Aerosol
An aerosol is a suspension of fine solid particles or liquid droplets in air or another gas. This mixture includes both the particles and the suspending gas, which is usually air. Frederick G. Donnan presumably first used the term aerosol during World War I to describe an aero-solution. He applied this term analogously to hydrosol, a colloid system with water as the dispersed medium. The liquid or solid particles in an aerosol have diameters typically less than 1 micrometer. Larger particles with a significant settling speed make the mixture a suspension. Meteorologists and climatologists often refer to them as particle matter. The classification in sizes ranges like PM2.5 or PM10. PM2.5 refers to the mass of particles with sizes between 0 and 2.5 micrometers. PM10 covers sizes between 0 and 10 micrometers. These size ranges play a role in ascertaining harmful effects on human health.
Examples of natural aerosols are fog, mist, and dust from geological or biological sources. Certain infections spread by means of small droplets in the breath, sometimes called bioaerosols. Volcanic aerosol forms in the stratosphere after an eruption as droplets of sulfuric acid that can prevail for up to two years. Desert dust consists of mineral particles blown to high altitudes. Human-made sulfate aerosols come primarily from burning oil and coal. Other examples include perfume from atomizers and medical treatments for respiratory illnesses. Dust from material or industrial sources creates visible plumes. Smoke rises from combustion processes while irrigation mist sprays over crops. Particulate air pollutants linger in urban ecosystems alongside cigarette smoke. Mist from the discharge at hydroelectric dams adds moisture to the atmosphere. Soot or fumes in car exhaust contribute to local pollution levels.
Volcanic eruptions release large amounts of sulphuric acid into the atmosphere. These gases represent aerosols and eventually return to earth as acid rain. When aerosols absorb pollutants, it facilitates the deposition of pollutants to the surface of the earth. This has the potential to be damaging to both the environment and human health. Aerosols interact with the Earth's energy budget in two ways, directly and indirectly. A direct effect is that aerosols scatter and absorb incoming solar radiation. This will mainly lead to a cooling of the surface but may also contribute to warming. Ship tracks are clouds that form around the exhaust released by ships into still ocean air. Water molecules collect around tiny particles from exhaust to form a cloud seed. The resulting clouds resemble long strings over the ocean where wind blows the ship's exhaust. The warming caused by human-produced greenhouse gases has been somewhat offset by the cooling effect of human-produced aerosols. In 2020, regulations on fuel significantly cut sulfur dioxide emissions from international shipping by approximately 80%. This led to an unexpected global geoengineering termination shock.
Aerosol particles with an effective diameter smaller than 10 micrometers can enter the bronchi. Those with an effective diameter smaller than 2.5 micrometers can enter as far as the gas exchange region in the lungs. This can be hazardous to human health. Aerosols in the 20 micrometer range show a particularly long persistence time in air conditioned rooms due to their jet rider behavior. Such aerosols may contribute to the pandemic since this size is most effectively adsorbed in the human nose. The location of deposition of aerosol particles within the respiratory system strongly determines the health effects of exposure. PM10 corresponds to the thoracic convention while PM2.5 corresponds to the high-risk respirable convention. The United States Environmental Protection Agency replaced older standards for particulate matter based on Total Suspended Particulate with another standard based on PM10 in 1987. They then introduced standards for PM2.5 in 1997.
The particle size distribution defines the relative amounts of particles sorted according to size. A more widely chosen log-normal distribution gives the number frequency. This distribution has no negative values and covers a wide range of values. It fits many observed size distributions reasonably well. For low values of the Reynolds number, true for most aerosol motion, Stokes' law describes the force of resistance on a solid spherical particle in a fluid. To account for failure of assumptions for small particles less than 1 micrometer, one can introduce the Cunningham correction factor. Including this factor allows calculation of terminal velocity of a particle undergoing gravitational settling in still air. Aerosol dynamics explains the evolution of complete aerosol populations over time. A differential equation called the Aerosol General Dynamic Equation characterizes the evolution of the number density of particles. Change in time equals convective transport plus brownian diffusion plus gas-particle interactions plus coagulation plus migration by external forces. The Knudsen number of the particle define three different dynamical regimes that govern behavior.
Aerosols can either be measured in-situ or with remote sensing techniques either ground-based or airborne-based. Some available in situ measurement techniques include Aerosol mass spectrometer and Differential mobility analyzer. Electrical aerosol spectrometers and Aerodynamic particle sizers provide data on concentration. Condensation particle counters detect tiny particles while Epiphaniometers measure surface area. Remote sensing approaches include Sun photometer and Lidar systems. Imaging spectroscopy helps identify composition from space. Two alternative size-selective criteria often used in atmospheric monitoring are PM10 and PM2.5. PM10 is defined by ISO as particles which pass through a size-selective inlet with a 50% efficiency cut-off at 10 micrometers aerodynamic diameter. PM2.5 refers to particles passing through an inlet with a 50% efficiency cut-off at 2.5 micrometers. To measure the respirable fraction, a pre-collector excludes larger particles before air reaches the sampling filter. It is common to use cyclonic separation for the pre-collector but other techniques include impactors and large pore membrane filters.
Common questions
What is the definition of an aerosol?
An aerosol is a suspension of fine solid particles or liquid droplets in air or another gas. This mixture includes both the particles and the suspending gas, which is usually air.
When did Frederick G. Donnan first use the term aerosol?
Frederick G. Donnan presumably first used the term aerosol during World War I to describe an aero-solution. He applied this term analogously to hydrosol, a colloid system with water as the dispersed medium.
How do PM2.5 and PM10 differ in particle size ranges?
PM2.5 refers to the mass of particles with sizes between 0 and 2.5 micrometers while PM10 covers sizes between 0 and 10 micrometers. These size ranges play a role in ascertaining harmful effects on human health.
Why are volcanic aerosols significant for climate change?
Volcanic aerosols form in the stratosphere after an eruption as droplets of sulfuric acid that can prevail for up to two years. They interact with Earth's energy budget by scattering and absorbing incoming solar radiation, leading to surface cooling.
What regulations were implemented regarding shipping emissions in 2020?
In 2020, regulations on fuel significantly cut sulfur dioxide emissions from international shipping by approximately 80%. This led to an unexpected global geoengineering termination shock due to reduced cooling effects from aerosols.