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 O2 and CO2 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 (NH2CONH2), present in animal urine, as a typical biologically generated nitrogen compound in soil. Hydrolysis converts NH2CONH2 to ammonia (NH3) and CO2 according to the equation:

If the soil where this hydrolysis occurs is alkaline (Box 3.3), gaseous NH3 can be released, although in acidic conditions it will react to form the non-volatile ammonium ion (NH+):

Plants can absorb soil NH3 or NH+ directly and some microorganisms, such as Nitrosomonas, oxidize NH3, using it as an energy source for respiration, in the same way that other cells use reduced carbon compounds. One possible reaction would be:

Here we can see a biological source for nitrous oxide (N2O), an important and rather stable trace gas in the troposphere. In nature there are many other

(a) Dimethyl sulphide

(b) Trichlorofluoromethane (Freon-II)

(d) Toluene

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Fig. 3.4 Representations of some of the organic molecules discussed in Chapter 3. (a) Dimethyl sulphide. (b) Trichlorofluoromethane (Freon-11), one of the important CFCs. (c) Retene, a tricyclic compound derived from higher plant resins. (d) Toluene, a methylated aromatic compound.

reactions of nitrogen compounds in soils that produce the gases: NH3, N2, N2O and nitric oxide (NO).

Microorganisms in the oceans also prove to be an enormous source of atmospheric trace gases. Seawater is rich in dissolved sulphate and chloride (and to a lesser extent salts of the other halogens: fluorine (F), bromine (Br) and iodine (I)). Marine microorganisms metabolize these elements, for reasons that are not properly understood, to generate sulphur (S)- and halogen-containing trace gases. However, the nitrate concentration of surface seawater is so low that the oceans are effectively a nitrogen desert. This means that seawater is not such a large source of nitrogen-containing trace gases.

Organosulphides produced by marine microorganisms make a particularly significant contribution to the atmospheric sulphur burden. The most characteristic compound is dimethyl sulphide (DMS; (CH3)2S; Fig. 3.4a). This volatile compound is produced by marine phytoplankton, such as Phaeocystis pouchetii, in the upper layers of the ocean by the hydrolysis of beta-dimethylsulphoniopropionate (DMSP; (CH3)2S+CH2CH2COO-) to DMS and acrylic acid (CH2CHCOOH):

(CH3)2S+CH2CH2COO+aq) CH3)2S(g) + CH2CHCOOH(aq) eqn. 3.10 Another important sulphur compound released from the oceans is carbonyl sul-

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