Atmospheric Biogeochemistry - Ecological Systems - page 13

Similar to carbon species, nitrogen is also contained in organic gas species and to
a smaller extent in inorganic and organic aerosols. These species can act as
reservoirs of reactive nitrogen (e.g., [ 17 ], and thus be important for atmospheric
chemistry. The fraction of nitrogen species that are in aerosols is dependent on
chemical equilibrium with other aerosols, especially the strong acid sulfate [ 20 ]. In
the future, as sulfate emissions decrease, nitrogen aerosols may become more
important in terms of the radiative forcing of the climate as the fraction of nitrogen
in aerosol form increases [ 20 ].
Riverine inputs to oceans are thought to be 55-80 Tg/year [ 21 ], which is of the
same magnitude as the fluxes of many nitrogen compounds discussed above,
suggesting that the atmosphere plays an important role in the cycling of nitrogen
between the land and ocean.
Because many terrestrial ecosystems are nitrogen limited, the impact of nitrogen
deposition, especially from nitrogen oxides and ammonia, is thought to be large for
land ecosystems [ 3 , 22 - 25 ]. Nitrogen is also a limiting nutrient for ocean systems,
and there have been large increases in nitrogen deposition to ocean systems [ 21 ];
however, because of the large reservoir of reactive nitrogen in the oceans,
deposition to the oceans is not thought to substantially modify the ocean nitrogen
budget [ 26 ].
Phosphorus Cycle
Unlike carbon or nitrogen, most of the phosphorus in the atmosphere is in the
aerosol form, with little phosphorus in the gas phase (e.g., [ 27 ]. As phosphorus is
approximately 700 ppm in crustal material, and mineral aerosols (soil particles
suspended in the atmosphere) represent a major source of aerosols, most of the
phosphorus in the atmosphere is in mineral aerosols [ 28 ] at about 1.15 TgP/year.
The next largest source is thought to be primary biogenic particles (0.16 TgP/year),
but these aerosols are very poorly understood [ 28 ]. Combustion sources from fossil
fuels, biomass burning, and biofuels represent a small source (0.0.7 TgP/year),
while volcanoes and sea salts are thought to provide small amounts as well [ 28 ].
Thus, continental regions, especially deserts are the dominant source of atmo-
spheric phosphorus [ 28 ].
Aerosols deposited to ocean have a limited residence time in the ocean mixed
layer, before they sink to the ocean floor and become incorporated into the sedi-
ment. Only the fraction of phosphorus which is soluble or bioavailable can contrib-
ute to the phosphorus cycle of the ocean [ 28 , 29 ]. The bioavailable fraction is often
considered phosphate or soluble-reactive phosphorus [ 28 ], and the fraction of
phosphorus in aerosols that is soluble varies from 7% to 100% [ 28 , 29 ]. Transects
of the North Atlantic suggest that desert dust aerosols average about 10% solubility
[ 30 , 31 ]; however a gradient in solubility is observed as the particles move away
from the desert dust source area, suggesting that atmospheric processing may make
the phosphorus in aerosols more soluble [ 30 ]. Studies in Mediterranean suggest that
acidification, partly due to anthropogenic emissions of sulfur and nitrogen species,