concepts of chemistry in the atmosphere - universität · pdf filen. k ampfer concepts of...

N. Kampfer

Concepts ofchemistry in the

atmosphere

Thermodynamicalaspects

Basics of reactionkinetics

Chemical lifetime

Photochemistry

Ozone chemistry

Chapman model

Catalytic cycles

Ozone hole

Ozone on otherplanets

Concepts of chemistry in the atmosphere

N. Kampfer

Institute of Applied PhysicsUniversity of Bern

N. Kampfer


atmosphere



Chemical lifetime

Photochemistry

Ozone chemistry

Chapman model

Catalytic cycles

Ozone hole


Outline

Thermodynamical aspects

Basics of reaction kinetics

Chemical lifetime

Photochemistry

Ozone chemistryChapman modelCatalytic cyclesOzone holeOzone on other planets

N. Kampfer


atmosphere



Chemical lifetime

Photochemistry

Ozone chemistry

Chapman model

Catalytic cycles

Ozone hole


Introduction

I Chemical reactions between stable molecules are quiteslow in planetary atmospheres

I Absorption of solar UV-radiation leads to the productionof radical species: atoms, ions, excited molecules

I Radicals are extremely reactive

Bulk of atmospheric chemistry involves the reaction betweenthe radicals themselves and between the radicals and stablemolecules

I Main question:

1. Is a specific reaction possible?2. How fast is a reaction?

I Atmospheric reactions are classified into four typesI Unimolecular reactions: A −−→ B + CI Bimolecular reactions: A + B −−→ C + DI Termolecular reactions: A + B + M −−→ C + MI Photochemical reactions A + (hν) −−→ B + C

N. Kampfer


atmosphere



Chemical lifetime

Photochemistry

Ozone chemistry

Chapman model

Catalytic cycles

Ozone hole


Enthalpy of formation

In every chemical reaction either heat is liberated or heat hasto be addedReaction where energy is released is called exothermicReaction which requires energy is called endothermic

Energy Q released or consumed in a reaction, resp. enthalpychange is

Q = ∆HR = ∆H0f (products)−∆H0

f (educts)

Enthalpy of most stable form is normally taken as zero

If reaction is exotherm → ∆HR < 0An exothermal reaction may proceed spontaneously ifchange in free Gibbs energy is negative: ∆G = ∆H − T∆Sand

∆GR = ∆G 0f (products)−∆G 0

f (educts)

Tables for ∆HR and ∆GR are found in the literature

N. Kampfer


atmosphere



Chemical lifetime

Photochemistry

Ozone chemistry

Chapman model

Catalytic cycles

Ozone hole


Enthalpy of formation

N. Kampfer


atmosphere



Chemical lifetime

Photochemistry

Ozone chemistry

Chapman model

Catalytic cycles

Ozone hole


Examples

Reaction: NO3 + H2O −−→ HNO3 + OH∆GR = +17.8 kcal/Mol → reaction not possible

Reaction: O + O2 + M −−→ O3 + MFormation of ozone O3

∆H0R = ∆H0

O3+ ∆H0

M −∆H0O −∆H0

M = −25.4kcal/Mol

→ energy is released → heating the atmosphere

Reaction: O2 + hν → 2 O(3P) photochemical reaction

∆HR = 2(59.55)− hcλ = 119.10kcal/Mol−hc

λ

→ reaction will work if λ < 240nm i.e. UV radiation

N. Kampfer


atmosphere



Chemical lifetime

Photochemistry

Ozone chemistry

Chapman model

Catalytic cycles

Ozone hole


Unimolecular reaction

Unimolecular reaction: A −−→ B + CReaction rate R is

R = −d [A]

dt=

d [B]

dt=

d [C ]

dt= k[A]

k is called rate coefficientThe symbol [X ] is used for number densities i.e. number ofmolecules per volumeIt follows for the decay of A

d [A]

[A]= −kdt

and

[A] = [A0]e−kt

Chemical lifetime: τ = 1/k

N. Kampfer


atmosphere



Chemical lifetime

Photochemistry

Ozone chemistry

Chapman model

Catalytic cycles

Ozone hole


Bimolecular reactionBimolecular reaction: A + B −−→ C + DReaction rate R is

R =d [C ]

dt=

d [D]

dt= −d [A]

dt= −d [B]

dt= k[A][B]

In contrast to unimolecular reactions the rate coefficient hashere dimension of cm3molecule−1sec−1

In order to interact with each other A and B must collide

To do so they must overcome some activation energy Eact

Reaction rate is temperaturedependent and given byArrhenius law

k(T ) = Ae−EactRT

N. Kampfer


atmosphere



Chemical lifetime

Photochemistry

Ozone chemistry

Chapman model

Catalytic cycles

Ozone hole


Bimolecular reaction rates

N. Kampfer


atmosphere



Chemical lifetime

Photochemistry

Ozone chemistry

Chapman model

Catalytic cycles

Ozone hole


Termolecular reactionSome bimolecular reactions need an additional partner M,any air molecule, to proceed. Such reactions are thusdependent on pressureTermolecular reaction: A + B + M −−→ C + MThe reaction rate is a complicated function

k = k0[M](1 +k0[M]

k∞)−1Fc(1 + (N−1 log k0[M]/k∞)2)−1

where k0 und k∞ reaction rates for small and very highpressure regimes

k0(T ) = k3000 (

T

300)−n

and

k∞(T ) = k300∞ (

T

300)−m

N. Kampfer


atmosphere



Chemical lifetime

Photochemistry

Ozone chemistry

Chapman model

Catalytic cycles

Ozone hole


Termolecular reaction rates

N. Kampfer


atmosphere



Chemical lifetime

Photochemistry

Ozone chemistry

Chapman model

Catalytic cycles

Ozone hole


Chemical lifetime

In an atmosphere many constituents react , e.g

A + B→ P k1

A + C + M→ P k2

A + F→ P k3

G + H→ A + P k4

For the change of species A we get

d [A]

dt= −k1[A][B]− k2[A][C ][M]− k3[A][F ] + k4[G ][H]

and for the lifetime

τA =1

k1[B] + k2[C ][M] + k3[F ]

N. Kampfer


atmosphere



Chemical lifetime

Photochemistry

Ozone chemistry

Chapman model

Catalytic cycles

Ozone hole


Chemical lifetime

For steady state:

d [A]

dt= 0 =

∑i

Qi −∑

i

Si [A]

and therefore

[A] =

∑i Qi∑i Si

For our example above

[A] =k4[G ][H]

k1[B] + k2[C ][M] + k3[F ]

The chemical lifetime extends from fraction of sections tocenturies!According to this the distribution in the atmosphere canextend from meters to global scales

N. Kampfer


atmosphere



Chemical lifetime

Photochemistry

Ozone chemistry

Chapman model

Catalytic cycles

Ozone hole


Chemical lifetime

N. Kampfer


atmosphere



Chemical lifetime

Photochemistry

Ozone chemistry

Chapman model

Catalytic cycles

Ozone hole


Photochemical reactions: A + hν → B + CThe reaction rate of a photochemical reaction is given by

d [A]

dt= −j [A]

The inverse of j is the photochemical lifetime

In an atmosphere j is determined by the amount of photons,actinic flux, I (λ) = F ↓λλ/hc , the absorption cross section, σa

and the quantum efficiency Φ

j =

λmax∫λmin

σa(λ)Φ(λ)I (λ)dλ

Important examples in ozone chemistry are:

O2 + hν → O + O j2O3 + hν → O2 + O j3

N. Kampfer


atmosphere



Chemical lifetime

Photochemistry

Ozone chemistry

Chapman model

Catalytic cycles

Ozone hole


Examples from ozone photochemsitry

N. Kampfer


atmosphere



Chemical lifetime

Photochemistry

Ozone chemistry

Chapman model

Catalytic cycles

Ozone hole


Chapman model

Reactions in a pure oxygen atmosphere according Chapman:

O2 + hν → O + O j2 (1)

O + O2 + M→ O3 + M k2 (2)

O3 + hν → O2 + O j3 (3)

O + O3 → O2 + O2 k3 (4)

There are two types of reactions:

I Reaction (1) and (4) create and destroy odd oxygen

I Reaction (2) and (3) interconvert O and O3

Evaluating reaction rates → d [O]dt and d [O3]

dt andevaluating steady state i.e. equilibrium, it can be shown:

[O3] = [O2]

(k2

k3· j2j3· [M]

)1/2

N. Kampfer


atmosphere



Chemical lifetime

Photochemistry

Ozone chemistry

Chapman model

Catalytic cycles

Ozone hole


Vertical distribution of ozoneMeasurements with balloon sondes are performed twice aweek in Payerne

http://www.meteoschweiz.admin.ch/web/de/wetter/ozone_layer.html

N. Kampfer


atmosphere



Chemical lifetime

Photochemistry

Ozone chemistry

Chapman model

Catalytic cycles

Ozone hole


O3 distribution over Bern measured bymicrowave radiometry

N. Kampfer


atmosphere



Chemical lifetime

Photochemistry

Ozone chemistry

Chapman model

Catalytic cycles

Ozone hole


O3 global average column density

N. Kampfer


atmosphere



Chemical lifetime

Photochemistry

Ozone chemistry

Chapman model

Catalytic cycles

Ozone hole


O3 forecast

150 175 200 225 250 275 300 325 350 375 400 425 450 475 500

[DU]

KNMI / ESASCIAMACHY

Forecast total ozone (D+2)14 Mar 2008

12 UTC

N. Kampfer


atmosphere



Chemical lifetime

Photochemistry

Ozone chemistry

Chapman model

Catalytic cycles

Ozone hole


O3 distribution

Measured ozone distribution shows:

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 asfunction of time and location

I Chapman model is far too simple, particularly it predictsmore ozone→ additional processes must act:

I Chemistry must be modified

I Transport processes must be considered

http://www.temis.nl/protocols/O3forecast.html

N. Kampfer


atmosphere



Chemical lifetime

Photochemistry

Ozone chemistry

Chapman model

Catalytic cycles

Ozone hole


Catalytic cycles

In addition to pure oxygen chemistry:

X + O3 → XO + O2

XO + O→ X + O2

net:O3 + O→ O2 + O2

X can be a radical as H, OH, NO, Cl, Br,...X stems from source gases that are transported upwards tothe stratosphere where they are destroyed by UV-radiationliberating the radicals

In addition radicals can be converted to so called reservoirgases such as HCl or ClONO2

Also heterogeneous reactions on particles such as on cloudsare important→ ozone hole

N. Kampfer


atmosphere



Chemical lifetime

Photochemistry

Ozone chemistry

Chapman model

Catalytic cycles

Ozone hole


Ozone hole

In the 1980-ties extremely low values of ozone overantarctica were observedLater a similar effect was observed also in the arctic

N. Kampfer


atmosphere



Chemical lifetime

Photochemistry

Ozone chemistry

Chapman model

Catalytic cycles

Ozone hole


Ozone-hole as seen by microwave limb sounder

N. Kampfer


atmosphere



Chemical lifetime

Photochemistry

Ozone chemistry

Chapman model

Catalytic cycles

Ozone hole


O3 hole details

http://www.esrl.noaa.gov/gmd/dv/spo_oz/movies/anim2007.html

N. Kampfer


atmosphere



Chemical lifetime

Photochemistry

Ozone chemistry

Chapman model

Catalytic cycles

Ozone hole


O3 hole schematics

N. Kampfer


atmosphere



Chemical lifetime

Photochemistry

Ozone chemistry

Chapman model

Catalytic cycles

Ozone hole


O3 on Mars

N. Kampfer


atmosphere



Chemical lifetime

Photochemistry

Ozone chemistry

Chapman model

Catalytic cycles

Ozone hole


O3 on Mars

Zonally averaged ozone column density in µm-atm

N. Kampfer


atmosphere



Chemical lifetime

Photochemistry

Ozone chemistry

Chapman model

Catalytic cycles

Ozone hole


O3 on Ganymed

concepts of chemistry in the atmosphere - universität · pdf filen. k ampfer concepts of...

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