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…