HR: 08:45h
AN: A31D-03    [Abstracts]
TI: Temperature Thresholds for Polar Stratospheric Ozone Loss
AU: * Drdla, K
EM: Katja.Drdla@nasa.gov
AF: NASA Ames Research Center, Mailstop 245-4, Moffett Field, CA 94035 United States
AB: Cold stratospheric temperatures are known to be responsible for the chlorine activation that leads to polar ozone depletion. It is commonly assumed that the formation of polar stratospheric clouds (PSCs) is a necessary prerequisite for chlorine activation. In particular, T_NAT, the temperature at which NAT (nitric acid trihydrate) can theoretically condense to form PSCs, is widely used as the critical temperature threshold for chlorine activation and ozone loss. This paper discusses the relevance of T_NAT, demonstrating that in the context of chlorine activation T_NAT is an inappropriate parameter that was adopted based on outdated and incorrect assumptions about PSC formation. Using current PSC theories, the parameters controlling chlorine activation are examined in a detailed model of microphysics and heterogeneous chemistry. Background liquid-phase aerosols alone are shown to control the onset of chlorine activation, without any significant HNO₃ condensation, i.e. without PSC formation. Even when NAT particles are present, their contribution to chlorine activation is generally insignificant compared with the liquid particles. Using liquid-phase reaction rates, a chlorine activation threshold temperature, T_ACl, is derived. While similar in value to T_NAT at typical stratospheric conditions, perturbations can cause T_ACl to differ from T_NAT: T_ACl is dependent upon H₂O, potential temperature, and the sulphate aerosol loading, but unlike T_NAT is not dependent upon HNO₃. A parameterization of T_ACl is provided, allowing it to be calculated over a comprehensive range of stratospheric conditions. T_ACl provides a more accurate description of the temperature conditions necessary for polar ozone depletion that can readily be used in place of T_NAT in future studies.
DE: 0305 Aerosols and particles (0345, 4801, 4906)
DE: 0320 Cloud physics and chemistry
DE: 0340 Middle atmosphere: composition and chemistry
SC: Atmospheric Sciences [A]
MN: Fall Meeting 2005