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

Abstract

To examine the characteristics of polar stratospheric clouds (PSCs) in the Antarctic, we have analyzed shorttime ( 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 m. IntroductionPolar stratospheric clouds (PSCs) that are formed in winter polar vortices play a crucial role in determining rates of heterogeneous chemical reactions, which occur on the surface of the cloud particles and relate to the ozone destruction (e.g., WMO 2007). The compositions of PSC particles are mainly categorized as solid nitric acid trihydrate (NAT; HNO3-3•H2O), supercooled ternary (H2 SO4 /HNO3 /H2 O) solution (STS), and ice. In the Antarctic stratosphere, there are observational evidences that the nitric acid (HNO3) concentrations were significantly suppressed in an airmass after the gravitational sedimentation of particles (e.g., Fahey et al. 1990). This phenomenon is called "permanent" denitrification. When the PSC particles in an airmass are small so as not to sediment, the gas-phase HNO3 value decreases, although the total HNO3 (= gas phase + condensed phase) in the airmass is conserved. This is called "temporary" denitrification.The purpose of this study is to examine the time scale of temporary denitrification and characteristics of PSCs that caused the denitrification. For this purpose, short-time ( 5 days) changes in HNO3 and aerosol extinction coefficient (AEC) at 780 nm were analyzed with data taken by the Improved Limb Atmospheric Spectrometer (ILAS)-II aboard the Advanced Earth Observing Satellite (ADEOS)-II. An attempt has also been made to derive the size of PSC particles by applying theoretical calculations. ILAS-II dataILAS-II is a satellite-borne stratospheric observation sensor based on the solar occultation technique . ILAS-II continuously measured vertical profiles of 10 gases including O3, HNO3, NO2, N2 O, CH4, and H2O by the onion-peeling method. Its resolutions were 1.3 1.9 km at tangent altitudes between 15 and 25 km at 14 circumpolar points in each hemisphere at high latitudes (54 71°N and 64 88°S) from April through October 2003. Vertical profiles of aerosol extinction coefficients (AEC) at 780 nm were retrieved by the global-fit approach (Yokota et al. 2002). In this analysis, Version 1.4 HNO3, N2O, and AEC data were used.The accuracy in the HNO3 data is estimated to be better than 1...