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
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