Revisiting Stratospheric Ozone Depletion

Stratospheric Ozone Depletion Hobe 2007 Lance Richards METEO 6030 02/19/08 http://polar%20stratospheric%20clouds&FORM=BIRE#focal=Fpolar_stratospheric_cloud.jpg Overview Ozone and our Atmosphere  Processes leading to Ozone Depletion  Antarctic Ozone Hole  ClO-dimer Catalytic Cycle  Controversial Pope et al. Conclusions  Future Health of Ozone Layer  Role of Ozone  90% of O3 is found in Stratosphere  15-50 km above surface with max concentration at 25 km   Absorbs lethal UV rays from the Sun In 1985, Farman et al. discovered an “ozone hole” in SH spring over the Antarctic http://ess.geology.ufl.edu/HTMLpages/ESS/GLY1033_notes/lecture4.html Importance of Ozone   Ozone blocking of most UV light incident at TOA Very important to maintain levels since no other gases help block this wavelength of radiation http://en.wikipedia.org/wiki/Absorption_band Ozone Depletion  Emission of CFC’s and other O3 depleting substances (ODS’s)  Broken apart by strong UV light  CFC’s release Chlorine atoms, while halons release Bromine atoms  1 Cl atom can destroy ≈ 105 O3 atoms  1 Br atom can destroy ≈ 6·106 O3 atoms  O3 levels remained relatively stable until the past few decades  Large increases in emissions of Cl and Br upset the natural balance Ozone Depletion http://jwocky.gsfc.nasa.gov/multi/oz_hole_area.gif Antarctic Ozone Polar vortex isolates stratospheric air over Antarctica  Polar Stratospheric Clouds (PSC’s) provide a surface to convert inert chlorine compounds to reactive species  Dramatic O3 depletion occurs in early SH spring (Sep) as the Sun rises and sparks the photochemical reactions  O3 loss coincides with temps low enough to produce PSC’s (195 K)  Antarctic Ozone   TOMS – Total Ozone Mapping Spectrometer O3 minimum contained within polar vortex  Traps air in it and prevents influx of warm air from outside  High ozone concentration air moving in from North ftp://toms.gsfc.nasa.gov/pub/omi/images/spole/Y2007/IM_ozspl_omi_20071001.png Ozone Depletion Processes  Two processes are key in cold polar stratosphere  HCl & ClONO2 are activated on PSC’s to form compounds which catalyze O3 destruction  Then these participate in ClO-dimer and ClO-BrO catalytic cycles ○ Rapidly destroy O3 at cold temperatures and high solar zenith angles ○ Responsible for 80% of spring-time polar ozone loss ClO-Dimer Cycle  Important in springtime polar stratosphere  High concentrations of chlorine above the Antarctic  Rapid procession of process  Chlorine activation and photochemical reactions convert the chlorine into active form responsible for ozone destruction Pope et al. 2007 Absorption Cross Section  The absorption cross section determines efficiency of light absorption  Controls the rate of photolysis & speed of the catalyzing cycles  Pope et al. suggests a much smaller cross section of the ClO-dimer  Would produce slower photolysis rate and less O3 destruction  Affects our ability to forecast future depletion Pope et al.  Pope et al. is consistent up to 300 nm, where it then diverges away from past results  Rates are far slower than those found in the field and with observed O3 losses in the stratosphere Hobe 2007 Pope et al.  High disagreement in Pope et al. when compared to previous studies and observations  50% reduction in O3 losses In situ & satellite observations best agree with Burkholder et al.  Possibility of missing O3 depletion reactions that must somehow be discovered and accounted for in models  Recommendations  Hobe recommends that “Modelers not use the new Pope et al. photolysis rate until the problem is completely resolved.”  By establishing new O3 destruction processes or refuting Pope et al.  Current techniques do a fine job modeling current losses and should continue to do so into the future Future Outlook    2006 Scientific Assessment of Ozone Depletion Return to pre-1980 levels around the year 2050, at which time all ODS’s would be phased out Long road to recovery of ozone layer Scientific Assessment of Ozone Depletion 2006 Conclusions Cold Temperatures (<195 K) High Zenith Angle Sunlight O3 Depleting Substances PSC Surfaces Isolation of Air Intensified Antarctic O3 Depletion Ozone layer is crucial for life on planet  Speed of catalytic cycles is a key process to understanding ozone formation & destruction  Pope et al. results should not yet be trusted 