Same as Fig. 3, but over-plotted with a XO-2b specific model of the transmission spectrum, which uses a dayside-averaged temperature-pressure profile. The model is dominated in the optical by Na and K absorption signatures. Compared with the isothermal model of Fig. 3, the lower temperatures at higher-altitudes in this model lead to a better fit of the K-core 7664.9 Å measurement. Additional observations are planned at blue wavelengths (blue boxes), which will be highly sensitive to atmospheric sodium, and will help provide a more complete picture of the transmission spectrum.