How Do Aerosols Affect Global Warming?

What are aerosols? Are they pollutants? What effect do they have on albedo, cloud formation and climate change? Do they reflect sunlight? On balance do aerosols have a warming or cooling effect? Will their numbers increase or decrease in the future? What effects does aerosol air pollution have on human health? We answer these questions and more.
Crop burning creates aerosols
Crop burning near Lahore, Pakistan, contributes to large quantities of aerosols. Photo: © Imran Shahid/imaggeo.egu.eu

What Are Aerosols?

Aerosols are small particles (solid or liquid) emitted into the atmosphere by a variety of natural and industrial processes and then carried around the globe by wind and weather.

Strictly speaking, an ‘aerosol’ (aero-solution) is defined as a particle or collection of particles (solid or liquid) suspended in the air, or a particular gas. It includes both the particle and the gas in which it is suspended. By contrast, the term ‘particulate matter‘ (scientific name for particles) refers only to the suspended solid or liquid matter.

However, in common parlance – and also in this article – the two terms are used interchangeably to mean any solid or liquid particles of matter in the air.

Aerosols can be microscopically small, or big enough to see with the naked eye. They can be natural or man-made. They are believed to exert a slight cooling effect on Earth’s temperature, which helps to restrain global warming, but they are also a major contributor to air pollution and pose a serious threat to human respiratory health.

Most aerosols (about 90 percent) are natural in origin. Common types include: dust, sea salt, phytoplankton-produced dimethyl sulfide (the main source of sulfate aerosol in the Arctic atmosphere), smoke from wildfires, or ash particles from volcanic eruption. Man-made aerosols include: diesel exhaust particles, bits of fine black carbon, droplets of acid, aerosolized nitrogen products from agriculture, soot from the combustion of fossil fuels, or from the burning of wood or other biomass. 1

Aerosol optical depth (AOD) – how much direct sunlight is prevented from reaching the ground by dust, haze and other aerosol particles – varies considerably. At present, the largest values are in Asia and the tropical regions of Africa.

Aerosol particles are either emitted or blown directly up into the atmosphere (primary aerosols), or else they’re produced in the atmosphere from chemical reactions with other particles or gases. (secondary aerosols).

Although certain classes of industrial emissions – like sulfate aerosols emitted by coal-fired power plants – have been reduced, cars, trucks, motorcycles, planes, factories and fires continue to release more particles into the atmosphere.

So although man-made aerosols only account for 10 percent of the total number of atmospheric particles, they can exert a huge effect on the air quality of cities and industrial areas. For example, the amount of “PM2.5” – particulate matter less than 2.5 microns across (about thirty times smaller than that of a human hair) has increased by about 60 percent since the period 1850-1900.

According to the State of Global Air Report (2019), issued by the Health Effects Institute, aerosol air pollution accounts for nearly 3 million deaths, or 5.2 percent of all global deaths. More than half of these deaths (52 percent) occurred in China and India. Major sources of PM2.5 in India include the burning of solid fuels in homes, dust from roads and construction, and industrial burning of coal in coal-fired power plants.

Aerosols Distribution Worldwide: Map
Global distribution of aerosols. Intense colors indicate a thick layer of aerosols. Yellow areas are mostly coarser particles, like dust, while red areas are mostly fine aerosols, such as smoke and pollution. Photo: NASA map by Robert Simmon, based on MODIS data from NASA Earth Observations

What Affect Do Aerosols Have On Climate Change?

Aerosols affect Earth’s climate in two ways: by reflecting sunlight back into space, or by affecting the way clouds form.

Aerosols Reflect Sunlight

When it comes to the reflection of light, not all aerosols behave in the same way. Most of them reflect it, but some absorb it. The difference is usually down to the composition and color of the aerosol particles. Typically, brighter-colored particles reflect sunlight (cooling the atmosphere), while darker aerosols absorb it (warming atmosphere). 2 Note also that small particles are far more successful at reflecting sunlight than large ones. This is because large aerosols fall out of the air much faster. 3

Thus, lighter-colored dust reflects light, darker dust absorbs it. Salt reflects sunlight as do sulfate and nitrate particles. For example, the Mt. Pinatubo volcanic eruption (1991) discharged billions of tons of fine-grained magma along with 20 million tons of sulfur dioxide into the stratosphere. Here the magma formed a dust cloud that entered the stratosphere, and then spread out to cover the entire Northern Hemisphere. Meantime, the sulfur dioxide formed a haze of light-reflecting sulfuric acid droplets. Strong stratospheric winds spread these aerosol particles around the globe cooling the planet by about 0.6°C (1°F) for two years. 4 5

Note that, while aerosols in the troposphere (the lower atmosphere) are usually removed by rainfall within a week, those in the dryer stratosphere, can remain active for several years. 6

The Pinatubo eruption and its impact on Earth’s climate system was a dramatic example of the impact that aerosols can have. It also marked a landmark in the scientific study of aerosols as an influential but complex factor in climate change.

Black carbon (soot) absorbs sunlight – and in this respect acts as a heating agent much like a greenhouse gas – but it also shades and cools the surface below. 7

Organic or brown carbon behaves in less understood ways. To begin with, it absorbs large amounts of ultraviolet light, but less, visible light. At the same time, at least one study has shown that wintertime emissions of brown carbon aerosols absorb almost as much sunlight as black carbon, but summertime emissions absorb far less. 8 The latter is consistent with an Indian study which showed that black carbon absorbs 70 percent of light compared to brown carbon’s 15 percent. 9

The light absorption capacity of black carbon was re-confirmed in 1991, by a study conducted during the burning of the Kuwaiti oil fields during the invasion of Kuwait by Iraq. The study showed that surface temperatures beneath the burning oil fires were up to 10°C cooler than temperatures of other local areas under clear skies. 10

In areas with high levels of black carbon aerosols (e.g. from coal burning or wood fires), such as rural India, as much as half the warming effect of greenhouse gas emissions may be masked by atmospheric clouds of aerosols. 11

Aerosols Act As Cloud Condensation Nuclei

When water evaporates and is carried up into the atmosphere by rising currents of air, it finds it easier to condense into water droplets when it has something to condense upon, like an aerosol. In this way, aerosols have an indirect effect on the Earth’s radiation budget. Aerosols act as cloud condensation nuclei and cause clouds to contain more and smaller water droplets. Clouds in general – being white on top – have a high albedo (light reflective capability), but clouds with more and smaller water droplets have a higher albedo and reflect solar radiation more efficiently. 12

However, at night, clouds make Earth’s temperature warmer by trapping heat radiating from the surface. (A cloudy night is invariably much warmer than a cloudless night.)

So by stimulating the formation of more clouds, aerosols affect both sides of Earth’s radiative equilibrium. (See also: How do Clouds Affect Climate?)

Satellite image of smoke and haze aerosols over India, NASA
Satellite image of fog and haze in the Indo-Gangetic Plain. This plain, located south of the Himalaya, is affected by dense fog nearly every year in the winter months. This fog is considered radiational fog as it forms overnight as the air near the ground cools. It sometimes becomes smog when it mixes with smoke. Photo: © Joshua Stevens, NASA Earth Observatory

Does Black Carbon Lower The Albedo Of Ice And Snow?

Scientists used to think that black carbon aerosols darkened the snow cover on Arctic sea ice, thus lowering its albedo. 13 However, a recent study of satellite data from 1982 to 2014, confirmed that although the Arctic temperatures increased almost three times faster than the global average, the resulting loss of surface albedo (about 1.4 percent per decade) was due almost entirely to melting ice, not soot absorption. 14

Despite the findings of this study, large amounts of atmospheric aerosols continue to land on glaciers and ice sheets around the world, reducing their reflective capacity and thus their ability to limit the amount of solar heat reaching the surface of the Earth.

On Balance Do Aerosols Have A Warming Or Cooling Effect?

On balance, scientists believe that aerosols end up cooling the planet. In addition to current evidence showing this to be the case, there are three historical factors that are relevant. 

First, volcanoes have a history of spewing out gases and dust particles into the atmosphere which have proceeded to shade the planet from incoming solar heat. The cooling effect can last up to years, depending on the characteristics of the eruption and how high the ash cloud rises. Usually, small particles of ash and rock dust combine to form a dark cloud that shades and cools the area below, albeit it temporarily. Although most of them are washed out of the sky by rainfall within a few days, the smallest particles are so light that they can travel vast distances, forming a haze and blocking sunlight as they circle the globe.

Second, current theory surrounding the Cretaceous-Tertiary extinction event, which led to the die-off of the dinosaurs and most other species, 65.5 million years ago, suggests it was caused by a giant asteroid or comet which slammed into the Yucatan Peninsula, in Mexico. The impact, it is believed, caused an explosion of dust and debris obliterating the sun and throwing the planet into a cold darkness, from which few species emerged alive. If true, this theory is a powerful illustration of the shading and cooling power of aerosols.

Third, from around 1940 to the late 1950s the Earth’s average temperature fell by about 0.2°C. Later, from about 1960 to 1990, a gradual fall in the amount of sunlight reaching the Earth’s surface was observed – known as global dimming. 15 Scientists have attributed both these events to the effect of anthropogenic aerosols. This view was seemingly confirmed when the decline of aerosols (from about 1990 onwards) coincided with increased global brightness and a significant warming of the Earth’s surface.

What Are Primary And Secondary Aerosols?

Aerosols are categorized as either primary (emitted) or secondary (produced). Thus, if particles are emitted directly into the atmosphere (e.g. blown by the wind or emitted from a factory chimney) they are called “primary aerosols”. If they are produced in the atmosphere as a result of a chemical or physical reaction with (say) a gas or another aerosol, they are called “secondary aerosols”.

Examples of primary particles are those emitted through volcanic eruptions, wind-driven sea salt, soil, or mineral dust, fuelwood or biomass burning, or incomplete combustion of coal and other fossil fuels. Also included are biological aerosols such as pollen particles, or fragments of plants and microorganisms. Inorganic primary aerosols (often larger than 1 micron) like sea spray, mineral dust, and volcano ash usually have short atmospheric life spans measured in days. Carbonaceous primary aerosols – that is, those derived from organic carbon (OC) or solid black carbon (BC) – are generally smaller than 1 micron and therefore have longer lifetimes. For more on this point, please see our article: Environmental Effects of Fossil Fuels.

Examples of secondary aerosol particles include, first and foremost, tropospheric ozone (O3) which is produced in the atmosphere from exhaust fume gases (like nitrous oxide, carbon monoxide and volatile organic compounds) and sunlight. Secondary aerosols typically range in size from a few nanometers up to 1 micron and have a lifespan of days to weeks. Secondary aerosols often derive from reactions with sulphates, nitrates, and organic carbon. 16

What Is The Aerosol Content Of Air?

Clean continental air typically contains fewer than 3,000 particles per cubic centimeter; polluted continental air typically has 50,000 particles per cubic centimeter (of which roughly 66 percent are soot, and the remainder water-soluble). Urban air typically contains 160,000 particles per cubic centimeter (mostly soot, with only 20 percent water-soluble). Desert air has about 2,300 particles per cubic centimeter on average (almost all water-soluble). Clean ocean air usually contains about 1,500 particles per cubic centimeter (about all water-soluble). The lowest aerosol values occur over the oceans near the subtropical highs. Here, they average 600 particles per cubic centimeter, but occasionally below 300 particles per cubic centimeter). Arctic air has about 6600 particles per cubic centimeter (including 5,300 soot) and on the Antarctic plateau only 43 particles per cubic centimeter occur (about all sulphate). 17

What Effects Do Aerosols Have On Air Quality And Human Health?

Pollution from microscopic aerosols is associated with a wide range of serious non-communicable diseases (NCDs), including strokes, coronary heart disease, kidney disease, chronic obstructive pulmonary disease (COPD) – including chronic bronchitis, emphysema and chronic obstructive airways disease – type 2 diabetes, hypertension, lung cancer, pneumonia, certain birth defects, and dementia.

Some shocking statistics on the health effects of air pollution:
Specifically, air pollution accounts for 41 percent of deaths globally from COPD, 20 percent of type 2 diabetes deaths, 19 percent of deaths from lung cancer, 16 percent of deaths from ischemic heart disease, and 11 percent of deaths from stroke. 18

The World Health Organization (WHO) say that, in 2016, airborne particles – notably microscopic particles less than 2.5 microns in diameter, known as PM2.5 – accounted for more than 4 million premature deaths.

According to a 2019 report from Swiss company IQAir, Indian and Pakistani cities again led the world in PM2.5 pollution, in 2019. Twenty-one out of the top 30 most polluted cities are located in India. Five are in Pakistan. South Korea had the worst PM2.5 pollution of all OECD countries. 18

Crop burning in India causes smog
Burning rice residues in Punjab, India, prior to the wheat season. The smog creates a smog problem in the area. Photo: CIAT/CC BY-SA 2.0

So, irrespective of whether they raise or lower Earth’s temperature, aerosols are implicated in a range of adverse health conditions affecting people around the world.

Aerosol pollution is estimated to kill 107,000 people a year in the United States, at a cost to society of $886 billion. 19

These fatalities illustrate the global nature of the aerosol problem, since they are caused in part by prevailing winds from East Asia that cross the Pacific to the West Coast of America carrying unhealthy levels of sulphates, carbon grit and industrial chemicals. 20

Almost one third of the air over San Francisco and Los Angeles can be traced directly to Asia, and this air contains 75 percent of the black carbon aerosol pollution that reaches the West Coast. 21 22

Will Man-Made Aerosol Emissions Increase Or Decrease In The Future?

Man-made aerosol emissions have altered significantly during the 20th century in quantity, composition, and geographical distribution. Although it is difficult to predict future trends, scientists anticipate that in the West, aerosol emissions will continue to reduce as technology improves, the use of electric vehicles (EVs) increases, and the composition of marine fuel changes.

In the East, where most of the aerosol emissions currently occur, worries about air quality as well as rapid technological development are both thought to be acting to lower emissions. Which is why some experts are saying that “the age of aerosols may soon be over”. 23

A similar view is reflected in the aerosol projections considered for the 5th Assessment Report of the Intergovernmental Panel on Climate Change. All projections assume major reductions relative to those in 2000: beginning in Europe, then in America, then, after 2020, in Asia. All forecasts are predicting a cut in half of man-made aerosol emissions by 2100.

References

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