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A chamber study of secondary organic aerosol formation by linalool ozonolysis
Atmospheric Environment 43 (2009) 3935–3940
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A chamber study of secondary organic aerosol formation by linalool ozonolysis Xi Chen, Philip K. Hopke* Center for Air Resources Engineering and Science, Clarkson University, Potsdam, NY 13699-5708, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 January 2009 Received in revised form 12 April 2009 Accepted 14 April 2009
The formation of secondary organic aerosol (SOA) produced from linalool ozonolysis was examined using a dynamic chamber system that allowed the simulation of ventilated indoor environments. Experiments were conducted under room temperature (22–23 C) and air exchange rate of 0.67 h1. An effort was made to maintain the product of the concentrations of the two reagents constant. The results suggest that under the conditions when the product of the two reagent concentrations was constant, the relative concentrations play an important role in determining the total SOA formed. A combination of concentrations somewhere in ozone limiting region will produce the maximum SOA concentration. The measured reactive oxygen species (ROS) concentrations at linalool and ozone concentrations relevant to prevailing indoor concentrations ranged from 0.71 to 2.53 nmol m3 equivalents of H2O2. It was found that particle samples aged for 24 h lost a significant fraction of the ROS compared to fresh samples. The residual ROS concentrations were around 15–69%. Compared with other terpene species like a-pinene that has one endocyclic unsaturated carbon bond, linalool was less efficient in potential SOA formation yields. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Linalool SOA ROS Indoor aerosol Yield
1. Introduction Linalool, a biogenic unsaturated terpene alcohol, has been identified in the emissions from orange blossoms and certain vegetation (Grosjean et al., 1993; Shu et al., 1997). It is commonly used in foods as flavor and aroma enhancer especially for fruits and vegetables (Ba¨cktorp et al., 2006). Linalool is also the most important flavor component in orange juice (Bazemore et al., 2003). Moreover, widespread use as a fragrance in household products like air fresheners and detergents makes it ubiquitous in indoor air. These active fragrance ingredients are volatilized by the use of household products or consumption of foods. These reactive volatile organic compounds can result in the formation of secondary organic aerosol (SOA) and gas phase products by reaction with ozone. Ozone is present in indoor environments owing to its infiltration from the ambient environment at 20–70% of outdoor concentrations (Weschler, 2000). The application of certain electronic devices including photocopiers and printers as well as some air purifiers would introduce extra ozone to indoor environments (Lee et al., 2001; Singer et al., 2006). Previous studies have reported SOA formation from commercial scented-oil air fresheners that contain linalool as an active constituent (Singer et al., 2006; Destaillats et al., 2006). Gas phase
* Corresponding author. Tel.: þ1 315 268 3861; fax: þ1 315 268 4410. E-mail address: [email protected] (P.K. Hopke). 1352-2310/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2009.04.033
products from reaction of linalool with OH radicals, NO3 and, ozone were analyzed by Shu et al. (1997). The major identified compounds from OH and ozone reactions include two products with 7 carbon atoms which also might be important constituents of SOA formed. Gas-particle partitioning of semivolatile compounds was described by Pankow (1994a,b). From absorptive partitioning theory, in which the partitioning between two phases is governed by an equilibrium partitioning coefficient, the fraction of semivolatile species in the particle phase (F) is given by:
F ¼
MKp 1 þ MKP
(1)
where M is the absorbing material or here as the SOA mass, and Kp is partitioning coefficient. The model developed by Odum et al. (1996) related the SOA yield defined as the mass of SOA formed per mass of hydrocarbon consumed, with the SOA mass loading.
Y ¼
X ai Kp;i DM ¼ M DHC 1 þ MKp;i
(2)
where Y is SOA yield, Kp,i and ai are partitioning coefficient and mass yield of compound i, respectively. This approach has been found to successfully fit the yield data for many experiments (Odum et al., 1996; Yu et al., 1999; Ng et al., 2006; Chen and Hopke, 2009). Under conditions of minimal photochemical activity, O3/terpene reactions have been suggested to be an important pathway to
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indoor H2O2 formation (Fan et al., 2005) as well as producing organic peroxide species (Docherty et al., 2005). These reactive oxygen species (ROS) may produce adverse health effect when they partition into the particulate phase and penetrate deeper into lungs than if present in the gas phase (Weschler, 2006). Thus, the cooccurrence of H2O2 and SOA could produce potential health effects associated with oxidative stress in the lung. Oxidative stress has been implicated in the pathogenesis of many pulmonary diseases (Kehrer, 1993) increasing the potential role of particle-bound ROS. In typical sampling programs, samples are collected but are not analyzed immediately because of the difficulties in making in situ measurement. Most collected samples are kept refrigerated until shipped to the laboratory to be analyzed. The time interval between sample collection and analysis conducted may allow samples to undergo decay or re-equilibrium. The bias introduced from such time delays is expected to produce substantial underestimation of the true oxidant concentrations of collected samples. The comparison between fresh and 24 h aged samples in this study is expected to provide information on the time effect on collected samples in terms of ROS intensities. Although many investigations have looked at chemistry of terpene/ozone system, given the complexity of reactive chemistry between linalool and ozone, there is still limited knowledge regarding exposure to SOA formation at concentrations relevant to indoor situations. The aim of this study is to provide new information on SOA yields and ROS concentrations under various combinations of reactant concentrations. In addition, the role of hydroxyl radicals in SOA formation will be explored.
2. Experimental procedures Linalool ozonolysis was studied in a 2.5 m3 stainless steel chamber in a temperature controlled room. Details regarding the chamber and experimental protocols were given elsewhere (Chen and Hopke, 2009) so only a brief description is presented here. Ozone-loaded air from a UV ozone generator (UV550, Ozone Solutions Inc., Sioux Center, IA) was first introduced into chamber using the UV ozone generator and sufficient time was allowed for the concentration inside the chamber to reach steady state. The experiments were conducted under low relative humidity (RH) conditions with the RH around 7%. The low humidity prevented any hygroscopic growth of the resulting SOA species and permitted the study of behavior of the undiluted SOA. Linalool was delivered from an impinger at an approximately constant concentration. The flow rates and temperature were controlled to get specific VOC concentrations to the chamber. Muffin fans inside the chamber were used to permit thorough mixing. Because of the low vapor pressure of linalool, thorough cleaning of the chamber was performed for at least 36 h in presence of ozone prior to each set of experiments to remove any residue from last experiment. The resultant background contained less than
100 particles cm3, volume concentrations less than 0.1 mm3 cm3 and negligible NOx with concentrations lower than 5 ppb. Particle number and size distributions were measured with a Scanning Mobility Particle Sizer (SMPS). A TSI model 3071 DMA coupled with TSI model 3775 Condensation Particle Counter (CPC) operating at 3 L min1 and 0.3 L min1 for sheath and aerosol flows measures particles range from 14 to 700 nm. Size distribution data were recorded and analyzed using TSI AIM software v 8.1. The ozone concentration inside the chamber was measured by a UV absorption ozone analyzer (49i, Thermo Fisher Scientific Inc, MA) that sampled air at a rate of 1.5 L min1. Gas phase samples were collected from the sampling port at the face plate planted in the center and analyzed by GC–FID. Gas tight syringes (A2, VICI Precision sampling In. IL, USA) were used to collect hydrocarbon samples. Samples for ROS measurement were collected on Teflon filters (Pall, Teflo 25 mm, 3 mm) over 30 min intervals after the system reached steady state using a flow rate of 23 LPM. The determination of ROS was performed by using a fluorogenic probe, dichlorofluorescin (DCFH) (Hung and Wang, 2001; Venkatachari et al., 2005). To catalyze the reaction between DCFH and ROS species, horseradish peroxidase (HRP) was added to the DCFH solution. Fluorescent intensities were converted to equivalent H2O2 concentrations by conducting an assay of standard calibration. Fluorescence intensities were measured by a Turner Quantech Digital Filter Fluorometer (#FM109535, Barnstead Thermolyne Corp, Dubuque, IA, USA). The excitation and emission wavelength used were 485 nm and 530 nm respectively. The wall deposition coefficient was estimated which was described in detail elsewhere before (Chen and Hopke, 2009) as 0.08 0.01 h1 at an air exchange rate (AER) of 0.67 0.01 h1. The wall loss correction was used in the next section when calculating the overall SOA mass yields. Five sets of experiments were conducted under room temperature (22–23 C) and air exchange rate of 0.67 0.01 h1. Experimental conditions are listed in Table 1. Efforts were made to maintain a constant product of reagent concentrations. 3. Results and discussion 3.1. Time evolution of SOA The time evolution of the SOA was similar to the pattern observed for a-pinene (Chen and Hopke, 2009). Fig. 1 shows the typical evolution of the SOA size and volume concentrations as well as ozone concentration beginning at the time when the linalool was added to the chamber that had attained a steady-state concentration of ozone. This point is defined as the time equal to zero point in the figure. With an AER at 0.67 h1, it took around 8 h to reach steady-state concentrations for the number size distribution. The development of the number and volume size distributions for all of the experiments conducted showed similar trends that consisted of four stages with regard to number size distribution. Upon adding
Table 1 Experimental condition details. Date
Initial linalool (ppb)
Initial ozone (ppb)
Number conc (# cm3)
Mass conc (mg m3)
ROS0a (nmole m3)
ROS24b (nmole m3)
s.s. linalool (ppb)
s.s ozone (ppb)
OHs.s (ppb)
11142008 11182008 11212008 11252008 12092008
32 75 106 24 39
110 50 34 150 92
1640 5280 4850 1960 1970
0.71 3.75 3.18 0.30 0.95
2.00 0.93 0.71 2.53 1.17
0.29 0.26 0.49 1.22 0.20
12 40 72 6.2 16
69 8 4 116 46
8.2E05 1.1E05 6.3E06 1.1E04 5.7E05
2 3 6 1 1
2 2 1 2 2
70 110 610 70 70
s.s ¼ steady state. a ROS0: ROS concentrations for samples measured immediately after collection. b ROS24: ROS concentrations for samples measured 24 h after collection.
0.09 0.24 0.17 0.05 0.06
0.30 0.14 0.04 0.41 0.08
0.06 0.05 0.04 0.16 0.04
1 2 3 0.2 1
2 1 1 1 1
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Fig. 2. Plot of the steady-state number and mass concentrations with initial linalool concentrations.
Fig. 1. Typical evolution profile of SOA produced and time series of ozone concentrations for experiment 11182008.
linalool to the chamber, there was a sudden burst of small particles but with a time lag compared to those of a-pinene (Chen and Hopke, 2009). Then the total number and volume concentrations increased but with a shift in size to larger particles until the number concentration started to decline when the second stage began. The number concentration continued to decrease while the volume or mass concentration continued to increase. There was a second nucleation burst that was smaller than the first burst in the number concentration. The number concentration increased representing a third stage in the process. Then both the number and mass concentrations approached steady-state values that represented the fourth stage. The volume concentrations followed a different pattern. There was an exponential increase in mass that eventually reached a steady-state value. 3.2. Particle concentrations For the five experiments, the concentration products of linalool and ozone were approximately 3612 [ppb O3 ppb linalool] (the differences are within 5%). Keeping the concentration products constant while varying the combination of the two reagents helped to examine the effect of changing the limiting reagent with regards to the number concentrations and SOA mass produced. Since the reaction of linalool with ozone is a second order reaction, the produced SOA would be expected to be proportional to the initial concentrations of the precursors. The five sets of experiments can be separated into two groups: linalool limiting or ozone limiting according to initial ozone/linalool concentration ratios (if >2, linalool limiting, otherwise ozone limiting). Fig. 2 shows the relationship of the steady-state number
and mass concentrations with the initial linalool concentrations. The resulting SOA number size distribution showed that the two groups fell in two groups with different concentration combinations of the reagents. However, within each group, the number concentrations were approximately constant such that those with ozone limiting values were about 3 times higher than those for linalool limiting. Compared to a-pinene ozonolysis experiments with the reagent products at approximately 3557 [ppb O3 ppb a-pinene], the number concentrations at steady state for all a-pinene experiments were relatively constant with the variations being only 11% (Chen and Hopke, 2009). These differences of linalool over a-pinene might be attributed to a-pinene being monoterpene with one double bond while linalool with two double bonds. The attack on the second unsaturated bond is expected to produce different product compositions that then nucleate and condense. Thus, under the conditions where the products of the two reagent concentrations were constant, the relative concentrations play an important role in determining the total SOA formed. The trend indicated from Fig. 2 suggests that a combination somewhere in ozone limiting region would produce the maximum SOA concentrations. The consumed linalool/ozone was approximately 1:2 for linalool limiting group; while the ratios were between 1 and 2 for those of ozone limiting sets. Such observed stoichimometry suggested that for linalool limiting scenarios ozone was the dominant oxidation reagent to react with the two carbon double bonds given its abundance; while for ozone limiting sets ozone attacked one of the double bond to form primary first generation products, which were further oxidized by either OH radicals or ozone. For the steady-state concentration products of linalool and ozone, a similar pattern was observed as that for a-pinene (Chen and Hopke, 2009). For the two reagent limited regions, similar steady-state concentration products were observed within the region, but different products were obtained in each region. The steady-state concentration products for the two reagents under linalool limited were found to be about 2.5 times higher than those of ozone limited sets. The measured steady-state concentrations of linalool and ozone for each experiment are listed in Table 1. 3.3. Linalool ozonolysis chemistry and yields The gas phase products of linalool with OH radicals and ozone were studied by Shu et al. (1997). Among the identified major gas phase products, there are species that are possible candidates for partitioning between the gas and particle phases to form additional SOA. The reaction of linalool [(CH3)2C]CHCH2CH2C(CH3)(OH)
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CH]CH2] with ozone is by the initial addition of ozone to one of the carbon double bonds; mainly to (CH3)2C]CHe, minor to CH2]CHe with an approximately 97%/3% ratio(Atkinson et al., 1995; Shu et al., 1997). The primary ozonide is expected to undergo rapid decomposition into formation of biradicals. OH radicals would arise from reaction pathways from one of the biradicals generated ½ðCH3 Þ2 ,CO O* with the measured OH yield in literature , ranges from 0.66 to 0.85 (Atkinson et al., 1995; Aschmann et al., 2002; Shu et al., 1997). This pathway will form a co-product 4-hydroxy-4-methyl-5-hexenal, that will cyclize and be present as 2-ethenyl-2-methyl-5-hydroxyterahydrofuran. The observed formation of 5-ethenyldihydro-5-methyl-2(3H)-furanone suggests the other biradical ½CH2 ]CHCðCH3 ÞðOHÞCH2 CH2 ,CHO O* undergo , cyclization as well as the loss of water (Shu et al., 1997). The reaction of linalool with OH radicals is by the addition of OH to an unsaturated carbon bond with the dominant addition to (CH3)2C]CHe over CH2]CHe with a 77%/23% ratio (Shu et al., 1997; Kwok and Atkinson, 1995). The two major gas phase products identified from the reactions with ozone and OH radicals are 2-ethenyl-2-methyl-5-hydroxyterahydrofuran and 5-ethenyldihydro-5-methyl-2(3H)-furanone (Shu et al., 1997), and are likely to nucleate or condense to form SOA. Both of the compounds have 7 carbon atoms and are expected to be primary first generation products. The further attack of OH or ozone on the second unsaturated carbon bond will lead to further fragmentation of carbon backbones yielding products with even fewer carbon atoms, and therefore less likely to contribute to SOA formation and growth. SOA mass concentrations were calculated from volume size distributions. The density was chosen to be 1.5 g cm3 based on prior density measurements for the a-pinene – ozone system (Chen and Hopke, 2009). The SOA yields derived from this study ranged from 0.3 to 1.9%. Attempts were made to relate SOA yield with SOA mass loading using eq. (2) using one and two hypothetical products. Fig. 3 shows the experimental data points and fitted curves. The differences between the one and two hypothetical products curves are not very large for the low SOA region of interest. This region would represent SOA formation under conditions typically encountered in indoor environments. Only one set of Kp,i and ai was found to be sufficient to fit the experimental data for SOA mass concentrations in low region (Chen and Hopke, 2009). With other terpene species like a-pinene that has one endocyclic unsaturated carbon bond, the primary oxidation products are of 10 or 9 carbon atoms from ozonolysis. For linalool, the major
product molecules have 7 or fewer carbon backbone atoms and therefore the potential of SOA formation is expected to be smaller. Linalool SOA yields measured in this study as well as those from previously in literature are consistent with this expectation. Ng et al. (2006) conducted a series of experiments on terpene ozonolysis and photooxidation including linalool. The SOA yields Ng et al. derived for ozonolysis with cyclohexane present as OH scavenger at 20 C for 106 ppb linalool is around 1.2%, while for 124 ppb linalool under photooxidation, the yield is approximately 13%, where both systems had (NH4)2SO4 particles as seeds. Studies conducted by Destaillats et al. (2006) on SOA from ozonolysis of terpene-based household products reported yields ranging from 4 to 6% for a plug-in scented-oil air freshener that contained linalool as one of the active ingredients. Hoffmann and coworkers (1997) estimated w5% SOA yield for open-chain hydrocarbon products of about 100 ppb linalool under ozonolysis, which is the lowest compared to other monounsaturated cyclic monoterpenes like a-pinene and cyclic monoterpenes with two double bonds like D-limonene. The linalool ozonolysis experiments conducted by Hoffmann et al. (1997) were under similar temperature and (NH4)2SO4 as seeds as those conducted by Ng et al. (2006). The only difference between the experiments was the absence of OH scavenger cyclohexane in the Hoffmann et al. (1997) study. These differences resulted in higher SOA yields; higher yields were also observed from photooxidation experiments by Ng et al. (2006) where OH, NO3 and ozone were present. 3.4. OH radicals The OH radical concentrations in the chamber at steady state are calculated as follows. The OH source reaction is ozone/linalool, and the sinks include reaction with linalool and its dominant primary ozonolysis products as well as physical loss pathways.
½OH ¼
yOH kO3 LINA ½O3 ½LINA AER þ kd VA þ kOHLINA ½LINA þ kOHPLOP ½PLOP
(3)
where LINA ¼ linalool, PLOP ¼ primary linalool ozonolysis products; yOH is molar yield of OH from ozone/linalool reaction; kd is OH deposition velocity (0.00007 m s1) (Fan et al., 2003); A/V is chamber surface to volume ratio (4.5 m1); kO3 LINA is secondary order rate constant for reaction of linalool with ozone; kOH-LINA is secondary order rate constant for reaction of linalool with OH; kOH-PLOP is secondary order rate constant for reaction of primary linalool ozonolysis products with OH. Comparing the reaction of OH with linalool and its primary ozonolysis products, the OH losses resulting from ventilation and deposition are negligible. All of the concentrations for species in eq. (3) refer to their steady-state values. Presumably PLOP here refers to two compounds identified as 4-hydroxy-4-methyl-5-hexenal and 5-ethenyldihydro-5-methyl2(3H)-furanone. The OH yield estimated by Aschmann et al. (2002) was taken to be 0.66 for calculation. Reaction rate constants and parameters associated with eq. (3) are list in Table 2. Steady-state PLOP concentrations were not measured. Thus, it is estimated from the yield reported by Shu et al. (1997). Table 2 Rate constants and parameters. Hydrocarbon
kO3 (ppb1 s1)
kOH (ppb1 s1)
yOH
LINA PLOP
1.1 105a
3.98b 1.32d
0.66c
a b
Fig. 3. Experimental data and fitted curves for yield and SOA mass; yields corrected for wall losses.
c d
Singer et al. (2006) and references therein. Ng et al. (2006). Aschmann et al. (2002). Shu et al. (1997) and references therein.
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They reported that the molar yield of PLOP from ozonolysis to be 0.85 and 0.126 for 4-hydroxy-4-methyl-5-hexenal and 5-ethenyldihydro-5-methyl-2(3H)-furanone respectively. Thus, the sum of molar yields of 0.976 was used. It was assumed that PLOP production was solely from ozone oxidation and the decomposition by OH was neglected for rough estimation. These assumptions might produce an overestimation of PLOP concentrations. However, for the [OH] estimation, the dominant term (in eq. (3)) was the reaction with linalool. Therefore, this approach to the estimation of the PLOP concentrations should not introduce significant error into these calculations. The [OH]s.s concentration was closely related to the ozone concentration, which can be seen from eq. (3) if only the dominant OH decomposition pathway was considered (reaction with linalool). Similar pattern was observed for a-pinene ozonolysis experiments (Chen and Hopke, 2009). A similar correlation between residual ozone and measured [OH] concentrations was also observed by Destaillats et al. (2006) in their study of the reactions of pine oil with ozone. The estimated [OH]s.s concentrations in this study were consistent in magnitude with previous studies conducted at comparable ozone concentrations (Chen and Hopke, 2009; Fan et al., 2003; Singer et al., 2006). Sarwar et al. (2002) estimated indoor [OH] to be lower than outdoor summer time [OH] but similar or greater than nighttime outdoor levels using an atmospheric chemistry model with reactive indoor pollutants including a-pinene. Estimated [OH] concentrations in this study were within range using modeling in literature [Sarwar et al., 2002; Weschler and Shields, 1996]. 3.5. ROS intensities Several collected samples were immediately extracted and measured for ROS concentrations; while several others were kept in petri dishes at room temperature for 24 h following collection to investigate the effect of time on the ROS concentrations. The ROS concentrations are tabulated in Table 1. Holding the collected fresh samples for 24 h was to investigate potential of ROS loss over time. The changes of ROS intensities over a period of 24 h are shown in Fig. 4. The ROS decay caused by analysis delay upon sample collection in this study might overestimate the effect since samples are usually kept refrigerated until measurement. However, keeping fresh samples at room temperature in this study to estimate time effect on ROS intensities was done to examine two processes: 1) loss from reactions over the time between the sample collection and the analysis; 2) the exposure of the aerosol produced in chamber environment to prevailing conditions in the room for possible volatilization losses. The sampling duration was 30 min. Those short-lived ROS species with lifetimes shorter than 30 min are expected to be lost and therefore the overall ROS concentrations
Fig. 4. Plot of relative ROS with [OH]s.s concentrations.
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derived may be underestimated. The measured ROS concentrations ranged from 0.71 to 2.53 nmol m3. For the linalool limited experiments, higher ROS0 concentrations were observed than those for ozone limited scenarios. A relatively higher percentage of hydroperoxides and organic peroxides might be the reason. In addition, with excess ozone present, OH radical concentrations were also higher for the linalool limited experiments. The observed higher ROS0 concentrations might be attributed to two possibilities combined mentioned above and exploring of chemical compositions would help better understand the pattern observed. The 24 h-aged aerosols lost a significant fraction of ROS concentration compared to fresh counterparts. The residual ROS concentrations ranged from 15 to 69% of the fresh sample values. The particle-bound ROS intensities may decrease over time because the short-lived free radical species like OH, RO, HO2 and RO2 disappear and re-equilibrium of the collected material between gas/particle phases might result in losses to the gas phase. The results observed in this study were consistent with previous literature. Antonini et al. (1998) investigated fresh and aged welding fumes and found the ROS of welding fumes decreasing exponentially with a half-life of 10 days. Ambient samples collected on a sidewalk in Taipei showed a decay ranging from 62 to 73% comparing 1 h fresh samples to 115 h-aged samples over different size intervals (Hung and Wang, 2001). Fig. 4 presents the observed residual ROS (ROS measured 24 h later compared to the fresh samples) with steady-state OH radical concentrations. The complexity of the mixture of radical constituents prevents the further investigation on specific contributions to ROS intensities in this study. Future efforts will be needed to identify the specific ROS related to help understand and unveil the mystery. The highest residual was found in the experiments with the lowest ROS but highest initial linalool concentrations. Linalool can form hydroperoxides that are known to be allergenic and sensitizer as contact allergens (Ba¨cktorp et al., 2006) from air oxidation at normal temperature and pressure. Keeping the collected filter samples exposed to air for 24 h until measurement might provide an opportunity for auto-oxidation of linalool, particularly for the cases with high linalool concentrations like experiment 11212008. The ROS concentrations were found to be lower compared to those obtained from a-pinene ozonolysis (Chen and Hopke, 2009). Converting to ROS intensities per unit SOA mass (ROS/SOA mass), the opposite situation was observed. Since linalool is a terpene alcohol with two double bonds it is not surprising that arisen ROS intensities per unit SOA mass were higher than those from a-pinene. The results emphasize the importance of rapid analysis of the collected samples in terms of interpreting the ROS content. 4. Conclusions This study adds to the potential for SOA formation from use of linalool-containing household products upon reaction with ozone. Characteristics of the resulting SOA for initial precursor concentrations relevant to the indoor environments were investigated and quantified in term of particle-bound ROS. Linalool ozonolysis was found to produce species that have low enough volatility to nucleate and condense. While the SOA yields were found to be much lower than other terpene species like a-pinene, higher concentrations of ROS per unit SOA mass were observed. Thus, potential health effect cannot be overlooked given the observed elevated ROS intensities OH radicals produced from linalool/ozone were estimated to be important with respect to the final SOA yield and merits further investigation of its indoor chemistry. Results from this study suggest that SOA in higher quantity will result from events when linalool is present in excess over ozone. Exposure to SOA can be reduced by means such as appropriate
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choice of household products with less ozone reactive constituents; adequate ventilation during and following the application of those products to dilute as much as possible those reactive VOC species and this might be more effective and easier compared to ozone control measures. Acknowledgment This work was supported by U.S Environmental Protection Agency under cooperative agreement X-83269001-0 through a subcontract from Syracuse University. Although the research described in this article has been funded wholly or in part by the United States Environmental Protection Agency, it has not been subjected to the Agency’s required peer and policy review and therefore, does not necessarily reflect the views of the Agency and no official endorsement should be inferred. References Antonini, J.M., Clarke, R.W., Krishna Murthy, G.G., Sreekanthan, P., Jenkins, N., Eagar, T.W., Brain, J.D., 1998. Freshly generated stainless steel welding fume induces greater lung inflammation in rats as compared to aged fume. Toxicology Letters 98, 78–86. Aschmann, S.M., Arey, J., Atkinson, R., 2002. OH radical formation from the gas-phase reactions of O3 with a series of terpenes. Atmospheric Environment 36, 4347–4355. Atkinson, R., Arey, J., Aschmann, S.M., Corchnoy, S.B., Shu, Y.,1995. Rate constants for the gas-phase reactions of cis-3-hexen-1-ol, cis-3-hexenylacetate, trans-2-hexenal, and linalool with OH and NO3 radicals and O3 at 296 2 K, and OH radical formation yields from the O3 reactions. International Journal of Chemical Kinetics 27, 941–955. Ba¨cktorp, G., Tobias Johanson Wass, J.R., Panas, I., Sko¨ld, M., Bo¨rje, A., Nyman, G., 2006. Theoretical investigation of linalool oxidation. Journal of Physical Chemistry A 110, 12204–12212. Bazemore, R., Rouseff, R., Naim, M., 2003. Linalool in orange juice: origin and thermal stability. Journal of Agricultural and Food Chemistry 51, 196–199. Chen, X., Hopke, P.K., 2009. Secondary organic aerosol from a-pinene ozonolysis in dynamic chamber system. Indoor Air (accepted). Destaillats, H., Lunden, M.M., Singer, B.C., Coleman, B.K., Hodgson, A.T., Weschler, C.J., Nazaroff, W.W., 2006. Indoor secondary pollutants from household product emissions in the presence of ozone: a bench scale chamber study. Environmental Science and Technology 40, 4421–4428. Docherty, K.S., Wu, W., Lim, Y.B., Ziemann, P.J., 2005. Contributions of organic peroxides to secondary aerosol formed from reactions of monoterpenes with O3. Environmental Science and Technology 39, 4049–4059. Fan, Z., Lioy, P., Weschler, C., Fiedler, N., Kipen, H., Zhang, J., 2003. Ozone-initiated reactions with mixtures of volatile organic compounds under simulated indoor conditions. Environmental Science and Technology 37, 1811–1821.
Fan, Z., Weschler, C.J., Han, I., Zhang, J., 2005. Co-formation of hydroperoxides and ultra-fine particles during the reactions of ozone with a complex VOC mixture under simulated indoor conditions. Atmospheric Environment 39, 5171–5182. Grosjean, D., Grosjean, E., Williams II, E.L., 1993. Atmospheric chemistry of unsaturated alcohols. Environmental Science and Technology 27, 2478–2485. Hoffmann, T., Odum, J.R., Bowman, F., Collins, D., Klockow, D., Flagan, R.C., Seinfeld, J.H., 1997. Formation of organic aerosols from the oxidation of biogenic hydrocarbons. Journal of Atmospheric Chemistry 26, 189–222. Hung, H.-F., Wang, C.-S., 2001. Experimental determination of reactive oxygen species in Taipei aerosols. Aerosol Science 32, 1201–1211. Kehrer, J.P., 1993. Free radicals as mediators of tissue injury and disease. Critical Review of Toxicology 23, 21–48. Kwok, E.S.C., Atkinson, R., 1995. Estimation of hydroxyl radical reaction rate constants for gas-phase organic compounds using a structure-reactivity relationship: an update. Atmospheric Environment 29, 1685–1695. Lee, S.C., Lam, S., Fai, H.K., 2001. Characterization of VOCs, ozone, and PM10 emissions from office equipment in an environmental chamber. Building and Environment 36, 837–842. Ng, N.L., Kroll, J.H., Keywood, M.D., Bahreini, R., Varubangkul, V., Flagan, R.C., Seinfeld, J.H., 2006. Contribution of first versus second generation products to secondary organic aerosols formed in the oxidation of biogenic hydrocarbons. Environmental Science and Technology 40, 2283–2297. Odum, J.R., Hoffmann, T., Bowman, F., Collins, D., Flagan, R.C., Seinfeld, J.H., 1996. Gas/particle partitioning and secondary organic aerosol yields. Environmental Science and Technology 30, 2580–2585. Pankow, J.F., 1994a. An absorption model of gas-particle partitioning of organic compounds in the atmosphere. Atmospheric Environment 28, 185–188. Pankow, J.F., 1994b. An absorption model of gas aerosol partitioning involved in the formation of secondary organic aerosol. Atmospheric Environment 28, 189–193. Sarwar, G., Corsi, R., Kimura, Y., Allen, D., Weschler, C.J., 2002. Hydroxyl radicals in indoor environments. Atmospheric Environment 36, 3973–3988. Shu, Y., Kwok, E.S.C., Tuazon, E.C., Atkinson, R., Arey, J., 1997. Products of the gasphase reactions of linalool with OH radicals, NO3 radicals, and O3. Environmental Science and Technology 31, 896–904. Singer, B.C., Coleman, B.K., Destaillats, H., Hodgson, A.T., Lunden, M.M., Weschler, C.J., Nazaroff, W.W., 2006. Indoor secondary pollutants from cleaning product and air freshener use in the presence of ozone. Atmospheric Environment 40, 6696–6710. Venkatachari, P., Hopke, P.K., Grover, B.D., Eatough, D.J., 2005. Measurement of particle-bound reactive oxygen species in rubidoux aerosols. Journal of Atmospheric Chemistry 50, 49–58. Weschler, C.J., 2000. Ozone in indoor environments: concentration and chemistry. Indoor Air 10, 269–288. Weschler, C.J., 2006. Ozone’s impact on public health: contributions from indoor exposures to ozone and products of ozone-initiated chemistry. Environmental Health Perspectives 114, 1489–1496. Weschler, C.J., Shields, H.C., 1996. Production of hydroxyl radicals in indoor air. Environmental Science and Technology 30, 3250–3258. Yu, J., Cocker, D.R., Griffin, R.J., Flagan, R.C., Seinfeld, J.H., 1999. Gas phase ozone oxidation of monoterpenes: gaseous and particulate products. Journal of Atmospheric Chemistry 34, 207–258.
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A chamber study of secondary organic aerosol formation by linalool ozonolysis Xi Chen, Philip K. Hopke* Center for Air Resources Engineering and Science, Clarkson University, Potsdam, NY 13699-5708, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 January 2009 Received in revised form 12 April 2009 Accepted 14 April 2009
The formation of secondary organic aerosol (SOA) produced from linalool ozonolysis was examined using a dynamic chamber system that allowed the simulation of ventilated indoor environments. Experiments were conducted under room temperature (22–23 C) and air exchange rate of 0.67 h1. An effort was made to maintain the product of the concentrations of the two reagents constant. The results suggest that under the conditions when the product of the two reagent concentrations was constant, the relative concentrations play an important role in determining the total SOA formed. A combination of concentrations somewhere in ozone limiting region will produce the maximum SOA concentration. The measured reactive oxygen species (ROS) concentrations at linalool and ozone concentrations relevant to prevailing indoor concentrations ranged from 0.71 to 2.53 nmol m3 equivalents of H2O2. It was found that particle samples aged for 24 h lost a significant fraction of the ROS compared to fresh samples. The residual ROS concentrations were around 15–69%. Compared with other terpene species like a-pinene that has one endocyclic unsaturated carbon bond, linalool was less efficient in potential SOA formation yields. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Linalool SOA ROS Indoor aerosol Yield
1. Introduction Linalool, a biogenic unsaturated terpene alcohol, has been identified in the emissions from orange blossoms and certain vegetation (Grosjean et al., 1993; Shu et al., 1997). It is commonly used in foods as flavor and aroma enhancer especially for fruits and vegetables (Ba¨cktorp et al., 2006). Linalool is also the most important flavor component in orange juice (Bazemore et al., 2003). Moreover, widespread use as a fragrance in household products like air fresheners and detergents makes it ubiquitous in indoor air. These active fragrance ingredients are volatilized by the use of household products or consumption of foods. These reactive volatile organic compounds can result in the formation of secondary organic aerosol (SOA) and gas phase products by reaction with ozone. Ozone is present in indoor environments owing to its infiltration from the ambient environment at 20–70% of outdoor concentrations (Weschler, 2000). The application of certain electronic devices including photocopiers and printers as well as some air purifiers would introduce extra ozone to indoor environments (Lee et al., 2001; Singer et al., 2006). Previous studies have reported SOA formation from commercial scented-oil air fresheners that contain linalool as an active constituent (Singer et al., 2006; Destaillats et al., 2006). Gas phase
* Corresponding author. Tel.: þ1 315 268 3861; fax: þ1 315 268 4410. E-mail address: [email protected] (P.K. Hopke). 1352-2310/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2009.04.033
products from reaction of linalool with OH radicals, NO3 and, ozone were analyzed by Shu et al. (1997). The major identified compounds from OH and ozone reactions include two products with 7 carbon atoms which also might be important constituents of SOA formed. Gas-particle partitioning of semivolatile compounds was described by Pankow (1994a,b). From absorptive partitioning theory, in which the partitioning between two phases is governed by an equilibrium partitioning coefficient, the fraction of semivolatile species in the particle phase (F) is given by:
F ¼
MKp 1 þ MKP
(1)
where M is the absorbing material or here as the SOA mass, and Kp is partitioning coefficient. The model developed by Odum et al. (1996) related the SOA yield defined as the mass of SOA formed per mass of hydrocarbon consumed, with the SOA mass loading.
Y ¼
X ai Kp;i DM ¼ M DHC 1 þ MKp;i
(2)
where Y is SOA yield, Kp,i and ai are partitioning coefficient and mass yield of compound i, respectively. This approach has been found to successfully fit the yield data for many experiments (Odum et al., 1996; Yu et al., 1999; Ng et al., 2006; Chen and Hopke, 2009). Under conditions of minimal photochemical activity, O3/terpene reactions have been suggested to be an important pathway to
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indoor H2O2 formation (Fan et al., 2005) as well as producing organic peroxide species (Docherty et al., 2005). These reactive oxygen species (ROS) may produce adverse health effect when they partition into the particulate phase and penetrate deeper into lungs than if present in the gas phase (Weschler, 2006). Thus, the cooccurrence of H2O2 and SOA could produce potential health effects associated with oxidative stress in the lung. Oxidative stress has been implicated in the pathogenesis of many pulmonary diseases (Kehrer, 1993) increasing the potential role of particle-bound ROS. In typical sampling programs, samples are collected but are not analyzed immediately because of the difficulties in making in situ measurement. Most collected samples are kept refrigerated until shipped to the laboratory to be analyzed. The time interval between sample collection and analysis conducted may allow samples to undergo decay or re-equilibrium. The bias introduced from such time delays is expected to produce substantial underestimation of the true oxidant concentrations of collected samples. The comparison between fresh and 24 h aged samples in this study is expected to provide information on the time effect on collected samples in terms of ROS intensities. Although many investigations have looked at chemistry of terpene/ozone system, given the complexity of reactive chemistry between linalool and ozone, there is still limited knowledge regarding exposure to SOA formation at concentrations relevant to indoor situations. The aim of this study is to provide new information on SOA yields and ROS concentrations under various combinations of reactant concentrations. In addition, the role of hydroxyl radicals in SOA formation will be explored.
2. Experimental procedures Linalool ozonolysis was studied in a 2.5 m3 stainless steel chamber in a temperature controlled room. Details regarding the chamber and experimental protocols were given elsewhere (Chen and Hopke, 2009) so only a brief description is presented here. Ozone-loaded air from a UV ozone generator (UV550, Ozone Solutions Inc., Sioux Center, IA) was first introduced into chamber using the UV ozone generator and sufficient time was allowed for the concentration inside the chamber to reach steady state. The experiments were conducted under low relative humidity (RH) conditions with the RH around 7%. The low humidity prevented any hygroscopic growth of the resulting SOA species and permitted the study of behavior of the undiluted SOA. Linalool was delivered from an impinger at an approximately constant concentration. The flow rates and temperature were controlled to get specific VOC concentrations to the chamber. Muffin fans inside the chamber were used to permit thorough mixing. Because of the low vapor pressure of linalool, thorough cleaning of the chamber was performed for at least 36 h in presence of ozone prior to each set of experiments to remove any residue from last experiment. The resultant background contained less than
100 particles cm3, volume concentrations less than 0.1 mm3 cm3 and negligible NOx with concentrations lower than 5 ppb. Particle number and size distributions were measured with a Scanning Mobility Particle Sizer (SMPS). A TSI model 3071 DMA coupled with TSI model 3775 Condensation Particle Counter (CPC) operating at 3 L min1 and 0.3 L min1 for sheath and aerosol flows measures particles range from 14 to 700 nm. Size distribution data were recorded and analyzed using TSI AIM software v 8.1. The ozone concentration inside the chamber was measured by a UV absorption ozone analyzer (49i, Thermo Fisher Scientific Inc, MA) that sampled air at a rate of 1.5 L min1. Gas phase samples were collected from the sampling port at the face plate planted in the center and analyzed by GC–FID. Gas tight syringes (A2, VICI Precision sampling In. IL, USA) were used to collect hydrocarbon samples. Samples for ROS measurement were collected on Teflon filters (Pall, Teflo 25 mm, 3 mm) over 30 min intervals after the system reached steady state using a flow rate of 23 LPM. The determination of ROS was performed by using a fluorogenic probe, dichlorofluorescin (DCFH) (Hung and Wang, 2001; Venkatachari et al., 2005). To catalyze the reaction between DCFH and ROS species, horseradish peroxidase (HRP) was added to the DCFH solution. Fluorescent intensities were converted to equivalent H2O2 concentrations by conducting an assay of standard calibration. Fluorescence intensities were measured by a Turner Quantech Digital Filter Fluorometer (#FM109535, Barnstead Thermolyne Corp, Dubuque, IA, USA). The excitation and emission wavelength used were 485 nm and 530 nm respectively. The wall deposition coefficient was estimated which was described in detail elsewhere before (Chen and Hopke, 2009) as 0.08 0.01 h1 at an air exchange rate (AER) of 0.67 0.01 h1. The wall loss correction was used in the next section when calculating the overall SOA mass yields. Five sets of experiments were conducted under room temperature (22–23 C) and air exchange rate of 0.67 0.01 h1. Experimental conditions are listed in Table 1. Efforts were made to maintain a constant product of reagent concentrations. 3. Results and discussion 3.1. Time evolution of SOA The time evolution of the SOA was similar to the pattern observed for a-pinene (Chen and Hopke, 2009). Fig. 1 shows the typical evolution of the SOA size and volume concentrations as well as ozone concentration beginning at the time when the linalool was added to the chamber that had attained a steady-state concentration of ozone. This point is defined as the time equal to zero point in the figure. With an AER at 0.67 h1, it took around 8 h to reach steady-state concentrations for the number size distribution. The development of the number and volume size distributions for all of the experiments conducted showed similar trends that consisted of four stages with regard to number size distribution. Upon adding
Table 1 Experimental condition details. Date
Initial linalool (ppb)
Initial ozone (ppb)
Number conc (# cm3)
Mass conc (mg m3)
ROS0a (nmole m3)
ROS24b (nmole m3)
s.s. linalool (ppb)
s.s ozone (ppb)
OHs.s (ppb)
11142008 11182008 11212008 11252008 12092008
32 75 106 24 39
110 50 34 150 92
1640 5280 4850 1960 1970
0.71 3.75 3.18 0.30 0.95
2.00 0.93 0.71 2.53 1.17
0.29 0.26 0.49 1.22 0.20
12 40 72 6.2 16
69 8 4 116 46
8.2E05 1.1E05 6.3E06 1.1E04 5.7E05
2 3 6 1 1
2 2 1 2 2
70 110 610 70 70
s.s ¼ steady state. a ROS0: ROS concentrations for samples measured immediately after collection. b ROS24: ROS concentrations for samples measured 24 h after collection.
0.09 0.24 0.17 0.05 0.06
0.30 0.14 0.04 0.41 0.08
0.06 0.05 0.04 0.16 0.04
1 2 3 0.2 1
2 1 1 1 1
X. Chen, P.K. Hopke / Atmospheric Environment 43 (2009) 3935–3940
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Fig. 2. Plot of the steady-state number and mass concentrations with initial linalool concentrations.
Fig. 1. Typical evolution profile of SOA produced and time series of ozone concentrations for experiment 11182008.
linalool to the chamber, there was a sudden burst of small particles but with a time lag compared to those of a-pinene (Chen and Hopke, 2009). Then the total number and volume concentrations increased but with a shift in size to larger particles until the number concentration started to decline when the second stage began. The number concentration continued to decrease while the volume or mass concentration continued to increase. There was a second nucleation burst that was smaller than the first burst in the number concentration. The number concentration increased representing a third stage in the process. Then both the number and mass concentrations approached steady-state values that represented the fourth stage. The volume concentrations followed a different pattern. There was an exponential increase in mass that eventually reached a steady-state value. 3.2. Particle concentrations For the five experiments, the concentration products of linalool and ozone were approximately 3612 [ppb O3 ppb linalool] (the differences are within 5%). Keeping the concentration products constant while varying the combination of the two reagents helped to examine the effect of changing the limiting reagent with regards to the number concentrations and SOA mass produced. Since the reaction of linalool with ozone is a second order reaction, the produced SOA would be expected to be proportional to the initial concentrations of the precursors. The five sets of experiments can be separated into two groups: linalool limiting or ozone limiting according to initial ozone/linalool concentration ratios (if >2, linalool limiting, otherwise ozone limiting). Fig. 2 shows the relationship of the steady-state number
and mass concentrations with the initial linalool concentrations. The resulting SOA number size distribution showed that the two groups fell in two groups with different concentration combinations of the reagents. However, within each group, the number concentrations were approximately constant such that those with ozone limiting values were about 3 times higher than those for linalool limiting. Compared to a-pinene ozonolysis experiments with the reagent products at approximately 3557 [ppb O3 ppb a-pinene], the number concentrations at steady state for all a-pinene experiments were relatively constant with the variations being only 11% (Chen and Hopke, 2009). These differences of linalool over a-pinene might be attributed to a-pinene being monoterpene with one double bond while linalool with two double bonds. The attack on the second unsaturated bond is expected to produce different product compositions that then nucleate and condense. Thus, under the conditions where the products of the two reagent concentrations were constant, the relative concentrations play an important role in determining the total SOA formed. The trend indicated from Fig. 2 suggests that a combination somewhere in ozone limiting region would produce the maximum SOA concentrations. The consumed linalool/ozone was approximately 1:2 for linalool limiting group; while the ratios were between 1 and 2 for those of ozone limiting sets. Such observed stoichimometry suggested that for linalool limiting scenarios ozone was the dominant oxidation reagent to react with the two carbon double bonds given its abundance; while for ozone limiting sets ozone attacked one of the double bond to form primary first generation products, which were further oxidized by either OH radicals or ozone. For the steady-state concentration products of linalool and ozone, a similar pattern was observed as that for a-pinene (Chen and Hopke, 2009). For the two reagent limited regions, similar steady-state concentration products were observed within the region, but different products were obtained in each region. The steady-state concentration products for the two reagents under linalool limited were found to be about 2.5 times higher than those of ozone limited sets. The measured steady-state concentrations of linalool and ozone for each experiment are listed in Table 1. 3.3. Linalool ozonolysis chemistry and yields The gas phase products of linalool with OH radicals and ozone were studied by Shu et al. (1997). Among the identified major gas phase products, there are species that are possible candidates for partitioning between the gas and particle phases to form additional SOA. The reaction of linalool [(CH3)2C]CHCH2CH2C(CH3)(OH)
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CH]CH2] with ozone is by the initial addition of ozone to one of the carbon double bonds; mainly to (CH3)2C]CHe, minor to CH2]CHe with an approximately 97%/3% ratio(Atkinson et al., 1995; Shu et al., 1997). The primary ozonide is expected to undergo rapid decomposition into formation of biradicals. OH radicals would arise from reaction pathways from one of the biradicals generated ½ðCH3 Þ2 ,CO O* with the measured OH yield in literature , ranges from 0.66 to 0.85 (Atkinson et al., 1995; Aschmann et al., 2002; Shu et al., 1997). This pathway will form a co-product 4-hydroxy-4-methyl-5-hexenal, that will cyclize and be present as 2-ethenyl-2-methyl-5-hydroxyterahydrofuran. The observed formation of 5-ethenyldihydro-5-methyl-2(3H)-furanone suggests the other biradical ½CH2 ]CHCðCH3 ÞðOHÞCH2 CH2 ,CHO O* undergo , cyclization as well as the loss of water (Shu et al., 1997). The reaction of linalool with OH radicals is by the addition of OH to an unsaturated carbon bond with the dominant addition to (CH3)2C]CHe over CH2]CHe with a 77%/23% ratio (Shu et al., 1997; Kwok and Atkinson, 1995). The two major gas phase products identified from the reactions with ozone and OH radicals are 2-ethenyl-2-methyl-5-hydroxyterahydrofuran and 5-ethenyldihydro-5-methyl-2(3H)-furanone (Shu et al., 1997), and are likely to nucleate or condense to form SOA. Both of the compounds have 7 carbon atoms and are expected to be primary first generation products. The further attack of OH or ozone on the second unsaturated carbon bond will lead to further fragmentation of carbon backbones yielding products with even fewer carbon atoms, and therefore less likely to contribute to SOA formation and growth. SOA mass concentrations were calculated from volume size distributions. The density was chosen to be 1.5 g cm3 based on prior density measurements for the a-pinene – ozone system (Chen and Hopke, 2009). The SOA yields derived from this study ranged from 0.3 to 1.9%. Attempts were made to relate SOA yield with SOA mass loading using eq. (2) using one and two hypothetical products. Fig. 3 shows the experimental data points and fitted curves. The differences between the one and two hypothetical products curves are not very large for the low SOA region of interest. This region would represent SOA formation under conditions typically encountered in indoor environments. Only one set of Kp,i and ai was found to be sufficient to fit the experimental data for SOA mass concentrations in low region (Chen and Hopke, 2009). With other terpene species like a-pinene that has one endocyclic unsaturated carbon bond, the primary oxidation products are of 10 or 9 carbon atoms from ozonolysis. For linalool, the major
product molecules have 7 or fewer carbon backbone atoms and therefore the potential of SOA formation is expected to be smaller. Linalool SOA yields measured in this study as well as those from previously in literature are consistent with this expectation. Ng et al. (2006) conducted a series of experiments on terpene ozonolysis and photooxidation including linalool. The SOA yields Ng et al. derived for ozonolysis with cyclohexane present as OH scavenger at 20 C for 106 ppb linalool is around 1.2%, while for 124 ppb linalool under photooxidation, the yield is approximately 13%, where both systems had (NH4)2SO4 particles as seeds. Studies conducted by Destaillats et al. (2006) on SOA from ozonolysis of terpene-based household products reported yields ranging from 4 to 6% for a plug-in scented-oil air freshener that contained linalool as one of the active ingredients. Hoffmann and coworkers (1997) estimated w5% SOA yield for open-chain hydrocarbon products of about 100 ppb linalool under ozonolysis, which is the lowest compared to other monounsaturated cyclic monoterpenes like a-pinene and cyclic monoterpenes with two double bonds like D-limonene. The linalool ozonolysis experiments conducted by Hoffmann et al. (1997) were under similar temperature and (NH4)2SO4 as seeds as those conducted by Ng et al. (2006). The only difference between the experiments was the absence of OH scavenger cyclohexane in the Hoffmann et al. (1997) study. These differences resulted in higher SOA yields; higher yields were also observed from photooxidation experiments by Ng et al. (2006) where OH, NO3 and ozone were present. 3.4. OH radicals The OH radical concentrations in the chamber at steady state are calculated as follows. The OH source reaction is ozone/linalool, and the sinks include reaction with linalool and its dominant primary ozonolysis products as well as physical loss pathways.
½OH ¼
yOH kO3 LINA ½O3 ½LINA AER þ kd VA þ kOHLINA ½LINA þ kOHPLOP ½PLOP
(3)
where LINA ¼ linalool, PLOP ¼ primary linalool ozonolysis products; yOH is molar yield of OH from ozone/linalool reaction; kd is OH deposition velocity (0.00007 m s1) (Fan et al., 2003); A/V is chamber surface to volume ratio (4.5 m1); kO3 LINA is secondary order rate constant for reaction of linalool with ozone; kOH-LINA is secondary order rate constant for reaction of linalool with OH; kOH-PLOP is secondary order rate constant for reaction of primary linalool ozonolysis products with OH. Comparing the reaction of OH with linalool and its primary ozonolysis products, the OH losses resulting from ventilation and deposition are negligible. All of the concentrations for species in eq. (3) refer to their steady-state values. Presumably PLOP here refers to two compounds identified as 4-hydroxy-4-methyl-5-hexenal and 5-ethenyldihydro-5-methyl2(3H)-furanone. The OH yield estimated by Aschmann et al. (2002) was taken to be 0.66 for calculation. Reaction rate constants and parameters associated with eq. (3) are list in Table 2. Steady-state PLOP concentrations were not measured. Thus, it is estimated from the yield reported by Shu et al. (1997). Table 2 Rate constants and parameters. Hydrocarbon
kO3 (ppb1 s1)
kOH (ppb1 s1)
yOH
LINA PLOP
1.1 105a
3.98b 1.32d
0.66c
a b
Fig. 3. Experimental data and fitted curves for yield and SOA mass; yields corrected for wall losses.
c d
Singer et al. (2006) and references therein. Ng et al. (2006). Aschmann et al. (2002). Shu et al. (1997) and references therein.
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They reported that the molar yield of PLOP from ozonolysis to be 0.85 and 0.126 for 4-hydroxy-4-methyl-5-hexenal and 5-ethenyldihydro-5-methyl-2(3H)-furanone respectively. Thus, the sum of molar yields of 0.976 was used. It was assumed that PLOP production was solely from ozone oxidation and the decomposition by OH was neglected for rough estimation. These assumptions might produce an overestimation of PLOP concentrations. However, for the [OH] estimation, the dominant term (in eq. (3)) was the reaction with linalool. Therefore, this approach to the estimation of the PLOP concentrations should not introduce significant error into these calculations. The [OH]s.s concentration was closely related to the ozone concentration, which can be seen from eq. (3) if only the dominant OH decomposition pathway was considered (reaction with linalool). Similar pattern was observed for a-pinene ozonolysis experiments (Chen and Hopke, 2009). A similar correlation between residual ozone and measured [OH] concentrations was also observed by Destaillats et al. (2006) in their study of the reactions of pine oil with ozone. The estimated [OH]s.s concentrations in this study were consistent in magnitude with previous studies conducted at comparable ozone concentrations (Chen and Hopke, 2009; Fan et al., 2003; Singer et al., 2006). Sarwar et al. (2002) estimated indoor [OH] to be lower than outdoor summer time [OH] but similar or greater than nighttime outdoor levels using an atmospheric chemistry model with reactive indoor pollutants including a-pinene. Estimated [OH] concentrations in this study were within range using modeling in literature [Sarwar et al., 2002; Weschler and Shields, 1996]. 3.5. ROS intensities Several collected samples were immediately extracted and measured for ROS concentrations; while several others were kept in petri dishes at room temperature for 24 h following collection to investigate the effect of time on the ROS concentrations. The ROS concentrations are tabulated in Table 1. Holding the collected fresh samples for 24 h was to investigate potential of ROS loss over time. The changes of ROS intensities over a period of 24 h are shown in Fig. 4. The ROS decay caused by analysis delay upon sample collection in this study might overestimate the effect since samples are usually kept refrigerated until measurement. However, keeping fresh samples at room temperature in this study to estimate time effect on ROS intensities was done to examine two processes: 1) loss from reactions over the time between the sample collection and the analysis; 2) the exposure of the aerosol produced in chamber environment to prevailing conditions in the room for possible volatilization losses. The sampling duration was 30 min. Those short-lived ROS species with lifetimes shorter than 30 min are expected to be lost and therefore the overall ROS concentrations
Fig. 4. Plot of relative ROS with [OH]s.s concentrations.
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derived may be underestimated. The measured ROS concentrations ranged from 0.71 to 2.53 nmol m3. For the linalool limited experiments, higher ROS0 concentrations were observed than those for ozone limited scenarios. A relatively higher percentage of hydroperoxides and organic peroxides might be the reason. In addition, with excess ozone present, OH radical concentrations were also higher for the linalool limited experiments. The observed higher ROS0 concentrations might be attributed to two possibilities combined mentioned above and exploring of chemical compositions would help better understand the pattern observed. The 24 h-aged aerosols lost a significant fraction of ROS concentration compared to fresh counterparts. The residual ROS concentrations ranged from 15 to 69% of the fresh sample values. The particle-bound ROS intensities may decrease over time because the short-lived free radical species like OH, RO, HO2 and RO2 disappear and re-equilibrium of the collected material between gas/particle phases might result in losses to the gas phase. The results observed in this study were consistent with previous literature. Antonini et al. (1998) investigated fresh and aged welding fumes and found the ROS of welding fumes decreasing exponentially with a half-life of 10 days. Ambient samples collected on a sidewalk in Taipei showed a decay ranging from 62 to 73% comparing 1 h fresh samples to 115 h-aged samples over different size intervals (Hung and Wang, 2001). Fig. 4 presents the observed residual ROS (ROS measured 24 h later compared to the fresh samples) with steady-state OH radical concentrations. The complexity of the mixture of radical constituents prevents the further investigation on specific contributions to ROS intensities in this study. Future efforts will be needed to identify the specific ROS related to help understand and unveil the mystery. The highest residual was found in the experiments with the lowest ROS but highest initial linalool concentrations. Linalool can form hydroperoxides that are known to be allergenic and sensitizer as contact allergens (Ba¨cktorp et al., 2006) from air oxidation at normal temperature and pressure. Keeping the collected filter samples exposed to air for 24 h until measurement might provide an opportunity for auto-oxidation of linalool, particularly for the cases with high linalool concentrations like experiment 11212008. The ROS concentrations were found to be lower compared to those obtained from a-pinene ozonolysis (Chen and Hopke, 2009). Converting to ROS intensities per unit SOA mass (ROS/SOA mass), the opposite situation was observed. Since linalool is a terpene alcohol with two double bonds it is not surprising that arisen ROS intensities per unit SOA mass were higher than those from a-pinene. The results emphasize the importance of rapid analysis of the collected samples in terms of interpreting the ROS content. 4. Conclusions This study adds to the potential for SOA formation from use of linalool-containing household products upon reaction with ozone. Characteristics of the resulting SOA for initial precursor concentrations relevant to the indoor environments were investigated and quantified in term of particle-bound ROS. Linalool ozonolysis was found to produce species that have low enough volatility to nucleate and condense. While the SOA yields were found to be much lower than other terpene species like a-pinene, higher concentrations of ROS per unit SOA mass were observed. Thus, potential health effect cannot be overlooked given the observed elevated ROS intensities OH radicals produced from linalool/ozone were estimated to be important with respect to the final SOA yield and merits further investigation of its indoor chemistry. Results from this study suggest that SOA in higher quantity will result from events when linalool is present in excess over ozone. Exposure to SOA can be reduced by means such as appropriate
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