Temperature dependence of ternary solution particle volumes as observed by lidar in the Arctic stratosphere during winter 1992/1993

Multiwavelength lidar measurements of stratospheric aerosols performed at the Arctic Network for the Detection of Stratospheric Change station on Spitsbergen during winter 1992/1993 are analyzed. Altitude profiles of particle median radius and volume density are derived for measurements with aerosol depolarization smaller than 0.01. Below an altitude corresponding to 450 K potential temperature the Pinatubo aerosol layer dominated the stratospheric aerosol content with volume densities df more than 5 mu m(3) cm(-3), whereas above 450 K, volume densities were close to background values of 0.1 mu m(3) cm(-3). However, at all altitude levels between 350 and 550 K, volume densities consistently increased by a factor of 2-30 when temperatures approached the frost point. The observations are compared to results from thermodynamic model calculations at altitude levels of 400, 440, and 480 K. Good agreement between the observed and theoretically derived temperature dependencies of volume density suggests that nondepolarizing polar stratospheric cloud particles, as well as volcanic aerosols, at low temperatures are composed of a ternary liquid solution of sulfuric and nitric acid. At all altitude levels, model results indicated more than 90% HNO3 gas phase depletion as temperatures approached the frost point. A mean profile of total H2SO4 volume mixing ratio is derived, decreasing from about 4 parts per billion by volume (ppbv) at 350 K to about 0.5 ppbv above 450 K.