Atmospheric chemistry Atmospheric chemistry Set of Reactions that - - PowerPoint PPT Presentation

Atmospheric chemistry

Atmospheric chemistry

• Set of Reactions that happens in the

atmosphere.

• The atmosphere is an oxidizing medium
• Most of reactive species in the atmosphere

became from radiation reaching the earth.

Reactions

• Formation of breaking of bonds (or both)

– A+B  AB – AC + D – A+ BCAB + C – …

• Bonds are formed when 2 electrons are shared

between two atoms.

Electronegativity

Electronegativity

• All atoms want to be like a noble gas == have

a close shell (8 electrons)

• If they have few electrons the will want to give

those, if the have a may they will want to take them.

Forming bonds

Forming bonds

Forming bonds

Polar bonds

Breaking bonds

• Different bonds have different energies.
• Energy of O-O2 bond in Ozone is 150 KJ/mol
• Energy of O-NO bond in NO2 is 300 KJ/mol
• If we want to break a bond we nee to give at

least that energy to it.

Charge

• Excess or defect of electrons in a molecule

Radicals

• Molecules with an odd electron (or 2, bi-radicals)
• They are extremely reactive, the are represented

using · in front of the molecule

• ·OH, ·NO3…

What is radiation?

• Radiation is energy transmitted by electromagnetic

waves; all objects emit radiation

• One refers to electromagnetic waves equivalently

as photons, representing quantized packets of energy with zero mass traveling at the speed of light

• We use “electromagnetic waves” to stress the

wave nature of radiation and “photons” to emphasize its quantized nature

Energy of photons

• E = hν = h c / λ

– Where h is the Planck constant , c is light speed and lambda is the wave length

• Using that expression we can calculate the

energy of the photon depending on the wavelength

– 700 nm  170 KJ/mol – 530 nm  230 KJ/mol – 420 nm  280 KJ/mol – …

Atmospheric photochemistry

• When a photon collide with a molecule A, the

molecule reach an excited state

– A +hv  A*

• Molecule A may release the energy excess in

different way:

– Dissociation A* B+C – Reaction A*+BC+D – Fluorescence A*A + hv – Collision A* + M  A + M – Ionization A* A+ + e-

Quantum yield

• Defined for each process
• Ratio of the number of molecules A* that

follows that process

• So, the sum of the quantum yields for all

possible process must be 1

• Is a function of wavelength

Photolysis

• To calculate de rate of photolysis we need to know the photons

absorbed by molecule A.

• This amount is given by absorption cross section of A, σA(λ)
• So, if we multiply the absorption cross section by the number of

photons that reach the molecule and the concentration of A

– σA(λ) I (λ) d (λ) [A]

• But photolysis is not the only process, so we have to include

quantum yield of photolysis ( ϕA)

– ϕA (λ) σA(λ) I (λ) d (λ) [A]

• The Photolysis rate JA will be the integral over Lambda

Actinic Flux

What is I(λ)?

• The photon flux from ALL directions
• The actinic flux is determined by the

solar radiation entering the atmosphere and by any changes in this due to atmospheric gases and particles (e.g. Rayleigh scattering absorption by stratospheric ozone, scattering and absorption by aerosols and clouds), and reflections from the

• ground. It is therefore dependent on

the wavelength of the light, on the altitude and on specific local environmental conditions.

A

Actinic flux

Absorption cross section

Quantum yield

Oxidizing atmosphere

• The main oxidants in the atmosphere are

– OH radical (·OH) – Ozone – Nitrate Radical (·NO3)

The OH radical: main tropospheric oxidant

O3 + hvO2 + O(1D) (1)‏ O(1D) + M O + M (2)‏ O(1D) + H2O 2OH (3)‏ Primary source: Sink: oxidation of reduced species CO + OH CO2 + H CH4 + OH CH3 + H2O HCFC + OH H2O +‏… Major OH sinks GLOBAL MEAN [OH] = 1.2x106 molecules cm-3 and a lifetime of about 100 ms Earths surface, 30º N

D.J. Jacob

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

30 equinox midday Solar spectrum

D.J. Jacob

O(1D) RO2 HONO HCHO OH HO2 H2O2 + O2 HNO3 H2SO4 RO2 + O2

RO2

NO hν NO2

SO2

NO O3

CH4 HCHO H2 CO NMHC O3 HO2

hν

VOC

H2O

(Boy et al., ACP, 2005)

Formation of HO2 Radical

HCHO + hv ·H + HCO ·H+O2 +M·HO2+M HCHO + hv  H2 + CO CO + ·OH CO2 + ·H ·H+O2 +M·HO2+M

Formation of Sulfuric acid

• SO2 + ·OH + M ·HOSO2 + M
• ·HOSO2 + O2  ·HO2 + SO3
• H2O + SO3+ M  H2SO4 + M

Monoterpenes oxidation by OH radicals