148 S. Bohler et al.
larger part due to anthropogenic atmospheric pollution, primarily brought on by the
combustion of fossil fuels. Molecules like carbon dioxide (CO 2 ), methane (CH 4 ),
nitric and sulphur oxides (NOx, SOx) are directly emitted into the atmosphere, while
other molecules like ozone (O 3 ) are formed from reactions between pollutants and
atmospheric constituents. CO 2 and ozone are two of the main greenhouse gases that
cause the retention of heat in the atmosphere and lead to the observed increase in
temperatures.
7.2 Ozone
Ozone is a secondary pollutant, i.e. it is not directly emitted, but produced as a
consequence of primary pollutants. Nitric dioxide (NO 2 ), mostly emitted by car
and industrial exhausts, reacts with solar UV radiation and fragments to form nitric
monoxide (NO) and an activated oxygen atom (O•−). The reaction between O•− and
atmospheric oxygen (O 2 ) leads to the formation of ozone (Andreae and Crutzen
1997 ; Renaut et al. 2009 ; Fig. 7.1). Molecules like volatile organic compounds
(VOC) also intervene in the complex reactions. Background ozone concentrations
in the troposphere (the layer of air that expands from the earth’s surface up to an
altitude of about 10 km) have increased by 500 % during the past century (Marenco
1994 ). Even though, in recent years, steps have been taken to reduce the emission of
ozone-forming pollutants, results are inconclusive (Jonson et al. 2006 ).
An ozone-enriched atmosphere induces a situation of stress in plants. Ozone is
easily absorbed through stomata and instantly fragments in contact with the plant
Fig. 7.1 Simplified scheme
of the formation of the
secondary pollutant ozone
in the troposphere. NO 2
nitric oxide; O 3 ozone; O•−
activated oxygen atom; O 2
atmospheric dioxygen