Using gas-phase nitric acid as an indicator of PSC composition

Miriam von König, Holger Bremer, Armin Kleinböhl, Harry Küllmann, and Klaus F. Künzi
Institute of Environmental Physics, University of Bremen, Bremen, Germany

Albert P. H. Goede
Space Research Organisation of the Netherlands (SRON), Utrecht, The Netherlands

Edward V. Browell and William B. Grant
Atmospheric Sciences Research, NASA Langley Research Center, Hampton, Virginia, USA

John F. Burris, Thomas J. McGee, and Laurence Twigg
NASA Goddard Space Flight Center, Greenbelt, Maryland, USA

Abstract
The composition of polar stratospheric cloud particles is investigated using data from several remote sensing instruments: gas-phase HNO₃ measured by the Airborne Submillimeter Radiometer ASUR, temperature measured by the Airborne Raman Ozone, Temperature and Aerosol Lidar AROTEL, and aerosol backscatter wavelength dependence and depolarisation measured by the UV Differential Absorption Lidar DIAL. All three instruments have been operated onboard the NASA DC-8 during the SOLVE winter 1999/2000. A simple thermodynamical model is used to calculate the gas-phase amount of HNO₃ in the presence of polar stratospheric clouds along the flight-track of the DC-8 for one flight into the polar vortex on December 7, 1999. Three types of PSCs are considered in the model: Nitric acid trihydrate (NAT), nitric acid dihydrate (NAD) and supercooled ternary solutions (STS). The comparison of the modeled and measured gas-phase HNO₃ in the presence of PSCs shows a very good agreement if a NAT composition of the particles is assumed. However, sensitivity studies show that while the PSCs observed on this flight are not in agreement with a STS composition, the model is very sensitive to temperature, and a NAD composition of the PSC cannot be ruled out.