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Aerosol Size-Dependent Impaction

Scavenging in Warm, Mixed, and IceClouds in the ECHAM5-HAM GCM

Betty Croft, and Randall V. Martin Dalhousie University, Canada

Ulrike Lohmann ETH Zurich, Switzerland

Philip Stier Oxford University, U.K.

Sabine Wurzler Landesamt fur Umwelt, Natur, und Verbrauchershutz, Germany

Johann Feichter Max Planck Institute for Meteorology, Germany

Corinna Hoose University of Oslo, Norway

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MOCA 09 - Clouds in Global Models Session, July 21, 2009

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Aerosol Scavenging Processes:

(Figure from

Hoose et al. (2008))Wet scavenging accounts for 50-95% of

aerosol deposition, and strongly controls

aerosol 3-dimensional distributions, which

influence climate both directly and indirectly.

Sedimentation and

dry deposition

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Modeling In-Cloud Impaction Scavenging:

Global climate model methodologies -

1) Prescribed coefficients (e.g., Stier et al. (2005))

2) Size-dependent impaction with diagnostic nucleationscavenging (e.g., this study)

3) Prognostic in-droplet and in-crystal aerosol modes withprescribed impaction coefficients(e.g., Hoose et al. (2008))

Questions we will address in this talk:1) Are certain aerosol species more strongly influenced by

in-cloud impaction scavenging on a global scale?

2) Are there certain geographic regions where in-cloud

impaction contributes more to aerosol scavenging?

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All results shown are for a 1-year simulation of the ECHAM5-HAM global aerosol-

climate model, at T42 resolution, nudged to the meteorological conditions of the

year 2001, and following a 3 months spin-up period.

SU:sulfate; BC:black carbon; POM:particulate organic matter; DU:dust; SS:sea salt

The 7 lognormal modes of the ECHAM5-HAM GCM:

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0

0.2

0.4

0.6

0.8

1

1.2

Pr

r

R

LiquidMixed

Ice

Thecurrent in-cloud scavenging in the ECHAM5-HAM GCMuses

prescribed ratios. Since the ECHAM5-HAM GCM predicts aerosol size,

wecan replace these ratios with size-dependent in-cloud scavenging

NS KS AS CS KI AI CI

Pr r r f h

r E HAM5 HAM ( + mp ):

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Size-Dependent Impaction Scavenging by Cloud Droplets:

Solid lines: Number scavenging coefficients

Dashed lines: Mass scavenging coefficients

Data sources described in Croft et al. (2009)

Example for

CDNC 40 cm-3,

assuming a

gamma

distribution

Prescribed

coefficients of

Hoose et al.

(2008)

prognostic

scheme areshown with

red steps10

310

210

110

010

110

8

107

106

105

104

103

102

101

100

Geometric Mean Aerosol Radius [m]

MeanScav.Coeff.[s1](mas

s:dashed;number:solid)

Cloud Droplet Impaction [CDNC=40 cm3

]

5 m10 m

15 m

20 m

25 m

30 m

35 m

40 m

45 m50 m

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Impaction Scavenging by Column and Plate Ice Crystals:

Prescribed

coefficients of

Hoose et al.(2008)

(red steps)

Assume columns for T

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Diagnostic 2-Moment Nucleation Scavenging:

Assume each cloud droplet and ice crystal

scavenge 1 aerosol by nucleation, and apportionthis number between thejsoluble modes,

based on the fractional contribution of each mode

to the total number of soluble aerosols having radii

greater than 35 nm, which are the aerosols that

participate in the Ghan et al. (1993) activation

scheme.

nm

jjjact

N

NfracICNCCDNCN

35

, )(u

y

!

Find the radius that contains exactly Nact,jin the lognormal tail, using cumulativelognormal size-distribution,

Scavenge all mass above this radius for nucleation scavenging. Thus, we typically

scavenge a higher fraction of the mass versus number distribution.

? A))))(2(1(ln2exp(35

1

n

jggcrit

fracIerfrr

"

y

y W

Find rcritthat contains Nact,j in

the lognormal tail.

102

101

100

101

102

0

0.1

0.2

0.3

0.4

0.5

0.6

Aerosol Radius (m)

Number:Blue;Mass:Black rcrit

Number Distribution

Mass Distribution

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-20

-15

-10

-5

0

5

10

SU BC POM SS DU KS AS CS KI AI CI

NS

Percent Change in Global Aerosol Mass and Number

Burdens (With versus Without In-Cloud Impaction):

The global and annual mean dust mass burden, and the number

burden for the nucleation and accumulation mode aerosols are

sensitive to in-cloud impaction scavenging.

[%]

Change in Mass Burdens Change in Number Burdens

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Annual and

Zonal Mean

Mass Mixing

atios:

Black carbon,particulate organicmatter, and dust

concentrationsreduce by near to25% with inclusionof in-cloud (IC)impaction,particularly in the

regions of mixedand ice clouds.

Sea salt and sulfateare changed by lessthan 10% (not

shown).

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Inclusion of in-cloud impaction scavenging increases the zonal and annual mean

black carbon scavenged mass in the upper troposphere by up to 100%.

Zonal and Annual Mean Black Carbon Scavenged Mass:

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Stratiform Nucleation Stratiform Impaction

Annual and Global Mean Dust and BC In-Cloud Scavenging:

Contributions to global and annual mass deposition by process (%)

T>273K 238

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Standard: Prescribed impaction and nucleation scavenging (Stier et al.(2005))

DIAG1: 1-moment nucleation scavenging + prescribed impaction

DIAG2: 2-moment nucleation scavenging + prescribed impaction

DIAG2+Imp: 2-moment nucleation scavenging + size-dependent impaction

PROG: prognostic aerosol processing scheme with prescribed impaction (Hoose et al. (2008))

Global Burden Sensitivity to In-Cloud Scavenging

Parameterizations

0

10

20

0

0

50

60

70

ChangeRelativeto100%

Cloud-BorneAerosol[%

Standard

DIAG1

DIAG2DIAG2+Imp

PROG

SO BC POM SS DU AS

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Summary and Outlook:

1) Size-dependent in-cloud impaction scavenging reduced

zonal and annual mean carbonaceous and dustconcentrations by up to 25% and 75%, respectively, in theregions of mixed phase and ice clouds.

2) Prediction of climate change due to absorbing aerosols

requires consideration of in-cloud impaction scavenging.

3) Impaction scavenging enhanced scavenged mass of blackcarbon by up to 100% in the upper troposphere.

4) Impaction scavenging in convective clouds will be

investigated in future work.

Acknowledgments:Thanks!

Questions ?