Black carbon emitted in the aircraft exhaust plume has the potential to seed cirrus clouds, as well as modify optical properties of existing clouds. Optical differences between natural- and aviation-induced cirrus are not well characterized or understood. This study combines datasets containing advected aircraft locations with two sources of LIDAR observations. We find that ice clouds that correspond to the locations of aircraft exhaust plumes show higher depolarization ratios (mean increase of 3.36% [95% CI: 3.19% to 3.54%]). This increase in depolarization occurs without a proportional increase in backscatter, but with a large increase in extinction (mean increase from 5.58e-5 [95% CI: 3.70e-5 to 7.50e-5] to 1.78e-4 [95% CI: 1.38e-4 to 2.17e-4]). Using linear optical scattering theory, we show that these changes are well explained by the inclusion of black carbon within the ice crystals. No suitable explanation has previously been offered to explain this measured increase in depolarization.