Influence of OH Scavenger on the Water Effect on Secondary Organic

(white circles) and mass (dark circles) when changing the RH stepwise (10, 40, 60, 80, and 60%). Vertical lines indicate the time of the change in RH...
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Environ. Sci. Technol. 2008, 42, 5938–5944

Influence of OH Scavenger on the Water Effect on Secondary Organic Aerosol Formation from Ozonolysis of Limonene, ∆3-Carene, and r-Pinene ÅSA M. JONSSON,* MATTIAS HALLQUIST, ¨ M AND EVERT LJUNGSTRO Department of Chemistry, Atmospheric Science, University of Gothenburg, SE-412 96 Go¨teborg, Sweden

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

The effect of OH scavengers on how water vapor influences the formation of secondary organic aerosol (SOA) in ozonolysis of limonene, ∆3-carene, and R-pinene at low concentrations hasbeeninvestigatedbyusingalaminarflowreactor.Cyclohexane and 2-butanol (3-40 × 1013 molecules cm-3) were used as scavengers and compared to experiments without any scavenger. The reactions were conducted at 298 K and at relative humidities between 2-butanol > cyclohexane. The effect of water vapor was similar for 2-butanol and without a scavenger, with an increase in particle number and mass concentration with increasing relative humidity. The water effect for cyclohexane was more complex, depending on the terpene, 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 suggested mechanism for alkene ozonolysis and to atmospheric importance. The results imply that the ozone-initiated oxidation of terpenes needs revision in order to fully account for the role of water in the chemical mechanism.

Although there have been numerous studies on the oxidation of monoterpenes, a detailed mechanistic description of how the oxidation leads to aerosol formation is still lacking. In particular, the understanding of its ozone-initiated oxidation is challenging. The generally accepted mechanism for ozonolysis of these unsaturated compounds is, in brief (10), the initial addition of ozone to a double bond forming an ozonide that proceeds via cleavage of the former double bond and an O-O bond, producing a carbonyl species and a Criegee intermediate (CI), that is, a biradical, in an vibrationally excited state (CI*). The fate of the CI* is complicated, and four pathways have been postulated: (1) collisional relaxation to a stabilized CI, CIs, (2) hydroperoxide channel, (3) ester-/“hot acid” channel, or (4) O atom elimination. One important aspect of the CI* is the production of OH radicals from the hydroperoxide channel (11–13). In laboratory experiments of alkene ozonolysis, the OH radical will induce secondary chemical degradation by reaction with the precursor molecule and/or the products. To reduce the effect of this secondary degradation, most recent studies use an OH scavenger, for example, cyclohexane (11, 12), 2-butanol (14), or CO (15, 16). The chemistry without a scavenger and with cyclohexane or 2-butanol is indeed different (4). This could be the reason for different amounts of SOA formed when using different scavengers (4, 17–19). The ambition to reduce complexity by removing OH radical chemistry with the parent compounds is partly counteracted by the addition of the degradation chemistry of the scavenger itself. Cyclohexane and 2-butanol have well-established major degradation pathways (reactions I-V), resulting in higher HO2/RO2 ratios for 2-butanol compared to cyclohexane (4, 18).

Introduction A central issue in atmospheric science is to elucidate the effect of organic aerosols on, for example, climate and health. Especially the secondary aerosol, that is, the fraction of the organic aerosol that is produced by gas-to-particle conversion in the atmosphere, has been difficult to quantify due to its complexity (1–3). One important source of compounds taking part in secondary organic aerosol (SOA) formation is the gas-phase oxidation of terpenes. Here, the monoterpenes (C10H16) have attracted great interest due to their huge emission and appropriate molecular mass, that is, light enough to be volatilized and still sufficiently heavy to give condensable products (4–6). Among all monoterpenes, R-pinene is the most studied with regard to kinetics, reaction mechanisms, and aerosol formation (7). R-Pinene is not necessarily the monoterpene dominating SOA formation. Two other terpenes responsible for a significant SOA mass are limonene and ∆3-carene (8). Limonene is of additional interest due to its use in various household products, affecting indoor SOA formation (9). * Corresponding author e-mail: [email protected]. 5938

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The effect of water on the number and mass of particles produced in the SOA formation process has recently been summarized (8), and there are discrepancies in the literature. One explanation could be the use of different scavengers and their concentrations among studies. In order to elucidate how the water effect is influenced by the use of a scavenger, this paper reports on SOA formation at room temperature from the ozonolysis of R-pinene, ∆3-carene, and limonene, obtained in a flow reactor where systematic changes in water 10.1021/es702508y CCC: $40.75

 2008 American Chemical Society

Published on Web 07/11/2008

TABLE 1. Experimental Conditionsa exp

organic precursor

OH scavenger

[Terpene]0 1011 molecules cm-3

[Scavenger]0 1013 molecules cm-3

[O3]0 1013 molecules cm-3

RH(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

limonene limonene limonene limonene limonene limonene limonene limonene limonene ∆3-carene ∆3-carene ∆3-carene ∆3-carene ∆3-carene ∆3-carene ∆3-carene ∆3-carene R-pinene R-pinene R-pinene R-pinene R-pinene

2-butanol 2-butanol 2-butanol 2-butanol cyclohexane cyclohexane cyclohexane cyclohexane none 2-butanol 2-butanol 2-butanol 2-butanol cyclohexane cyclohexane cyclohexane none 2-butanol 2-butanol 2-butanol 2-butanol none

4.37 ( 0.17 4.37 ( 0.17 4.37 ( 0.17 4.37 ( 0.17 4.37 ( 0.17 4.37 ( 0.17 4.37 ( 0.17 4.37 ( 0.17 4.37 ( 0.17 4.32 ( 0.65 4.32 ( 0.65 4.32 ( 0.65 4.32 ( 0.65 4.32 ( 0.65 4.32 ( 0.65 4.32 ( 0.65 4.32 ( 0.65 4.54 ( 0.54 4.54 ( 0.54 4.54 ( 0.54 4.54 ( 0.54 4.54 ( 0.54

3.5 ( 0.2 6.8 ( 0.4 12 ( 0.6 32 ( 1.7 3.9 ( 0.2 8.6 ( 0.5 19 ( 1.0 36 ( 1.9

1.19 ( 0.01 1.19 ( 0.01 1.19 ( 0.01 1.19 ( 0.01 1.19 ( 0.01 1.19 ( 0.01 1.19 ( 0.01 1.19 ( 0.01 1.19 ( 0.01 6.23 ( 0.01 6.23 ( 0.01 6.23 ( 0.01 6.23 ( 0.01 6.23 ( 0.01 6.23 ( 0.01 6.23 ( 0.01 6.23 ( 0.01 2.53 ( 0.01 2.53 ( 0.01 2.53 ( 0.01 2.53 ( 0.01 2.53 ( 0.01

10.6-80.0 10.8-76.6 ∼10-78.2 10.9-76.7 11.0-76.9 11.1-77.0 10.6-77.2 10.1-77.0 9.1-77.3 9.5-74.2 10.5-75.8 10.3-76.2 10.3-75.1 6.3-76.8 8.0-72.9 6.7-75.6 6.2-75.0 10.0-73.6 10.4-76.6 9.1 -71.9 9.5-73.7 9.6-74.5

3.5 ( 0.2 6.8 ( 0.4 12 ( 0.6 32 ( 1.7 3.9 ( 0.2 8.6 ( 0.5 36 ( 1.9 3.5 ( 0.2 6.8 ( 0.4 12 ( 0.6 32 ( 1.7

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 was varied between the given limits in four steps. Stated errors are at the statistical 95% confidence level. a

concentration and with 2-butanol and cyclohexane as scavengers were made.

Experimental Section The Experimental Setup. The experimental setup is a combination of a vertical laminar flow reactor and a scanning mobility particle sizer (SMPS) system, named G-FROST (Go¨teborg-Flow Reactor for Oxidation Studies at low Temperatures). The setup, procedure, and analysis have been described elsewhere (8). Some changes compared to earlier work are that the reactor was modified and positioned in a chamber with an extended working temperature range, 238-323 K. The reactor is 1.91-m-long and 10 cm in diameter. In addition, at the end of the flow reactor, there is a centered sampling funnel, which means that only the center part of the laminar flow is conveyed to the SMPS system. This is to reduce the impact of the slower flow near the reactor walls. The excess flow is passed to the vent. The mixing of the reactants at the outflow of the injector was upgraded by using a mixing plunger in line with Bonn et al. (20). Additionally, two stainless steel nets were added to the outlet of the plunger in order to rapidly obtain a laminar flow. Ozone is added through a Teflon tube running inside the stainless steel tube that makes up the injector. The scavenger is added to the system by passing N2 gas through a gas-wash bottle containing the substance, and the concentration is controlled by varying the temperature of the scavenger compound. The bulk flow is humidified by passing it through a Gore-Tex tube submerged in thermostatted deionized water. Experimental Procedure. The formation of particles was investigated at experimental conditions summarized in Table 1. In the experiments, 1.8-1.9 × 1011 molecules cm-3 (∼7 ppb at 298 K, 1 atm) of the terpene had reacted, and the initial rate of reaction was ∼1 × 109 molecules cm-3 s-1. The rate coefficients used for the rate calculation were 200 × 10-18, 37 × 10-18, and 86.6 × 10-18 cm3 molecule-1 s-1 for limonene, ∆3-carene, and R-pinene, respectively (21). The average reaction time was kept constant (238 s), and the total flow in the system was 1.6 standard liters per minute

(SLPM; 298 K and 1 atm), where 0.94 SLPM was conveyed via the sampling funnel. Particle number (N10-300nm) and mass (M10-300nm) concentrations are given as averages of five consecutive distributions. Estimates of SOA density have been presented (22). However, since this quantity may change between monoterpenes and experimental conditions, a density of unity has been applied. The scan time was 5 min, comprising of an up-scanning time of 240 s, a down-scanning time of 45 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, for example, from 10% up to 80% over 10 h, as is shown in Figure 1. It is noteworthy that, when running a dynamic change in relative humidity, the aerosol and the system may not have had enough time to equilibrate, and the measured values may not represent the actual SOA production at the specified RH. However, as shown in Figure 1a, there is good reproducibility between the two methods. Figure 1b shows a full stepwise experiment in which also the stability of the system is demonstrated. The long-time stability of the system was excellent in that a stable aerosol could be produced for hours and even days. However, the resulting aerosol formation is extremely temperature-sensitive, and variations in temperature of more than 0.4 K induced significant scatter. It was also noted that the air quality influenced the results. For reproducible results, highly purified air (Laboratory Zero Air Generator, Linde Gas, Model N-GC6000) in conjunction with a NOx absorbent, Sofnox R (Molecular Products Limited), was needed. In this study, three main types of experiments were performed: using no OH scavenger, cyclohexane as a scavenger, and 2-butanol as a scavenger. The concentration of the scavenger was varied and the relative humidity dependence measured for each combination and scavenger concentration. The amount of terpene consumed and the anticipated concentrations of HO2, RO2, and ROOH were estimated using the R-pinene, cyclohexane, and 2-butanol subset codes available from VOL. 42, NO. 16, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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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 results when changing RH stepwise are shown with triangles (dark symbols, mass; white symbols, number). (b) Change in number (white circles) and mass (dark circles) when changing the RH stepwise (10, 40, 60, 80, and 60%). Vertical lines indicate the time of the change in RH. the Master Chemical Mechanism (MCM v.3.1, http://mcm. leeds.ac.uk/MCM).

Results Table S1 in the Supporting Information contains, in addition to experimental conditions, summarized results of the number and mass concentrations of the SOA formed in all experiments. In a previous paper, the impact of humidity on the number and mass of particles formed in the ozonolysis of limonene, ∆3-carene, and R-pinene was investigated using 2-butanol as a scavenger (8). Using the new system with a slightly different delivery and sampling system, the main features from the previous study were confirmed, that is, more aerosols, both regarding number and mass under humid conditions for all three terpenes, in the text called a positive water effect. Figure 2 gives a comparison with previous data for the limonene experiments, using 2-butanol as an OH scavenger. There is a clear difference in the shape of the number size distributions as a result of only the center flow being analyzed in the new system. Here, the size distribution is narrowed, and less mass is recorded, due to shorter average contact time in the reactor and less influence of the wall. A general trend is that OH scavengers suppress the aerosol formation, and the lowest SOA yields were measured when cyclohexane was used, Figure 3 and Table S1 (Supporting Information). The system without an OH scavenger will consume more of the organic precursor and possibly also oxidize the products further (MCM v.3.1 calculations for Exp#22 show 41% more R-pinene reacted compared to Exp#21, whereas the increase in SOA mass was 10-fold). This could partly explain the increase in aerosol yield, while some of the increase could be attributed to products with low volatility, formed from OH-initiated oxidation. 5940

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FIGURE 2. Effect of the concentration of 2-butanol on particle number (a) and mass (b) in the ozonolysis of limonene (Exp#1-4). White circles, 3.5; white diamonds, 6.8; white triangles, 12; and crosses, 32 × 1013 molecules cm-3. For comparison, data from Jonsson et al. (8) is added (dark circles, low conc.; triangles, high conc.). (c) Size distributions obtained with the upgraded system (broad line), where only the centered part 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 OH scavengers or using no scavenger (triangles, Exp#9) in the ozonolysis of limonene. It is evident that the RH effect is present and positive for systems with 2-butanol and without a scavenger, whereas the RH effect in the cyclohexane case is more complex. Figure 2 shows the relative humidity dependence of SOA formation from limonene for four concentrations of 2-butanol. All cases show similar relative humidity dependence, where the amount of SOA formed is increased with a decreasing concentration of 2-butanol. As expected,

FIGURE 4. Number (a-d) and mass (e-h) of particles as a function of scavenger concentration. The concentration is increasing in the order: circles, diamonds, triangles, and crosses, cf. Table 1. the scavenger effect is diminished for lower 2-butanol concentrations, and the system is moving toward the nonscavenger 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 all 2-butanol concentrations, but the amount of SOA formed increased with an increasing concentration of 2-butanol (Figure 4b,f). Figure 4c shows the analogue to Figure 2a with cyclohexane as the OH scavenger in the ozonolysis of limonene. The number of particles formed seems to be the lowest at intermediate relative humidity (40 and 60%). Even though these variations are within the experimental uncertainties, this trend is reproduced in all experiments. Additionally, the number of particles increased with a decreasing concentration of cyclohexane. Regarding the mass of SOA, there is a slight positive water effect for low concentrations of cyclohexane, while in the high cyclohexane case, there is no significant trend going from dry to wet conditions (Figure 4g). For ∆3-carene, the trend is the same for all concentrations of cyclohexane, where the number of particles is increased with increasing relative humidity (Figure 4d). However, the number of particles is increasing with increasing scavenger concentration. As for the R-pinene/2-butanol case, this was unexpected, but it

appears that this positive scavenger effect is only present in cases where a low mass and low number of particles are formed. The effect on mass is also difficult to establish due to the small number of particles formed, resulting in large errors (Figure 4h). For R-pinene, not enough particles were formed to enable analysis when using cyclohexane as a scavenger. However, it was established that cyclohexane gave fewer particles and a lower mass than the corresponding experiment with 2-butanol.

Discussion and Atmospheric Implications Water in the Ozonolysis Mechanism. In recent ozonolysis studies, it has been shown that water affects both gas-phase chemistry, for example, OH (16), end products, (23, 24) and SOA formation, see, for example, ref 8. Mechanistically, water is believed to react with CIs, forming an R-hydroxy hydroperoxide that possesses excess energy (10). This peroxide can either be collisionally stabilized or decompose into a carbonyl and H2O2 (reaction VIa) or a carboxylic acid and H2O (reaction VIb). For the acid to be formed, the R-hydroxy hydroperoxide must contain an R-hydrogen, that is, R1 or R2 must be an H atom. VOL. 42, NO. 16, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Anglada et al. (13)have suggested decomposition to form OH and an alkoxy or alkyl radical as another possibility for the R-hydroxy hydroperoxide as the peroxide bond may be cleaved due to the excess internal energy. According to Anglada et al. (13), water can react with CIs and form OH radicals in two ways: the cycloaddition of water (reaction VII) and water-catalyzed H migration (reaction VIII). The water-catalyzed channel (VIII) is only active for CIs with no R-H in the syn position.

Another pathway forming OH radicals is directly from CI*, involving no water. Here, a β-hydrogen migrates to the terminal oxygen followed by a breakage of the O-OH bond. This pathway proceeds for a monosubstituted CI* almost entirely from the syn configuration (25) and does not involve CIs (reaction IX).

Consequently, the two suggested pathways of OH formation should have different H2O dependences. The OH yield from R-pinene has been measured in several studies and, for example, Atkinson et al. (12) did not see a dependence on water. Furthermore, experiments with ∆3-carene and limonene showed an OH yield that was independent of the water concentration in the range 5-40% RH, using 2-butanol as a scavenger (26). However, a problem is that the scavenger or its products in high concentration experiments can react with CIs and diminish the role of water. For simpler alkene systems, it has also been demonstrated that the water dependence on OH yield is still an unresolved issue. Hasson et al. (27) presented OH yields being independent of water, whereas a recent study of Wegener et al. (16) with CO as a scavenger saw increased yields of OH under humid conditions. OH Scavenger and SOA Formation. Several studies have to some extent treated the OH scavenger effect on aerosol formation from ozonolysis of terpenes and compounds with related chemical structures, see for example refs 18, 19, and 28. When not using an OH scavenger, more organic material is converted due to the additional OH reaction with the organic precursor (e.g., the terpene) and its products. In addition, the products formed in OH-initiated oxidation could possess additional polar groups (e.g., -OH). This is in line with volatility measurements of SOA particles from the ozonolysis of R-pinene in the presence of cyclohexane. Such particles are significantly more volatile than the corresponding particles produced without an OH scavenger (29). It could be noted that Jenkin (4), when using the MCM v.3.1 model, showed contradictory results, where the system without scavenger gave more volatile products. 5942

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SOA formation also depends on which scavenger is used; however, the reason is not completely understood and quantified. The most widely used OH scavengers are CO, 2-butanol, and cyclohexane. As outlined in the Introduction, the mechanistic complexity of the degradation of these compounds is increasing in the order CO, 2-butanol, and cyclohexane. Carbon monoxide gives only HO2, while 2-butanol and, even more so, cyclohexane in addition give organic peroxy radicals (RO2). Table 2 is a summary of the observed effects on aerosol formation in a number of studies, including the present one. From this comparison, it is clear that endocyclic alkenes (cyclohexene, R-pinene, limonene, and ∆3-carene) behave differently compared to exocyclic alkenes (β-pinene and sabinene). The endocyclic alkenes all have higher 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 has been under debate, where Keywood et al. (18) stress the importance of acylperoxy radical reactions. These authors calculated enhanced organic acid production (possibly responsible for the SOA formation) in systems with enhanced HO2 chemistry by including acylperoxy radicals as a direct product of the “hot acid” channel. Alternatively, Docherty et al. (17) measured the formation of organic peroxides and described them as having an important role in SOA formation. They observed a significant negative SOA and peroxide dependence on the HO2/RO2 ratio for exocyclic β-pinene, while the possibly positive trend for endocyclic compounds was not found to be significant. Implication of Scavenger and Water in SOA Formation and the Ozonolysis Mechanism. The effect of increased water concentration varied depending on which scavenger was used. Without an OH scavenger and with 2-butanol in the system, the results resembled earlier observations (8). When using cyclohexane, the water effect was not consistent between the three monoterpene systems. In the ozonolysis of limonene, the number of particles seemed to decrease when going from low to intermediate relative humidity, which is in line with other studies using cyclohexane to scavenge the OH formed (20). However, when extending to high relative humidity (i.e., 80%), the number of particles increased. In the case of the ozonolysis of ∆3-carene in the presence of cyclohexane, the number and mass of particles produced was very small, but a positive water dependence was noted. The conclusions from differences between the scavenger situations are that the effect of water is most probably influenced by the HO2/RO2 chemistry and dependent on the HO2/RO2 ratio and absolute concentrations. Adding water enables the formation of acylperoxides for R-pinene, limonene, and ∆3-carene (this is not the case for β-pinene, due to steric hindrance) via Anglada’s pathways (13). The effect of RO2 and HO2 was investigated by modeling our reaction system using Facsimile and the R-pinene, cyclohexane, and 2-butanol subset codes available from the MCM v.3.1. The results confirmed previous results (18, 30), where the highest HO2 levels were obtained when 2-butanol was used as an OH scavenger. The concentration of RO2 was in the order no OH scavenger > cyclohexane > 2-butanol. The concentration of ROOH species was also affected by the use of a scavenger: 2-butanol > cyclohexane > no OH scavenger. One feature in the modeling of the systems is shown in Figure 5, where the HO2 profiles for the three cases are shown. The system without an OH scavenger and with 2-butanol creates an initial peak in HO2 that is absent in the cyclohexane case. This again indicates that the cyclohexane system is special and that HO2/RO2 chemistry is involved in the water dependence for ozone-initiated oxidation. An alternative explanation for the positive water effect that is most pronounced for 2-butanol systems could include involvement of the reaction between 2-butanol and CIs,

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

org. precursor

Keywood et al. 2004 (18) Iinuma et al. 2005 (19) Docherty and Ziemann, 2003 (30) Docherty et al. 2005 (17)

limonene R-pinene ∆3-carene cyclohexene R-pinene β-pinene R-pinene β-pinene ∆3-carene sabinene

this study

scavenger cyclohexane, cyclohexane, cyclohexane, cyclohexane, cyclohexane, cyclohexane, cyclohexane, cyclohexane, cyclohexane, cyclohexane,

producing R-butyloxy hydroperoxides in competition with water, forming R-hydroxy hydroperoxides. If that is the case, it would imply that the hydroperoxide or subsequent products from water are less volatile than the corresponding peroxides from the butanol reaction. A caveat is that the 2-butanol system gives a higher aerosol yield than the corresponding cyclohexane experiments where water is more or less the only fate for the CIs. Carbonyl compounds, for example, produced from decomposition of the hydroxy hydroperoxides, can react with CIs and form secondary ozonides (31). If the amount of carbonyl products is enhanced via waterinduced reaction, the importance of secondary ozonides may increase. However, from current experimental findings and for water to be in competition for the available CIs, any conclusion of secondary ozonides is so far speculative, and more specific experiments on this topic are needed. It should be noted that the use of different scavengers and different scavenger concentrations may partly explain deviations between previous studies on terpene ozonolysis. During the course of this study, it was also noted that conducting SOA formation experiments on the ozonolysis of terpenes is a delicate matter, and small details, such as temperature stability and the quality of the carrier gas, may influence 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 wall eventually will be saturated and in equilibrium with the gas phase, and the observed SOA formation is not underestimated due to wall loss. It was observed that, in order to get a stable aerosol production after a sudden change in flow, temperature, relative humidity, or concentrations, the system needed about 1 h. The system was therefore allowed at least 3 h of stabilization before making the final measurements. The lack of a solid mechanistic description of the water influence prevents its proper incorporation into atmospheric chemistry models. Thus, it appears that experiments conducted at low concentrations and high HO2/RO2 ratios in appropriate temperature and relative humidity ranges would

2-butanol, without 2-butanol, without 2-butanol, without 2-butanol, CO 2-butanol 1-propanol 1-propanol, HCHO 1-propanol, HCHO 1-propanol, HCHO 1-propanol, HCHO

SOA effect SOAWO > SOA2-B > SOACH SOAWO > SOA2-B > SOACH SOAWO > SOA2-B > SOACH SOACO > SOA2-B > SOACH SOA2-B > SOACH SOACH >SOA1-P SOA1-P g SOACH g SOAHCHO SOACH > SOA1-P > SOAHCHO SOA1-Pg SOACH SOACH g SOA1-P

be the safest source of information for real atmospheric applications. Generally, terpene ozonolysis with subsequent SOA formation would be of high relevance in rural, unpolluted atmospheres where the NOx levels are low, for example, the boreal forest (32). It should be noted that NOx chemistry will change the oxidation pathways and influence the SOA yield, as pointed out in several recent studies (33–35). To extrapolate experimental results to atmospheric conditions, it is important to realize the limitations of the conducted studies. As is shown by the results in this paper, for example, the effect of OH scavengers, relative humidity and concentrations need to be considered before results are applied, for example, to models. In addition, the water effect and its relation to the scavenger effect is possibly altered when lowering the amount of converted precursors down to levels when less than 0.01 µg m-3 of SOA is produced. Regarding the mechanism, the most promising way, so far, of describing the water effect on aerosol and OH production, in addition to the effect of a scavenger, is the suggestion of a H2O reaction with the CIs giving a hydroxy hydroperoxide that gives an alkoxy radical and OH (13). The alkoxy radical produced is substituted by both a carbonyl and an OH group and will most probably isomerize and give an additional OH group and become a peroxy radical, as shown in reaction X.

The HO2/RO2 ratio and the change in HO2 kinetics, for example, due to a water-HO2 complex (36, 37), will influence the fate of RO2 and can at least partly explain the combined water and scavenger effect, for example, via the acyl-RO2 produced in reaction X. The increase in the number of particles formed with increasing RH is then due to the increase in the rate of HO2-acyl-RO2 reactions compared to RO2-acyl-RO2 reactions (the idea is that multisubstituted hydroperoxides ends up in the condensed phase).

Acknowledgments FIGURE 5. Modeled formation of HO2 in the ozonolysis of r-pinene for different reaction systems studied by using Facsimile and subsets from MCM v.3.1 (RH 60%). Thin line: cyclohexane (36 × 1013 molecules cm-3). Coarse line: without scavenger. Dashed line: 2-butanol (32 × 1013 molecules cm-3).

This work was supported by The Swedish Foundation for Strategic Environmental Research MISTRA, Formas under contract 214-2006-1204 and the Graduate school “Climate and Mobility”, University of Gothenburg. We are grateful for the donation of the temperature chamber from SaabTechGo¨teborg. Senior Research Engineer, Benny Lo¨nn, is acknowledged for skilful technical support. VOL. 42, NO. 16, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Supporting Information Available Additional detailed information of experimental conditions and summarized results of produced number and mass concentrations of the SOA formed in all experiments. This information is available free of charge via the Internet at http://pubs.acs.org.

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