1. 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

2. BC exported to the free troposphere is a major component of BC direct radiative forcing frontal 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 km

3. 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) Observed Models 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 This has major implications for IPCC radiative forcing estimates

4. 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 uncertainty

5. GEOS-Chem BC simulation: source regions and outflow NMB= -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 fires

6. 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

7. 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

8. BC top-of-atmosphere direct radiative forcing (DRF) Emission Tg C a -1 Global load (mg m -2 ) [% above 5 km] BC AAOD x100 Forcing efficiency (W m -2 /AAOD) Direct radiative forcing (W m -2 ) fuel+fires This work6.50.15 [8.7%]0.171140.19 (0.17-0.31) AeroCom [2006] 6.30.23 ± 0.07 [21±11%] 0.18±0.08168 ± 530.27 ± 0.06 Chung et al. [2012] 0.77840.65 Bond et al. [2013] 170.550.601470.88 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)

9. 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