Experimental and theoretical study of hydrocarbon photochemistry applied to Titan stratosphere

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1 Icarus 185 (2006) Experimental and theoretical study of hydrocarbon photochemistry applied to Titan stratosphere V. Vuitton a,b,, J.-F. Doussin b,y.bénilan b,f.raulin b, M.-C. Gazeau b a Lunar & Planetary Laboratory (LPL), University of Arizona, 1629 E. University Blvd., Tucson, AZ , USA b Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA), Université Paris 12 Val de Marne, 61 Ave. du Général de Gaulle, Créteil Cedex 94010, France Received 6 October 2005; revised 8 June 2006 Available online 2 August 2006 Abstract None of the Titan photochemical models currently available have been able to reproduce the full set of stratospheric molecular mixing ratios inferred from observations. In order to assess how well reaction sets describe hydrocarbon chemistry, theoretical modeling predictions were compared to the results of a laboratory experiment. A CH 4 C 2 H 2 mixture was irradiated at 185 nm in an atmospheric simulation chamber and the evolution of the gas mixture was followed in situ and in real time by infrared spectroscopy. In parallel, a 0D theoretical model of the laboratory experiment was developed. A new reaction set describing Titan s chemistry was built and incorporated in the model. Lebonnois et al. [Lebonnois, S., Toublanc, D., Hourdin, F., Rannou, P., Icarus 152, ] reaction set was also used for comparison. The presence of small amounts of atmospheric O 2 in the experiment was properly accounted for and led us to suggest that oxygenated chemistry might be a source of C 2 H 4 in Titan s atmosphere. With Lebonnois et al. [Lebonnois, S., Toublanc, D., Hourdin, F., Rannou, P., Icarus 152, ] reaction set, the model could not fit at all the experimental evolution of the compounds. This is explained by some of the choices made for crucial kinetic parameters such as the quantum yield of photolysis of C 2 H 2. Also, the absence of some reactions led to the enhancement of pathways that would otherwise be negligible. For example, the lack of reactions between C 4 H 4 and radicals induced an erroneously high photolysis rate for this species. With the reaction set built in this study, the model much better fits the experiment, especially when the soot, which includes C 4 H 4, is recycled into C 2 H 2. This shows that photochemistry of the larger species has a role in determining the lighter species concentrations and that considering that they are simply lost from the system is not a valid assumption. Including even an abridged set of C 4 + hydrocarbon reactions will be required in future photochemical models. Especially, photolysis rates and yields for C 2 H 2,C 4 H 2,andC 4 H 4, are important parameters in need of a better determination Elsevier Inc. All rights reserved. Keywords: Photochemistry; Titan; Experimental techniques; Atmospheres, composition 1. Introduction Titan has an extended (from 1.5 bar at the surface down to bar at 1400 km) and cold (from 70 K at the tropopause to 180 K at the stratopause) atmosphere. Its principal constituents are N 2, a few percent of CH 4 and per mil of H 2. High-energy electrons from Saturn s magnetosphere, UV photons from solar radiation and cosmic rays are deposited throughout the atmosphere. The minor constituents resulting from the action of * Corresponding author. Fax: + 1 (520) address: vvuitton@lpl.arizona.edu (V. Vuitton). these energy sources on N 2 and CH 4 include hydrocarbons and nitriles. The most abundant, C 2 H 6,C 2 H 2,C 2 H 4, and HCN, are in the order of the ppm in the stratosphere (Coustenis et al., 2003). The main oxygenated species, CO, is at the 10 ppm level and is possibly of cometary or meteoritic origin. Several haze layers have been observed at about 550 km and below. They probably result from the polymerization of unsaturated gaseous species such as HC 3 N, C 4 H 2,orC 6 H 6 (Lebonnois et al., 2002; Wilson and Atreya, 2003), which have been detected at the ppb level. Then Titan is of interest because it is one of the places where the most complex atmospheric organic chemistry is taking place at present in the Solar System /$ see front matter 2006 Elsevier Inc. All rights reserved. doi: /j.icarus

2 288 V. Vuitton et al. / Icarus 185 (2006) Yung et al. (1984) published the first comprehensive photochemical model of Titan s atmosphere. The model describes the chemical and physical processes taking place in the atmosphere, and computes the vertical profiles of the chemical compounds. This model shows that while N 2 and CH 4 are readily dissociated in the upper atmosphere by electrons and short-wavelength photons, producing HCN, C 2 H 4, and C 2 H 2, the bulk of the CH 4 dissociation occurs indirectly in the stratosphere. There, long-wavelength photons (λ<235 nm) dissociate C 2 H 2, forming C 2 H radicals that can abstract an H atom from CH 4 as well as regenerate C 2 H 2 (catalytic dissociation). The CH 3 radicals produced react with each other and with other radicals to form heavier species such as C 2 H 6. Even though Yung et al. (1984) provide a full description of the chemical processes, the choice made for the eddy diffusion coefficient does not allow the model to fit correctly the observational data (Coustenis, 1990). Other models (Lara et al., 1996; Lebonnois et al., 2001; Toublanc et al., 1995; Wilson and Atreya, 2004) were subsequently developed with improved chemistry and physics (energy deposition, radiative transfer, condensation, transport, etc.) but none of them has been able to fully reproduce the available set of observations [see Fig. 1 in Romanzin et al. (2005)]. This drawback can be explained in part by the fact that it is very difficult to model all the physical processes influencing the distribution of chemical constituents. Moreover, all the models but one published to date (Lebonnois et al., 2001) are 1D models in which dynamical processes are parameterized as a vertical eddy diffusion. This mixing coefficient has to be inferred as a function of altitude in order to reproduce the observations and is a major source of uncertainties in the models. The only way to avoid this parameterization is to develop higher dimensional models, in which transport is accounted for explicitly, as in Lebonnois et al. (2001). Another major source of uncertainties in photochemical models comes from the set of chemical reactions describing the chemical transformations of the gaseous species. Such a model includes tens of compounds and hundreds of reactions, for which rate constants and reaction products have to be retrieved from the literature. Some of these parameters are unknown or highly uncertain at the low temperatures of Titan s atmosphere. Hébrard et al. (2005) showed that imprecision carried by the kinetic parameters introduces significant uncertainties in computed mole fractions of chemical species. The purpose of the work recently undertaken at LISA (Laboratoire Interuniversitaire des Systèmes Atmosphériques) was to assess how well reaction sets currently available describe hydrocarbon chemistry, by coupling theoretical modeling with laboratory experiments. The main advantage of this new approach is that the composition of a gas mixture can be constrained much better in the laboratory than in a planetary atmosphere, where observations are scarce. Moreover, the system can be modeled without having to take into account all the complex physical processes occurring in a real environment. Our goal was not to develop a laboratory simulation reproducing the whole Titan s atmospheric chemistry but rather to perform a simple experiment dedicated to the study of a particular mechanism. C 2 H 2 is produced in the higher atmosphere and subsequently induces the dissociation of CH 4 in the lower stratosphere. Since this catalytic cycle initiates the bulk of the formation of the other species, we decided to focus on this process. Smith et al. (1999) first performed experiments in a small reaction cell: length 50 cm, diameter 6 cm. They irradiated 100 Torr of CH 4 C 2 H 2 (1000:1) at 185 nm and analyzed the reaction mixture by gas chromatography. Then they compared the experimental evolution of the species with the prediction ofa0dmodelusingreactionsetspreviouslypublished (Lara et al., 1996; Toublanc et al., 1995; Yung et al., 1984). They were able to validate experimentally the catalytic dissociation of CH 4 by C 2 H 2 postulated in photochemical models. However, they showed that heterogeneous processes on the walls of the cell (surface/volume ratio of 65 m 1 ) were perturbing the gas phase chemistry. Moreover, the gas mixture could not be analyzed in situ and the evolution of the species with time had to be retrieved from independent experiments obtained after different irradiation times, increasing the scatter of the data points. In order to improve this previous experiment, we irradiated the same CH 4 C 2 H 2 mixture in a large atmospheric simulation chamber in order to reduce the wall effects. Moreover, we followed in situ the evolution of the gas mixture in real time by infrared spectroscopy. In parallel, a 0D theoretical model of this new laboratory experiment was developed. We incorporated in the model our own reaction set describing Titan s chemistry (subsequently referred to as model V) and compared the theoretical predictions to the experimental results. We also used Lebonnois et al. (2001) reaction set for comparison (subsequently referred to as model L). We specifically focused on C 4 H 2 chemistry and highlighted the important mechanisms involving this potential intermediate in the haze formation. 2. Experimental methods 2.1. Atmospheric simulation chamber The laboratory experiments were performed in an atmospheric simulation chamber. The experimental and analytical devices were described previously (Doussin et al., 1997, 1999). The reactor is a 6 m long/45 cm diameter Pyrex cylinder with a surface/volume ratio of 9 m 1. This small ratio minimizes the importance of the heterogeneous reactions susceptible to happen on the walls of the reactor. Two turbo molecular pumps provide an oil-free system and vacuum in the range of the µbar. The irradiation system consists of 32 Hamamatsu low-pressure mercury lamps of 13 cm length uniformly dispatched inside the reactor, with principal emissions at 185 and 254 nm. The flux of each lamp at 254 nm is 80 µw cm 2 at 30 cm, according to the manufacturer s specifications. A Bomem FTIR spectrometer ( cm 1 ) is coupled to the reactor in order to follow in situ the evolution of the reaction mixture in real time. The infrared beam is introduced in a White cell, increasing the optical pathlength and consequently decreasing considerably the detection limit of molecular species. The reaction products were identified and quantified

3 Hydrocarbon photochemistry in Titan stratosphere 289 Fig. 1. Evolution of the concentrations (cm 3 )ofc 2 H 2 (open circles) and CH 4 (stars). τ i indicates the instants at which the injections were made. The concentrations introduced are: τ 0 : [C 2 H 2 ]= cm 3, [CH 4 ]= cm 3 ; τ 1 : [C 2 H 2 ]= cm 3 ; τ 2 : [C 2 H 2 ]= cm 3. by comparison with reference spectra of the pure compounds previously obtained with the same apparatus. A software developed in our laboratory was used to automatically calculate the variation of the concentrations of the compounds with time. We used a resolution of 0.5 cm 1, an optical pathlength of 180 m, a Hamming apodization function and a sampling time step of 15 min Experimental conditions An atmospheric simulation chamber is mainly dedicated to the study of the Earth troposphere and it is usually filled with synthetic air (N 2 O 2 (0.8:0.2)). O-bearing compounds being extremely reactive, a specific protocol was used in order to remove the species potentially adsorbed on the reactor walls and the residual O 2. This involved irradiating the reactor under vacuum for about a day and flushing its content with N 2 (Air Liquide, Research grade, %) for a few hours. The experiments were performed at a pressure of 1200 ± 20 mbar and a temperature of 297 ± 2 K. The pressure was higher than that in Titan s stratosphere ( mbar) in order to obtain enough products to be detected by the spectrometer. It was higher than atmospheric pressure to reduce the diffusion of atmospheric O 2 in the reactor. Also, experimental constraints did not allow us to work at Titan s stratospheric temperature ( 175 K). The gas mixture had Titan mixing ratios of N 2 and CH 4 (0.98 and 0.02) and varying amounts of C 2 H 2. The mixing ratio of C 2 H 2 was greater than that on Titan in order to increase the destruction of CH 4. Three injections were made over the course of the experiment. First, before the lamps were switched on, about 10 mbar of a CH 4 C 2 H 2 (1000:1) mixture (Linde, Research grade, %) were introduced in the reactor (τ 0 ). After 12 h of irradiation, 99.5% of C 2 H 2 had been destroyed. In order to enhance the products formation and to study the influence of the C 2 H 2 /CH 4 ratio on the chemistry, 10 1 mbar of C 2 H 2 (AGA, Research grade, 99.99%) was added to the gas mixture (τ 1 ),increasing the previous C 2 H 2 /CH 4 ratio by a factor of 10. Finally, after 44 h of irradiation, 10 2 mbar of C 2 H 2 were injected to perform another simulation with a CH 4 C 2 H 2 ratio of 1000:1 (τ 2 ). The time variations of C 2 H 2 and CH 4 inside the reactor are presented in Fig Theoretical model 3.1. Model description The model solves the continuity equation for the concentration of species i: dc i = P i L i C i, dt where C i is the concentration of species i (cm 3 ) and P i and L i (cm 3 s 1 ), the chemical production and loss rate, respectively. P i and L i depend on the thermal reaction and photolysis rates for each reaction present in the photochemical scheme. For a reaction between species i and j, the thermal reaction rate is defined as: dc i = k ij C i C j, dt where k ij is the bimolecular rate constant (cm 3 s 1 )orthe reaction between species i and j. The photolysis rate of species i, is described as: dc i = J i C i, dt

4 290 V. Vuitton et al. / Icarus 185 (2006) where J i is the photodissociation coefficient for species i (s 1 ). In our laboratory experiment, because of the disposition of the lamps, the gas is not uniformly irradiated in different parts of the reactor. Consequently, we have to calculate an effective value of J i to be used in our 0D model, as follows: J i = Φ i I 0 (1 exp( σ i C i L)) C i V σ i C i, σi C i where Φ i and σ i are the total primary quantum yield of dissociation and the absorption cross section (cm 2 ) of species i respectively, I 0 is the total number of photons emitted by the lamps (s 1 ), L is the effective optical path length and V is the volume of the reactor (cm 3 ). The computation of this equation was detailed previously in Smith and Raulin (1999). The coupled differential equation system of the box model was solved using the Facsimile computer program (Curtis, 1979) Chemical mechanisms The reaction set was built from a review of the available literature. Hydrocarbons (stable molecules and radicals) containing up to 3 carbon atoms and C 4 H, C 4 H 2, and C 4 H 3 are included. Any heavier molecule produced is being removed in the form of soot and no longer participates in the chemistry. We did not try to include a set of larger chemistry, as some other authors have done, for 2 reasons. First, there are very few kinetic data available in the literature when it comes to species having more than 3 carbon atoms. One can still evaluate rate constants and products from similar reactions or extrapolate data obtained in combustion conditions to lower temperatures but this is a very uncertain procedure. The second reason is that the goal here is to test the photochemical scheme by comparison with the laboratory results. Since none of these heavier species were detected in the experiment (see Section 5.1.1), no constrain was available for these reactions. The model includes the absorption of all the unsaturated species at 185 nm. Photolysis of the alkanes (CH 4,C 2 H 6, and C 3 H 8 ) is not taken into account since their absorption coefficients at 185 nm is small (σ <10 20 cm 2 ). None of the species considered in the model absorb significantly at 254 nm. The photolysis of radicals is very slow by comparison to their reaction rates and is also neglected. The kinetic rates were preferentially retrieved from review articles (Baulch et al., 1992, 1994). Experimental studies were always preferred to theoretical calculations. Finally, if no data were available, the rate was assumed equal to the rate of a similar reaction, the rate of which is known. In Titan s stratosphere, the pressure is low (0.1 1 mbar). Consequently, low-pressure as well as high-pressure rate constants are necessary to describe the pressure dependence of 3-body reactions. For this reason, low-pressure rate constants were included in the model. However, in our experimental conditions (1200 mbar), the reactions were always close to their high-pressure limit and the rate constants were virtually independent of their low pressure counterpart. Likewise, in order to have a set of reactions applicable to Titan s conditions, we used expressions describing the temperature dependence of the rate constants when available. Finally, 26 species involved in 245 reactions are included in the reaction set. This set of reactions can be found as a supplementary material (Tables S1 and S2). Regular updates are available from the authors. 4. Experimental results 4.1. Products formation The time variations of the products formed are presented in Fig. 2. Right from the beginning of the experiment, the formation of C 4 H 2 (diacetylene) is observed and after 1 h 30 min of irradiation (time noted τ ), C 2 H 4 (ethylene), C 2 H 6 (ethane) and C 3 H 6 (propene) also reach detectable concentrations. The main chemical processes leading to the formation of these compounds and their subsequent chemistry are presented in Fig. 3 and we refer to Smith et al. (1999) for further details. At time τ, a change in the slope of the C 2 H 2 evolution is also observed. After the first addition of C 2 H 2 (time τ 1 ), the abundance of C 2 H 4,C 2 H 6, and C 3 H 6 increases by a factor of about 10 while that of C 4 H 2 increases by a factor of 70. No other hydrocarbons are observed. Finally, after the second addition, the maximum concentrations of C 2 H 4 and C 3 H 6 increase slightly (+25 and 60%, respectively) with respect to the initial injection while C 4 H 2 decreases by a factor of 2.5. Five oxygenated species are also detected along the course of the experiment: H 2 O, CO, CO 2, HCHO, and CH 3 OH. These species are mainly produced during the first 1 h 30 min of irradiation and subsequently somewhat level off Influence of oxygenated species The detection of oxygenated species shows that either the experimental protocol of elimination of the residual O 2 is insufficient, or there is an additional source of oxygen in the reactor. This is a problem because we are interested in hydrocarbon chemistry, which is probably modified by the presence of these compounds. In order to determine to what extend they are perturbing the system, a set of reactions describing the chemical reactions of oxygenated species was developed and added to the hydrocarbon reaction set. This represents 54 species and 376 reactions, for which kinetic rates and products were retrieved from the literature. This set of reactions can be found as a supplementary material (Tables S3 and S4). The model shows that the instant τ at which the change in the evolution of the species occurs depends on both the amount of oxygen and the light intensity. If the amount of oxygen in the reactor is increased, τ happens at a longer irradiation time. A lower limit for the oxygen input can be estimated from the concentration of the oxygenated species. For a given amount of oxygen, the higher the light intensity is, the faster τ occurs. The total light intensity is estimated to be (2±1) photons s 1 from the specifications provided by the manufacturer of the lamps.

5 Hydrocarbon photochemistry in Titan stratosphere 291 (a) (b) Fig. 2. Evolution of C 2 H 2 (a), C 2 H 4 (b), C 2 H 6 (c), C 3 H 6 (d), and C 4 H 2 (e) (cm 3 ). τ i indicates the instants at which the injections were made and τ the instant of the slope change. Empty circles: experimental measurements. Thick plain line: simulation with model V, I = photons s 1. Thin plain line: simulation with model L, I = photons s 1. Both theoretical simulations were run with RO 2 = cm 3 s 1. We find that the irradiation time at which the change in the evolution of the species occurs and the general evolution of the oxygenated compounds match the experimental data when assuming a total intensity of photons s 1 at 185 nm and a constant flux of O 2 in the reactor (RO 2 )of cm 3 s 1 ( mbar h 1 ). The value of O 2 flux is compatible with a diffusion of atmospheric air inside the reactor that occurs despite the overpressure. Diffusion of CO 2 and H 2 O, as well as the presence of residual O 2 were also considered but their effects were negligible with respect to the diffusion of O 2. The fact that the model reproduces the change in slope at the right time with realistic O 2 input and light intensities is a good indication that the reaction set is adequate to represent the chemistry occurring in the system. Specific calculations showed that the event τ is caused by the sudden destruction of O 2 that had previously accumulated

6 292 V. Vuitton et al. / Icarus 185 (2006) (c) (d) Fig. 2. (continued) in the reactor. This is linked to the build-up of hydrocarbon radicals, which can subsequently destroy O 2. Prior to this event, the chemistry is dominated by the high concentration of O 2 ( molecules cm 3 or 20 ppb). For the following 65 h of experiment, the calculations show that the mixing ratio of O 2 stays a factor of 10 3 lower. The complete reaction set for oxygenated species is then no longer required to properly describe the chemistry occurring in the system. However, the rate of 4 reactions involving oxygenated species was still fast enough to influence the chemistry and their addition in the hydrocarbon reaction set was necessary. These reactions are presented in bold in Fig. 3. Fortunately, they are well constrained because of their importance in the Earth atmospheric chemistry. Consequently after τ = 1h30min,the oxygenated chemistry is properly accounted for in our model and the comparison of the experimental and modeled species evolution provides a test of our hydrocarbon reaction set, as desired.

7 Hydrocarbon photochemistry in Titan stratosphere 293 (e) Fig. 2. (continued) Table 1 Experimental detection limits and theoretical predictions at the maximum peak of production (molecules cm 3 and ppb) of the principal hydrocarbons predicted by the model but not observed in the experiment. C 4 H 4,C 6 H 2,and C 6 H 6 are not explicitly described in the model and their abundance is not retrieved Compound Detection limit Model prediction C 3 H 4 (methylacetylene) (340) (140) C 3 H 4 (allene) (100) (70) C 3 H (3420) (2050) C 4 H (300) C 6 H (140) C 6 H (140) Fig. 3. Main formation and destruction reactions for the hydrocarbons in the second part of the experiment. The reactions involving oxygenated compounds are in bold. 5. Model predictions 5.1. New reaction set (model V) Comparison to experimental results The theoretical evolutions of C 2 H 2,C 2 H 4,C 2 H 6,C 3 H 6, and C 4 H 2 obtained with our set of chemical reactions are compared to the experimental evolutions in Fig. 2. The profiles are globally similar but the model systematically underestimates the products. The theoretical concentrations of C 2 H 4,C 2 H 6, and C 3 H 6 after injection τ 1 are twice smaller than those obtained experimentally. The concentration of C 4 H 2 is almost 4 times smaller at the same time. The model also predicts the formation of C 3 H 4 (both allene and methylacetylene) and C 3 H 8 (propane), and of species included in soot such as C 4 H 4 (vinylacetylene), C 6 H 2 (triacetylene), and C 6 H 6 (benzene). None of these species were detected experimentally, in agreement with the model results. Detection limits and calculated abundances at the maximum peak of production for these species are presented in Table 1. In a previous study, it was showed that recombination of hydrogen atoms on the walls of the reactor was perturbing the gas phase chemistry (Smith and Raulin, 1999). In order to perform sensitivity studies on this phenomenon, a removal term for H atoms was added in the reaction set. This did not improve the fit of the data, indicating that wall effects are small in our experimental conditions. This is in agreement with the small surface/volume ratio of the reactor. Reaction rates and products were chosen to provide a consistent set of chemical reactions, based on chemical arguments. However, uncertainties carried by the kinetic parameters can introduce significant imprecision in the computed mole fractions. A sensitivity analysis is currently underway to estimate

8 294 V. Vuitton et al. / Icarus 185 (2006) Fig. 4. Evolution of C 2 H 2 (cm 3 ). τ i indicates the instants at which the injections were made and τ the instant of the slope change. Empty circles: experimental measurements. Thick plain line: simulation with Φ(C 2 H 2 ) = 0.3, I = photons s 1. Thin plain line: simulation with Φ(C 2 H 2 ) = 0.08, I = photons s 1. Dotted line: simulation with Φ(C 2 H 2 ) = 0.3, I = photons s 1 ; Φ(SOOT) = 0, σ(soot) = cm 2 molecule 1 ; Φ(OSOOT) = 0, σ(osoot) = cm 2 molecule 1. The 3 theoretical simulations were run with RO 2 = cm 3 s 1. to which extend this variability affects the predicted mole fractions. However, the systematic underestimation of the observed concentrations (hydrocarbons and oxygenated species) by the model suggests that uncertainties are not the main source of disagreement between theory and experiment but rather that some process is not properly accounted for in the model. We propose that either some initial chemical process, such as the rate of C 2 H 2 photolysis, is not properly described or that too much material is irreversibly lost as soot Photolysis quantum yield of C 2 H 2 The chemistry being initiated by C 2 H 2 photolysis, the production rates directly depend on its photolysis quantum yields. Unfortunately, these are not well constrained. The C 2 H production yield was measured to be 0.08 at 185 nm (Okabe, 1983). However, many studies performed at 147 nm (Fahr and Laufer, 1986; Okabe, 1981) and 193 nm (Satyapal and Bersohn, 1991; Seki and Okabe, 1993; Shin and Michael, 1991) found a value of 0.3, suggesting a constant yield over this wavelength range. Consequently, a quantum yield of 0.3 was first adopted in the reaction set. Recently, a new study was published in which the quantum yield was found to be 1 at 122 and 193 nm (Läuter et al., 2002). In order to solve the discrepancies observed between model and experiment, sensitivity studies of the quantum yield were performed by varying its value from 0.08 to 1. The light intensity and O 2 input were adjusted accordingly in order to keep the slope change in the species evolution at the right time. For example, the evolution of C 2 H 2 for a quantum yield of 0.08 and a light intensity of photons s 1 is presented in Fig. 4. Unfortunately, we were not able to find a realistic set of parameters that would significantly improve the experimental fit. Consequently, the uncertainties in the C 2 H production yield from C 2 H 2 photolysis cannot explain the discrepancies between model and experiment Photolysis of heavier products According to carbon balance calculations, at least carbon atoms cm 3 were incorporated into non-identified species during the experiment. This is a lower limit since the percentage of CH 4 consumed is too low to be quantified. (An upper limit of 3 % that is molecules cm 3 could only be retrieved). This is in agreement with the amount of carbon contributing to the formation of soot and heavier oxygenated species in the model. The compounds included in soot are very good absorbers at 185 nm. For example, the absorption coefficient of C 4 H 4 and C 6 H 6 is cm 2 molecule 1 (Fahr and Nayak, 1996) and cm 2 molecule 1 (Pantos et al., 1978), respectively. Consequently, it is possible that these gaseous species shield the light available for photolysis (see Section 3.1). The formation of solid particles (tholins) in the course of the experiment was not observed, probably because of the high dilution of the CH 4 C 2 H 2 mixture in N 2. However, even a thin coating of this material on the lamps would also contribute to the absorption of the UV light. We assumed that in the model the soot is lost for the system but actually some of these species may be dissociated back to radicals or lighter species such as C 2 H 2. Then, not having taken into account the photolysis of the heavier species in the model may be the reason why the lighter species are underestimated. In order to test this hypothesis, we ran a new simulation in which the soot and heavier oxygenated species absorb the UV

9 Hydrocarbon photochemistry in Titan stratosphere 295 Fig. 5. Evolution of C 2 H 2 (cm 3 ) with model L. τ i indicates the instants at which the injections were made and τ the instant of the slope change. Empty circles: experimental measurements. Thick plain line: simulation with RO 2 = cm 3 s 1, I = photons s 1. Thin plain line: simulation with RO 2 = cm 3 s 1, I = photons s 1. Dotted line: simulation with RO 2 = cm 3 s 1, I = photons s 1. light but are not dissociated. In these conditions, it was not possible to reproduce the C 2 H 2 decrease. However, by assuming that a soot molecule dissociates back to C 2 H 2 when it absorbs a photon, the C 2 H 2 decrease can be very well fitted, as shown in Fig. 4. The concentration of all the other hydrocarbons is subsequently increased, up to 50% for C 2 H 6. In conclusion, with our new reaction set, the model can reproduce the general evolution of the products but it systematically underestimates their abundance. The incorporation of the absorption of the heavier compounds improves the results but without further details in the reaction sets, still does not allow a satisfactory retrieving of the data related to all the hydrocarbons formed experimentally. In order to test the sensitivity of the results to the chemical scheme, we decided to replace our hydrocarbon reaction set by another recent scheme developed by Lebonnois et al. (2001) Lebonnois reaction set (model L) The reaction set presented in Lebonnois et al. (2001) takes into account 30 hydrocarbons including two excited states ( 1 CH 2 and 3 C 4 H 2 ), 243 reactions and 36 photolysis. The compounds containing more than 4 carbon atoms are treated as organic solid material and called soot. Other publications by the same author have followed this initial work. Revisions of the initial reaction set include parameterizations of the production rate of macromolecules (and the loss of the parent molecules) from the gas to the solid phase (Lebonnois et al., 2002), heterogeneous recombination of hydrogen atoms (Lebonnois et al., 2003) and C 6 H 6 (benzene) formation (Lebonnois, 2005). Production of macromolecules (or soot ) definitely occurs in the experiment. Actually, including in the model the recycling of the soot into C 2 H 2 through photolysis was necessary to better reproduce the experimental data (see Section 5.1.3). However, adding another loss of C 2 H 2 to the soot would require increasing even more the recycling process, without providing any information on the actual chemical pathways. We showed that heterogeneous recombination of H does not occur in our experimental conditions (see Section 5.1.1). Finally, the production of benzene is very small, with an upper limit of 150 ppb or molecules cm 3 (see Table 1). Consequently, these processes, while relevant to Titan s conditions, do not need to be taken into account to properly model the experiment and the reaction rates, branching ratios and photolysis quantum yields of Lebonnois et al. (2001) were used. In our experimental conditions, both excited states could be neglected since the photons available are not energetic enough to form 1 CH 2, and 3 C 4 H 2 is quickly deactivated by collisions with other species. The set of oxygenated compounds reactions used before was added to the set of hydrocarbon reactions. With this new set of reactions, the light intensity has to be increased by 75% ( photons s 1 ) for the change in slope in the C 2 H 2 evolution to occur at the right time. The concentration of C 2 H 2 with the previous and new light intensity is presented in Fig. 5 and the concentration of the 5 species is compared to the results obtained with model V in Fig. 2. With the new value, the decrease of C 2 H 2 after τ 1, where the perturbation by oxygenated compounds is small, is too slow. Also, the calculated concentrations of C 2 H 6 and C 4 H 2 are overestimated by 50 and 100%, respectively, while those of C 2 H 4 and C 3 H 6 are underestimated by 70%. In order to better match the experimental C 2 H 2, the light intensity and O 2 rate have to be further increased. For example, the C 2 H 2 decrease obtained for an O 2 input of cm 3 s 1 and a flux of photons s 1

10 296 V. Vuitton et al. / Icarus 185 (2006) is presented in Fig. 5. However, the light intensity is then significantly higher than the value estimated from the manufacturer s specifications. Consequently, no fully satisfactory combination of light intensity and oxygen input were able to match the experimental C 2 H 2 evolution. Moreover, the subsequent evolution of the products is not consistent with the experimental data. This suggests that model L is not able to fit the experimental observations C 2 H 4 formation 1D models tend to underestimate the mixing ratio of C 2 H 4 in Titan s stratosphere. However, in general circulation model simulations, the C 2 H 4 abundance is well fitted, due to transport of C 2 H 4 downward over the polar region, which yields enrichment of the lower stratosphere at all latitudes (Crespin et al., 2005). The influence of latitudinal transport would then solve the problem of 1D models to reproduce the C 2 H 4 abundance. Our experimental results indicate that while the production of most hydrocarbons is suppressed in the presence of oxygenated compounds, C 2 H 4 abundance increases. This is because the radicals CH 3 O and HCO react with C 2 H 3 to form C 2 H 4 (Tsang and Hampson, 1986). Few tens of ppm of CO have been detected in Titan s stratosphere. This mixing ratio is probably temporarily higher when comets ablate in Titan s upper atmosphere. Addition of H atoms on CO molecules could be an efficient source of HCO in Titan s atmosphere, subsequently increasing the C 2 H 4 mixing ratio. Addition of representative deposition and chemical reactions of oxygenated species in Titan photochemical models is required in order to investigate the importance of this additional source of C 2 H Comparison of the two reaction sets Model L highly overestimates the amount of C 4 H 2 formed during the experiment, as well as its mixing ratio observed in Titan s stratosphere. On the opposite, model V tends to underestimate the experimental amount of C 4 H 2. Understanding the loss and production pathways of C 4 H 2 is very interesting since this species has been proposed to be an intermediate in the formation of the heavier organic material leading to the aerosols (Allen et al., 1980; Lebonnois et al., 2002; Wilson and Atreya, 2003; Zwier and Allen, 1996). Consequently, in order to understand the difference observed in the modeled abundance of this compound for reaction sets L and V, the models were run with the same light intensity ( photons s 1 ) and without O 2.TheC 2 H 2 and C 4 H 2 evolutions obtained by both models are presented in Fig. 6. Once formed, C 4 H 2 concentration stays constant with model L but decreases rapidly with model V. With model L a maximum of 50% of C 2 H 2 is consumed, while with model V it is almost totally destroyed. In both models, insertion of C 2 H radicals into C 2 H 2 is the main formation pathway to C 4 H 2. The evolution of C 4 H 2 is then directly related to the evolution of C 2 H 2 and the slow C 2 H 2 decrease in model L explains the constant C 4 H 2 concentration over the course of the experiment. Photolysis and reaction with H and C 2 H radicals are the main loss processes for C 2 H 2 and C 4 H 2. In Titan s atmosphere, the radicals formed in the photolysis of C 2 H 2 and C 4 H 2 catalyze the dissociation of CH 4 in the stratosphere, control the H/H 2 ratio and initiate the formation of heavier molecules through polymerization reactions. These three mechanisms are presented in Fig. 7 and detailed here after. Reaction sets presented in previous photochemical models of Titan and the giant planets are discussed Photocatalysis of CH 4 dissociation (a) As mentioned in Section 5.1.2, we chose 0.3 for the quantum yield of production of C 2 H in the photolysis of C 2 H 2, while Lebonnois et al. (2001) used With model L, the destruction of C 2 H 2 by photolysis is then almost 4 times smaller. C 2 H 2 loss by reaction with C 2 H and H is smaller as well since the principal source of these radicals is C 2 H 2 photolysis. This is one reason why, for the same light intensity, C 2 H 2 does not decrease as much with model L as with model V. Recently, Läuter et al. (2002) measured that the quantum yield of formation of H atoms along with C 2 H radicals in the and nm photolysis of C 2 H 2 is close to 1, a value in strong disagreement with the older measurements. In Titan s stratosphere, C 2 H abstracts H atoms from CH 4, leading to the formation of CH 3 radicals that subsequently recombine. Thus, the conversion rate of CH 4 to higher molecular weight organics directly depends on the C 2 H production rate. A first order calculation indicates that the 3 times higher quantum yield suggested by Läuter et al. (2002) would lead to an increase of the C 2 H 6 /C 2 H 2 ratio by one order of magnitude Control of H/H 2 ratio (b) Atomic hydrogen adds onto C 4 H 2 to form C 4 H 3 radicals (Klippenstein and Miller, 2005). Information on the reactivity of C 4 H 3 radicals is very scarce. Analogy with C 2 H 3 would suggest that they readily react with H atoms, the reaction products being C 2 H 2,C 4 H 2,orC 4 H 4. In model V, C 4 H 4 is considered as soot and does not react further. In model L, C 4 H 4 is photolysed to form back C 2 H 2 (20%) and C 4 H 2 (80%). This is another reason why with model L, the decrease of C 2 H 2 is much slower and the production of C 4 H 2 is higher. Addition of H atoms onto C 4 H 3 following addition onto C 4 H 2 converts H into H 2, either directly or via C 4 H 4 photolysis (Fig. 7). While H is extremely reactive, H 2 is very stable and is ultimately lost by escape from Titan s upper atmosphere. This prevents any recycling of the unsaturated species to CH 4, contrary to the giant planets. The H 2 escape rate is then a good tracer of the rate of CH 4 destruction. This suggests that comparison of the escape rate predicted by the photochemical models to the observations can provide indication on the overall accuracy of the chemical scheme. In order to properly model the conversion of H to H 2, measurements of the rate constant and products of the reactions of H atoms with C 4 H 3, especially at low temperature and low pressure are required.

11 Hydrocarbon photochemistry in Titan stratosphere 297 (a) (b) Fig. 6. Evolution of C 2 H 2 (a) and C 4 H 2 (b) (cm 3 ). τ i indicates the instants at which the injections were made. Thick plain line: simulation with model V. Thin plain line: simulation with model L. The simulations were run with RO 2 = 0andI = photons s Polymers formation (c) Only a single study of C 4 H 2 photolysis is available to date in the literature (Glicker and Okabe, 1987). At 185 nm, the authors could only determine an upper limit for the C 4 H quantum yield (0.08). Since the total quantum yield at this wavelength is small (0.08), the branching ratio assumed for C 4 H can modify the C 4 H 2 photolysis rate by as much as 100%. The values taken in models L and V are 0 and 0.08, respectively, which partially explains the difference in C 4 H 2 concentration observed in the 2 cases. In Titan s atmosphere, C 4 H 2 is the principal absorbent of long UV photons and its photolysis would be an important source of C 4 H radicals if their quantum yield of formation were substantial. This is interesting because C 4 H radicals are expected to insert into unsaturated bonds as C 2 H does and could contribute to the formation of higher molecular weight species. The alternative product to C 4 H is a metastable state 3 C 4 H 2,

12 298 V. Vuitton et al. / Icarus 185 (2006) Fig. 7. Scheme of the main reactions involving C 4 H 2. The letters identify the different mechanisms: (a) photocatalysis of CH 4 dissociation, (b) control of H/H 2 ratio, (c) polymers formation. which has also been suggested to be an intermediate to heavier compounds (Zwier and Allen, 1996). However, its lifetime seems too low to allow it to react significantly with other molecules (Vuitton et al., 2003). Consequently, knowledge of C 4 H 2 photolysis branching ratios is crucial in order to understand the formation pathways to complex organics and further experimental studies are required, especially at longer wavelength. The addition of C 4 H 3 with C 2 H 2 to form C 6 H 5 radicals has been suggested as an important step in benzene formation mechanisms in Titan s atmosphere (Lebonnois et al., 2002; Wilson et al., 2003). In these models, rate coefficients and pathways for the possible mechanisms were obtained from modeling studies in combustion conditions (Wang and Frenklach, 1997; Westmoreland et al., 1989). However, this reaction is likely to have a large energy barrier that makes it unimportant at low temperatures (Moses et al., 2005), leaving the recombination of C 3 H 3 as the main pathway to benzene formation in model V. Lebonnois (2005) found that reaction of C 3 H 3 and C 3 H 2 (which otherwise ends up as C 3 H 3 by addition with H atoms) with C 2 H 2 is a large sink for these radicals and can inhibit benzene formation as well as increase C 4 H 2 by a factor of approximately 4 in Titan s stratosphere. However, in our laboratory experiment, C 3 H 3 and C 3 H 2 are very minor species since their precursor (C 3 H 4 ) is not efficiently produced (on Titan, C 3 H 4 is mostly formed by insertion of CH radicals produced in the photolysis of CH 4 into C 2 H 4 ). Consequently, we find that in model L, the reaction of C 3 H 3 with C 2 H 2 is only responsible for 10% of the total C 4 H 2 production. According to Moses et al. (2000) these reactions are endothermic and are unlikely to occur at low temperature. We have therefore omitted this C 4 H 2 formation pathway from our model. We showed that including the photolysis of C 4 H 4 in the reaction set is definitely necessary to improve photochemical models. However, we did not find any measurement of the quantum yields of photolysis of C 4 H 4 in the literature. Dissociation pathways leading to C 4 H 2 and C 2 H 2 with branching ratios of 0.8 and 0.2, respectively, are used in model L and other recent models (Moses et al., 2005; Wilson and Atreya, 2004). This is based on Yung et al. (1984) assumption that the C 4 H 2 branch is the most probable. However, it is not clear why, and on what grounds, this statement was made. It is conceivable that the breakage of the central C C bond to produce acetylene and vinylidene (CH 2 =C) is the most important dissociation pathway. Consequently, it is quite possible that the amount of C 4 H 2 formed in C 4 H 4 photolysis is overestimated in model L. If one wants to correctly describe C 4 H 4 loss processes, its reactions with radicals should be taken into account as well as photolysis. In Titan s atmosphere, C 4 H 4 is expected to react with H atoms and C 2 H radicals to produce higher molecular weight organics (C 4 H 5 and C 6 H 4, respectively). These reactions are presented in Fig. 7. However, experimental (or theoretical) studies are required before these reactions can be safely introduced in the reaction set. The comparison of both reaction sets explains why the C 4 H 2 concentration calculated with model L is higher than that calculated with model V. While some of Lebonnois et al. (2001) choices are genuine, it seems that some reactions are missing in his reaction set. For example, the lack of reactions of C 4 H 4 with any radicals probably leads to the overestimation of C 4 H 4 photolysis products, namely C 2 H 2 and C 4 H 2, as is observed by comparison with the laboratory experiments. On the other hand, model V tends to underestimate the experimental C 4 H 2 concentration. Imprecision carried by the different kinetic parameters governing C 4 H 2 concentration is very high. This introduces significant uncertainties in its computed mole fractions, which can explain the discrepancy between models and experiment. A sensitivity analysis of the model for the main hydrocarbons is currently underway to help improve the reaction set further (Hébrard et al., in preparation). 7. Conclusions We irradiated with long-uv light a CH 4 C 2 H 2 gas mixture in order to study the catalytic dissociation of CH 4 occurring in Titan s stratosphere. We developed a 0D model of the experiment and built a new set of reactions describing hydrocarbon chemistry. We then compared the experimental results to the model predictions obtained with our reaction set and Lebonnois et al. (2001) scheme. The experimental setup has the advantages to limit heterogeneous reactions on the chamber walls and to follow the evolution of the compounds in situ and in real time. This allowed us to observe experimentally the formation of all the major species predicted by the model. However, atmospheric O 2 was found to diffuse inside the chamber, modifying the hydrocarbon chemistry under study. We were able to identify the major reactions perturbing the system and to include them in our chemical model. Consequently, the presence of O 2 in the experiment was properly accounted for and did not prevent from studying the hydrocarbon chemistry we were interested in. Moreover, it allowed us to suggest that oxygenated chemistry might be a source of C 2 H 4 in Titan s atmosphere. With Lebonnois et al. (2001) reaction set, the model could not fit at all the experimental evolution of the compounds. This was found to be due to some of the choices made for

13 Hydrocarbon photochemistry in Titan stratosphere 299 crucial kinetic parameters such as the quantum yield of photolysis of C 2 H 2. Also, the absence of some reactions led to the enhancement of pathways that would otherwise be negligible. For example, the lack of reactions between C 4 H 4 and radicals induced an erroneously high photolysis rate for this species. We built our own reaction set, by exclusively retrieving rate constants and branching ratios from a critical review of the kinetic literature. We never attempted to fit the experimental data by tuning one kinetic parameter or the other. With this set of reactions, the model can much better fit the experiment, especially when the soot, which includes C 4 H 4, is recycled into C 2 H 2. This shows that considering that heavier compounds are lost from the system is probably not a valid assumption. Including their loss processes will be required to better describe the chemistry of the lighter species. In order to do so, new data (rate constants, branching ratios, absorption cross sections) are strongly required, especially at the low temperature and pressure of Titan s atmosphere, for which uncertainties are even higher. Testing this reaction set and others with new reactants, at lower pressure and more representative temperature and at other wavelengths is necessary. Simpler, better-constrained experimental conditions would be helpful. For example, high-vacuum experimental setups will be required to avoid O 2 contamination. Actinometry measurements to measure the intensity of the lamps would allow better constraining the quantum yields of photolysis of the reactants. Flow systems seem particularly attractive since the gas mixture under photolysis is constantly renewed, preventing the accumulation of secondary photolysis products that are poorly described by models. Also, the mixing ratio of the reactants can be kept low, preventing heterogeneous reactions on the reactor walls. Also the development at LISA of a new analytical system allowing the detection of transient species (SETUP, the French acronym for Experimental and Theoretical Simulations Useful for Planetary Sciences) will allow to better constrain and consequently further improve the models. Even if our reaction set includes a limited set of species and was tested in very specific conditions, it was built in order to be directly applicable to the atmospheric conditions of Titan and the giant planets. Now that a lot of new data becomes available from Cassini and Huygens observations (CH 4 mixing ratio, eddy diffusion coefficient, aerosol opacity and distribution), it would be particularly interesting to use our reaction set in a new photochemical model of Titan. Titan s atmospheric composition has also been measured in both the lower stratosphere and upper atmosphere providing good constraints to the modeled vertical profiles of the chemical species. 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