How well do we understand multiphase oxidation in the
troposphere?
• At the begining…• Phase transfer• Bulk and surface reactivity• Conclusions…
Christian GEORGEIRCELYON
Institut de Recherches sur la Catalyse et l'Environnement de Lyon
1754: Joseph Black identifies CO2 in ambiant air.
CO2
1839: Christian Schönbein identifies ozone.
CO2
O3
1872: Publication of Robert Angus Smith’s book on acid rain.
CO2
O3
Acid rain
1878: Alfred Cornu measures the solar spectrum at the Earth’s surface. Walter Hartley identifies ozone in this spectrum
CO2
O3
Acid rain
1896: First climate model by Svante Arrhenius showing the role played by CO2 on surface temperature.
CO2
O3
Acid rain
1950: Arie Haagen-Smit identifies ozone formation duringthe irradiation of hydrocarbons/NOx mix.(Los Angeles smog)
CO2
O3 RHO2 ROO
NO
NO2
O3
h
RORCHO
Acid rain
1970: Paul Crutzen identifies a stratosphericozone sink involving nitrogen oxides.Chemistry Nobel Price in 1995
CO2
O3
NOx
RHO2 ROO
NO
NO2
O3
h
RORCHO
Acid rain
1972: Hiram Levy demonstrates the importance of thehydroxyl radicals (OH) during the oxidation of pollutants.
CO2
O3
NOx
OHh
+ RHO2 ROO
NO
NO2
O3
h
RORCHO
Acid rain
1970: Acid rain is a major preocupation.
CO2
O3
NOx
OHh
+ RHO2 ROO
NO
NO2
O3
h
RORCHO
Acid rainSO2 H2SO4
NO2 HNO3
Acidity of atmospheric water...
1 2
3
4
5
6
7
Battery acid
Lemon juice
Vinegar
Tomato juice
Milk
Fog (LA-USA)
Cloud (Whiteface Mt., USA)
Rain (Whiteface Mt., USA)
Rain in remote areas
(Crutzen and Graedel, 1993)
Where is this acidiy coming from?
Heterogeneous chemistry…
How do we describe such processes?
Earth's cloud coverage...
This image shows the average global cloud cover during the past month. For example, 50% cloud cover indicates that of all the times the satellite passed over a certain area, it detected clouds half of the time. The cloud cover image was created using data from the Special Sensor Microwave Imager (SSM/I). This is one of the instruments on a Defense Meteorological Satellite Program (DMSP) satellite.
The water cycle...
Gas Phase
Liquid
Deflection
Uptake
Diffusion intobulk
Reaction
Evap
ora
tion
What is water looking like?...
Temperature
Wat
er v
apor
pre
ssur
e (T
orr)
Vapor
Solid Liquid Marine boundary layer
Lower stratosphere
Lower troposphere
Upper troposphere
Examples of solutes - water interactions.
Representation of an ionic solid dissolving in water. From S.S. Zumdahl, Chemistry, 3rd ed., copyright © 1993 by D.C. Heath and Company
(A) The structure of the ethanol molecule. (B) The interaction between ethanol and water molecules. From S.S. Zumdahl, Chemistry, 3rd ed., copyright © 1993 by D.C. Heath and Company.
The hydration of a sodium ion.
• Upper limit...
net g
1
4c A
Uptake Coefficient
Kinetic Theory
Gas
Liquids
in g
1
4c A
Accommodation Coefficient Limitation due to the interface
Molecular flux across the interface..
• Net flux...
Challenge 1: understand phase transfer kinetics
Analysis
GasConcentration decay due to exposure to the aqueous phase
Experimental procedures...
Liquid jett ~ 1 msS~0.01 cm2
P=760 Torr10-5 < < 10-2
Aerosolt ~ 10 sS~0.1-1000 cm2
P=760 Torr10-7 < < 10-1
Wetted-wallt ~ 10 sS~100 cm2
P=5-760 Torr10-7 < < 10-2
Droplet traint~ 10 msS~0.2 cm2
P=5-50 Torr10-4 < < 1
=
g
out
g
in
g
FcS
A
A
4ln
Uptake coefficient determination
S=0
Scan number
Tra
ce g
as d
ensi
ty
S 0
Ammonia: mass accommodation coefficient
Shi et al. JPC-A, 1999
HCl: mass accommodation coefficient
Température (K)
260 265 270 275 280 285 290 295
0,0
0,1
0,2
0,3
0,4
0,5Ce travailVan Doren et al., 1990
Postulated free energy diagram
Postulated free energy diagram
Ene
rgy
Cluster sizeDistance
Gas
Surface
Aqueous
Gobs
Gsolv
N=1 N=N*Nathanson et al., JPC, 1996
G*Gvap
kdesorb
ksolng ns ns* naq
kads
Nathanson et al., JPC, 1996
Intermolecular forces Interaction of a molecule in a medium
From J.N. Israechvili, Intermolecular and surface forces
A) Displacement of solvent by two approaching moleculesInteraction energies between two solute molecules must not only direct solute-solute interactions but also any changes in the solute-solvent and solvent-solvent interactions
B) SolvationSolute molecules often perturb local ordering, producing new interactions between solutes and solvents
C) Cavity formationCavity energy expended by the medium when it forms a cavity to accommodate a guest molecule
Gas
Entry of a gas...
Gas
Liquid
in-coming molecule
Cavity formation modelCavity formation model
Molar volume (cm3 mol-1)
30 40 50 60 70 80 90 100
G (
kcal
/mol
)
0
1
2
3
4
Gcav
~tension
bulk properties
Experimental resultsNathanson et al, JPC, 1996
ln1
G
RTobs
Description of the mass accommodation process...
• From the experiments: exhibit a negative temperature dependance
• the process may involve a pre-equilibrium
• The postulated concept (Davidovits et al., JPC, 1991) – Interface is a (thin) dynamic region– aggregates are formed, falling apart, re-forming…– liquidlike "clusters" merge with the nearby liquid
• notion of critical size for the cluster (N*)
• hability for hydrogen bounding
– Solvation is the rate limiting step
• Use of the nucleation theory
Nucleation theory based model...Nucleation theory based model...D
ensi
tyGas Interface Liquid
Theory and experiments...
Capillary-wave model of gas-liquid exchange
Knox and Phillips, JPC-B, 1998
Predicts a linear relationship between H and S!
Mechanism: continuous mixing of the surface by thermally induced capillary waves leading to an increase of the coordination number
S and H relationship...
Sobs (cal mol-1 K-1)
-70 -60 -50 -40 -30 -20 -10 0
Hob
s (
kcal
mol
-1)
-16
-14
-12
-10
-8
-6
-4
-2
0
Cluster model
Capillary-wave model(slope fitted to exp. data)
Data from: ARC/BC and CGS
This relationship is a highly striking feature!
Coordination number as a function of in-coming gas position
Somasundaram et al., PCCP, 1999
Water density
CO2
N2
CH3CN
Ar
Bulk
Surface
Molecular dynamic simulation:• coord. number increases smoothly during uptake• coord. numbers are much larger (considering the first coordination shell)
•Why?
Water distribution function around...
Somasundaram et al., PCCP, 1999
Energy maxMolecule still surronded by waterSolvation shell are perturbedCoord. Number is decreased
Surface stateSurface is perturbedsolvation increases water density
Film centre2 solvation shells can be seenthey occupy all the thickness!!
Outside filmwater layering?
Solute at:CO2 N2
Å
Contour interval: 0.345 g cm-3
Dynamics of solvation at the air/water interface
Zimdars et al., Chem. Phys. Lett., 1999
Technique: femtosecond time-resolved surface second harmonic generation (TRSHG)
Characteristic solvation time: about 800 fsSo once adsorbed the molecules are rapidly solvated!
Ethanol on the surface...KE= kinetic energy
Equilibrium KE at 310 K
Acceleration due to the attraction well near the surface
Thermal equilibrium is reached after 20 psSurface state stable for more than 10 ns!Do adsorbed EtOH posses enough energy to leave the surface?
EtOH
Gas
Liquid
Wilson and Pohorille, JPC-B, 1997
Once equilibrium is reached...
Taylor and Garrett, JPC-B, 1999
Density
water
water
Ethylene glycol
Ethanol
Orientation
CO bond
CC bond
Adsorption of gases at the interface:
surface tension
Surface tension of aqueous solutions of 1-propanol at 298 K as a function of the alcohol concentration
Surface excess of 1-propanol in aqueous solution as a function of its concentration at 298 K
Donaldson and Anderson, JPC-A, 1999
Gibbs equationLangmuir isotherm
Simulated free energy profile...
Taylor and Garrett, JPC-B, 1999
Water density
H2OEtOH
Glycol
Interface=surface minimum
Molecular dynamics yields different free energy profiles:• no significant energy barrier to solvation meas. ~ 0.01)• Scattering of EtOH:only 18 molecules over 1000 trajectories i.e.,
Other free energy profiles...
Wilson and Pohorille, JPC-B, 1997
MeOH
EtOH
Somasundaram et al., PCCP, 1999
Maximum present?
No max. ?
Free energy profiles for anesthetics
Chipot et al., JPC-B, 1997
dichlorodifluoromethane (a),
1,2-dichloroperfluoroethane (b),
1-chloro-1,2,2-trifluorocyclobutane (c),
1,2-dichloroperfluorocyclobutane (d),
perfluorocyclobutane (e),
n-butane (f),
1,1,2,2,3,3,4,4-octafluorobutane (g),
2,3-dichloroperfluorobutane (h),
1,2,3,4-tetrachloroperfluorobutane (i).
water
vapoura
b
c
de
f,g,i
h
hexane vapour
cd
e
hexane
water
a
b
c
d
e
f,ig
h
Another model for the accommodation process:
Wilson and Pohorille, JPC-B, 1997
Water density
From exp.
From MDS
After 20 ps
After 60 ps
Molecular dynamic trajectory
Diffusion model
Molecule is adsorbed with unit probabilitythen diffuses “simply” into the bulkTime to diffuse out of the surface ~nsPb: temperature dependence?
MDS and temperature effects...
Increasing temperature leads to a lowering of the energy required to escape from the surface i.e., decreases with increasing temperature
Water density
Taylor and Garrett, JPC-B, 1999
Finally, what do we know?• From the experiments:
decreases with temperature• surface adsorption
– relationship between S and H
• From a theoretical approach:– increase of coordination numbers– energy barrier is not too large (?)– long lived (?) surface state– surface solvation is fast
• solvation is not the rate limiting step (?)
• Various models– cluster
• predict the slope between S and H
– capillary-wave• can be fitted to the experimental data
– diffusion
Challenge 2: understand bulk chemistry
What can happen once in the liquid?
• As already mentioned, the solute can undergo – solvation– acid-base dissociation
• The solute can also react with various partners– water (the most abundant!)
• aldehydes undergo gem-diol formation – (affecting both solubility and reactivity)
• N2O5 is hydrolysed "instantaneously"!
– (while quite slow in the gas phase)
• RCOX (X being an halogen) are slowly hydrolysed– (still affecting their tropospheric lifetimes and their impact on stratospheric ozone)N2O5 + H2O 2 NO3
- + 2 H+
RCOX + H2O RCOOH + X- + H+
What can happen once in the liquid?
• The solute can also react with various partners– ions (nucleophilic attack)
• with HCHO
– forming hydroxymethanesulfonate (HMSA)– need high pH for its formation (decomposition is OH- driven)– "stable" in acidic solutions– has been observed in the field, "stabilises" S(IV) and increases its solubility
– light (photolysis)
– forming radicals
HCHO + HSO3- HOCH2SO3
-
Absorption spectra...
Ionic environment...
• Existence of charge exchange reactions– For example:
• 1992 Nobel Prize in Chemistry: R.A. Marcus for his theory for charge exchange reactions: calculation of free energy changes
From Herrmann, 1997
Ionic strength and reactivity...
In a classical "Physical chemistry" textbook (e.g. Atkins)
Debye-Hückel limiting law
Lg(
k/k 0)
I
+
-
+ +
+ ++
+
- ++
NO3 + Cl-
•Debye and Mc Aulay•Ion pairing
Hydroxyl radicals
• OH is certainly the most important radical• Sources
– uptake from the gas phase– photolysis of
• nitrite, nitrate, H2O2
– "dark" reactions of reduced metal ions• Fe2+ + H2O2 Fe3+ + OH + OH-
• Reactivity, OH undergoes all possible pathways– H abstraction
• polar compounds (alcools, ethers,, acids,…) have similar reactivities as in the gas phase
• alkanes, DMS have higher aqueous reactivities
• OH + HSO3- H2O + SO3
-
– addition to double bonds– charge exchange
• OH + SO3-- OH- + SO3
-
Hydroperoxyl and Superoxide radicals
• HO2…
– is very abundant in the troposphere
– is quite soluble (H~103 M atm-1)
– have a large • uptake will be only limited by gas phase diffusion
– in-cloud HO2 concentration decreases by a factor 2-3
• clouds suppresses the reaction: – HO2 + NO OH + NO2
• increases the NO/NOx ratio
• in the liquid phase
Acting as oxidant Acting as reductant
Halides radicals
• Ubiquitous – many potential sources
• marine, erosion...
• and very reactive
From Buxton et al., 2000
Nitrate radical
• Key reactant also in the aqueous phase• May be taken up by clouds
– solubility is only moderate (~0.6 M atm-1)
• May be formed in-situ– OH + HNO3 NO3 + H2O– SO4
- + NO3- SO4
2- + NO3
– SO4- + Cl- SO4
2- + Cl– NO3 + Cl- NO3
- + Cl
• Will undergo a full set of reactions– NO3 + OH- NO3
- + OH Charge exchange– RH + NO3 R + HNO3 H abstraction– addition to doubles bonds
Sulfur oxide radicals SOx-
• Four basic radicals– SO2
-
• SO2- + O2 SO2 + O2
-
– will not form in atmospheric droplets
– SO3-
• SO3- + O2 SO5
-
– SO5-
• SO5- + SO5
- 2 SO4- + O2
• SO5- + SO3
-- SO4- + SO4
--
– SO4-
• SO4- + HSO3
- SO3- + SO4
--
– The latter will undergo• H abstraction• addition to double bonds• electron transfer
Sulfate radical• Electron transfer
• H abstraction
– correlation with BDE
(with data from H. Herrmann)
Laser photolysis...
PC
0765
Pulse
Lase
r
SpectrographCCD
Xe Lamp
Optical fiber coupling
SolutionLens
Filters Irradiation cell
SO4- + Cl2- SO4
2- + Cl
time (s)
0 2x10-5 4x10-5 6x10-5 8x10-5 10-4
Abs
orba
nce
Cl
2-
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
Abs
orba
nce
SO
4-
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
SO4-
Cl2-
Peroxy radicals
• Under atmospheric conditions oxygen addition is mostly irreversible
• ROO will react...– unimolecular decomposition
• strongly dependent on the nature of R– ROO R'CO + HO2
– bimolecular reactions• produces a full or carbonyl containing species
– ROO + ROO R'CO + R"CHO + R'"OH
– also, electron transfer and H abstraction (slow process)
RH + X R + HX R + O2 ROO
Clouds support acidity formation...• Nitrogen oxides
• Sulfur (IV) to Sulfur (VI) oxidationS(IV): SO2.H2O, HSO3
-, SO3-- / S(VI): SO4
--
– by dissolved O3
S(IV) + 03 S(VI) + O2
– by dissolved H2O2
S(IV) + H2O2 S(VI) + H2Oproceeds according to:HSO3
- + H2O2 SO2OOH- + H2OSO2OOH- + H+ H2SO4
– both exhibit a complex pH dependency
N2O5 + H2O 2 NO3- + 2 H+
HNO3 + H2O NO3- + H3O+
S(IV) oxidation by OH, O2 and Transition Metal Ions...
SO3-
S(IV)
OH
SO5-
S(IV)
HSO5-
S(IV)
SO4--
S(IV)
SO4-
S(IV)
Summary of HOx/TMI chemistry
OH
H2O2 HO2
O2
O2-
M(n-1)+ Mn+
Summary of nitrogen oxides chemistry...
NO2 HONO
NO2-
NO3-
NO3N2O5
Summary of "organic" in-cloud chemistry...
RH
R'''COOH
RX
ROO
O2
R''OH
ROOROOH
HSO3-, HO2
HSO3
-
R'CHO
X
Summary of cloud chemistry
RHR
ROO
R'CHO
ROOH
R'''COOH
R''OH
O2
ROO
HSO3-, HO2
HSO3
-
X
H2O2
OH
HO2 O2-
O2
M(n-1)+ Mn+
NO2 HONO
NO2-
NO3-
NO3N2O5
S(IV)
SO3-
SO5-
SO4-
HSO5-
SO4--
S(IV)
S(IV)
S(IV)S(IV)
Sulfate formation…
SO2 SO42-
CondensationNucleationOH
O H OHOSO2 SO3 H2 SO4 (g)
2 2
Homogeneous conversion
H2O2, O3, O2, OH, NO2
Aqueous AEROSOLS
Heterogeneous Conversion
Adapté de S. Pandis, 2001
Nitrate formation…
NO2 HNO3
OHHNO3
Photolysis CloudsNuages NO3-
AerosolsNO3
-
Aérosols
GasesHONO, NO2, ClONO2, etc.
GasHONO, NO2, ClONO2, etc.
Réactio
ns
hétéro
gèn
es
NH3NH3
NO2 N2O5N2O5NO3NO3
O3
HCRCHO
Inorganic chemistry ok
Complex radical chemistry partly ok, partly discussed
OH and NO3 radical reactions with organics up to C4 (Herrmann et al., Atmos. Env., 2005)SOx
-, Cl/Cl2- and CO3
- radical reactions with C1 and C2 (Ervens et al., JGR, 2003)
Organic chemistry in its beginnings
C1-C2 chemistry: (Herrmann et al. J. Atm. Chem., 2000), (Ervens et al., JGR, 2003)e. g. formation of small dicarboxylic acids: (Warneck et al., Atmos. Env., 2003) (Ervens et al., JGR, 2004) C1-C4 chemistry: (Herrmann et al., Atmos. Env., 2005)Multiphase conversion of aromatics (Lahoutifard et al, ACP, 2002)First simple model of SOA formation: (Gelencser and Varga, ACP, 2005)
Aqueous phase chemistry for clouds
1980: Halogen activation in the troposphere
CO2
O3
NOx
OHh
+ RHO2 ROO
NO
NO2
O3
h
RORCHO
Acid rainSO2 H2SO4
NO2 HNO3
Jour (avril 1986)
2 4 6 8 10 12 14 16 18
O3
(ppb
)
0
10
20
30
40
50
Jour (avril 1986)
2 4 6 8 10 12 14 16 18f-
Br
(ng
m-3
)
020406080
100120140160
Finlayson-Pitts et al., Nature, 343, 622, 1990
Source de BrNO2
0 ppt BrNO2 20 ppt BrNO2 100 ppt BrNO250 ppt BrNO2
Impact on the oxidation capacity
Hebestreit et al., Science, 1999
DOAS Latitude moyenne
Cl2… observations…
Spicer et al., Nature, 1998
From space…
At UC Irvine
Surface reaction on sea–salt
Knipping et al., Science, 2000
Knipping et al., Science, 2000
Challenge 3: understand surface chemistry
Chloride surface availability
20-Å water lamella
Snapshot of molecular dynamics predictions of typical open surface of a slab consisting of 96 NaCl molecules and 864 water molecules. The large yellow balls are Cl- ions, the smaller green balls Na+, and the red and white balls are water molecules.
Knipping et al., Science, 2000
Rad
ical
dis
trib
utio
n fu
nctio
n th
e ce
nter
mas
s of
the
Cl(H
2O) 255 water molecule cluster
Stuart and Berne, JPC-A, 1999
Cl-
O
PMT«Reflected» signal
Focusing andfiltering optics
150 W XenonArc Lamp
PMT«Bulk» signal
KrF laser
248 nmmirrors
Suspendeddroplet
Reaction Chamber
Irradiated surface
Laser
sh
eet
Focu
sed X
e
lam
p
HCA
differentiating circuit
Time
AB
S
DG535
Oscilloscope
PC
HCA
Diffuse Reflectance Laser Flash Photolysis
Reaction mechanism…
I. Cl2- + ethanol
S2O82- + h 2 SO4
●-
SO4●- + Cl- Cl• + SO4
2-
Cl- + Cl• Cl2-
Cl2- Cl- + Cl•
Cl2- + C2H5OH products
Cl2- productsCl• products
Cl2- + Cl2- products
Example: The reaction of Cl2- with EtOH
Absorbance at 350 nm [NaCl] = 50 x 10-3 M and [Ethanol] = 0.3 M
Bulk decays in agreement with literatureSurface decays faster?
50
40
30
20
10
0
Abs
(10
-3)
6050403020100
time (10-6
sec)
Bulk
Surface
50
40
30
20
10
0
Abs
(10
-3)
6050403020100
time (10-6
sec)
50
40
30
20
10
0
Abs
(10
-3)
6050403020100
time (10-6
sec)
Bulk
Surface
EtOH + Cl2- : 1st order plot
Absorbance at 350 nm
[NaCl] = 50 x 10 -3 M and [Ethanol] = 0.3 M
-7
-6
-5
-4
-3lo
g si
gnal
403020100
Time (10-6
sec)
Surface reflectance
Bulk
Absorbance at 350 nm
[NaCl] = 50 x 10 -3 M and [Ethanol] = 0.3 M
-7
-6
-5
-4
-3lo
g si
gnal
403020100
Time (10-6
sec)
-7
-6
-5
-4
-3lo
g si
gnal
403020100
Time (10-6
sec)
Surface reflectance
Bulk
Absorbance at 350 nm
[NaCl] = 50 x 10 -3 M and [Ethanol] = 0.3 M
-7
-6
-5
-4
-3lo
g si
gnal
403020100
Time (10-6
sec)Absorbance at 350 nm
[NaCl] = 50 x 10 -3 M and [Ethanol] = 0.3 M
-7
-6
-5
-4
-3lo
g si
gnal
403020100
Time (10-6
sec)
Surface reflectance
Bulk
Absorbance at 350 nm
[NaCl] = 50 x 10 -3 M and [Ethanol] = 0.3 M
-7
-6
-5
-4
-3lo
g si
gnal
403020100
Time (10-6
sec)
Surface reflectance
Bulk
Absorbance at 350 nm
[NaCl] = 50 x 10 -3 M and [Ethanol] = 0.3 M
-7
-6
-5
-4
-3lo
g si
gnal
403020100
Time (10-6
sec)
-7
-6
-5
-4
-3lo
g si
gnal
403020100
Time (10-6
sec)
-7
-6
-5
-4
-3lo
g si
gnal
403020100
Time (10-6
sec)
Surface reflectance
Bulk
Bimolecular plot
Bulk
20
15
10
5
0
k obs
erve
d(1
04s-1
)
6005004003002001000
[Ethanol] (10-3
M)
Bulk
20
15
10
5
0
k obs
erve
d(1
04s-1
)
6005004003002001000
[Ethanol] (10-3
M)
20
15
10
5
0
k obs
erve
d(1
04s-1
)
6005004003002001000
[Ethanol] (10-3
M)
Bulk
20
15
10
5
0
k obs
erve
d(1
04s-1
)
6005004003002001000
[Ethanol] (10-3
M)
Bulk
20
15
10
5
0
k obs
erve
d(1
04s-1
)
6005004003002001000
[Ethanol] (10-3
M)
20
15
10
5
0
k obs
erve
d(1
04s-1
)
6005004003002001000
[Ethanol] (10-3
M)
Surface
Why a curvature?
Surface tension and Gibbs surface excess for ethanol solutions
70
60
50
40
30
Sur
face
te
nsio
n, d
yne
s/cm
86420
Ethanol, M
70
60
50
40
30
86420
4
3
2
1
0S
urfa
ce C
ove
rage
(1
014 m
ole
c cm
-2)
543210
Ethanol, M
4
3
2
1
0
543210
Langmuir type adsorption of Ethanol at the interface
Surface tension
Surface concentration
How do we convert from surface to bulk…
Gas Phase Liquid phase
Density
Interfacea few Å
Ethanol concentration
a few nm?
Detector
Xe Lamp
We can assume an interface thickness d (a few Å), then Sd volume unitsWe ignore our sounding depth: on what length are integrating the signal?
Why faster at the Interface?
• Solvation shells are incomplete– Less water to remove before reaction
• Costs less energy
• Mobility is higher– More reaction encounters
• Concentrations may be higher– Surface tension and surface excess– Particular cases: some anions
Nowadays: Secondary organic aerosols
CO2
O3
NOx
OHh
+ RHO2 ROO
NO
NO2
O3
h
RORCHO
Acid rainSO2 H2SO4
NO2 HNO3
Aerosols
SOA =
Secondary organic aerosols
SOA particles undergo constant changes
(=aging, processing)
that modify their properties and chemical composition
during atmospheric residence time (and also affect it!).
from: John Tyndall, “Fragments of Science” 1892, 96-109, experiments from 1868-69, New chemical reactions produced by light
arc lamp(„electric lamp“)
glass tube(length 1m, 8 cm )
particle filter(cotton wool)
ambient air
CO2 trap (KOH) (caustic potash)
dryer (H2SO4)
S, S‘ rocksalt plates
• C5H11ONO (nitrite of amyl)• benzene• C3H5I (iodide of allyl)
formation of ‚sky matter‘ (organic germs)
observation of blue clouds
From T. Hoffmann - Mainz
low volatile productse.g.
gas phasechemistry
(e.g. ozone formation)
Gi Ai
gas/particlepartitioning
new particleformation
condensation
cond
ensa
tion
homogeneousnucleation
ki
semivolatile productse.g.
gaseous productse.g. HCHO acetone glyoxal
Mechanisms
oligomeric productse.g.
CHO
O
CHO
OHHOO
O
O O
OH
COOH
COOH
mesitylene
OH
+ OH
+ O3
+ OH
+ NO3
-pinene
OHO-O
ONO2
O-O
radical intermediates
COO
O
OHO-O
ONO2
O-O
radical intermediates
COO
O
COO
O
From T. Hoffmann - Mainz
Markku Kulmala, How Particles Nucleate and Grow, Science, 2003, VOL 302, 1000-1001
Concepts to explain atmospheric new particle formation
TSCs (H2SO4 – H2O – NH3 )
e.g. alkenes
2) heterogeneous reactions
e.g. alkyl sulfates
growth and lowering surface tension
2) condensation of low volatile organics
activation („nano-Köhler“)
bonding energy ~ 20 kcal mol-1
H2SO4 – H2O ~ 10 kcal mol-1
H2SO4 – H2O – NH3 ~ 25 kcal mol-1
~ 1 nm
3) condensation
of low volatile organics
1) Formation of TSCs
1) Formation of TSCs
heteromolecularhomogeneous nucleationinvolving organic acidsand sulphuric acid
condensation of low volatile organics
A) Kulmala, Pirjola and Mäkelä (2000) Nature, Kulmala et al. (2004) JGR
B) Zhang and Wexler (2002) JGR
C) Zhang et al. (2004) Science
TSCs (H2SO4 – H2O – NH3 )
D) Berndt et al. (2005) Science
TSCs (H2SO4 – H2O (organics?))
growth by carbonylic oxidation products ?
1) Freshly formed H2SO4
very similar for different VOC precursors
atmospheric lifetime ~ 1-2 minutes
aerosol yield ~ 100 %
saturation vapour pressure < 9×10-11 Torr
Shu and Atkinson (1994) Intern. J. Chem. Kinet.
Hoffmann et al. (1997) J. Atmos. Chem.
Bonn and Moortgat (2003) Geophys. Res. Lett.
From T. Hoffmann - Mainz
Where are the organics?
Everywhere!!!
As coatings
Droplet
Inorganic core
As particles
Viscous liquid and solid
Internally mixed
Very abundant in fine particles(just after sulphate)
Very high chemical complexityRequires model systems
Cecinato et al, J. Sep. Sci, 2003
Uptake of water
Water adsorbs to hydrophobic surfaces
Thomas et al, JGR, 1999OTS: octadecyltrichlorosilane
adsorption
desorption Depends on rhIs reversible
CH3
(CH2)17
SiCl3
Where does it adsorb?
Rudich et al, JPC-A,2000µdroplet
The morphology governs the amount of water being adsorbed
rougher = wetter!Extent of coverage increases
Rough guidelines
• Water soluble large organic– Deliquescence type behaviour
• Similar to inorganic salts
• Organic liquid at room temperature– Smooth water uptake– Reversible
• Very hydrophobic– Very reduced water uptake– Adsorptive in nature (surface defects?)
Organic films and mass transfer
Evaporation rates
Cruz et al, Atmos. Environ., 2000
decreases up to a factor 2
DOP: dioctyl phthalate
Deactivation of aqueous aerosol surfaces
Uptake of N2O5
NH4HSO4 = 1.82·10-2 (60% rel. humidity)
NH4HSO4
1.22 ppm -pinene + O3
11 ppb -pinene + O3
O3 + NO2 N2O5
= 5.9·10-4
= 3.4·10 -3
NH4HSO4
Reference:
sulfate aerosol
Folkers et al, GRL, 2003
Mass accommodation on water surfaces
Schweitzer et al, JPC-A, 2000
Mass accommodation on 1-octanol surfaces
Zhang et al, JPC-A, 2003
HBr and HI uptake are favoured on octanol surfaces
Effect of water on mass accommodation
Zhang et al, JPC-A, 2003
Langmuir Hinshelwood type uptake
Is this an evidence for some role played by microdroplets?
Reactive uptake on organics
Ozone on films and monolayers
Moise and Rudich, JPC-A, 2002
Changes in hydrophobicity…
Produces gas phase aldehydes
OPPC:1-oleoyl-2-palmitoyl-sn-glycero-3-phosphocholineWadia et al., Langmuir, 2000
C8= octenyltrichlorosilane, terminal alkeneThomas et al., JGR, 2001
OH on films and monolayers
Bertram et al., JPC-A, 2001
Multiphase SOA formation
Jang et al, Science, 2002
A few unnecessary comments…
Finally what’s an aerosol?Aerosol: particles suspension (solid or liquid) in a gas
Do not isolate the particles from its bath gas, the object to consider is the aerosol (and not simply the particle)!
Indeed particles are physically and chemically changing withy time
This system is hyghly dynamic
External/internal mixing
External mixing Internal mixing
The life of a particle…
NH3 NOx
Photochemistry
HNO3
Inorganic primary particles
Salts (marine)
COV
Semi-volatils VOCsPhotochemistry
Primary organic particle
SO2
H2SO4
H2SO4
Photochemistry
H2O
Adapté de Meng et al., Science, 1997
Conclusions
• Multiphase chemistry is– Complex– Still poorly understood in many aspects– Is a sink for gases– Is a source for other gases– Reaction mechanism differ from the gas
phase (not necessarily the kinetics)
• Many questions still open…