Sulfate Aerosols

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 marine plankton.

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 dimethyl sulfone. 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 following steps:

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 DMS emissions