Aerosol pollutants

Aerosols are among the least well understood influences on global climate, but anthropogenic aerosols, especially sulfate aerosols released by fossil fuel combustion, seem to exert a cooling influence on the climate. This cooling effect, however, appears insufficient to counteract the warming caused by GHGs.

Background

Effects of aerosol pollutants, such as volcanic dust, have been debated for a long time. A 1783 eruption of a volcanic fissure in Iceland seemed related to an unusually cool summer in France that year. In 1883, the volcanic dust from the explosion of Krakatoa in the East Indies dimmed the sunlight for months, as had the 1815 eruption of Tambora. Some scientists perceived a pattern of temporary cooling from such events. Others asked if pollutants should be expected to warm, rather than cool, the atmosphere.

Aerosols are minute airborne solid or liquid particles suspended in the atmosphere, typically measuring between 0.01 and 10 microns. They may be of either natural or origin. Natural aerosol sources include salt particles from sea spray; clay particles from the weathering of rocks; volcanically produced sulfur dioxide, which oxidizes to form sulfuric acid molecules; and desert dust. Anthropogenic (human-produced) aerosol sources include industrial pollutants such as sulfates, created by burning oil and coal; smoke from large-scale burning of biomass, such as occurs in slash-and-burn clearing of tropical forests; and pollution from naval vessels’ smokestacks.

Normally, most aerosols rise to form a thin haze in the troposphere; rain washes these out within about a week’s time. Some aerosols, however, are found in the higher stratosphere, where it does not rain. They can remain in this atmospheric layer for months. Aerosols may influence climate in several ways: directly, through scattering and absorbing radiation, and indirectly, by acting as or by modifying the optical properties and lifetimes of clouds.

On windy days, bubbles created by breaking waves toss salt into the air when they burst, forming aerosols. Salt aerosols scatter sunlight, lessening the amount of energy that reaches Earth’s surface and cooling the climate. The interaction of sea salt with clouds also causes cooling. The resulting whitening of the Earth further reduces the amount of sunlight that can reach the ground. Oceans cover over 70 percent of Earth’s surface, and sea salt is a major source of aerosols in areas far distant from land.

Wind also helps form aerosols over land. Particles carried by the wind push and bounce over one another, abrading the surfaces of rock and other landforms. These particles wear rocks down progressively over time, converting their surfaces into dust and other particles. When these particles are incorporated into the air, they too form aerosols.

Following major volcanic eruptions, sulfur dioxide gas vented during the eruptions is converted into sulfuric acid droplets. These droplets form an aerosol layer in the stratosphere. Winds in the scatter the aerosols over the entire globe, and they may remain in the atmosphere for about two years. Since they reflect sunlight, these aerosols reduce the amount of energy that reaches the troposphere and the Earth’s surface, resulting in cooling.

Another significant natural aerosol is desert dust. “Veils” of dust stream off deserts in Asia and Africa, and they have also been observed on the American continent. These particles fall out of the atmosphere after a short flight, but intense dust storms often blow them to altitudes of 4,500 meters or higher. Since the dust is made up of minerals, the particles both absorb and scatter sunlight. Absorption warms the layer of the atmosphere where they are located, possibly inhibiting the formation of storm clouds and contributing to desertification.

Early Speculation About Aerosols

Long before there was much interest in aerosols as a factor in climate change or any equipment capable of adequately analyzing aerosol data, a few individuals speculated about a possible aerosol-climate connection. The first man credited with reporting his ideas was Mourgue de Mondtredon, a French naturalist who in 1783 documented the eight-month-long Laki eruption in southern Iceland. The eruption caused the grass to die: Three-quarters of the region’s livestock and one-quarter of its people starved to death. For months, a haze hovered over western Europe. When Benjamin Franklin was visiting in France in 1783, he experienced an unseasonably cold summer and speculated that the Laki volcanic “fog” had noticeably dimmed the sunlight.

A century later, in 1883, the eruption of the Indonesian volcano Krakatoa (Krakatau) sent up a veil of volcanic dust that reduced sunlight globally for months. Scientists were unable to determine what effect the eruption might have had on the average global temperature, but scientists thereafter acknowledged volcanoes as a possible natural influence on Earth’s climate.

A few scientists who examined temperatures after major volcanic eruptions between 1880 and 1910 perceived a pattern of temporary cooling. Only later would older records reveal that the 1815 eruption of Tambora in Indonesia had affected the climate more severely than had the Krakatoa eruption. Speculation led some to ask if volcanic eruptions had precipitated ice ages or had cooled the Earth to the extent that dinosaurs became extinct.

Early Twentieth Century Aerosol Research

Throughout the first half of the twentieth century, it was known that volcanic aerosols could affect climate. As a result, some scientists suspected that other kinds of dust particles could have similar climatic effects. Physics theory seemed to support the notion that these particles should scatter radiation from the Sun back into space, thereby cooling the Earth. These ideas remained largely speculative, though some researchers began to focus on the possibility that human activity might be a major source of atmospheric particles.

In the 1950s, nuclear bomb tests provided improved data on aerosol behavior in the stratosphere. It was determined that stratospheric dust would remain for some years, but would stay in one hemisphere. Research in the early 1960s indicated that large volcanic eruptions lowered average annual temperatures. Some researchers, however, deemed those results enigmatic, since temperatures had fallen during a period of few eruptions. Meteorologists acknowledged that other small, airborne particles could influence climate, but throughout the first half of the century, speculation fell short of conclusion.

Gradually, scientists shifted their focus to anthropogenic atmospheric particles. Measurements by ships between 1913 and 1929 noted that sea air showed an extended decrease in conductivity, apparently caused by stack smoke and gases from ships and possibly from industry on land. Even in 1953, however, scientists were uncertain about the significance of the pollution.

During the 1950s, some scientists asked whether aerosols might affect climate by helping form clouds. Since cloud condensation nuclei are essential for providing a surface for water droplets to condense around, the notion of seeding clouds with silver iodide smoke to make rain was widespread. By this time, aerosol science was just coming into its own as an independent field of study, having been given impetus by the concern that disease-carrying aerosols and poisonous gas could be lethal. Public concern over urban smog also fueled studies by aerosol experts. By and large, however, scientists avoided the study of cloud formation. Field testing often produced contradictory results and was extremely expensive, and many researchers believed that aerosols’ effects on clouds were too complex to comprehend.

Aerosol Research in the Later Twentieth Century

By the early 1960s, the scientific community was beginning to pay more attention to the possibility that humans influenced clouds. One noted astrophysicist had long had an interest in aerosols after seeing the effects of the Dust Bowl in the 1930s. He noticed changes in the skies over Boulder, Colorado, and pointed out jet airplane contrails, predicting correctly that they would spread, thin, and become indistinguishable from cirrus clouds. The apparent ability of aircraft to create cirrus clouds revealed the possibility that they might be causing climate changes along major air routes. Others questioned the possibility of anthropogenic activity as the source of pollution settling on polar ice caps. At the time, the theory did not receive much credence.

Around 1970, the British meteorologist Hubert Horace Lamb’s Dust Veil Index established a connection between dust and lower temperatures. While scientific studies at this time did not yet find strong evidence for an increase in global turbidity, they did document regional hazes that spread in a radius of up to one thousand kilometers or more from industrial centers. The scientific debate shifted from the existence of anthropogenic dust to the effects of that dust. It remained a subject of controversy whether and under what circumstances dust would cool or heat the climate, especially after a spacecraft on Mars in 1971 found that a large dust storm had caused substantial warming of the Martian atmosphere.

Deadly droughts in Africa and South Asia in 1973 caused public concern about climate change, but it was not confirmed that sulfate pollution had contributed to the Sahel drought until the end of the century. Scientific publications in the mid- to late 1970s discussed warming or cooling effects without reaching accord, although a majority felt that greenhouse warming would dominate. At this time, only a few researchers noted that aerosol pollution might cancel out some greenhouse warming and thus temporarily mask its effects. Others denied that industrial pollution could mitigate the caused by emissions.

The 1980s brought the realization that additional factors contributed to climate and climate change. For example, climate scientists generally treated aerosols as a globally uniform background, largely of natural origin, when, in fact, different aerosol properties obtained in different regions based on relative humidity. Many questions remained.

By 1990, it was acknowledged that from one-fourth to one-half of all tropospheric aerosol particles were anthropogenic. These included industrial soot and sulfates, smoke from fires, and dust from overgrazed or semiarid land turned to agriculture. Impressive advances in laboratory instrumentation made possible much more sophisticated satellite observations, greatly increasing the resolution of climate models. The key paper establishing the net effect of aerosols on Earth’s heat balance was published in the early 1990s; it concluded that radiation scattering due to anthropogenic sulfate emissions was counterbalancing CO₂-related greenhouse warming in the Northern Hemisphere.

It became apparent that earlier climate projections might be erroneous, because they had not factored in sulfate aerosol increases. Climatologists redoubled their efforts to produce accurate models and projections of Earth’s climate. In 1995, for the first time, new results that took into account aerosol influence yielded a consistent and plausible picture of twentieth-century climate. According to this picture, industrial pollution had temporarily depressed Northern Hemisphere temperatures around the mid-century. A 2008 study found that black carbon aerosols had exerted a much greater warming effect than had been earlier estimated, because the combined effects of black carbon with sulfate aerosols had not been taken into account. It seemed clear that reducing sooty emissions would both delay global warming and benefit public health.

According to NASA, aerosol pollution was significantly reduced during the COVID-19 global pandemic in 2020 because air travel, driving, electricity use, and industrial activity decreased sharply because of lockdowns. While this caused a slight warming, the air was cleaner and clearer. This reduction in aerosol pollution was believed to have saved roughly 11,000 lives in Europe and 77,000 lives in China. In some cities, people had clear views for the first time in years.

Context

A number of aerosol specialists have questioned whether they have underestimated the cooling effect of aerosols. If they had, they would have underestimated those aerosols’ restraint of greenhouse warming, significantly underestimating the extent of global warming in the absence of anthropogenic aerosol pollution. Much uncertainty remains, and each new study introduces new complexities. It seems clear that reducing sooty emissions would both delay global warming and benefit public health, yet nagging questions remain: Since aerosols and clouds, unlike gases, are not distributed evenly throughout the atmosphere, uniform samples cannot be obtained. Further, the properties of clouds and aerosols are incompletely understood, and scientists are only beginning to understand some of the interactions that take place between aerosols, clouds, and climate. Thus, these interactions have not yet been incorporated into their models. According to NASA in 2023, aerosols such as dust may influence the formation of ice particles in colder clouds, and this was an area of active research.

Key Concepts

• cloud condensation nuclei: atmospheric particles such as dust that can form the centers of water droplets, increasing cloud cover
• Dust Veil Index: a numerical index that quantifies the impact of a volcanic eruption’s release of dust and aerosols
• global dimming: the effect produced when clouds reflect the Sun’s rays back to space
• stratosphere: part of the atmosphere just above the troposphere that can hold large amounts of aerosols produced by volcanic eruptions for many months
• troposphere: location in the lower atmosphere where the majority of aerosols form a thin haze before being washed out of the air by rain

Bibliography

"Aerosols: Small Particles with Big Climate Effects." NASA, June 12, 2023, science.nasa.gov/science-research/earth-science/climate-science/aerosols-small-particles-with-big-climate-effects/. Accessed 10 Dec. 2024.

Levin, Z., and William R. Cotton, eds. Aerosol Pollution Impact on Precipitation: A Scientific Review. New York: Springer, 2009.

Massel, Stanislaw R. Ocean Waves Breaking and Marine Aerosol Fluxes. Sapot, Poland: Institute of Oceanology of the Polish Academy of Sciences, 2007.

Spury, Kvetoslav R., ed. Aerosol Chemical Processes in the Environment. New York: Lewis, 2000.