1. The effect of stratospheric sulfur from Pinatubo on the oxidizing capacity of the troposphere Narcisa Bândă Maarten Krol Thomas Röckmann Twan van Noije Michiel van Weele 10 May 2011

2. http://www.esrl.noaa.gov/

3. OH - the tropospheric oxidant OH Source – ozone photolysis ( O 3 + hv 2OH ) Sink – reaction with CH4, CO, NMHCs, etc + H 2 O CH 4 Source – Emissions (natural, anthropogenic) Sink – reaction with OH ( NO 2 + hv NO + O 3 ) + O 2

4. SO2 aerosol SW UV Decrease in photolysis stratosphere troposphere

5. OH - the tropospheric oxidant OH Source – ozone photolysis ( O 3 + hv 2OH ) Sink – reaction with CH 4, CO, NMHCs, etc + H 2 O CH 4 Source – Emissions (natural, anthropogenic) Sink – reaction with OH ( NO 2 + hv NO + O 3 ) + O 2 JO 3 JNO 2

6. Model setup TM5 global chemistry model 3x2, 60 levels ECMWF ERA-Interim meteorology fields Aerosol module M7 coupled to photolysis M7 setup as recommended in Kokkola et al. 2009: no soluble coarse mode, accumulation mode σ=1.2 instead of 1.59 no tropospheric aerosols 18.5 Tg SO 2 emitted in the grid-cell containing Pinatubo on 15 th June 1991 between 20-24 km The effect on photolysis in the period June-December 1991: no Pinatubo with Pinatubo, without SO 2 absorption with Pinatubo, with SO 2 absorption

7. Injection 20-24 km Injection 23-27 km TOMS SO 2 plume evolution SO 2 lifetime: 36 days (MLS: 33 days, TOMS: 25 days) http://so2.gsfc.nasa.gov/

8. Sulphate aerosols Aerosol size (14-20 N) TM5 SAGE II

9. Effect on photolysis rates and OH at the surface SO 2 absorption on 22 June 1991 Aerosol scattering on 1 December 1991

10. Effect on photolysis rates and OH at the surface Aerosol scattering SO 2 absorption

11. Effect on photolysis rates and OH Average profile change 30S-30N due to aerosol scattering and SO 2 +OH Average profile change 0-30N due to SO 2 absorption SO 2 +OH

12. Tropospheric CH 4 sink Total: 0.9 Tg Total: 6.7 Tg

13. Conclusions The evolution of the SO 2 and sulphate aerosols can be modelled reasonably well The effect on global OH is less than 1% for SO 2 and 3% for aerosols, with larger effects regionally This results in decreases in the CH 4 sink of 1 Tg and 7 Tg respectively in the first 6 months after the eruption Future work: Improve plume evolution (injection height) Look at a 2-year period Study the effects of stratospheric ozone depletion, climate change after Pinatubo on OH, CH 4 (see Banda et al. ACP 2013) TM5 + EC-Earth

14. OH - the tropospheric oxidant OH Source – ozone photolysis ( O 3 + hv 2OH ) Sink – reaction with CH 4, CO, NMHCs, etc + H 2 O CH 4 Source – Emissions (natural, anthropogenic) Sink – reaction with OH ( NO 2 + hv NO + O 3 ) + O 2 JO 3 JNO 2

15. Acknowledgements Philippe le Sager (KNMI) Jason Williams (KNMI) Joost Aan de Brugh (SRON) Jeroen van Gent (BISA) Ulrike Niemeyer (MPI-M) NWO SSiRC