Page 262 - Materials Chemistry, Second Edition

246 4 Life Cycle Impact Assessment

Because there are only data available for the human health effect, only this one has
been quantified. 204)

4.5.2.4 Formation of Photo Oxidants (Summer Smog)
The photochemical smog (= photo smog), also called summer smog or Los Angeles
Smog with a history of about 60 years to correlate to the air in California particularly
in the region of Los Angeles. 205) High motorisation following a de facto removal of
rail traffic, high intensities of solar radiation and geographical conditions hindering
the exchange of air masses (inversion weather conditions) are the ideal basis for the
formation of photochemical smog initiated by the following reaction sequence 206) :
( c )
NO + ℎ → NO + O = ν < 405nm (4.17a)
2
O + O → O 3 (4.17b)
2
As long as NO is present in sufficient concentration, ozone and NO react back to
NO . Therefore secondary reactions with reactive hydrocarbons particularly with
2
alkenes or carbon monoxide (CO) are also necessary for photo smog formation.
These compounds bind NO through oxidation by the radical intermediate product
HO forming NO again. Overall the concentration of NO is reduced resulting in a
2 2
surplus of ozone which is part of human- and phyto-toxicity of the summer smog.
The reaction cycle of ozone formation is exemplified in Equation 4.18 for CO
forming OOH:
CO + OH + O (+M) → CO + HO (+M) (4.18a)
2 2 2

NO + HO → NO + OH (4.18b)
2 2

( c )
NO + ℎ → NO + O = ν < 405nm (4.18c)
2
O + O (+M) → O (+M) (4.18d)
2
3
net ∶ CO + 2O + ℎ → CO + O 3
2
2
(M: inactive impact partner).
Ozone, which is beneficial in the stratosphere for the absorption of short-wave
UV radiation (see Section 4.5.2.3) is a threat to the environment and to human
health if formed at ground level. In addition to the well-known toxic ozone, other
human and ecotoxic substances are produced. Together they form the group of
‘photooxidants’; hence the name of this impact category.

204) Struijs et al. (2010).
205) McCabe (1952).
206) Fabian (1992), Kl¨ opffer, Potting and Meilinger (2001b) and Barnes, Becker and Wiesen (2007).