N2 and H2 broadened isobutane infrared absorption cross sections and butane upper limits on Titan

New Constraints on Titan’s Stratospheric n-Butane Abundance

“…These were obtained for a variety of atmospheric profiles either photochemically predicted or set as constant above 70 km (Krasnopolsky 2009;Loison 2019). In all cases, our results are consistent with previous upper limits for n-butane established in the literature (Hewett et al 2020). Though n-butane has certainly not been firmly detected through our work, we have demonstrated tentative statistical evidence for its presence within Titan's stratosphere, particularly in observations of Titan's equator.…”

“…They used the cross sections from the Pacific Northwest National Laboratory (Sharpe et al 2004), which were obtained at 278 K. As that temperature is closest to (although still significantly warmer than) our retrieved stratospheric temperatures for the EQ data, it is useful to compare those results as a starting point. From Figure 8 and Table 3, it is seen that each of our 3σ upper limits is consistent with the result of Hewett et al (2020). For instance, using the Loison profile, we find abundances less than 200 ppb in the stratosphere.…”

“…The colors of the upper-limit points correspond to the similarly colored profile curves. The vertical blue dashed line marks the upper limit (513 ppb) derived by Hewett et al (2020), discussed further in Section 5.…”

“…In some cases, however, our results do provide tentative evidence for the presence of n-butane in Titan's atmosphere. Recently, Hewett et al (2020) used infrared cross sections to derive an upper limit of 513 ppb for n-butane's abundance. They used the cross sections from the Pacific Northwest National Laboratory (Sharpe et al 2004), which were obtained at 278 K. As that temperature is closest to (although still significantly warmer than) our retrieved stratospheric temperatures for the EQ data, it is useful to compare those results as a starting point.…”

“…The butane molecule (Figure 1) has been predicted to be present in detectable abundances in Titan's atmosphere. In spite of this, it remains elusive to remote detection efforts, though its abundance has been constrained by Hewett et al (2020), who found an upper limit of 513 ppb. Photochemically predicted abundances for butane at 200 km in altitude range from order 1 (Krasnopolsky 2009) to 370 ppb (Loison 2019) to even as high as 1100 ppb (Dobrijevic et al 2016).…”