Modeling of aerosol processes in the

atmospherePeter Tunved

Department of Environmental Science and Analytical ChemistryStockholm University

Sweden

GOALS:

- Develop a fundamental understanding of processes affecting aerosols in the atmosphere- Apply knowledge gained to atmospheric data collected during the

GoAmazon campaign.

Atmospheric measurements Process models

Modeling the atmospheric aerosols

Nucleation

Chemical reactions

Coagulation

Dry deposition

Gas-to-particle conversion

(“condensation”)

Primary emissions

Cloud processing

Wet removal

Why important to describe aerosol dynamics?

The aerosol is continously changed via a number of dynamical (both physical and chemical) processes,

altering the properties of the size distribution

Knowledge of these processes is necessary in order to accurately assess both smaller scale interactions

(e.g. aerosol cloud interactions) as well as large scale transport and processing

Thus, aerosol processes are relevant on both micro and synoptic scale

Sources of atmospheric aerosols

Sinks

Dry depositionTransport through air to a surface: Turbulent transport, transport over the laminar surface

layer, surface properties

Wet depositionUp-take in cloud droplets (”in-cloud scavenging”), up-take in falling rain droplets (”below-

cloud scavenging”)

Chemical reactions in the atmosphere

Reactions of different compounds with OH, ozone och NO3

C2H6 CO2+H2O

Sources of secondary and primary aerosols

Inorganic

SO2 (DMS,COS, CS2)H2SO4

NOx, NH3 HNO3, NH4NO3

Organic

Biogenic VOC (BVOC’s; eg Monoterpenes)

Anthropogenic VOC’s (generally high Mw)

Terpenoids in boreal forests

Monoterpenes

Sesquiterpenes

Isoprene

Kesselmeier and Staudt, 1999.

Terpene chemistry: α-pineneVery complex chemistry!

Most well studied for α-pinene and β-pinene

For most compounds, secondary oxidation steps are largely unknow

α-pinene most commonly abundant monoterpene

The ozone reaction believed to dominate SOA productionKanakidou et al., 2005

Sources of atmospheric particles:Mass based assessment

Few but heavy! (contains much mass, but lifetime is short)

Small but many!(visibility, climate, longer lifetime)

Recall:The mass of ONE 1µm particle is equal to the mass of ONE THOUSAND 100nm particles, or ONE MILLION 10nm particles

Aerosol dynamics

SO2

CONDENSATION/COAGULATION

OXIDATION

SO2+H2SO4(g)

NUCLEATION

SO2+H2SO4(g)+H2SO4(l)

FURTHER AGEING; TRANSPORT; DEPOSITION

Basic processes acting on single aerosol particles • Gravitational settling• Drag force• Brownian motion

Gravitational settling

tv p

cpt D

gCmv

3

n.b. that effects of Drag need to be corrected for if Re>0.1, i.e Dp>20um

Single particle dynamics and Knudsen number

0Kn Kn1Kn

pDKn

2 Knudsen number

Continuum regime Free molecular regimetransition regime

Brownian diffusion

0Kn Kn1Kn

Continuum regime Free molecular regimetransition regime

p

c

D

kTCD

3

Dry Deposition

Only removal path in the dry atmosphereDepends on:

• Atmospheric turbulence• Phase of species (gas or particle)• Physio-chemical properties of depositing species• Particles: size, density• Gases: water solubility, reactivity

• Surface properties (Reactive? Sticky? Irregular?(eg vegetation)

Dry deposition

ssbaba

st

d vvrrrr

vr

v

11

2*uUra

)10(*

13

32

Stb

Scur

Diffusion ImpactionTurbulence

p

pcs D

gmCV

3

Sedimentation

Dry deposition

18

2

p

s

gDv

Ageing due to dry deposition

Brownian Diffusion

Sedimentation, impaction

32

Sc

s

23V and

*,10

g

VuSt sSt

pair DiffSc /0

,0

dt

dNdt

dM

Dry depsoitionF=-vd*Cwhere:

F=flux to surface m-2s-1

vd=deposition velocity (m/s)C=concentration of particles

smcmsmF

CVF d

23##*

Resistance analogy

sbatd rrrr

v

11

C3

ra

rb

rs

C1

C2

C0=0

Laminar resistance; laminar surface layer: ~f(molecular or brownian diffusivity)

Aerodynamic resistance; surface layer: ~f(temperature; windspeed; surface roughness)

Surface resistance; surface properties: ~f(reactivitiy, solubility, pH etc.)

Resistance analogy cont’d

Coagulation

• Mainly the result of Brownian motion, although other forces may come into play (electrical, gravitational etc)

Key concepts:

Coagulation

• Free molecular regime

)(

)(22

22

ji

ji

rrCC

vvRMS

Coagulation

• Continuum regimethe kernel in this regime is found by solving the time-dependent diffusion

equation around a stationary spherical absorber in an infinite medium with suspended particles. Transport and collision through “random walk”

• Transition regime (~1<Kn<50)Semi-empirical solution to the collision kernels (Fuchs 1964, “Flux matching”)

Coagulation; Fuchs form of the coagulation coefficient

2112 NNKdt

dN

1

212/1

21

212/12

22121

21212112

)(8

2)(2

DpDpcc

DD

ggDpDp

DpDpDpDpDDK

3

2,1

1,

1,

DpTfC

DpTfD

K12=COAG_COEFF(Dp1, Dp2, T, P)

Aerosol dynamics: coagulation

0

,0

dt

dNdt

dM

Gas-to-particle production

Condensation on existing particles

SIZE

NU

MBE

R

Precursorgases(e.g. SO2)

High saturation vapor pressure

Nucleation

Oxidation products(e.g. H2SO4)

Low saturation vapor pressure

([H2SO4]>[H2SO4]sat)

Secondary particle production: Saturation ratio

)(Tp

pS

as

a

S=saturation ratiopa=partial pressure of apa

s=saturation vapor pressure of a at temperature T

SaturationS

ationSupersaturS

ionSubsaturatS

,1

,1

,1

122,

1, 11ln

TTR

H

p

p vap

as

as

Clausius-Clapeyron relation

Concepts of gas-to-particle conversion

1 0 Satdt

dr

1 0 Satdt

dr

1 0 Satdt

dr

Gas-to-particle conversion:How is supersaturation reached

(hot) gas[x]<[x]sat

cool

ing

[x]>[x]sat [x]=[x]sat

[x]<[x]sat [x2]>[X2]sat

Transformation: XX2 [x]<[x]sat [x2]=[X2]sat

Particle number or particle mass?Role of condensation sink

• Amount of pre-existing aerosol surface crucial• Generation of supersaturated conditions + low surface area

of pre-existing particles favours formation of particle number via nucleation• Generation of supersaturated conditions + High

concentration of pre-existing particles favours formation of particle mass via condensation• This is often referred to as ”condensation sink”

Condensation

KnKnKn

Kn

where

ccDDrrdt

dM

ccDrdt

dM

M

sgpgp

sgp

121

34

34

1377.0

1

))()((4

)(4

β=Fuchs and Sutugin non-contnuum correction factor

c

effectKelvin ,c

surfaceover on oncentratipressure/c vapor

0s

kkc

saturationcs

Aerosol dynamics: condensation

0

,0

dt

dNdt

dM

Wet depositon

In-cloud scavenging

SO2(g)

SO2(aq)

Below-cloud scavenging

Cloud droplet Formation

(”nucleation”)

Nucleation scavenging

Aerosol

Particles

Scavenging coefficient, gases

phase gasin ion Concentrat,

(i) gas oft coefficien gscavengnin,

liquid togas of ratetransfer

,,

gasiC

gi

raingasW

gasiCgirain

gasW

t

gasigasi

raingas

gasieCC

Ct

C

Wt

C

,0

,,

Scavenging of aerosols: Impaction, diffusion and interception

Below cloud scavengning of particles

pDdpppp dDNdDEDUDdp

),()(4

)( 2

0

efficiency )d E(Dp,

diameter d

diameterdroplet rain

d

d

scavenging

particle

Dp

Scavengning

• Thus, we need a droplet distribution

• Marshal palmer droplet distribution

Scavengning coefficient, Λ

Simulating below cloud scavengning

Simple approach: Assuming constant, linear and irreversible scavengning

t

gasigasi

gasigasirain

gas

raingas

gasieCC

Ct

C

mgsgeCW

REWt

C

,

0

,,

31,, )/* ..(

: as and E,or R no Assuming

resp.) emissions, and reactions additional are E and R (where

Somewhat more complicated , though mor accurate, approach: mass transfer and Henrys Law

“At a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid”

atmatml

mol

l

mol

pAHaqA

aqAgA

A

**

)]([

)()(

[A(aq)] pA(g)

Wet removal of gases

)),(

),((

),(*),(C

as ),(

),( Since

),(),((),(

,aq

,

H

tzCtzCKW

tzCHtzH

tzCtzC

tzCtzCKtzW

aqgc

raingas

gaseq

aqgaseq

eqgc

[A(aq)] pA(g)A(eq)

raingasW

t

C

Z

Where Kc is a mass transfer coefficient:

(cm/s) 6.023/12/1

Dg

DU

Dp

DgK

air

air

air

ptairc

However, both Caq and Cg is function of altitude and time

Aitkenmode

Accumulation mode

Coarse Particles

0.001 0.01 0.1 1 10 100

Particle Diameter ( m)

Sedimentation

Rainoutand

Washout

Wind blown dust+

Emissions+

Sea spray+

Volcanoes+

Plant ParticlesCoagulation

Condensation nuclei

AgglomeratesDropletsCoagu-

lation

Primaryparticles

Homogenous nucleationand condensation

Hot vapoursLow volatility gases

Gas phase chemical reactions

Coagu-lation

Nucleationmode

Activation

Condensation

Diffusion

All processes

Aerosol dynamics:coagulation, condensation and dry deposition

Cloud formation

Aerosolpartiklar

Precipitation

SO2

Liquid phase chemistry

OZONE + SO2

H2O2

H2SO4

O3H2O2

SO2

H2SO4

Gas phase chemistry

Aerosol dynamics:Cloud processing

Explaining the residence time

NucleationCoagulation

CondensationDeposition(Brownian Diffusion)

Deposition(sedimentation)

Clouds!Wet deposition

Creating your own Lagrangian model

Processes includedNucleation Coagulation

CondensationDry deposition

Meteorological input from trajectory

Temperature (T)Pressure (Pa)

Relative humidity (RH)