Atmospheric Gas Phase Reactions. U. Platt

Transcription

1 SOLAS Summer School 009 Atmospheric Gas Phase Reactions (Tue, August 11) U. Platt Institute for Environmental Physics Ruprecht-Karls-University of Heidelberg

2 The Composition of the Atmosphere N O Noble gases H O Xe Kr He Ne Ar CO CH 4 H CO O 3 Hydrocarbons NO X SO HO OH?? 100 s-1000 s s ppt ppb ppm No. of different spezies

3 Trace Species in the Atmosphere Gaseous components of the atmosphere: 1) Permanent components (noble gases, N ) ) Transient components (any other gas) - primary emitted species (e.g. O, CO, NO, SO,...) - secondary species, formed in the atmosphere by chemical reactions (e.g. O 3, CH O, BrO,...) Even Species only ocurring at mixing ratios below 10-1 can have a decisive influence on the atmosphere as a whole. Quantitative treatment of chemical processes Reaktion Kinetics

4 Reaction Kinetics Chemical reactions in the atmosphere are relevant for (e.g.): Ozone formation Degradation of air pollutants (self cleaning of the atmosphere) Degradation of climate gases We categorise chemical reactions in: Homogeneous Reactions: The reactands are all in the same phase (in the atmosphere usually in the gas phase). A Bang! usually in the gas phase). B D A Heterogeneous Reactions: The reactands are in different phases (e.g. reactions of gas molecules at aerosol surfaces, cloud droplets or ice crystals). Photochemical Reactions, i.e. the chemical transformation of gas molecules by solar radiation (can be homogeneous or heterogeneous). C A B B C C

5 Outline Evolution of reactant concentrations as a function of time (reaction velocity) Elementary reactions vs. complex reactions Reaction order Chemical equilibria How can the thousands of chemical reactions occuring in the atmosphere simultaneously be captured in a numerical model? Some examples (HO X radicals, NO X chemistry, Bromine Explosion) Summary

6 Reaction Velocity and Reaktion Order (1) Reactions of zero th ' Order: A Products Educt A decays with constant reaction velocity. Def: reaction velocity: d[ A] dt With the reaction rate constant k in units of Molek./(cm 3 s). = k Not normally ocurring Reactions of 1 st Order (unimolekular Reactions) A Products reaction rate constant k in 1/s d[ A] dt = k [ A] Thermal decay, photochemistry, pseudo 1 st order Def: [A] denotes the concentration i.e. amount of matter per unit volume of the atom or molecular species A. Units: Molecules per cm 3 or Mols per liter

7 Temporal Evolution of the Concentration () Reaction Velocity Reaction of 1 st Order: d[ A] dt = k [ A] [A](t) [A] 0 d dt ([ ]( )) [ ] Integration: k t ( ) [ ] A t = A k e k A t= 0 0 t= 0 0 [A] 1 d[a] k dt [A] = [A] 0 0 t [A] 0 / [A] 0 /e [ A] ln [ A] 0 = k t ( ) 0 [A] t = [A] e k t t [ A]( t) = [ A] 0 1 τ 0 t ½ τ=1/k time, t 1 1 τ =, t = ln τ τ k

8 Reactions Velocity and Reaktion Order (3) Reactions of nd order: A+B C+D A collides with B: 1) Reactions can only occur during collisions ) Usually only a small fraction of the collisions leads to reactions Reaction velocity: d[ A] d[ B] d[ C] d[ D] = = = = k [ A ] [ B ] dt dt dt dt Reaction rate constant of nd order: k in s -1 (konz.) -1, e.g. cm 3 molec. -1 s -1

9 Snapshot of a Volume of Air (10-18 cm 3 ) Volume: 10 x 10 x 10 nm At 1000hPa and 17 o C It contains on average: 5 O molecules 0 N molecules Blue trajectory: mean free path (60 nm) of an air molecule (red arrow) until it hits another molecule (black arrow). In reality the mean free In reality the mean free path is a straight line!

10 Reactions of nd Order A + B Products (Example: O 3 + NO O + NO ) In this case A and B are consumed in equal amounts, and the velocity of the reaction is proportional to the product of the concentrations of A and B: d [ A ] dt = d [ B ] dt = k [ A ] [ B ] [ A ] with the Reaction rate constant k in units of: 1/((Molek/cm 3 ) sec)= cm 3 /(Molek. s) Solution for special case [A]=[B]: d[ A] d[ A] = k [ A] = k dt dt [ A] 0 d [ A ] t [ A ] 0 [ A ] 0 = 4 [ ] = = + = [ ] k dt k t k t A A [ A] [ A] 0 [ A] [ A] 0 4 k t [ A] t [ A] 0 t

11 Reactions Velocity and Reaction Order (4) 1) A Products d[a [ ] ) A + B Products dt d[ A] dt = k[ A] { k } = d[ B] = = k [ A ][ B ] dt { k } k = 1 s cm 3 molec. s 3) A + B + C Products d [ A ] dt Pseudo 1 st Order reactions: = d [ B ] dt = d [ C ] dt = k [ A ][ B ][ C ] { k} = cm molec 6. s A + B Products, but [B] >> [A] [B] const. (e.g. O ) d [ A ] d [ B ] = = k[ A][ B] = k '[ A] dt dt 1 d[ B] 0 [ B] dt

12 Elementary vs. Complex Reactions Thesis: Only Reactions of nd Order of the type: A + B C + D Are Elementary reactions Any other type of reaction (in particular the unimolecular decay and reactions of the type A+B C) are Complex Reactions, i.e. reactions, which are not occurring within a single collision, but rather proceed in a series of steps.

13 Example for a Complex Reaction: Oxyhydrogen Gas Explosion Atmospheric mixing ratios of radicals are exceedingly low e.g. about 1 OH-radical for every air molecules But: In chemical reactions with molecules the radical status is hereditary e.g.: OH + CO CO + H Example (oxyhydrogen gas): H + O HO + H Start H + HO H + OH OH + H O H + H O Propagation (inherit radical status) H + O O + H OH + O OH + H Branching (from one radical we get two) Does that also OH + HO H O + O Termination happen in the Net: H + O H O Consequence: The radikal concentration and thus the speed of reaction can increase exponentially. Oxyhydrogen gas atmosphere?

14 Reactions of 3 rd Order (Termolecular Reactions) In general: A + B + C Products In the atmosphere: C usually N or O M ( Molecule ) Popular special case: Rekombination reactions A + B + M AB + M M absorbs the reaction energy (+part of momentum) In detail: A + B (AB)* (a) (AB)* A + B (b) (AB)* + M AB + M (c) Thus: Not a elementary reaction High pressure limit: k =k a reaction (b) can be neglected Low pressure limit: k 0 =k a k c /k b reaction (a)+(c) slower than (b) Overall Reaction rate constant (Lindemann Hinshelwood Formula): k k [ M] k k [ M] a c 0 ([ ]) = ( ) = = k M k p Pressure: p [ M] kb + k [ c M ] k0 [ M ] + k Hence the name: pressure dependent reaction

15 Pressure dependent Reactions (Termolekular Reactions ) Pressure and altitude dependence of two pseudobimolecular reactions. The scale on the right-hand side denotes the lifetimes of NO and SO, respectively, which would result if these compounds were only removed by reaction with 10 6 OH cm -3 at all altitudes. From: U. Schurath High pressure limit: k = const. Low pressure limit: k [M]

16 What Happens During a Chemical Reaction? A Bang! C B D The Excited Transition State Activation energy E a

17 The Maxwell-Boltzmann-Distribution The velocity distribution function, i.e. the number of gas molecules in the velocity interval [v, v+dv] given by the Maxwell-Boltzmann - distribution: 3 m mv E f ( v) dv = v exp dv with v = π kt kt m We obtain: dv dv = de = de de E m f(v) v v 1 E f ( E) de = E exp de π k T k T ( ) 3 T > T 1 1 E = mv > E a Fraction of molecules with E>E a : 1 E f (E > E a ) = f ( E) de = E exp de 3 π k T k T E ( ) 0 0 E

18 Fraction of Molecules with E>E a The integration becomes simple, if the term E 1/ in the integrand is assumed to be constant in comparison to the exponential: E ( ) a Ea f (E > E a ) = exp π k T k T Thus the reaction rate constant k should be ist proportional to n(e>e a ). In 1889 Svante Arrhenius derived the following expression for the T-dependence of reaction rate constants: ( E ) ( ) a = A exp k T K = Boltzmann constant. k T The constant A depends on the collision rate of the molecules (as well as of further conditions, e.g. the orientation of the molecules during the collision). The maximum value of A thus is the collision rate. Svante Arrhenius

19 On the Collision Rate A molecule can be thought to traverse a collision cylinder with radius r = diameter of the molecule d. Number of collisions per time interval t is given by the number of moleculs in the volume V=πd tv Molec. Collision Cross Section σ, [10-15 cm²] H.7 N 4.3 O 4.0 CO 5. If each collision would lead to a chemical reaction (A+B C+D) it would proceed with a reaction rate of: d A k AB A B dt = [ ] [ ][ ] = 1443 σ v k AB [ A][ B]

20 Estimating Reaction Rate Constants Three conditions for a gas-phase reaction to occur: 1) Molecules must collide ) Energy (in the centre of gravity system) must exceed barrier (if present) 3) Steric factor must be right Mean kinetic energy E kin of air molecules at temperature T and corresponding mean velocity: E kin 3 = k T Collision frequency (T = 93K, m = kg): 3 k T m v v = 500 ( air at T = 93 K ) m s N z = = ρ σ v t 9 Hz With these numbers we obtain a maximum reaktion rate constant (for P AB = 1, i.e. a reaction ocurring at each collision): k AB = σ v cm Molec. s However, in reality usually P AB << 1, e.g. because of the energy barrier, thus the Arrhenius Eq. Becomes: ( ) ( E ) k exp k T a = AB k T

21 Some Important Reaction Rate Constants Reaction k in cm 3 molecule -1 s -1 (at 98K) OH + CO H + CO (1 atm) OH + CH 4 H O + CH OH + C H 6 H O + C H CH 3 + O + M CH 3 O +M (k 0, Molek - cm 6 s -1 ) (k ) OH + NO + M HNO 3 + M (k 0, Molek - cm 6 s -1 ) (k ) HO + O 3 OH + O NO + O 3 NO + O NO + O 3 NO 3 + O O + O 3 O + O Atkinson et al. 1997, J. Phys. Chem. Ref. Data, 6, JPL compilations: IUPAC compilations: There are many more relevant reactions in the atmosphere: The Master Chemical Mechanism (MCM), contains 1600 reactions and 4500 chemical species

22 Photo Chemistry Absorption of a photon with frequency ν by a molecule can lead to a chemical reaction, e.g. break-up of the molecule (Photolysis). A + hν Products Analogous to first order reactions we can describe the reaction velocity of photolysis as: d[ A] dt = J [ [ A ] unitsof J : s 1 The reaction rate constant J is called Photolysis Frequency. Not to be confused with the photolysis rate which is defined as: d [ A ] 3 1 RJ = = J [ A] unitsof R : molec. cm s dt

23 The Photolysis Frequency (1) The absolute value of the photolysis frequency J depends on three factors: 1) The intensity of the radiation ) The property of the molecule, to absorb radiation of a given frequency ν (or wavelength λ). Quantitatively: I(ν) = Intensity of the radiation field σ(ν)= Absorption cross section of A at the frequency ν ds = Thickness of the absorbing layer di ( ν ) = I ( ν ) σ ( ν ) [ A ] ds bzw. di ( λ ) = I ( λ ) σ ( λ ) [ A ] ds 3) The probability, that the absorption of a photon will lead to a reaction (e.g. to the dissociation) of the molecule. Prerequisit: Photon energy > Binding energy of the molecule (or the activiation energy E a. This probability is called Quantum Efficiency (quantum yield) φ. Frequently φ can be approximated by a step funktion, i.e. φ( ν ) 0; h ν < E 1; hν E a a

24 Example: UV Photolysis of O 3 : O 3 + hν O( 1 D) + O ( 1 ) J ( λ ) = φ ( λ ) σ ( λ ) F ( λ ) J = J ( λ) dλ 0 = φ( λ) σ ( λ) F( λ) dλ 0 Typical daily mean: OH-Production: cm -3 s -1 Daily integral: cm -3 The Photolysis Frequency () 13 insolation at sea level Summer (1. 6.) 10 (rel. units) Winter (1.1.) ,0 0,8 0,6 0,4 0, 0,0 10 F( λ) ozone absorption cross section σ ( λ) 10 cm 19 Quantum efficiency 0,6 for O('D) - Formation φ( λ) J(λ)=I(λ)*σ(λ)*φ(λ) in 10-8 s -1 nm J=.5*10-5 s x 10 J=8*10-7 s wavelength in nm

25 NO x O 3 Reactions: The Photo - Stationary State NO + hv NO +O( 3 P) O( 3 P) + O O 3 very fast O 3 + NO NO + O k = No net reaction The stationary state NO - concentration is a function of the UV-flux J d[ NO ] dt = J[ NO ] k[ NO ][ O 3 ] Stationary state: d[ NO] = 0 dt [ NO 14 ] k = [ O 3] [ NO] J 8 10 Also known as Leighton-ratio (thermodynamic) Equilibrium Stationary State

26 The Thermodynamic Equilibrium Equilibrium reaction: A + B C + D Law of Mass Action (Massenwirkungsgesetz): G Equilibrium constant: [ C ][ D ] [ A ][ B ] k RT K = = e = [ A][ B] [ C][ D] k [ ] G = H T S = G + G G + G fc fd fa fb ( H TS ) ( H TS ) ( H TS ) ( H TS ) = + + c c D D A A B B The equilibrium constant is only determined by the difference in the Gibbs free energy (i.e. the heat of formation and the entropy) of the involved species and the temperature. However: The time it takes to reach the equilibrium depends on the magnitude of the reaction rate konstants! Example: CH 4 + OH CH 3 + H O, see listings of GG f in JPL: G(CH 4 )=-17.9 kcal mol -1, G(OH)=9.3 kcal mol -1, G(CH 3 )=35.0 kcal mol -1, G(HO)=-57.8 kcal mol -1 G react = G(CH 4 )+ G(OH)- G(CH 3 )- G(H O) =-17.9 kcal mol kcal mol kcal mol kcal mol -1 = 14. kcal mol -1

27 Reaction Systems (very simple example) NO x O 3 Reactions: NO + hv NO +O( 3 P) O( 3 P) + O O 3 O 3 + NO NO + O d[ NO ] dt d[ NO] dt d[ O] dt d[ O ] dt 3 = J [ NO ] + k [ NO ][ O ] 3 = J[ NO ] k[ NO][ O ] 3 = J[ NO ] k [ O][ O ] = k [ O][ O ] k[ NO][ O ] 3 System of coupled differential equations Numerical generation by reaction compilers + Numerical integration e.g. CHEMC (

28 Reaction Kinetics at Work - Some Examples from the Atmosphere

29 The Origin of Tropospheric Ozone Initial Idea: Stratosphere O 3 Flux Troposphere No Chemistry, since Φ(λ 4 nm) = Molec.cm - s -1 O 3 Depos Molec.cm - s -1 Today: Stratosphere Troposphere CO, HC O 3 O 3 + hν + H O OH O 3 Flux Molec.cm - s Molec.cm - s -1 O 3 Depos Molec.cm - s -1

30 Smog Chamber Experiments --> Ozon 'Isoplets' Lines of constant O 3 mixing ratios (ppb) NO X -limited Hydrocarbon-limited

31 NO X - and HO X - Catalysis of the Photochemical Ozone Production in the Troposphere

32 Disturbance of the Leighton Ratio and the rate of O 3 Formation There is no net ozone production. However, if other reactions, in particular by Peroxy radicals oxidise NO to NO : RO + NO NO + RO (R4) Ozone will be formed at the rate P O3. Also the Leighton Ratio will be reduced: L 1 = [NO] J [NO ] = k [O ]+k [RO ] 3 R The rate of ozone formation, P O3 in a given airmass can be calculated from measurements of the Leighton Ratio [NO]/ [NO ]=L 1 together with [O 3 ], and J: 1 1 PO3 = j1 NO L 1 L 0 [ ] Since P O3 relates to the concentration of peroxy radicals (RO ), there concentration can also be inferred from these measurements. [ ] P = k RO O3 R

33 Observed O 3 Production Rates 'Southern Oxidant Study June 15 to July 15, 1999 at Cornelia Fort Airpark, Nashville, Tennessee [Thornton et al. 00] Averaged O 3 production rates P O3 calculated from simultaneous observations of NO, NO, O 3, OH, HO, H CO, actinic flux, and T. Data were placed into three P HOx bins: high (0.5 < P HOx < 0.7 ppt/s, circles), moderate (0. < P HOx < 0.3 ppt/s, squares), and low (0.03 < P HOx < 0.07 ppt/s, triangles), and then averaged as a function of NO. All three P HOx regimes demonstrate the expected dependence on NO: P O3 increases linearly with NO for low NO (<600 ppt NO), P O3 becomes independent of NO for high NO (>600 ppt NO). Crossover between NO X - limited and NO X - saturated P O3 occurs at different levels of NO in the three P HOx regimes.

34 Diurnal Variation of O 3 Levels in Different Air Masses: Forest (Weltzheimer Wald) and City of Heilbronn (southern Germany). August 11-14, 000 Welzheimer Wald, rural Heilbronn, urban Landesanstalt für Umweltschutz Baden-Württemberg and UMEG)

35 Removal of CH 4 CH 4 + OH CH 3 + H O CH 3 + O + M CH 3 O + M CH 3 O + NO CH 3 O + NO CH 3 O + O CH O + HO Formaldehyde Another Radical CH O + hν CO + H CHO + H CHO + O CO + HO H + O + M HO + M HO + NO OH + NO CH 4 + OH + NO + hν CO + OH

36 Free Radicals Which are important in atmospheric chemistry? First suggestion of OH reactions in the atmosphere by B. Weinstock Formation of OH by: O 3 + hν O( 1 D) + O ( 1 ) O( 1 D) + H O OH + OH Role of OH in the formation of CO and formaldehyde suggested [H. Levy 1971]. Role of OH in tropospheric ozone formation (P. Crutzen 1974) Other Radicals (by either definition)? O ( 1 ), Cl-atoms, NO 3... HO X - Cycle OH Degradation of most VOC Key intermediate in O 3 formation NO X NO Y conversion HO Intermediate in O 3 formation Intermediate in H O formation RO Intermediate in ROOR formation Aldehyd precursor PAN precursor Intermediate in O 3 formation NO 3 Degradation of certain VOC NO X NO Y conversion (via N O 5) RO precursor XO Catalytic O 3 destruction (X = Cl, Br, I) Degradation of DMS (BrO) Particle formation (IO) Change of the Leighton Ratio X Degradation of most (some) VOC: Cl (Br) Initiates O 3 formation RO precursor

37 Simplified Outline of the HO X (=OH + HO + RO ) Cycles N O (O ) C H 3 O (C n H n+1 -O ) C H 3 O O 3 (O ) C H O P(O H ) O H (O ) C H O C H 4 (N M H C ) C O (O ) H (O ) C H O (O ) hν(o ) O H (O ) O 3 hν H O O 3 O H H O H O N O O 3 O lefines O 3 H O O H O 3 P rim ary O H S ource N O hν [O H ] P (O H ) H N O 3 Final O H S inks N O H O [O H ] P (O H )

38 DiurnalVariation of OH - Concentration Hofzumahaus et al. FZ Jülich

39 NO X Dependence of OH - Model [NO ] = 0 X O 3+ hν + H O 86 % 14 % OH 6-3 4*10 cm τ 0.9 sec OH + CO + KW + HO 4 % H O 96 % HO H O (Peroxides) O3 [NO ] = ppb X O 3+ hν + H O 0 % 80 % OH *10 cm τ 0.6 sec OH 8 % HO + NO 17 % + HO 1 % % 80 % HNO 3 H O H O (Peroxides) from Ehhalt 1999 NO [NO ] = 100 ppb X O 3+ hν + H O CH O + hν (+ O ) 70 % 30 % 0 % OH 6-3 3*10 cm τ 0.03 sec OH 10 % HO + NO 90 % HNO 3 NO

40 NO X Dependence of OH -Observation Forschungs zentrum Jülich OH / 10 6 cm ALBATROSS 96 POPCORN 94 BERLIOZ NO x / ppb

41 OH the Cleaning Agent of the Atmosphere A + OH products + OH Although usually [OH] << [A] this can be treated as Pseudo 1 st Order reaction, since OH is recycled Lifetime of A (e.g. CO, CH 4,...): d[a] 1 = k A+ OH[OH][A] = [A] dt 1443 τ τ = A k A+ OH const. 1 [OH] Examples: average [OH]: 10 6 cm -3 τ NO (10 6 x ) -1 A 10 5 s or 1 day τ CO (10 6 x ) s or 3 weeks τ CH4 (10 6 x ) s or 6 years

42 Simplified Outline of the NO X - (=NO + NO ) and NO Y - Cycles oxidation stage: NO, NO, NO 3 O 3 Emission NO NO NO 3 RO OH hν hν H O liq. O 3 hν N O 5 VOCs Temperature dependent! N O OH H O liq. HNO NO 3 Aerosol HNO 3 <10% >90% RO HO NO PAN?

43 NO 3 Field Measurements Cavity Ringdown (Boulder, CO) Brown et al. 001 DOAS (Edwards AFB) Platt et al. 1984

44 Contribution of NO 3 to the Atmospheric Oxidation Capacity in Pabstthum (BERLIOZ 1998) oc : = d dt VOCs = [ NO3 / OH / O3] - average of all BERLIOZ-data - 4h- mean values: O 3 16 % f NO 3 / OH / O3 VOCs oc [cm -3 s -1 ] NO OH O NO 3 30 % OH 54 % Geyer et al. 001 oc( NO3 ) = oc( OH ) 1

45 Simplified Outline of the XO X (=X + XO, X = I, Br, Cl) Cycles Primary Emission (Algae, etc.) CH 3 X CHX 3 CH X etc. OH, hν XO X = X + XO, X = I, Br, Cl HX RH,HO OH XNO YO NO X hν O 3, NO XO hν, hν YO hν hν HO hν, HOX OXO XY, X [ ] [ ] XO 1000 ( X Cl ), X = ( ) ( ) 100 X = Br, 10 X = I XONO X O N O 5 HX XONO X Y - O HOX H + + X - Sea-Salt (Snow Pack or Aerosol)

46 Observation of Reactive Halogens in the Troposphere Reactive halogen species: Cl, Br, I, ClO, BrO, IO, OClO, OBrO, OIO, I O,... Domain ClO BrO IO ppt ppt ppt Free Troposphere? 1-? Coastal Regions? <... 6 up to 30 Polar Regions (springtime) several 10? ? Salt Lakes/Pans up to 15 up to 00 up to 10 Volcanic Plumes? up to 1000?

47 Halogen Catalysed Destruction of Tropospheric Ozone 1) BrO + BrO Br + Br + O (rate determining) Br + O 3 BrO + O net: O 3 3 O at high BrO levels ) BrO + HO HOBr + O (rate determining) HOBr + hν Br + OH OH + AO HO + A (AO = O 3, CO,...) net: O 3 3 O at low BrO levels 3) BrO + ClO Br + Cl + O (rate determining) BrCl + O Br + OClO + O net: O 3 3 O if ClO available 4) BrO + IO Br + I + O (rate determining) BrI + O Br + OIO + O very fast, if IO available

48 Halogen anti-catalysis of Tropospheric O 3 Production XO + HO HOX+O HOX + hν OH + X XO XO + NO NO + X X + O 3 XO + O The product: [HO ]x[no] and thus the rate of O 3 formation is reduced XO

49 Observation of Reactive Halogen Species in the Troposphere Jun 3 Jun 4 Jun 5 Jun 6 Jun 7 Jun 10 BrO-clouds in the Arctic, Jens Hollwedel et al. 004 Adv. Space Res. 34, IO at a coastal site, IO concentration [ppt] Christina Peters et al. 005, ACP 5, Jun 3 Jun 4 Jun 5 Jun 6 Jun 7 Jun Date Tidenhöhe [m]

50 A Chemical Instability: The "Bromine Explosion" (Polar) Clean Air Well mixed Boundary Layer (up to 1000m) Br + hν Br Inversion (Mit-Latitudes) Polluted Air BrO + NO BrONO Br + O 3 BrO + O BrO + HO HOBr + O net: BrO BrO (Bromine Explosion Mechanism) Snow/Aerosol Surface: HOBr + Br - + H + Br + H O net: BrO BrO (Bromine Explosion Mechanism) Surface: BrONO HOBr + NO - 3 HOBr + Br - + H + Br + H O Polar Boundary Layer Salt Lakes Coastal Areas Volcanic Plumes... Frost flowers? Tang & McConnel 1996, Vogt et al Platt & Lehrer 1997 Wennberg 1999 v. Glasow et al. 000

51 BrO and Ozone During the 'Polar Sunrise 000' Experiment at Alert, Northern Canada ALERT000 GAW ozone / DOAS BrO time series G AW station O 3 [ppb] BrO DSCD [10 14 cm - ] periods of low clouds/snow BrO mixing ratio * [ppt] 1 Ap r 3 Ap r 5 Ap r 7 Ap r 9 Ap r 1 May 3 May 5 May 7 May 9 May DATE [UT] * assuming a hom ogeneous surface layer of 1 km thickn ess BrO by Multi-Axis DOAS (MAX-DOAS) Hönninger and Platt 00, Atmos. Environ. 36,

52 Satellite (GOME) Observations of BrO Plumes Eckehard Lehrer, Doctoral Thesis, Univ. Heidelberg, 1999

53 Summary Reactions of importance in the atmosphere are: Homogeneous Reactions They are always of the type A+B B+C (elementary reactions). Each reaction can be treated independently from the others. Heterogeneous Reactions, they are difficult to quantify Photochemical Reactions Theoretical determination of reaction rate constants is extremely difficult because of the steric factor, to be shure: Lab. measurements Elementary reactions interact to form complex schemes. Usually numerical models have to be used to integrate the very large systems (hundreds to thousands of equations) of differential equations. Free radicals (OH, HO, NO 3, BrO, IO,...) are the driving force of atmospheric chemistry