Page 241 - Materials Chemistry, Second Edition

10 Life Cycle Impact Assessment 227
1. Alkenes (decreasing with chain length) and aromatics (increasing with the
degree of alkyl substitution, decreasing with the length of the chain in the
substituted alkyl group)
2. Aldehydes (the strongest is formaldehyde; benzaldehyde has no or even a
negative ozone formation potential)
3. Ketones
4. Alkanes (almost constant from a chain length of three carbon atoms and
upwards), alcohols and esters (the more oxygen in the molecule, the weaker)
5. Halocarbons (decreasing with the degree of halogen substitution and the weight
of the halogen element)
Animals and humans are mainly exposed to the photochemical oxidants through
inhalation of the surrounding air, and the effects therefore appear in their respiratory
organs. Ozone is detectable by its odour at a concentration of ca. 20 ppb in pure air,
but only at somewhat higher concentrations we start to see acute symptoms like
increased resistance of the respiratory passages and irritation of the eyes, followed
at even higher concentrations by more serious effects like oedema of the lungs,
which can lead to long-term incapacity. Smog episodes with extreme concentrations
of photochemical oxidants in urban areas are known to cause increased mortality.
Chronic respiratory illness may result from long-term exposure to the photo-
chemical oxidants.
Plants rely on continuous exchange of air between their photosynthetic organs
(leaves or needles) and the atmosphere to absorb the carbon dioxide which is
needed for photosynthesis. Ozone and other photooxidants enter together with the
air and through their oxidative properties damage the photosynthetic organelles,
leading to discolouration of the leaves followed by withering of the plant. The
sensitivity of the plant varies with the season and also between plant species, but
considerable growth reductions are observed in areas with high ozone concentra-
tions during the growth season. Agriculture yield losses of 10–15% have been
estimated for common crop plants.
Figure 10.16 summarises the impact pathway for photochemical ozone forma-
tion linking emissions of VOCs, CO and NO x to the resulting damage to the areas
of protection.

10.10.3 Emissions and Main Sources

In some cases the emissions of individual substances are known, but in the case of oil
products the emissions will often be composed of many different substances and will
be specified under collective designations like VOCs or nmVOCs (non-methane
VOCs, i.e. VOCs apart from methane which is typically reported separately due to
its nature as a strong greenhouse gas) and sometimes also HCs (hydrocarbons), or
nmHCs (non-methane hydrocarbons, i.e. hydrocarbons excluding methane).