96
E. Pelizzetti . P. Calza
Reactions of sodium chloride, the major component of sea salt particles, with nitrogen oxides generate chlorine atom precursors. Chlorine atoms formed from the
reactions of sea salt particles can destroy ozone and greenhouse gas through direct
reactions. Alternatively, CI reacts rapidly with organic molecules, which can, in turn,
lead to ozone formation in the presence of sufficient nitrogen oxides (Finlayson-Pitts
and Pitts 1986).
The injection into the atmosphere of sea salt aerosol generated by breaking waves
on the ocean surface is the major global source of tropospheric chlorine. Most of this
material remains in the aerosol and is redeposited to the ocean surface, but important fractions ranging from averages of 3 to 35% are released from the aerosol as inorganic CI vapour (Keene 1995). Another precursor of atomic CI is HCI (Behnke et al.
1995). Although HN0 3 and H2S04 displace HCI from sea salt particles, this is not a significant source of atomic CI, because the subsequent reaction of HCI with OH is relatively slow (Singh and Kasting 1988).
Recent evidence indicates that other chemical processes may also dechlorinate sea
salt aerosol. These processes generate highly reactive CI gases that during the daytime
undergo rapid photochemical conversion to HCI via CI atoms. The aerosol eventually
scavenges HCI. Processes involved in CI cycling are very uncertain, however, partially
because of the difficulties in reliable measures of principal reactant and product species.
3.4.2
Role of Iron in Surface Waters
In waters several reductants coming from biodegradation of the biomasses are present.
Among these, it has to be reminded of iron(II) and H2S. Even if those species are able
to directly interact with the organic compounds, but usually through slow reactions,
redox reactions in natural systems may occur more easily due to a microbial action
and mediation through one or more electron transporter. The iron(III)/iron(II) system acts as a electron transporter. The oxido-reductive processes mediated by iron
compounds are important, because in anaerobic conditions, such as sediments and
deep waters, iron(III) oxides are the more abundant oxidant species. In various ecosystems the iron cycle depends on several physical, chemical and biological processes
(Stumm 1992). The iron cycle is interdependent and often connected with the cycles
of phosphorus, sulphur, heavy metals, oxygen and carbon; moreover, it is dependent
on living matter and light intensity.
The importance of iron can be stressed by such considerations:
1. High quantities are present in rocks and it possesses a rapid rate of transformation.
The ability of the species containing iron to oxidize or reduce and contemporary to
precipitate or to be solubilized, links the iron cycle to the oxygen cycle (oxidant) and
to the carbon cycle (reductant).
2. High surface areas of the iron oxides and their surface reactivity easily makes the
adsorption of various solutes. This is one of the causes of interdependence between
the iron cycle and the cycles of several other elements, such as heavy metals and
phosphate.
E. Pelizzetti . P. Calza
Reactions of sodium chloride, the major component of sea salt particles, with nitrogen oxides generate chlorine atom precursors. Chlorine atoms formed from the
reactions of sea salt particles can destroy ozone and greenhouse gas through direct
reactions. Alternatively, CI reacts rapidly with organic molecules, which can, in turn,
lead to ozone formation in the presence of sufficient nitrogen oxides (Finlayson-Pitts
and Pitts 1986).
The injection into the atmosphere of sea salt aerosol generated by breaking waves
on the ocean surface is the major global source of tropospheric chlorine. Most of this
material remains in the aerosol and is redeposited to the ocean surface, but important fractions ranging from averages of 3 to 35% are released from the aerosol as inorganic CI vapour (Keene 1995). Another precursor of atomic CI is HCI (Behnke et al.
1995). Although HN0 3 and H2S04 displace HCI from sea salt particles, this is not a significant source of atomic CI, because the subsequent reaction of HCI with OH is relatively slow (Singh and Kasting 1988).
Recent evidence indicates that other chemical processes may also dechlorinate sea
salt aerosol. These processes generate highly reactive CI gases that during the daytime
undergo rapid photochemical conversion to HCI via CI atoms. The aerosol eventually
scavenges HCI. Processes involved in CI cycling are very uncertain, however, partially
because of the difficulties in reliable measures of principal reactant and product species.
3.4.2
Role of Iron in Surface Waters
In waters several reductants coming from biodegradation of the biomasses are present.
Among these, it has to be reminded of iron(II) and H2S. Even if those species are able
to directly interact with the organic compounds, but usually through slow reactions,
redox reactions in natural systems may occur more easily due to a microbial action
and mediation through one or more electron transporter. The iron(III)/iron(II) system acts as a electron transporter. The oxido-reductive processes mediated by iron
compounds are important, because in anaerobic conditions, such as sediments and
deep waters, iron(III) oxides are the more abundant oxidant species. In various ecosystems the iron cycle depends on several physical, chemical and biological processes
(Stumm 1992). The iron cycle is interdependent and often connected with the cycles
of phosphorus, sulphur, heavy metals, oxygen and carbon; moreover, it is dependent
on living matter and light intensity.
The importance of iron can be stressed by such considerations:
1. High quantities are present in rocks and it possesses a rapid rate of transformation.
The ability of the species containing iron to oxidize or reduce and contemporary to
precipitate or to be solubilized, links the iron cycle to the oxygen cycle (oxidant) and
to the carbon cycle (reductant).
2. High surface areas of the iron oxides and their surface reactivity easily makes the
adsorption of various solutes. This is one of the causes of interdependence between
the iron cycle and the cycles of several other elements, such as heavy metals and
phosphate.