Global budget and radiative forcing of black carbon aerosol: constraints from pole-to-pole

(HIPPO) observations across the Pacific

Qiaoqiao Wang, Daniel J. Jacob, J. Ryan Spackman, Anne E. Perring, Joshua P. Schwarz, Nobuhiro Moteki, Eloïse A. Marais, Cui

Ge, Jun Wang, Steven R.H. Barrett

Research funded by NSF

AGU talk on Dec 10, 2013

BC exported to the free troposphereis a major component of BC direct radiative forcing

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frontallifting

deep convection

scavenging

BC source region (combustion)

Ocean

Export to free troposphere

Global mean BC profile(Oslo CTM)

BC forcingefficiency

Integral contributionTo BC forcing

Samset and Myhre [2011]

50% fromBC > 5 km

Multimodel intercomparison and comparison to

observations

Multimodel intercomparisons and comparisons to observations

Koch et al. [2009], Schwarz et al. [2010]

BC, ng kg-1

TC4 (Costa Rica, summer)

ObservedModels

Large overestimate must reflect model errors in scavenging

Free tropospheric BC in AeroCom models is ~10x too highPr

essu

re, h

Pa

obsmodels60-80N

obsmodels20S-20N

Pres

sure

, hPa

HIPPO over Pacific (Jan)

BC, ng kg-1 BC, ng kg-1

This has major implications for IPCC radiative forcing estimates

Previous application to Arctic spring (ARCTAS)

CCN

Cloud updraft scavenging

Large scale precipitationAnvil precipitation

IN+CCN

entrainment

detrainment

GEOS-Chem aerosol scavenging scheme

CCN+IN,impaction • Below-cloud scavenging (accumulation mode aerosol),

different for rain and snow• BC has 1-day time scale for conversion from

hydrophobic (IN but not CCN) to hydrophilic (CCN but not IN)

• Scheme evaluated with aerosol observations worldwide• 210Pb tropospheric lifetime of 8.6 days (consistent with best estimate of 9 days)• BC tropospheric lifetime of 4.2 days (vs. 6.8 ± 1.8 days in AeroCom models)

Dealing with freezing/frozen clouds is key uncertainty

GEOS-Chem BC simulation: source regions and outflow

NMB= -27%

NMB= -12%

NMB= -28%

Observations (circles) and model (background)

surfacenetworks

AERONETBC AAOD NMB= -32%

Aircraft profiles in continental/outflow regionsHIPPO(US)

Arctic(ARCTAS)

Asian outflow(A-FORCE)

US(HIPPO)

observedmodel

Wan

g et

al.,

acc

epte

d

Normalized mean bias (NMB) in range of -10% to -30%

BC source (2009): 4.9 Tg a-1 fuel + 1.6 Tg a-1 open fires

Comparison to HIPPO BC observations across the Pacific

• Model doesn’t capture low tail, is too high at N mid-latitudes

• Mean column bias is +48%

• Still much better than the AeroCom models

Wang et al., accepted

Observed Model PDF

Zonal mean BC in GEOS-Chem Direct Radiative Forcing due to BC

• A four-stream broadband radiative transfer model for DRF estimates• Global BC DRF=0.19 W m-2 (AAOD=0.0017)• Uncertainty range based on atmospheric distribution

• AAOD: 0.0014-0.0026• DRF: 0.17- 0.31 W m-2

BC top-of-atmosphere direct radiative forcing (DRF)

EmissionTg C a-1

Global load(mg m-2)[% above 5 km]

BC AAODx100

Forcing efficiency(W m-2/AAOD)

Direct radiative forcing (W m-2)fuel+fires

This work 6.5 0.15 [8.7%] 0.17 114 0.19 (0.17-0.31)AeroCom [2006]

6.3 0.23 ± 0.07[21±11%]

0.18±0.08 168 ± 53 0.27 ± 0.06

Chung et al. [2012]

0.77 84 0.65

Bond et al. [2013]

17 0.55 0.60 147 0.88

• Our best estimate of 0.19 W m-2 is at the low end of literature and of IPCC AR5 recommendation of 0.40 (0.05-0.8) W m-2 for fuel-only

• Models that cannot reproduce observations in the free troposphere should not be trusted for DRF estimates Wang et al., accepted

DRF = Emissions X Lifetime X Mass absorption coefficient X Forcing

efficiencyGlobal load

Absorbing aerosol optical depth (AAOD)

Zonal mean BC in

• Observed BC concentrations across the Pacific range is very low, implying much more efficient scavenging than is usually implemented in models.

•The model with updated scavenging is able to reproduce the observed seasonality and latitudinal, and overall agrees with the HIPPO data within a factor of 2

• The simulation yields global mean BC AAOD of 0.0017 and DRF of 0.19 W m-2, reflecting low BC concentrations over the oceans and in the upper troposphere

• Previous estimates of DRF are biased high because of excessive BC concentrations over oceans and in the free troposphere

Conclusions