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Aerosol
Aerosol is a suspension of fine solid particles or liquid droplets in air or another gas, yet for most of human history, this invisible mixture remained a scientific abstraction rather than a tangible reality. The term itself was likely coined by Frederick G. Donnan during World War I to describe aero-solutions, clouds of microscopic particles in air, drawing an analogy to the term hydrosol which describes a colloid system with water as the dispersed medium. While the word now evokes images of spray cans and perfume, the scientific definition encompasses everything from the sulfuric acid droplets formed in the stratosphere after a volcanic eruption to the dust particles blown from the Sahara that can travel thousands of miles across the Atlantic. These particles, typically less than 1 micrometer in diameter, are so small that they defy the laws of gravity that govern larger objects, instead behaving like gas molecules and diffusing rapidly through Brownian motion. The distinction between an aerosol and a suspension is often blurred, as larger particles with significant settling speeds make the mixture a suspension, yet the scientific community continues to debate the precise boundary where one becomes the other. This ambiguity is not merely semantic; it dictates how we measure the health risks of air pollution and how we model the Earth's climate response to human activity.
Volcanoes And Dust
When Mount Pinatubo erupted in the Philippines on the 15th of June 1991, it injected massive amounts of sulfur dioxide into the stratosphere, which rapidly converted into droplets of sulfuric acid that formed a global aerosol layer. This volcanic aerosol persisted for up to two years, reflecting sunlight back into space and lowering global temperatures by approximately 0.5 degrees Celsius, a phenomenon that temporarily masked the warming effects of greenhouse gases. Unlike the cooling effect of volcanic eruptions, desert dust from geological sources absorbs heat and may be responsible for inhibiting storm cloud formation, creating a complex interplay between different types of atmospheric aerosols. Sea-salt aerosols, originating from the ocean, and biogenic aerosols, released by living organisms, further complicate the picture by interacting with water vapor to form clouds. These natural aerosols are not merely passive components of the atmosphere; they actively shape the Earth's energy budget by scattering and absorbing incoming solar radiation. The presence of these particles in the atmosphere can influence climate patterns for decades, as seen in the aftermath of the 1815 eruption of Mount Tambora, which caused the Year Without a Summer in 1816. The scientific understanding of these natural aerosols has evolved from simple observations of fog and mist to sophisticated models that account for their chemical composition and physical properties.
Who coined the term aerosol and when was it first used?
Frederick G. Donnan likely coined the term aerosol during World War I to describe aero-solutions and clouds of microscopic particles in air. The word was created by drawing an analogy to the term hydrosol which describes a colloid system with water as the dispersed medium.
What happened to global temperatures after the Mount Pinatubo eruption on the 15th of June 1991?
The eruption injected massive amounts of sulfur dioxide into the stratosphere which rapidly converted into droplets of sulfuric acid that formed a global aerosol layer. This volcanic aerosol persisted for up to two years and lowered global temperatures by approximately 0.5 degrees Celsius while reflecting sunlight back into space.
How did the 2020 fuel regulations affect global warming trends in 2023 and 2024?
Regulations on fuel significantly cut sulfur dioxide emissions from international shipping by approximately 80% in 2020. This sudden reduction removed the cooling effect that had been masking the true extent of global warming and resulted in unexpectedly large global warming in 2023 and 2024.
When did the United States Environmental Protection Agency introduce standards for PM2.5?
The United States Environmental Protection Agency replaced the older standards for particulate matter based on Total Suspended Particulate with another standard based on PM10 in 1987. The agency then introduced standards for PM2.5 in 1997 to reflect the growing understanding of the health risks associated with different sizes of aerosol particles.
Why do scientists use a log-normal distribution to model aerosol size distributions?
Scientists use a log-normal distribution because the normal distribution usually does not suitably describe particle size distributions due to the skewness associated with a long tail of larger particles. The log-normal distribution has no negative values and can cover a wide range of values to fit many observed size distributions reasonably well.
How do aerosols influence the Earth's energy budget through indirect effects?
Aerosols interfere with formations that interact directly with radiation by modifying the size of cloud particles in the lower atmosphere. This process changes the way clouds reflect and absorb light thereby modifying the Earth's energy budget and influencing climate patterns.
In the vast expanse of the ocean, invisible lines of clouds known as ship tracks stretch for hundreds of miles, formed by the exhaust released by ships into the still ocean air. Water molecules collect around the tiny particles, or aerosols, from the ship's exhaust to form a cloud seed, and as more water accumulates on the seed, a visible cloud is formed. These cloud seeds are stretched over a long narrow path where the wind has blown the ship's exhaust, so the resulting clouds resemble long strings over the ocean, a phenomenon that has been observed from space. The formation of ship tracks demonstrates how human activity can alter the atmosphere in ways that were previously unknown, as the aerosols from ship exhaust act as cloud condensation nuclei, modifying the size of cloud particles and changing the way clouds reflect and absorb light. This process, known as the indirect effect of aerosols, has significant implications for the Earth's energy budget, as it can lead to a cooling effect that offsets the warming caused by greenhouse gases. The discovery of ship tracks in the 1970s and 1980s provided a natural experiment that helped scientists understand the complex interactions between aerosols and clouds, leading to new insights into the climate system.
The Faustian Bargain
The warming caused by human-produced greenhouse gases has been somewhat offset by the cooling effect of human-produced aerosols, creating what James Hansen called a Faustian bargain. In 2020, regulations on fuel significantly cut sulfur dioxide emissions from international shipping by approximately 80%, leading to an unexpected global geoengineering termination shock. This sudden reduction in aerosol emissions removed the cooling effect that had been masking the true extent of global warming, resulting in unexpectedly large global warming in 2023 and 2024. The regulation of aerosols improved air quality, but the aerosols' cooling effect became inadequate to temper the increasing warming effect of greenhouse gases, explaining the rapid rise in global temperatures. This paradox highlights the delicate balance between human health and climate change, as the same particles that cause respiratory problems also help to cool the planet. The scientific community has struggled to predict the consequences of this shift, as the removal of aerosols has revealed the full extent of the warming caused by greenhouse gases. The situation has led to a reevaluation of climate models and a greater understanding of the role of aerosols in the Earth's climate system.
The Breath Of Life
Aerosol particles with an effective diameter smaller than 10 micrometers can enter the bronchi, while the ones with an effective diameter smaller than 2.5 micrometers can enter as far as the gas exchange region in the lungs, which can be hazardous to human health. The location of deposition of aerosol particles within the respiratory system strongly determines the health effects of exposure to such aerosols, leading to the invention of aerosol samplers that select a subset of the aerosol particles that reach certain parts of the respiratory system. The inhalable fraction of particles, defined as the proportion of particles originally in the air that can enter the nose or mouth, depends on external wind speed and direction and on the particle-size distribution by aerodynamic diameter. The thoracic fraction is the proportion of the particles in ambient aerosol that can reach the thorax or chest region, while the respirable fraction is the proportion of particles in the air that can reach the alveolar region. The United States Environmental Protection Agency replaced the older standards for particulate matter based on Total Suspended Particulate with another standard based on PM10 in 1987 and then introduced standards for PM2.5 in 1997, reflecting the growing understanding of the health risks associated with different sizes of aerosol particles. The study of aerosols has thus become a critical component of public health, as the particles that we breathe can have profound effects on our well-being.
The Science Of Size
For a monodisperse aerosol, a single number, the particle diameter, suffices to describe the size of the particles, but more complicated particle-size distributions describe the sizes of the particles in a polydisperse aerosol. The normal distribution usually does not suitably describe particle size distributions in aerosols because of the skewness associated with a long tail of larger particles, so scientists often use a log-normal distribution to model the aerosol size distributions. The log-normal distribution has no negative values, can cover a wide range of values, and fits many observed size distributions reasonably well, providing a practical advantage for modeling the aerosols size distributions. The particle size distribution can be approximated using various methods, including the moment method, modal/sectional method, and Monte Carlo method, each of which has its own advantages and limitations. The study of aerosol size distributions has led to a deeper understanding of the physical properties of aerosols, such as their terminal velocity, aerodynamic diameter, and dynamic shape factor. These properties are critical for predicting the behavior of aerosols in the atmosphere and for understanding their impact on human health and the environment.
The Cloud Maker
Aerosols interact with the Earth's energy budget in two ways, directly and indirectly, with the indirect effects referring to the aerosol interfering with formations that interact directly with radiation. For example, they are able to modify the size of the cloud particles in the lower atmosphere, thereby changing the way clouds reflect and absorb light and therefore modifying the Earth's energy budget. The presence of aerosols in the atmosphere can influence its climate, as well as human health, as they can act as cloud condensation nuclei and modify the properties of clouds. The study of aerosols has thus become a critical component of climate science, as the particles that we breathe can have profound effects on the Earth's climate system. The interaction between aerosols and clouds is a complex process that involves the condensation of water vapor onto the surface of aerosol particles, leading to the formation of cloud droplets. The size and composition of the aerosol particles determine the properties of the cloud droplets, which in turn affect the way clouds reflect and absorb light. The study of aerosols has thus led to a deeper understanding of the Earth's climate system and the role of aerosols in shaping the planet's climate.