1. The impact of volcanic aerosols on stratospheric chemistry with implications for geoengineering Simone Tilmes (tilmes@ucar.edu) WACCM team, Doug Kinnison, Rolando Garcia, Anne Smith, Ryan Neely, Andrew Conley, Jean-Francois Lamarque Rolf Müller, Ross Salawitch, Tim Canty, Julia Jee-Taylor, Sasha Madronich, Kelly Chance SSiRC: 28 - 30 October 2013, Atlanta, Georgia, USA

2. The impact of volcanic aerosols on stratospheric chemistry with implications for geoengineering Impact of volcanic aerosols on total ozone Changes in chemical reaction rates with enhanced aerosols Importance of the aerosol distribution of polar ozone loss Importance of very short-lived species on the example of geoengineering

3. Volcanic aerosols have an important impact on ozone, result in a reduction of column ozone of about 3% for El Chichón and 5% for Mt Pinatubo Influence up to 5 years Column Ozone Change Ground based Observations, WMO 2006 ElChichón MtPinatubo Observations, WMO 2010 Total ozone deviations from 1964-80 mean

4. WACCM (SD) WACCM (no volcanoes, 2000-01 condition) ElChichón MtPinatubo NCAR WACCM CCMI Simulations WACCM results in slightly lower reduction (1-3%). Changes of chemistry do not explain reduced values of ozone 4 years after the eruption ElChichón MtPinatubo Column Ozone Change Observations, WMO 2010 Total ozone deviations from 1964-80 mean

5. Increase of heterogeneous reactions -> decrease of the NO x /NO y equilibrium (Fahey et al., 1993) (1) N 2 O 5 + H 2 O -> 2HNO 3 (2)ClONO 2 + H 2 O -> HOCl + HNO 3 T < 200 K as important as (1) (3)ClONO 2 + HCl -> HNO 3 + Cl 2 (4)HOCl + HCl -> Cl 2 + H 2 O -> increase in the ClO x, BrO x and HO x Impact on Ozone Destroying Cycles Surface Area Density (1992) WACCM Ozone Change (1992) Difference Volcanic – Clean NOx ClOx/BrOx HOx Ox, Total Loss ppm μm 2 /cm 3 New SAD Dataset for CCMI

6. Impact on Ozone Destroying Cycles Surface Area Density (1992) WACCM Ozone Change (1992) ppm μm 2 /cm 3 New SAD Dataset for CCMI Total Column change

7. Ozone depletion depends on temperatures in the polar vortex 1992, 1993: Low PSC Formation Potential, however larger Ozone Loss Arctic Polar Ozone Loss Ozone depletion dependent on surface area density (Drdla and Müller, 2012) 1992, 1993: line up in relation to ozone Are models able to simulate high ozone loss in 1993? Estimation from Observations (HALOE satellite) Tilmes et al., 2008

8. Arctic Polar Ozone Loss Estimation from Observations WACCM Volcanic – Clean, March 1993  expected impact of volcanic aerosols ~70 DU ~Impact of aerosols (~70DU)

9. Arctic Polar Ozone Loss Estimation from Observations WACCM Volcanic – Clean, March 1993 SAD μm 2 /cm 3  expected impact of volcanic aerosols ~70 DU (~30%)  simulated reduction around 40 DU Issue: prescribed surface area density field (monthly and zonal averages) are not in alignment with vortex dynamics Maximum around 40 DU

10. ~Impact of aerosols (70DU) Arctic Polar Ozone Loss Estimation from Observations WACCM Volcanic – Clean, March 1993  expected impact of volcanic aerosols ~70 DU (~30%)  simulated reduction around 40 DU Issue: prescribed surface area density field (monthly and zonal averages) are not in alignment with vortex dynamics  Modified SAD: Ozone loss up to 65 DU  MERRA temperatures too warm? Maximum around 40 DUMaximum around 65 DU SAD μm 2 /cm 3 SAD from 1992 + increased high latitudes

11. % Difference Equivalent Latitude Impact of Geo-engineering SAD on Ozone in 2040 Impact of different assumptions of VSL halogens in the stratosphere Feb/March/April % Change in Column Ozone μm 2 /cm 3 Rasch et al., 2008: 2Tg S yr -1, Tropical Injection 10 o N-10 o S Surface Area Density (2040) Aug/Sep/Oct Equivalent Latitude Tilmes et al., 2012 Considering the impact of VSLS on ozone loss: -Tropics: sign change with very short-lived species -Mid-latitudes: doubling of ozone loss; enhancement of the BrO and HOx catalytic cycles -Polar LMS: significant increase of ozone loss; enhancement of BrOx and ClOx cycles

12. % Difference Equivalent Latitude Impact of Geo-engineering SAD on Ozone in 2040 Impact of different assumptions of VSL halogens in the stratosphere Feb/March/April % Change in Column Ozone μm 2 /cm 3 Rasch et al., 2008: 2Tg S yr -1, Tropical Injection 10 o N-10 o S Surface Area Density (2040) Aug/Sep/Oct Equivalent Latitude Tilmes et al., 2012 Considering the impact of VSLS on ozone loss: -Tropics: sign change with very short-lived species -Mid-latitudes: doubling of ozone loss; enhancement of the BrO and HOx catalytic cycles -Polar LMS: significant increase of ozone loss; enhancement of BrOx and ClOx cycles

13. Volcanic aerosols impact global ozone for up to 5 years Largest local reduction in the Arctic polar Aerosol loading critical for models to estimate the impact of aerosols on ozone Understanding of the amount of Chlorine and Bromine critical for impact of potential geoengineering applications on ozone Summary