How Measurements of Rate Coefficients at Low Temperature Increase the Predictivity of Photochemical Models of Titan’s Atmosphere

Hébrard, Éric; Dobrijévic, M.; Pernot, Pascal; Carrasco, Nathalie; Bergeat, Astrid; Hickson, Kevin M.; Canosa, André; Picard, Sébastien D. Le; Sims, Ian

doi:10.1021/jp905524e

Cited by 91 publications

(142 citation statements)

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“…The methodology used to study the propagation of uncertainties in the model is described in Hébrard et al (2007) and Hébrard et al (2009). There are many sources of uncertainty in a 1D photochemical model.…”

Section: Methodsmentioning

confidence: 99%

“…The knowledge of both F(300 K) and g parameters allows to quantify the temperature-dependent uncertainties carried by each reaction rate present in the standard sets of reaction rates in a temperature range adequate for Titan's atmosphere. Most of the reaction rate coefficients and their associated uncertainty factors used in the present study are extracted from our previous reviews (Hébrard et al 2006;Hébrard et al 2009). Reaction rates and uncertainties for new processes are estimated according to the methodology introduced in Sect.…”

Section: Methodsmentioning

confidence: 99%

“…The basis of the chemical scheme is presented in Hébrard et al (2006) and Hébrard et al (2009). In the present work, many rate constants have been updated and numerous reactions have been added.…”

Section: Chemical Schemementioning

confidence: 99%

“…It has been previously used and described in Carrasco et al (2007); Dobrijevic et al (2008); Hébrard et al (2009). The greater the absolute value of a RCC is, the more important the contribution a given reaction rate has on the abundance uncertainty for a given species.…”

Section: Global Sensitivity Analysis: Determination Of Key Reactionsmentioning

confidence: 99%

“…Since HNC was not considered in previous photochemical models, it is necessary to build a new chemical scheme devoted to HNC chemistry and its putative interactions with other species. The chemistry of nitrogen compounds at low temperatures (between 100 and 200 K), either in cold planetary atmospheres or in protoplanetary disks, is not well known (Hébrard et al 2006;Vasyunin et al 2008;Hébrard et al 2009). Therefore, it is also critical to evaluate the uncertainties attached to the reviewed reaction rates.…”

Section: Introductionmentioning

confidence: 99%

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Neutral production of hydrogen isocyanide (HNC) and hydrogen cyanide (HCN) in Titan’s upper atmosphere

Hébrard

Dobrijévic

Loison

et al. 2012

A&A

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116

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Aims. Following the first detection of hydrogen isocyanide (HNC) in Titan's atmosphere, we have devised a new neutral chemical scheme for hydrogen cyanide (HCN) and HNC in the upper atmosphere of Titan. Methods. Our updated chemical scheme contains 137 compounds (with C, H, O and N elements) and 788 reactions (including 91 photolysis processes). To improve the chemistry of HNC and HCN, a careful review of the literature has been performed to retrieve critical reaction rates and to evaluate their uncertainty factors. We have also estimated the reaction rates of 48 new reactions using simple capture theory. Results. Our photochemical model gives abundances of HNC and HCN in reasonable agreement with observations. An uncertainty propagation study shows large uncertainties for HNC and relatively moderate uncertainties for HCN. A global sensitivity analysis pinpoints some key reactions to study as a priority to improve the predictivity of the model. Conclusions. In particular, our knowledge of the isomerization of HNC via the reaction H + HNC → HCN + H and the chemistry of H 2 CN needs to be improved. This study of the neutral chemistry taking place in the upper atmosphere of Titan is a prerequisite for future ionospheric models since ion-neutral reactions may also contribute significantly to HNC and HCN production.

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Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Section: Chemical Schemementioning

confidence: 99%

Section: Global Sensitivity Analysis: Determination Of Key Reactionsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

Neutral production of hydrogen isocyanide (HNC) and hydrogen cyanide (HCN) in Titan’s upper atmosphere

Hébrard

Dobrijévic

Loison

et al. 2012

A&A

Self Cite

116

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Determination of rate parameters based on both direct and indirect measurements

Turányi

Nagy

Zsély

et al. 2012

Int J of Chemical Kinetics

137

132

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The determination of rate parameters of gas-phase elementary reactions is usually based on direct measurements. The rate parameters obtained in many independent direct measurements are then used in reaction mechanisms, which are tested against the results of indirect experiments, like time-to-ignition or laminar flame velocity measurements. We suggest a new approach that takes into account both direct and indirect measurements and optimizes all influential rate parameters. First, the domain of feasibility of the Arrhenius parameters is determined from all of the available direct measurements. Thereafter, the optimal Arrhenius parameters are sought within this domain to reproduce the selected direct and indirect measurements. Other parameters of a complex mechanism (third-body efficiencies, enthalpies of formation, parameters of pressure dependence, etc.) can also be taken into account in a similar way. A new fitting algorithm and a new method for error calculation were developed to determine the optimal mean values and the covariance matrix of all parameters. The approach is demonstrated on the calculation of Arrhenius parameters of reactions (R1): H + O 2 = OH + O and (R2): H + O 2 + M = HO 2 + M (low-pressure limit, M = N 2 or Ar). In total, 9 direct measurements for reaction (R1) (745 data points), 10 direct measurements for reaction (R2) (258 data points), and 11 ignition time measurements (79 data points) were taken into account. The application of the method resulted in the following rate parameters for the investigated reactions-(R1): A = 3.003 × 10 10 cm 3 mol −1 s −1 , n = 0.965, E/R = 6158 K (T = 950-3550 K) DETERMINATION OF RATE PARAMETERS BASED ON DIRECT AND INDIRECT MEASUREMENTS 285and (R2): A = 7.856 × 10 18 cm 6 mol −2 s −1 , n = −1.100, E/R = 0 K (low-pressure limit, M = N 2 , T = 300-1850 K). The optimized third-body efficiency of Ar relative to N 2 is m = 0.494 (standard deviation σ = 0.010). The uncertainty parameter f as a function of temperature was also calculated. Average uncertainty parameter values are f = 0.025 and 0.049 for reactions (R1) and (R2) (corresponding to 6% and 12%), respectively, which are much lower than those of the previous evaluations. C 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 284-302, 2012

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Inference Given Summary Statistics

Najm¹,

Chowdhary²

2015

Handbook of Uncertainty Quantification

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How Measurements of Rate Coefficients at Low Temperature Increase the Predictivity of Photochemical Models of Titan’s Atmosphere

Cited by 91 publications

References 95 publications

Neutral production of hydrogen isocyanide (HNC) and hydrogen cyanide (HCN) in Titan’s upper atmosphere

Neutral production of hydrogen isocyanide (HNC) and hydrogen cyanide (HCN) in Titan’s upper atmosphere

Determination of rate parameters based on both direct and indirect measurements

Inference Given Summary Statistics

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