Atmospheric Biogeochemistry - Ecological Systems - page 14

can make the phosphorus in desert dust more soluble [ 32 ]. Non-dust phosphorus is
thought to be much more soluble ( 50%), based on studies of European-derived

aerosols in the Mediterranean [ 28 , 33 - 36 ].
Phosphine gas (PH 3 ) has been measured in limited studies, with concentrations
of between 0.39 and 2.45 ng/m 3 in remote regions and up to 178 ng/m 3 in urban
locations or near paddy fields [ 37 - 39 ]. While traditionally, the gas phase transfer of
phosphorus has been considered negligible [ 28 , 40 ], some of the values listed here
are similar in magnitude as those found for phosphorus in aerosols [ 28 ]. This
suggests that phosphine could be an important mechanism for transferring phos-
phorus, yet is not well understood. Small amounts of phosphine could be generated
in soils, agricultural and industrial processes, and lightning, but phosphine in the
presence of sunlight is converted to phosphoric acid [ 37 , 38 ].
Because phosphorus limitation is thought to be widespread in tropical forests
and savannahs [ 41 , 42 ], atmospheric deposition of desert dust may play a role in the
long-term viability of tropical soils. For example, it has been suggested that
the atmospheric deposition of desert dust from North Africa is responsible for
the maintenance of the Amazon forest [ 43 ], and that deposition from Asia is
important source of phosphorus to the tropical forests in Hawaii on long timescales
[ 44 ]. The atmospheric deposition of phosphorus is likely to be important in many
land ecosystems on geological timescales [ 45 ]. Forest and grassland ecosystems
can also lose phosphorus through the atmosphere, as primary biogenic particles or
biomass burning particles contain a large proportion of the phosphorus in tropical
forests [ 46 , 47 ].
The ocean tends to be a net sink of phosphorus from the atmosphere [ 28 ], and
since productivity in the ocean is thought to be phosphorus limited on long
timescales [ 48 ] and in some regions [ 49 , 50 ], the atmospheric deposition of
phosphorus could be an important source. Riverine inputs of phosphorus to the
oceans are much larger than the atmospheric fluxes described here (11 Tg/year)
[ 51 ]; however this phosphorus is input to the oceans in the coastal regions, and it is
unclear how much of this phosphorus is advected into the open oceans [ 52 ].
However, on short timescales, large increases (25%) in phosphorus deposition to
oceans which could be due to human activity [ 28 ] is not thought to substantially
impact ocean biogeochemistry, because of the large reservoir of phosphorus already
in the oceans [ 26 ].

Sulfur Cycle

Sulfur is an important trace compound in the earth system, used by some
microorganisms for energy. It is commonly found in rocks with smaller quantities
measured in the atmosphere [ 27 ]. The atmospheric portion of the sulfur cycle is
important because of the climate interactions of the sulfur species, especially in the
sulfate aerosol form. The sulfur compound with the longest lifetime (5 years) and
most common by mass is carbonyl sulfide (OCS) [ 27 ]. The dominant sources
(10 GgS/year) of carbonyl sulfide are the oceans and soils, with small contributions