Atmospheric Biogeochemistry - Ecological Systems - page 15

from combustion processes and the chemical oxidation of CS 2 [ 27 ]. Carbonyl
sulfide is destroyed by oxidation in the atmosphere, as well as by stratospheric
phytolysis and uptake by vegetation at the surface [ 27 , 53 ]. Carbonyl sulfide is
thought to contribute about half of the sulfur that forms the stratospheric aerosol
layer, the other half coming from volcanic emissions [ 54 ].
Sulfur dioxide is a naturally occurring compound released from volcanoes and
biomass burning ( < 10% total) as well as oxidation of the naturally emitted

dimethyl sulfide ( 20%), but the dominant source currently are human combustion

activities with a total emission of about 91-125 TgS/year [ 16 , 18 ]. Once released
into the atmosphere, sulfur dioxide is oxidized within a few days to form sulfate
aerosols. Because sulfate is highly hydrophilic, sulfate aerosols take up water and
make a larger contribution to aerosol-radiation interactions than their mass would
suggest [ 16 ]. In addition these aerosols readily interact with cloud droplets to
modify cloud optical properties (aerosol indirect effect on clouds) [ 16 ]. Sulfur
dioxide from human emissions is responsible for a large portion of the anthropo-
genic radiation forcing of aerosols [ 16 ].
Dimethyl sulfide is a natural source of sulfur gas to the atmosphere from the
ocean, with a magnitude of about 15 TgS/year, which is readily oxidized to sulfur
dioxide and then to sulfate aerosol (lifetime about 1.5 days). This is the dominant
gas phase species of sulfur released from the ocean to the atmosphere. Because of
sulfate aerosol interactions with clouds, it was hypothesized that biota could
modulate their temperature by modifying emissions of dimethyl sulfide [ 55 ];
however, recent studies suggest this mechanism is not a dominant mechanism, for
example, under anthropogenic climate change [ 56 ].
Sea salts are 7.7% sulfate by weight [ 57 ], and represents the largest source of
sulfur to the atmosphere. Approximately 10,000 Tg/year of sea salt aerosols [ 58 ]is
emitted into the atmosphere, which translates to 770 Tg SO 4 /year or 250 TgS/year.
Sea salt aerosols reside in the atmosphere from hours to days, and tend to redeposit
back onto oceans [ 58 ], and are important as cloud condensation nuclei [ 59 ].
Sulfur moves from the land to the ocean at a rate of approximately 130 TgS/year
[ 27 ], which is similar in magnitude to many of the atmospheric fluxes, suggesting
that atmospheric sulfur is an important part of the global biogeochemical cycle.

Iron Cycle

Iron is required in small quantities by almost all organisms (e.g., [ 48 , 60 ]), and is
often considered a micronutrient. Because some regions of the oceans are iron
limited [ 61 ], atmospheric deposition of iron is important for providing new iron to
ocean biota [ 62 ]. Riverine inputs of iron are large, but are largely removed in
coastal regions [ 63 ]. There is some evidence that the nitrogen-fixing organisms
have larger iron requirements
Similar to the phosphorus cycle, iron resides almost entirely in atmospheric
aerosols, predominately in desert dust. Combustion and other sources of iron are