The NASA ER‐2 aircraft flew on January 24, 1989, from Stavanger to Spitsbergen, Norway, at the 430–440 K potential temperature surface (19.2–19.8 km pressure altitude). Aerosols were sampled continuously by an optical particle counter (PMS‐FSSP300) for concentration and size analyses, and during five 10‐min intervals by four wire and one replicator impactor for concentration, size, composition, and phase analyses. During sampling, the air saturation of H₂O with respect to ice changed from 20% to 100%, and of HNO₃ with respect to nitric acid trihydrate (NAT) from subaturation to supersaturation. Data from both instruments indicate a condensation of hydrochloric acid and, later, nitric acid on the background aerosol particles as the ambient temperature decreases along the flight track. This heterogeneous nucleation mechanism generates type I polar stratospheric cloud particles of 10‐fold enhanced optical depth, which could play a role in stratospheric ozone depletion.