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 H 2 O) 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 μ m in diameter. Condensed HNO 3 /H 2 O molar ratios observed in the cloud particles were 1–2 orders of magnitude greater than ratios observed previously in cirrus clouds at similar HNO 3 partial pressures. Nitric acid trihydrate saturation ratios were 10 or greater during much of the cloud encounter, indicating that HNO 3 may be present in the cloud particles as a stable condensate and not simply physically adsorbed on or trapped in the particles.