Geoscience Reference
In-Depth Information
1. Introduction
The element sulfur is essential for the sustenance of life. So it is necessary
to take stock of supply of sulfur in the sulfur cycle. There are a number of
natural sources of atmospheric sulfur. Apart from volcanic emission and the
Aeolian generation of sulfate particles, the biogenic emissions from land and
sea are important sources.
1
At one time, H
2
S was considered as the main
representative of biogenic source, as it is copiously produced in anaerobic
marshlands and tidal flats. However, since the early 1970s, the scenario has
changed. A number of additional sulfides were discovered in the atmosphere
in that decade, like carbonyl sulfide (OCS),
2
carbonyl disulfide (CS
2
),
3
dimethyl sulfide (DMS) (CH
3
SCH
3
)
4
and dimethyl disulfide (CH
3
S
2
CH
3
).
5
In 1972 Lovelock
et al.
6
first discovered that sea water is saturated with
DMS and then, in 1977, Maroulis and Bandy
4
discovered it in marine
atmosphere. After the works of Nguyen
et al.
7
and Barnard
et al.
,
8
the
ubiquitous presence of DMS in the atmosphere was established.
In the atmosphere, DMS is supposed to be oxidized into SO
2
and
methane sulfonate. SO
2
is converted into sulfuric acid and particulate
sulfate. However, laboratory experiments identify that there may be various
other important end products during oxidation. These are HCHO, methane
sulfonic acid (MSA: CH
3
SO
3
H), dimethylsulfoxide (DMSO: CH
3
SOCH
3
),
and methane sulfinic acid (MSIA: CH
3
S(O)(OH)).
9
,
10
All sulfides except
OCS react rapidly with OH radicals. DMS reacts with OH in two pathways,
either by abstraction of a hydrogen atom or by addition of OH to sulfur
atom (Ref. 1, p. 603):
OH + CH
3
SCH
3
→
H
2
O+CH
3
SCH
2
↔
CH
3
S(OH)CH
3
In the presence of O
2
, the addition product reacts with it to produce
DMSO
11
:
CH
3
S(OH)CH
3
+O
2
→
CH
3
SOCH
3
+HO
2
DMSO has also been produced in the laboratory by photochemical
reactions of DMS in the presence of OH.
12
There exist reports about
the possibility of formation of DMSO due to microbial oxidation and
photo-oxidation of DMS, as well as due to the direct biosynthesis by
phytoplanktons.
13
In fact, both DMS and DMSO have been detected in the marine
atmosphere.
14
Many
sulfates
or
sulfur-containing
aerosols
are
now
