Outline. Chemical lifetime. Photochemistry. Ozone chemistry Chapman model Catalytic cycles Ozone hole. Institute of Applied Physics University of Bern

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Neal Laurence Tyler
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1 Institute of Applied Physics University of Bern Outline

2 Introduction Chemical reactions between stable molecules are quite slow in planetary s Absorption of solar UV-radiation leads to the production of radical species: atoms, ions, excited molecules Radicals are extremely reactive Bulk of atmospheric chemistry involves the reaction between the radicals themselves and between the radicals and stable molecules Main question: 1. Is a specific reaction possible? 2. How fast is a reaction? Atmospheric reactions are classified into four types Unimolecular reactions: A B + C Bimolecular reactions: A + B C + D Termolecular reactions: A + B + M C + M Photochemical reactions A + (hν) B + C Enthalpy of formation In every chemical reaction either heat is liberated or heat has to be added Reaction where energy is released is called exothermic Reaction which requires energy is called endothermic Energy Q released or consumed in a reaction, resp. enthalpy change is Q = H R = H 0 f (products) H0 f (educts) Enthalpy of most stable form is normally taken as zero If reaction is exotherm H R < 0 An exothermal reaction may proceed spontaneously if change in free Gibbs energy is negative: G = H T S and G R = Gf 0 0 (products) Gf (educts) Tables for H R and G R are found in the literature

3 Enthalpy of formation Examples Reaction: NO 3 + H 2 O HNO 3 + OH G R = kcal/mol reaction not possible Reaction: O + O 2 + M O 3 + M Formation of ozone O 3 H 0 R = H0 O 3 + H 0 M H0 O H0 M = 25.4kcal/Mol energy is released heating the Reaction: O 2 + hν 2 O( 3 P) photochemical reaction H R = 2(59.55) hc λ = kcal/Mol hc λ reaction will work if λ < 240nm i.e. UV radiation

4 Unimolecular reaction Unimolecular reaction: A B + C Reaction rate R is R = d[a] = d[b] = d[c] = k[a] k is called rate coefficient The symbol [X ] is used for number densities i.e. number of molecules per volume It follows for the decay of A and d[a] [A] = k : τ = 1/k [A] = [A 0 ]e kt Bimolecular reaction Bimolecular reaction: A + B C + D Reaction rate R is R = d[c] = d[d] = d[a] = d[b] = k[a][b] In contrast to unimolecular reactions the rate coefficient has here dimension of cm 3 molecule 1 sec 1 In order to interact with each other A and B must collide To do so they must overcome some activation energy E act Reaction rate is temperature dependent and given by Arrhenius law k(t ) = Ae E act RT

5 Bimolecular reaction rates Termolecular reaction Some bimolecular reactions need an additional partner M, any air molecule, to proceed. Such reactions are thus dependent on pressure Termolecular reaction: A + B + M C + M The reaction rate is a complicated function k = k 0 [M](1 + k 0[M] ) 1 F c (1 + (N 1 log k 0 [M]/k ) 2 ) 1 k where k 0 und k reaction rates for small and very high pressure regimes k 0 (T ) = k ( T 300 ) n and k (T ) = k 300 ( T 300 ) m

6 Termolecular reaction rates In an many constituents react, e.g A + B P k 1 A + C + M P k 2 A + F P k 3 G + H A + P k 4 For the change of species A we get d[a] = k 1 [A][B] k 2 [A][C][M] k 3 [A][F ] + k 4 [G][H] and for the lifetime τ A = 1 k 1 [B] + k 2 [C][M] + k 3 [F ]

7 For steady state: d[a] = 0 = i Q i i S i [A] and therefore For our example above [A] = [A] = i Q i i S i k 4 [G][H] k 1 [B] + k 2 [C][M] + k 3 [F ] The chemical lifetime extends from fraction of sections to centuries! According to this the distribution in the can extend from meters to global scales

8 Photochemical reactions: A + hν B + C The reaction rate of a photochemical reaction is given by d[a] = j[a] The inverse of j is the photochemical lifetime In an j is determined by the amount of photons, actinic flux, I (λ) = F λ λ/hc, the absorption cross section, σ a and the quantum efficiency Φ j = λ max λ min σ a (λ)φ(λ)i (λ)dλ Important examples in ozone chemistry are: O 2 + hν O + O j 2 O 3 + hν O 2 + O j 3 Examples from ozone photochemsitry

Reactions in a pure oxygen according Chapman: O 2 + hν O + O j 2 (1) O + O 2 + M O 3 +

reactions: Reaction (1) and (4) create and destroy odd oxygen Reaction (2) and (3)

9 Reactions in a pure oxygen according Chapman: O 2 + hν O + O j 2 (1) O + O 2 + M O 3 + M k 2 (2) O 3 + hν O 2 + O j 3 (3) O + O 3 O 2 + O 2 k 3 (4) There are two types of reactions: Reaction (1) and (4) create and destroy odd oxygen Reaction (2) and (3) interconvert O and O 3 Evaluating reaction rates d[o] d[o and 3 ] and evaluating steady state i.e. equilibrium, it can be shown: [O 3 ] = [O 2 ] ( ) 1/2 k2 j2 [M] k 3 j 3 Vertical distribution of ozone Measurements with balloon sondes are performed twice a week in Payerne

10 O3 distribution over Bern measured by microwave radiometry N. K ampfer O3 global average column density N. K ampfer

11 O3 forecast N. K ampfer KNMI / ESA SCIAMACHY Forecast total ozone (D+2) 14 Mar UTC [DU] O3 distribution Measured ozone distribution shows: N. K ampfer I Maximum at approx. 22 km for number density I Maximum at approx. 35 km for volume mixing ratio (remember: VMR=pO3 /p) I Column density aprrox. 3mm=300 Dobson units I Distribution of ozone is variable and changes as function of time and location I is far too simple, particularly it predicts more ozone additional processes must act: I Chemistry must be modified I Transport processes must be considered

12 In addition to pure oxygen chemistry: X + O 3 XO + O 2 XO + O X + O 2 net:o 3 + O O 2 + O 2 X can be a radical as H, OH, NO, Cl, Br,... X stems from source gases that are transported upwards to the stratosphere where they are destroyed by UV-radiation liberating the radicals In addition radicals can be converted to so called reservoir gases such as HCl or ClONO 2 Also heterogeneous reactions on particles such as on clouds are important ozone hole In the 1980-ties extremely low values of ozone over antarctica were observed Later a similar effect was observed also in the arctic

13 Ozone-hole as seen by microwave limb sounder O 3 hole details

14 O 3 hole schematics O 3 on Mars

15 O 3 on Mars Zonally averaged ozone column density in µm-atm O 3 on Ganymed

Introduction to Chemical Kinetics AOSC 433/633 & CHEM 433 Ross Salawitch

Introduction to Chemical Kinetics AOSC 433/633 & CHEM 433 Ross Salawitch Class Web Site: http://www.atmos.umd.edu/~rjs/class/spr2017 Goals for today: Loose ends from last lecture Overview of Chemical Kinetics