Geography Reference
In-Depth Information
atmosphere to generate sulphurous and sulphuric
acids and nitric and nitrous acids, as shown in
Figure 3.1. Although small amounts of these gases
are produced naturally through volcanic eruptions,
which are fluxes within the natural
biogeochemical cycles of nitrogen and sulphur
(Figure 3.2), new fluxes between the lithosphere
and atmosphere have been created by human
activity, notably the combustion of fossil fuels. As
Figure 3.2 shows, the anthropogenic influence on
these biogeochemical cycles has been considerable
(see discussion in Mannion 1997; 1998). For
example, fossil fuel combustion fluxes 70-80 Tg
yr
-1
of sulphur from the lithosphere to the
atmosphere, which is ten times as much as the
volcanic flux. Biomass burning also accelerates the
flux of both sulphur and nitrogen from the
biosphere to the atmosphere.
Once they have formed in the troposphere (the
lower atmosphere), the acids become incorporated
into clouds and can produce a pH as low as 2.6.
This can have a significant impact on high-altitude
ecosystems that experience a low cloud cover or
mist for relatively long periods. This is occult
deposition. In addition, the entrainment of acids
within clouds and air masses means that they can
be transported hundreds of kilometres beyond
their initial site of production by prevailing winds
before they are deposited. This and occult
deposition are both forms of wet deposition
(Figure 3.3). Alternatively, the dry deposition of
oxides of sulphur and nitrogen as gases, aerosols or
particulates may take place. This is known as dry
deposition (see Figure 3.3) and usually occurs
close to the source of nitrous and sulphurous oxide
production.
Both wet and dry deposition have an
environmental impact, the magnitude of which
depends on the capacity of the environment to
neutralise, or buffer, the acidic precipitation. For
example, in areas of alkaline bedrock such as
limestone or chalk, soils have a relatively high pH,
usually
c
. 6 to 7.5, and a high cation exchange
capacity. They have the capacity to neutralise
acidic precipitation because of the presence of
substances such as calcium carbonate and
magnesium carbonate. Similarly, lakes and rivers
Figure 3.2
The major reservoirs and fluxes in the global
biogeochemical cycle of (A) sulphur and (B) nitrogen.
Sources:
Based on (A) Charlson
et al
. (1992) and
Schlesinger (1997); (B) Jaffe (1992) and
Schlesinger (1997).
in areas of alkaline bedrock are less susceptible to
acidification because of the presence of
bicarbonate anions (HCO
3
-
). Areas of acidic
bedrock such as granite or those with acidic soils,
peats and lakes are particularly susceptible to
further acidification because there is little or no
buffering capacity. As a result, hydrogen ions (H
+
)
accumulate in the system, while sulphate and
nitrate anions (SO
4
2-
and NO
3
-
) freely combine
with cations such as sodium and potassium to
