How well do we understand multiphase
oxidation in the troposphere?
Christian GEORGE
IRCELYON
Institut de Recherches sur la Catalyse et l'Environnement de Lyon
•
•
•
•
At the begining…
Phase transfer
Bulk and surface reactivity
Conclusions…
CO2
1754: Joseph Black identifies CO2 in ambiant air.
CO2
O3
1839: Christian Schönbein identifies ozone.
Acid rain
CO2
O3
1872: Publication of Robert Angus Smith’s book
on acid rain.
Acid rain
CO2
O3
1878: Alfred Cornu measures the solar spectrum at the
Earth’s surface.
Walter Hartley identifies ozone in this spectrum
Acid rain
CO2
O3
1896: First climate model by Svante Arrhenius showing
the role played by CO2 on surface temperature.
Acid rain
CO2
O3
NO
RH
O2
O3
ROO
hn
RCHO
RO
NO2
1950: Arie Haagen-Smit identifies ozone formation during
the irradiation of hydrocarbons/NOx mix.
(Los Angeles smog)
Acid rain
CO2
O3
NO
RH
O2
O3
ROO
hn
NOx
RCHO
RO
NO2
1970: Paul Crutzen identifies a stratospheric
ozone sink involving nitrogen oxides.
Chemistry Nobel Price in 1995
Acid rain
CO2
O3
NO
hn
OH + RH
O2
O3
ROO
hn
NOx
RCHO
RO
NO2
1972: Hiram Levy demonstrates the importance of the
hydroxyl radicals (OH) during the oxidation of
pollutants.
Acid rain
CO2
O3
SO2
H2SO4
NO2
HNO3
hn
OH + RH
NO
O2
O3
ROO
hn
NOx
RCHO
RO
1970: Acid rain is a major preocupation.
NO2
Acidity of atmospheric
water...
Milk
Tomato juice
Vinegar
Rain in remote areas
Lemon juice
Battery acid
Rain (Whiteface Mt., USA)
Cloud (Whiteface Mt., USA)
Fog (LA-USA)
(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
Deflection
Uptake
Liquid
Reaction
Evaporation
Diffusion into
bulk
Water vapor pressure (Torr)
What is water looking like?...
Solid
Liquid Marine boundary layer
Lower troposphere
Upper troposphere
Lower stratosphere
Vapor
Temperature
Examples of solutes - water interactions.
The hydration of a sodium ion.
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.
Molecular flux across the
interface..
Gas
• Upper limit...
Liquids
 
1
 in   c  Ag
4
• Net flux...
 net

Accommodation Coefficient
Limitation due to the interface
Kinetic Theory
1
  c  Ag
4

Uptake Coefficient
Challenge 1: understand phase transfer kinetics
Experimental procedures...
Gas
Concentration decay due to
exposure to the aqueous
phase
Wetted-wall
Liquid jet
Droplet
train
Aerosol
1 ms
ms
tt~~ 10
10
s
222 cm2
S~100
cm
S~0.01cm
cm
S~0.2
S~0.1-1000
P=5-760
Torr
P=760 Torr
P=5-50
Torr
-2
-5 <  < 10
-4
10-7
1 -1
Analysis
Uptake coefficient determination
 A
ln
 A
in
g
out
S=0
g
S  c 

4 Fg
ln[A] gentré /[A] gsortie
2
,
0
H
C
l
T
=
2
6
7
K
7
0
0
6
0
0
5
0
0
4
0
0
1
,
0
=
0
,
5
0
,
0
0
2
4
6
8
1
0
1
2
1
4
1
6
3
0
0
densitédugaztraces
S 0
Trace gas density
1
,
5
2
0
0
1
0
0
2
0
0
0
2
4
0
0
2
8
0
0
3
2
0
0
Scan
number
n
o
m
b
r
e
d
e
s
c
a
n

S
<
c
>
/
4
F
g
Ammonia: mass accommodation coefficient
Shi et al. JPC-A, 1999
HCl: mass accommodation coefficient
0,5
Ce travail
Van Doren et al., 1990
0,4

0,3
0,2
0,1
0,0
260
265
270
275
280
285
Température (K)
290
295
Energy
Postulated free energy diagram
ng
kads
ns
ns*
ksol
naq
kdesorb
Gobs
Gas
Gvap
G*
Gsolv
Surface
Aqueous
Distance
N=1
N=N*
Cluster size
Nathanson et al., JPC, 1996
Nathanson et al., JPC, 1996
A)
B)
Intermolecular forces
Interaction of a molecule in a medium
Displacement of solvent by two approaching molecules
Interaction energies between two solute molecules must not only direct solute-solute
interactions but also any changes in the solute-solvent and solvent-solvent interactions
Solvation
Solute molecules often perturb local
ordering, producing new interactions
between solutes and solvents
C)
Cavity formation
Cavity energy expended by the
medium when it forms a cavity to
accommodate a guest molecule
Gas
From J.N. Israechvili, Intermolecular and surface forces
Entry of a gas...
4
Cavity formation model
Gas
Gcav~tension
3
in-coming molecule
G (kcal/mol)
bulk properties
2

Gobs
  
ln

RT
 1  
1
Experimental results
Nathanson et al, JPC, 1996
Liquid
0
30
40
50
60
70
80
Molar volume (cm3 mol-1)
90
100
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...
Density
Gas
Interface
Liquid
Theory and experiments...
0
(kcal mol-1)
-2
N*=1
Experiments
MSA
-4
N*=1.5
H2O2
-6
MHP
N*=2
-8
MeOH
Theory
-10
N*=2.5
N*=2.8
-12
-14
-60
N*=3
-50
-40
-30
-20
-10
Sobs (cal-1 mol-1 K-1)
0
Capillary-wave model of gas-liquid
exchange
Mechanism: continuous mixing of the surface
by thermally induced capillary waves leading to an increase
of the coordination number
Predicts a linear relationship between H and S!
Knox and Phillips, JPC-B, 1998
S and H relationship...
0
-1
Hobs (kcal mol )
-2
-4
Capillary-wave model
(slope fitted to exp. data)
-6
Cluster model
-8
-10
-12
Data from: ARC/BC and CGS
-14
-16
-70 -60
-50
-40
-30
-20
-10
0
Sobs (cal mol K )
-1
-1
This relationship is a highly striking feature!
Coordination number as a function of
in-coming gas position
Bulk
CH3CN
N2
Ar
CO2
Molecular dynamic simulation:
• coord. number increases smoothly
during uptake
• coord. numbers are much larger
(considering the first coordination
shell)
•Why?
Surface
Water density
Somasundaram et al., PCCP, 1999
Water distribution function around...
CO2
N2
Solute at:
Film centre
2 solvation shells can be seen
they occupy all the thickness!!
Energy max
Molecule still surronded by water
Solvation shell are perturbed
Coord. Number is decreased
Surface state
Surface is perturbed
solvation increases water density
Outside film
water layering?
Å
Contour interval: 0.345 g cm-3
Somasundaram et al., PCCP, 1999
Dynamics of solvation at the air/water
interface
Technique: femtosecond time-resolved surface second harmonic
generation (TRSHG)
Characteristic solvation time: about 800 fs
So once adsorbed the molecules are rapidly solvated!
Zimdars et al., Chem. Phys. Lett., 1999
Ethanol on the surface...
KE= kinetic energy
EtOH
Gas
Acceleration due to the
attraction well near the
surface
Liquid
Equilibrium KE at 310 K
Thermal equilibrium is reached after 20 ps
Surface state stable for more than 10 ns!
Do adsorbed EtOH posses enough energy to leave the surface?
Wilson and Pohorille, JPC-B, 1997
Once equilibrium is reached...
Density
Orientation
CO bond
water
Ethylene glycol
CC bond
water
Ethanol
Taylor and Garrett, JPC-B, 1999
Adsorption of gases at the interface:
surface tension
Gibbs equation
Langmuir isotherm
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
Simulated free energy profile...
Molecular dynamics yields
different free energy profiles:
• no significant energy barrier
to solvation
1!(meas. ~ 0.01)
• Scattering of EtOH:
only 18 molecules over 1000
trajectories i.e., 0.98!
Water density
Interface=surface minimum
H2O
EtOH
Glycol
Taylor and Garrett, JPC-B, 1999
Other free energy profiles...
Maximum present?
MeOH
EtOH
Wilson and Pohorille, JPC-B, 1997
No max. ?
Somasundaram et al., PCCP, 1999
Free energy profiles for
anesthetics
b
vapour
water
e
d
water
e
e
d
c
d
c
c
h
hexane
a
vapour
hexane
a
f,g,i
b
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).
Chipot et al., JPC-B, 1997
h
f,i
g
Another model for the accommodation process:
From exp.
Water density
After 20 ps
Molecular
dynamic
trajectory
Diffusion model
From MDS
Molecule is adsorbed with unit probability
then diffuses “simply” into the bulk
Time to diffuse out of the surface ~ns
Pb: temperature dependence?
Wilson and Pohorille, JPC-B, 1997
After 60 ps
MDS and temperature effects...
Water density
Increasing temperature leads
to a lowering of the energy
required to escape from the
surface i.e.,
 decreases with increasing
temperature
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)
HCHO + H2O
CH3CHO + H2O
CH2(OH)2 99.99%
CH3CH(OH)2 58%
• N2O5 is hydrolysed "instantaneously"!
– (while quite slow in the gas phase)
N2O5 + H2O 2 NO3- + 2 H+
• RCOX (X being an halogen) are slowly hydrolysed
– (still affecting their tropospheric lifetimes and their impact on
stratospheric ozone)
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
HCHO + HSO3-  HOCH2SO3–
–
–
–
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)
H2O2 + hn 2 OH
CH3O2H + hn CH3O + OH
NO2- + hn  NO + OH
NO3-+ hn  NO2 + OH
C2O4--+ hn  C2O4- + e– forming radicals
 (L mol-1 cm-1)
Absorption spectra...
20000
18000
16000
14000
12000
10000
8000
6000
4000
2000
0
100
e-aq
-
Cl2
Cl
OH
200
300
400
-
SO4
500
Wavelength (nm)
NO3
600
700
800
Ionic environment...
• Existence of charge exchange reactions
– For example:
SO4- + ClNO3 + Cl-
SO42- + Cl
NO3- + Cl
• 1992 Nobel Prize in Chemistry: R.A. Marcus for his theory for
charge exchange reactions: calculation of free energy changes
10x109
-HSO3
1x109
NO2 N3
SCN
-SO3
100x106
-1 -1
s )
NH3
OH
HCOO
10x106
k (M
Br
Cl
-
CH2FCOO
-SO4
CHF2COO
CH3COO
1x106
100x103
10x103
CF3COO
1x103
OCN
From Herrmann, 1997
100x100
-200
-150
-100
-50
0
50
100
Gibbs energy for charge exchange (kJ mol-1)
Ionic strength and reactivity...
9.2
9.1
9.0
8.9
8.8
log (k)
In a classical "Physical chemistry" textbook (e.g. Atkins)
8.7
8.6
Debye-Hückel limiting law
8.5
8.4
+ ++
8.3
8.2
0.00
0.05
0.10
0.15
0.20
0.25
0.30
I1/2/(1+I1/2)
+ +
100x106
I
- +
k (M-1s-1)
+
NO3 + Cl•Debye and Mc Aulay
•Ion pairing
- ++
10x106
0
1
2
3
I (M)
4
5
0.35
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 (0.2)
• 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
2 H
HO2
H+ + O2-
 H2O2 + Fe3+
+ Fe 
H
2+
HO2 + Fe  H2O2 + Fe3+
O2-
2+
Acting as reductant
HO2 + Cu2+  O2 + Cu+ + H+
O2- + Cu2+  O2 + Cu+
Halides radicals
• Ubiquitous
– many potential sources
• marine, erosion...
• and very reactive
10
HCO2-
CH3CO2HCHO
9
log k Cl.
(CH3)3COH
CH3CH2OH
CH3CH(OH)CH3
CH3OH
CH3CHO
HCO2H
8
From Buxton et al., 2000
CH3CO2H
CH3COCH2Cl
7
CH3COCH3
7
8
log k .OH
Cl + ClCl2Br + BrBr2Cl- + OH
ClOHClOH- + H+
Cl + H2O
ClOH + Cl
Cl2- + OHBr- + OH
BrOHBrOH- + H+
Br + H2O
BrOH + Br
Br2- + OH-
9
10
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-  SO42- + NO3
SO4- + Cl-  SO42- + 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
SO4- + ClSO42- + Cl
SO4- + Br-  SO42- + Br
SO4- + O2-  SO42- + O2
SO4- + NO3-  SO42- + NO3
• H abstraction
8
+
– correlation with BDE
(with data from H. Herrmann)
log (kH)
SO42-
2-butanol
7
SO4 + H2O2 
+ H + HO2
SO4- + CH3OOH  SO42- + H+ + CH3O2
-
9
Diethylether
THF
Ethanol
2-propanol 1-propanol
Glyoxal
HOOH
Formaldehyde
HCOOH
6
MTBE
Methanol
Chloroform
2-butanone
Dimethyl succinate
CH2Cl2
Dimethyl malonate
tert-butanol
5
CH3COOH
Acetone
4
360
380
400
420
Bond dissociation energy (kJ mol-1)
440
460
Laser photolysis...
Laser
0765
Pulse
Solution
Lens
Filters
Irradiation cell
Xe Lamp
PC
CCD
Spectrograph
Optical fiber coupling
SO4- + Cl2-
SO42- + Cl
0.18
0.07
0.16
0.06
0.05
0.10
0.04
Cl2-
0.08
0.03
0.06
0.04
0.02
SO4-
0.02
0.01
0.00
0.00
0
2x10-5
4x10-5
time (s)
6x10-5
8x10-5
10-4
-
0.12
Absorbance SO4
Absorbance Cl2
-
0.14
Peroxy radicals
RH + X  R + HX
R + O2  ROO
• 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)
Clouds support acidity formation...
• Nitrogen oxides
N2O5 + H2O 2 NO3- + 2 H+
HNO3 + H2O  NO3- + H3O+
• Sulfur (IV) to Sulfur (VI) oxidation
S(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) + H2O
– both exhibit a complex pH dependency
Relative reaction rate
proceeds according to:
HSO3- + H2O2  SO2OOH- + H2O
SO2OOH- + H+  H2SO4
10
8
6
Na2SO4
4
NaCl
2
0
0.0
0.5
1.0
I
1.5
2.0
S(IV) oxidation by OH, O2 and Transition Metal Ions...
S(IV)
OH
SO3-
SO5S(IV)
SO4S(IV)
S(IV)
HSO5-
S(IV)
SO4--
Summary of HOx/TMI chemistry
H2O2
M(n-1)+
OH
O2-
HO2
Mn+
O2
H2O2 + Fe2+  OH + OH- + Fe3+
O2- + Fe3+  O2 + Fe2+
2 H
2+
 H2O2 + Fe3+
O2 + Fe 
H
2+
HO2 + Fe  H2O2 + Fe3+
O3 + O2-  O3- + O2
HO3  OH + O2
HO2 + HO2  O2 + H2O2
H
- 
 H2O2 + O2
HO2 + O2
HO2 + OH  H2O + O2
O2- + OH  OH- + O2
H2O2 + OH  HO2 + H2O
Fe2+ + O3  FeO2+ + O2
OH + O3  O2 + HO2
O3P + O2  O3
Summary of nitrogen oxides chemistry...
NO2
HONO
NO2-
NO3-
NO3
N2O5
N2O5 + H2O  2 H+ + 2 NO3NO3 + OH-  NO3- + OH
NO3 + Fe2+  NO3- + Fe3+
NO3 + Mn2+  NO3- + Mn3+
NO3 + H2O2  NO3- + H+ + HO2
NO3 + CH3OOH  NO3- + H+ + CH3O2
NO3 + HO2  NO3- + H+ + O2
NO3 + O2-  NO3- + O2
NO3 + HSO3-  NO3- + H+ + SO3+ O2
NO2 + O2-  NO
2
H2O
 HNO + NO - + H+
NO2 + NO2 
3
2
NO2 + OH  NO2 + OH
NO2- + SO4-  SO42- + NO2
NO2- + NO3  NO3- + NO2
NO2- + Cl2-  2 Cl- + NO2
Summary of "organic" in-cloud
chemistry...
X
RH
R
O2
ROO
HSO3-, HO2
ROOH
X
HSO
3
ROO
-
R''OH
R'CHO
R'''COOH
CH3OH + OH  H2O + CH2OH
CH2OH + O2  O2CH2OH
O2CH2OH + OH- HCHO + H2O + O2O2CH2OH + O2CH2OH  CH3OH + O2 + HCHO
CH2(OH)2 + OH  H2O + CH(OH)2
CH(OH)2 + O2  HO2 + HCOOH
HCOOH + OH  H2O + CO2H
HCOO- + OH  OH- + CO2H
CO2H + O2  CO2 + HO2
CH3 + O2  CH3O2
CH3O2 + CH3O2  CH3OH + HCHO + O2
CH3O2 + CH3O2  CH3O + CH3O + O2
CH3O2 + HSO3-  CH3OOH + SO3Fe2+ + CH3O2  FeCH3O22+
FeCH3O22+ + H+  Fe3+ + O2
FeCH3O22+  Fe3+ + CH3OOH + OHCH3O + O2  HCHO + HO2
CH3O  CH2OH H 2 O


2 O2CH2COO2 CH(OH)2COO- + H2O2
2 O2CH2COO- H22 OHCHO + H2O2 + 2 OH- + 2 CO2


2 O2CH2COO- H 2 O CH(OH)2COO- + CH2OHCOO- + O2



2 O2CH2COO2 O2- + CH(OH)2COO- + 2 H2O
CO2- + O2  CO2 + O2-
Summary of cloud chemistry
S(IV)
O2-
HO2
H2O2
SO3-
Mn+
M(n-1)+
HSO5-
SO5-
OH
O2
S(IV)
S(IV)
S(IV)
SO4-
SO4-NO2
S(IV)
R
HONO
RH
O2
NO2-
ROO
HSO3-, HO2
ROOH
NO3-
ROO
X
HSO3
-
R''OH
R'CHO
R'''COOH
NO3
N2O5
Sulfate formation…
Homogeneous conversion
O2
HOSO2
H2 O
SO3
H2 SO4 (g)
OH
Condensation
Nucleation
SO42-
SO2
Aqueous AEROSOLS
H2O2, O3, O2, OH, NO2
Heterogeneous Conversion
Adapté de
S. Pandis, 2001
Nitrate formation…
OH
NO2
HNO3
NH3
NO3
Photolysis
NO2
N2O5
Nuages
Clouds
Réactions
hétérogènes
O3
HC
RCHO
Gas
Gases
HONO, NO2,
ClONO2, etc.
NO3Aérosols
Aerosols
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
Acid rain
CO2
O3
SO2
H2SO4
NO2
HNO3
hn
OH + RH
NO
O2
O3
ROO
hn
NOx
RCHO
RO
NO2
1980: Halogen activation in the troposphere
Finlayson-Pitts et al., Nature, 343, 622, 1990
50
f-Br (ng m-3)
O3 (ppb)
40
30
20
10
0
2 4 6 8 10 12 14 16 18
Jour (avril 1986)
160
140
120
100
80
60
40
20
0
2 4 6 8 10 12 14 16 18
Jour (avril 1986)
Source de BrNO2
0 ppt BrNO2
20 ppt BrNO2
50 ppt BrNO2 100 ppt BrNO2
Impact on the oxidation capacity
DOAS Latitude moyenne
Hebestreit et al., Science, 1999
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
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 Clions, the smaller green balls Na+, and the red and white
balls are water molecules.
Knipping et al., Science, 2000
Radical distribution function the center mass of the Cl(H2O)
Chloride surface availability
255 water molecule cluster
ClO
20-Å water lamella
Stuart and Berne, JPC-A, 1999
Diffuse Reflectance Laser Flash Photolysis
150 W
Xenon
Arc Lamp
Focusing and
filtering optics
DG535
HCA
PMT
«Reflected» signal
KrF laser
differentiating
circuit
Suspended
droplet
PMT
«Bulk» signal
248 nm
mirrors
Oscilloscope
ABS
Reaction Chamber
HCA
Time
PC
Reaction mechanism…
I.
Cl2- + ethanol
S2O82- + hn  2 SO4●SO4●- + Cl-  Cl• + SO42Cl- + Cl•  Cl2Cl2-  Cl- + Cl•
Cl2- + C2H5OH  products
Cl2-  products
Cl•  products
Cl2- + Cl2-  products
Example: The reaction of Cl2- with EtOH
50
Bulk
Abs (10 -3 )
40
30
Surface
20
10
0
0
10
20
30
40
-6
Absorbance at 350 nm
time (10 sec)
[NaCl] = 50 x 10-3 M and [Ethanol] = 0.3 M
50
60
Bulk decays in agreement with literature
Surface decays faster?
EtOH + Cl2- : 1st order plot
-3
Bulk
log signal
-4
-5
-6
Surface reflectance
-7
0
10
20
-6
Time (10 sec)
Absorbance at 350 nm
[NaCl] = 50 x 10 -3 M and [Ethanol] = 0.3
M
30
40
Bimolecular plot
Why a curvature?
4
-1
kobserved (10 s )
20
Surface
15
10
Bulk
5
0
0
100
200
300
400
-3
[Ethanol] (10 M)
500
600
Surface tension and Gibbs surface
excess for ethanol solutions
4
6
8
70
Surface tension
60
1
2
3
4
5
4
4
3
3
2
2
14
60
50
50
40
40
30
30
0
2
4
Ethanol, M
6
8
Surface Coverage (10
Surface tension, dynes/cm
70
0
-2
2
molec cm )
0
Surface concentration
1
1
0
0
0
1
2
3
Ethanol, M
Langmuir type adsorption of Ethanol at the interface
4
5
How do we convert from surface to bulk…
Detector
Density
Liquid phase
Gas Phase
Xe Lamp
Ethanol concentration
a few nm?
Interface
a few Å
We can assume an interface thickness d (a few Å), then Sd  volume units
We 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
Acid rain
CO2
Aerosols
O3
SO2
H2SO4
NO2
HNO3
hn
OH + RH
NO
O2
O3
ROO
hn
NOx
Nowadays:
RCHO
RO
NO2
Secondary organic 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 T. Hoffmann - Mainz
glass tube
(length  1m,  8 cm )
S, S‘ rocksalt plates
particle filter
(cotton wool)
ambient air
dryer (H2SO4)
arc lamp
(„electric lamp“)
CO2 trap (KOH)
(caustic potash)
• C5H11ONO (nitrite of amyl)
• benzene
• C3H5I (iodide of allyl)
 observation of blue clouds
 formation of ‚sky matter‘
(organic germs)
from: John Tyndall, “Fragments of Science” 1892, 96-109, experiments from 1868-69, New chemical
reactions produced by light
gaseous products
e.g.
HCHO
acetone
glyoxal
Mechanisms
OH
+ OH
gas phase
chemistry
(e.g. ozone formation)
oligomeric products
e.g.
semivolatile products
e.g.
O
CHO
mesitylene
radical intermediates
HOO
OH
CHO
O
COO
+ O3
O
O-O
OH
OH
O
O
condensation
+ OH
-pinene
gas/particle
partitioning
Gi
O-O
ONO 2
+ NO3
ki
Ai
low volatile products
e.g.
COOH
COOH
From T. Hoffmann - Mainz
homogeneous
nucleation
new particle
formation
Concepts to explain atmospheric new particle formation
Markku Kulmala, How Particles Nucleate and Grow, Science, 2003,
VOL 302, 1000-1001
A) Kulmala, Pirjola and Mäkelä (2000) Nature, Kulmala et al. (2004) JGR
2) condensation of low volatile organics
1) Formation of TSCs
activation („nano-Köhler“)
TSCs (H2SO4 – H2O – NH3 ) ~ 1 nm
B) Zhang and Wexler (2002) JGR
1) Formation of TSCs
e.g. alkyl sulfates
2) heterogeneous reactions
e.g. alkenes
TSCs (H2SO4 – H2O – NH3 )
3) condensation
of low volatile
organics
 growth and lowering surface tension
C) Zhang et al. (2004) Science
heteromolecular
homogeneous
nucleation
involving organic acids
and sulphuric acid
condensation of low volatile organics
bonding energy ~ 20 kcal mol-1
H2SO4 – H2O ~ 10 kcal mol-1
H2SO4 – H2O – NH3 ~ 25 kcal mol-1
D) Berndt et al. (2005) Science
1) Freshly formed H2SO4
TSCs (H2SO4 – H2O (organics?))
growth by carbonylic oxidation products ?
very similar for different VOC precursors
aerosol yield ~ 100 %
Hoffmann et al. (1997) J. Atmos. Chem.
saturation vapour pressure < 9×10-11 Torr
Bonn and Moortgat (2003) Geophys. Res. Lett.
From T. Hoffmann - Mainz
atmospheric lifetime ~ 1-2 minutes
Shu and Atkinson (1994) Intern. J. Chem. Kinet.
Where are the organics?
As coatings
Internally mixed
As particles
Very abundant in fine particles
(just after sulphate)
Viscous liquid and solid
Droplet
Inorganic core
Everywhere!!!
Very high chemical complexity
Requires model systems
Cecinato et al, J. Sep. Sci, 2003
Uptake of water
Water adsorbs to hydrophobic
surfaces
adsorption
desorption
Depends on rh
Is reversible
CH3
(CH2)17
Si
Cl3
OTS: octadecyltrichlorosilane
Thomas et al, JGR, 1999
Where does it adsorb?
The morphology governs the
amount of water being
adsorbed
rougher = wetter!
Extent of coverage increases
µdroplet
Rudich et al, JPC-A,2000
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
 decreases
up to a factor 2
DOP: dioctyl phthalate
Cruz et al, Atmos. Environ., 2000
Deactivation of aqueous aerosol surfaces
Uptake of N2O5
Reference:
sulfate aerosol
 = 1.82·10-2
NH4HSO4
(60% rel. humidity)
NH4HSO4
1.22 ppm -pinene + O3
O3 + NO2  N2O5
11 ppb -pinene + O3
NH4HSO4
Folkers et al, GRL, 2003
Mass accommodation on water surfaces
Schweitzer et al, JPC-A, 2000
Mass accommodation on 1-octanol
surfaces
HBr and HI uptake are
favoured on octanol
surfaces
Zhang et al, JPC-A, 2003
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-phosphocholine
Wadia et al., Langmuir, 2000
C8= octenyltrichlorosilane, terminal alkene
Thomas 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:
gas
particles suspension (solid or liquid) in a
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…
Photochemistry
Semi-volatils VOCs
Primary organic
particle
COV
SO2
Salts (marine)
Photochemistry
Inorganic primary
particles
H2SO4
HNO3
H2O
Photochemistry
NOx
H2SO4
NH3
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…