Detection of reactive nitrogen containing particles in the tropopause region? Evidence for a tropical nitric acid trihydrage (NAT) belt

The detection of nitric acid trihydrate (NAT, HNO<sub>3</sub>×3H<sub>2</sub>O) particles in the tropical transition layer (TTL) harmonizes our understanding of polar stratospheric cloud formation. Large reactive nitrogen (NO<sub>y</sub>) containing particles were observed on 8 August 2006 by instruments onboard the high altitude research aircraft M55-Geophysica near and below the tropical tropopause. The particles, most likely NAT, have diameters less than 6 μm and concentrations below 10<sup>-4</sup> cm<sup>−3</sup>. The NAT particle layer was repeatedly detected at altitudes between 15.1 and 17.5 km ove…