Environ. Sci. Technol. 2008, 42, 7138–7145
Accretion Reactions of Octanal Catalyzed by Sulfuric Acid: Product Identification, Reaction Pathways, and Atmospheric Implications YONG JIE LI,† ALEX K. Y. LEE,‡ ARTHUR P. S. LAU,§ AND C H A K K . C H A N * ,‡ Environmental Engineering Program, Department of Chemical Engineering, and Insitute for the Environment, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
Received December 14, 2007. Revised manuscript received June 2, 2008. Accepted July 21, 2008.
Atmospheric accretion reactions of octanal with sulfuric acid as a catalyst were investigated in bulk liquid-liquid experiments and gas-particle experiments. In bulk studies, trioxane, R,βunsaturated aldehyde, and trialkyl benzene were identified by gas chromatography-mass spectrometry as major reaction products with increasing sulfuric acid concentrations (0-86 wt%). Cyclotrimerization and one or multiple steps of aldol condensation are proposed as possible accretion reaction pathways. High molecular weight (up to 700 Da) oligomers were also observed by electrospray ionization-mass spectrometry in reactions under extremely high acid concentration conditions (86 wt%). Gas-particle experiments using a reaction cell were carried out using both high (∼20 ppmv) and low (∼900 ppbv) gas-phase octanal concentrations under a wide range of relative humidity (RH, from 80 wt% to 43 wt% H2SO4) and long reaction durations (24 h). One or multiple steps of aldol condensation occurred under low RH (80 wt% and 64 wt% H2SO4, respectively) and high octanal concentration (∼20 ppmv) conditions. No cyclotrimerization was observed in the gas-particle experiments even under RH conditions corresponding to similar sulfuric acid concentration conditions that favor cyclotrimerization in bulk studies. No accretion reaction product was found in the low octanal concentration (∼900 ppbv) experiments, which indicates that the accretion reactions are not significant as expected when the gas-phase octanal concentration is low. A kinetic analysis of the first-step aldol condensation product was performed to understand the discrepancies between the bulk and gas-particle experiments and between the high and low octanal concentrations in the gas-particle experiments. The comparisons between experimental results and kinetic estimations suggest that caution should be exercised in the extrapolation of laboratory experiment results to ambient conditions.
* Corresponding author phone: (852)2358-7124; fax: (852)23580054; e-mail:
[email protected]. † Environmental Engineering Program. ‡ Department of Chemical Engineering. § Insitute for the Environment. 7138
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1. Introduction Organic aerosols have become a main research focus in atmospheric chemistry because of their effects on visibility, human health, and global climate (1). The compositions of organic aerosols are complex (2, 3) and change continuously over their atmospheric lifetime. The heterogeneous interactions between gases and aerosols affect the organic burden in aerosols and alter their physical/chemical properties. These chemical processes include the oxidation of gas-phase organics followed by partitioning into the particle phase (4-6), aging of particulate organics in the presence of oxidants (7-9), and reactive uptake of volatile organics onto preexisting particles (10-14). While the former two processes are well documented as important routes of organic aerosol formation, the last one has only been recognized in the past decade, and its impacts are still under open debate. One particularly important issue in the reactive uptake of volatile organics into preexisting particles is the uptake of carbonyl compounds into acidic particles. Carbonyls are ubiquitous in the troposphere due to numerous sources such as direct emissions (15) and photodegradation of other organics (16, 17). Because of their high reactivity, they are considered as important precursors (or intermediates when they are photochemically formed) of secondary organic aerosol (SOA). Moreover, the much higher observed aerosol organic mass increases than those predicted by partitioning theories further suggest that the uptake of carbonyls involves chemical processes, especially when the particles are acidic (18). A number of studies have been aimed at ascertaining the role of acid-catalyzed carbonyls reactions under ambient conditions. The term “accretion reaction” is used to describe the collection of reactions that result in significantly higher molecular weight products formation (19). An increase in the organic burden of atmospheric aerosols is thus expected as those products so formed are normally less volatile than their precursors and tend to remain in the aerosol phase. Several reaction mechanisms, including aldol condensation, acetal/hemicetal formation (in the presence of alcohols), and polymerization, have been proposed as acid-catalyzed accretion reactions that lead to an increase in the organic aerosol mass (20). Recent studies, however, have found that most of these mechanisms may not be kinetically (21-24) or thermodynamically (19, 25) favorable to have large effects on the properties of aerosols under atmospheric conditions. The results from field measurements (26-28) also raised questions on the hypothesis that acid-catalyzed reactions of organics will significantly increase the organic burden of aerosols. Uncertainties and inconsistencies still exist regarding whether or not acid-catalyzed aldehyde reactions significantly affect the properties of aerosols. To this end, we raise two fundamental questions: 1) How do acids catalyze aldehyde reactions? 2) Do these reactions occur under atmospheric conditions? Although there is a large body of literature on organic synthesis in acid-catalyzed aldehyde reactions (29, 30), it is not surprising that most of them focus on obtaining high yields of the end products, rather than on the product formation pathways. As a result, comprehensive product information is not available to answer the first question, not to mention that the conditions (e.g., high acid and aldehyde concentrations etc.) are not likely applicable in the atmosphere. In most of the laboratory experiments concerning this issue, relatively high gas-phase aldehyde concentrations and short reaction durations remain as 10.1021/es7031373 CCC: $40.75
2008 American Chemical Society
Published on Web 08/27/2008
limitations. Controlled experiments under more realistic conditions are needed so that laboratory findings can be extrapolated to the real atmosphere. In the present work, we examined sulfuric acid catalyzed accretion reactions of octanal in bulk liquid-liquid and gasparticle experiments. Bulk experiment results were used for product identification and reaction pathways interpretation. Gas-particle experiments were performed to determine the conditions under which those reactions occurred. Long reaction durations (24 h) and different octanal concentrations were employed in the gas-particle experiments. The discrepancies between the results in the low (∼900 ppbv) and high (∼20 ppmv) octanal concentrations and between those from the gas-particle experiments and the bulk experiments are addressed. Comparisons of the experimental results with kinetic estimations are also presented.
2. Experimental Section All chemicals, including octanal (99%, Aldrich) and sulfuric acid (95-98 wt%, Mallinckrodt Chemicals), were used as received, and all solvents were of analytical grade. 2.1. Bulk Experiments. A volume of 0.3 mL octanal was added to 10 mL sulfuric acid solutions (0%, 31%, 54%, 72%, and 86% in weight percentage) in 50 mL conical flasks. The mixtures were shaken for 24 h at 180 rpm and room temperature in an automated shaker sealed with aluminum foil and then extracted by a solvent mixture (hexane/ dichloromethane)4/1, v/v; 5 mL × 4)20 mL). Residual water was removed from the extracts by adding anhydrous sodium sulfate followed by syringe filtration. The filtrates were analyzed by gas chromatography mass spectrometry (GCMS, Clarus 500, Perkin-Elmer) directly. To characterize the polar products from reactions at high sulfuric acid concentrations (86 wt%), the filtrate was also separated by a normalphase solid-phase extraction (SPE) cartridge (Extract-Clean SPE Si, Alltech) by hexane/dichloromethane (4/1, v/v) and subsequently by methanol (31). The methanol fraction was subjected to direct electrospray ionization mass spectrometry (ESI-MS, Micromass, Waters) analysis. TOC and EC/OC analyses were also performed for the extracts from the bulk experiments (Supporting Information). 2.2. Gas-Particle Experiments. A RH-controlled flow system and a custom-made stainless steel reaction cell (32) were used to study the reactions between the gas-phase octanal and sulfuric acid particles, as illustrated in Figure S1 (Supporting Information). Sulfuric acid droplets (aqueous solution, 0.1% vol.) were deposited onto a hydrophobic membrane (Model 5793, Yellow Springs Inc., OH) by an autopipette. The original diameters of the particles were several hundred microns. As these droplets were equilibrated in a large flow (>1 L/min) of clean air under a desired RH (2 h, they shrank and became smaller particles with diameters less than 100 µm. The sulfuric acid concentrations in those particles after water evaporation equilibrium were estimated by the AIM model (33) to be >80, 64, 53, and 43 wt%, respectively. After equilibrium, high (∼20 ppmv) or low (∼900 ppbv) octanal concentration flow, calibrated by adsorption tubes and thermal desorption GCMS, was introduced into the reaction cell at around 600 mL/ min. The RH ((1%RH) and temperature (25-27 °C) were monitored by respective sensors (HIH 3610 and LM 35, RS Components) during the reaction periods. After 24 h of exposure to octanal, the particles on the substrate were extracted by 5 mL of hexane/dichloromethane (4/1, v/v) under ultrasonication (5 min), and water was removed by adding anhydrous sodium sulfate, followed by filtration. The filtrates were concentrated to around 100 µL under gentle nitrogen flow before GC-MS analysis.
FIGURE 1. Total ion chromatograms from gas chromatography-mass spectrometry (GC-MS) analysis of reaction products from bulk experiments. a: 0 wt%; b: 31 wt%; c: 54 wt%; d: 72 wt%; e: 86 wt% H2SO4 solutions. A: monomer region; B: dimer region; C: trimer region; D: tetramer or higher oligomers region.
3. Results and Discussion 3.1. Product Identification. Figure 1 shows the GC-MS total ion chromatograms (TIC) of the extracts from different sulfuric acid concentration (0, 31, 54, 72, and 86 wt%) bulk experiments. The chromatograms are divided into four regions (A, B, C, and D). Region A contains the reactant (octanal, ∼8.3 min) and its oxidation product (octanoic acid, ∼13 min). To identify the products other than octanoic acid, we searched the NIST Mass Spec. database (ref 34, as NIST database hereafter) for spectra of chemicals with similar structures as the possible products from clues such as molecular weights and retention times. As the spectra from the NIST database and our GC-MS analysis are both obtained from electron ionization (EI), we can compare their fragmentation patterns and verify our proposed structures. Fragments of ion peaks of exactly the same proposed species from the literature, if available, were also used for further confirmation. The product in region B (∼23.7 min) has a molecular ion peak of m/z 238 (Figure 2a). It is reasonable to consider it as the R,β-unsaturated aldehyde formed from aldol condensation (2 × octanal-H2O)2 × 128-18)238). The spectrum of a similar R,β-unsaturated aldehyde with two fewer carbon atoms, 2-pentylnon-2-enal (PNE), was found in the NIST database (Figure 2b). Their similar fragmentation patterns, including the formation of molecular ions, ion peaks with one side chain lost and with the other side chain lost, are listed in Table 1. Given that the same EI MS fragmentation VOL. 42, NO. 19, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. EI MS (from GC-MS) spectra of r,β-unsaturated aldehyde (a), trioxane (c), trialkyl benzene (e), and the corresponding EI MS spectra of their reference compounds from the NIST database (b, d, and f, respectively), fully dehydrated tetramer (g), and the ESI-MS spectrum of polar products (h). pattern (m/z 238, 167, 153, and 139 etc.) of the identical R,β-unsaturated aldehyde (2-hexyldec-2-enal, HDE) was also observed in an early study (35), we believe that the product in region B is the aldol condensation product (HDE). The spectrum of the first product (∼31.6 min) in region C is shown in Figure 2c. From its easily distinguishable mass differences of peaks 126 (m/z 383-257) and 128 (m/z 257-129), we deduce that it should be the heterocyclic trimer 7140
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(MW)384) of octanal (MW)128), from which one or two octanal monomer units are readily lost in EI. The spectrum of the heterocyclic trimer of acetaldehyde, namely 2,4,6trimethyl-1,3,5-trioxane (TMT), is available from the NIST database and is shown in Figure 2d for comparison. The formation of a weak quasi-molecular ion ([M-H]+) and an ion peak with one substituted side chain lost are observed in the two spectra (Figures 2c and 2d and Table 1). Fragments
TABLE 1. Major Ion Peaks of MS Spectra from Bulk Experiments and the NIST Database (Ref 34)a experimental MS spectra species b
HDE
THTc
THBd
ion peaks [M]+
[M-C5H11]+ [M-C7H15]+ [M-H]+ [M-C7H15]+ [M-C8H15O]+ [M-C8H15O-C8H16O]+ [M]+ [M-C5H10]+ [M-C5H10-C6H13]+
reference MS spectra (from the NIST database) m/z
species
238 167 139 383 285 257 129 330 260 175
b
PNE
TMTc
TBBd
ion peaks [M]+
[M-C4H9]+ [M-C6H13]+ [M-H]+ [M-CH3]+ [M-C2H3O]+ [M-C2H3O-C2H4O]+ [M]+ [M-C3H6]+ [M-C3H6-C4H9]+
m/z 210 153 125 131 117 89 45 246 204 147
a For the description of fragmentation patterns, please refer to the section entitled Product Identification. R,β-Unsaturated aldehydes: HDE (2-hexyldec-2-enal, Figure 2a) and PNE (2-pentylnon-2-enal, Figure 2b). c Trioxanes: THT (2,4,6-triheptyl-1,3,5-trioxane, Figure 2c) and TMT (2,4,6-trimethyl-1,3,5-trioxane, Figure 2d). d Trialkyl benzenes: THB (1,3,5-trihexylbenzene, Figure 2e) and TBB (1,3,5-tributylbenzene, Figure 2f). b
FIGURE 3. Proposed reaction pathways in bulk experiments. Path A: cyclotrimerization; Path B: the first step aldol condensation; Path C: further steps of aldol condensation. Species 3, 6, and 7 are products identified by GC-MS. The abbreviations of the names of those species can be found in the footnotes of Table 1. that have lost one and two monomer units are also observed in both spectra. The trioxane of butanal (2,4,6-tripropyl-1,3,5trioxane) from the NIST database also has a similar fragmentation pattern (not shown). Therefore, we conclude that the first product (∼31.6 min) in region C is 2,4,6-triheptyl1,3,5-trioxane (THT), the heterocyclic trimer from octanal,. The spectrum of the other product (∼30.2 min) in region C has three fingerprint peaks of m/z 91, 105, and 119 (Figure 2e), indicating that this product contains an aromatic ring (36). With its molecular ion peak at m/z 330, this observation leads us to believe that this product should be related to a trialkyl benzene, the aromatic trimer of octanal with all oxygen atoms lost in the form of water molecules (3 × octanal3H2O)3 × 128-3 × 18)330). The spectrum of 1,3,5tributylbenzene (TBB) from the NIST database is shown in Figure 2f for comparison. The formation of the molecular ions and ion peaks that have lost one substituted side chain (one carbon less than the side chain, forming a benzyl ion) and those that have lost two substituted side chains suggest that they have similar fragmentation patterns (Figure 2e,f
and Table 1). In addition, an early synthesis study (37) provided the same EI mass fragments (m/z 330, 287, 260, and 245 etc.) for the identical compound (1,3,5-trihexylbenzene, THB) as the one we propose here. On the basis of the above observations, we attribute the second product (∼30.2 min) in region C to 1,3,5-trihexylbenzene (THB). The product in region D (∼37.1 min) is expected to be a fully dehydrated tetramer of octanal, with a molecular weight of 440 (4 × octanal-4H2O)4 × 128-4 × 18)440). It has typical alkyl chain losses (m/z 369)440-71 as [M-C5H11]+; m/z 355)440-85 as [M-C6H13]+) via alpha cleavages as shown in Figure 2g. However, with the available information, it only leads us to a conclusion that this product should be an unsaturated hydrocarbon with four double bonds and probably an aromatic. Figure 2h shows the positive mode ESI mass spectrum of the polar products from the high acid concentration (86 wt%) catalyzed octanal reactions. “Cluster” patterns can be easily seen in the spectrum, which indicates the presence of oligomers (MW up to 700 Da) of octanal under such a high VOL. 42, NO. 19, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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acid concentration condition (86 wt%). Furthermore, some minor peaks in the GC-MS chromatograms under high (72 wt% and 86 wt%) acid concentration conditions (regions C and D in Figure 1d,e) contain products with molecular ion peaks at m/z 348 and 366 etc. (spectra not shown). Although not identified, they are believed to be polar oligomers, which gave low responses in the GC-MS analysis, with oxygen atoms partially lost. The formation of reaction products in bulk experiments is strongly acidity-dependent as shown in Figure 1. Trioxane (∼31.6 min) was formed under 31 wt% conditions (Figure 1b) and became the major product under 54 wt% conditions (Figure 1c). The R,β-unsaturated aldehyde (∼23.7 min) took over the role as the dominant product under 72 wt% conditions (Figure 1d). Under extremely high acid concentration conditions (86 wt%), fully dehydrated trimer (aromatic) and tetramer (∼30.2 min and ∼37.1 min, respectively) became major products (Figure 1e), together with other partially dehydrated oligomers. The formation of these products under different acidic conditions was also supported by TOC and EC/OC analyses, from which thermal evaporation and/or decomposition were rationalized by the products formed under different acidic conditions (Supporting Information). 3.2. Bulk Experiment Reaction Pathways. From the products identified in bulk experiments, several accretion reaction pathways are proposed as illustrated in Figure 3. Oxidation occurs under all conditions, but it is not included in this figure as it is not considered as an accretion reaction (19). 3.2.1. Cyclotrimerization. Cyclotrimerization is a wellknown process for aldehydes, and it is catalyzed by acids (38, 39). Following the poly(oxymethylene) glycol (species 2 in Figure 3) formation mechanism, trioxanes are formed from cyclization of protonated poly(oxymethylene) glycols with three or more monomer units (38). Trioxane (THT, species 3 in Figure 3) so formed can be stabilized under appropriate acidic conditions (11). However, trioxane formation is not favorable in stronger acidic conditions (38, 40). This strong acidity dependence was observed in our bulk experiments (∼31.6 min at region C in Figure 1). In terms of the role of acid (proton), this phenomenon can be explained by the efficiency of poly(oxymethylene) glycol formation (path A in Figure 3), which is facilitated by hydration. Hydration of octanal is limited for this long aliphatic aldehyde (41, 42), and the equilibrium favors the parent aldehyde under neutral conditions. Therefore, poly(oxymethylene) glycol formation and hence cyclotrimerization would be inhibited under neutral conditions (0 wt% H2SO4) due to the reversibility of the hydration and poly(oxymethylene) glycol formation (38). Trioxane (species 3 in Figure 3) was thereby observed only under mild (31 wt% and 54 wt%) acidic conditions in which both hydration and stabilization of trioxane are efficient. In high acid concentration solutions where water activity is low, the equilibrium is driven to protonation and dehydration for other reactions (path B in Figure 3). 3.2.2. Aldol Condensation. Many studies have considered aldol condensation (path B in Figure 3) as an accretion reaction pathway of acid-catalyzed aldehyde reactions (11, 18, 22, 43, 44). Although increases in organic mass have been widely attributed to this reaction pathway, direct identification of products from this pathway is rare (21). Little attention has been paid to the acidity dependence of the first-step aldol condensation products (HDE, species 6 in Figure 3). In our bulk experiments, HDE dominated under high acid concentration conditions (72 wt%, ∼23.7 min at region B in Figure 1). A negligible amount of HDE under low acid concentration conditions (30.7 min). This is consistent with the bulk experiments that a high acid concentration favors one-step or multiple-step aldol condensation pathways. However, aldol condensation products were not observed at low octanal concentration (∼900 ppbv) experiments under the two low RH conditions studied (Figure S5). The above observations support the hypothesis that aldol condensations can only occur at low RH and at high octanal concentrations. At 30%RH (53 wt% H2SO4) in the gas-particle experiments, no accretion reaction products were observed even with a high octanal concentration (∼20 ppmv, Figure S4c). Under a similar sulfuric acid concentration (54 wt%) condition, cyclotrimerization was dominant in our bulk experiments, and the first-step aldol condensation product (HDE) was observable (Figure 1c). Yet neither trioxane nor HDE was observed in gas-particle experiments under 30%RH (53 wt% H2SO4), probably due to the limited availability of octanal in the sulfuric acid particles. The formation of trioxane from acid-catalyzed aldehyde reactions has been observed by HNMR analysis (11) and has also been used to explain the high aerosol yield in chamber experiments (46). Jang et al. (46), however, argued that trioxane formation was an artifact from octanal cyclotrimerization over the glass surface of the injection tube. We performed a further check in the bulk experiments by using a polyethylene container, rather than glass vials. This still yielded trioxane as the main product with the 54 wt% acid concentration reaction. Therefore, we believe that the smaller amount of octanal that can partition onto the particle phase in the gas-particle experiments is a more rational explanation for the discrepancy in our bulk and gas-particle experiment results. 3.4. Kinetic Analysis. According to Casale et al. (21), the first-step aldol condensation reaction is a second-order reaction with respect to the total aldehyde concentration ([A]tot, including the concentrations of molecular aldehyde, hydrated aldehyde, protonated aldehyde, and the enol form of aldehyde), and the reaction rate constant (kaldol) is strongly dependent on acidity. We used Casale’s method to calculate the reaction rate constants (kaldol) under the different reaction
FIGURE 4. Estimated aldol condensation product (HDE) final concentrations as a function of sulfuric acid concentration in different scenarios. bulk: bulk experiment; G-P/20 ppmv: gasparticle experiment with octanal concentration at 20 ppmv; G-P/900 ppbv: gas-particle experiment with octanal concentration at 900 ppbv. conditions used in this study. The total concentrations of octanal ([A]tot) under the bulk liquid-liquid and gas-particle interactions were calculated using the available data and the appropriate approximations for S* (the apparent solubility) and H* (the apparent Henry’s law constant), respectively (Supporting Information). Reaction rate constants were estimated from steady state calculations (eq 6, Supporting Information) instead of full integration, as the differences of calculated rate constants from the two methods are less than 10% in acid concentrations under concern. Finally, we incorporated the reaction time (24 h) and the dilution factors during sample preparation to estimate the first-step aldol condensation product (HDE, species 6 in Figure 3) concentrations, whose values are shown as a function of the sulfuric acid concentrations in Figure 4. In Figure 4, we can see that the estimated HDE concentration increases exponentially with increasing sulfuric acid concentration under all conditions. In our bulk experiments, a small amount of HDE was detected under the 54 wt% H2SO4 condition (23.7 min, Figure 1), while the estimated HDE concentration was around 10-5 M in bulk liquid-liquid interactions (Figure 4, black solid line). Under the high octanal concentration (20 ppmv) gas-particle interactions (Figure 4, red dashed line), the sulfuric acid concentration had to be higher than 64 wt% (10%RH or below) to produce HDE with concentrations larger than 10-5 M. This estimation is in good qualitative agreement with our experimental observations, which also showed that HDE was present at 10%RH or below (Figure S4). The low octanal concentration (900 ppbv) gasparticle interactions (Figure 4, blue dotted line) also predicted HDE concentrations at around 10-5 M when the sulfuric acid concentration exceeds 80 wt% (RH < 1%). In our experiments, however, no HDE was observed with such a low octanal concentration, even at RH < 1%. The discrepancy can be explained by the overestimation of the HDE formation under the low octanal concentration (900 ppbv) gas-particle interactions. Alternatively, it is also possible that HDE formation may be underestimated under the bulk and high octanal concentration (20 ppmv) gas-particle interactions, in which case the GC-MS detection limit for HDE would be higher than 10-5 M, the estimated concentrations under these conditions. This higher detection limit cannot be reached under the low octanal concentration (900 ppbv) gas-particle interactions, even at RH < 1% (Figure 4). It is important to point out that the estimated [A]tot (2.0 × 10-4 M at 10%RH) for the 900 ppbv gas-particle interactions are much smaller than the apparent solubility S* (4.7 × 10-3 M at 72 wt% H2SO4)
in bulk liquid-liquid interactions, while [A]tot (4.5 × 10-3 M at 10%RH) for the 20 ppm gas-particle interactions are comparable to S* (Table S2 in the Supporting Information). Hence, [A]tot is limited by the availability of gas-phase octanal for gas-particle interactions. The effective initial uptake of octanal in the 20 ppmv gas-particle experiments may help sustain a further uptake of octanal into the particle phase and approximate the “unlimited” octanal supply as in the bulk experiments. Furthermore, our calculations did not account for the possible enhancement of gas-phase octanal uptake by hydrophobic interactions between octanal and the reaction products. Evidence for this uptake enhancement in the high octanal concentration gas-particle experiments, probably by rapid accumulation of the reaction products, was observed in the levitated single particles experiments using an electrodynamic balance (EDB) (47). On the basis of these unaccounted octanal transport mechanisms, it is possible that our estimations underpredicted HDE formation under the bulk and high octanal concentration (20 ppm) gas-particle interactions.
4. Implications We investigated the acid-catalyzed accretion reactions of octanal in both bulk liquid-liquid and gas-particle experiments. Possible accretion reaction pathways, including cyclotrimerization and one- or multiple-step aldol condensation, were proposed. In the gas-particle experiments, accretion reactions only occurred with high octanal concentrations (∼20 ppmv) and under low RH conditions (53 wt% H2SO4). No accretion reaction products were observed in the low octanal concentration (∼900 ppbv) gas-particle experiments, even at RH < 1% (>80 wt% H2SO4). Kinetic analysis was performed to rationalize the observations from different experimental scenarios, which suggests that the carbonyl concentration is a critical parameter, in addition to acidity, to be considered when extrapolating experimental data to ambient conditions. Several conclusions can be drawn from our experiments: 1) trioxane was formed with medium (∼50 wt%) sulfuric acid concentrations in the bulk experiments but not in the gasparticle experiments with similar acid concentrations; 2) the strong acidity dependence of the first-step aldol condensation product (HDE) formation deserves more attention in future researches; and 3) further steps of aldol condensations happened at high sulfuric acid concentrations in both bulk (>72 wt% H2SO4) and high octanal concentration (∼20 ppmv) gas-particle experiments (80 wt% H2SO4), forming fully dehydrated trimer (aromatic), tetramer, and other partially dehydrated HMW products. In the lower troposphere, gas-phase concentrations of aldehydes or ketones may be relatively high. At such a low altitude, however, it is not common to find aerosols with sulfuric acid concentrations up to 70 wt%. Therefore, aldol condensation may not be very important in those environments. In the upper troposphere, on the other hand, the sulfuric acid concentration can be as high as 60-80 wt% (44), and the temperature is much lower than our experimental temperature. Carbonyl compounds, however, are not expected to be in high concentrations in that region, because of their high reactivity toward photo-oxidation and other reactions. An at most 3-fold increase in the apparent Henry’s law constant (H*) is expected if the temperature drops from 296 to 263 K for octanal (44). Assuming octanal has the same temperature dependence of the aldol condensation rate constant (kaldol) as hexanal, kaldol of octanal at 263 K decreases to 1/10 of that at 295 K (21). Although the squared term of the aldehyde concentration (eq 6, Supporting Information) leads to a close to 10-fold increase in the formation of HDE if H* increases by 3 times, the decrease (to 1/10) of kaldol offsets this effect. Furthermore, a relatively low octanal VOL. 42, NO. 19, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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concentration (∼900 ppbv) did not lead to any accretion reactions in our gas-particle experiments even under extremely low RH conditions (
