tokens: (above 500 km) and 4 K at 200 km. Each observation was modeled independently and results for all five emission lines were combined using the technique provided in Nixon et al. ( 2008a ) Section 4 . A systematic error in the CO line intensity of 2% (Rothman et al. 2013 ) was also added to the measurement error. The best-fitting abundance for CO was found to be 49.6 1.8 ppm, which is consistent with and more accurate than previous measurements. Retrieved vertical temperature profiles from CO models were then used as fixed temperature profiles for modeling of the less abundant CO isotopologues ( 13 CO, C 18 O, and C 17 O.) For each isotopologue transition, the retrieved temperature profile which was closest in time to that observation was used as the fixed temperature profile with errors from that temperature retrieval propagated into the isotopologue retrieval. For a priori abundances, we assumed terrestrial isotopic ratios of 89.3 for 12 C/ 13 C, 498.8 for 16 O/ 18 O, and 2680.6 for 16 O/ 17 O (Lodders 2003 ). Best-fitting models of CO isotopologues were found using the same modeling procedures as CO (aside from the fixed temperature profile) and are presented in Figure 2 . The combined best-fitting 12 C/ 13 C ratio of 89.9 3.4 is consistent with similar studies of other molecules (Courtin et al. 2011 ; Mandt et al. 2012 ; Nixon et al. 2012 ), suggesting no significant carbon fractionation occurred during CO formation. Best-fitting ratios of 16 O/ 18 O and 16 O/ 17 O are 486 22 and 2917 359, respectively. Although the C 18 O (2–1) peak appears slightly under-fitted, the spectral residuals (model minus observation) for this transition indicate an optimal fit given the noise. The mixing ratios for C 18 O, derived independently from the J = 3–2 and J = 2–1 transitions, are consistent within the observational errors. Figure 2. Top: spatially integrated spectra of 13 CO with best-fitting models overlaid. Bottom: spatially integrated spectra of C 18 O and C 17 O (3–2) with best-fitting models overlaid. C 18 O (2–1) and C 17 O (3–2) are each combinations of two observations taken close in time to each other, while C 18 O (3–2) is a combination of three observations. Observations were combined in CASA before deconvolution. Additional systematic errors that have not been quantified in the final error estimate could exist. Sources of systematic errors include our assumption that Titan has the same temperature profile at all locations in our disc-averaged calculation, as well as our assumption that the Voigt profile is correct. Additional errors could exist in the line-broadening coefficients and temperature-dependence exponent used in the line database (Rothman et al. 2013 ), which could alter the line strength and shape. An accurate estimation of these systematic errors is difficult and beyond the scope of this paper, but these possible sources are mentioned for completeness. Our results constitute the most accurate determinations of atmospheric
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