1 The 1D model of IPSL : IPSL05 Intercomparison of 1D photochemical models of Titan atmosphere Nathalie Carrasco Workshop ISSI Bern th march 2009

2 Inputs presently implemented Turbulent transport K(z): Hidayat 1997Hidayat 1997 Photochemical database ~Hébrard 2006 Temperature profile T(z): Vervack 2004Vervack 2004 P profiles, based on N 2 +CH 4 density profiles  km N 2 +CH 4 Yelle  km N 2 Vervack

3 History of the model Based on the model of D. Toublanc, developed in Bordeaux, then Toulouse  Toublanc et al. 1995, Icarus 113 Same root as Dobrijevic’s model  Identical grid (no, rougher : 10 km altitude instead of 5 for MD)  Same solver for the resolution of the continuity equation (LU method) Noticeable difference : coded in C language instead of fortran

4 Evolution of the 1D model since SL thesis UV flux description, dependence in latitude  Lebonnois, 2000, Thesis  Lebonnois and Toublanc, 1999, JGR Extension to a 2D chemistry-transport model  Lebonnois et al, 2001, Icarus Parameterization of polymerization  Lebonnois et al, 2002, Icarus  Lebonnois, 2005, PSS Heterogeneous reactions  Lebonnois et al, 2003, Icarus

5 UV flux description Aim : model the UV absorption in the night-side of Titan at higher altitudes for latitudinal and seasonal dependence of the UV fluxes

6 UV flux description 3D grid

7 UV flux description Axial symmetry along the Sun-Titan axis  Hypothesis : global homogeneous distribution of the absorbing and diffusing species (gazeous+aerosols)  1D profiles from model, adjusted with observational data Gazeous compounds : Vervack et al 2004, Icarus ; Coustenis and Bezard, 1995, Icarus (observations) ; Lebonnois et al (model) Aerosols : … (observations) ; Rannou et al, 1995, Icarus ; 2002 Nature (model)  Monte-Carlo computation of the actinic fluxes in each cell

8 UV flux description Calculation of the diurnal mean of the actinic flux vertical profile as a function of the wavelength, latitude and season

9 UV flux description Within the photochemical model, computation of the photodissociation rates from these profiles

10 Evolution of the 1D model since SL thesis UV flux description, dependence in latitude  Lebonnois, 2000, Thesis  Lebonnois and Toublanc, 1999, JGR Extension to a 2D chemistry-transport model  Lebonnois et al, 2001, Icarus Parameterization of polymerization  Lebonnois et al, 2002, Icarus  Lebonnois, 2005, PSS Heterogeneous reactions  Lebonnois et al, 2003, Icarus

11 Extension to a 2D model Latitudinal grid : 10°  17 1D columns (central column at the equator) Vertical diffusion in each 1D box between 0 and 1300 km Columns horizontally coupled  Horizontal turbulent diffusion between 0 and 1300 km : Kh adjusted with…?  2D advection between 0 and 550 km Analytical description of 2D winds based results from the GCM of IPSL (Hourdin et al, 1995)

12 Extension to a 2D model

13 Evolution of the 1D model since SL thesis UV flux description, dependence in latitude  Lebonnois, 2000, Thesis  Lebonnois and Toublanc, 1999, JGR Extension to a 2D chemistry-transport model  Lebonnois et al, 2001, Icarus Parameterization of polymerization  Lebonnois et al, 2002, Icarus  Lebonnois, 2005, PSS Heterogeneous reactions  Lebonnois et al, 2003, Icarus

14 Polymerization 3 polymer growth schemes  Acetylene polymer (linear)  PAH growth (aromatic formation)  HCN and nitrile polymer

15 Polymerization Precursor level  Threshold chosen : N1=N2=N3 = 20 gaz molecules incorporated in one polymer macromolecule  Corresponds to a nucleus of ~7-8 Å  Beyond this threshold the growth is no longer chemical, but microphysically modelled

16 Evolution of the 1D model since SL thesis UV flux description, dependence in latitude  Lebonnois, 2000, Thesis  Lebonnois and Toublanc, 1999, JGR Extension to a 2D chemistry-transport model  Lebonnois et al, 2001, Icarus Parameterization of polymerization  Lebonnois et al, 2002, Icarus  Lebonnois, 2005, PSS Heterogeneous reactions  Lebonnois et al, 2003, Icarus

17 Heterogeneous reactions Hypothesis  Balance of the hydrogen budget : only conversion 2H->H2 no sink or source through heterogeneous chemistry, neither in the gazeous phase, nor on the aerosol surface  Estimations N : number of adsorption sites. Taken from the model of Toon et al 1992, and related to an average size of the aerosol r haze = 0.1µm H : speed of collision, assimilated to the thermal kinetic speed of atomic hydrogen  : cross section for H2 desorption : taken from Bakes et al H H H H H H

18 Conclusion For the purpose of the first 1D models intercomparison meeting  Same root as MD  But some significant changes Programmed in C language UV flux calculated for a 3D absorbing and diffusing atmosphere, with a 1D description of the gazeous and aerosol profiles