Presentation on theme: "How well do we understand multiphase oxidation in the troposphere? At the begining… Phase transfer Bulk and surface reactivity Conclusions… Christian."— Presentation transcript:
How well do we understand multiphase oxidation in the troposphere? At the begining… Phase transfer Bulk and surface reactivity Conclusions… Christian GEORGE IRCELYON Institut de Recherches sur la Catalyse et l'Environnement de Lyon
1754:Joseph Black identifies CO 2 in ambiant air. CO 2
1839:Christian Schönbein identifies ozone. CO 2 O3O3
1872:Publication of Robert Angus Smiths book on acid rain. CO 2 O3O3 Acid rain
1878: Alfred Cornu measures the solar spectrum at the Earths surface. Walter Hartley identifies ozone in this spectrum CO 2 O3O3 Acid rain
1896: First climate model by Svante Arrhenius showing the role played by CO 2 on surface temperature. CO 2 O3O3 Acid rain
1950: Arie Haagen-Smit identifies ozone formation during the irradiation of hydrocarbons/NO x mix. (Los Angeles smog) CO 2 O3O3 RH O2O2 ROO NO NO 2 O3O3 h RO RCHO Acid rain
1970: Paul Crutzen identifies a stratospheric ozone sink involving nitrogen oxides. Chemistry Nobel Price in 1995 CO 2 O3O3 NO x RH O2O2 ROO NO NO 2 O3O3 h RO RCHO Acid rain
1972: Hiram Levy demonstrates the importance of the hydroxyl radicals (OH) during the oxidation of pollutants. CO 2 O3O3 NO x OH h + RH O2O2 ROO NO NO 2 O3O3 h RO RCHO Acid rain
1970: Acid rain is a major preocupation. CO 2 O3O3 NO x OH h + RH O2O2 ROO NO NO 2 O3O3 h RO RCHO Acid rain SO 2 H 2 SO 4 NO 2 HNO 3
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.
Dissociation of a molecule... Possible energy diagram for the dissociation of a covalent molecule, E-N, into its ions E + and N - In water, ions are not necessarily independent from each other … ion pairing.
Description of gas/liquid equilibrium… In dilute solutions: Henry's law P= H [solute] or [solute] = H*P may apply to the minor constituents P vap pure In ideal solutions: Raoult's law P= x P vap pure may apply to the solvent x (mole fraction) 0 1 P Deviation from both laws Real solutions: solute activity a solute = [solute] is the activity coefficient, dimensionless (different from the uptake coefficient)
A few examples... Gas/Aqueous Phase distribution factor. Fractions of A that exist either in the gas or aqueous phase are: or
Atmospheric water is not "pure"... Ionic strength changes also the aqueous solubility of gases Using Setchenow expressions: H=H 0 exp(-hI) where h is the Setchenow coefficient H 0 the real solubility For NaCl solutions: h= h Na+ + h Cl-
Upper limit... Uptake Coefficient Kinetic Theory Gas Liquids Accommodation Coefficient Limitation due to the interface Molecular flux across the interface.. Net flux...
Mass transport considerations... Paper(s) by S.E. Schwartz reviewed and described most of the mathematical basis needed to describe uptake kinetics - NATO Series 1986 He stated following sentences –Knudsen (1913): it is not possible a priori to specify –Sherwood (1975) Not only is there no useful theory to employ in predicting, there is no easy way to experimentally measure it prior to the 90s, values were very dispersed...
Challenge 1: understand phase transfer kinetics
Analysis Gas Concentration decay due to exposure to the aqueous phase Experimental procedures... Liquid jet t ~ 1 ms S~0.01 cm 2 P=760 Torr 10 -5 < < 10 -2 Aerosol t ~ 10 s S~0.1-1000 cm 2 P=760 Torr 10 -7 < < 10 -1 Wetted-wall t ~ 10 s S~100 cm 2 P=5-760 Torr 10 -7 < < 10 -2 Droplet train t~ 10 ms S~0.2 cm 2 P=5-50 Torr 10 -4 < < 1
= Uptake coefficient determination S=0 Scan number Trace gas density S 0
Ammonia: mass accommodation coefficient Shi et al. JPC-A, 1999
Postulated free energy diagram k desorb k sol ngng nsns ns*ns* n aq k ads 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 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 B) 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
Entry of a gas... Gas Liquid in-coming molecule Cavity formation model
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
k desorb k sol ngng nsns ns*ns* n aq k ads Nathanson et al., JPC, 1996 Some more details... Competition between evaporation and clustering... In terms of transition-state theory... Nucleation theory gives the free energies...
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... This relationship is a highly striking feature!
Coordination number as a function of in-coming gas position Somasundaram et al., PCCP, 1999 Water density CO 2 N2N2 CH 3 CN 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 max Molecule still surronded by water Solvation shell are perturbed Coord. Number is decreased Surface state Surface is perturbed solvation increases water density Film centre 2 solvation shells can be seen they occupy all the thickness!! Outside film water layering? Solute at: CO 2 N2N2 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 fs So 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 ps Surface 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 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 equation Langmuir isotherm
Simulated free energy profile... Taylor and Garrett, JPC-B, 1999 Water density H2OH2O EtOH 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 vapour a b c d e f,g,i h hexane vapour c d e hexane water a b c d e f,i g 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 probability then diffuses simply into the bulk Time to diffuse out of the surface ~ns Pb: 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
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) N 2 O 5 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) N 2 O 5 + H 2 O 2 NO 3 - + 2 H + RCOX + H 2 O 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 + HSO 3 - HOCH 2 SO 3 -
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 + - + + + ++ + - NO 3 + 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, H 2 O 2 –"dark" reactions of reduced metal ions Fe 2+ + H 2 O 2 Fe 3+ + 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 + HSO 3 - H 2 O + SO 3 - –addition to double bonds –charge exchange OH + SO 3 -- OH - + SO 3 -
Hydroperoxyl and Superoxide radicals HO 2 … –is very abundant in the troposphere –is quite soluble (H~10 3 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: –HO 2 + NO OH + NO 2 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 + HNO 3 NO 3 + H 2 O –SO 4 - + NO 3 - SO 4 2- + NO 3 –SO 4 - + Cl - SO 4 2- + Cl –NO 3 + Cl - NO 3 - + Cl Will undergo a full set of reactions –NO 3 + OH - NO 3 - + OHCharge exchange –RH + NO3 R + HNO 3 H abstraction –addition to doubles bonds
Sulfur oxide radicals SO x - Four basic radicals –SO 2 - SO 2 - + O 2 SO 2 + O 2 - –will not form in atmospheric droplets –SO 3 - SO 3 - + O 2 SO 5 - – SO 5 - SO 5 - + SO 5 - 2 SO 4 - + O 2 SO 5 - + SO 3 -- SO 4 - + SO 4 -- –SO 4 - SO 4 - + HSO 3 - SO 3 - + SO 4 -- –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)
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 + HO 2 –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 + O 2 ROO
Clouds support acidity formation... Nitrogen oxides Sulfur (IV) to Sulfur (VI) oxidation S(IV): SO 2.H 2 O, HSO 3 -, SO 3 -- / S(VI): SO 4 -- –by dissolved O 3 S(IV) + 0 3 S(VI) + O 2 –by dissolved H 2 O 2 S(IV) + H 2 O 2 S(VI) + H 2 O proceeds according to: HSO 3 - + H 2 O 2 SO 2 OOH - + H 2 O SO 2 OOH - + H + H 2 SO 4 –both exhibit a complex pH dependency N 2 O 5 + H 2 O 2 NO 3 - + 2 H + HNO3 + H 2 O NO 3 - + H 3 O +
S(IV) oxidation by OH, O 2 and Transition Metal Ions... SO 3 - S(IV) OH SO 5 - S(IV) HSO 5 - S(IV) SO 4 -- S(IV) SO 4 - S(IV)
Summary of HOx/TMI chemistry OH H2O2H2O2 HO 2 O2O2 O2-O2- M (n-1)+ M n+
Summary of nitrogen oxides chemistry... NO 2 HONO NO 2 - NO 3 - NO 3 N2O5N2O5
Summary of "organic" in-cloud chemistry... RH R'''COOH R X ROO O2O2 R''OH ROO ROOH HSO 3 -, HO 2 HSO 3 - R'CHO X
Summary of cloud chemistry RHR ROO R'CHO ROOH R'''COOH R''OH O2O2 ROO HSO 3 -, HO 2 HSO 3 - X H2O2H2O2 OH HO 2 O2-O2- O2O2 M (n-1)+ M n+ NO 2 HONO NO 2 - NO 3 - NO 3 N2O5N2O5 S(IV) SO 3 - SO 5 - SO 4 - HSO 5 - SO 4 -- S(IV)
Sulfate formation… SO 2 4 2- Condensation Nucleation OH OHO HOSO 2 SO 3 H 2 4 (g) 22 Homogeneous conversion H 2 O 2, O 3 2, OH, NO 2 Aqueous AEROSOLS Heterogeneous Conversion Adapté de S. Pandis, 2001
Nitrate formation… NO 2 HNO 3 OH HNO 3 Photolysis CloudsNuages NO 3 - Aerosols NO 3 - Aérosols Gases HONO, NO 2, ClONO 2, etc. Gas HONO, NO 2, ClONO 2, etc. Réactions hétérogènes NH 3 3 NO 2 N 2 O 5 N 2 O 5 3 3 O 3 HC RCHO
Inorganic chemistry ok Complex radical chemistry partly ok, partly discussed OH and NO 3 radical reactions with organics up to C4 (Herrmann et al., Atmos. Env., 2005) SO x -, Cl/Cl 2 - and CO 3 - 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 CO 2 O3O3 NO x OH h + RH O2O2 ROO NO NO 2 O3O3 h RO RCHO Acid rain SO 2 H 2 SO 4 NO 2 HNO 3
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 Radical distribution function the center mass of the Cl(H 2 O) 255 water molecule cluster Stuart and Berne, JPC-A, 1999 Cl - O
PMT «Reflected» signal Focusing and filtering optics 150 W Xenon Arc Lamp PMT «Bulk» signal KrF laser 248 nm mirrors Suspended droplet Reaction Chamber Irradiated surface Laser sheet Focused Xe lamp HCA differentiating circuit Time ABS DG535 Oscilloscope PC HCA Diffuse Reflectance Laser Flash Photolysis
Reaction mechanism… I. Cl 2 - + ethanol S 2 O 8 2- + h 2 SO 4 - SO 4 - + Cl - Cl + SO 4 2- Cl - + Cl Cl 2 - Cl 2 - Cl - + Cl Cl 2 - + C 2 H 5 OH products Cl 2 - products Cl products Cl 2 - + Cl 2 - products
Example: The reaction of Cl 2 - with EtOH Absorbance at 350 nm [NaCl] = 50 x 10 -3 M and [Ethanol] = 0.3 M Bulk decays in agreement with literature Surface decays faster?
Surface tension and Gibbs surface excess for ethanol solutions 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 Interface a few Å Ethanol concentration a few nm? Detector Xe Lamp 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
Nowadays: Secondary organic aerosols CO 2 O3O3 NO x OH h + RH O2O2 ROO NO NO 2 O3O3 h RO RCHO Acid rain SO 2 H 2 SO 4 NO 2 HNO 3 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 CO 2 trap (KOH) (caustic potash) dryer (H 2 SO 4 ) S, S rocksalt plates C 5 H 11 ONO (nitrite of amyl) benzene C 3 H 5 I (iodide of allyl) formation of sky matter (organic germs) observation of blue clouds From T. Hoffmann - Mainz
low volatile products e.g. gas phase chemistry (e.g. ozone formation) GiGi AiAi gas/particle partitioning new particle formation condensation homogeneous nucleation kiki semivolatile products e.g. gaseous products e.g. HCHO acetone glyoxal Mechanisms oligomeric products e.g. CHO OH HOO O O O OH mesitylene OH + OH + O 3 + OH + NO 3 -pinene OH O-O ONO 2 O-O radicalintermediates COO O OH O-O ONO 2 O-O radicalintermediates COO O 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 (H 2 SO 4 – H 2 O – NH 3 ) 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 H 2 SO 4 – H 2 O ~ 10 kcal mol -1 H 2 SO 4 – H 2 O – NH 3 ~ 25 kcal mol -1 ~ 1 nm 3) condensation of low volatile organics 1) Formation of TSCs heteromolecular homogeneous nucleation involving organic acids and 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 (H 2 SO 4 – H 2 O – NH 3 ) D) Berndt et al. (2005) Science TSCs (H 2 SO 4 – H 2 O (organics?)) growth by carbonylic oxidation products ? 1) Freshly formed H 2 SO 4 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 complexity Requires model systems Cecinato et al, J. Sep. Sci, 2003
Water adsorbs to hydrophobic surfaces Thomas et al, JGR, 1999 OTS: octadecyltrichlorosilane adsorption desorption Depends on rh Is reversible CH 3 (CH 2 ) 17 Si Cl 3
Where does it adsorb? Rudich et al, JPC-A,2000 µdroplet 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?)
Finally whats 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
The life of a particle… NH 3 NOx Photochemistry HNO 3 Inorganic primary particles Salts (marine) COV Semi-volatils VOCs Photochemistry Primary organic particle SO 2 H 2 SO 4 Photochemistry H2OH2O 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…