Black Carbon Ageing in the CCCma GCM

Betty Croft and Ulrike LohmannDepartment of Physics and Atmospheric Science

Dalhousie University, Halifax, N.S. Canada

Knut von SalzenCanadian Centre for Climate Modelling and Analysis

University of Victoria, Victoria, B.C. Canada

8th International Conference on Carbonaceous Particles in the Atmosphere Vienna, Austria

September 14, 2004

Outline

IntroductionBC ageing schemesCCCma GCM burdens, concentrations and lifetimesComparison with surface layer observationsConclusions

Direct effect

Indirect cloud lifetime effect

Anthropogenic emissions

SO2

SO4--

BCOC

Indirect cloud albedo effect

Cloud evaporationSemi-direct effect

Climate Effects of Carbon Aerosol

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.

BC treatment in CCCma GCM

80% 20%

λ=24h

Primary Emissions

Hydrophobic BC Hydrophylic BC

Deposition

Transport

Deposition

Transport

BC Burden – fixed 24h halflife

24h Halflife vs. No Ageing

Global and annual mean: 0.23 Tg C vs. 3.26 Tg C

Physical Processes related to ageing

Aerosols

H2SO4

HNO3

O3 OH

Coagulation Condensation

Oxidation

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

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 coefficientConsider - number of surface sites on BC

- surface residence times - sticking coefficients of O3 and H2O - O3 and H2O concentrations.

Condensation/coagulationbased 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

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

Internally mixed concentration

BC global and annual means

No-ageing

Fix-life

Cond-coag

Mod- cond-coag

Oxid Mod-cond-coag-oxid

Emissions (Tg C/yr)

13.09 13.09 13.09 13.09 13.09 13.09

Burden (Tg C)

3.26 0.23 0.17 0.19 0.32 0.18

Lifetime (d)

90.8 6.5 4.7 5.4 8.9 5.1

Wet Deposition (Tg C/yr)

7.74 9.19 9.14 9.12 9.10 9.09

Dry Deposition (Tg C/yr)

4.95 3.90 3.95 3.97 3.99 4.00

Alert, Canada

Data from Sharma et al. (2004)

Amsterdam Island

(Wolff and Cachier 1998)

Mace Head, Ireland

(Cooke et al. 1997)

Uji, Japan

Data from Höller et al. (2002)

IMPROVE USA domain mean

Vienna & Streithfen data

Data courtesy of H. Puxbaum & A. Salam

Model/observations ratios

Ratios of annual mean surface layer concentration (model/obs)

Austria USA Japan Alert No-ageing

1.96 4.18 0.38 23.85

Fix-life 1.27 0.77 0.18 3.84 Cond-coag

1.11 0.66 0.16 2.99

Mod-cond-coag

1.16 0.73 0.17 3.22

Oxid 1.30 0.84 0.19 4.33 Mod-cond-coag-oxid

1.13 0.63 0.16 3.02

Model/observations ratios

Ratios of annual mean surface layer concentration (model/obs)

Amsterdam Island

Mauna Loa

Mace Head

Antarctica

No-ageing

66.16 157.7 5.83 308.37

Fix-life 3.84 4.04 1.03 19.97 Cond-coag

2.99 2.73 0.86 17.60

Mod-cond-coag

3.22 3.36 0.90 21.64

Oxid 4.33 5.76 1.18 23.68 Mod-cond-coag-oxid

3.02 3.16 0.86 20.15

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.