Concepts of chemistry in the atmosphere - Universität · PDF fileN. K ampfer Concepts of...
Transcript of 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
Concepts ofchemistry in the
atmosphere
Thermodynamicalaspects
Basics of reactionkinetics
Chemical lifetime
Photochemistry
Ozone chemistry
Chapman model
Catalytic cycles
Ozone hole
Ozone on otherplanets
Outline
Thermodynamical aspects
Basics of reaction kinetics
Chemical lifetime
Photochemistry
Ozone chemistryChapman modelCatalytic cyclesOzone holeOzone on other planets
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
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
Concepts ofchemistry in the
atmosphere
Thermodynamicalaspects
Basics of reactionkinetics
Chemical lifetime
Photochemistry
Ozone chemistry
Chapman model
Catalytic cycles
Ozone hole
Ozone on otherplanets
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
Concepts ofchemistry in the
atmosphere
Thermodynamicalaspects
Basics of reactionkinetics
Chemical lifetime
Photochemistry
Ozone chemistry
Chapman model
Catalytic cycles
Ozone hole
Ozone on otherplanets
Enthalpy of formation
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
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
Concepts ofchemistry in the
atmosphere
Thermodynamicalaspects
Basics of reactionkinetics
Chemical lifetime
Photochemistry
Ozone chemistry
Chapman model
Catalytic cycles
Ozone hole
Ozone on otherplanets
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
Concepts ofchemistry in the
atmosphere
Thermodynamicalaspects
Basics of reactionkinetics
Chemical lifetime
Photochemistry
Ozone chemistry
Chapman model
Catalytic cycles
Ozone hole
Ozone on otherplanets
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
Concepts ofchemistry in the
atmosphere
Thermodynamicalaspects
Basics of reactionkinetics
Chemical lifetime
Photochemistry
Ozone chemistry
Chapman model
Catalytic cycles
Ozone hole
Ozone on otherplanets
Bimolecular reaction rates
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
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
Concepts ofchemistry in the
atmosphere
Thermodynamicalaspects
Basics of reactionkinetics
Chemical lifetime
Photochemistry
Ozone chemistry
Chapman model
Catalytic cycles
Ozone hole
Ozone on otherplanets
Termolecular reaction rates
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
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
Concepts ofchemistry in the
atmosphere
Thermodynamicalaspects
Basics of reactionkinetics
Chemical lifetime
Photochemistry
Ozone chemistry
Chapman model
Catalytic cycles
Ozone hole
Ozone on otherplanets
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
Concepts ofchemistry in the
atmosphere
Thermodynamicalaspects
Basics of reactionkinetics
Chemical lifetime
Photochemistry
Ozone chemistry
Chapman model
Catalytic cycles
Ozone hole
Ozone on otherplanets
Chemical lifetime
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
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
Concepts ofchemistry in the
atmosphere
Thermodynamicalaspects
Basics of reactionkinetics
Chemical lifetime
Photochemistry
Ozone chemistry
Chapman model
Catalytic cycles
Ozone hole
Ozone on otherplanets
Examples from ozone photochemsitry
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
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
Concepts ofchemistry in the
atmosphere
Thermodynamicalaspects
Basics of reactionkinetics
Chemical lifetime
Photochemistry
Ozone chemistry
Chapman model
Catalytic cycles
Ozone hole
Ozone on otherplanets
Vertical distribution of ozoneMeasurements with balloon sondes are performed twice aweek in Payerne
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
O3 distribution over Bern measured bymicrowave radiometry
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
O3 global average column density
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
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
Concepts ofchemistry in the
atmosphere
Thermodynamicalaspects
Basics of reactionkinetics
Chemical lifetime
Photochemistry
Ozone chemistry
Chapman model
Catalytic cycles
Ozone hole
Ozone on otherplanets
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
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
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
Concepts ofchemistry in the
atmosphere
Thermodynamicalaspects
Basics of reactionkinetics
Chemical lifetime
Photochemistry
Ozone chemistry
Chapman model
Catalytic cycles
Ozone hole
Ozone on otherplanets
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
Concepts ofchemistry in the
atmosphere
Thermodynamicalaspects
Basics of reactionkinetics
Chemical lifetime
Photochemistry
Ozone chemistry
Chapman model
Catalytic cycles
Ozone hole
Ozone on otherplanets
Ozone-hole as seen by microwave limb sounder
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
O3 hole details
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
O3 hole schematics
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
O3 on Mars
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
O3 on Mars
Zonally averaged ozone column density in µm-atm
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
O3 on Ganymed