introduction into atmospheric chemistrylpc2e.cnrs-orleans.fr/~specatmos/documents... · source:...

Introduction into Atmospheric Chemistry

Christa Fittschen

June 12, 2017 Christa Fittschen 1

• Introduction to Atmospheric Composition and Pollution Phenomena

• Composition and structure of the Atmosphere

• Key trace gases in Atmospheric Chemistry

Brief History of Air pollution

Long long time ago … First pollution event during the Roman Empire

“Greek and Roman lead and silver mining and smelting activities polluted the middle troposphere “ (Hong et al, 1994)

From ice cores analyses

End of the 19th Century: Industrial Revolution

(Source: M. Pidwirny)

Intense use of fossil fuels ⇒ Emission of pollutants (CH4, CO2)

At the same period, ozone concentration increases …

• Ozone is a secondary pollutant, produced by photochemical processes involving primary pollutants

• Ozone concentration has been increased by a factor of 5 since the late 19th Century

- London smog

- L.A. smog

- Global Troposheric pollution

- Particles

- Acid deposition

- Stratospheric Ozone depletion

- Global Climate Change

Interest in atmospheric composition

Climate Change (…1990s…)

Regional Air Pollution (…1950s…)

Acid rain (1970s…)

Stratospheric Ozone depletion

(1985…)

Atmospheric CHEMISTRY

Jacobson et al., Atmospheric pollution

Youngstown, Ohio (c. 1910)

Reading, Pennsylvania (c. 1909)

Gary, Indiana (c. 1912)

Noontime, Donora, Pennsylvania, October 29, 1948

Pedestrians in Los Angeles (1950s)

M. Jacobson, Air pollution

Los Angeles (July 23, 2000)

Arie Haagan-Smit demonstrates smog formation. (undated photo, Caltech Archives)

Ozone pollution has become commonplace

Ozone concentration in µg/m3 (8th August 2003)

Number of hours the threshold value of 180µg/m3 was exceeded (1-14 August 2003)

Ozone trend from European mountain observations, 1870-1997

Air Pollution and Visibility

Pictures taken from a same location, at same time of day, on two different days (source: Qi Zhang)

Reduction of visibility by aerosols

Visibility reduction due to particles Bryce Canyon Great Smokies NP

Acid rain

Getting better in the U.S.

… but worsening in Asia

Stratospheric ozone depletion observed in the 80’s

Data from NASA (TOMS instrument) June 12, 2017 Christa Fittschen 28

Rowland, Molina and Crutzen (1974)

• Discovered that CFCs can last 10-100s of years in atmosphere

• CFCs are susceptible to break down by UV

• Predicted that CFCs will reduce ozone inventories

• Proof that this was occurring came in 1985

• Montreal Protocol 1987

• Absorption of UV light is sufficiently energetic to cause a photochemical reaction in which the DNA breaks down

Effect of UV Light on DNA

Absorption Spectrum of DNA • Each of the base pairs have slightly different

absorption spectra

Absorption Spectrum of DNA • The absorption spectrum of DNA is an average of the

spectra of its base pairs with a peak near 260 nm

• It is the UV-B region where DNA will absorb UV light

Absorption Spectrum of DNA • In the UV-C region virtually no UV light

reaches the earth’s surface

• The UV-B region will expand as there will be insufficient ozone to remove all the UV light

Effect of Ozone Depletion

• 1-2% increase in skin cancer for each 1% decrease in ozone

• Currently, one in five Americans will develop skin cancer in their lifetime

Large increases in greenhouse gases and aerosols since pre-industrial times

Climate change

Date Speaker / Intervenant 37

Scales of air pollution

Local pollution (urban and industrial) - Primary pollutants: NOx, VOCs, PAHs, dioxines… - Time scale: 1h - 1 week

Regional and continental pollution - Secondary pollutants: NO2, tropospheric O3, CO, acid rains - Time scale: 1 day - 1 month

Global pollution - Long-life trace species : CO2, CH4, N2O, SF6, CFCs, …

- Time scale: few months - 100 years

-Temperature and Pressure structure of the Atmosphere

-Chemical composition of the Atmosphere

- Different sources of Traces Gases

- Transport and Distribution of Traces Gases

Atmosphere is very thin...

Atmospheric Structure

Temperature structure • In absence of local heating, T decreases with

height • Exceptions: Stratosphere: Chapman Cycle (1930s) O2 + hν → 2 O O + O2 (+ M) → O3 (+ heat) O + O3 → 2 O2 (+ heat) O3 + hν → O + O2 • Why is there an ozone “layer”? • Inversion layers are stable: no vertical motion • Tropopause is coldest region: no transport of

condensable molecules (H2O)

Pressure structure • Scale height H(z) = RT(z) / M x g • Dalton’s law: each component behaves as if it was

alone in the atmosphere • H(z) depends on mass M - O2 lower than N2?? • No, gravitational separation much slower than

turbulent diffusion - only at > 100km enriched in lighter gases • H = 8.5km at 290 K, 6 km at 210 K • 99% of atmospheric mass is below 50km

(troposphere + stratosphere)

Definitions Mixing ratio:

Volume mixing ratio:

number of moles(A)/number of moles(air) = molecules(A)/molecules(air).

Units: ppmv = 10-6 mole/mole (parts per million) ppbv = 10-9 mole/mole (parts per billion)

Attention with the word billion: can be 109 or 1012 !!! ppt = 10-12 mole/mole (parts per trillion)

Mass mixing ratio: mass(A)/mass(air)

Atmospheric Composition

Nitrogen

Oxygen

H2O Argon

N2O 310

30 ppb

CH4 (1.8)

18 He (5)

HCHO 300

Ethane

SO2 NOx

200 100

NH3 400

CH3OOH 700

H2O2 500

HNO3 300

others

TRANSPORT of gases

Source: Introduction to Atmospheric Chemistry, Jacob, D. J., 1999

Residence times and spatial variability

- Hydroxyl radical (OH)

- Reactive nitrogen species

- Hydrocarbons

- Ozone

Some important Trace gases

• Hydroxyl radical (OH)

• Reactive nitrogen species

• Hydrocarbons

• Ozone

troposphere

stratosphere

H2O,O3 OH

View prior to 1970s: Inert troposphere (only source of oxidants in the stratosphere)

With an inert troposphere: With increasing emissions from fossil fuel combustion will CO accumulate in the troposphere?

View of an inert troposphere challenged [Weinstock, 1969]: 14CO measurements in troposphere imply a 2-month lifetime for CO there MUST be a tropospheric sink!

Primary source of tropospheric OH [Levy, 1971] O3 + hν O2 + O(1D) 290 nm <λ<330 nm O(1D ) + M O(3P) + M M (“third body”) = O2 or N2 O(3P) + O2 + M O3 + M O(1D) + H2O OH + OH

2 OH Solar radiation (290<λ<330 nm)

(~99%)

~1% Only 10% of total O3 are found in the troposphere!

Sinks of OH Dominant sinks of OH in the global troposphere: 70%: CO + OH CO2 + H 30%: CH4 + OH CH3 + H2O …CO Over continents, reactions with non-methane hydrocarbons (NMHCs) dominates: NMHC + OH products Lifetime of OH ~ 1 second!

CO NMHCs

Has human activities changed the oxidizing capacity of the atmosphere?

Since pre-industrial times CO has increased by factors of 3-4; NO has increased by factors of 2-8 …. What has happened to OH?

NO ↑ increase in OH CO ↑ decrease in OH

troposphere

OH HO2 hν, H2O

CO, hydrocarbons

Wang & Jacob, 1999

Increase in OH near sources driven by increases of NOx (~days)

Decrease in OH away from sources driven by increases of CO (~months) Little change in OH

globally (buffering)

• Hydrocarbons

• Ozone

Reactive nitrogen species and the nitrogen cycle

• Molecular nitrogen: N2 , 78% (~99.99% of all nitrogen) • Nitrous oxide: N2O, 310 ppmv • Reactive nitrogen species: NOy (pptv-ppbv) NOy = NO+NO2(=NOx) + NO3+N2O5+HNO3+ PAN

(=CH3C(O)OONO2) • Ammonia: NH3 pptv-ppbv

The crucial role of NOx

The catalytic ozone formation cycle NO + O3 → NO2 + O2 NO2 + hν + O2 → NO + O3

NO + HO2 → NO2 + OH NO + CH3O2 → NO2 + CH3O these are the key reactions! NO + RO2 → NO2 + RO

NO2 NO + O3

Unperturbed cycle leads to

equilibrium concentration of

NO2 / NO / O3

Net formation of 1 molecules O3

Net formation of 2 molecules O3

NO2 NO + O3

NO2 NO

NO NO2

hν O3

Hydrocarbons - biogenic - anthropogenic

hν NO2 NO + O3

NO2 NO

NO NO2

hν O3

Hydrocarbons - biogenic No NOx is available

Change in chemistry between “polluted” and “clean” atmospheres

OH HO2

Hydrocarbons - biogenic

+ → mostly stable products

1015 cm-2

Tropospheric NO2 columns from the GOME satellite

August 2000

Concentration of NOx

• Hydrocarbons

• Ozone

Methane, CH4

Fluxes in TgC/year

Historical methane trend

Anthropogenic sources: ~70%

The budget of CH4

Nonmethane hydrocarbons Alkanes (C-C single bonds) Alkenes (C-C double bonds)

ethene ethane

Alkynes (C-C triple bonds)

ethyne Benzene

Aromatic compounds

Oxygenated hydrocarbons: Aldehydes, alcohols, ketones, etc…

Global emissions of non-methane hydrocarbons ANTHROPOGENIC SOURCES ~100 TgC/yr Energy use and transfer 43 TgC/yr Biomass burning 45 TgC/yr Organic solvents 15 TgC/yr NATURAL SOURCES ~1170 TgC/yr Emissions from vegetation isoprene (C5H8) 500 TgC/yr monoterpenes 125 TgC/yr other VOC 520 TgC/yr Oceanic emissions 6-36 TgC/yr

Brasseur et al., 1998 June 12, 2017 Christa Fittschen 68

June 12, 2017 69

Satellite observations of HCHO Hours/ days

Hours/ days

HCHO NMHC CO

Isoprene emissions

CxHy + (x + 0.25y) O2 x CO2 + H2O y 2

Fate of organic carbon : what says thermodynamics ?

[CH4] = 1.8 ppm

[CO2] = 370 ppm [CO2] ≈ 102 [CH4]

Atmospherique concentrations of CO2 et CH4 :

Kinetic limitations !

Atmosphere = oxidizing environment H → H2O ; N → HNO3 ; S → H2SO4

∆G° ≈ -800 kJ/mol

K= [CO2][H2O]2

[CH4][O2]2 ≈ 10140 At equilibrium : [CO2] ≈ 10144 [CH4]

Oxydation of organique carbon is complet

Example : atmospheric combustion of methane

CO2 + 2 H2O CH4 + 2 O2

C → CO2

Bond energy - collision energy

For CH4 : ECH3-H = 435 kJ/mol For O2 : EO=O = 492 kJ/mol

Collision energy Photon energy

Ecollision ≈ 3 kJ/mol E300 nm ≈ 400 kJ/mol

Bond energy :

Collisions between CH4+O2 are not « energetic » enough to break the bond

No reaction between CH4 and O2

How do they get over the barrier??

In the stratosphere : O2 + hν (λ < 240 nm) 2 O

In the troposphere : catalytiques (photochemical) cycles

Tropospheric OH production takes place in a narrow UV window (300-320 nm)

30o equinox midday Solar spectrum

λ300nm= 400 kJ mol-1

λ400nm= 300 kJ mol-1

O-NO=306 kJ mol-1

O-OO=105 kJ mol-1

Carbon monoxide, CO

CH4 CO2

Fluxes in TgC/year

CO sources: ~25% Fossil fuel, ~25% Biomass burning ~25% CH4 oxidation, ~25% NMHC oxidation

• Hydrocarbons

• Ozone

Transport from the Stratosphere: 475 Tg/yr

Deposition: 1165 Tg/yr

Chemically produces in the Troposphere: 4920 Tg/yr

Chem loss: 4230 Tg/yr

Ozone concentration is measured in Dobson units

All the Ozone over a certain area is compressed down to 0°C and 1 atm pressure. A slab of 1mm thick corresponds to 100 DU.

O3 Area covered by Column

Tropospheric ozone column seen from space

Historical records imply a large anthropogenic contribution to the present-day ozone background at northern midlatitudes

Ozone trend from European mountain observations, 1870-1990

[Marenco et al.,1994]

Increase is important from pollution and climate perspectives

• Hydrocarbons

• Ozone

Lab studies

Modeling Atmosphere’s observations

Provide data + validation of 0D models Help for the

definition of the experiments

Help for the preparation of field campains

Validation of chemical-transport models (3D)

Identification of new species

How to study atmospheric chemistry?

Lab studies

• Provide kinetic and mechanistic data in order to:

– Identify major reactions

– Determine lifetimes of trace species

– Identify oxidation products, which can themselves affect air quality or play an important role in atmospheric chemistry

– Provide data for chemical schemes in models

Thank you very much for your

attention!!

Any questions??

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