Sulfate aerosols
Sulfate aerosols are tiny particles, either in liquid or solid form, that are suspended in the atmosphere, primarily originating from anthropogenic sources such as sulfur impurities in fossil fuels like coal and natural gas. They contribute to a phenomenon known as global dimming, which can partially offset global warming by reflecting sunlight back into space while allowing infrared radiation to pass through. This dual role highlights the complexity of their impact on climate change. Natural sources also play a role in the sulfur cycle, including emissions from volcanic eruptions and organic decay, with significant climatic effects.
Notably, major volcanic eruptions can inject large amounts of sulfate aerosols into the stratosphere, leading to temporary cooling periods, as seen after the eruptions of Mount Tambora and Krakatoa. Current concerns about climate change have prompted discussions around potential geoengineering solutions, such as intentional sulfate injection into the stratosphere to mimic volcanic effects. However, these approaches raise questions about their effectiveness and possible unintended consequences. Ultimately, the relationship between sulfate aerosols and climate change is complex, intertwining environmental health and the need for effective pollution control in both developed and developing nations.
Sulfate aerosols
Definition
Sulfate aerosols are tiny (submicron) liquid or solid particles suspended in the atmosphere. emissions are probably the leading source of sulfate aerosols in the atmosphere. The global dimming from sulfates and certain other aerosols may partly counterbalance global warming, masking the greenhouse effect. The most important anthropogenic sources of sulfate aerosols are sulfur impurities in coal, petroleum products, and natural gas. Health and environmental concerns have caused Western countries to sharply curtail these emissions, but rapidly industrializing developing nations are more than compensating for this reduction in the developed world.
In addition to anthropogenic emissions of sulfur into the environment, the world has a natural sulfur cycle analogous to the carbon cycle. Sulfur compounds, especially, hydrogen sulfide (H2S) and sulfur dioxide (SO2), escape from volcanoes and eroding rocks. These and other sulfur compounds eventually redeposit. However, some of this sulfur passes through the atmosphere, with significant climatic effects. Non-SO2 sulfur compounds entering the atmosphere are oxidized into SO2. The SO2 combines with hydrogen in water to form sulfuric acid (H2SO4) or with ammonium ions to form ammonium sulfate (NH3)2SO4. These form liquid or gas aerosols that eventually fall to the ground or are washed down in rain.
Before returning to the ground, sulfate aerosols influence climate by reflecting visible light back up into space while allowing infrared light (heat waves) to pass through and by providing nucleation sites for water into clouds. The clouds also reflect visible light and allow infrared to pass. The resulting cooling is the opposite of the warming greenhouse effect.
Sulfur is also a small but significant fraction of the mass in organic matter. The most significant terrestrial source of sulfur-compound emissions is organic decay, particularly in marshlands, but all organisms emit some by-product sulfur compounds. At sea, plankton metabolism dominates sulfur emissions, with dimethyl sulfide (DMS or CH3SCH3) and carbonyl sulfide (COS) being the most common.
Volcanic sulfur emissions average significantly less than anthropogenic emissions. However, major explosive volcanic eruptions insert large pulses of sulfate aerosols into the stratosphere. Reaching the greater altitude and drier air of the allows these aerosols to remain aloft longer. These pulses have been associated with significant cooling, including the 1816 “year without a summer” after the 1815 eruption of Mount Tambora and the milder cooling after the smaller 1883 Krakatoa eruption. The much smaller (but better documented) 1991 eruption of Mount Pinatubo put roughly 18 million metric tons of sulfates into the stratosphere and caused cooling for several months.
Significance for Climate Change
Although aerosol analyses are even hazier than are most climate science, humanity may be balancing a significant fraction of greenhouse-gas-related warming with sulfate-aerosol-related cooling. Increased industrialization in areas without pollution controls could tip the balance into cooling. Conversely, humanity may opt to attempt planetary climate change or if global warming is seen as a major threat. Two methods have been seriously proposed, direct sulfate injection into the stratosphere and ocean fertilization. The arguments for both are that they could be stopped at any time. The arguments against them are that they might have only limited effects and might have unexpected side effects.
In 1974, Russian geologist Mikhail I. Budyko suggested that, if global warming became a problem, humanity could burn sulfur in the stratosphere to create a haze similar to that arising from volcanic eruptions. Recent calculations suggest that such an “artificial volcano” could offset the warming of twofold or even threefold increased atmospheric carbon dioxide (CO2), and the price might be as low as one billion dollars annually. The sulfates would also stop some ultraviolet radiation. This ability might prove important, because sulfates attack ozone, and the need to preserve the may be a limitation. Continued sulfate injections might be needed to compensate for the lost ozone.
In 1988, California oceanographer John Martin said “Give me a half tanker of iron, and I will give you an ice age.” Less ambitious fertilization of nutrient-limited areas in the ocean would cause plankton blooms emitting sulfate aerosols for greater and capturing CO2 from the atmosphere. The plan could even be self-funding through increased fishery yields.
However, ocean fertilization has potential limitations. The effects may be less efficient because of predator activity on the plankton, and some plankton emissions may be greenhouse agents rather than cooling agents. Most worrisome, an attempt to compensate for a major fraction of yearly greenhouse warming might cause so much organic material to rain into deeper waters that they could become toxic from excess CO2.
Bibliography
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Kondratyev, Kirill Y., et al. Atmospheric Aerosol Properties: Formation, Processes, and Impacts. Chichester, West Sussex, England: Praxis, 2006.
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