General Structure and Properties of the Earth’s Atmosphere

*global circulation

*atmospheric radiation

*weather patterns

*atmospheric composition

Dr Tony Cox ERCA 2004 -Lecture 1

Temperature structureof the Atmosphere

Rising Limb doldrums

Horse latitudesDescending Limb

Hadley Cell

Jetstreams at (~12km)

Max.Outgoing earth radiation

Max. Solar radiation

Absorbtion and re-emission up- and downwards

Warming at the surface

Black body emission at ~280 K

Coriolis Force - This is a force which is caused by the rotation of the earth and acts perpendicular to the direction of motion. It results from the change in radius of rotation with latitude and the need to conserve angular momentum, by developing zonal motion, I.e. in the direction of the earths rotation. The hypothetical force producing this motion perpendicular to the initial direction of transport is called the Coriolis force. The horizontal component of the Coriolis force is directed perpendicular to the horizontal velocity vector: to the right in the N.Hemisphere and to the left in the S.Hemisphere.

The Coriolis force has the magnitude: Fc = 2ΩVhsin

(Ω = angular velocity; Vh = horizontal velocity; = latitude)

The force is thus a minimum at the equator and maximum at the poles.

The Coriolis and the horizontal pressure force tend to balance each other, see examples above for cyclonic and anticyclonic pressure systems

Cirrus

Cumulus

Stratus

10 km

0 km

Orographic Clouds

Cross-section of a Tropical Cyclone

Issues in Atmospheric Chemistry

Tropics High Latitudes

Reservoir Atmospheric Non-atmospheric

Carbon cycle CO2 CaCO3 (carbonate)

CO, CH4, VOC biomass

Oxygen cycle O2 sulphate

CO2 CaCO3

Nitrogen cycle NOx nitrate

N2O, N2,NH3 fixed organic N

Sulphur cycle H2S, OCS sulphate, sulphides

SO2, H2SO4 sulphur in biomass

diameterFall speed

110 ms-1

<10-3 ms-1

Compound Sources Emission rate

g/yr x 1012.

CH4 enteric fermentation, wetlands, natural gas

leakage, combustion

400-500

CO atm. oxidation of VOC, combustion 800

Isoprene Natural vegetation 500

VOC* solvents, combustion, fermentation, natural

vegetation

>>100

NO/NO2 soil micro-organisms, lightning, combustion 40

N2O soil and marine micro-organisms, industrial

processes, combustion

4.4 -10.5

NH3 breakdown of animal waste, soil micro-organisms 82

SO2 Oxidation of DMS, volcanoes, fossil fuel

combustion, refining & smelting

110

CH3SCH3** Marine micro-organisms 40

H2S Terrestrial micro-organisms 10

CH3Cl Marine and terrestrial micro-organisms 1.5

CH3Br Marine micro-organisms, agricultural application 0.1

CFCs/HCFCs solvents and refrigerants 1.1***

* VO C = volatile organic compounds (hydrocarbons, halocarbons, oxygenated

organics);

** dimethyl sulphide; *** emission rates for 1990.

Sources of the Minor Constituents

Sources of the Minor Constituents

• The majority of the minor constituents of the troposphere originate from emissions from the Earth's surface.

• Natural emissions are primarily biogenic although volcanism accounts for significant amounts of atmospheric sulphur. Man made emissions result from energy production, industrial activity and agricultural practices.

• It frequently occurs that the chemical transformation of one minor constituent in the atmosphere creates one or more products which may themselves have significant roles in the overall chemical system. Knowledge of atmospheric degradation pathways is therefore important for understanding the behaviour of many minor constituents, gases and aerosols.

• Several important trace species enter the troposphere from the stratosphere. Most notable is O3 which plays a central role in tropospheric chemistry. Other species include HNO3 and HCl which result from stratospheric NOx and ClOx chemistry.

Sinks of the Minor Constituents

Sink process Minor constituent removed

Physical Processes

Dry Deposition to water surfaces SO2, NH3, HNO3, CO2, H2O2, HCl

Dry deposition to land surfaces SO2, O3, HNO3, CO2, H2O2, H2, HCl

Wet deposition in precipitation HCl, H2SO4, NH3, SO2, HNO3, H2O2

aerosol particles

Chemical Processes

Oxidation by OH radicals VOC, CO, SO2, NO2, H2O2, H2S, DMS

Oxidation by ozone NO, alkenes

Direct Photolysis O3, HCHO, CH3I, H2O2, NO2, NO3

Cloud & aerosol reactions H2SO4, NH3, SO2, N2O5, HNO3

These sink processes can be highly variable with time of day, season and

geographical distribution.

Deposition to the underlying surface

Removal at t he Earth's oceanic and t errestrial surfaces (water, soil and

vegetation) is termed 'dry deposition'. The rate of this process is controlled

by transport through the a tmospheric boundary layer and by reaction or

absorption at the surface.

The flux of a tr ace gas to the su rface, F (molecule cm-2s-1), and its

concentration c ( molecule cm-3), both measured at the same height above

the surface, are related by the deposition velocity, vg:

vg = F/c (cm s-1)

The lifetime of a trace gas with respect to deposition is related to the

deposition velocity and the height to which the trace gas is mix ed by the

equation:

τ = h/vg (s)

Trac e gase s whic h ar e remove d efficient ly a t thesurface, . e g SO2, HNO3, O3

(lan d surface s on )ly , value s o f vg ar e typica llyo f the orde r 1 of cm s-1.

Chemical Removal

Chemical removal of t race gases is mainly by oxidation, which occurs

mainly by gas phase reactions involving either attack by OH radicals or

other oxidising species such as ozone and NO3 r adicals, or by direct

photolysis. The OH radical, which is generated photochemically and

maintained in a s teady state in the sunlit atmosphere, is the principal gas

phase oxidising agent for many atmospheric trace gases. The rate of

removal is given by the equation:

-d [ X ]

dt

= kr

[ OH ] [ X ]

where [OH] is the local steady state concentrati on of OH (or other oxidising

specie )s . T he ra te ofphoto lysis isgive n b y :

-d [ X ]

dt

= Jx

[ X ] Jx = Σ Iλ σλ Φλ where J is the photolysis coefficient, obtained by integrating the product of thesolar flux, I, the absorption cross section, σ, and the quantum yield fordissociation, Φ, over all wavelengths where the molecule absorbs.

Lifetimes and Atmospheric Concentrations

• The atmospheric concentration of a particular gas emitted to the atmosphere is determined by its emission rate, and its atmospheric lifetime.

• For well mixed gases (those with lifetimes of ~ several months or greater), the time evolution of concentration can be represented by a simple box model.

• [A] is the concentration of the gas of interest, emitted into the atmosphere at rate R.

[A] [B]

source

physical

removal

chemical

conversion

R

Kc

Kd Kd'

Lifetimes and Atmospheric Concentrations

The concentration of [A] and [B] as a function

of time are given by:

d [ A ]

dt

= R − k

c

[ A ] − k

d

[ A ] = R − k

"

[ A ]

I n stea dy state:

[ A ]ss

=

R

k

"

The time dependence of [A] is obtained by solving the eqn:

dydt

-1k" = y where y = R - k".[A],

Assuming [A] = 0 at t = 0 gives

[ A ] =

R

k

' '

1 − e

− k

' '

t

Thu s [A] wi ll reach a steady stat e valu e o f R/k" , wit h an

e-foldin g t ime o f 1/ " k years. Ifemission s wer e the n to

cea , se [A] would fall exponentia , lly wit h a n e-foldin g ti meo 1f / "k

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0.00 2.00 4.00 6.00 8.00 10.00

Concentration

time (years)

lifetime = 4 years

lifetime = 1 year

Measurements of surface concentrations of atmospheric CH3CCl3

Emissions regulated under Montreal Protocol

kr= 6.8x10-15 s-1

OH + CH3CCl3 products

kII = kr[OH]mean

[OH] = 9.2x105 molecule cm-3

τ1/2 ~ 5 yr

i.e. kII = 0.20 yr-1