Observations of Gas- and Condensed-Phase Nitric Acid in the Tropical Upper Troposphere and Lower Stratosphere

Nitric acid (HNO₃) serves as the primary reservoir species for nitrogen oxides, which are directly involved in the photochemical production of tropospheric ozone. Gas- and condensed-phase HNO₃ were observed for the first time in the tropical upper troposphere and lower stratosphere (UT/LS) during the NASA Pre-Aura Validation Experiment (Pre-AVE) in January, 2004, onboard the WB-57F high-altitude research aircraft. Vertical profiles of HNO₃ in the UT/LS reveal low values (150 pptv or less) near the tropical tropopause. Individual nitric acid-containing particles were observed between 3\deg S and 7\deg N during two Pre-AVE flights in the LS. These particles were observed at 18 km altitude and temperatures of 188-192 K, with number densities of 10^-4-10^-6 cm^-3. Understanding the formation mechanism of these particles might provide insight into ice particle formation at the tropical tropopause and the selective nucleation process for large nitric acid trihydrate (NAT) particles previously observed in the Arctic lower stratosphere.