Geology Reference
In-Depth Information
(iii) The chemistry of soils and rocks.Thereareverysignificant overlaps
with freshwater chemistry but the main differences arise from the
very large quantities of solid matter providing very large surfaces
and often restricting access of oxygen so that conditions readily
become anoxic. However, many of the basic issues such as carbon-
ate equilibria and redox properties overlap very strongly with the
field of freshwater chemistry. Soils can, however, vary very greatly
according to their location and the physical and chemical processes
which have affected them during and since their formation.
(iv) Environmental organic chemistry. Much of the traditional study
of the aquatic and soil environment has been concerned with its
inorganic constituents. Increasingly, however, it is recognised
that organic matter plays a very important role both in terms
of the contribution of natural organic substances to the proper-
ties of waters and soils, but also that specific organic compounds,
many of them deriving from human activity, show properties in
the environment which are not easily understood from traditional
approaches and therefore these have become a rather distinct
area of study.
(v) Atmospheric chemistry. The atmosphere contains both gas phase
and particulate material. The study of both is important and the
two interact very substantially. However, as outlined previously,
chemical processes in the atmosphere tend to be very strongly
influenced by kinetic factors, and to a large extent are concerned
with rather small molecules, which play only a minor part in the
chemistry of the aquatic environment or solid earth. Inevitably,
there are important processes at the interface between the atmos-
phere and the land surface or oceans, but these are dealt with more
substantially in the companion volume on Pollution Science.
1.4 UNITS OF CONCENTRATION
1.4.1 Atmospheric Chemistry
Concentrations of trace gases and particles in the atmosphere can be
expressed as mass per unit volume, typically mgm 3 . The difficulty with
this unit is that it is not independent of temperature and pressure. Thus, as
an airmass becomes warmer or colder, or changes in pressure, so its
volume will change, but the mass of the trace gas will not. Therefore, air
containing 1 mgm 3 of sulfur dioxide in air at 01C will contain less than 1
mgm 3 of sulfur dioxide in air if heated to 251C. For gases (but not
particles), this difficulty is overcome by expressing the concentration of
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