Atmospheric Biogeochemistry - Ecological Systems - page 18

Oxidation of organic and reduced species in the troposphere is the sink for many
important greenhouse gases (e.g., methane), and other important trace species in the
atmosphere (e.g., sulfur dioxides, nitrogen oxides) [ 17 ]. Much of this oxidation
does not occur directly by ozone, but rather by the hydroxyl radical, a by-product of
ozone photochemistry in the troposphere [ 17 ]. Because this constituent reacts so
quickly, it is difficult to determine its concentration directly, but it can be deduced
from changes in concentrations in some of the compounds it reacts with [ 97 ]. It is
unclear whether humans are increasing or decreasing the hydroxyl radical, and will
continue to do so, because of the complex nonlinear chemistry [ 16 ].
Other Cycles
Most elements are cycled in the atmosphere, and because long-range transport is much
faster in the atmosphere than on land or oceans, this transport can be important for
many elements. Here the focus is on a few critical elements for global biogeochemistry.
However, some elements are important for their negative impacts, especially heavy
metals. For example, mercury emissions from humans impacts human and ecosystem
health [ 98 ]. Some studies have suggested that atmospheric deposition of heavy metals
far from source areas can adversely impact ocean biogeochemistry [ 99 ]. In addition,
atmospheric deposition of acid is thought to reduce the ability of some ecosystems to
retain some nutrients and to negatively impact land and aquatic ecosystems [ 101 - 103 ],
and to enhance ocean acidification [ 100 ].
Anthropogenic Modifications to Atmospheric
Biogeochemical Cycles
Direct human emissions of carbon dioxide, sulfur dioxide, and nitrous oxides gases,
as well carbonaceous aerosols, through the increase in combustion that humans use
to generate energy, has substantially increased the sources and atmospheric
concentrations of these compounds [ 16 ]( Fig. 2.3 ). In addition, changes in land
use have increased carbon dioxide, methane, nitrous oxide, and ammonia emissions
[ 16 ]( Fig. 2.3 ). These modifications are likely to continue into the future, with some
decrease in emissions of combustion-sourced compounds estimated ( Fig. 2.3 )
[ 101 - 107 ]. A combination of climate change and land-use change seems to have
resulted in an increase in mineral aerosol over the past 100 years [ 89 ], which
increases the flux of phosphorus and iron. In addition, bioavailable iron and
phosphorus are likely to be even more susceptible to acidity and combustion
sources; thus these compounds have changed more than simply from mineral
aerosol changes over the past 100 years ( Fig. 2.3 ). All of these factors have
accelerated the atmospheric biogeochemical cycling of these compounds.