Aerosol 1st indirect forcing in the coupled CAM-IMPACT model: effects from primary-emitted particulate sulfate and boundary layer nucleation

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Aerosol 1st indirect forcing in the coupled
CAM-IMPACT model effects from primary-emitted
particulate sulfate and boundary layer nucleation

• Minghuai Wang and Joyce E. Penner
• Department of Atmospheric, Oceanic and Space
  Sciences
• University of Michigan
• AMWG 2008, Boulder, CO
• Thanks to Xiaohong Liu (PNNL)

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Introduction Aerosol microphysics
10 km
1 km
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Motivation
Boundary layer (BL) nucleation mechanism

• Most of global models only include binary
  homogeneous nucleation (BHN, H2SO4-H2O)
  mechanism. But observed new particle formation
  events in the BL cannot be explained by BHN.
• A boundary layer nucleation mechanism is added
  into our model and increases the simulated
  aerosol number concentration, and improves the
  comparison with observations (Wang et al., in
  preparation).

Primary-emitted particulate sulfate

• Most of global models assumed a certain amount of
  anthropogenic sulfur emitted as particulate
  sulfate, to take account of sub-grid scale
  nucleation and growth near strong sources of
  sulfur emissions
• It can increase aerosol number concentration
  significantly.

In this study

• We examine how primary-emitted particulate
  sulfate and boundary nucleation mechanism affect
  1st AIE

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Calculation Method for Indirect Effect
Size-resolved aerosol number concentration
Coupled CAM-IMPACT
NCAR CAM3
IMPACT (Liu et al., 2005)
Aerosol microphysics
Nucleation parameterization (Abdul-Razzak and
Ghan, 2000 2002)
Cloud droplet number concentration, effective
radius
Updraft velocity (Morrison et al. 2005)
re and rv relationship (Rotstayn and Liu 2003)
Met fields (CAM-IMPACT)
The aerosol first indirect forcing (1st AIE)
Radiative transfer model (CAM3)
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IMPACT aerosol model (Liu et al., 2005)

• Pure sulfate aerosol in 2 modes
• Nucleation (Rlt0.05 ?m), Accumulation
  (0.05?mltR)
• OM/BC fixed size distribution (Rlt1 ?m)
• Sea salt and dust in 4 size bins between 0.05
  and 10 µm
• Binary homogeneous nucleation (BHN, Vehkamaki et
  al. 2002)
• 2 of anthropogenic sulfur is emitted as
  particulate sulfate in two modes (PAR)
• Mode 1 rg 0.013µm, 1.6, 15(m)
• Mode 2 rg 0.068µm, 2.0, 85(m)

A boundary layer nucleation (BLN) mechanism is
added

• BL particle formation rate (1nm) is taken from
  observations (Kulmala et al., 2006)
• j1nmAH2SO4 , A1.0e-6/s (Sihto et al.
  2006)

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Cases

• REF no primary-emitted sulfate particle and no
  BL nucleation
• PAR REF primary-emitted sulfate particles
• BLN REF BL nucleation in the BL.
• BLN_PAR BLN primary-emitted sulfate particles

The effect of boundary layer nucleation
BLN vs. REF BLN_PAR vs. PAR
The effect of primary-emitted sulfate particles
PAR vs. REF BLN_PAR vs. BLN
The coupled CAM-IMPACT model was run for two
scenarios of aerosols Present day (PD) and
preindustrial period (PI).
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Cloud top CDNC, Reff in PD, 1st AIE in PAR
(primary-emitted sulfate)
High cloud droplet number over land, and low
droplet number over ocean Small droplet effective
radius over land, and large over ocean Change in
Reff is large over polluted regions 1st AIE is
large over regions with large change in Reff and
large cloud forcing
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Change in Reff (PDPI), and 1st AIE
Change in Reff between PD and PI (µm)
1st AIE (w/m2)
Primary-emitted sulfate (PAR) increases 1st
AIE Boundary layer nucleation (BLN) decreases 1st
AIE
So Why?
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Primary-emitted sulfate (PAR) Increase 1st AIE
Change in anthropogenic fraction of CCN (0.2)
(PD-PI)/PD
Change in 1st AIE from PAR
1st AIE NO BLN -1.55 to -2.02 w/m2 BLN
-1.49 to -1.65 w/m2
Anthropogenic fraction of CCN NO BLN 49 to
59 BLN 51 to 55
Reason primary-emitted sulfate forms CCN-size
particles more efficiently, and the percentage
change between PD and PI in primary-emitted
sulfate emissions is larger than the percentage
change for other primary emissions Boundary layer
nucleation offsets some effects of primary
emitted sulfate
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Boundary layer nucleation (BLN) Decreases the
1st AIE
Change in 1st AIE (w/m2) from BLN
Global
Large decreases over ocean
Small decreases over land (PAR) Increases
over land (NOPAR)
S_Ocean
S_Land
Change the spatial pattern of 1st AIE
N_Ocean
N_Land
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Boundary layer nucleation (BLN) Anthropogenic
fraction of CCN (0.2)
NOPAR (BLN REF)
Regime I decreases. The relative increase of SO2
from PI to PD is small and the relative increase
of pre-existing particle number is large.
Regime II increases. The relative increase of
SO2 from PI to PD is large and the relative
increase of the particle number is small.
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How sensitive is the 1st AIE to cloud types, and
assumed minimum droplet number, Nmin?
1st AIE (w/m2)
STD case PAR, standard configurations (liquid
cloud, total cloud fraction, Nmin20/cm3), four
months (Jan., Apr., Jul., and Oct.) WARM only T
gt 273K STRAT only stratiform clouds N40
Nmin 40/cm3 N10 Nmin 10/cm3
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Conclusion

• The decreases in the effective radius from
  anthropogenic emission can range from -0.86 to
  -1.23 µm depending whether primary-emitted
  sulfate, and BLN is included. The results for the
  1st AIE range from -1.49 to -2.03 w/m2.
• Primary-emitted particulate sulfate increases the
  1st AIE because it produces CCN-size particles
  more efficiently than does formation of particles
  by nucleation and because the percentage change
  between PD and PI in sulfur emissions is large.
• Boundary layer nucleation decreases the 1st AIE
  over ocean. Over land it slightly decreases the
  1st AIE when primary-emitted sulfate is included,
  but it increases the 1st AIE when primary-emitted
  sulfate is not included. It changes the pattern
  of the 1st AIE.
• Different assumptions regarding cloud types and
  the minimum value for cloud droplet number
  concentration have large impact on the estimation
  of 1st AIE.

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Aerosol size distribution in the marine boundary
layer model vs. observations (Heintzenberg et
al. 2000)
Enhanced boundary layer nucleation rates are not
able to explain the number concentrations of
small particles between 30S and 60S, but number
concentrations are reasonable in the
NH. Primary emitted sulfate causes a large
increase at northern latitudes.
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CDNC, Reff at cloud top in present day
Reff (µm)
CDNC (/cm3)
Both boundary layer nucleation (BLN) and primary
emitted sulfate (PAR) increase CDNC, decrease Reff
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Boundary layer nucleation Anthropogenic
fraction of CCN at 930 hPa
PAR (BLN_PAR PAR)
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Anthropogenic fraction of CCN