Polar Stratospheric Clouds -Properties and Climate Impacts Haibin Li

Outline  Why these clouds?  Properties of PSCs Definition; Occurrence; Type; Characteristics Definition; Occurrence; Type; Characteristics  Measurements of PSCs Methods, example Methods, example  Climate Impacts Ozone depletion with PSCs; mechanism Ozone depletion with PSCs; mechanism

Why??  Relationship with ozone depletion  Understand the past climate (Sloan et al., 1998)  They are beautiful… “mother of pearl”

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. ( 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. ( Large, diffuse, ice-particle clouds that form in the stratosphere usually over polar regions.( Large, diffuse, ice-particle clouds that form in the stratosphere usually over polar regions.(  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. 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 TypeTemperature Particle Size Composition Type Ia < 195 K ~ 1 μm characterized by NAT (HNO3.3H2O) crystals (depolarizing solid) Type Ib nitric acid/water/sulphuric acid Supercooled ternary solution (STS)(nondepolarizing liquid) Type II ~ 188 K ~ 10 μm Small Ice Crystals PSCs Type and physical properties ( Crutzen, 1986; Hofmann, 1989;Voigt et al., 2000;Toon et al., 1995)

Measurements of PSCs Measuring Methods: 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: 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. 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;

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…

HNO 3 ·3H 2 O HCl ClONO 2 HCl ClONO 2 ClONO 2 ClH Cl-Cl HNO 3 Vaporize HNO 3 Cl-Cl Cl * * Cl Net: 2O 3 →3O 2 ClONO 2 +HCl → Cl 2 +HNO 3 ICE Hv ≤450 nm O 3 + Cl → ClO + O 2 O 3 +hv(≤ 1140nm)→O 2 +O ClO + O → Cl + O 2

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.