Tropical cirrus clouds: a possible sink for ozone.

Using 7 years of ozone, humidity and temperature radiosoundings performed e very two-weeks at Reunion Island (21 degrees S, 55 degrees E), a systematic diminution of ozone mixing ratio at level of tropical cirrus clouds is evi denced. The average depletion of ozone concentration, as compared to adjace nt levels, is above 25%. This effect is further documented using concomitan t Lidar and radiosounding data. Ice cloud particles evidenced from polariza tion and backscattering Lidar data are observed at the same level than the ozone depletion in a detailed case study. The ozone concentration is 30% we aker at this level than outside the cirrus cloud. Backtrajectory analyses f urther indicate that the advection of O-3-poor layer injected from lower le vels is not likely.