Polar Stratospheric Clouds -Properties and Climate Impacts Haibin Li Outline Why these clouds? Properties of PSCs Definition; Occurrence; Type; Characteristics Measurements of PSCs Methods, example Climate Impacts Ozone depletion with PSCs; mechanism Why?? Relationship Understand with ozone depletion the past climate (Sloan et al., 1998) They are beautiful… “mother of pearl” http://www.sundog.clara.co.uk/atoptics/nacr1.htm http://alphais.inep.ksc.ru/knet_in/sky.htm http://www.nilu.no/projects/theseo2000/images/general/psc8.jpg Properties of PSCs Definition: High altitude clouds that form in the stratosphere above Antarctica during the Southern Hemisphere winter. Their presence seems to initiate the ozone loss experienced during the ensuing Southern Hemisphere spring. (http://earthobservatory.nasa.gov/Library/glossary.php3) Large, diffuse, ice-particle clouds that form in the stratosphere usually over polar regions.(http://www.grida.no/climate/ipcc/aviation/158.htm) Formation: During the polar night (i.e. winter) a strong circumpolar wind develops in the middle to lower stratosphere. This creates a vortex, which isolates the air above the poles. Due to a lack of sunlight the air inside the vortex cools and once the temperature falls below about 195K the probability of PSC formation increases very rapidly. Temperature decided Type PSCs Type and physical properties Type Temperature Particle Size Type Ia < 195 K ~ 1 μm Type Ib Type II Composition characterized by NAT (HNO3.3H2O) crystals (depolarizing solid) nitric acid/water/sulphuric acid Supercooled ternary solution (STS)(nondepolarizing liquid) ~ 188 K ~ 10 μm Small Ice Crystals ( Crutzen, 1986; Hofmann, 1989;Voigt et al., 2000;Toon et al., 1995) Measurements of PSCs Measuring Methods: ground-based and aircraft LIDAR systems (Stein et al., 1999) in-situ balloons-borne and aircraft instruments measuring particle sizes and densities through sampling and backscatter laser devices ( Voigt et al., 2000; Kivi et al., 2001) Satellite observations of absorption, emission and scattering of light by PSC particles (Foschi et al., 2002) Interested in: Particle size; surface area; volume; frequency of PSC events; different PSC types; the relation between the PSC occurrence and the environmental atmospheric conditions. Example of PSC Measurements X-axis :the backscatter ratio R (red line), the depolarization*20 (thin line), and the color ratio from two wavelength measurements. A: Early stage: few highly aspherical particles; B: mid winter: strong PSC layer of spherical particles; C: late winter: decaying PSC signals; http://www.awi-potsdam.de/www-pot/atmo/psc/psc.html#Lidar_Measurements_of_PSCs Climate Impacts:Ozone depletion and PSCs Phenomenon Mechanisms PSCs act as catalysts converting chlorine from the inert "reservoir" species to active species by heterogeneous chemical reactions on their surface. Details… HNO3·3H2O HCl ClONO2 HCl ClONO2 Cl-Cl Vaporize Cl-Cl HNO3 HNO3 ClONO2+HCl ICE → Cl2+HNO3 Hv ≤450 nm Cl-Cl O3 + Cl → ClO + O2 O3+hv(≤ 1140nm)→O2+O ClO + O → Cl + O2 Cl* *Cl Net: 2O3→3O2 ClONO2 ClH Future Stuff ??? Chlorine activation chemistry is sensitive to the chemical position and the sizes of PSC particles but PSC properties have never been adequately quantified, either observationally or theoretically. So far, size distributions are among the best known PSC properties(light scattering or parameterized lidar measurements). PSC chemical compositions are still very poor known --- the best guesses are based on equilibrium thermodynamic models ( Carslaw, 1995). How to understand them more realistically? More advanced&consistent satellite observation + improved models simulation.