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J. Phys. Chem. C 2009, 113, 2120–2127
Heterogeneous Loss of Gas-Phase Ozone on n-Hexane Soot Surfaces: Similar Kinetics to Loss on Other Chemically Unsaturated Solid Surfaces† J. McCabe and J. P. D. Abbatt* Department of Chemistry, UniVersity of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada ReceiVed: July 30, 2008; ReVised Manuscript ReceiVed: September 22, 2008
The heterogeneous loss of ozone on n-hexane soot surfaces has been studied in a coated-wall flow tube connected to a mass spectrometer for gas-phase analysis. Uptake measurements confirm earlier studies that the initial uptake is primarily noncatalytic and that the number of reactive surface sites is close to that of a full monolayer. The initial uptake kinetics exhibit an inverse dependence on ozone gas-phase concentration, as expected in the surface-saturated limit if the reaction proceeds via a Langmuir-Hinshelwood mechanism. Support for this reaction’s not being an Eley-Rideal process comes from the lack of temperature dependence of the initial uptake coefficient from 260 to 360 K and that a saturated surface coverage of adsorbed dodecane does not affect the kinetics. It is demonstrated that there is a strong similarity between the initial uptake kinetics for ozone loss on a wide variety of surfaces, including soot; 1-hexadecene (as studied in this work); metal oxides, including atmospheric mineral dust; and PAHs adsorbed on a variety of surfaces. This suggests that the ozone loss may proceed through a common reaction pathway on such surfaces. Introduction Freshly emitted soot consists of hydrophobic aerosol particles made of elemental and organic carbon. These particles can often comprise a significant portion of the total aerosol population,1 especially in urban areas where diesel engines are abundant. How they affect human health is therefore especially a concern. Soot aerosols have large surface areas and may be adept at importing toxic compounds, such as carcinogenic PAHs, deep into the lungs.2 The roles that soot particles play in climate change are associated with a high degree of uncertainty.3 It is known that these aerosols absorb and scatter radiation by direct effect and alter clouds by indirect effect.3 Hydrophobic soot particles are not good cloud condensation nuclei because they do not effectively take up water. However, aged soot particles that have been oxidized will be more hygroscopic. In the troposphere, soot encounters reactive oxidants, such as the hydroxyl radical, ozone, and nitrate radical. Although several studies examining the oxidation of soot by ozone have been performed,4-13 there are still uncertainties concerning the rate and mechanism at which these particles oxidize. Reactions that take place on the surface of the particle may be generally characterized as following Eley-Rideal or Langmuir-Hinshelwood mechanisms, depending upon whether the oxidant directly reacts with the particle or partitions to the surface before undergoing a surface reaction. Recent studies have suggested that ozone oxidizes soot by the latter, two-step, LangmuirHinshelwood mechanism.10,12,14 This paper has three goals: First, it focuses on well characterizing the fast initial uptake of ozone on n-hexane soot, where n-hexane soot can be conveniently made in the laboratory and has been recommended as a surrogate for atmospheric black carbon.15 Although there is some debate whether ozone is catalytically destroyed during heterogeneous loss at high † Part of the special section “Physical Chemistry of Environmental Interfaces”. * To whom correspondence should be addressed. E-mail: jabbatt@ chem.utoronto.ca.
exposures, we address the initial uptake because such processing will determine the overall surface properties of an aged soot particle and so also its health and climate impacts. Our second goal is to evaluate the reaction in terms of the two ozone loss reaction mechanisms just mentioned. Given the similarity in the ozone kinetics displayed on soot and a variety of other solid surfaces, we also ask the important question of whether a common pathway for ozone loss prevails for these reaction systems. Finally, as the aerosol ages, it also encounters semivolatile and low-volatility compounds which may adsorb to the surface. As a result, the oxidation kinetics of soot aerosol particles may be affected by species partitioned to the surface. For instance, as soon as a diesel soot particle is created in an engine, organic compounds of varying volatilities will condense upon the particle’s surface16 and may compete with atmospheric oxidants for surface sites. The effects these coatings have on the oxidation rate and, therefore, the oxidation lifetime of the particle’s surface are unknown. We have performed preliminary investigations of the effect of dodecane and sulfuric acid coatings on ozone oxidation kinetics. Sulfuric acid is much less volatile than dodecane, but both serve as proxies for compounds that partition to soot aerosols in the troposphere. Experimental The methods of soot generation, specific surface area (SSA) characterization, and measurement of gas-surface uptake were the same as those used in the past.17 Briefly, soot is collected on the inner walls of 2.6-cm-i.d. pyrex cylinders situated above the flame of an alcohol burner containing n-hexane, where the air flow was controlled to generate a sooting flame. Thicker films were produced by lengthening the collection time; the flame conditions were not altered. The soot film appeared somewhat granular and powdery with no visible interstitial spaces exposing the pyrex surface. Prior unpublished SEM work on similar films showed a lot of surface roughness. Chemical analysis of n-hexane films from the same apparatus were analyzed by thermal optical transmission to have an elemental carbon to total carbon ratio of 0.90 ( 0.04.18
10.1021/jp806771q CCC: $40.75 2009 American Chemical Society Published on Web 12/24/2008
Loss of Gas-Phase Ozone on n-Hexane Soot Surfaces
Figure 1. The specific surface area of n-hexane soot films.
SSAs of films were measured using an in situ technique of krypton adsorption at 77 K.17 The SSA of the pyrex tube alone, without soot, was below the detection limit. Whereas other studies report SSAs of soot samples scraped from their reaction surface and assume that they scale with mass, our method measures the SSA of the sample that is accessible to uptake. Buried sites are eliminated from consideration. The SSA of soot was determined to be 17.1 ( 0.6 m2/g for films with masses from 0.1 to 0.4 g, as illustrated in Figure 1. Soot films with masses below 0.1 g were not measured experimentally and SSAs were determined by extrapolation using the soot mass because the relationship was linear. SSA values agreed with previous experiments in our group17 but were significantly smaller than those of other groups investigating heterogeneous soot chemistry.4,7,8 For surfaces such as soot with large SSAs, the accuracy of kinetic measurements and total uptake measurements hinge in part on the accuracy of the samples’ SSA. For this reason, we made a considerable effort to obtain reliable SSA measurements. Oxidation and coating experiments were performed in a flow tube, operated at 0.8-1.0 torr of He carrier gas, that contained the coated pyrex cylinder insert and was coupled to an electron impact quadrupole mass spectrometer, which detected ozone at m/z 48.17,19 We could not detect reaction products that other studies observed,4,5,12 such as CO, CO2, and O2, because the background signals in the mass spectrometer at the masses at which they would have been observed were too high. Ozone was generated and detected prior to injection into the flow tube using a homemade system and Beer’s Law, as described previously.20 On the basis of prior experience, we know that there is negligible loss of ozone along the lines to the flow tube system. A few experiments were also performed on frozen 1-hexadecene films. In particular, pyrex cylinders were coated with liquid 1-hexadecene at room temperature and then placed into the flow tube at 263 K, where the 1-hexadecene froze to form a film that was smooth to the eye. To deliver dodecane to the flow tube for coating experiments, a small, variable flow of He carrier gas (10-200 sccm) became saturated with dodecane by flowing through a room-temperature glass reservoir of dodecane-coated glass beads. The flow then passed through the moveable injector into the flow tube. Dodecane was observed at m/z 57 by the mass spectrometer, and its partial pressure was determined by using dilution ratios and the room temperature vapor pressure of dodecane. For sulfuric acid coating experiments, the moveable injector was modified to deliver sulfuric acid vapor to the soot surface. In particular, a small glass reservoir at the tip of the injector was resistively heated with nichrome wire. We indirectly quantified the amount of sulfuric acid delivered to the flow tube using conductivity analysis. Clean pyrex cylinders were inserted
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Figure 2. Typical uptake profile for ozone oxidation of soot.
TABLE 1: Number of Active Sites for Ozone Oxidation of Soot Surfaces, for This Work and from the Literature Poschl et al.10 Kamm et al.7 Lelievre et al.8 This study
5.6 × 1014 6.5 × 1014 8.0 × 1014 (4 ( 2) × 1014
into the flow tube and coated under typical operating conditions using the moveable injector. In particular, the injector was pulled back over the surface in small increments (1-2 cm), allowing time to coat the section immediately downstream from the injector tip each step. Sulfuric acid was then rinsed from the cylinder and was detected using a calibrated conductivity meter. Ozone uptake experiments were then carried out on sulfuric acid-coated soot films where the sulfuric acid had been deposited using the same conditions as for the clean pyrex tubes. Results and Discussion A typical uptake experiment is shown in Figure 2. The steady state signal indicative of ozone flow through the injector dropped after the injector was pulled back across the soot surface. The signal then recovered toward the steady state signal. When the injector was pushed back across the surface, the signal immediately jumped back to its steady state. No desorption peak was observed. This behavior is similar to that observed in other equivalent experiments.8,9,12 To confirm the pyrex and flow tube surfaces were inert, the injector was pulled back across a bare pyrex surface, and the steady state signal was unaffected. The amount of ozone taken up by the soot surface was calculated from the uptake area below the steady state signal, whereas the initial reaction rate was derived from the drop in signal concurrent with the injector being pulled back. 1. Reactive Surface Sites. Soot films were exposed to ozone sufficiently long that the ozone signal recovered to within at least 85% of its steady state value. Due to drift in the system, it was difficult to run the experiment longer than the time required to do this. The area between the recovery curve and steady state signal was integrated to determine the number of ozone molecules lost on the surface. There was significant variability from film to film ((50%, 1-σ precision), probably arising from different chemical states of the films. Variability within the same film as demonstrated by measuring uptakes on each half of one film (as done by reversing the position of the cylinder in the flow tube) was smaller ((25%). The results are in good agreement with previously reported values,7,8,10,17 as illustrated in Table 1. There has been some debate in the literature whether ozone is catalytically lost in heterogeneous reaction with solid soot particles, with some groups reporting such behavior.7,11 How-
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ever, the experimental approaches used in these studies were more sensitive to the long-term exposure behavior than was ours. At short times, as studied by Knudsen cells and coated-wall flow tubes, the loss of ozone is primarily noncatalytic, with the uptake rate decreasing with exposure. For a catalytic loss, the mass spectrometer signal would remain constant with time after its initial drop rather than recovering toward steady state, as observed in Figure 2. Without addressing whether there is a small degree of loss at long exposures, Table 1 illustrates that the surface has a high density of reactive sites. For our results, if one ozone molecule is lost on each active surface site, then there is an active site every 0.25 nm2. Many reactive surface sites are likely PAHs or PAH-like species, since these compounds are the precursors to the elemental carbon framework of soot. PAHs such as anthracene and benzo[a]pyrene have molecular cross sections of about 1 nm2.10,21 If the fresh soot surface were covered with a complete monolayer of these PAHs then roughly 2-4 molecules of ozone would be lost per PAH molecule. Kwamena et al. has outlined a mechanism in which anthracene could be oxidized to anthraquinone by two ozone molecules.20 A similar process may be taking place on the soot surface, with a variety of specific sites available. This analysis is similar to that of Kamm et al.7 These authors suggested that the soot surface could be modeled as a very large PAH and determined the number of six-membered rings on such a surface would be 1.9 × 1015 cm-2. A total uptake of 5 × 1014 molecules cm-2 indicates that approximately one in three rings take up an ozone molecule. In general then, the soot surface appears to be consuming ozone primarily in a noncatalytic manner. 2. Initial Uptake Coefficient: Soot. The uptake coefficient, γ, describes the ratio of successful collisions removing ozone from the gas phase to total collisions with the surface. Uptake includes both sorption to the surface and chemical reaction. In a flow tube, the magnitude of the drop in signal from steady state to a given level arises from both wall loss due to collision with the surface and resistance arising from diffusion to the surface. For our conditions, radial diffusion amounted to a correction of up to 30% using the same method as Thornberry and Abbatt;22 therefore, the dominant contributor to the observed reaction rate was loss of ozone on soot at the wall. The uptake coefficient, γ, was calculated by
γ)
4V · kw,corr ω · SSA
(1)
where V is the volume of the soot-coated pyrex cylinder, kw,corr is the observed and diffusion-corrected first-order loss of ozone upon pulling the injector back, and ω is the mean molecular speed of ozone. Initial uptake coefficients from 38 different fresh soot films were analyzed. The uptakes were performed over 2 orders of magnitude of ozone gas-phase concentration. There was significant variability in the initial uptake coefficient from film to film and over different concentrations of ozone. The data from the 38 films were reduced into five bins covering different partial pressures of ozone and plotted on a log-log scale (see Figure 3). The large variation in initial uptake coefficients was not related to the mass, shelf life, or length of the film. Some uptakes were performed on either side of the same film. In these instances, the moveable injector was pulled back over half of the film in one experiment. The film was removed from the flow tube, flipped, and reinserted. Then another experiment was performed on the other, fresh half of the film. The initial uptake
Figure 3. Initial uptake coefficients for ozone loss on soot at room temperature as a function of gas-phase ozone concentration. The solid black line has a -1 slope, which reflects the predicted behavior of the Langmuir-Hinshelwood model under saturated surface conditions. Error bars represent 1-σ precision.
coefficient varied by up to about 25% in these cases. This is significantly less than the differences between different films and was likely caused by small variations in the speed at which the injector was pulled back. There were likely differences from film to film in the manner the soot adhered to the pyrex surface and in the combustion conditions of the flame. The inverse dependence of the uptake coefficient on the ozone concentration is consistent with a Langmuir-Hinshelwood mechanism, in which ozone molecules reversibly sorb to the surface before undergoing a reaction. At sufficiently high concentrations, the soot surface becomes saturated with sorbed ozone molecules that had not yet reacted. In this instance, any further collisions between ozone and the surface would be unsuccessful and result in a lowering of the uptake coefficient. Thus, a decrease in initial uptake coefficient with increasing ozone concentration could be commensurate with a LangmuirHinshelwood mechanism on a soot film saturated with sorbed ozone. We observe this behavior, illustrated in Figure 3, at all ozone concentrations tested, down to 7 × 1010 molecules cm-3. For an Eley-Rideal mechanism on an entirely fresh surface, the initial uptake coefficient would be independent of ozone partial pressure. The addition of more ozone-surface collisions would simply result in faster ozone loss, and the proportion of successful collisions would remain the same. For our experimental approach in which uptakes are measured after the finite time it takes to withdraw the injector (1 s at most), it is possible that more of the surface has been oxidized at higher partial pressures of ozone and that the surface has been converted to a more inert form. In this manner, we can not rule out an Eley-Rideal mechanism on the basis of the partial pressure dependence data alone. Our uptake coefficients are generally similar to previous studies (see Table 2). We note that most ozone-soot experiments also used the Brunauer, Emmett, and Teller (BET)isotherm-derived23 surface areas to calculate uptake coefficients (see Table 2), whereas two studies expressed the initial uptake coefficient relative to geometric surface area only.9,13 These groups reported BET surface areas of 460 m2 g-1 and 25 m2 g-1, respectively. Thus, their uptake coefficients relative to the BET surface area would likely fall into the same range as the other studies. Some other groups have also reported that the initial uptake coefficient was dependent upon the ozone concentration. In particular, Stephens et al. detected O2 as a reaction product, indicating oxygen atoms were being deposited on the soot
Loss of Gas-Phase Ozone on n-Hexane Soot Surfaces
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TABLE 2: Values of Initial Uptake Coefficient for Ozone Oxidation of Soot Surfaces, for This Work and from the Literaturea author
method 12
Stephens et al. Fendel et al.6 Rogaski et al.13 Disselkamp et al.5 Longfellow et al.9 Lelievre et al.8 Lelievre et al.8 this experiment
Knudson cell flow chamber Knudson cell aerosol chamber flow tube flow tube flow tube flow tube
γo BET
soot surface ground charcoal spark generated Degussa carbon black Degussa carbon black methane kerosene toluene hexane
γo geo -4
(2-40) × 10 (2.2-33) × 10-4 10-3 (1.8 ( 0.7) × 10-4 (3.8 ( 1.4) × 10-4 (8-100) × 10-5
1 × 10-3 7 × 10-2
a Note that “BET” refers to values calculated using the Brunauer, Emmett, Teller surface area23 of the substrate whereas “geo” refers to those using the geometric surface area.
surface. They also showed that the initial uptake coefficient and initial rate constant decreased with increasing ozone flow rate through a Knudsen cell reactor coupled to a mass spectrometer.12 The increased flow rate was proportional to an increased ozone concentration. Their observations support a mechanism more complex than Eley-Rideal. In particular, Stephens et al. observed a steady state loss in addition to a deactivation process and suggested a separate catalytic oxidation mechanism was taking place in competition with their postulated LangmuirHinshelwood type mechanism. Given that the uptake coefficients deviate slightly from inverse dependence on the ozone partial pressure (Figure 3), this may be the case in our system as well. Fendel et al. measured the change in particle size of spark generated soot aerosols exposed to ozone for 15-30 s.6 They found that soot particles grew in the presence of ozonated air and shrunk in ozone-free air as CO and CO2 were released. The initial uptake coefficient decreased with increasing initial ozone concentrations and were in agreement with Stephens et al. that ozone was adsorbing to the soot surface. Finally, Lelievre et al. presented data (Figure 11 of ref 8) that showed uptake curves involving different initial ozone concentrations normalized to their steady state signal. The initial drop in signal was smaller for each trial with increasing ozone concentration. Assuming that other variables, such as flow tube pressure and flow rates, were the same in each trial, the drop in signal must be proportional to the initial uptake coefficient. It can therefore be qualitatively concluded that the initial uptake coefficients decreased with increasing ozone concentration in their study, as well. By contrast, several studies have analyzed the heterogeneous reaction between ozone and soot but have not reported that the initial uptake coefficient varies with ozone concentration, perhaps because the ozone concentration was not changed significantly so that any effects were lost in the inherent variability of the experiment.5,7,9,13 Although this was not the focus of our study, we also note that some groups observed a small, steady state uptake coefficient that occurred in addition to the initial rapid uptake of ozone onto the film.5,7,12 It is possible that there was a small, steady state reaction which our flow tube system was not able to accurately quantify. 3. Temperature Dependence. The flow tube apparatus was jacketed for circulation of temperature-controlled fluid. This enabled us to perform uptakes at 260 and 360 K to see how the heterogeneous ozone-soot reaction was affected. Through 31 experiments, the initial uptake coefficients were found to have no temperature dependence over the experimental range within experimental uncertainties (see Figure 4). The data displayed the same trend in decreasing initial uptake coefficient with increasing ozone concentration. Under a simple Eley-Rideal mechanism in which there is no strong steric hindrance in the reaction coordinate, the initial
Figure 4. Initial uptake coefficients for ozone loss on soot as a function of temperature and gas-phase ozone concentration. The solid black line reflects the predicted behavior of the Langmuir-Hinshelwood model for an ozone-saturated surface. Error bars represent 1-σ precision.
uptake coefficient would be expected to increase with temperature. At higher temperatures, more ozone molecules would have sufficient energy to overcome the reaction’s activation energy barrier that we know must be present, given that the uptake coefficient is significantly smaller than unity. On the other hand, a Langmuir-Hinshelwood mechanism could have a more complex temperature relationship. At lower temperatures, more ozone molecules would partition to the surface, but on the surface, fewer ozone molecules would have sufficient activation energy to undergo surface reaction. Thus, a LangmuirHinshelwood mechanism may not display a positive temperature dependence with its initial uptake coefficient. The Arrhenius equation relates temperature (T), activation energy (Ea), a pre-exponential factor (A), and the rate constant (k) by Ea
k ) Ae- RT
(2)
For Eley-Rideal kinetics, if we assume the uptake coefficient is directly proportional to the rate constant and that all unsuccessful collisions were the result of the failure of ozone molecules to overcome the activation energy barrier, then the activation energy for a given uptake coefficient can be calculated. This assumes that the A factor has a negligible temperature dependence. At 260 K, an initial uptake coefficient of 1 × 10-4 yields an activation energy of 20 KJ mol-1. With this activation energy barrier set, a temperature of 360 K would be expected to increase the uptake coefficient to 1.3 × 10-3. This increase was not observed in our data. Although this is a rather simple analysis that excludes the possibility that steric factors could control an Eley-Rideal mechanism (i.e., set a small A factor), the lack of temperature-dependent results lends support to a multistep mechanism such as Langmuir-Hinshelwood and is
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Figure 5. Initial uptake coefficients for ozone loss on soot at room temperature and on frozen 1-hexadecene surfaces at 263 K. The solid black line reflects the predicted behavior of the Langmuir-Hinshelwood model for an ozone-saturated surface. Error bars represent 1-σ precision.
in agreement with previous research.8 There may be a temperature dependence associated with the longer, slower reaction,5,7,9 but this was beyond the scope of the present study. 4. Initial Uptake Kinetics: Other Surfaces. To study kinetics similar to those between ozone and soot and yet with a surface that could be formed with reproducible chemical composition, we decided to perform similar ozone uptake experiments on a solid 1-hexadecene surface. 1-Hexadecene is a 16-carbon terminal alkene that is a liquid at room temperature and freezes at 277 K.24 The single unsaturated bond is the only reactive site with ozone and will react with only one ozone molecule. Since the 1-hexadecene surface was less complex than the soot surface and because it has been previously studied for its heterogeneous reactivity,24 it was chosen as a simple proxy for a surface with unsaturated carbon-carbon bonds that could be prepared reproducibly from run to run. The data from uptake experiments on this surface were analyzed using the geometric surface area of the pyrex cylinders to calculate initial uptake coefficients. In particular, frozen water surfaces have specific surface areas equal to their geometric area.25 Although we do not know that frozen 1-hexadecene films are smooth at the molecular level, we note that there is only a small (15%) increase in density upon freezing,26 and so we expect that this surface would not be highly fractal and porous like soot. For 1-hexadecene, the total uptakes were quite small, with changes in the ozone signal of only 15% occurring for just a few seconds after the injector was pulled out. Since the surface was relatively smooth, there were far fewer active sites than those on the soot surfaces. It is therefore not surprising that small total uptakes were observed. This small uptake also gives rise to scatter in the calculated uptake coefficients. Nevertheless, once plotted versus ozone partial pressures, the binned and averaged values decreased with increasing ozone concentration (see Figure 5). For reasons not entirely clear, our results are not in agreement with Moise and Rudich, who measured smaller uptake coefficients of (2.5 ( 0.4) × 10-5 and found them to be relatively constant over ozone concentrations of 1 × 109 to 1 × 1011 molecules cm-3.24 When plotted together, the soot and 1-hexadecene data indicate similar ozone loss kinetics. In particular, the initial uptake coefficients calculated using vastly different surface areas reveal, within the experimental uncertainties, the same values at the same ozone concentrations. This suggests a common pathway for ozone loss involving ozone alone in the rate-
McCabe and Abbatt determining step, as opposed to involvement of specific features of the surface, such as the chemical composition of active sites. To take this argument further, we note that initial ozone-loss uptake coefficients that show a partial pressure dependence on ozone partial pressure have been reported on other solid substrates, from experiments conducted by either watching the disappearance of gas-phase ozone or the loss of a surface species that is reactive with ozone. Starting with the studies reporting gas-phase loss, using the same static absorption cell method, Sullivan et al. measured initial uptake coefficients of ozone on alumina, and Chang et al. studied loss on Saharan dust.27,28 In both cases, the same trend of decreasing initial uptake coefficient with increasing ozone concentration was reported. Hanisch and Crowley used a Knudsen cell to measure initial uptake coefficients of ozone on Saharan dust29 and reported LangmuirHinshelwood-like kinetics. All of these substrates were vastly different from soot, with active sites composed of metal oxides, such as SiO2, Al2O3, and FeO. Several heterogeneous oxidation studies involving ozone in which the loss of surface species was observed have been completed. Poschl et al. examined the loss of benzo[a]pyrene (BaP)-coated soot particles exposed to ozone.10 The BaP-loss rate constants increased with increasing ozone exposure until saturating beyond 1 × 1013 molecules/cm3 of ozone, indicating a Langmuir-Hinshelwood mechanism with surface saturation. The initial uptake coefficient was calculated by
γ)
4k1 σω[O3]
(3)
where σ is the cross section of a surface molecule and k1 is the rate constant for its loss. At ozone concentrations where the rate constant is maximized, the initial uptake coefficient decreases with increasing ozone concentration because all other variables are fixed. Other groups have followed the analysis framework of Poschl et al. to conduct similar experiments on different substrates, and all report kinetic behavior consistent with Langmuir-Hinshelwood kinetics. These include Kwamena et al., who analyzed PAH loss on different organic substrates.30,31 Dubowski et al. measured the loss of alkene functional groups concurrent with the creation of carbonyl groups using attenuated total reflectance FTIR.32 They used self-assembled monolayers with alkene functional groups as a proxy for organic species adsorbed to an aerosol surface. Donaldson and co-workers spectroscopically examined the loss of surface-bound PAHs by ozone oxidation on the air-aqueous interface33 and on organic films.34,35 McNeill et al. used an aerosol flow tube to detect the loss of surface-bound oleate using a chemical ionization mass spectrometer.36 Saturation-like behavior in the observed firstorder rate constant indicated a possible Langmuir-Hinshelwood mechanism. Figure 6 shows the combined results from all these literature reports, along with the results from this paper for soot and 1-hexadecene. For the studies conducted by measuring loss of surface species, we have plotted the calculated uptake coefficients over an order-of-magnitude span in the region of ozone saturation. The similarity of the uptake coefficients is remarkable both in their absolute magnitude and in their partial pressure dependence, especially given the very different experimental techniques used and the extremely wide range of surface-phase reactants and reaction substrate types. Initial uptake coefficients appear to follow the same trend whether the loss of ozone is measured or the loss of surface bound species. Experiments measuring the loss of ozone on solid surfaces typically involved lower ozone concentrations, whereas those measuring surface
Loss of Gas-Phase Ozone on n-Hexane Soot Surfaces
Figure 6. Initial uptake coefficients for ozone loss on a variety of surfaces (all at room temperature except for the 263 K results for 1-hexadecene): black triangles, this study, 1-hexadecene; blue triangles, this study, n-hexane soot; pink squares, Sullivan et al.,27 alumina; green circles, Hanisch and Crowley,29 Saharan dust; cyan diamonds, Chang et al.,28 Saharan dust; green solid line, Dubowski et al.,32 octene SAM; green dots, Dubowski et al.,32 allyl SAM; blue solid line, Kahan et al.,34 anthracene on octanol; blue dots, Mmereki and Donaldson,33 anthracene on water; blue dashes, Clifford et al.,35 chlorophyll on water; red solid line, Kwamena et al.,30 benzo[a]pyrene on azelaic acid; red dots, Kwamena et al.,31 anthracene on azelaic acid; red dashes, Kwamena et al.,31 anthracene on diffusion pump oil; orange solid line, McNeill et al.,36 oleate on NaCl; cyan solid line, Poschl et al.,10 benzo[a]pyrene on soot. The solid black line reflects the predicted behavior of the Langmuir-Hinshelwood model for an ozone-saturated surface.
species loss are done at higher ozone. The two methods complement each other and enable kinetic information to be obtained over a much larger range. 5. Surface Coating Experiments. Coating experiments were conducted to determine whether the presence of adsorbed molecules would affect the initial uptake kinetics. The general procedure was to conduct separate ozone uptake experiments on both halves of the same soot films, one with an adsorbed surface layer and one without, but with the same ozone concentration. Dodecane was chosen as a semivolatile species that might partition to the surface of diesel soot particles immediately upon emission. Although somewhat smaller than some of the species present in true diesel emissions, dodecane was selected for the study because it was volatile enough to easily evaporate in our system and yet sufficiently involatile that it partitioned significantly to the surface at flow tube gasphase concentrations. Total uptake measurements onto fresh soot surfaces were performed with different concentrations of dodecane in the flow tube, using an approach described in ref 37 for the uptake of aromatics onto the soot surfaces. All uptakes of dodecane were reversible, and as the concentration increased, the total amount sorbed to the soot surface at equilibrium increased in a nonlinear fashion so that the soot surface became saturated at high dodecane concentrations. The uptakes were well-modeled by a Langmuir isotherm, and the surface was found to have a saturated surface capacity of about 1 × 1014 molecules/cm2, as shown in Figure 7. As shown in Figure 8, a saturated surface coating of dodecane does not affect the oxidation kinetics with ozone. In particular, the data in this figure were obtained by first measuring an ozone uptake on one-half of an uncoated film. Then, the pyrex tube was reversed, a dodecane flow was established at a concentration of 1.0 × 1013 molecules/cm3 for a sufficiently long time that steady state will have been reached, and then an oxidation experiment was repeated. This experiment was repeated five
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Figure 7. Uptakes of dodecane as a function of dodecane gas-phase concentration at room temperature. Saturated surface coverage for the Langmuir fit (shown as a solid line) is 1 × 1014 molecules/cm2.
Figure 8. Uptake profiles for ozone loss on a bare soot surface and one saturated with dodecane (gas-phase concentration of 1.0 × 1013 molecules/cm3) at room temperature.
times, and in each case, the kinetics on the coated and uncoated films were the same within the precision of the experiment. It was concluded that unreactive semivolatile compounds partitioned to a soot aerosol surface at monolayer amounts do not affect the rate of ozone loss. The dodecane was able to adsorb onto the surface but may have had a weaker attraction than that of ozone. Thus, although dodecane was in rapid dynamic adsorption/desorption equilibrium with active adsorption sites on the soot surface, ozone molecules were able to see reactive sites as if dodecane was not there. Note that these results are strong evidence against the Eley-Rideal mechanism given that the saturated surface coverage of 1 × 1014 molecules/cm2 represents a large fraction of a monolayer, given the large size of the dodecane molecule. An ozone molecule would not react with dodecane, and so one would expect the initial uptake coefficient to be smaller under these coating conditions, should the reaction follow an Eley-Rideal mechanism. Although our experimental conditions were limited to about one monolayer coverage of dodecane, it is possible that under certain conditions in the atmosphere that the partial pressures of semivolatile alkanes are much greater and that adsorption is pushed into the multilayer regime. Consequently, a liquid film may form on the surface and have a more substantial effect on the kinetics. Sulfuric acid was selected as a second coating substance, given that it is ubiquitous in the troposphere, much less volatile than dodecane and not reactive with ozone. Using the same approach as with dodecane, uptake experiments were conducted on different sides of the same soot films. Suppression in the uptake was observed when the sulfuric acid coating was approximated to be 1 monolayer and was almost exclusive to
2126 J. Phys. Chem. C, Vol. 113, No. 6, 2009 the fast initial reaction in the first few minutes of ozone exposure. Beyond the initial stages, the uptakes with and without sulfuric acid coatings appeared to converge and follow the same slow recovery toward steady state. We did not know whether the sulfuric acid from the injector evenly coated the surface or was immobile after condensation. The molecules may have congregated together and formed tiny pools on the hydrophobic surface, which would have further enabled ozone molecules to access the vast majority of reactive surface sites. Follow-up experiments with sulfuric acid, preferably with particulates, would be desirable to subsantiate these qualitative observations. In the atmosphere, soot aerosol particles interact with both reactive molecules, such as ozone, and unreactive molecules, such as dodecane and sulfuric acid. When particles have up to a monolayer of semivolatile organics partitioned to their surfaces, ozone is still able to efficiently oxidize the surface and transform the particle from hydrophobic to hygroscopic. Less volatile compounds may provide more competition for active sites on the surface of soot. Nevertheless, ozone is still able to oxidize the surface with significant amounts of lowvolatility species present. 6. Pathway for Heterogeneous Ozone Loss. Although no one experimental measurement can definitively establish a reaction mechanism, the pathway for ozone loss on soot most consistent with the suite of measurements performed here is a multistep process in which ozone first absorbs and then reacts to oxidize the surface. In particular, this mechanism is consistent with the inverse relationship between the uptake coefficient and the ozone partial pressure, the lack of a positive temperature dependence, and with the observation that a saturated surface coverage of dodecane has no impact on the initial uptake coefficient. We note that we do not have information on the nature of the heterogeneous oxidation, nor of the products that result from the ozone exposure, so we focus only on the pathway leading to ozone loss. In addition, we find considerable similarity in the reported kinetics for ozone heterogeneous chemistry involving not only soot; solid 1-hexadecene; and metal oxides, including atmospheric mineral dust; but also PAHs coated to soot, organic, and water substrates. This concurrence of kinetics results arises from a range of experimental techniques conducted over short timescales (Knudsen cells, flow tubes, static absorption cells) to longer-term experiments (aerosol flow tubes, spectroscopic monitoring of thin films). This similarity is very difficult to rationalize with an Eley-Rideal mechanism, given the wide range of reactants. For a Langmuir-Hinshelwood mechanism, we believe these data support there being a common barrier along the reaction coordinate that does not involve the different substrates/actives sites that are involved later in the reaction. Instead, the barrier is likely related to the ozone molecule alone. A first possibility, suggested in ref 31, is that the barrier to heterogeneous ozone oxidation is surface diffusion. Once on the surface, an ozone molecule must find a reactive surface site. Adsorbed ozone molecules may travel across a particle’s surface much more slowly than they react on the surface once a suitable reaction site is found. Thus, different types of reactive sites, such as alkenes and PAHs, do not alter the overall reaction rate. A counter-argument against surface diffusion being rate-limiting is that the dodecane coating had no effect on the kinetics. A second possibility is that once ozone is partitioned to the surface, multiple reaction steps take place before a site is oxidized. The rate-limiting step of this process and the overall reaction would involve ozone only. For example, a process involving some component of charge transfer could occur
McCabe and Abbatt whereby adsorbed ozone molecules obtain an electron to form a negatively charged O3- ion. Once the ion is formed, the reaction with a surface site could be significantly faster. Support for this comes from Nelander and Nord, who observed charge transfer complexes between ozone and benzene.38 Alternatively, dissociation of ozone into a surface-bound oxygen molecule and atomic oxygen may take place, as supported by the observation of gas-phase O2 as a product.12,27 The energy barrier in creating the reactive atomic oxygen would be much higher to overcome than the subsequent reaction between the radical and a surface site. Our experiments do not allow us to distinguish between these possibilities, and so further studies are needed to better determine the rate-limiting step of heterogeneous ozone loss, as well as further measurements of the products formed. We note also that attention needs to be paid to the number of surface sites that might be available for reaction, since they also affect the uptake coefficient,
γ)
4k2[SS] σω[O3]
(4)
where the first-order surface-phase rate constant is expressed as a second-order rate constant (k2) multiplied by the number of sites ([SS]). With accounting for the number of surface sites some of the variability in the uptake coefficients in Figure 6 for a specific ozone concentration may be reduced. As an example, the number of reactive surface sites were measured to be 1.4 × 1014 cm-2 and 2 × 1013 cm-2 in the work of Sullivan et al. and Chang et al. for the loss of ozone on alumina and Saharan dust, respectively.27,28 The measured uptake coefficients for loss on alumina are about an order of magnitude larger than those on Saharan dust, perhaps because the number of surface sites differs by about the same factor. Conclusions and Atmospheric Implications From an atmospheric perspective, with the initial uptake coefficient and concentration of reactive surface sites experimentally determined for n-hexane soot, we can now estimate the lifetime of such a particle freshly emitted into the urban troposphere. The lifetime describes the amount of time it would take for a particle to be completely oxidized, by ozone only, under the initial fast kinetic regime. In an urban area, ozone concentrations are frequently as high as 50 ppb. This corresponds to an initial uptake coefficient of about 2 × 10-4. The reciprocal of the concentration of reactive surface sites generates an effective surface site area, σ ) 2.5 × 10-15 cm2. Therefore, the first order rate loss constant can be estimated,10
kI )
γoσω[O3] ) 0.00581 s-1 4
(6)
where ω is the molecular speed of ozone. From the rate constant, the lifetime for oxidation of the particle surface is only a few minutes:
τ)
1 ) 172 s kI
(7)
This simple calculation suggests that the surfaces of soot particles of this type emitted into the urban troposphere, in the absence of any coating materials, are rapidly oxidized. In comparison, oxidation by hydroxyl radicals could have the maximum possible initial uptake coefficient of 1. In an urban area, the concentration of these radicals may be as high as 107 cm-3, and so the estimated lifetime can be estimated similar to
Loss of Gas-Phase Ozone on n-Hexane Soot Surfaces eqs 6 and 7 to be about 1 h. Accordingly, ozone appears to be the most important oxidant to soot aerosols under these conditions. Although straightforward, to our knowledge, these timescales have not been highlighted previously. Other studies have reported the expected impact of soot aerosols on ambient ozone concentrations is negligible5,7-9,11 but we point out that ambient ozone rapidly oxidizes the surface of soot particles. In addition, if the coatings are only a monolayer thick of semivolatile materials, such as dodecane, then this lifetime estimate probably remains valid. However, if they are thicker and made of less volatile material, then the lifetimes could be lengthened. Acknowledgment. The authors acknowledge financial support from CFCAS and NSERC. References and Notes (1) Seinfeld, J.; Pandis, S. Atmospheric Chemistry and Physics; John Wiley & Sons, Inc: New York, 1998. (2) Heyder, J.; Gebhart, J.; Rudolf, G.; Schiller, C. F.; Stahlofen, W. J. Aerosol Sci. 1986, 17, 811. (3) IPCC “IPCC Fourth Assessment Report: Working Group I Report The Physical Science Basis”, 2007. (4) Chughtai, A.; Kim, J. M.; Smith, D. J. Atmos. Chem. 2003, 45, 231. (5) Disselkamp, R. S.; Carpenter, M. A.; Cowin, J. P.; Berkowitz, C. M.; Chapman, E. G.; Zaveri, R. A.; Laulainen, N. S. J. Geophys. Res. 2000, 105, 9767. (6) Fendel, W.; Matter, D.; Burtscher, M. H.; Schmidt-Ott, A. Atmos. EnViron. 1995, 29, 967. (7) Kamm, S.; Mohler, O.; Naumann, K.-H.; Saathoff, H.; Schurath, U. Atmos. EnViron. 1999, 33, 4651. (8) Lelievre, S.; Bedjanian, Y.; Pouvesle, N.; Delfau, J.; Vovelle, C.; Le Bras, G. Phys. Chem. Chem. Phys. 2004, 6, 1181. (9) Longfellow, C. A.; Ravishankara, A. R.; Hanson, D. R. J. Geophys. Res. 2000, 105, 24345. (10) Poschl, U.; Letzel, T.; Schauer, C.; Niessner, R. J. Phys. Chem. A 2001, 105, 4029. (11) Smith, D.; Chughtai, A. J. Geophys. Res. 1996, 101, 19607. (12) Stephens, S.; Rossi, M.; Golden, D. M. Int. J. Chem. Kinet. 1986, 18, 1133. (13) Rogaski, C. A.; Golden, D. M.; Williams, L. R. Geophys. Res. Lett. 1997, 24, 381.
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