An Introduction to Environmental Chemistry

3.4.2 Biological sources

Unlike the geological sources, biology does not appear to be a large direct source
of particles to the atmosphere, unless we consider forest fires to be a biological
source. Table 3.2 shows that forest fires are quite an important source of carbon
(C), i.e. soot particles.
The living forest also plays an important role in exchanging gases with the
atmosphere. The major gases O 2 and CO 2 are, of course, involved in respiration
and photosynthesis. However, forests also emit enormous quantities of trace
organic compounds. Terpenes, (a class of lipids) such as pinene and limonene,
give forests their wonderful odour. Forests are also important sources of organic
acids, aldehydes (see Table 2.1) and other organic compounds (see Section 2.7).
Although forests are obvious as sources of gas, it is the microorganisms
that are especially important in generating atmospheric trace gases. Methane,
which we have already discussed, is generated by reactions in anaerobic
systems. Damp soils, as found in marshes or rice paddies, are important micro-
biologically dominated environments, as are the digestive tracts of ruminants such
as cattle.
The soils of the Earth are rich in nitrogen compounds, giving rise to a whole
range of active nitrogen chemistry that generates many nitrogenous trace gases.
We can consider urea (NH 2 CONH 2 ), present in animal urine, as a typical bio-
logically generated nitrogen compound in soil. Hydrolysis converts NH 2 CONH 2
to ammonia (NH 3 ) and CO 2 according to the equation:

eqn. 3.7

If the soil where this hydrolysis occurs is alkaline (Box 3.3), gaseous NH 3 can be
released, although in acidic conditions it will react to form the non-volatile
ammonium ion (NH 4 +):

eqn. 3.8

Plants can absorb soil NH 3 or NH 4 +directly and some microorganisms, such as
Nitrosomonas, oxidize NH 3 , using it as an energy source for respiration, in the
same way that other cells use reduced carbon compounds. One possible reaction
would be:

eqn. 3.9

Here we can see a biological source for nitrous oxide (N 2 O), an important
and rather stable trace gas in the troposphere. In nature there are many other

22 NH32 2()gg+Æ +O() N O()g 3 H O 2 ()g

NH 34 ()g+ÆH()+aq NH()+aq

NH CONH 222 ()aq+Æ +H O()l 2 NH 32 ()g CO()g

The Atmosphere 41

Table 3.2Sources for particulate material in the atmosphere. From Brimblecombe (1986).

Source Global flux (Tg yr-^1 )

Forest fires 35

Dust 750

Sea salt 1500

Volcanic dust 50

Meteoritic dust 1