Sulfate aerosols are produced by chemical reactions in the atmosphere from
gaseous precursors (with the exception of sea salt sulfate and gypsum dust particles).
The two main sulfate precursors are sulfur dioxide (SO2) from anthropogenic
sources and volcanoes, and dimethyl sulfide (DMS) from biogenic sources, especially
SO2 as a precursor gas
SO2 must be oxidized to SO4-2 (sulfate)
before it plays a role in aerosol formation. The oxidation can take place while
SO2 is still in the gas phase, or after SO2 becomes dissolved
in cloud droplets (aqueous production). In the latter case, the sulfate becomes
aerosol after the cloud droplets evaporate. Both pathways produce sulfate aerosols
in the submicron size range that are efficient light scatterers.
Positively charged ions must be present on sulfate aerosols to neutralize
the sulfate anions. Thus, the sulfate in aerosol particles is present as sulfuric
acid (H2SO4), ammonium sulfate (NH3)2SO4,
or intermediate compounds, depending on the availability of gaseous ammonia
to neutralize the sulfuric acid formed from SO2.
Most of the SO2 emissions globally result from fossil fuel burning.
For this reason, most of the aerosol produced from the oxidation of SO2
is considered to be anthropogenic sulfate aerosol (ASA). The source strengths
for this trace gas are fairly well known compared to other aerosol precursor
gases, and recent estimates differ by no more than ~20- 30%. Volcanic
eruption and degassing is another source of SO2, although its average
source strength is probably only one fifth that of ASA. The radiative (and thus
climatic) imact of volcanic SO2 from major eruptions can be very
high if the gases are injected into the stratosphere, where sulfate particles
have a longer residence time.
Sulfate is known to condense onto larger aerosol particles with lower scattering
efficiencies and shorter atmospheric lifetimes. At least two pathways for this
interaction are recognized. The first is heterogeneous reactions of SO2
on mineral aerosols. The second is oxidation of SO2 to sulfate in
sea salt-containing cloud droplets and deliquesced sea salt aerosols. This process
can result in a substantial fraction of non-sea-salt sulfate to be present on
large sea salt particles, especially under conditions where the rate of photochemical
H2SO4 production is low and the amount of sea
salt aerosol surface available is high.
The calculated residence times of SO2, defined as the global burden
divided by the global emission flux, range between 0.6 and 2.1 days as a result
of different deposition parameterizations. Because of losses due to SO2
deposition, only 46-82% of the SO2 emitted undergoes chemical transformations
and forms sulfate. The residence time of sulfate is mainly determined by wet
removal and is estimated to be between 2.7 and 7.2 days.
Biogenic sulfate aerosol
Three reduced sulfur gases of biogenic origin play a
role in the formation of sulfate aerosol. Hydrogen sulfide (H2S)
is produced by sulfate reducing bacteria and is emitted from swamps and sediments.
H2S is converted rapidly in the atmosphere to SO2. Marine
phytoplankton emit dimethyl sulfide (CH3SCH3, or DMS).
Carbonyl sulfide (COS) is released in much smaller amounts from the biosphere,
but has a much longer atmospheric lifetime (ca. 44 years).
Both COS and DMS serve as precursor gases for sulfate aerosol production.
This requires that DMS or COS be first oxidized to SO2 in the atmosphere.
DMS is oxidized by free radicals that contain oxygen (such as NO3-
and OH-). OH- forms during the day via photochemical processes.
During daylight hours, reaction with OH- is the primary vehicle for
DMS oxidation. In polluted atmospheres, NO3 can build up at night
and lead to DMS+NO3 reaction. Intermediate oxidation products are
methanesulfonic acid (CH3SO3H), dimethyl sulfoxide, and
Although anthropogenic emissions are the main source of SO2 over
continents and in polluted marine airspace, DMS is the main precursor of SO2
in remote oceanic air. DMS is not subject to dry deposition and can therefore
be converted to SO2 far enough from the ground to avoid large deposition
losses. In contrast, most anthropogenic SO2 is released near the
ground and much of it is lost by deposition before oxidation can occur.
The reactivity of COS in the troposphere is low, allowing COS to enter the
stratosphere by diffusion. During periods lacking volcanic activity (and thus
direct injection of SO2 into the stratosphere), oxidation of COS
dominates the production of stratospheric sulfate aerosol.
The DMS global climate feedback
The CLAW hypothesis was published in 1987 by Charlson, Lovelock, Andreae and
Warren. It states that biogenic dimethyl sulfide (DMS) production by marine
phytoplankton influences global climate through a feedback mechanism with the
1. DMS diffuses from the sea surface to the atmosphere
2. DMS is oxidized to a sulfate aerosol
3. Sulfate aerosols act as the main source of cloud condensation nuclei
over the oceans
4. Cloud albedo is increased
5. Global temperature is lowered
6. Phytoplankton production decreases
7. DMS emissions decrease in turn
8. Sulfate aerosol loads decrease
9. Cloud albedo decreases
10. Global temperature rises, increasing both phytoplankton production and