Aerosol Size-Dependent Below-Cloud Scavenging in the ECHAM5-HAM GCM

1
Aerosol Size-Dependent Below-Cloud Scavenging in
the ECHAM5-HAM GCM 1B.
Croft, 2U. Lohmann, 1R. Martin, 3P. Stier, 4J.
Feichter, 5S. Wurzler, and 2S. Ferrachat 1Dalhousi
e University, Dept. Physics Atmospheric
Science, Halifax, Canada, croft_at_mathstat.dal.ca
2ETH Zurich, Zurich, Switzerland 3California
Institute of Technology, Pasadena, U.S.A. 4Max
Planck Institute for Meteorology, Hamburg,
Germany
5Landesumweltamt Nordrhein-Westfalen, Essen,
Germany

AGU Fall Meeting 2006, Presentation
Number A33B-0983
4. Annual-Global Aerosol Deposition Processes
A) Constant Modal Scavenging B) Aerosol
Size-Dependent Scavenging I) Sulphate Burdens
A) 0.86
Tg S B) 0.81 Tg S II) Black Carbon Burdens
A) 0.113 Tg C B) 0.106 Tg C III) Dust
Burdens A) 18.7 Tg B) 16.5 Tg IV) Sea
Salt A) 11.6 Tg B) 11.0 Tg

• Abstract
• Wet scavenging processes largely control the
  global aerosol abundance and, ultimately, aerosol
  effects on the radiation budget of the climate
  system. Present-day general circulation models
  (GCMs) treat the below-cloud scavenging processes
  in a manner that neglects much of the significant
  dependency of aerosol scavenging on size. A
  below-cloud aerosol scavenging parameterization
  that represents the details of the aerosol
  size-dependency of the rain scavenging
  coefficients is presented and applied in the
  ECHAM5-HAM GCM. The relative contribution of
  below-cloud scavenging to the total deposition is
  quantified for sulphate, black carbon, sea salt,
  and dust. A comparison is made with the common
  assumption that constant scavenging coefficients
  may be applied in GCMs for the nucleation,
  Aitken, accumulation, and coarse aerosol size
  modes. Comparison with observed deposition is
  also shown.

6. Comparison with Observed Deposition A)
Constant Modal Scavenging B) Aerosol
Size-Dependent Scavenging
Figure 4 Relative contributions of wet
deposition (in-cloud and below-cloud components),
dry deposition, and sedimentation to the total
mass deposition of the various aerosol species,
including sulphate, black carbon, dust and sea
salt. Same legend applies to all figures.
Figure 6 Year 2000 observed U.S. sulphate
deposition from the U.S. National Atmospheric
Deposition Program (NADP) is compared to the
sulphate deposition predicted by the ECHAM5-HAM
GCM. One to one correspondence is shown by the
dashed red line, and the solid red lines show a
factor of two. The regression line is shown in
blue. Red and green data points show that the
model over-estimated, or under-estimated the
observed precipitation by a factor of two or
more, respectively. The green and red points are
excluded from the regression and correlation
coefficients in order to remove deposition that
is biased due to errors in the modeled
precipitation.
2. Introduction and Model Description The ECHAM5
GCM (Roeckner et al., 2003), coupled to the HAM
aerosol model (Stier et al., 2005), predicts
aerosol modal radii for seven lognormal modes.
All results shown are from a one-year simulation
following a three month spin-up period. In the
standard model version, the below-cloud constant
modal scavenging coefficients, normalized by the
precipitation rate, are 5x10-4, 1x10-4, 1x10-3,
and 1x10-1 mm-1 for the nucleation, Aitken,
accumulation and coarse modes, respectively.


7. Thermophoretic Effects on Scavenging


Black Carbon Deposition
Legend Below-cloud
In-cloud Dry Deposition
Sedimentation
Figure 1 The 7 aerosol modes of the ECHAM5-HAM
GCM. Mass and number transfer follow the green
arrows.
5. Geographic Distribution of Aerosol Deposition
A) Constant Modal Scavenging B) Aerosol
Size-Dependent Scavenging
3. Below-Cloud Scavenging Coefficients .
The mean mass below-cloud scavenging
coefficients are where



and the
Marshall-Palmer distribution is assumed for the
raindrops and a lognormal distribution is
assumed for the aerosols.

Figure 7 Scavenging coefficients using the
collision efficiencies of Wang et al. (1978)
that account for particle motion due to thermal
gradients associated with evaporative cooling.
The far right panel shows deposition following
same legend as Figure 4.
Figure 5 Geographic distribution of sulphate ion
and black carbon wet deposition is shown by the
upper and lower panels, respectively. The left
and right panels show deposition for the two
below-cloud scavenging parameterizations.
8. Conclusion and Summary The ECHAM5-HAM GCM
is the first atmospheric general circulation
model to predict aerosol modal radius for seven
lognormal modes. As a result, the size dependency
of below-cloud scavenging can now be explicitly
represented in the model. As opposed to using
constant modal below-cloud scavenging
coefficients that are scaled by rainfall rate,
the revised scavenging approach suggests that
previous studies may have under-estimated the
importance of below-cloud scavenging as a
process for the removal of aerosols from the
atmosphere.
Figure 3 Mean mass (solid) and number (dashed)
scavenging coefficients as a function of aerosol
modal size and rainfall rate.
Figure 2 Collection efficiency (E) as a function
of aerosol size (d) and collector drop size (D).
9. Acknowledgements
References Roeckner et al., The atmospheric
general circulation model ECHAM5, Part IModel
description, Report 349, MaxPlanck Institute
for Meteorology, Hamburg, Germany, available at
http//www.mpimet.mpg.de, 2003. Stier et al., The
aerosol-climate model ECHAM5-HAM, Atmos. Chem.
Phys., 5, 1125-1156, 2005. Wang et al., On the
effect of electric charges on the scavenging of
aerosol particles by clouds and small raindrops,
J. Atmos. Sci., 35, 1735-1743, 1978