Of the two solar lines, K I D_1 and D_2, almost all attention so far has been devoted to the D_1 line, as D_2 is severely affected by an O_2 atmospheric band. This, however, makes the latter appealing for balloon and space observations from above (most of) the Earth's atmosphere. We estimate the residual effect of the O_2 band on the K I D_2 line at altitudes typical for stratospheric balloons. Our aim is to study the feasibility of observing the 770 nm window. Specifically, this paper serves

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... a preparation for the third flight of the Sunrise balloon-borne observatory. The results indicate that the absorption by O_2 is still present, albeit much weaker, at the expected balloon altitude. We applied the obtained O_2 transmittance to K I D_2 synthetic polarimetric spectra and found that in the absence of line-of-sight motions, the residual O_2 has a negligible effect on the K I D_2 line. On the other hand, for Doppler-shifted K I D_2 data, the residual O_2 might alter the shape of the Stokes profiles. However, the residual O_2 absorption is sufficiently weak at stratospheric levels that it can be divided out if appropriate measurements are made, something that is impossible at ground level. Therefore, for the first time with Sunrise III, we will be able to perform polarimetric observations of the K I D_2 line and, consequently, we will have improved access to the thermodynamics and magnetic properties of the upper photosphere from observations of the K I lines.