3.5 Reactivity of trace substances in the atmosphere
Gases with short residence times in the atmosphere are clearly those that can be
removed easily. Some of these gases are removed by being absorbed by plants or
solids or into water. However, chemical reactions are the usual reason for a gas
having a short residence time.
What makes gases react in the atmosphere? It turns out that most of the trace
gases listed in Table 3.3 are not very reactive with the major components of air.
In fact, the most important reactive entity in the atmosphere is a fragment of a
water molecule, the hydroxyl (OH) radical. This radical (a reactive molecular
fragment) is formed by the photochemically initiated reaction sequence, started
by the photon of light, hn:
eqn. 3.12
eqn. 3.13
The OH radical can react with many compounds in the atmosphere and thus
it has a short residence time. The rates are faster than with abundant gases such
as O 2.
The reaction between nitrogen dioxide (NO 2 ) and the OH radical leads to the
formation of HNO 3 , an important contributor to acid rain.eqn. 3.14
By contrast, kinetic measurements in the laboratory (which aim at determining
the speed of reaction) show that gases that have slow rates of reaction with the
OH radical have a long residence time in the atmosphere. Table 3.3 shows thatNO 23 ()gg+ÆOH() HNO()gOHO OH()gg g+Æ 2 () (^2) ()
OOO 32 ()ggg+Æhv ()+()
44 Chapter Three
Table 3.3Naturally occurring trace gases of the atmosphere. From Brimblecombe (1986).
Residence time Concentration (ppb)
Carbon dioxide 4 years 360 000
Carbon monoxide 0.1 year 100
Methane 3.6 years 1600
Formic acid 10 days 1
Nitrous oxide 20–30 years 300
Nitric oxide 4 days 0.1
Nitrogen dioxide 4 days 0.3
Ammonia 2 days 1
Sulphur dioxide 3–7 days 0.01–0.1
Hydrogen sulphide 1 day 0.05
Carbon disulphide 40 days 0.02
Carbonyl sulphide 1 year 0.5
Dimethyl sulphide 1 day 0.001
Methyl chloride 30 days 0.7
Methyl iodide 5 days 0.002
Hydrogen chloride 4 days 0.001