modeling of aerosol processes in the atmosphere peter tunved department of environmental science and...
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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)