1. Black Carbon Ageing in the CCCma GCM Betty Croft and Ulrike Lohmann Department of Physics and Atmospheric Science Dalhousie University, Halifax, N.S. Canada Knut von Salzen Canadian Centre for Climate Modelling and Analysis University of Victoria, Victoria, B.C. Canada 8 th International Conference on Carbonaceous Particles in the Atmosphere Vienna, Austria September 14, 2004

2. Outline Introduction BC ageing schemes CCCma GCM burdens, concentrations and lifetimes Comparison with surface layer observations Conclusions

3. Climate Effects of Carbon Aerosol

4. CCCma AGCM15b overview 3 year model simulations using T47 horizontal resolution (3.75x3.75). 35 vertical hybrid p-σ levels ( lid=50Pa). Prognostic variables include T, qv, ln(SP), vorticity, divergence, LWC and IWC.

5. BC treatment in CCCma GCM 80% 20% λ=24h Primary Emissions Hydrophobic BCHydrophylic BC Deposition Transport Deposition Transport

6. BC Burden – fixed 24h halflife

7. 24h Halflife vs. No Ageing Global and annual mean: 0.23 Tg C vs. 3.26 Tg C

8. Physical Processes related to ageing Aerosols H 2 SO 4 HNO 3 O 3 OH Coagulation Condensation Oxidation

9. BC ageing schemes - exponential decay - Lohmann et al. (1999) - Pöschl et al. (2002), Tsigaridis & Kanakidou (2003) - ozone and water vapour compete. - Riemer et al. (2004) - Day: fixed e-folding time (8h and 2h) - Night: use aerosol number concentration Fixed 24h half life Condensation/coagulation Oxidation

10. Oxidative parameterization Assume ageing occurs by oxidation of a layer of organic material (benzo(a)pyrene) that coats the soot. Use pseudo-first order decay rate coefficient Consider - number of surface sites on BC - surface residence times - sticking coefficients of O 3 and H 2 O - O 3 and H 2 O concentrations.

11. Condensation/coagulation based parameterization Day-time: above 250m τ = 2h below 250m τ = 8h Modification: if N<300 then τ = 35h Night-time: if N<4100/cm3 then τ = exp(-aN + b) if N>4100/cm3 then τ = (c + d/N) where a = 2.3012e4 ; b = 4.4428 c = 3.8585 ; d = 1.48e5

12. Internal mixture of: - sulfate - BC (hydrophylic) - OC (hydrophylic) - dust (acc mode) - sea salt (acc mode)

13. Internally mixed concentration

15. BC global and annual means

17. Alert, Canada Data from Sharma et al. (2004)

18. Amsterdam Island (Wolff and Cachier 1998)

19. Mace Head, Ireland (Cooke et al. 1997)

20. Uji, Japan Data from Höller et al. (2002)

21. IMPROVE USA domain mean

22. Vienna & Streithfen data Data courtesy of H. Puxbaum & A. Salam

23. Model/observations ratios Ratios of annual mean surface layer concentration (model/obs)

24. Model/observations ratios Ratios of annual mean surface layer concentration (model/obs)

25. Conclusions Preliminary results show that ageing based on a condensation/coagulation scheme (Riemer et al. 2004) gave the lowest annual and global mean burdens. An oxidative scheme gave the highest burdens. A physically based BC aging treatment is desirable in a GCM and allows the BC life to respond to regional conditions. Reasonable BC burdens can be predicted with the coagulation/condensation/oxidation parameterization. The global BC burden and ultimately the climate forcing is highly sensitive to the BC ageing parameterization in a global climate model.