Condensed-phase nitric acid in a tropical subvisible cirrus cloud

In situ observations in a tropical subvisible cirrus cloud during the Costa Rica Aura Validation Experiment on 2 February 2006 show the presence of condensed-phase nitric acid. The cloud was observed near the tropopause at altitudes of 16.3 – 17.7 km in an extremely cold (183 – 191 K) and dry (<5 ppm H2O) air mass. Relative humidities with respect to ice ranged from 150– 250% throughout most of the cloud. Optical particle measurements indicate the presence of ice crystals as large as 90 mm in diameter. Condensed HNO3/H2O molar ratios observed in the cloud particles were 1 – 2 orders of magnitude greater than ratios observed previously in cirrus clouds at similar HNO3 partial pressures. Nitric acid trihydrate saturation ratios were 10 or greater during much of the cloud encounter, indicating that HNO3 may be present in the cloud particles as a stable condensate and not simply physically adsorbed on or trapped in the particles.