Page 263 - Materials Chemistry, Second Edition

4.5 Impact Categories, Impact Indicators and Characterisation Factors 247

The basic mechanism of Equations 4.17 and 4.18 has long been known 207) but not
as long as the ‘London Smog’. This is because of the smoke and fog which originated
at the time of the introduction of coal heating (sulphur content responsible for
subsequent formation of sulphuric acid) and the open fireplaces of those days, and
was feared for its health-damaging threats. 208) Although both phenomena are called
smog the impacts are caused by different pollutants, different reaction mechanisms
and refer to different impact categories (see also ‘Acidification’, Section 4.5.2.5).
For the formation of summer smog the following circumstances are thus
necessary:
1. intense solar radiation with a high UV contribution
2. reactive nitrogen oxides NO (= NO + NO )
x 2
3. reactive volatile organic compounds (VOC, especially alkenes) and/or CO.
Point 1
Intensities of necessary solar radiation have long been exaggerated: the radiation
intensity as well as the spectral composition (UV + short-wave visible radiation)
in central Europe is sufficient for the formation of summer smog, as has been
known since the 1970s. 209) The effect increases downwind many kilometres off the
formation of primary smog. In Europe, the metropolitan area of Athens, the capital
of Greece, comes closest to the meteorological situation and radiation climate of
Los Angeles.
Point 2
Reactive nitrogen oxides NO are mostly released by car traffic (also by diesel
x
engines, high contribution of trucks). Nitrogen dioxide provides oxygen atoms by
photolysis in the short wavelength spectrum (red gas, absorption within the blue
part of the solar spectrum), and in the near UV (see Equation 4.18). NO also
x
occurs in pure air areas in small but increasing concentrations in recent years.
Point 3
Unsaturated hydrocarbons mostly originate from traffic but also from industrial
plants. The releases of motor vehicles have been reduced by means of a suitable
catalytic converter (in California since the mid 1970s) but have not been completely
avoided. In California, many years after the introduction of the catalytic converter,
red smog has been observed again. There are also reactive natural hydrocarbons
(terpenes) which react with traces of NO to form a summer smog with particulate
x
follow-up products (aerosol, blue haze) in sunny woods. The proverbial ‘ozone’ of
the spicy air of woods and forests seems to have another (more real) meaning
besides a metaphorical one.
For an efficient abatement of summer smog the reduction of volatile organic
compounds (VOCs), of carbon monoxide and of NO releases is indispensable. 210)
x

207) Finlayson-Pitts and Pitts (1986), Fabian (1992, Chapter 4.1) and Barnes, Becker and Wiesen
(2007).
208) The word ‘Smog’ is an artificial word composed from smoke + fog. The definition by the Oxford
dictionary (smog = fog intensified by smoke) is relevant only for the winter variant, see also
Fabian (1992).
209) Becker et al. (1985) and Fabian (1992).
210) Finlayson-Pitts and Pitts (1986).