Figure 7:

Time constants for photochemical loss (Left) and net photochemical lifetime (Right) above the condensation region on Neptune for C₂H₆ (red solid line) and C₂H₂ (blue dashed line), along with the diffusion time constant for C₂H₆ (green dot-dashed line, nearly identical to that for C₂H₂), compared to a Neptune year (vertical black triple-dot-dashed line) and a Neptune season (orange dotted line) for −3° latitude, vernal equinox. The photochemical loss time scale is defined as the species concentration divided by the chemical loss rate; the net photochemical time scale is defined as the species concentration divided by the absolute magnitude of the chemical production rate minus the loss rate for the species. The latter gives a better measure of the species stability in the presence of efficient recycling processes; the sharp peaks occur where production and loss rates are nearly equal. The diffusion time scale is defined as the square of the generalized scale height divided by the generalized diffusion coefficient (see Moses and Greathouse, 2005). Note that chemical and diffusion time scales are shorter than a Neptune season in the peak production region from 1 to a few μbar but the net chemical lifetimes become longer than a Neptune season in the middle and lower stratosphere. (For interpretation of the references to color in this figure legend, the reader is referred to the online version of this article.)