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This paper focuses on polar stratospheric cloud (PSC)measurements by the CALIOP (Cloud-Aerosol LIdar with OrthogonalPolarization) lidar system onboard the CALIPSO (Cloud-Aerosol Lidar andInfrared Pathfinder Satellite Observations) spacecraft, which has beenoperating since June 2006. We describe a second-generation PSC detectionalgorithm that utilizes both the CALIOP 532-nm scattering ratio (ratio oftotal-to-molecular backscatter coefficients) and 532-nm perpendicularbackscatter coefficient measurements for cloud detection. The inclusion ofthe perpendicular backscatter measurements enhances the detection of tenuousPSC mixtures containing low number densities of solid (likely nitric acidtrihydrate, NAT) particles and leads to about a 15% increase in PSC arealcoverage compared with our original algorithm. Although these low numberdensity NAT mixtures would have a minimal impact on chlorine activation dueto their relatively small particle surface area, these particles may play asignificant role in denitrification and therefore are an important componentof our PSC detection. In addition, the new algorithm allows discriminationof PSCs by composition in terms of their ensemble backscatter anddepolarization in a manner analogous to that used in previous ground-basedand airborne lidar PSC studies. Based on theoretical optical calculations,we define four CALIPSO-based composition classes which we call supercooledternary solution (STS), ice, and Mix1 and Mix2, denoting mixtures of STSwith NAT particles in lower or higher number densities/volumes,respectively. We examine the evolution of PSCs for three Antarctic and twoArctic seasons and illustrate the unique attributes of the CALIPSO PSCdatabase. These analyses show substantial interannual variability in PSCareal coverage and also the well-known contrast between the Antarctic andArctic. The CALIPSO data also reveal seasonal and altitudinal variations inAntarctic PSC composition, which are related to changes in HNO<sub>3</sub> andH<sub>2</sub>O observed by the Microwave Limb Sounder on the Aura satellite

CALIPSO polar stratospheric cloud observations: second-generation detection algorithm and composition discrimination