John Lee Grenfell Technische Universität Berlin · John Lee Grenfell Technische Universität Berlin. Atmospheres and Habitability (Earthlike) Atmospheres:-support complex life (respiration)

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


CO2

N2

O2

CO2CO2 N2


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


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


C

H

HHH O-H

C

H

HHH O-H

METHANE

C.H

HH H-O-H


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



Gas-phaseADSORBS Solid or liquid phase







Chemisorption-chemical bondsPhysisorption-Van der Waal Bonds



Some adsobedspecies can movealong surface



Adsorbed speciesreact with surfaceor with other gas-phasemolecules to form productswhich are desorbed



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



rt = residence time

Massive amount of water in oceansstored for ~3000 years



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

Documents

John Lee Grenfell Technische Universität Berlin · John Lee Grenfell Technische Universität Berlin. Atmospheres and Habitability (Earthlike) Atmospheres:-support complex life (respiration)