Observations of condensed-phase nitric acid in a tropical subvisual cirrus cloud

Observations in a tropical subvisual cirrus cloud during the Costa Rica Aura Validation Experiment (CR-AVE) on 2 February 2006 show the presence of condensed-phase HNO3. These measurements were made with the NOAA chemical ionization mass spectrometer onboard the NASA WB-57F high-altitude research aircraft. The cloud was observed at altitudes of 16-17 km in an extremely cold air mass (183-190 K). Ambient water vapor measurements show that relative humidities with respect to ice ranged from 100-240%. Optical particle measurements indicate the presence of large ice crystals in this cloud (50-100 μm diameter), as well as a particle mode in the 10-20 μm range. Ice particle surface area densities were typically less than 50 μm2 cm-3. Despite gas-phase HNO3 mixing ratios of less than 100 pptv, the cold ambient temperatures caused nitric acid trihydrate saturation ratios to be 10 or greater during much of the cloud encounter. This result raises the possibility that HNO3 may be present in the cloud particles as a stable condensate, and not simply physically adsorbed on the particles. Understanding the role of HNO3 in the formation and growth of cirrus ice particles may be essential in GCMs for correctly simulating the contribution of subvisual cirrus clouds to the radiative forcing of climate.