An aerosol is a suspension of fine solid particles or liquid droplets in air or another gas. Aerosols can be generated from natural or human causes. The scientific term aerosol refers to the system consisting of particles in gas, and not to the particulate matter alone. The liquid or solid particles in an aerosol have diameters typically less than 10 μm. Larger particles, with a significant settling speed, make the mixture an unstable suspension, although the distinction is somewhat arbitrary.
Prominent examples of natural aerosols are sea spray, haze from biological sources, windblown dust, smoke from natural wildfires; particles directly emitted by volcanoes or formed from volcanic emissions of gases; and fog, mist, and clouds. Other examples include pollens and fungus spores and particles ejected from coughing or sneezing.
Anthropogenic aerosols include smoke composed of particles directly emitted from combustion sources (including cigarette smoke), secondary aerosols in which the particles are formed by chemical reactions in air, notably photochemical smog, windblown dust from disturbed soils. Other examples include comminuted material from grinding processes, dust from material or industrial sources, irrigation mist, sprayed pesticides, and mist from the discharge at hydroelectric dams.
Commonly, the term aerosol is used to refer to a dispensing system that delivers a consumer product in aerosol form from a spray can. Other examples of such aerosols include perfume from atomizers, and medical treatments for respiratory illnesses.
Aerosol effects
Exposure to aerosols results in deleterious effects on human health through deposition in the pulmonary system.
Epidemiologcal studies show that prolonged, repeated exposure raises the risk of cardiorespiratory death and chronic respiratory illness. Short-term particle pollution exposure can aggravate pre-existing pulmonary and cardiovascular diseases increasing illness and death. Arfin, T., Pillai, A.M., Mathew, N., Tirpude, A., Bang, R. and Mondal, P., 2023. An overview of atmospheric aerosol and their effects on human health. Environmental Science and Pollution Research, 30(60), pp.125347-125369.
Notable episodes of extraordinarily high loadings of aerosol particulate matter include the Great Smog of London of 1952 and the 1948 Donora smog event of 1952, the great smog of Delhi, India of 2016 Air pollution in Delhi.
Certain infections are transmitted by means of small droplets in the breath, sometimes called bioaerosols.
Several types of atmospheric aerosol have a significant effect on Earth's climate: volcanic, desert dust, sea-salt, originating from natural and anthropogenic sources. Volcanic aerosol forms in the stratosphere after an eruption as droplets of sulfuric acid that can prevail for up to two years, and reflect sunlight, lowering temperature. Desert dust, mineral particles blown to high altitudes, absorb heat and may be responsible for inhibiting storm cloud formation. Key effects are scattering and absorption of solar radiation, influencing Earth's radiation budget and modification of cloud properties.
Human-made sulfate aerosols, primarily from burning sulfur-containing oil and coal, affect the behavior of clouds by reducing the number concentration of cloud drops, increasing the persistence and reflectivity of clouds and also cloud dynamics and precipitation formation.
A manifestation of aerosol influences on clouds, readily seen in satellite images, is Ship tracks, clouds that form around the exhaust released by ships into the still ocean air. Enhanced concentrations of aerosol particles result in greater reflectivity of clouds in which the exhaust is embedded. In ship tracks, the clouds 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.
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%, leading to an unexpected global geoengineering termination shock.
Uptake of gaseous pollutants, mainly sulfur and nitrogen oxides, by aerosols and clouds increases the deposition of acidic pollutants to the surface of the earth in precipitation . with resultant environmental degradation Acid Rain.
Definitions
Aerosol is defined as a suspension system of solid or liquid particles in a gas. An aerosol includes both the particles and the suspending gas, which is usually air. Meteorologists and climatologists often refer to them as particle matter, while 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, and PM10 for sizes between 0 and 10 micrometers. is useful in the field of atmospheric pollution as these size range play a role in ascertain the harmful effects in human health. Frederick G. Donnan presumably first used the term aerosol during World War I to describe an aero-solution, clouds of microscopic particles in air. This term developed analogously to the term hydrosol, a colloid system with water as the dispersed medium. Primary aerosols contain particles introduced directly into the gas; secondary aerosols form through gas-to-particle conversion.
Key aerosol groups include sulfates, organic carbon, black carbon, nitrates, mineral dust, and sea salt, they usually clump together to form a complex mixture. Various types of aerosol, classified according to physical form and how they were generated, include dust, fume, mist, smoke and fog.
There are several measures of aerosol concentration. Environmental science and environmental health often use the mass concentration (M), defined as the mass of particulate matter per unit volume, in units such as μg/m3. Also commonly used is the number concentration (N), the number of particles per unit volume, in units such as number per m3 or number per cm3.
Particle size has a major influence on particle properties, and the aerosol particle radius or diameter (dp) is a key property used to characterise aerosols.
Aerosols vary in their dispersity. A monodisperse aerosol, producible in the laboratory, contains particles of uniform size. Most aerosols, however, as polydisperse colloidal systems, exhibit a range of particle sizes. Liquid droplets are almost always nearly spherical, but scientists use an equivalent diameter to characterize the properties of various shapes of solid particles, some very irregular. The equivalent diameter is the diameter of a spherical particle with the same value of some physical property as the irregular particle. The equivalent volume diameter (de) is defined as the diameter of a sphere of the same volume as that of the irregular particle. Also commonly used is the aerodynamic diameter, da.
Generation and applications People generate aerosols for various purposes, including: as test aerosols for calibrating instruments, performing research, and testing sampling equipment and air filters; to deliver deodorants, paints, and other consumer products in sprays; for dispersal and agricultural application for medical treatment of respiratory disease; and in fuel injection systems and other combustion technology.
Some devices for generating aerosols are: Aerosol spray Atomizer nozzle or nebulizer Electrospray Electronic cigarette Vibrating orifice aerosol generator (VOAG)
In the atmosphere
Several types of atmospheric aerosol have a significant effect on Earth's climate: volcanic, desert dust, sea-salt, that originating from biogenic sources and human-made. Volcanic aerosol forms in the stratosphere after an eruption as droplets of sulfuric acid that can prevail for up to two years, and reflect sunlight, lowering temperature. Desert dust, mineral particles blown to high altitudes, absorb heat and may be responsible for inhibiting storm cloud formation. Human-made sulfate aerosols, primarily from burning oil and coal, affect the behavior of clouds.
Although all hydrometeors, solid and liquid, can be described as aerosols, a distinction is commonly made between such dispersions (i.e. clouds and precipitation) containing relatively large water drops or ice crystals, and aerosol particles. The atmosphere of Earth contains aerosols of various types and concentrations, including quantities of: natural inorganic materials: fine dust, sea salt, or water droplets natural organic materials: smoke, pollen, spores, or bacteria anthropogenic products of combustion such as: smoke, ashes or dusts
Aerosols can be found in urban ecosystems in various forms, for example: Dust Cigarette smoke Mist from aerosol spray cans Soot or fumes in car exhaust
The presence of aerosols in the Earth's atmosphere can influence its climate, as well as human health.
Effects
Volcanic eruptions release large amounts of sulphuric acid, hydrogen sulfide and hydrochloric acid into the atmosphere. These gases represent aerosols and eventually return to earth as acid rain, having a number of adverse effects on the environment and human life.
When aerosols absorb pollutants, it facilitates the deposition of pollutants to the surface of the earth as well as to bodies of water. 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. E.g., a direct effect is that aerosols scatter and absorb incoming solar radiation. This will mainly lead to a cooling of the surface (solar radiation is scattered back to space) but may also contribute to a warming of the surface (caused by the absorption of incoming solar energy). This will be an additional element to the greenhouse effect and therefore contributing to the global climate change. The indirect effects refer 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. There is evidence to suggest that anthropogenic aerosols actually offset the effects of greenhouse gases in some areas, which is why the Northern Hemisphere shows slower surface warming than the Southern Hemisphere, although that just means that the Northern Hemisphere will absorb the heat later through ocean currents bringing warmer waters from the South.Anthropogenic Aerosols, Greenhouse Gases, and the Uptake, Transport, and Storage of Excess Heat in the Climate System On a global scale however, aerosol cooling decreases greenhouse-gases-induced heating without offsetting it completely.GIEC AR6 WG1 - Figure SPM.2 https://www.ipcc.ch/report/sixth-assessment-report-working-group-i/ Ship tracks are clouds that form around the exhaust released by ships into the still ocean air. Water molecules collect around the tiny particles (aerosols) from exhaust to form a cloud seed. More and more water accumulates on the seed until a visible cloud is formed. In the case of ship tracks, the 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.
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%, leading to an unexpected global geoengineering termination shock.