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

Direct effect

Indirect cloud lifetime effect

Anthropogenic emissions

SO2

SO4--

BC

OC

Indirect cloud albedo effect

Cloud evaporationSemi-direct effect

ColumnBC

emissionsY2000

(kg C/m2/s)Bond et al.

(2004)van der Werfet al. (2003) (up to 6km)

ColumnPOM 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 34.66 Tg POM/yr

secondary 19.11 Tg POM/yr

Assumed 50% insoluble

BC/POM treatment in a GCM

80% (BC) 20% (BC)

50%(POM) 50%(POM)

Primary Emissions

Insoluble BC/POM Soluble/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 Emissions

(Tg C/yr) Burden (Tg C)

Lifetime (days)

NO-AGE 8.0 2.15 98.1 ALLSOL 8.0 0.09 4.5 FIX-LIFE 8.0 0.15 6.6 COND-COAG 8.0 0.11 5.0 Stier et al. (2005) 7.7 0.11 5.2 Chung-Seinfeld (2002) 12.5 0.22 6.4 Koch (2001) 12.4 0.15 4.4 Lohmann et al. (1999) 11.7 0.26 8.1

Annual and global mean POM burdens Emissions

(Tg POM/yr) Burden (Tg POM)

Lifetime (days)

NO-AGE 66.4 10.54 57.9 ALLSOL 66.4 0.76 4.2 FIX-LIFE 66.4 1.02 5.6 COND-COAG 66.4 0.80 4.4 Stier et al. (2005) 66.3 0.99 5.4 Chung-Seinfeld (2002) 95.7 1.39 5.3 Koch (2001) 89.9 0.95 3.9 Lohmann et al. (1999) 136.6 2.43 6.5

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

Model:Obs ratio

Model:Obs correlation

NO-AGE 77.6 0.50 ALLSOL 2.08 0.78 FIX-LIFE 1.96 0.80 COND-COAG 1.73 0.82

Surface layer POM: Model vs. observed

IMPROVE domain

BC and POM Model vs. observed IMPROVE summary

BC Model:Obs ratio

Model:Obs correlation

NO-AGE 2.84 0.20 ALLSOL 0.62 0.57 FIX-LIFE 0.78 0.63 COND-COAG 0.67 0.58

POM Model:Obs

ratio Model:Obs correlation

NO-AGE 2.07 0.85 ALLSOL 0.63 0.88 FIX-LIFE 0.74 0.87 COND-COAG 0.67 0.88

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?