Page 239 - Materials Chemistry, Second Edition

10 Life Cycle Impact Assessment 225

differences, but in essence they all address the impacts from ozone and other
reactive oxygen compounds formed as secondary contaminants in the troposphere
by the oxidation of the primary contaminants volatile organic compounds (VOC),
or carbon monoxide in the presence of nitrogen oxides (NO x ) under the influence of
light. VOCs are here defined as organic compounds with a boiling point below
250 °C (WHO 1989). NO x is a joint name for the nitrogen monoxide NO and
nitrogen dioxide NO 2 .

10.10.1 Problem

The negative impacts from the photochemically generated pollutants are due to their
reactive nature which enables them to oxidise organic molecules in exposed sur-
faces. Impacts on humans arise when the ozone and other reactive oxygen com-
pounds, which are formed in the process, are inhaled and come into contact with the
surface of the respiratory tract, where they damage tissue and cause respiratory
diseases. Impacts on vegetation arise when the reactive compounds attack the
surfaces of plants or enter plant leaves and cause oxidative damage on their pho-
tosynthetic organs. Impacts on man-made materials are caused by oxidation and
damage many types of organic materials which are exposed to ambient air. It is thus
not the VOCs per se which cause the environmental problems associated with
photochemical ozone formation, but the products of their transformation in the
troposphere which is the lower stratum of the atmosphere, from the surface of the
earth to the tropopause 8–17 km above us. Direct toxic effects on humans from
VOCs are treated separately in the impact category human toxicity (see
Sect. 10.12). Apart from a general increase in the tropospheric ozone concentration,
photochemical ozone formation may cause smog episodes on a more local scale in
and around cities with a combination of large emissions and the right meteoro-
logical conditions. During smog episodes, the concentrations of ozone and other
photooxidants reach extreme levels causing immediate damage to human health.

10.10.2 Environmental Mechanism

The photochemical formation of ozone and other reactive oxygen compounds in the
troposphere from emissions of VOCs and NO x follows rather complex reaction
schemes that depend on the nature of the specific organic compound emitted.
A simplified presentation of the fundamental elements of the schemes is given in
Fig. 10.15 and can be summarised as:
•
1. VOCs (written as RH) or CO react with hydroxyl radical OH in the troposphere
and form peroxy radicals, ROO •
2. The peroxy radicals oxidise NO to NO 2