Computational Modelling of Aerosol Cycle An Integrated Environmental Modelling System and Its Applications Dr. Yaping Shao CEMAP, School of Mathematics The University of New South Wales Sydney, Australia Tel: 61 2 9385 5746; Fax: 61 2 9386 7123 email: [email protected] Aerosols – Aerosols are small particles suspended in air. The sizes of aerosols range between 0.1 - 20 microns; – Aerosol sources include natural and human induced ones. Aerosol Research: Climate and Weather • Directly, aerosols affect atmospheric radiation budget through scattering and absorbing; • Indirectly, aerosols modify the optical properties and lifetimes of clouds; • Dust (global emission): ~ 3000 Mt/yr. • Sea salt: ~ 1300 Mt/yr. • Dust (mean column load): ~ 65 mg m-2 • Sea salt: ~ 7 mg m-2 Dust storm in Africa: 27 July 1998, Algeria and Mali A severe dust storm over China (16 April 1998) Dust clouds seen from satellite picture (14 April 1998) Aerosol Research: Air quality • Aerosols cause air-quality hazards in populated areas, e.g., Beijing; • Many contaminants which pose significant risks to human health and the environment are found or associated with dust, including metal, pesticides, dioxins and radionuclides. A severe dust storm (acknowledgement) Land-Use Sustainability • In agricultural areas, soil erosion depletes fine particles which are rich in organic matters and soil nutrients. This leads to land degradation; • Wind erosion also reduces water, resulting in desertification. A dust storm in an agricultural area Soil Erosion Melbourne 08-02-1983 dust storm: Nutrient content in soil particles < 44 microns Site Soil Type Wamberra Box Creek Montarna Sandy Sand Clay Loam Loam Total N (%) 0.226 0.16 0.153 Totol P (%) 0.038 0.029 0.034 N enrichment ratio 19 2 2 P enrichment ratio 5.7 1.9 2.4 Mass fraction 0.003 0.11 0.06 Melbourne 08-02-1983 dust storm Total loss of top soil M=2 million tonnes Total loss of N M*0.0017=3400 tonnes Total loss of P M*0.000055=110 tonnes Cost of fertilizer (N:P:K=32:10:0) 0.37 dollars Cost of N 3.9 million dollars Cost of P 0.4 million dollars Mineral Aerosol Cycle – Entrainment: atmosphere and land-surface interactions; multi-disciplinary; – Transport: atmospheric circulation; atmospheric boundary layers; turbulence; two phase flow problem – Deposition: turbulent diffusion; clouds and precipitation. Integrated Environmental Modelling – How can such complex environmental problems be simulated and predicted? – Computational environmental modelling: the integration of dynamic models with spatially distributed data – Atmosphere-land surface interactions – Air quality – Aerosol cycle – Land surface hydrology and salinity Framework I Computational Environmental Modelling System (CEMSYS_3) – Atmospheric prediction model (HIRES): highresolution limited-area; nested in GCM, selfnested; 3rd order upwinding and semilagrangian schemes; clouds and radiation. – Land surface (ALSIS): Soil moisture, temperature; fluxes of energy, mass and momentum; – Aerosol cycle: entrainment, transport and deposition. – Air quality, etc Framework of CEMSYS_3 (partial) Physical processes involved in wind erosion Particle Motion – Saltation: hop motion of sand particles; – Suspension: small particles can remain suspended once airborne. Friction velocity & threshold friction velocity • The capability of wind to cause erosion is quantified by surface friction velocity, u*,depending on wind speed and surface roughness • The ability of the surface to resist erosion is quantified by threshold friction velocity u*t, depending on soil texture, compactness, moisture content and surface coverage • Modeling u*t is difficult Entrainment of Coarse Particles • Balance of aerodynamic, gravity and cohesive forces, fa, fg and fi, determines the entrainment; • For coarse particles, fa overcome fg and fi; • Friction velocity u* measures aerodynamic forces; • Threshold friction velocity u*t measures retarding forces. • Shao-Lu model for u*t is d u*t f (Re * )( p gd ) Entrainment of Fine Particles • The Entrainment mechanisms for coarse and fine particles differ as the importance of forces change. • fg d3, fa d2 and fi d; fi dominates. Dust Emission Mechanisms • Fa, aerodynamic lift. Particles can be lifted directly by fa, but emission is weak; • Fb, saltation bombardment. Striking particles cause local impacts, overcome fi, result in strong emission; • Fc, aggregates disintegration. Fine particles exist as aggregates. Weak events, they behave as grains. Strong events, they disintegrate. • Dust-emission rate: F = Fa + Fb + Fc Particle-size Distribution • • • • • • Soil particle size ranges: 0.1 m - 2 m Gravel: 2000 m < d 2m Sand: 63 < d 2000 m Silt: 4 < d 63 m Clay: d 4 m Silt and clay particles are dust. Particle-size Distributions • ps(d): sediment particle-size distribution (psd); • pm(d): in-situ soil psd; minimally dispersed analysis; • pf(d): fully-disturbed soil psd; fully-dispersed analysis. Model for ps(d) • Limiting cases ps(d) pm(d) ps(d) p (d) f u u * *t u u * *t Model ps(d) pm(d) (1 )p (d) f Weight exp[ k(u u )n ] * *t Example of ps(d) Fractions of Fine Particles dd m pm(d)d 0 dd p (d)d f 0 f dd s ps(d)d 0 dd c p (d) pm(d)d 0 f m: free dust, lower limit for dust emission from unit soil mass; f: not free dust, released through saltation impact and aggregates disintegration, upper limit for dust emission from unit soil mass; s: aerosol in suspension Theory of Saltation • Saltation plays a critical role in the process of dust emission. • Two quantities are of particular importance, namely, the streamwise saltation flux, Q, and the number flux of striking particles, ns u *t 3 Q c 0 u * (1 2 ) g u* 2 Qg ns mc 0 u * ( U1 cos 1 U 2 cos 2 ) Volume Based Model for Fb Particle trajectory is (XT, YT) in soil, forms a crater of volume Volume Based Model for Fb •Trajectory from equation of particle motion; •cbf: fraction released; • (1-cb)f : fraction retained; dX T b YT dt 0 dt du p dv p dX T dYT m a x p x 0; m a y p y 0; - u p 0; - vp 0 dt dt dt dt Fb (d s ) c b f s n s tc Volume Based Model for Fb •Particle trajectory is (XT, YT) in soil, forms a crater of volume ; •Trajectory from equation of particle motion; •cbf: fraction released; (1-cb)f : fraction retained; dX T b YT dt 0 dt du p dv p dX T dYT m a x p x 0; m a y p y 0; - u p 0; - vp 0 dt dt dt dt Fb (d s ) c b f s n s tc Aggregates Disintegration: Fc • Aggregates disintegration occurs as they strike surface. • Corresponding to ns, the mass flux of particles striking surface is mns. Fc(ds) = cc fc m ns • cc: a coefficient Total Dust Emission: F • Divide particles into I size groups, mean di, increment di; Consider emission of i group ~ F (d , ds ) c~ ns; b i b fi b ~ Fc(d , ds ) c~c m ns i ci c~ c~c c E b (d ) i ~ c F(d , ds ) 6.5 c Qg ( m ) i ci fi b 0 Emission of ith group : Fˆ (d ) dd12 F(d , d) ps(d) d i i I ˆ Total : F F(d ) i1 i Model of Particle Size Distribution • Emission model requires pm(d) and pf(d). • Express as sum of J log-normal pdfs with parameters wj, Dj and j; both for pm(d) and pf(d) for sand, loam and silty clay. w (ln d ln D )2 J j •exp j p(d) 1 2 d j12 2 j j c • Model requires ; • cE fi /: fraction of release; • cY = 1/7 co, order 0.1. ~ F(d , ds ) c E ( m)ns i fi b si ps (di ) E c p ( d ) fi f i pm(d ) Qg ~ i ] F(d ,ds ) c [(1-) ( m) i ci Y p (d ) u2m b fi f i * Quantities Required • • • • • • • u*: friction velocity; u*t: threshold friction velocity for surface; pm(d): minimally-dispersed psd; pf(d): fully-dispersed psd; b, p: bulk soil and particle density; s: soil drag coefficient; pys:vertical component of plastic pressure. Results Conclusions for Emission Model • Concept: F is related to Q; • Mechanisms: saltation bombardment and aggregates disintegration; • Models for Fb and Fc; • Soft soils, Fb dominates;Hard soils, Fc dominates; • psds are used to eliminate empirical parameters; • psds modeled using log-normal pdfs; • Emission rates compare well with observations. Transport: Lagrangian • Particles are individuals; Trajectories are determined by integrating equations of motion; • Isentropic trajectories on surface of constant potential temperature; • Fluid parcel and particle are at height zft-1 = zpt-1 at t-1, fluid moves to zft, particle to zpt=zft+wtt. Transport: Eulerian • Particulate phase is a continuum; • Particle concentration obeys advection-diffusion type of conservation equation; • Kpx: particle eddy diffusivity; Sr: wet and dry removal; Sc: dry and wet convection; F0: dust flux at surface c u c v c (w w )c K c K c t z x px x y py y t x y K pz c Sr Sc z z (w wt )c - K pz c F z 0 K pz c 0 z Inertial and Trajectory Crossing Particle Eddy Diffusivity Deposition • Dry-deposition flux Fd = -wd[c(z)-c(0)] • c(0), c(z): concentration at surface and reference level; wd: dry-deposition velocity. • Single-layer dry-deposition model wd=-wt+gbb+gbm • gbb: molecular conductance; gbm: impaction conductance; fr: ratio of pressure drag to total drag; wd=-wt+ga[fr ap em+(1-fr)avSc-2/3] Wet Deposition Wet deposition is the removal of aerosols by precipitation. The processes is extremely complicated, but is commonly calculated using Fw=w pr0s0c0 s0: scavenging ratio is a function of many parameters, but ranges from 100 to 2000. pr0: rain received at the surface; c0: concentration in rain water . Example 1: How does the Scheme Work Comparison with Field Measurements Land Surface Data Weather pattern Feb. 1996 Soil Erosion Threshold Friction Velocity Friction Velocity Concentration Cross Section Total Suspended Dust Time Series Comparison with Satellite Image Aerosol Concentration Surface Concentration: Birdsvill, Feb. 1996 Higher Resolution Higher Resolution Summary • A comprehensively integrated system has been developed for the simulation and prediction of the entire mineral dust cycle, from entrainment, transport to deposition. CEMSYS_3 has a much wider range of applications; • I have illustrated how the entire cycle can be modeled. Each of the modeling components constitutes an interesting research area. I have concentrated on dust emission in this talk; • Coupling dynamic models with spatially distributed data has enabled the predictions of dust storm events.
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