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Atmospheric Ion Chemistry: From Top to

Bottom

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Historical Background• Benjamin Franklin’s experiment

with kite.• Marconi’s first transmission of radio

waves from England to Canada in 1901.

• Kennelly and Heaviside attributed the transmission to reflection due to electrons in the atmosphere, and the relevant layer of the ionosphere.

• In 1923, Appleton confirmed the existence of the ionosphere by studying the frequency modulation resulting from radio wave propagation at sunset. (Nobel Prize in 1947)

Guglielmo Marconi

Benjamin FranklinOliver Heaviside

Arthur E. Kennelly

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Background Basics of the Atmosphere• Atmosphere is divided

into different regions following different conventions:

1. Temperature.2. Nature of mixing of

constituent gases.3. Charged particle

density.

Heterosphere

Homosphere

• Homosphere : Uniformly mixed. Relative concentration of long lived species like CO2, O2, N2, etc is more or less constant at a familiar ratio.

• Heterosphere: Species separate on basis of weight and photo dissociation reactions. Mixing ratios become altitude dependent.

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Charged particle density based atmospheric profile

• Net charge of atmosphere at all altitudes is zero.

• Based upon the peak in the charged particle density, certain regions are named as D, E and F.

• Below D-region, all negative charges are in form of ions which interact weakly with radio waves.

D

E

F

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How are charged particles generated in atmosphere?• Degree of ionization to a large extent is

determined by solar radiation in atmosphere.• Solar flux decreases with altitude due to

absorption by atmospheric species.

For much of ionosphere, the neutral reactant partners for charged species are N2, O2, NO, N and O.H and He become significant at very high altitudes.

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Layer

• E- region (90-150 km)

• F- (150-500km) Region

• D-Region (60-90 km)

Species Ionization potential

• N2 (15.58eV or 80 nm)

• O (13.62eV or 91nm)

• O2 (12.07eV or 103 nm)

• NO (9.26eV or 134nm)

Absorbed radiation

• UV & X-rays. (λ<100nm or 12.4 eV)

• UV & X-rays. (λ<100nm or 12.4 eV)

• Layman-α (121.6nm or 10.2eV)

• Cosmic rays

Features

• Reflects Short waves (3-30MHz)

• Reflects HF radio waves (30- 100 MHz)

• ionized during the daylight hours, completely disappearing at night. Absorbs MF and LF radiowaves. (30kHz-3MHz)

• O, O2 and N2

• O, O2 and N2

• NO

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Exosphere

• Molecular diffusion dominates in higher altitudes and concentration of species decreases exponentially.

• At a critical altitude, mean free path b/w collisions is equal to scale height.

• This critical altitude defines the start of exosphere (500 km). The region is essentially Collison less.

• Neutral species: H & He.• Ionic species: H+ , He+, trace

amounts of O+ & N+.• Production of H+ : Photoionization

of H or H2. Concentration controlled via following equilibrium:

O+ + H H+ + O + heat• k= 6.0 x 10-10 cm3s-1; I.E. of H = 13.59

eV ; I.E. of O = 13.62 eV

No negative ions. Only free electrons contribute towards –ve charge density.

• He+ is less than H+ by an order of 1.• I.E. of He+ = 24.59 eV, i.e., very

large.• Formed only because of

photoionization of He+ by high energy solar radiations.

• Charge transfer reactions also don’t occur as energy difference is too high.

k

scale height: It is the vertical distance over which the density and pressure fall by a factor of 1/e

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Loss process of H

• Combination of monoatomic cations and free electrons in exosphere has a rate constant of 10-12cm3s-1. Thus, chemical loss process is negligible.

• Loss processes:1. Some ions are lost to space via

diffusion (Jeans escape).Since, the exosphere is essentially Collison less, so particles with sufficient velocity (escape velocity = 10.8 kms-1) will escape. Such particles are very less ,1%.

2. Non-thermal process : H+* + H H* + H

Loss process of He

• Primary loss mechanism of He : He+ undergoes charge exchange reactions with O and N2.; gets translationally excited and escapes earth.

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Thermosphere

Composition• All –ve charge is in form of

electrons.• O+, N+, NO+, N2

+ & O2+ are main

cationic species.• Neutral species: O, N2, O2 .• Covers both E and F regions of

ionosphere.• Electron density is max of 106 cm-3

at 300km (Middle of F-region). It is responsible for reflecting HF radio waves (3-30MHz) back to earth.

• NO+ has small I.E. and its loss process involve dissociative recombination.

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ION-MOLECULE REACTIONS IN THERMOSPHERE

July 22, 2012

O2+ REACTIONS:

• O2+ + NO O2 + NO+

• Exothermic, k = 4.6 x 10-10 cm3 s-1 .

• Empirically , the rate constant has been parametrized for temperature as follows:

+ve temp dependence

-ve temp dependence

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Reactions of O2

+

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Reactions of O+

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Reactions of N+

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Reactions of N2

+

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Dissociative recombination

• Reaction of +ve ions with electrons to yield neutral products. • Radiative recombination :

X+ + e- X + hν• Collisional recombination:

X+ + e- + M X + M*• Dissociative recombination:

XY+ + e- X + Y

Dissociative recombination of O+, N+ and NO+ produces excited state atomic N and O. These excited atoms then decay radiatively to give rise to phenomenon of air glow and aurora.

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AIR GLOW• Air glow is a constant luminosity

of the sky or a faint emission of light by a planetary atmosphere.

• Cause: Radiative decay of excited states.

Wavelength( Å)

Transitions responsible

Altitude

5200 (green) N(2D) N(4S) 80-100 km5577 (Green) O(1S) O(1D)  90-100

km5890 (yellow) Na(3P) Na(3S) 92 km6300 (Red) O(1D) O(3P) 150 - 300

km6364 (red) OH vibrational &

rotational transitions~ 86-87 km

7620 (infra red)

~95 kmAirglow in the Central of France, 2015

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• Aurorae are at similar heights and are also the light of excited atoms.

• aurorae excitation is by collisions with energetic particles. Oxygen: red-orange, nitrogen: blue-green.

• daytime short wavelength solar radiation produces the airglow via chemical excitation.

July 22, 2012

The airglow above the horizon, captured from the ISS

Airglow vs Aurorae

aurora borealis in Norway

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MesosphereComposition• Coincident with D-region.• O+, N+, NO+, N2

+ & O2+ are main

cationic species. NO is photo ionized by Lyman-α, rest are generated by cosmic rays.

• H3O+(H2O)n found and dominant <80km.

• Anionic species: O-, O2-, HCO3

-, CO3

-.• Cluster formation is enhanced

because of low temperature an increasing H2O concentrations.

Initiation by O2+

fast exchange reaction

switching of H2O for OH

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O2 + e- + M O2- + M

O2- + O O3 + e-

O- + O2

O- + O O2 + e-

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Stratosphere

• Stratospheric +ve ion chemistry consists essentially of Bronsted acid-base reaction.

• Any base with proton affinity greater than water (691kJ/mol) will form a +ve ion even if its concn is less.

July 22, 2012

Proton transfer reactions:

LPA and HPA refer to lower and higher proton affinities

Troposphere• Dominated by amines with

very high proton affinity.• -ve ion chemistry dominated

by acid-base chemistry.• NO3

- and HSO4-

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Junge Layer/ Stratospheric Aerosol layer• While searching for cosmic dust and

debris from nuclear bomb tests, Christian Junge discovered in 1960 a layer of microscopic aerosol particles between the tropopause and about 18 miles (30 km) altitude.• These particles are composed of

sulfuric acid and water and are formed by the chemical transformation of sulfur-containing gases . This layer is called the Junge Layer or the Stratospheric Aerosol Layer.• Sustained because of Carbonyl sulfide

and volcanic SO2 emissions.

July 22, 2012

Ion induced Nucleation• The main nucleation agents

in the atmosphere are water and sulfuric acid.

• Other species such as ammonia and organic species also figure into both the ion chemistry and the nucleation.

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HOW ARE THESE STUDIES DONE????Atmospheric tracer experiments

• Certain substances that scatter sunlight are added artificially into the atmosphere such that they can be tracked using various optical and spectroscopical tools.

• Examples:a) TMA (trimethylaluminium, Al2(CH3)6 : chemiluminescence.b) Barium: photo ionizes in sunlight and scatters at 4554 Åc) Samarium : Plasma enhancementsd) CF3Br, Ni(CO)4, SF6: Plasma depletion; SF6

- neutralizes with O+ to produce O* which radiates at 7774 Å

July 22, 2012

Albert A. Viggiano

Senior researcher at the Air Force ResearchLaboratory.

Donald E. Hunton

Chief Technologist, ASTRA, LLC

Nicholas S. Shuman

Research chemist, Space VehiclesDirectorate of the Air Force Research Laboratory, Albuquerque, NM.

Reference

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THANK YOU!!!