radiative forcing of black carbon aerosol constraints from poletopole HIPPO observations across the Pacific Qiaoqiao Wang Daniel J Jacob J Ryan Spackman Anne E Perring Joshua P Schwarz ID: 395913
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Slide1
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, 2013Slide2
BC exported to the free troposphereis a major component of BC direct radiative forcing
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f
rontal
lifting
deep
convection
scavenging
BC source
region
(combustion)
Ocean
Export to free
troposphere
Global mean
BC profile
(Oslo CTM)
BC forcing
efficiency
Integral contribution
To BC forcing
Samset
and
Myhre
[2011]
50% from
BC > 5 kmSlide3
Multimodel
intercomparison and comparison to observationsMultimodel intercomparisons and comparisons to observations
Koch et al. [2009], Schwarz et al. [2010]
BC, ng kg
-1
TC4 (Costa Rica, summer)
Observed
M
o
d
e
l
s
Large overestimate must reflect
model
errors in scavenging
Free tropospheric BC in
AeroCom
models is ~10x too high
Pressure, hPa
obs
models
60-80N
obs
models
20S-20N
Pressure, hPa
HIPPO over Pacific (Jan)
BC, ng kg
-1
BC, ng kg
-1
This has major implications for IPCC radiative forcing estimatesSlide4
Previous application to Arctic spring (ARCTAS)
CCN
Cloud
updraft
scavenging
Large
scale
precipitation
Anvil
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
210
Pb
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 uncertaintySlide5
GEOS-
Chem BC simulation: source regions and outflowNMB= -27%NMB= -12%
NMB=
-28%
Observations (circles) and model (background)
surface
networks
AERONET
BC AAOD
NMB=
-32%
Aircraft profiles in continental/outflow regions
HIPPO
(US)
Arctic
(ARCTAS)
Asian outflow
(A-FORCE)
US
(HIPPO)
observed
model
Wang et al., accepted
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 firesSlide6
Comparison to HIPPO BC observations across the PacificModel doesn’t capture low tail, is too high at N mid-latitudes
Mean column bias is +48%Still much better than the AeroCom modelsWang et al., accepted
Observed Model
PDFSlide7
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 Slide8
BC top-of-atmosphere direct radiative forcing (DRF)
EmissionTg C a-1Global load(mg m-2)[% above 5 km]
BC AAOD
x100Forcing
efficiency
(W m
-2
/AAOD)
Direct radiative forcing (W m
-2
)
fuel+fires
This work
6.5
0.15 [8.7%]0.17114
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
efficiency
Global load
Absorbing aerosol optical depth (AAOD)Slide9
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