1. Modeling Carbon Species in the Atmosphere of Neptune and Comparison with Spitzer Observations Xi Zhang 1, Mao-Chang Liang 2, Daniel Feldman 1, Julianne Moses 3, Victoria Meadows 4, Glenn Orton 5, and Yuk L Yung 1 1 Caltech 2 Academia Sinica 3 LPI 4 UW 5 JPL ABSTRACT. Recent observations made by the IRS instrument onboard Spitzer provide new constraints on the abundances of hydrocarbons in the atmosphere of Neptune. A one-dimensional, diurnally averaged photochemical model has been used to simulate the Spitzer results. We show that the molar fractions of complex hydrocarbons, such as benzene (C6H6), are extremely sensitive to the prescribed eddy diffusion profile, but simple hydrocarbons like ethane (C2H6) and acetylene (C2H2) are less sensitive to the eddy diffusivity. For example, reducing the eddy coefficients by a factor of 5 would enhance hydrocarbon abundances such as C4H2, CH3C2H, and C6H6 by more than an order of magnitude. In order to give the best fit to the methane data from Spitzer, we must enhance the concentration of methane at lower boundary over that given by Moses et al (2005). A sensitivity study with respect to the changes of eddy mixing, selected chemical reactions and CH4 concentration at the lower boundary are presented.. Results from Chemical Model Our model uses Moses et al. (2005) as reference. The mixing ratio of CH4 at the lower boundary is increased to 5.6e-2. Enhanced flux of CO (200 times) at the top is required to reproduce the JCMT observations. The crosses in the figure are from Spitzer measurements. The CO profile (dashed line) is taken from Hesman et al. (2006). ►► Sensitivity Study The sensitivity of species to the eddy diffusivity is studied in the Caltech/JPL 1-D photochemical model. The results showing here are based on three typical cases. Solid line is based on Moses et. al. (2005), dotted line is the case for 5 times of the eddy diffusion profile, while dashed line is the case for one tenth of the eddy diffusion profile. Note that the heavy hydrocarbons, such as benzene (C6H6), are very sensitive to changes in the the eddy diffusivity. Conclusions ▲ The revised chemistry by Moses et al. (2005) can account for the abundances of hydrocarbons on Neptune. Sensitivity study shows that some hydrocarbons are extremely sensitive to the eddy diffusivity in the atmosphere. ▲ The methane concentration at the tropopause is well above that of saturation. However, the rough balance between heating and cooling rates suggest provides independent confirmation of methane supersaturation.. ▲ A large flux of oxygen in the form of CO (not H2O or CO2) is needed to explain the CO profile. References Hesman, B.E., et al., Icarus 186, 342, 2007 Meadows, V. S., G. Orton, M. Line, M.-C. Liang, Y. L. Yung, J. Van Cleve, M. Burgdorf, First Spitzer Observations of Neptune: Detection of New Hydrocarbons, Geophys. Res. Lett. 2007 (submitted). Moses, J. I., et al. J. Geophys. Res. 110, Art. No. E08001, 2005 Orton, G. S., et al. Icarus 189, 544, 2007