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Environmental Impact of Light

Fundamental processes in soil, atmospheric and aquatic systems

Chemical and biochemical changes2.iii.c (2 hrs)

2Environmental processes / Chemical and biochemical changes / Impact of light

‘Dark’ Reactions

• Most organic compounds react very slowly, even with oxygen, at normal temperature.

• As a rule, they can be considered generally nonreactive.

• BUT !...

Environmental processes / Chemical and biochemical changes / Impact of light

3

Let Be Light !

• Visible and ultraviolet radiation can promote the reactivity of almost all compounds.

• Recall a well known fact that items exposed (northern hemisphere) on southern side of buildings decay more quickly than those on northern side.


On the side facing south, the wooden (and metal) items decay faster


Museum item

Side exposed to light Side not exposed to light


Basics of Photochemistry

• Primary photophysical process.

• Subsequent chemical change(s)


Photophysical process

• Actually, it means absorption of light quanta.• It promotes molecule from ground- to excited

state (energy rich one).

• But, to catch the light quanta, molecule has to have a CHROMOPHORE !


Chromophores

• Chromophores are structural details in molecule(s) that enable high(er) probability of light absorption.

• Normally, it comprises double bonds in molecule, i.e., π-electrons:


Chromophore

• Double bond

• Aromatic ring

• Far more efficient are polarized double bonds.

C=O:

C=N-

C=C


Photophysical process• Once molecule acquires light quantum, it has

many ways to decay, involving the chemical change.

• Simplified approach tells that molecule can exist in ground state singlet or triplet– The same holds for excited state


Ground-statesinglet

Excited-statesinglet

Excited-statetriplet

S1 T10S


hν(d)(b)

(c)E

S1

S0

T1

hν hν(a)

ES1S0 T1

hν

hν

hν

(a)

(b)(c)

(d)

r


• Singlets and triplets are known as multiplicities of state.

• Electronic transition among states of different multiplicities is not likely to happen (is ‘forbidden’),

• But, as many other things, something forbidden still could happen.


• These ‘violations’ could be promoted by, so called, SENSITIZERS.

OHO OH

COO

+

S

N

NMe 2Me 2 N+

CN

CN

Fluoresceineee

Methylene blue Dicyanoanthracene


• Sensitizers quickly, and in good yields populate excited triplet states.

• Triplets live long enough to encounter other molecules.

hν(d)(b)

(c)E

S1

S0

T1

hν hν(a)


Sensitization

h ν

in te rs ys te m

C r o ss in g

S e n s (S 0) S e n s (S 1)

S e n s (S 1) S e n s (T 1)

S e n s (T 1) + V O C (S 0) S e n s (S 0) + V O C (T 1)))))


Photophysics → Photochemistry

C=O:

CH3-CO-CH3 hν CH3CO + CH3 ))CH3COCOCH3 + CH3CH3

Norrish type I

+

R

O H

R

OH

hνR

R

R

OHR

Enol

Norrish type II


• Primary chemical intermediates in photochemical metathesis are organic free radicals.

• Free radicals are highly reactive.• Radicals react with almost

everything!


Additional sensitization(singlet oxygen)

h ν

in te rs y s te m

C r o ss in g

S e n s (S 0) S e n s (S 1)

S e n s (S 1) S e n s (T 1)

S e n s (T 1) + O 2 (T 0) S e n s (S 0) + O 2 (S 1)))))


Coatings & Erosion

• VOC-s exposed to light can be promoted to more reactive compounds.

• Oxygen-containing derivatives can be aggressive to living organisms and to artifacts.

• Reactive intermediates may lead to polymers forming sticky coating.


Everything matters

• Ozone is not emitted directly from industrial sources and vehicles.

• It is formed in troposphere as a result of reactions involving oxides of nitrogen and volatile organic compounds.


What pollution we can control?

–Industry,–Traffic,–Agriculture– and (….. leisure)


Internet Resources

• http://www.iaq.dk/index.html• http://www.airclim.org/airAndEnvironment/A

E_chp3.htm• http://www.epa.gov/region4/airqualitytoolkit/1

0_Glossaries/US NPS - Air Quality Glossary.pdf

• http://www.heritage.xtd.pl/pdf/full_czop.pdf• http://www.epa.gov/iaq/voc.html

http://www.iaq.dk/index.html

http://www.airclim.org/airAndEnvironment/AE_chp3.htm

http://www.airclim.org/airAndEnvironment/AE_chp3.htm

http://rds.yahoo.com/_ylt=A0geu6OwnCFK.5sA_nFXNyoA;_ylu=X3oDMTEzOGgxdW85BHNlYwNzcgRwb3MDNgRjb2xvA2FjMgR2dGlkA0Y4NjFfMTIx/SIG=144ore530/EXP=1243803184/**http:/www.epa.gov/region4/airqualitytoolkit/10_Glossaries/US%20NPS%20-%20Air

http://rds.yahoo.com/_ylt=A0geu6OwnCFK.5sA_nFXNyoA;_ylu=X3oDMTEzOGgxdW85BHNlYwNzcgRwb3MDNgRjb2xvA2FjMgR2dGlkA0Y4NjFfMTIx/SIG=144ore530/EXP=1243803184/**http:/www.epa.gov/region4/airqualitytoolkit/10_Glossaries/US%20NPS%20-%20Air


• http://www.environment.gov.au/soe/2006/publications/commentaries/atmosphere/glossary.html

• http://www.airimpacts.org/documents/local/aqbook.pdf

http://www.environment.gov.au/soe/2006/publications/commentaries/atmosphere/glossary.html



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