appropriate molecular mass, that is, lightenough to be volatilized and still sufficiently heavy to givecondensable products (4–6). Among all monoterpenes,R-pinene is the most studied with regard to kinetics, reactionmechanisms, and aerosol formation (7). R-Pinene is notnecessarily the monoterpene dominating SOA formation.Two other terpenes responsible for a significant SOA massare limonene and ∆3-carene (8). Limonene is of additionalinterest due to its use in various household products, affectingindoor SOA formation (9).

Although there have been numerous studies on theoxidation of monoterpenes, a detailed mechanistic descrip-tion of how the oxidation leads to aerosol formation is stilllacking. In particular, the understanding of its ozone-initiatedoxidation is challenging. The generally accepted mechanismfor ozonolysis of these unsaturated compounds is, in brief(10), the initial addition of ozone to a double bond formingan ozonide that proceeds via cleavage of the former doublebond and an O-O bond, producing a carbonyl species anda Criegee intermediate (CI), that is, a biradical, in anvibrationally excited state (CI*). The fate of the CI* iscomplicated, and four pathways have been postulated: (1)collisional relaxation to a stabilized CI, CIs, (2) hydroperoxidechannel, (3) ester-/“hot acid” channel, or (4) O atomelimination.

One important aspect of the CI* is the production ofOH radicals from the hydroperoxide channel (11–13). Inlaboratory experiments of alkene ozonolysis, the OH radicalwill induce secondary chemical degradation by reactionwith the precursor molecule and/or the products. Toreduce the effect of this secondary degradation, most recentstudies use an OH scavenger, for example, cyclohexane(11, 12), 2-butanol (14), or CO (15, 16). The chemistrywithout a scavenger and with cyclohexane or 2-butanol isindeed different (4). This could be the reason for differentamounts of SOA formed when using different scavengers(4, 17–19). The ambition to reduce complexity by removingOH radical chemistry with the parent compounds is partlycounteracted by the addition of the degradation chemistryof the scavenger itself. Cyclohexane and 2-butanol havewell-established major degradation pathways (reactionsI-V), resulting in higher HO2/RO2 ratios for 2-butanolcompared to cyclohexane (4, 18).

The effect of water on the number and mass of particlesproduced in the SOA formation process has recently beensummarized (8), and there are discrepancies in the literature.One explanation could be the use of different scavengersand their concentrations among studies. In order to elucidatehow the water effect is influenced by the use of a scavenger,this paper reports on SOA formation at room temperaturefrom the ozonolysis of R-pinene, ∆3-carene, and limonene,obtained in a flow reactor where systematic changes in water* Corresponding author e-mail: asajon@chem.gu.se.

Environ. Sci. Technol. 2008, 42, 5938–5944

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concentration and with 2-butanol and cyclohexane asscavengers were made.

Experimental SectionThe Experimental Setup. The experimental setup is acombination of a vertical laminar flow reactor and ascanning mobility particle sizer (SMPS) system, namedG-FROST (Goteborg-Flow Reactor for Oxidation Studiesat low Temperatures). The setup, procedure, and analysishave been described elsewhere (8). Some changes com-pared to earlier work are that the reactor was modifiedand positioned in a chamber with an extended workingtemperature range, 238-323 K. The reactor is 1.91-m-longand 10 cm in diameter. In addition, at the end of the flowreactor, there is a centered sampling funnel, which meansthat only the center part of the laminar flow is conveyedto the SMPS system. This is to reduce the impact of theslower flow near the reactor walls. The excess flow is passedto the vent. The mixing of the reactants at the outflow ofthe injector was upgraded by using a mixing plunger inline with Bonn et al. (20). Additionally, two stainless steelnets were added to the outlet of the plunger in order torapidly obtain a laminar flow. Ozone is added through aTeflon tube running inside the stainless steel tube thatmakes up the injector. The scavenger is added to the systemby passing N2 gas through a gas-wash bottle containingthe substance, and the concentration is controlled byvarying the temperature of the scavenger compound. Thebulk flow is humidified by passing it through a Gore-Textube submerged in thermostatted deionized water.

Experimental Procedure. The formation of particles wasinvestigated at experimental conditions summarized in Table1. In the experiments, 1.8-1.9 × 1011 molecules cm-3 (∼7ppb at 298 K, 1 atm) of the terpene had reacted, and theinitial rate of reaction was ∼1 × 109 molecules cm-3 s-1. Therate coefficients used for the rate calculation were 200 ×10-18, 37 × 10-18, and 86.6 × 10-18 cm3 molecule-1 s-1 forlimonene, ∆3-carene, and R-pinene, respectively (21). Theaverage reaction time was kept constant (238 s), and thetotal flow in the system was 1.6 standard liters per minute

(SLPM; 298 K and 1 atm), where 0.94 SLPM was conveyedvia the sampling funnel.

Particle number (N10-300nm) and mass (M10-300nm) con-centrations are given as averages of five consecutive distri-butions. Estimates of SOA density have been presented (22).However, since this quantity may change between monot-erpenes and experimental conditions, a density of unityhas been applied. The scan time was 5 min, comprisingof an up-scanning time of 240 s, a down-scanning time of45 s, and a delay time of 15 s. In all sets of experiments,the relative humidity (RH) was varied between 10 and 80%.With the new flow reactor system, it was, in addition,possible to slowly change the relative humidity, forexample, from 10% up to 80% over 10 h, as is shown inFigure 1. It is noteworthy that, when running a dynamicchange in relative humidity, the aerosol and the systemmay not have had enough time to equilibrate, and themeasured values may not represent the actual SOAproduction at the specified RH. However, as shown inFigure 1a, there is good reproducibility between the twomethods. Figure 1b shows a full stepwise experiment inwhich also the stability of the system is demonstrated.The long-time stability of the system was excellent in thata stable aerosol could be produced for hours and evendays. However, the resulting aerosol formation is extremelytemperature-sensitive, and variations in temperature ofmore than 0.4 K induced significant scatter. It was alsonoted that the air quality influenced the results. Forreproducible results, highly purified air (Laboratory ZeroAir Generator, Linde Gas, Model N-GC6000) in conjunctionwith a NOx absorbent, Sofnox R (Molecular ProductsLimited), was needed. In this study, three main types ofexperiments were performed: using no OH scavenger,cyclohexane as a scavenger, and 2-butanol as a scavenger.The concentration of the scavenger was varied and therelative humidity dependence measured for each com-bination and scavenger concentration. The amount ofterpene consumed and the anticipated concentrations ofHO2, RO2, and ROOH were estimated using the R-pinene,cyclohexane, and 2-butanol subset codes available from

TABLE 1. Experimental Conditionsa

exporganic

precursor OH scavenger[Terpene]0 1011

molecules cm-3[Scavenger]0 1013

molecules cm-3[O3]0 1013

molecules cm-3 RH(%)

1 limonene 2-butanol 4.37 ( 0.17 3.5 ( 0.2 1.19 ( 0.01 10.6-80.02 limonene 2-butanol 4.37 ( 0.17 6.8 ( 0.4 1.19 ( 0.01 10.8-76.63 limonene 2-butanol 4.37 ( 0.17 12 ( 0.6 1.19 ( 0.01 ∼10-78.24 limonene 2-butanol 4.37 ( 0.17 32 ( 1.7 1.19 ( 0.01 10.9-76.75 limonene cyclohexane 4.37 ( 0.17 3.9 ( 0.2 1.19 ( 0.01 11.0-76.96 limonene cyclohexane 4.37 ( 0.17 8.6 ( 0.5 1.19 ( 0.01 11.1-77.07 limonene cyclohexane 4.37 ( 0.17 19 ( 1.0 1.19 ( 0.01 10.6-77.28 limonene cyclohexane 4.37 ( 0.17 36 ( 1.9 1.19 ( 0.01 10.1-77.09 limonene none 4.37 ( 0.17 1.19 ( 0.01 9.1-77.310 ∆3-carene 2-butanol 4.32 ( 0.65 3.5 ( 0.2 6.23 ( 0.01 9.5-74.211 ∆3-carene 2-butanol 4.32 ( 0.65 6.8 ( 0.4 6.23 ( 0.01 10.5-75.812 ∆3-carene 2-butanol 4.32 ( 0.65 12 ( 0.6 6.23 ( 0.01 10.3-76.213 ∆3-carene 2-butanol 4.32 ( 0.65 32 ( 1.7 6.23 ( 0.01 10.3-75.114 ∆3-carene cyclohexane 4.32 ( 0.65 3.9 ( 0.2 6.23 ( 0.01 6.3-76.815 ∆3-carene cyclohexane 4.32 ( 0.65 8.6 ( 0.5 6.23 ( 0.01 8.0-72.916 ∆3-carene cyclohexane 4.32 ( 0.65 36 ( 1.9 6.23 ( 0.01 6.7-75.617 ∆3-carene none 4.32 ( 0.65 6.23 ( 0.01 6.2-75.018 R-pinene 2-butanol 4.54 ( 0.54 3.5 ( 0.2 2.53 ( 0.01 10.0-73.619 R-pinene 2-butanol 4.54 ( 0.54 6.8 ( 0.4 2.53 ( 0.01 10.4-76.620 R-pinene 2-butanol 4.54 ( 0.54 12 ( 0.6 2.53 ( 0.01 9.1 -71.921 R-pinene 2-butanol 4.54 ( 0.54 32 ( 1.7 2.53 ( 0.01 9.5-73.722 R-pinene none 4.54 ( 0.54 2.53 ( 0.01 9.6-74.5a The reaction temperature was 298 ( 1 K, and the temperature stability within an experiment was (0.15 K. [Terpene]0,

[Scavenger]0 ,and [O3]0 are initial concentrations (1 ppb ) 2.46 1010 molecules cm-3 at 298 K and 1 atm). The RH wasvaried between the given limits in four steps. Stated errors are at the statistical 95% confidence level.

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the Master Chemical Mechanism (MCM v.3.1, http://mcm.leeds.ac.uk/MCM).

ResultsTable S1 in the Supporting Information contains, in additionto experimental conditions, summarized results of thenumber and mass concentrations of the SOA formed in allexperiments. In a previous paper, the impact of humidity onthe number and mass of particles formed in the ozonolysisof limonene, ∆3-carene, andR-pinene was investigated using2-butanol as a scavenger (8). Using the new system with aslightly different delivery and sampling system, the mainfeatures from the previous study were confirmed, that is,more aerosols, both regarding number and mass underhumid conditions for all three terpenes, in the text called apositive water effect. Figure 2 gives a comparison withprevious data for the limonene experiments, using 2-butanolas an OH scavenger. There is a clear difference in the shapeof the number size distributions as a result of only the centerflow being analyzed in the new system. Here, the sizedistribution is narrowed, and less mass is recorded, due toshorter average contact time in the reactor and less influenceof the wall.

A general trend is that OH scavengers suppress theaerosol formation, and the lowest SOA yields weremeasured when cyclohexane was used, Figure 3 and TableS1 (Supporting Information). The system without an OHscavenger will consume more of the organic precursorand possibly also oxidize the products further (MCM v.3.1calculations for Exp#22 show 41% more R-pinene reactedcompared to Exp#21, whereas the increase in SOA masswas 10-fold). This could partly explain the increase inaerosol yield, while some of the increase could be attributedto products with low volatility, formed from OH-initiatedoxidation.

It is evident that the RH effect is present and positivefor systems with 2-butanol and without a scavenger,whereas the RH effect in the cyclohexane case is morecomplex. Figure 2 shows the relative humidity dependenceof SOA formation from limonene for four concentrationsof 2-butanol. All cases show similar relative humiditydependence, where the amount of SOA formed is increasedwith a decreasing concentration of 2-butanol. As expected,

FIGURE 1. (a) Change in particle number (N10-300nm) and mass(M10-300nm) concentration with relative humidity (RH; Exp#9).Circular symbols are the results when scanning RH (10 f 80%)(dark symbols, number; white symbols, mass), and the resultswhen changing RH stepwise are shown with triangles (darksymbols, mass; white symbols, number). (b) Change in number(white circles) and mass (dark circles) when changing the RHstepwise (10, 40, 60, 80, and 60%). Vertical lines indicate thetime of the change in RH.

FIGURE 2. Effect of the concentration of 2-butanol on particlenumber (a) and mass (b) in the ozonolysis of limonene(Exp#1-4). White circles, 3.5; white diamonds, 6.8; whitetriangles, 12; and crosses, 32 × 1013 molecules cm-3. Forcomparison, data from Jonsson et al. (8) is added (dark circles,low conc.; triangles, high conc.). (c) Size distributions obtainedwith the upgraded system (broad line), where only the centeredpart of the flow was taken for analysis, and the old system(thin line; Jonsson et al. (8)), where the whole flow was used.

FIGURE 3. Number (white symbols) and mass (dark symbols)concentrations of particles formed, when using 2-butanol(diamonds, Exp#4) and cyclohexane (circles, Exp#8) as OHscavengers or using no scavenger (triangles, Exp#9) in theozonolysis of limonene.

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the scavenger effect is diminished for lower 2-butanolconcentrations, and the system is moving toward the non-scavenger situation, that is, a higher aerosol production.A similar pattern was noted for ∆3-carene (Figure 4a,e).For R-pinene, the positive RH trend was present for all2-butanol concentrations, but the amount of SOA formedincreased with an increasing concentration of 2-butanol(Figure 4b,f). Figure 4c shows the analogue to Figure 2awith cyclohexane as the OH scavenger in the ozonolysisof limonene. The number of particles formed seems to bethe lowest at intermediate relative humidity (40 and 60%).Even though these variations are within the experimentaluncertainties, this trend is reproduced in all experiments.Additionally, the number of particles increased with adecreasing concentration of cyclohexane. Regarding themass of SOA, there is a slight positive water effect for lowconcentrations of cyclohexane, while in the high cyclo-hexane case, there is no significant trend going from dryto wet conditions (Figure 4g). For ∆3-carene, the trend isthe same for all concentrations of cyclohexane, where thenumber of particles is increased with increasing relativehumidity (Figure 4d). However, the number of particles isincreasing with increasing scavenger concentration. As forthe R-pinene/2-butanol case, this was unexpected, but it

appears that this positive scavenger effect is only presentin cases where a low mass and low number of particles areformed. The effect on mass is also difficult to establishdue to the small number of particles formed, resulting inlarge errors (Figure 4h). For R-pinene, not enough particleswere formed to enable analysis when using cyclohexaneas a scavenger. However, it was established that cyclo-hexane gave fewer particles and a lower mass than thecorresponding experiment with 2-butanol.

Discussion and Atmospheric ImplicationsWater in the Ozonolysis Mechanism. In recent ozonolysisstudies, it has been shown that water affects both gas-phasechemistry, for example, OH (16), end products, (23, 24) andSOA formation, see, for example, ref 8. Mechanistically, wateris believed to react with CIs, forming an R-hydroxy hydro-peroxide that possesses excess energy (10). This peroxidecan either be collisionally stabilized or decompose into acarbonyl and H2O2 (reaction VIa) or a carboxylic acid andH2O (reaction VIb). For the acid to be formed, the R-hydroxyhydroperoxide must contain an R-hydrogen, that is, R1 or R2

must be an H atom.

FIGURE 4. Number (a-d) and mass (e-h) of particles as a function of scavenger concentration. The concentration is increasing inthe order: circles, diamonds, triangles, and crosses, cf. Table 1.

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Anglada et al. (13)have suggested decomposition to formOH and an alkoxy or alkyl radical as another possibility forthe R-hydroxy hydroperoxide as the peroxide bond may becleaved due to the excess internal energy. According toAnglada et al. (13), water can react with CIs and form OHradicals in two ways: the cycloaddition of water (reactionVII) and water-catalyzed H migration (reaction VIII). Thewater-catalyzed channel (VIII) is only active for CIs with noR-H in the syn position.

Another pathway forming OH radicals is directly fromCI*, involving no water. Here, a �-hydrogen migrates to theterminal oxygen followed by a breakage of the O-OH bond.This pathway proceeds for a monosubstituted CI* almostentirely from the syn configuration (25) and does not involveCIs (reaction IX).

Consequently, the two suggested pathways of OH forma-tion should have different H2O dependences. The OH yieldfromR-pinene has been measured in several studies and, forexample, Atkinson et al. (12) did not see a dependence onwater. Furthermore, experiments with ∆3-carene and li-monene showed an OH yield that was independent of thewater concentration in the range 5-40% RH, using 2-butanolas a scavenger (26). However, a problem is that the scavengeror its products in high concentration experiments can reactwith CIs and diminish the role of water. For simpler alkenesystems, it has also been demonstrated that the waterdependence on OH yield is still an unresolved issue. Hassonet al. (27) presented OH yields being independent of water,whereas a recent study of Wegener et al. (16) with CO as ascavenger saw increased yields of OH under humid conditions.

OH Scavenger and SOA Formation. Several studies haveto some extent treated the OH scavenger effect on aerosolformation from ozonolysis of terpenes and compounds withrelated chemical structures, see for example refs 18, 19, and28. When not using an OH scavenger, more organic materialis converted due to the additional OH reaction with theorganic precursor (e.g., the terpene) and its products. Inaddition, the products formed in OH-initiated oxidation couldpossess additional polar groups (e.g., -OH). This is in linewith volatility measurements of SOA particles from theozonolysis of R-pinene in the presence of cyclohexane. Suchparticles are significantly more volatile than the correspond-ing particles produced without an OH scavenger (29). It couldbe noted that Jenkin (4), when using the MCM v.3.1 model,showed contradictory results, where the system withoutscavenger gave more volatile products.

SOA formation also depends on which scavenger is used;however, the reason is not completely understood andquantified. The most widely used OH scavengers are CO,2-butanol, and cyclohexane. As outlined in the Introduction,the mechanistic complexity of the degradation of thesecompounds is increasing in the order CO, 2-butanol, andcyclohexane. Carbon monoxide gives only HO2, while 2-bu-tanol and, even more so, cyclohexane in addition give organicperoxy radicals (RO2). Table 2 is a summary of the observedeffects on aerosol formation in a number of studies, includingthe present one. From this comparison, it is clear thatendocyclic alkenes (cyclohexene, R-pinene, limonene, and∆3-carene) behave differently compared to exocyclic alkenes(�-pinene and sabinene). The endocyclic alkenes all havehigher SOA yields for scavengers, leading to a larger HO2/RO2 ratio, while the exocyclic alkenes show the opposite trend.The actual role of HO2-RO2 chemistry in SOA formation hasbeen under debate, where Keywood et al. (18) stress theimportance of acylperoxy radical reactions. These authorscalculated enhanced organic acid production (possiblyresponsible for the SOA formation) in systems with enhancedHO2 chemistry by including acylperoxy radicals as a directproduct of the “hot acid” channel. Alternatively, Docherty etal. (17) measured the formation of organic peroxides anddescribed them as having an important role in SOA formation.They observed a significant negative SOA and peroxidedependence on the HO2/RO2 ratio for exocyclic �-pinene,while the possibly positive trend for endocyclic compoundswas not found to be significant.

Implication of Scavenger and Water in SOA Formationand the Ozonolysis Mechanism. The effect of increased waterconcentration varied depending on which scavenger wasused. Without an OH scavenger and with 2-butanol in thesystem, the results resembled earlier observations (8). Whenusing cyclohexane, the water effect was not consistentbetween the three monoterpene systems. In the ozonolysisof limonene, the number of particles seemed to decreasewhen going from low to intermediate relative humidity, whichis in line with other studies using cyclohexane to scavengethe OH formed (20). However, when extending to high relativehumidity (i.e., 80%), the number of particles increased. Inthe case of the ozonolysis of ∆3-carene in the presence ofcyclohexane, the number and mass of particles producedwas very small, but a positive water dependence was noted.The conclusions from differences between the scavengersituations are that the effect of water is most probablyinfluenced by the HO2/RO2 chemistry and dependent on theHO2/RO2 ratio and absolute concentrations. Adding waterenables the formation of acylperoxides for R-pinene, li-monene, and ∆3-carene (this is not the case for �-pinene,due to steric hindrance) via Anglada’s pathways (13).

The effect of RO2 and HO2 was investigated by modelingour reaction system using Facsimile and the R-pinene,cyclohexane, and 2-butanol subset codes available from theMCM v.3.1. The results confirmed previous results (18, 30),where the highest HO2 levels were obtained when 2-butanolwas used as an OH scavenger. The concentration of RO2 wasin the order no OH scavenger > cyclohexane > 2-butanol.The concentration of ROOH species was also affected by theuse of a scavenger: 2-butanol > cyclohexane > no OHscavenger. One feature in the modeling of the systems isshown in Figure 5, where the HO2 profiles for the three casesare shown. The system without an OH scavenger and with2-butanol creates an initial peak in HO2 that is absent in thecyclohexane case. This again indicates that the cyclohexanesystem is special and that HO2/RO2 chemistry is involved inthe water dependence for ozone-initiated oxidation.

An alternative explanation for the positive water effectthat is most pronounced for 2-butanol systems could includeinvolvement of the reaction between 2-butanol and CIs,

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producing R-butyloxy hydroperoxides in competition withwater, formingR-hydroxy hydroperoxides. If that is the case,it would imply that the hydroperoxide or subsequent productsfrom water are less volatile than the corresponding peroxidesfrom the butanol reaction. A caveat is that the 2-butanolsystem gives a higher aerosol yield than the correspondingcyclohexane experiments where water is more or less theonly fate for the CIs. Carbonyl compounds, for example,produced from decomposition of the hydroxy hydroperox-ides, can react with CIs and form secondary ozonides (31).If the amount of carbonyl products is enhanced via water-induced reaction, the importance of secondary ozonides mayincrease. However, from current experimental findings andfor water to be in competition for the available CIs, anyconclusion of secondary ozonides is so far speculative, andmore specific experiments on this topic are needed.

It should be noted that the use of different scavengersand different scavenger concentrations may partly explaindeviations between previous studies on terpene ozonolysis.During the course of this study, it was also noted thatconducting SOA formation experiments on the ozonolysis ofterpenes is a delicate matter, and small details, such astemperature stability and the quality of the carrier gas, mayinfluence the results. In a laminar flow reactor experiment,it is also important to stabilize the flow and avoid wall effects.One of the advantages of flow studies is that the walleventually will be saturated and in equilibrium with the gasphase, and the observed SOA formation is not underestimateddue to wall loss. It was observed that, in order to get a stableaerosol production after a sudden change in flow, temper-ature, relative humidity, or concentrations, the system neededabout 1 h. The system was therefore allowed at least 3 h ofstabilization before making the final measurements.

The lack of a solid mechanistic description of the waterinfluence prevents its proper incorporation into atmosphericchemistry models. Thus, it appears that experiments con-ducted at low concentrations and high HO2/RO2 ratios inappropriate temperature and relative humidity ranges would

be the safest source of information for real atmosphericapplications. Generally, terpene ozonolysis with subsequentSOA formation would be of high relevance in rural, unpollutedatmospheres where the NOx levels are low, for example, theboreal forest (32). It should be noted that NOx chemistry willchange the oxidation pathways and influence the SOA yield,as pointed out in several recent studies (33–35). To extrapolateexperimental results to atmospheric conditions, it is im-portant to realize the limitations of the conducted studies.As is shown by the results in this paper, for example, theeffect of OH scavengers, relative humidity and concentrationsneed to be considered before results are applied, for example,to models. In addition, the water effect and its relation to thescavenger effect is possibly altered when lowering the amountof converted precursors down to levels when less than 0.01µg m-3 of SOA is produced. Regarding the mechanism, themost promising way, so far, of describing the water effect onaerosol and OH production, in addition to the effect of ascavenger, is the suggestion of a H2O reaction with the CIs

giving a hydroxy hydroperoxide that gives an alkoxy radicaland OH (13). The alkoxy radical produced is substituted byboth a carbonyl and an OH group and will most probablyisomerize and give an additional OH group and become aperoxy radical, as shown in reaction X.

The HO2/RO2 ratio and the change in HO2 kinetics, forexample, due to a water-HO2 complex (36, 37), will influencethe fate of RO2 and can at least partly explain the combinedwater and scavenger effect, for example, via the acyl-RO2

produced in reaction X. The increase in the number ofparticles formed with increasing RH is then due to theincrease in the rate of HO2-acyl-RO2 reactions compared toRO2-acyl-RO2 reactions (the idea is that multisubstitutedhydroperoxides ends up in the condensed phase).

AcknowledgmentsThis work was supported by The Swedish Foundation forStrategic Environmental Research MISTRA, Formas undercontract 214-2006-1204 and the Graduate school “Climateand Mobility”, University of Gothenburg. We are grateful forthe donation of the temperature chamber from SaabTech-Goteborg. Senior Research Engineer, Benny Lonn, is ac-knowledged for skilful technical support.

TABLE 2. Effect of OH Scavenger on SOA Production from Selected Cyclic Precursors, a Comparison with Literature Data

reference org. precursor scavenger SOA effect

this study limonene cyclohexane, 2-butanol, without SOAWO > SOA2-B > SOACHR-pinene cyclohexane, 2-butanol, without SOAWO > SOA2-B > SOACH∆3-carene cyclohexane, 2-butanol, without SOAWO > SOA2-B > SOACH

Keywood et al. 2004 (18) cyclohexene cyclohexane, 2-butanol, CO SOACO > SOA2-B > SOACHIinuma et al. 2005 (19) R-pinene cyclohexane, 2-butanol SOA2-B > SOACHDocherty and Ziemann, 2003 (30) �-pinene cyclohexane, 1-propanol SOACH >SOA1-PDocherty et al. 2005 (17) R-pinene cyclohexane, 1-propanol, HCHO SOA1-P g SOACH g SOAHCHO

�-pinene cyclohexane, 1-propanol, HCHO SOACH > SOA1-P > SOAHCHO∆3-carene cyclohexane, 1-propanol, HCHO SOA1-Pg SOACHsabinene cyclohexane, 1-propanol, HCHO SOACH g SOA1-P

FIGURE 5. Modeled formation of HO2 in the ozonolysis ofr-pinene for different reaction systems studied by usingFacsimile and subsets from MCM v.3.1 (RH 60%). Thin line:cyclohexane (36 × 1013 molecules cm-3). Coarse line: withoutscavenger. Dashed line: 2-butanol (32 × 1013 molecules cm-3).

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Supporting Information AvailableAdditional detailed information of experimental conditionsand summarized results of produced number and massconcentrations of the SOA formed in all experiments. Thisinformation is available free of charge via the Internet athttp://pubs.acs.org.

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