1. Photochemical processes on Titan
A Revision from Cassini Observations
Cheng Li1, X. Zhang1, J. A. Kammer1, M. C. Liang2, Y. L. Yung1
1California Institute of Technology
2Research Center for Environmental Changes, Academia Sinica, Taiwan

2. Photochemistry on Titan
C2H2, C2H4,C2H6
C2H,C2H3,…
N2+,N,N+
C3Hx, C4Hx,…
CH,1CH2,3CH2
CH3 e-
CH4 N2
surface

3. The simplest 1D photochemical model
Chemical production
Chemical loss
Vertical transport
𝑃 − 𝐿 = − 𝜕 𝜕𝑧 (𝐾𝑛 𝜕𝑥 𝜕𝑧 )
Mixing ratio
Number density
Eddy diffusivity

4. CIRS limb view and FUV stellar occultation
C2H2
C2H4
C2H6
Altitude (km)
CH4
CH3CCH
C3H8
C4H2
C6H6
Mixing ratio
Vinatier et al., 2009; Kammer et al., 2011

5. Retrieval algorithm Levenberg–Marquardt Ar C2H2 1000 altitude
Mixing ratio
Levenberg–Marquardt

6. Molecular diffusivity
Initial guess
Retrieved eddy diffusivity

7. Altitude (km) Mixing ratio
Vinatier et al., 2009; Kammer et al., 2011; Yelle et al., 2008
model 1
model 2
Altitude (km)
CH4
C2H2
C2H4
C2H6
CH3CCH
C3H8
C6H6
C4H2
Mixing ratio

8. Haze formation process revealed by the eddy diffusivity
Liang et al., 2007
Stabilization due to haze heating
Aerosol growth
heating
Detached haze layer

9. Conclusion
Retrieved eddy diffusivity

10. How to estimate the sensitivity to chemical rate constants?
𝑆= 𝑖,𝑗 Δ 𝑥 𝑖 𝑥 𝑖 2
𝑖:𝑎𝑙𝑡𝑖𝑡𝑢𝑑𝑒
𝑗:𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛
𝑥 𝑖 :𝑎𝑏𝑢𝑛𝑑𝑎𝑛𝑐𝑒
Δ 𝑥 𝑖 :𝑐ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑎𝑏𝑢𝑛𝑑𝑎𝑛𝑐𝑒
𝑤ℎ𝑒𝑛 𝑟𝑎𝑡𝑒𝑠 𝑜𝑓 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 𝑗 𝑎𝑟𝑒 𝑑𝑜𝑢𝑏𝑙𝑒

11. Mixing tracer S

12. Reactions
Rate constants
Moses et al 2000
This work
H + C2H4 + M  C2H5 + M
𝑘 0 =1.3× 10 −29 𝑒 − 380 𝑇
𝑘 ∞ =6.6× 10 −15 𝑇 1.3 𝑒 − 650 𝑇
𝑘 0 =5.4× 10 −25 𝑇 −1.46 𝑒 − 1300 𝑇
𝑘 ∞ =1.8× 10 −13 𝑇 0.7 𝑒 − 600 𝑇
CH3 + C2H5 + M  C3H8 + M
𝑘 0 =7.5× 10 −17 𝑇 −3 𝑒 − 300 𝑇
𝑘 ∞ =6.64× 10 −11
Increase by 5 times
CH3 + C2H3 + M  C3H6 + M
𝑘 0 =5× 10 −27
𝑘 ∞ =1.1× 10 −11
H + C3H7 + M  C3H8 + M
𝑘 0 =4× 10 −19 𝑇 −3 𝑒 − 600 𝑇
𝑘 ∞ =2.49× 10 −10

13. How to estimate the eddy diffusivity?
Molecular diffusivity
adiabats
𝐾∝ 1 𝑛
Fulchignoni et al., 2005

14. Eddy diffusivity 𝑃−𝐿=− 𝜕 𝜕𝑧 (𝐾𝑛 𝜕𝑥 𝜕𝑧 ) Altitude(km)
adiabats
Altitude(km)
Eddy diffusivity (cm2/s)

15. Previous work -- mixing ratio v.s. altitude(km)
Lavvas et al, 07
Krasnopolsky, 09

16. Fractional change of C2H4 by doubling rate constants
CH3  1CH2+H
2CH3 + M  C2H6 + M
Altitude (km)
H + C2H2 + M  C2H3 + M
H + C2H4 + M  C2H5 +M

17. New reaction rate constants (solid lines)
H + C2H4 + M  C2H5 + M
604K / 511K / 400K / 285K / 170K (Titan)
Moses et al.,2000
This work
Lightfoot et al., 1987