In-situ observations and modeling of nitric acid-containing particles in a cirrus cloud formation region

Measurements in nascent ice forming regions are very rare and help understand cirrus cloud formation and the interactions of trace gases with ice crystals. A cirrus cloud has very likely been probed in its formation stage with in-situ and remote sensing instruments onboard the high altitude research aircraft Geophysica M55 in the tropical 5 upper troposphere. Besides microphysical and optical particle properties, water (H 2 O) and nitric acid (HNO 3) have been measured. In slightly ice supersaturated air between 14.2 and 15 km altitude, an unusually low ice water content of 0.026 mg m −3 and small ice crystals with mean radii of 5 µm have been detected. A high HNO 3 /H 2 O molar ratio in ice of 5×10 −5 has been observed in this region, about an order of magnitude higher 10 compared to previous observations in cirrus at similar temperatures (near 202 K). A model describing the trapping of HNO 3 in growing ice particles shows that a high HNO 3 content in ice crystals is expected during early growth stages, mainly originating from uptake in aerosol particles prior to freezing. Water vapor deposition on ice crystals and trapping of additional HNO 3 reduces the molar ratio to values close to the ratio of 15 HNO 3 /H 2 O in the gas phase while the cloud ages.