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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 highP
ress
ure
, h
Pa
obsmodels60-80N
obsmodels20S-20N
Pre
ssu
re,
hP
a
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 regions
HIPPO(US)
Arctic(ARCTAS)
Asian outflow(A-FORCE)
US(HIPPO)
observedmodel
Wan
g e
t al
., a
ccep
ted
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 XMass absorption
coefficientX
Forcingefficiency
Global 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