Page 265 - Materials Chemistry, Second Edition

4.5 Impact Categories, Impact Indicators and Characterisation Factors 249

4.5.2.4.1 Indicators and Characterisation Factors Impact indicator of the category
‘summer smog’ is the formation of photo smog mostly measured by the formation
of the leading substance of ozone. 216) A list of characterisation factors is used for a
quantification following similar considerations as the original list of Darnall et al.
(1976, loc. cit), but on the basis of model calculations. The photochemical ozone
creation potential (POCP) of ethene serves as reference, which is arbitrarily set to one.
Table 4.11 lists a selection of POCP values. 217) The calculation of the less recent
values is done according to three scenarios valid for Europe and for a time horizon
of 9 days. 218) ThevaluesbyLabouze et al., 2004 (Table 1 ‘POCP mean ’) refer to an
average daily ozone concentration from 0 to 2.2 km height neglecting limit values
for the environment and human health. Calculations considering those limits have
also been accomplished but are not listed. The relative sequence of organic pollutant
classes remains unchanged for various types of calculation. However, the POCP
of NO (as NO ), according to Labouze et al. (2004, loc. cit.), strongly depends on
x 2
the fundamentals of the calculations (POCP: 0.27–0.95 ethene equivalents). Values
averaged in space and time are graded as complementary to the values by Derwent
et al. 219) as they are independent of meteorological conditions and emissions at a
given time; they are valid for Europe only. They are usually below the values of
Derwent because they were deduced for average atmospheric conditions, not for
conditions that promote smog formation.
The low value of methane in Table 4.12 is owing to the low reactivity with OH rad-
icals. Because inventories often list aggregated values, factors like ‘sum of hydrocar-
bons’, ‘volatile hydrocarbons’ (VOC) or ‘non-methane hydrocarbons’ are of special
practical importance. The POCP of the majority of reactive substances are within the
range of 0.1–1 kg ethene equivalents per kilogram. This implies that the exact com-
position of the mixtures of VOCs is of minor importance for the result. However,
methane should not be integrated into the VOC mixture because of its inertness.
The concept of maximum incremental reactivity (MIR) 220) is an alternative for a
characterisation by POCP factors and was developed in California attempting to
quantify ozone formation under ‘optimum’ conditions. It is not region-specific but
simulates smog conditions with strong solar radiation and high pollution load.
So-called incremental reactivity is defined as an increase of ozone concentration
per C atom of a VOC. These values, however, depend on specific circumstances of
a smog episode and cannot be directly used for ranking. Thus a maximum value
was defined (MIR) (mg O /mg VOC). MIR values can be transformed into relative
3
values by arbitrarily assigning the value one to a substance in complete analogy to
POCP values and other characterisation factors. 221)

216) Potting et al. (2002), Norris (2002), Kl¨ opffer et al. (2001a) and Kl¨ opffer, Potting and Meilinger
(2001b).
217) Kl¨ opffer, Potting and Meilinger (2001b), Derwent, Jenkin and Saunders (1996), Derwent et al.
(1998), Wright et al. (1997) and Labouze et al. (2004).
218) UNO (1991).
219) Derwent et al. (1998).
220) Carter (1994, 2003), Kl¨ opffer (2002) and Potting et al. (2002).
221) Kl¨ opffer et al. (2001)