Geology Reference
In-Depth Information
natural sources with some gases also having geogenic sources. Table 1
lists a selection of the trace gases and their major sources.
4
For the
individual emission of a primary pollutant there are a number of factors
that need to be taken into account in order to estimate the emission
strength, these include the range and type of sources and the spatio- and
temporal-distribution of the sources. Often these factors are compiled
into the so-called emission inventories that combine the rate of emission
of various sources with the number and type of each source and the time
over which the emissions occur. Figure 2 shows the UK emission
inventory for a range of primary pollutants ascribed to different source
categories (see caption of Figure 2). It is clear from the data in Figure 2
that, for example, SO
2
has strong sources from public power generation
whereas ammonia has strong sources from agriculture. Figure 3 shows
the (2002) 1
1 km emission inventories for SO
2
and NO
2
for the UK.
In essence, the data presented in Figure 2 has been apportioned spatially
according to magnitude of each source category (e.g. road transport,
combustion in energy production and transformation, solvent use). For
example, in Figure 3a, the major road routes are clearly visible, showing
NO
2
has a major automotive source (cf. Figure 2). It is possible to scale
the budgets of many trace gases to a global scale.
It is worth noting that there are a number of sources that do not occur
within the boundary layer (the decoupled lowest layer of the tropo-
sphere, see Figure 1), such as lightning production of nitrogen oxides
and a range of pollutants emitted from the combustion-taking place in
aircraft engines. The non-surface sources often have a different chemical
impact owing to their direct injection into the free troposphere (the part
of the troposphere that overlays the boundary layer).
In summary, there are a range of trace species present in the atmos-
phere with a myriad of sources varying both spatially and temporally.
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It is the chemistry of the atmosphere that acts to transform the primary
pollutants into simpler chemical species.
2.3 INITIATION OF PHOTOCHEMISTRY BY LIGHT
Photodissociation of atmospheric molecules by solar radiation plays a
fundamental role in the chemistry of the atmosphere. The photodisso-
ciation of trace species such as ozone and formaldehyde contributes to
their removal from the atmosphere, but probably the most important
role played by these photoprocesses is the generation of highly reactive
atoms and radicals. Photodissociation of trace species and the subse-
quent reaction of the photoproducts with other molecules is the prime
initiator and driver for the bulk of atmospheric chemistry.
