Nitric acid uptake on subtropical cirrus cloud particles

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Abstract
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 the Cirrus Regional Study of Tropical Anvils and Cirrus Layers‐Florida Area Cirrus Experiment (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 197–224 K and pressures of 122–224 hPa. Maximum levels of condensed‐phase HNO3 approached the gas‐phase equivalent of 0.8 ppbv. Ice particle surface coverages as high as 1.4 × 1014 molecules cm−2 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 μm and particle fall velocities up to 10 m s−1, some observed clouds have significant potential to redistribute HNO3 in the upper troposphere.