Temporary Denitrification in the Antarctic Stratosphere as Observed by ILAS-II in June 2003

To examine the characteristics of polar stratospheric clouds (PSCs) in the Antarctic, we have analyzed short-time (<= 5 days) changes in nitric acid (HNO3) and aerosol extinction coefficient (AEC) at 780 nm, focusing near 20 km altitude in June 2003 as observed by the Improved Limb Atmospheric Spectrometer (ILAS)-II. The Match technique based on the air parcel trajectory was applied to the ILAS-II data. The several Match pairs have revealed decreased HNO3 values with increased AEC values within short times, indicating "temporary" denitrification. It is also suggested that the observed PSCs could be nitric acid trihydrate (NAT) particles, considering that the temperatures were above existence temperatures for supercooled ternary solution, but below those for NAT. Given appropriate size distributions for NAT particles, it is suggested that the median radius of particles was less than 3 mu m.