Potential alteration of ice clouds by aircraft soot Joyce E. Penner and Xiaohong Liu Department 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 –J haze = J w (T eff ); T eff = T+  T m Deposition nucleation –J s ’ =(4  2 r N 2 Z s e)/(2  ln(kT)) 1/2 a g 2 c l,s exp[-  F g,S /kT] –F g,S =[16  M w 2  i/v 3 ]/[3(RT  i ln S v,i ) 2 ]f(m i,v,x); m i,v =0.9 –or: Meyer’s empirical formulation: N id =exp{a+b[100(RH i -1]} Immersion nucleation –J s ’ =(4  2 r N 2 kT)/(h)  c 1,S exp[-  g * /(RT)-  F g,S /(kT)] –F g.S =[16  M w 2  i/v 3 ]/(3[L m,0  i ln (T 0 /T e )] 2 ) f(m i,w,x); m i,w =0.5 Contact nucleation

Parameterization for homogeneous ice formation T ≥ 6.07 ln w (fast growth; high T low w): –N i =min{exp(a 2 +b 2 T+c 2 lnw)N a a1+b1T+c1lnw, N a } T<6.07 ln w (slow growth; low T high w): –N i =min{exp(a 4 +(b 4 +b 5 lnw) T+c 4 lnw)N a a3+b3T+c3lnw, N a }

Homogeneous + deposition nucleation Lower updraft velocities and higher temperatures=> deposition nucleation only: –Threshold: T  ln(w) ; and w  0.3 m/s –S i (%) = 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 N s Immersion freezing: –N i,s =min{exp(a 22 )N s b22 exp(bT)w c, N s } –b, c are functions of ln N s Deposition freezing: –Maximum supersaturation ; S i max (%) = A T 2 + 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 W=0.5 m s -1 W=-0.04 m s -1  = -60C  = -40C Sulphate = 200 cm -3

Homogeneous nucleation: ice crystal number T=-80C T=-60C T=-40C T=-60C T=-80C W=0.04 m s -1 W=1.0 m s 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 BurdenwetdryLifetime (Tg)(Tg/yr)(Tg/yr)(days) DAO GISS NCAR GRANTOUR/CCM1 ffBC+bbBC: DAO*

GISSDAO NCAR Fuel tracer: ng/g BC Burdens: DAO3.3e-4 Tg GISS5.7e-4 Tg NCAR4.1e-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 sourcesSurface 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