Arctic polar stratospheric clouds observed with the Improved Limb Atmospheric Spectrometer during winter 1996/1997

The newest retrieval (version 4.20) of the Improved Limb Atmospheric, Spectrometer (ILAS) on board the Advanced Earth Observing Satellite (ADEOS) captured more than 60 polar stratospheric cloud (PSC) profiles during the winter and early spring of 1997 in the Northern Hemisphere. That winter is well known for its long-lasting polar vortex and significant ozone loss over the Arctic. The ILAS PSC measurements were the only spaceborne measurements made on a regular basis (about 14 times daily) during that period. PSC events were selected by comparing an individual profile with a threshold value at each altitude that was defined as an average of the extinction coefficient of background aerosols plus five standard deviations. Many of the selected PSC events correspond to temperatures lower than the nitric acid trihydrate (NAT) temperature, which was calculated using nitric acid and water vapor data observed with ILAS. The correlation between the aerosol extinction coefficient and temperature shows that the extinction data increase as the temperature decreases to a point several degrees lower than the NAT temperature, suggesting the formation of particles of a supercooled ternary solution. Some of the nitric acid profiles corresponding to intense PSC events showed a decreased mixing ratio, suggesting the uptake of nitric acid in the gas-phase into particles. The highest probability of sighting PSCs was obtained in mid-January at an altitude of approximately 23 km, and subsequent occurrences of PSCs were found intermittently at lower altitudes until mid-March. The 1997 Arctic winter was characterized by the prolonged appearance of PSCs until mid-March, associated with a long-lasting polar vortex. The PSC data presented in this paper compensate for the gap in the long-term PSC record from space and help to reveal the chemical mechanisms that caused the Arctic ozone loss observed that season.