Nitric Acid Uptake on Subtropical Cirrus Cloud Particles

The redistribution of HNO₃ via uptake and sedimentation by cirrus cloud particles is considered an important term in the upper tropospheric budget of reactive nitrogen. Numerous cirrus cloud encounters by the NASA WB-57F high-altitude research aircraft during CRYSTAL-FACE were accompanied by the observation of condensed-phase HNO₃ with the NOAA chemical ionization mass spectrometer. The instrument measures HNO₃ with two independent channels of detection connected to separate forward- and downward-facing inlets that allow a determination of the amount of HNO₃ condensed on ice particles. Subtropical cirrus clouds, as indicated by the presence of ice particles, were observed coincident with condensed-phase HNO₃ at temperatures of 197 K - 224 K and pressures of 120 hPa - 224 hPa. Maximum levels of condensed-phase HNO₃ approached the gas-phase equivalent of 0.8 ppbv. Ice particle surface coverages as high as 1.4x10 ¹⁴ molecules cm^-2 (14% of a HNO₃ monolayer) were observed. A dissociative Langmuir adsorption model, when using an empirically derived HNO₃ adsorption enthalpy of -11.0 kcal mol^-1, effectively describes the observed molecular coverages to within a factor of 5. The percentage of total HNO3 in the condensed phase ranged from near zero to 100% in the observed cirrus clouds. With volume-weighted mean particle diameters up to 700 microns and particle fall velocities up to 10 m s^-1, some observed clouds have significant potential to redistribute HNO₃ in the upper troposphere.