Carbonaceous aerosols – a global modeling view Betty Croft and Ulrike Lohmann * Department of Physics and Atmospheric Science Dalhousie University, Halifax, N.S. Canada *: now at ETH Zurich, Zurich, Switzerland Knut von Salzen Canadian Centre for Climate Modeling and Analysis University of Victoria, Victoria, B.C. Canada September Retreat – Propstei St Gerold, Austria – September 21, 2005

Outline BC and POM emissions Ageing of insoluble aerosols Burdens, concentrations and lifetimes Model to observation comparison Summary Future work - unanswered questions

Carbonaceous aerosols and climate

Column BC emissions Y2000 (kg C/m2/s) Bond et al. (2004) van der Werf et al. (2003) (up to 6km)

Column POM emissions Y2000 (kg POM/m2/s) Bond et al. (2004) van der Werf (2003) Guenther et al. (1995) (up to 6km)

BC and POM annual emissions BC: fossil fuel 3.04 Tg C/yr biofuel 1.63 Tg C/yr open burning 3.04 Tg C/yr Assumed 80% insoluble POM: fossil fuel 3.20 Tg POM/yr biofuel 9.09 Tg POM/yr open burning Tg POM/yr secondary Tg POM/yr Assumed 50% insoluble

BC/POM treatment in a GCM 80% (BC) 20% (BC) 50%(POM) Primary Emissions Insoluble BC/POMSoluble/Mixed BC/POM Deposition Transport Deposition Transport

Physical and chemical ageing Insoluble BC/POM  Soluble/mixed Aerosols H2SO4 HNO3 OH O3 Coagulation Condensation Oxidation

BC and POM ageing in GCMs Insoluble  Soluble/mixed Treatment options 1) Fixed exponential decay 2) Stier et al. (2005) condensation and coagulation explicit 3) Riemer et al. (2003) day: condensation – fixed e-folding time night: coagulation – e-folding time ~ number 4) Oxidation based on Pöschl et al. (2001)

GCM description Horizontal resolution: T47 (3.75° x 3.75°). Vertical resolution: 35 levels up to 50 hPa. Prognostic variables: temperature, specific humidity, surface pressure, vorticity, divergence, and liquid and ice water content. 3-year simulations following 5 month spin-up using the CCCma AGCM.

Annual mean BC burdens (mg C/m2)

Annual mean POM burdens (mg POM/m2)

Annual and global mean BC burdens

Annual and global mean POM burdens

Zonal and annual mean BC (ng C/m3)

Zonal and annual mean POM (ng POM/m3)

Emissions inventory issues Inventory uncertainty is at least a factor of two BC burdens using different inventories Emit : 8.0 Tg C /yr versus 13.1 Tg C/yr Burden: 0.15 Tg C 0.23 Tg C

Surface layer BC: Model vs. observed Red *- OBS Black o - NA Blue o - FL Green * - CC Black * - AS Blue * - FL2

BC Model vs. observed domain summary

Surface layer POM: Model vs. observed IMPROVE domain

BC and POM Model vs. observed IMPROVE summary

Summary Global and annual mean burdens (lifetimes) are 0.11 Tg C (5.0 days) and 0.80 Tg POM (4.4 days) for BC and POM, respectively. Physically based ageing is faster than use of a fixed e-folding time (24 h half life) and gives lower burdens. Chemically based ageing is not well understood and not modelled. BC and POM tend to be under-predicted at continental sites but over-predicted at remote sites. This suggests that emissions are low, but also either the transport is too diffusive or the deposition is too slow.

Future work Validation of carbon fields – (AERONET, satellite). Validation/improvement of scavenging parameterizations. Future climate studies– relatively more open burning emissions while sulphate production is controlled. What is the impact of increasing “carbon domination” on aerosol ageing, removal and concentrations?

Aerosol modeling questions What are the main chemical ageing processes for BC and POM? How does condensation compete with nucleation? Relative importance of physical versus chemical ageing on global scale (BC, POM and dust)? What are the main secondary organic aerosol production pathways and global yields? Can emissions inventory uncertainty be reduced, and how good are the assumptions about the insoluble fraction of BC and POM emissions?