14.1 Introduction
One of the main impacts of human activities on the global ecosystem is the change
caused on the chemistry of the atmosphere. Such a change is due mainly to the
emission of a wide range of chemicals, in the form of both gases and aerosols.
Some of them are potentially toxic (such as some trace metals and persistent organic
molecules) and are considered as pollutants. In many cases, these pollutants do not
exist in nature. Some of the emitted chemicals are present in nature, but the human
emissions have caused their levels in the atmosphere to exceed by far the natural
ones. This excess causes a disturbance in the planetary biogeochemical cycles, very
often with awkward effects. Sometimes there is a biogeochemical bottleneck that
causes the accumulation of a compound. An example of this is the rising atmospheric CO 2 (and other greenhouse gases) level, now very widely accepted to be
responsible of the human contribution to the global warming. In some other cases,
the addition of an element causes the acceleration of the natural cycling. For
instance, the introduction of large amounts of some compounds of nitrogen and
phosphorus (both are major nutrients for the living organisms) in the environment
has caused a nutrient imbalance in the ecosystems. This is of special importance in
nutrient-poor systems, such as remote oceanic areas and high mountains, where the
atmospheric inputs may have a fertilising effect that leads to an unnatural growth of
primary producers, basically plants and algae.
In addition to the emissions, human activities have also the effect of modifying
the natural atmospheric transport of substances. A direct way to do so is by changes
in the land use; for instance, agricultural practices and deforestation may enhance
production of dust and aeolian transport from land; changes in the wild fires regime
affect the emission of ashes and gases from burning biomass. But there are also
indirect ways that have to do with climate change, although in this case both natural
and human-induced causes play a role: droughts and losses of snow cover that
enhance dust production; melting of organic permafrost that increases CO 2 and
methane emissions from soil; changes in the prevailing winds and patterns of
circulation of air masses that carry airborne substances. All these are examples of
effects of climate change on the natural atmospheric fluxes of chemicals.
The long-range transport of atmospheric chemicals takes place in the free
atmosphere, that is, above the mixing boundary layer which generally has a depth
of 1,000–1,500 m. In this sense, the chemistry of the free atmosphere can be
considered to reflect the composition of the global atmosphere better than the
boundary layer below it. High mountains protrude above the boundary layer on the
lowlands around them, and thus intercept the “global” flux of chemicals. Such
interception is enhanced by altitude. Orographic precipitation is higher in the
mountains causing greater scavenging of airborne substances than in the lowlands.
Also, temperature is lower in the summits than in the piedmont and the deposition
of volatile compounds by condensation (cold trapping) is favoured. In summary,
mountains act as regional (or even global) convergence areas of atmospheric
chemical fluxes.
326
L. Camarero
One of the main impacts of human activities on the global ecosystem is the change
caused on the chemistry of the atmosphere. Such a change is due mainly to the
emission of a wide range of chemicals, in the form of both gases and aerosols.
Some of them are potentially toxic (such as some trace metals and persistent organic
molecules) and are considered as pollutants. In many cases, these pollutants do not
exist in nature. Some of the emitted chemicals are present in nature, but the human
emissions have caused their levels in the atmosphere to exceed by far the natural
ones. This excess causes a disturbance in the planetary biogeochemical cycles, very
often with awkward effects. Sometimes there is a biogeochemical bottleneck that
causes the accumulation of a compound. An example of this is the rising atmospheric CO 2 (and other greenhouse gases) level, now very widely accepted to be
responsible of the human contribution to the global warming. In some other cases,
the addition of an element causes the acceleration of the natural cycling. For
instance, the introduction of large amounts of some compounds of nitrogen and
phosphorus (both are major nutrients for the living organisms) in the environment
has caused a nutrient imbalance in the ecosystems. This is of special importance in
nutrient-poor systems, such as remote oceanic areas and high mountains, where the
atmospheric inputs may have a fertilising effect that leads to an unnatural growth of
primary producers, basically plants and algae.
In addition to the emissions, human activities have also the effect of modifying
the natural atmospheric transport of substances. A direct way to do so is by changes
in the land use; for instance, agricultural practices and deforestation may enhance
production of dust and aeolian transport from land; changes in the wild fires regime
affect the emission of ashes and gases from burning biomass. But there are also
indirect ways that have to do with climate change, although in this case both natural
and human-induced causes play a role: droughts and losses of snow cover that
enhance dust production; melting of organic permafrost that increases CO 2 and
methane emissions from soil; changes in the prevailing winds and patterns of
circulation of air masses that carry airborne substances. All these are examples of
effects of climate change on the natural atmospheric fluxes of chemicals.
The long-range transport of atmospheric chemicals takes place in the free
atmosphere, that is, above the mixing boundary layer which generally has a depth
of 1,000–1,500 m. In this sense, the chemistry of the free atmosphere can be
considered to reflect the composition of the global atmosphere better than the
boundary layer below it. High mountains protrude above the boundary layer on the
lowlands around them, and thus intercept the “global” flux of chemicals. Such
interception is enhanced by altitude. Orographic precipitation is higher in the
mountains causing greater scavenging of airborne substances than in the lowlands.
Also, temperature is lower in the summits than in the piedmont and the deposition
of volatile compounds by condensation (cold trapping) is favoured. In summary,
mountains act as regional (or even global) convergence areas of atmospheric
chemical fluxes.
326
L. Camarero