potential alteration of ice clouds by aircraft soot

Potential alteration of ice clouds by aircraft soot

Joyce E. Penner and Xiaohong LiuDepartment of Atmospheric, Oceanic and Space Sciences

University of Michigan

Aviation, Atmosphere and Climate

30 June - 3 July 2003

Friedrichshafen, Germany

Evidence for alteration of ice clouds by aircraft emissions

• Soot associated with increasing ice concentrations in regions of enhanced soot most probably due to aircraft (Ström and Ohlsson, 1998)

• Trend difference in high clouds observed over regions with Computed Contrail cover > 0.5% was 3.5%/decade (land) and 1.6%/decade (ocean) between 1984 and1990 (ISCCP data) (Fahey and Schumann et al. (2001))

• Model results:– Jensen and Toon [1997]– Lohmann [2000]

Mechanisms forming ice clouds

• Homogeneous nucleation– Jhaze = Jw(Teff); Teff= T+Tm

• Deposition nucleation– Js

’=(42rN2Zse)/(2ln(kT))1/2ag

2cl,sexp[-Fg,S/kT]– Fg,S=[16Mw

2i/v3]/[3(RTiln Sv,i)2]f(mi,v,x); mi,v =0.9

– or: Meyer’s empirical formulation: • Nid=exp{a+b[100(RHi-1]}

• Immersion nucleation– Js

’=(42rN2kT)/(h) c1,S exp[-g*/(RT)-Fg,S/(kT)]

– Fg.S=[16Mw2i/v

3]/(3[Lm,0i ln (T0/Te)]2) f(mi,w,x); mi,w =0.5

• Contact nucleation

Warm case (w=4 cm/s)

0

200

400

600

800

0.001 0.01 0.1 1 10 100

Ni (1/cc)

z (m

)

hf only

hf+deposition

hf+immersion

Warm case (w=20 cm/s)

0

200

400

600

800

0.001 0.01 0.1 1 10 100

Ni (1/cc)

z (m

)

hf only

hf+deposition

hf+immersion

Warm case (w=100 cm/s)

0

200

400

600

800

0.001 0.01 0.1 1 10 100

Ni (1/cc)

z (m

)

hf only

hf+deposition

hf+immersion

Cold case (w=4 cm/s)

0

200

400

600

800

0.001 0.01 0.1 1 10 100

Ni (1/cc)

z (m

)

hf only

hf+deposition

hf+immersion

Cold case (w=20 cm/s)

0

200

400

600

800

0.001 0.01 0.1 1 10 100

Ni (1/cc)

z (m

)

hf only

hf+deposition

hf+immersion

Cold case (w=100 cm/s)

0

200

400

600

800

0.001 0.01 0.1 1 10 100

Ni (1/cc)

z (m

)

hf only

hf+deposition

hf+immersion

Parameterization for homogeneous ice formation

• T ≥ 6.07 ln w - 55.0 (fast growth; high T low w):

– Ni=min{exp(a2+b2T+c2lnw)Naa1+b1T+c1lnw , Na}

• T<6.07 ln w - 55.0 (slow growth; low T high w):

– Ni=min{exp(a4+(b4+b5lnw) T+c4lnw)Naa3+b3T+c3lnw , Na}

Homogeneous + deposition nucleation

• Lower updraft velocities and higher temperatures=> deposition nucleation only:– Threshold: T 14.387 ln(w) - 18.825; and w 0.3

m/s

– Si (%) = a T + b;

– where a and b are a function of w

– Use with Meyer’s (1992) parameterization

• Use homogeneous parameterization at higher updrafts and lower temperatures

Homogeneous, deposition, and immersion freezing

• Threshold temperature for immersion, deposition freezing: – T a ln(w) + b– a, b are functions of the number of soot particles Ns

• Immersion freezing:– Ni,s=min{exp(a22)Ns

b22exp(bT)wc, Ns}– b, c are functions of ln Ns

• Deposition freezing:– Maximum supersaturation; Si

max(%) = A T2 + BT + C – A, B, C are functions of w– Number of ice crystals from Meyer’s (1992) parameterization for deposition

• Use homogeneous parameterization at lower T

Immersion nucleation: ice crystal number

0.001

0.01

0.1

1

10

100

0.001 0.01 0.1 1 10

Total soot concentration (cm-3)

Ice

nu

me

r co

nce

ntr

atio

n (

cm-3

)

W=0.5 m s-1

W=-0.04 m s-1

= -60C

= -40C

Sulphate = 200 cm-3

Homogeneous nucleation: ice crystal number

w = 0.04 m s -1

0.01

0.1

1

10

Ice

cry

sta

l nu

mb

er

de

nsi

ty (

cm-3

)

229.3 K214.2 K194.1 K

w = 1.0 m s-1

1

10

100

1000

10 100 1000Total sulfate aerosol concentration (cm -3)

Ice

crys

tal n

umbe

r de

nsity

(cm

-3)

229.3 K214.2 K194.1 K

T=-80C

T=-60C

T=-40CT=-40C

T=-60C

T=-80C

W=0.04 m s-1 W=1.0 m s-1

10 100 1000

Sulfate aerosol concentration (cm-3)

Sulfate aerosol concentration (cm-3)

IMPACT/DAO

• Uses NASA DAO 1997 meteorological fields

• Uses IPCC-recommended emissions inventories except for dust (from Ginoux for 1997 DAO winds)

• Emissions put into BL for dust and biomass burning

• Wet scavenging as in Harvard GEOS-CHEM model except that large scale scavenging uses 0.5 g/m3 for LWC

• Dry deposition as in Zhang, Gong et al. [AE, 2001]

Unique features

• DAO version has improved LWC for sulfate chemistry

• GMI model is based on IMPACT

• We can compare these results with more than one set of meteorological fields:

– IMPACT/DAO=GMI/DAO– GMI/MACCCM3– GMI/GISSII’

Comparison of burdens: GMI models for 1995 ff BC

Burden wet dry Lifetime (Tg) (Tg/yr) (Tg/yr) (days)

DAO 0.058 7.17 1.75 2.40

GISS 0.080 6.92 2.04 3.26

NCAR 0.060 7.31 1.88 2.4

GRANTOUR/CCM1 ffBC+bbBC:0.20 9.56 2.66 5.97

DAO* 0.14 5.00 1.65 7.52

GISSDAO

NCARFuel tracer: ng/g

BC Burdens:DAO 3.3e-4 TgGISS 5.7e-4 TgNCAR 4.1e-4 Tg

Zonal mean SO4 number concentration (cm-3)

Zonal mean ice number (cm-3), homogeneous nucleation only

Relative humidity wrt water (%)

Zonal mean soot number concentration (cm-3)

Zonal mean ice number (cm-3), heterogeneous + homogeneous nucleation, surface sources

Difference in ice concentration between heterogeneous + homogeneous and homogeneous only (cm-3), surface sources

Concentration of soot from aircraft (cm-3)

Concentration of ice (cm-3)

Aircraft + surface sources Surface aerosol sources

Difference in ice concentration between surface + aircraft aerosol sources and surface only sources (cm-3)

Conclusion

• An initial assessment of the potential impact of aircraft emissions on ice concentrations indicates significant increases (O˜100%) in zonal mean concentrations near flight corridors

• Better quantification requires a better simulation of upper tropospheric humidity together with full representation of all aerosol types and their mode of nucleation