Nitric acid trihydrate (NAT) in polar stratospheric clouds

A comprehensive investigation of polar stratospheric clouds was performed o n 25 January 2000 with instruments onboard a balloon gondola flown from Kir una, Sweden. Cloud layers were repeatedly encountered at altitudes between 20 and 24 kilometers over a wide range of atmospheric temperatures (185 to 197 kelvin). Particle composition analysis showed that a large fraction of the cloud layers was composed of nitric acid trihydrate (NAT) particles, co ntaining water and nitric acid at a molar ratio of 3:1; this confirmed that these long-sought solid crystals exist well above ice formation temperatur es. The presence of NAT particles enhances the potential for chlorine activ ation with subsequent ozone destruction in polar regions, particularly in e arly and late winter.