basic properties of the atmosphere greenhouse effect...

Remote sensing of the atmosphere [email protected] http://www.mpch-mainz.mpg.de/satellite

Physics and chemistry of the atmosphere- brief overview on important effects -

• Basic properties of the atmosphere

• Greenhouse effect

• Stratospheric chemistry: ozone layer

• Tropospheric chemistry:

- winter smog

- summer smog

- oxidation capacity


Basic properties of the atmosphere: composition


Basic properties of the atmosphere: composition

Trace gases are important. They control:

-radiation absorption/emission -energy supply for organisms

-chemical reactions -energy transport (latent heat)


Residence time of various atmospheric trace gases

Residence time

Global mixing hemisph. mixing


Basic properties of the atmosphere: pressure profile

dzzdpzg )()( =⋅− ρ

z

z + dz

p(z)

p(z+dz)

Hydrostatic equation


Basic properties of the atmosphere: pressure profile

z

z + dz

p(z)

p(z+dz)

NpV A=With ideal gas law

it follows:

and:

M⋅

dzzdpzg )()( =⋅− ρ Hydrostatic

equation

RT

N A=ρRTMp

=V


For isothermal atmosphere (t=const):

zRTMg

epzp−

⋅= )0()(

0zMgRT

=Scale height z0: ≈ 8km

=> Scale height is differentfor different molecules

0)0()( zz

epzp−

⋅=


Basic properties of the atmosphere: temperature profile

-100 -50 0 50Temperatur [°C]

0

20

40

60

80

100

120

Höh

e [k

m]

1E-4

1E-3

1E-2

1E-1

1E+0

1E+1

1E+2

1E+3

Dru

ck [m

b]

Troposphäre

Stratosphäre

Mesosphäre

Thermosphäre

Tropopause

Stratopause

Mesopause

Ozonschicht


The temperature profile in the troposphere is controlled by convective circulation. The energy originates from the warm surface which is heated by solar irradiation.

For the calculation of the temperature change with altitude it is assumed that the air parcels move adiabatically. That means they don’t exchange energy with their environment. This assumption is justified by comparison with the rates of radiative cooling with the transport times within the tropopause.

When an air parcel ascends it expands and thus does work against the air pressure.

The first thermodynamical law is:

(U = internal energy, Q = obtained heat, W = work done to the gas, p =pressure, V = volume)

For an ideal gas the internal energy only depends on the temperature T it is:

It follows:


With ideal gas law:

or

It follows:

With Cp = specific heat at constant pressure, Cp + R = Cv

With It follows:

For adiabatic processes: dQ = 0 and we get:


With barometric height formula: It follows:

And we get the dry adiabatic temperature gradient:

Values for air:

Moist adiabatic temperature gradient:

If air ascends and cools, it will eventually reach the temperature at which water vapor condenses. The condensation process releases latent heat which provides energy to the airparcel. Thus, the temperature decrease with height is smaller than for the dry adiabatic temperature gradient.

The typical temparture gradient in the lower atmospere (up to about 10km) is determined by the moist adiabatic temperature gradient. Typical values are ≈0.6K/100m


Long time it was known (e.g. from mountain climbing) that the temperature decreases with height. It was also assumed that the temperature decrease would continue until the‚upper edge‘ of the atmosphere.

About more than 100 years ago, several important observations were made, which showed that the actual temperature gradient differs from this expectation:

• 1880 - 1908: Discovery of the ozone absorption (especially in the UV)

• 1902: Balloon borne temperature measurements (Teisserenc de Bort and Richard Assmann) showed a temperature increase at about 11km.

• 1913: Balloon borne observations (Wigand) of the ozone absorption show nosignificant decrease of the UV intensity up to 10km

• 1926: Umkehr-Effekt (Paul Götz): Maximum of the ozone layer at about 25km


Basic properties of the atmosphere: temperature profile

-100 -50 0 50Temperatur [°C]

0

20

40

60

80

100

120

Höh

e [k

m]

1E-4

1E-3

1E-2

1E-1

1E+0

1E+1

1E+2

1E+3

Dru

ck [m

b]

Troposphäre

Stratosphäre

Mesosphäre

Thermosphäre

Tropopause

Stratopause

Mesopause

Ozonschicht


Potential temperature:

- measure of the (sum of the potential and inertial) energy of an air parcel

- potential temperature is the temperature of an air parcel which was adiabatically compressed to normal pressure (1013 hPa).

From Poisson law: with

It follows (with p0 = 1013 hPa):

Potential temperature: For air it is:

Is an unequivocal function of the state variables p and T; thus itself is a state variable.


Height profiles of the real temperature and the potential temperature(from radiosonde measurements, Kiruna northern Sweden)


Greenhouse effect

© W. Roedel

The solar irradiation is S = 1368 W/m². The earth cuts a cross section of πr² off the olar beam. It reflects a fraction A (albedo) back into space. The absorbed power (1-A)Sπr² has to be compensated by the terrestrial IR radiation to avoid a heating ofthe earth.

From the Stefan-Boltzmann-law it follows that the terrestrial radiation is σT4 · 4πr²

It follows: Taverage = -18°C

but Treal = +15°C Greenhouse effect


a) A roof of normal glass

b) A roof of rock salt

c) No roof

+50° +50° +25°

Some considerations to the ‚greenhouse effect‘

a) A roof of normal glass is transparent for solar radiation, but opaque forIR radiation.

b) A roof of rock salt is transparent for solar and IR radiation

c) No roof

To which model does the atmosphere fit?


Atmospheric radiation budget


Contribution of differentgreenhouse gases to the natural greenhouse effect(IPCC, 1995)


(C) ICCP


Report of the Intergovernmental Panel of Climate Change (IPCC), http://www.ipcc.ch/


Global circulation patterns redistribute energy

Average near surface temperature as function of latitude

Why notat the equator?


Global circulation patterns redistribute energy


Complex atmospheric circulation patterns

Convection-heat gradient-gravitation

Solarirradiance

cold

cold

hot


Tropopauserapid mixing

CFC + h -> Clν+ O1D

wintersummer

Schematic diagram of the Brewer-Dobson Circulation (adapted from Solomon et al. [1998]). Because the stratosphere contains only about 10% of the total atmosphere, the circulation must turn over many times to destroy all of the CFCs present, resulting in a long atmospheric live time of these species.


Ozone in the stratosphere

Chapman Cycle [1930].

O2 + hν → 2O (λ ≤ 240nm)

O + O2 +M → O3 + M

O3 + hν → O(1D) (λ ≤ 320nm)

O(1D) + M → O + M

O3 + hν → O + O2 (λ ≤ 1180nm)

O + O +M → O2 + M

O + O3 → 2O2

Odd oxygen is produced

Odd oxygen is conserved

Odd oxygen is destroyed


0 5 10 15 20Ozone mixing ratio [ppm]

10

20

30

40

Hei

ght [

km]

Chapman-cycle

measurements

Comparison of measured ozone profiles and modelled ones taking into account only the reactions of the Chapman-cycle (adapted from Röth[1994]).


Additional (catalytic) Ozone destruction mechanisms:

X + O3 → XO + O2

XO + O → X + O2

Net: O + O3 → 2O2

with:

X = OH, NO, Cl, Br


Organic chlorine (mostly CH3Cl)

Industrial emissionsBiomass

burningVolcanoes

CFCsTroposphericaccum.: 0.56

Inorganic chlorine compounds (mostly HCl)

Chloride ion in sea salt aerosol

Stratospheric Chlorine compoundsaccumulation: 0.08

600

Sedimentation, Precipation 5400

Wave generation 6000

Precipitation 610

HCl 73

0.81

1.5Oceanic emissions 2.5

Precipitation,HO-reaction 5

0.030.24 0.19 Precipitation

units:1012 g Cl yr-1

Global atmospheric chlorine cycle (adapted from Graedel and Crutzen [1993]).


Altitude profiles of CFC-11 (bottom) and CFC-12 (top) [NASA, 1994].

76 78 80 82 84 86 88 90 92 94year

100

200

300

[ppt

]

global averagedCFC-11 mixing ratios

Global averaged increase of CFC-11 (adapted from IPCC [1996]).


Ozone hole, Antarctica

Ground based measurements,Halley, Antarctis Farman et al., 1986, Jones and Shanklin, 1995


Over Antarctica, the ozone holeoccurs regularly

Over the Arctic, in some yearsa ‚small‘ ozone hole occurs


....what has been ignored so far...

Polar Stratospheric Clouds over Kiruna (Sweden), ©Carl-Fredrik Enell.


Dynamical and photochemical development in the stratosphere during polar winter

Abun

danc

e

Time

Surface reactions

Gas phasereactions

ClONO2

HCl

ClO + 2 Cl2O2

Fall Early winter Late winter Spring

End of polar nightphotochemical ozone destruction

DenitrificationDehydration

CFC → Stratosphere → Reservoir compounds → active comp. → OClO(HCl, ClONO2) (Cl, ClO)

Ozone destruction

Transport hv (UV) PSC BrO


Catalytic ozone destruction cycle through chlorine

2(Cl + O3 → ClO + O2)

ClO + ClO + M → Cl2O2 + M

Cl2O2 + hν → Cl + ClOO

ClOO + M → Cl + O2 +M

Net: 2O3 → 3O2

Quadratic dependenceon ClO

Dependenceon sun light


Temperature differences between both hemispheres

Envelope of minimum temperature 1980-1988 at about 90mb from MSU measurements [WMO 1991].


Predicted future atmospheric burden of chlorine (adapted fromBrasseur [1995]).


Questions:

• When will ozone hole close?(influence of climate change)

• How important are bromine compounds?

• Will there be an ozone hole over the Arctic?

• How strong does ozone change in mid-latiudes?


Ozone change at mid latitudes

Left: Observation of chlorine partitioning as a function of altitude [Zander et al., 1996].

Right: Observed vertical profiles of the ozone trend at northern mid-latitudes [SPARC, 1998] together with a current model estimate from Solomon et al. [1997].


’London Smog’ (John Evelyn, Fumifugium, 17th century)

‘It is this horried smoake which obscures our church and makes our places look old, which fouls our cloth and corrupts the waters, so as the very rain, and refreshing dews which fall in the several seasons, precipate to impure vapour, which, with its black and tenacious quality, spots and contaminates whatever is exposed to it.

But without the use of calculations it is evident to every one who looks on the yearly bill of mortality, that near half of the children that are born and bred in London die under two years of age. Some have attributed this amazing destruction to luxury and the abuse ofspirituos liquors: these, no doubts, are powerful assistants; but the constant and unremitting poison is communicated by the foul air, which, as the town still grows larger, has made regular and steady avances in its fatal influence.’

‘Es ist dieser schreckliche Rauch, der unsere Kirchen verdunkelt und unsere Paläste alt aussehen läßt; der unsere Kleider verdreckt und unser Wasser verdirbt. Insbesondere den Regen und denerfrischenden Tau, die sich zu unreinem Dunst niederschlagen, der schwarz und zäh alles benetzt, dasihm ausgesetzt ist.Aber ohne zu rechnen ist es auch für jeden offensichtlich, der in die Jährliche Todesstatistik liest, daßfast die Hälfte der in London geborenen Kinder im Alter von nicht einmal zwei Jahren stirbt. Manche glauben daß diese erschreckende Entwicklung auf Luxus und Mißbrauch von alkoholischen Getränken zurückzuführen sei. Diese Faktoren mögen die Entwicklung sicherlich begünstigen, aberdas wahre Gift erreicht uns durch die verdorbene Luft, die, solange die Stadt stetig wächst, ihren verderblichen Einfluß beständig erhöht.


’London Smog’ (also wintersmog, sulfur smog)

• In the 13th century coal (with high sulfur content) began to replace wood for domestic heating and industrial use in London

• Earliest massive human impact on the atmosphere

• The effects of smog on human health were evident, particularly when smog persisted for several days. Many people suffered respiratory problems and increased deaths were recorded, notably those relating to bronchial causes.

• The first smog-related deaths were recorded in London in 1873, when it killed 500 people. In 1880, the toll was 2000. London had one of its worst experiences with smog in December 1892. It lasted for three days and resulted in about 1000 deaths.

• London became quite notorious for its smog. By the end of the 19th century, many people visited London to see the fog.

• Despite gradual improvements in air quality during the 20th century, another major smog occurred in London in December 1952. The Great London Smog lasted for five days and resulted in about 4000 more deaths than usual.


Hazardous driving conditions due to smog(see also http://www.met-office.gov.uk/education/historic/smog.html)

The London smog disaster of 1952. Death rate with concentrations of smoke


Environmental damage due to sulfur emissions• earliest massive human impact on the atmosphere

• Anthropogenic sulfur emissions were in particular responsible for the first non-local pollution

• At the end of the 1960s in Scandinavia a massive fish-dying occurred in inland waters. It was found out that the reason was an acidification of the soil. Finally it turned out that it was caused by strong industrial SO2 emissions from Great Britain.

• The Waldsterben was also mainly caused by SO2 emissions

• Since the mid of the 1980s the SO2 -emissions were strongly reduced due to effective filter techniques.


Sulfur compounds in the atmosphere• The atmospheric sulfur budget is (was) strongly affected through the release of about 70 Tg S y-1 in the form of SO2.

• Sulfur is also emitted by natural sources, e.g. volcanoes (SO2) and biological activity in the oceans (DMS).

• In the atmosphere the sulfur exists as gas and in the aerosol phase. Especially the aerosols play a key role for the radiation budget of the earth. The sulfur aerosols provide also surfaces and volumes for many chemical reactions and act as condensation nuclei.

• Human health is affected by SO2 and aerosols


SO2 reacts rapidly with OH to form HSO3 (1), which reacts with O2 to form SO3 (2). It is soluble in clouds and aerosols, where it reacts with H2O2. As a result of these processes, SO2 is converted to H2SO4 (3), consequently causing Acid rain and deforestation. In general the maximum concentration of SO2 is close to its source and the amount of SO2 decreases rapidly as the distance from the source increases, indicating a short tropospheric lifetime of typically a few days.

SO2 + OH HSO3 (1)

HSO3 + O2 → SO3 + HO2 (2)

SO3 + H2 O2 H2SO4 (3)

In the dry stratosphere, particularly in the lower stratosphere where the concentration of OH is relatively small, the lifetime of SO2 is longer than in the troposphere being of the order of several weeks.

⎯→⎯M

⎯→⎯M


Typical conditions for winter smog:

-primary pollution: SO2, soot particles

-secundary pollution : H2SO4, Aerosols

-Temperature: ≤2°C

-Relative humidity: high, typically foggy

-kind of inversion: ground inversion

-time of maximum pollution: early morning


Los Angeles Smog

(Sommersmog, Ozonsmog)

• Ozone affects health

• Ozone damages plants

• Ozone destroys material

• Ozone determines the oxidation capacity of the atmosphere

Impact of Ozone on Rubber


Ozone production in the troposphere, summer smog

Penetration depth of UV radiation

-In the troposphere, not enough UV radiation is available for ozone formation through O2photolysis

-where does troposphericO3 originate from?

DeMore et al., 1997


Early assumption

Todays knowledge

Troposphere

Troposphere

Stratosphere

Stratosphere


Ozonsmog (Los Angeles, 1940s, Haagen-Smit, 1952)

Radical chemistry, Ozone production in the troposphere:

RO2 + NO → RO + NO2

NO2 + hν → NO + O (λ ≤ 410nm)

O + O2 + M → O3 + M

The occurance of ozone smog depends on the concentrationsof volatile organic compounds (VOC) an nitrogen dioxid(NO2)

Without pollution, ozone is produced by oxidation of CH4 and CO => tropospheric background ozone


NOx limited

VOC limited

A VOC to NOx ratio of 8 to 1 is often cited as an approximate decision point for determining the relative benefits of NOx vs. VOC controls. At low VOC to NOx ratios (< about 4 to 1), an area is considered to be VOC-limited; VOC reductions will be most effective in reducing ozone, and NOx controls may lead to ozone increases. At high VOC to NOx ratios (>about 15 to 1), an area is considered NOx limited, and VOC controls may be ineffective. When VOC to NOx ratios are at intermediate levels (4 to 15), a combination of VOC and NOx reductions may be warranted.


Importance of ‚local‘ effects?

Average concentrations of NO2 in Germany Emissions of NMHC in Germany

Days with

[O3] > 240 ug/m³

in Germany


What about tropospheric background ozone concentrations?

Average O3 concentration(Germany, all stations, (c) Umweltbundesamt)

30

32

34

36

38

40

42

44

46

48

1975 1980 1985 1990 1995 2000

Year

[ug/

m³]Montsouris

CapeArcona

Increase of background ozone in Europe(Volz and Kley, 1988)


CO and CH4 column above the Jungfraujoch station

1985 - 1996Mahieu et al., 1997


Discovery of the ‘Cleansing agent’ of the atmosphere (OH-radical, Levi, 1971)

O3 + hν → O(1D) + O2 (λ ≤ 310nm)

O(1D) + H2O → 2OH•

OH• ist a (free) radical. Radicals are molecule fragments with unpaired electrons. Thus their bound conditions are not required and they are very reactive. The production of radicals depends on the fission of molecules and requires high energy. Typically, this energy is supplied by photons. Radicals are the ‘active players’ of photochemistry.

OH reacts with almost all atmospheric trace gases which is the prerequisite for their destruction. This ability to destroy atmospheric trace gases is called oxidation capacity.

OH is very reactive (within seconds). Although its concentrations are very low, it is the most important atmospheric reactand.

OH exists only during day. Will concentrations of atmospheric pollutants increase during night?


Atmospheric mixing ratios of radicals are exceedingly low e.g. about 1 OH-radical for every 1013 air molecules

But: In chemical reactions with molecules the radical status is „hereditary“ e.g.:

OH• + CO CO2 + H•

Example (oxyhydrogen gas):

„Start“ Propagation (inherit radical

status)

Branching (from one radical

we get two)

Termination Net: H2 + O2 2 H2O Consequence: The radikal concentration and thus the speed of reaction can increase exponentially.

H2 + O2 HO2• + H•

H2 + HO2• OH• + H2O

H2 + OH• H• + H2O

H• + O2 OH• + O• O• + H2 OH• + H•

OH• + HO2• H2O + O2

Does that also happen in the atmosphere?

Why are Radicals important?


Average ROX diurnal profile (Chemical Amplifier) during BERLIOZ(July 14 to 15, 17 to 18, 24 to 26, Aug. 3, total of 8 days in 1998)

NO3 Data from Dieter Perner, in Platt et al. 2002

00:00 06:00 12:00 18:00 24:00

0

2

4

6

8

10

12

14

RO2 [p

pt]

NighttimeRO2 Peak:NO3 + VOC

Radical Gap:no J(O3) butstill J(NO3)

NoontimeRO2 Peak

OH54 %

O3

16 %

NO3

30 %

Contribution of NO3to the AtmosphericOxidation Capacityin Pabstthum(BERLIOZ 1998)

basic properties of the atmosphere greenhouse effect...

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