A8 Aerosol and Composition Anomalies

Topic

Aerosol and atmospheric composition anomalies are episodes in which the concentration of particles or reactive gases in the atmosphere rises dramatically above typical background levels, with impacts extending to the planet's radiative balance, cloud formation, precipitation, human health, and ecosystems. Four distinct sources generate these anomalies: explosive volcanic eruptions, which inject gases and ash directly into the stratosphere; Saharan dust storms, which transport vast quantities of mineral dust across the Atlantic Ocean; large-scale wildfires, whose most intense smoke plumes can also reach the stratosphere; and episodes of extreme pollution accumulating over megacities.

Of these four sources, explosive volcanic eruptions reaching the stratosphere have the greatest climatic impact: the sulfur dioxide injected by the eruption oxidizes in the stratosphere to become sulfuric acid, which in turn forms sulfate aerosols. These sulfate aerosols scatter solar radiation before it reaches the surface; the greater the mass of sulfate aerosols accumulating in the stratosphere, the more significant the resulting global cooling. The 1991 eruption of Mount Pinatubo is the best-documented case of this mechanism: it produced a negative radiative forcing of approximately three watts per square meter that persisted for one to two years, cooling the planet by an average of about half a degree. Large-scale wildfires can produce a similar effect when their most intense smoke plumes evolve into pyroconvective clouds capable of injecting smoke into the lower stratosphere—at altitudes of 15 to 20 kilometers—where the smoke can remain for periods ranging from several months to several years. Saharan dust storms—the second source—transport between 500 million and a billion tons of mineral dust annually from the Sahara to the tropical Atlantic, the Caribbean, and the Amazon basin. This mineral dust exerts direct radiative effects by scattering and absorbing solar radiation—much like volcanic sulfate aerosols—as well as indirect radiative effects by acting as ice nuclei and promoting hail formation within the clouds it traverses. Beyond these radiative effects, the mineral dust fertilizes the oceanic and terrestrial ecosystems it passes over, delivering iron and phosphorus—nutrients these ecosystems require but which are otherwise scarce.

The fourth source, extreme pollution from megacities, manifests primarily as urban photochemical smog: nitrogen oxides and volatile organic compounds emitted by these megacities react with sunlight to produce tropospheric ozone and secondary organic aerosols—the very aerosols that subsequently impact the health of those breathing the megacity air.

Thus, the four sources of aerosol and atmospheric composition anomalies—explosive volcanic eruptions, wildfires with their pyrogenic convection clouds, Saharan dust storms, and megacity photochemical smog—converge on the same five fronts mentioned at the beginning of this description: all alter the planet's radiative balance, either by cooling it via sulfate aerosols and mineral dust or by locally warming it through tropospheric ozone; all modify cloud formation, whether through dust-supplied condensation nuclei or smog-derived secondary organic aerosols; all ultimately influence precipitation; and all leave a mark—for better or worse—on human health and the ecosystems that receive their particles and gases.

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