Institute of Applied Physics University of Bern Outline
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
Enthalpy of formation Examples Reaction: NO 3 + H 2 O HNO 3 + OH G R = +17.8 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 = 119.10kcal/Mol hc λ reaction will work if λ < 240nm i.e. UV radiation
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
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 300 0 ( T 300 ) n and k (T ) = k 300 ( T 300 ) m
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 ]
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
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 + 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
O3 distribution over Bern measured by microwave radiometry N. K ampfer O3 global average column density N. K ampfer
O3 forecast N. K ampfer KNMI / ESA SCIAMACHY Forecast total ozone (D+2) 14 Mar 2008 12 UTC [DU] 150 175 200 225 250 275 300 325 350 375 400 425 450 475 500 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
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
Ozone-hole as seen by microwave limb sounder O 3 hole details
O 3 hole schematics O 3 on Mars
O 3 on Mars Zonally averaged ozone column density in µm-atm O 3 on Ganymed