(a) Profile of photon flux with wavelength at selected altitudes. (b) Absorption cross sections of CO₂ (black), CO¹⁷O (blue), CO¹⁸O (orange), and ¹³CO₂ (red) with wavelength. (c) Relative difference in absorption cross section between species i and CO₂, defined as ﹩({\sigma }_{i}/{\sigma }_{{{\rm{CO}}}_{2}}-1)\times 1000﹩, where σ_i and ﹩{\sigma }_{{{\rm{CO}}}_{2}}﹩ are the absorption cross sections of species i and CO₂, respectively (i = CO¹⁸O, CO¹⁷O, ¹³CO₂) (J. A. Schmidt et al. 2013). Relative differences are smoothed using a Gaussian window with FWHMs of 2.5 nm, as done by J. A. Schmidt et al. (2013).