Occurrence of solid particles in the winter polar stratosphere above the nitric acid trihydrate coexistence temperature inferred from ground-based polarization lidar observations at Ny-Alesund, Spitsbergen

Depolarization data of stratospheric aerosols obtained by ground-based pola rization lidar are analyzed. The measurements were performed at Koldewey st ation, Ny-Angstrom lesund, Spitsbergen (78.9 degreesN, 11.9 degreesE), duri ng the five winters of 1994-1995 to 1998-1999. The study is focused on stra tospheric sulfuric acid aerosols; interference by nitric acid trihydrate co ntaining particles is excluded by imposing a temperature threshold conditio n of 2 K above the nitric acid trihydrate coexistence temperature. Under th e assumption that large sulfuric acid particles freeze first, T-matrix scat tering calculations show that volume depolarizations start to increase sign ificantly if similar to1% Of the aerosols are frozen. Observations in Janua ry 1999 and February 1998 indicate the presence of nonspherical particles. We interpret these observations as signatures for the occurrence of sulfuri c acid tetrahydrate particles. Temperature histories obtained from back tra jectory model calculations and the corresponding polarization lidar measure ments are consistent with current knowledge of sulfuric acid tetrahydrate f ormation.