An Introduction to Environmental Chemistry

phide (OCS). This can be produced by reaction between carbon disulphide (CS 2 )

and water:

eqn. 3.11

and, although the flux to the atmosphere is smaller than that of DMS, its stabil-

ity means that it will accumulate to higher concentrations. These sulphur gases

have low solubility in water (Box 3.4), making them able easily to escape from

the oceans into the atmosphere.

Halogenated organic compounds are well known in the atmosphere. Although

these have an obvious human source, being present in cleaning fluids, fire extin-

guishers and aerosol propellants, they also have a wide range of biological sources.

Methyl chloride (CH 3 Cl) is the most abundant halocarbon in the atmosphere

and arises primarily from poorly understood marine sources, although terrestrial

microbiological processes and biomass burning also contribute. Bromine- and

iodine-containing organic compounds are also released from the oceans and the

distribution of this marine iodine over land-masses represents an important

source of this essential trace element for mammals. As one might predict, the

iodine-deficiency disease, goitre, has been common in regions remote from the

oceans.

CS 22 ()gg gg+Æ+H O() OCS() H S 2 ()

The Atmosphere 43

Box 3.4 Gas solubility

The solubility of gases in liquids is often
treated as an equilibrium process. Take the
dissolution of carbonyl sulphide (OCS) as an
example:

eqn. 1

where OCS(g)and OCS(aq)represent the
concentration of the substance carbonyl
sulphide in the gas and liquid phase. This
equilibrium relationship is often called
Henry’s law, after the English physical chemist
who worked c. 1800. The Henry’s law
constant (KH) describes the equilibrium.
Using pressure (p) to describe the
concentration (c) of OCS(g)in the gas phase,
we have:

eqn. 2

If we take the atmosphere as the unit of
pressure and mol l-^1 as the unit of
concentration, the Henry’s law constant will
have the units mol l-^1 atm-^1. The larger the
values of this constant, the more soluble the
gas. Table 1 shows that a gas like hydrogen

K c

H p

aq

g

OCS

OCS

= ()

()

OCS()gaªOCS( )q

peroxide is very soluble, oxygen very much

less so.

Many quite important gases have only

limited solubility, but often they can react in

water, which enhances their solubility. Take

the simple dissolution of formaldehyde

(HCHO) 2 which readily hydrolyses to

methylene glycol (H 2 C(OH) 2 ):

eqn. 3

eqn. 4

The second equilibrium lies so far to the right

that solubility is enhanced by a factor of

about 2000 (Box 3.2).

Table 1Some Henry’s law constants at 15°C

Gas KH(mol l-^1 atm-^1 )

Hydrogen peroxide 2 ¥ 105

Ammonia 90

Formaldehyde 1.7

Dimethyl sulphide 0.14

Carbonyl sulphide 0.035

Ozone 0.02

Oxygen 0.0015

Carbon monoxide 0.001

HCHO()aq+H O 22 ()lªH C OH( ) 2 ()aq

HCHO()gaªHCHO( )q