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Night OH in the Mesosphere of Venus and Earth

Night OH in the Mesosphere of Venus and Earth. Christopher Parkinson Dept. Atmospheric, Oceanic, and Space Sciences University of Michigan F. Mills, M. Allen, A. Brecht, S. Bougher, and Y.L. Yung DPS October 9, 2009. Motivation. Airglow emissions, such as NO and O 2 , have been observed

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Night OH in the Mesosphere of Venus and Earth

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  1. Night OH in the Mesosphere of Venus and Earth Christopher Parkinson Dept. Atmospheric, Oceanic, and Space Sciences University of Michigan F. Mills, M. Allen, A. Brecht, S. Bougher, and Y.L. Yung DPS October 9, 2009

  2. Motivation • Airglow emissions, such as NO and O2, have been observed • Airglow emissions provide insight into chemical and dynamical processes that control the composition and energy balance in the upper atmosphere • The OH airglow emission has been observed previously only in the Earth’s atmosphere

  3. Similarly, Venus airglow emissions have been detected at wavelengths of 1.40−1.49 and 2.6−3.14 μm in limb observations by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on the Venus Express spacecraft are attributed to the OH (2−0) and (1−0) Meinel band transitions as well (Piccioni et al., 2008). • The integrated emission rates for the OH (2−0) and (1−0) bands were measured to be 10040 and 88090 kR respectively, both peaking at an altitude of 962 km near midnight local time for the considered orbit.

  4. Key Reactions for OH • HO2 + O --> OH + O2 (71% OH) (1) • k = 3.00E-11 EXP( 200/T) • H + O3 --> OH + O2 (<1% OH) (2) • k = 1.40E-10 EXP( -470/T) • Cl + HO2 --> ClO + OH (22% OH) (3) • k = 4.10E-11 EXP( -450/T) • SO + HO2 --> SO2 + OH (6% OH) (4) • k = 2.80E-11

  5. Venus Earth

  6. we see that much more OH produced from (2) O + HO2 than (2) H + O3 • still have significant (3) and (4) contribution to OH (sulphur and Cl) • However, we see that this is not the case for OH(v) ……….

  7. In the region of >90 km for Venus (and >80km for Earth), three-body formation of HO2 becomes less important because the rate depends on square of the total pressure, making reaction (1) more “favourable” for the production of OH(v). • However, we find that the reaction rates for reaction (2) to be an order of magnitude greater than for reaction (1) in the production of OH(v)

  8. The two main reactions exciting OH in Venus atmosphere are: • H+ O3 --> OH(v) + O2, k =1.40e-10*exp(-470/ T), heat reaction: ~ 27,000 cm-1 (2) • HO2 +O --> OH(v) + O2, k=3.00e-11*exp(200.0/T) heat reaction: ~ 20,000 cm-1 (1) where T is the atmospheric kinetic temperature. • Given the exothermicities of these reactions, (2) has the potential to excite OH(v) levels up to v=9 and (1) up to levels v=6. • Due to exothermicity arguments, (3) and (4) do NOT contribute to OH(v)

  9. Here we use the well established values reported in Adler-Golden (1997): H + O3 --> OH(9), f (9) = 0.47 H + O3 --> OH(8), f (8) = 0.34 H + O3 --> OH(7), f (7) = 0.15 H + O3 --> OH(6), f (6) = 0.01 • Same as Adler-Golden but only for levels up to 6: HO2 + O --> OH(6), f (6) = 0.47 HO2 + O --> OH(5), f (5) = 0.34 HO2 + O --> OH(4), f (4) = 0.15 HO2 + O --> OH(3), f (3) = 0.01 • However, the literature (Kaye 1998): supposition is that v >= 4 not populated and 40% goes into v=1-3

  10. …this means… • The collisional removal rate constants for OH(v=1-9) by O2 and nitrogen has been measured by a number of groups • Adler-Golden or other nascent branching ratios/distributions are not really known for (2) for CO2 atm and (1) for any atmosphere • HENCE: there is no laboratory evidence showing which distribution we should consider for CO2, which is the case for Venus.

  11. Therefore, it is very important to consider the temperature dependence of the quenching rate constant for CO2. i.e. collisional cascading, sudden death, multi-quantum collisional quenching. • not exact yet: O3 not exactly correct due to eddy diffusion not exact. • H abundance does not vary diurnally, but O3 does plus is temperature dependent

  12. The Caltech 1-D KINETICS model (97 species/528 reactions) has been extended to include vibrational OH reactions for the Earth showing good agreement with MLS OH data and with observations of the Meinel bands (Pickett et al, 2006).

  13. KINETICS model (97 species/528 reactions) has been extended to include vibrational OH reactions, consistent with with observations of the Meinel bands (Piccioni et al, 2008).

  14. Conclusions and Future Work • Assess different branching ratios in parametric sensitivity study • Encourage lab work to determine removal rate constants and nascent branching ratio distributions • Run specific KINETICS cases matching VEx VIRTIS measurements and do airglow calculations using non-LTE model

  15. This increases the [H] fraction and increases the importance of the H + O3 --> OH + O2 reaction for Earth. • At slightly higher altitudes, [O] starts increasing rapidly with height because three-body formation of O3 from O also depends on the square of pressure. • The increase in [O] suppresses both [OH] and [HO2] through the reactions with O. • The lifetime of HOx increases with height because the loss depends on [OH] and [HO2], but not directly on [H].

  16. Conclusions • Observations of OH in its ground vibrational state show a night layer at ~82 km for the Earth and ~96 km for Venus • For Venus, we have dominance of reaction (1) over (2) above 80 km, which is different than for the Earth • (4) is also a significant source for OH above 80 km, it is small compared to reactions (1) and (3). • Further modeling will elucidate details, but results need to be researched further regarding temperature and latitudinal variations above this altitude

  17. A modified version of the Caltech 1-D model for Earth predicts a very narrow 1.6 km wide layer that is consistent with narrowest widths observed. However, the data show a distribution of widths from 1.6 km to 10.8 km with a global mean of 8.3 km that is most likely due to transport. A modified version of the Caltech 1-D model for Venus predicts a wider layer that is consistent with VIRTIS observations.

  18. Further modeling will elucidate details, but examination of the current model calculations show important H + O3 --> OH + O2 reaction for Earth apparently not important for Venus. • Modeling of OH at altitudes in lower pressure regions require explicit description of longer-term transport as well as explicit inclusion of OH vibrational state dependent chemistry (Bougher VTGCM). • VTGCM modeling simpler as no need to carry H along in the reaction set, or do H2O or HCl photolysis for production of H ……………………

  19. The pressure level of 0.42 Pa (85 km) is a critical pressure for OH. For altitudes below this pressure, more than 99% of the OH is in the ground vibrational state and chemistry is local for the Earth. • For altitudes above this pressure, there is an increasing fraction of OH in excited states and the chemical lifetime is longer than a month.

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