An Introduction to Environmental Chemistry

reactions of nitrogen compounds in soils that produce the gases: NH 3 , N 2 , N 2 O

and nitric oxide (NO).

Microorganisms in the oceans also prove to be an enormous source of atmos-

pheric 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 prop-

erly 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 char-

acteristic compound is dimethyl sulphide (DMS; (CH 3 ) 2 S; 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; (CH 3 ) 2 S+CH 2 CH 2 COO-) to DMS and

acrylic acid (CH 2 CHCOOH):

eqn. 3.10

Another important sulphur compound released from the oceans is carbonyl sul-

()CH 32 S CH CH COO+ (^22) ()+aqÆ()CH 32 S()ga+CH CHCOOH 2 ( )q
42 Chapter Three
(a) Dimethyl sulphide
H – C – S
H
H

• C – H

H

H

(b) Trichlorofluoromethane

(Freon-II)

F – C – Cl

Cl

Cl

(c) Retene

CH 3

(d) Toluene

Fig. 3.4Representations 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.