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Atmospheric Chemistry
John Lee GrenfellTechnische Universität Berlin
Atmospheres and Habitability
(Earthlike) Atmospheres:-support complex life (respiration)
-stabilise temperature-maintain liquid water
-we can measure their spectra hence life-signs
Modern Atmospheric Composition
CO2
Modern Atmospheric Composition
CO2
N2
O2
CO2CO2 N2
Modern Atmospheric Composition
CO2
N2
O2
CO2CO2 N2
Psurface 93bar 1bar 6mb 1.5barTsurface 735K 288K 220K 94K
Early Earth Atmospheric Compositions
CO2
Magma Hadean Archaean Proterozoic Snowball
Early Earth Atmospheric Compositions
CO2
Magma Hadean Archaean Proterozoic Snowball
Silicate CO2 CO2 N2 N2Steam H2O N2 O2 O2
Jurassic Earth Early Mars Early Venus
Jungleworld Desertworld Waterworld Superearth
Additional terrestrial-type atmospheres
Modern Atmospheric Composition
CO2
Today we will talk about these
Reading List
Yuk Yung (Caltech) and William DeMore“Photochemistry of Planetary Atmospheres”
Richard P. Wayne (Oxford)“Chemistry of Atmospheres”
T. Gredel and Paul Crutzen (Mainz)“Chemie der Atmosphäre”
OCEAN Biology Volcanism
Delivery
Photochemistry
Processes influencing Photochemistry
Escape
Surface
PhotonsProtection
Clouds
Some fundamentals…
ALKALI METALSOne outer electron
reactive
NOBLE GASES8 outer electrons:
unreactiveIncreasing atomic number
Rows called PERIODS
GROUPS: similar
chemical properties
C, Si etc. have 4 outer electronsSO CAN FORM STABLE CHAINS
The Periodic Table
Halogens
Chemical Structure and Reactivitys and p orbitals d orbitals
The Aufbau Method
EIGHT ELECTRON STABILITY RULENeon 1s2 2s2 2p6 Argon 1s2 2s2 2p6 3s2 3p6
works OK for the first 18 elements
USEFUL FOR UNDERSTANDING CHEMICAL PROPERTIES
Rules
Pauli exclusion principle(01,2 electrons per orbital,
with different different spins)
Hund’s Ruleshighest orbital fills SINGLY
with same spin
Electrons in Oxygen
Rules
Pauli exclusion principle(01,2 electrons per orbital,
with different different spins)
Hund’s Ruleshighest orbital fills SINGLY
with same spin
USEFUL FOR UNDERSTANDING
CHEMICAL PROPERTIES
Electrons in Oxygen
TWO Ways to form Chemical Bonds…
IONIC BONDCOVALENT BOND
Covalent bonds can be…..
Non-polar(hydrophobic) Polar
Molecular Orbitals
electrons and nucleii interact in twoways – in-phase and out of phase
Molecular Orbitals
electrons and nucleii interact in twoways – in-phase and out of phase
Useful to
predict
properties
of
molecules
such
as O 2,
H 2O
EQUILIBRIUM CHEMISTRY Consider the general reaction:
αA + βB σS + τT
At equilibrium:Rate forward = Rate backwards
Now, applying law of mass action:Reaction Rate = Rate Constant (k) * Concentrations
i.e. kforward[A]α[B]β = kbackwards[S]σ[T]τ
Equilibrium constant, K = (kforward/ kbackwards)i.e. K = [S]σ[T]τ / [A]α[B]β
It can be shown that: ΔG = -RT(lnK)Note: at equilibrium, all species are present in a
mixture determined by K
EQUILIBRIUM CHEMISTRY Consider the general reaction:
αA + βB σS + τT
At equilibrium:Rate forward = Rate backwards
Now, applying law of mass action:Reaction Rate = Rate Constant (k) * Concentrations
i.e. kforward[A]α[B]β = kbackwards[S]σ[T]τ
Equilibrium constant, K = (kforward/ kbackwards)i.e. K = [S]σ[T]τ / [A]α[B]β
It can be shown that: ΔG = -RT(lnK)Note: at equilibrium, all species are present in a
mixture determined by K
ONLY SAYS HOW MUCH
DOES NOT SAY HOW FAST
Energy (photon,cosmic ray, thermal)
HOW FAST depends on:Initial Concentration (C) ANDReaction Rate Constant (k)
e.g. d(O2)/dt = -k [O2]
e.g. O2
REACTION KINETICS – How fast?
Chemical Kinetics
Generally, three types of reactions:
A Products 1st orderA + B C + D 2nd orderA + B + M AB + M 3rd order
M = “third-body” = any species needed tocarry away excess vibrational energy
Rate constant, K=Aexp(-Eact/kT)K = rate constant
A = pre-exponential constantEact = activation energy
Rates (mostly) depend on Temperature: Arrhenius Equation
SvanteArrhenius(1859-1927)
Rate constant, K=Aexp(-Eact/kT)K = rate constant
A = pre-exponential constantEact = activation energy
Rates (mostly) depend on Temperature: Arrhenius Equation
SvanteArrhenius(1859-1927)
Why is there an activation energy?
C
H
HHH O-H
C
H
HHH O-H
METHANE
C.H
HH H-O-H
Why is there an activation energy?
C
H
HHH O-H
C
H
HHH O-H
METHANE
C.H
HH H-O-H
Why is there an activation energy?
energy needed to breakbonds and to overcome
electron-electron repulsion
C
H
HHH O-H
C
H
HHH O-H
METHANE
C.H
HH H-O-H
energy emitted vianewly-formed bond(s)
How strong are commonmolecules in Earth’s atmosphere?
Molecule Bond Strength (eV)
Nitric Acid (HNO3) 2.2 Weak moleculesNitrogen dioxide (NO2) 3.2 Broken in visible light
Hydrogen (H2) 4.5Methane (CH4) 4.6 Medium-strengthAmmonia (NH3) 4.7 Broken in UVOxygen (O2) 5.2Water (H2O) 5.2Carbon Dioxide (CO2) 5.5
Nitrogen (N2) 9.8 Strong moleculesCarbon monoxide (CO) 11.1 Broken in EUV
EQUILIBRIUM CHEMISTRY FAVOURED ATVERY HIGH T, P (e.g. deep under the Earth, on
Venus’ surface, deep in Jupiter and Saturn)
All substances present as a mixture governed by ΔG
EQUILIBRIUM THEORY PREDICTS ONLY FINALCOMPOSITION NOT HOW LONG IT TAKES TOBE REACHED - FOR THE RATES WE NEED
“REACTION KINETICS”
NON-EQUILIBRIUM CHEMISTRYe.g. photochemistry in Earth’s atmosphere
Species react and are removed to form products
SUMMARY
Thermodynamic Equilibrium in Troposphere
Non-Thermodynamic Equilibrium (“Photochemical”)
Photochemical Processes
Absorption AB+hv AB*Ionisation AB* AB+ + e-
Quenching AB*+M AB+MDissociation AB* A+BReaction AB*+C ProductsLuminescence AB* AB + hvPhotolysis AB + hv A + B
Photolysis
Photolysis Rate = σ (λ,T) φ (λ) Φ dλ
σ (λ,T) Absorption Cross-Sectionφ (λ) Quantum yieldΦ Actinic Fluxdλ Wavelength interval
AB + hv A + B
(1) Absorption Cross Section, σ (λ,T)σ = Π (rm)2 CROSS SECTIONAL AREA
σ is the AREA presented by a particular molecule to a flux of photons
Photolysis Rate = σ (λ,T) φ (λ) Φ dλ
(2) Quantum Yield, φ (λ)φ(λ) = Number of molecules reacting per total photons absorbed. Values range from 0.0 to 1.0
Photolysis Rate = σ(λ,T)φ(l)Φ dλ
Photolysis Rate = σ(λ,T)φ(l)Φ dλ
Actinic Flux (Φ )Total photons available to at a point
in the atmosphere - integral of spectralradiance (J m-2 s-1) over 3D space
Heterogeneous Chemistry
Particle (e.g. dust, pollen, seasalt)Aerosol (e.g. sulphate, cloud droplet)
Gas-phaseSolid or liquid phase
Heterogeneous Chemistry
Particle (e.g. dust, pollen, seasalt)Aerosol (e.g. sulphate, cloud droplet)
Gas-phaseADSORBS Solid or liquid phase
Heterogeneous Chemistry
Particle (e.g. dust, pollen, seasalt)Aerosol (e.g. sulphate, cloud droplet)
Gas-phaseADSORBS Solid or liquid phase
Heterogeneous Chemistry
Particle (e.g. dust, pollen, seasalt)Aerosol (e.g. sulphate, cloud droplet)
Gas-phaseADSORBS Solid or liquid phase
Chemisorption-chemical bondsPhysisorption-Van der Waal Bonds
Heterogeneous Chemistry
Particle (e.g. dust, pollen, seasalt)Aerosol (e.g. sulphate, cloud droplet)
Some adsobedspecies can movealong surface
Heterogeneous Chemistry
Particle (e.g. dust, pollen, seasalt)Aerosol (e.g. sulphate, cloud droplet)
Adsorbed speciesreact with surfaceor with other gas-phasemolecules to form productswhich are desorbed
Heterogeneous Chemistry
Particle (e.g. dust, pollen, seasalt)Aerosol (e.g. sulphate, cloud droplet)
Adsorbed speciesreact with surfaceor with other gas-phasemolecules to form productswhich are desorbed
Rate of Adsorption = kads[X(g)]
Kads = f( γ ν (πrx2) Νx )
γ = sticking coefficient (0 1)ν = molecular velocity
(πrx2) particle area
Νx = number density in gas-phase
Heterogeneous Reaction Rates
Atmospheric Regions
Ozone layer
“Strato”=layeredHeating (ozone
absorption)
“Tropo”=turningCooling
(adiabatic expansion)
“Mesos”=middleCooling (adiabatic
expansion)
“Thermos”=heatHeating (oxygen
absorption)
Earth’s Atmospheric Composition
Why so much N2 and O2?Original (primary) atmosphere: H2, He, H2O and CO2
Then, H2 and He LOST via escape
CO2 dissolved in rain to form carbonates in rocks
So, N2 (from volcanoes) came to dominate
why nitrogen?
-volatile, unreactive, stable to photolysis
O2 was input by PHOTOSYNTHESIS
Oxygen (O2) and Ozone (O3)
O=O1.207 A
Oxygen Ozone
Good biomarkers (indicators of life)O3 produced mainly from O2, O2 from life
MOST O2 STORED IN ROCKS
e.g. Zahnle and Catling (2003) quantify the cycle
Photochemistry
6CO2+6H2O+energyC6H12O6+6O2
O2 in3.8x1019 mol O2(atm) (0.5%)(Lasaga and Ohmoto, 2002)
(~91% from oceans, Holland 2006)
Respiration ~ balancesPhotosynthesis
Burial removesorganics – leads to
increase in O2
Ozone comes from oxygen…so study the Oxygen Cycle
Photochemistry of Biomarkers on Earth
O3
N2O
Source Sink
O2 + O + (N2) O3
LIFE
Catalytic cycles
spectrum
Denitrifyingbacteria
LIFEPhotolysis
Biomarker
FROM OXYGEN
FROM BACTERIA
Smog Ozone
Chapman Ozone(from O2 + h
30km
10km
~9x10-6volume mixing ratio
chlorinecycles
nitrogen andhydrogen cycles
40kmν)
1D Ozone (O3) Photochemistry
20km
50km
FROMLIFE
e.g. from pollution
faster at high hν
Daily variation (photolysis): 0.1-1.0ppm
(HOx) cycles important in mesosphere C
hem
ical
con
trol
(Ox,
HO
x, N
Ox,
ClO
x)D
ynam
ical
cont
rol
Tropospheric chemistry Bad ozone: Hydrocarbons, NOx, UV (smog)
2D Ozone Photochemistry on EarthTropospher iccol um
n=10%S
t r ato spher iccol um
n=90%
Ozone formed in the tropics via O2 photolysis Ozone transported from tropics to poleSouth
PoleNorthPole
N2O comes from bacteria as by-product of the NITROGEN CYCLE
Nitrogen Cycle
N2 (g) N2, N2O, energy
denitrificationnitrification
fixation
N2O comes from bacteria as by-product of the NITROGEN CYCLE
Nitrogen Cycle
N2 (g) N2, N2O, energy
denitrificationnitrification
fixation
biological N2O sources on Earthabout one billion times stronger
than non-biological (photochemistry)
Why is there an ozone “layer”?
Ozone formed from oxygen and UVOzone formation is a trade-off of two opposing factors
-leads to peak at 30km in ozone
Low in atmosphere -->lots of O2 but little UVHigh in atmosphere-->lots of UV, but little O2
Problem: Chapman scheme overestimatesobserved ozone by a factor 2-3
Solution: Add catalytic cycles, which destroy ozone:
X= catalyst e.g. NO, OH, Cl
Small amount of X can have big effect on ozonebecause X participates over and over in these cycles
X+O3-->XO+O2XO+O-->X+O2 ----------------------
Overall: O3+O-->2O2
Water (H2O) in Earth’s Atmosphere
Not a biomarker but a pre-requisite for life(because, a good solvent, high heat capacity etc.)
O-H = 0.957 Å
Water Cycle (1020g year-1)
Source: Baumgartner and Reichel, 1975)
rt = residence time
Water Cycle (1020g year-1)
Source: Baumgartner and Reichel, 1975)
rt = residence time
Massive amount of water in oceansstored for ~3000 years
Water Cycle (1020g year-1)
Source: Baumgartner and Reichel, 1975)
rt = residence time
Massive amount of water in oceansstored for ~3000 years
Atmospheric waterremoved in ~10 days
OUTER EDGEWater Freezes
Atmospheric Chemistry of H2O
OH
hv or O*
Evaporation
CH4 OH H2O
Precipitation
Strong greenhouse gas – but complicated budget
MethaneOxidation
“Cold Trap” at tropopause – FREEZES OUT WATERSource: UARS, USA
High values
increase via
methane oxidation
Atmospheric water (parts per million)
Venus
Surface Orography from Magellan
Three Views of Venus
UV VIS near IR
“Unknown absorber”S2O?
Polysulphur?
Cloudbaseat ~60km
See down
to lowerclouds
Venus (dry) Earth (wet)
2cm water 2.7km watercolumn column
90 bar CO2 CO2+waterin atmosphere carbonate
4x10-7 chlorine 4x10-9. Mostin atmosphere Cl in seasalt
2x10-6 sulphur ~1x10-9. Mostin atmosphere S in sulphate
Major features of Venus’ atmosphere are directly relatedto Venus being dry (oceans lost from runaway greenhouse)
Earth and Venus Atmospheres Compared
Atmospheric Composition of Venus
Lower Atmosphere (below clouds) Chemistry
CLOUDS
Thermochemical equilibriumPhotolysis negligible
hei g
ht
50km
100km
Temperature
Sulphur cycle
Sulphur Cycle
H2O, CO2SO2, COS
Oxidationof Sulphur
Compounds
Reductionof Sulphur
Compounds
Clouds on Venus
Property Venus EarthCoverage(%) 100 40Optical depth 25-40 5-7Composition H2SO4-H2O H2ONumber density 50-300 100-1000Radius 2-4 microns 10 micronsMain forms stratiform stratiform,cumulus
Venus clouds – efficient SW scatterers, LW absorbers(but this is a strong function of composition)
Picture: Clouds on Venus, Venus Express 2006
Cycles complex(chlorine-, sulphur-, hydrogen-oxides)
-need better kinetics?-missing catalytic cycles?
CO2 + hv CO + O
Answer: catalytic cycles regenerate CO2
Venus Stratosphere: CO2 photochemistry
CO2 stability problem
CATALYTIC CYCLES operate. Trace species take part in a cycle 1000s of times but is always regenerated at the end of the cycle.
In this way trace species affect species present in much greaterconcentrations than themselves.
Example of Catalytic Cycle
CO + XO CO2 + XX + O XO
---------------------------CO + O CO2
X, XO are catalysts
Catalytic cycles operate in the atmospheres of Earth, Venus and Mars
Venus Express – Chemical Data
Very important because little observedbelow clouds until now
Svedhem et al. (2007)
vertical gradient(CO2 photolysis)
COS and SO2-major S species
Mars
Hubble Space Telescope July 2001
CO2 and Water ICE
Dust Storm
Sand und Gravel
Mars’ Atmosphere
Mars Atmospheric T-P Profile
Atmosphere is thin (6mb) andcold (~250K) at surface
Water frozen out of atmosphere
Dusty Climate
Mainly CO2 (95.3%)
Seasonal Pressure Cycle
Mars’ Atmospheric Chemistry - Historical Overview
CO2 (Kuiper, 1952) (95.3%)
Carbon Monoxide (Kaplan et al. 1969) (7x10-4)
Ozone (Barth and Hord, 1971) (1-80x10-8)
Molecular Oxygen (Barker, 1972) (1x10-3)
Molecular Nitrogen (Owen et al. 1977) (2.7%)
Hydrogen Peroxide (H2O2) (Encrenaz et al. 2004) ~10-8
Methane (Formisano et al., 2004) (10-8)
CO2 + hv CO + O
catalytic cycles
BUT…CO oxidation too FAST in Mars models
CO2 photochemistry on Mars
HOx CycleCO+OH-->CO2+HH+O2+M-->HO2+MO+HO2-->O2+OH-----------------------
Overall:CO+O-->CO2
SAME CYCLES AS Venus
ClOx cycles
NOT IMPORTANT
UNLIKE Venus the HOx source on Mars is WATER (not HCl).
Example of Mars Catalytic Cycles
SOx cycles
NOT IMPORTANT
Titan
Source: Cassini Source:Huygens
Why study Titan’s atmosphere?
Source:Hugens
Only body in solar system other than Earth withthick (1.5 bar) nitrogen atmosphere
Conditions are thought to resemble the early Earthso understanding Titan could shed light on earthlikeatmosphere development and conditions favouring life
Titan – the moon that never grew up
Surface T=94KSurface
P=1.5bar97% N23% CH4
Titan's atmosphere
A methane ocean on Titan?
Source:Huygens
TITAN’s DIVERSE HYDROCARBON CHEMISTRY
Wide-range of carbon compoundsNitrogen chemistry leads to CN (NITRILE) compounds
PARALLELS WITH CHEMISTRY OF EARLY EARTH
CnH2n ALKENE FAMILY
Double bonds
e.g. Ethene (C2H4)
Reactive double bond
CnH2n+2 = ALKANE FAMILY
Single bonds
CnHn= ALKYNE FAMILYTriple bonds
e.g. Ethyne (C2H2)Very reactive triple bond
Titan’s Smile
ESO Image – the “smile” is believed to be relate dto changes in clouds on a global scaleTitan’s “smile” possibly related to transient
methane clouds in troposphere (Hirtzig et al. 2006)
Image: ESO
Thank you!
John Lee GrenfellTechnische Universität Berlin
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