  The optical properties of cold tropical tropopause clouds are examined on a
global scale, using two years of space-borne lidar observations from CALIPSO
(June 2006 ? May 2008). The linear depolarization ratio, color ratio and
backscatter signal are analyzed in tropical clouds colder than 200 K in a way
similar to recent studies of Polar Stratospheric Clouds (PSCs). It is found
that the three categories of particles encountered in PSC (Ice, Sulfate Ternary
Solutions or STS, and Nitric Acid Trihydrate or NAT) do also occur in tropical
cold cloud layers. Particles with optical properties similar to NAT are few,
but they cover the tropical belt and represent about 20% of cold cloud tropical
particles. The optical behavior of these particles requires them to be small,
non-spherical, optically thin, and persistent in the TTL at temperatures colder
than 200 K; NAT particles and very small ice crystals meet these criteria.
