An aerosol is a
suspension
Suspension or suspended may refer to:
Science and engineering
* Suspension (topology), in mathematics
* Suspension (dynamical systems), in mathematics
* Suspension of a ring, in mathematics
* Suspension (chemistry), small solid particles suspende ...
of fine
solid
Solid is one of the State of matter#Four fundamental states, four fundamental states of matter (the others being liquid, gas, and Plasma (physics), plasma). The molecules in a solid are closely packed together and contain the least amount o ...
particles or
liquid
A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure. As such, it is one of the four fundamental states of matter (the others being solid, gas, a ...
droplets
A drop or droplet is a small column of liquid, bounded completely or almost completely by free surfaces. A drop may form when liquid accumulates at the lower end of a tube or other surface boundary, producing a hanging drop called a pendant d ...
in
air
The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing f ...
or another
gas
Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma).
A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...
. Aerosols can be natural or
anthropogenic. Examples of natural aerosols are
fog or
mist,
dust
Dust is made of fine particles of solid matter. On Earth, it generally consists of particles in the atmosphere that come from various sources such as soil lifted by wind (an aeolian process), volcanic eruptions, and pollution. Dust in homes ...
, forest
exudates
An exudate is a fluid emitted by an organism through pores or a wound, a process known as exuding or exudation.
''Exudate'' is derived from ''exude'' 'to ooze' from Latin ''exsūdāre'' 'to (ooze out) sweat' (''ex-'' 'out' and ''sūdāre'' 'to ...
, and
geyser steam
Steam is a substance containing water in the gas phase, and sometimes also an aerosol of liquid water droplets, or air. This may occur due to evaporation or due to boiling, where heat is applied until water reaches the enthalpy of vaporization ...
. Examples of anthropogenic aerosols include
particulate
Particulates – also known as atmospheric aerosol particles, atmospheric particulate matter, particulate matter (PM) or suspended particulate matter (SPM) – are microscopic particles of solid or liquid matter suspended in the air. The te ...
air pollutants
Air pollution is the contamination of air due to the presence of substances in the atmosphere that are harmful to the health of humans and other living beings, or cause damage to the climate or to materials. There are many different types ...
, mist from the discharge at
hydroelectric dams
Hydroelectricity, or hydroelectric power, is electricity generated from hydropower (water power). Hydropower supplies one sixth of the world's electricity, almost 4500 TWh in 2020, which is more than all other renewable sources combined an ...
,
irrigation mist,
perfume from atomizers,
smoke
Smoke is a suspension of airborne particulates and gases emitted when a material undergoes combustion or pyrolysis, together with the quantity of air that is entrained or otherwise mixed into the mass. It is commonly an unwanted by-produc ...
, steam from a kettle,
sprayed pesticides, and medical treatments for respiratory illnesses. When a person inhales the contents of a vape pen or
e-cigarette
An electronic cigarette is an electronic device that simulates tobacco smoking. It consists of an atomizer, a power source such as a battery, and a container such as a cartridge or tank. Instead of smoke, the user inhales vapor. As such ...
, they are inhaling an
anthropogenic aerosol.
The liquid or solid particles in an aerosol have diameters typically less than
1 μm (larger particles with a significant settling speed make the mixture a
suspension
Suspension or suspended may refer to:
Science and engineering
* Suspension (topology), in mathematics
* Suspension (dynamical systems), in mathematics
* Suspension of a ring, in mathematics
* Suspension (chemistry), small solid particles suspende ...
, but the distinction is not clear-cut). In general conversation, ''aerosol'' often refers to a
dispensing system that delivers a consumer product from a can.
Diseases can spread by means of small droplets in the
breath
Breathing (or ventilation) is the process of moving air into and from the lungs to facilitate gas exchange with the internal environment, mostly to flush out carbon dioxide and bring in oxygen.
All aerobic creatures need oxygen for cellular ...
, sometimes called
bioaerosols Bioaerosols (short for biological aerosols) are a subcategory of particles released from terrestrial and marine ecosystems into the atmosphere. They consist of both living and non-living components, such as fungi, pollen, bacteria and viruses. Comm ...
.
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 usually refer them as particle matter - PM2.5 or PM10, depending on their size.
Frederick G. Donnan presumably first used the term ''aerosol'' during
World War I
World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, the United States, and the Ottoman Empire, with fightin ...
to describe an aero-
solution
Solution may refer to:
* Solution (chemistry), a mixture where one substance is dissolved in another
* Solution (equation), in mathematics
** Numerical solution, in numerical analysis, approximate solutions within specified error bounds
* Soluti ...
, clouds of microscopic particles in air. This term developed analogously to the term
hydrosol
Herbal distillates, also known as floral waters, hydrosols, hydrolates, herbal waters, and essential waters, are aqueous products of hydrodistillation. They are colloidal suspensions of essential oils as well as water-soluble components obtained ...
, 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
Environmental science is an interdisciplinary academic field that integrates physics, biology, and geography (including ecology, chemistry, plant science, zoology, mineralogy, oceanography, limnology, soil science, geology and physical geograp ...
and
environmental health
Environmental health is the branch of public health concerned with all aspects of the natural and built environment affecting human health. In order to effectively control factors that may affect health, the requirements that must be met in ...
often use the ''
mass concentration'' (''M''), defined as the mass of particulate matter per unit volume, in units such as μg/m
3. Also commonly used is the ''
number concentration
A number is a mathematical object used to count, measure, and label. The original examples are the natural numbers 1, 2, 3, 4, and so forth. Numbers can be represented in language with number words. More universally, individual numbers can ...
'' (''N''), the number of particles per unit volume, in units such as number per m
3 or number per cm
3.
Particle size has a major influence on particle properties, and the aerosol particle radius or diameter (''d
p'') is a key property used to characterise aerosols.
Aerosols vary in their
dispersity
In chemistry, the dispersity is a measure of the heterogeneity of sizes of molecules or particles in a mixture. A collection of objects is called uniform if the objects have the same size, shape, or mass. A sample of objects that have an inconsi ...
. 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'' (''d
e'') 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
An aerosol is a suspension of fine solid particles or liquid droplets in air or another gas. Aerosols can be natural or anthropogenic. Examples of natural aerosols are fog or mist, dust, forest exudates, and geyser steam. Examples of anthropo ...
, ''d
a''.
Size distribution
For a monodisperse aerosol, a single number—the particle diameter—suffices to describe the size of the particles. However, more complicated
particle-size distribution
The particle-size distribution (PSD) of a powder, or granular material, or particles dispersed in fluid, is a list of values or a mathematical function that defines the relative amount, typically by mass, of particles present according to size. Si ...
s describe the sizes of the particles in a polydisperse aerosol. This distribution defines the relative amounts of particles, sorted according to size. One approach to defining the particle size distribution uses a list of the sizes of every particle in a sample. However, this approach proves tedious to ascertain in aerosols with millions of particles and awkward to use. Another approach splits the size range into intervals and finds the number (or proportion) of particles in each interval. These data can be presented in a
histogram with the area of each bar representing the proportion of particles in that size bin, usually normalised by dividing the number of particles in a bin by the width of the interval so that the area of each bar is proportionate to the number of particles in the size range that it represents. If the width of the bins
tends to zero, the frequency function is:
:
where
:
is the diameter of the particles
:
is the fraction of particles having diameters between
and
+
:
is the frequency function
Therefore, the area under the frequency curve between two sizes a and ''b'' represents the total fraction of the particles in that size range:
:
It can also be formulated in terms of the total number density ''N'':
:
Assuming spherical aerosol particles, the aerosol surface area per unit volume (''S'') is given by the second
moment:
:
And the third moment gives the total volume concentration (''V'') of the particles:
:
The particle size distribution can be approximated. The
normal distribution
In statistics, a normal distribution or Gaussian distribution is a type of continuous probability distribution for a real-valued random variable. The general form of its probability density function is
:
f(x) = \frac e^
The parameter \mu ...
usually does not suitably describe particle size distributions in aerosols because of the
skewness
In probability theory and statistics, skewness is a measure of the asymmetry of the probability distribution of a real-valued random variable about its mean. The skewness value can be positive, zero, negative, or undefined.
For a unimodal ...
associated with a
long tail
In statistics and business, a long tail of some probability distribution, distributions of numbers is the portion of the distribution having many occurrences far from the "head" or central part of the distribution. The distribution could involv ...
of larger particles. Also for a quantity that varies over a large range, as many aerosol sizes do, the width of the distribution implies negative particles sizes, which is not physically realistic. However, the normal distribution can be suitable for some aerosols, such as test aerosols, certain
pollen
Pollen is a powdery substance produced by seed plants. It consists of pollen grains (highly reduced microgametophytes), which produce male gametes (sperm cells). Pollen grains have a hard coat made of sporopollenin that protects the gametophyt ...
grains and
spore
In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, f ...
s.
A more widely chosen
log-normal distribution
In probability theory, a log-normal (or lognormal) distribution is a continuous probability distribution of a random variable whose logarithm is normally distributed. Thus, if the random variable is log-normally distributed, then has a norma ...
gives the number frequency as:
:
where:
:
is the
standard deviation of the size distribution and
:
is the
arithmetic mean
In mathematics and statistics, the arithmetic mean ( ) or arithmetic average, or just the ''mean'' or the ''average'' (when the context is clear), is the sum of a collection of numbers divided by the count of numbers in the collection. The colle ...
diameter.
The log-normal distribution has no negative values, can cover a wide range of values, and fits many observed size distributions reasonably well.
Other distributions sometimes used to characterise particle size include: the
Rosin-Rammler distribution, applied to coarsely dispersed dusts and sprays; the Nukiyama–Tanasawa distribution, for sprays of extremely broad size ranges; the
power function distribution, occasionally applied to atmospheric aerosols; the
exponential distribution, applied to powdered materials; and for cloud droplets, the Khrgian–Mazin distribution.
Physics
Terminal velocity of a particle in a fluid
For low values of the
Reynolds number
In fluid mechanics, the Reynolds number () is a dimensionless quantity that helps predict fluid flow patterns in different situations by measuring the ratio between inertial and viscous forces. At low Reynolds numbers, flows tend to be domi ...
(<1), true for most aerosol motion,
Stokes' law describes the force of resistance on a solid spherical particle in a fluid. However, Stokes' law is only valid when the velocity of the gas at the surface of the particle is zero. For small particles (< 1 μm) that characterize aerosols, however, this assumption fails. To account for this failure, one can introduce the
Cunningham correction factor In fluid dynamics, the Cunningham correction factor or Cunningham slip correction factor is used to account for noncontinuum effects when calculating the drag on small particles. The derivation of Stokes' law, which is used to calculate the drag for ...
, always greater than 1. Including this factor, one finds the relation between the resisting force on a particle and its velocity:
:
where
:
is the resisting force on a spherical particle
:
is the dynamic
viscosity
The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water.
Viscosity quantifies the inte ...
of the gas
:
is the particle velocity
:
is the Cunningham correction factor.
This allows us to calculate the
terminal velocity
Terminal velocity is the maximum velocity (speed) attainable by an object as it falls through a fluid ( air is the most common example). It occurs when the sum of the drag force (''Fd'') and the buoyancy is equal to the downward force of grav ...
of a particle undergoing gravitational settling in still air. Neglecting
buoyancy
Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the p ...
effects, we find:
:
where
:
is the terminal settling velocity of the particle.
The terminal velocity can also be derived for other kinds of forces. If Stokes' law holds, then the resistance to motion is directly proportional to speed. The constant of proportionality is the mechanical mobility (''B'') of a particle:
:
A particle traveling at any reasonable initial velocity approaches its terminal velocity
exponentially
Exponential may refer to any of several mathematical topics related to exponentiation, including:
*Exponential function, also:
**Matrix exponential, the matrix analogue to the above
*Exponential decay, decrease at a rate proportional to value
*Expo ...
with an ''e''-folding time equal to the relaxation time:
:
where:
:
is the particle speed at time t
:
is the final particle speed
:
is the initial particle speed
To account for the effect of the shape of non-spherical particles, a correction factor known as the ''dynamic shape factor'' is applied to Stokes' law. It is defined as the ratio of the resistive force of the irregular particle to that of a spherical particle with the same volume and velocity:
:
where:
:
is the dynamic shape factor
Aerodynamic diameter
The aerodynamic diameter of an irregular particle is defined as the diameter of the spherical particle with a density of 1000 kg/m
3 and the same settling velocity as the irregular particle.
Neglecting the slip correction, the particle settles at the terminal velocity proportional to the square of the aerodynamic diameter, ''d
a'':
:
where
:
= standard particle density (1000 kg/m
3).
This equation gives the aerodynamic diameter:
:
One can apply the aerodynamic diameter to particulate pollutants or to inhaled drugs to predict where in the respiratory tract such particles deposit. Pharmaceutical companies typically use aerodynamic diameter, not geometric diameter, to characterize particles in inhalable drugs.
Dynamics
The previous discussion focused on single aerosol particles. In contrast, ''aerosol dynamics'' explains the evolution of complete aerosol populations. The concentrations of particles will change over time as a result of many processes. External processes that move particles outside a volume of gas under study include
diffusion
Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...
, gravitational settling, and
electric charge
Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons respe ...
s and other external forces that cause particle migration. A second set of processes internal to a given volume of gas include particle formation (nucleation), evaporation, chemical reaction, and coagulation.
A
differential equation
In mathematics, a differential equation is an equation that relates one or more unknown functions and their derivatives. In applications, the functions generally represent physical quantities, the derivatives represent their rates of change, an ...
called the ''Aerosol General Dynamic Equation'' (GDE) characterizes the evolution of the number density of particles in an aerosol due to these processes.
:
Change in time = Convective transport +
brownian diffusion + gas-particle interactions + coagulation + migration by external forces
Where:
:
is number density of particles of size category
:
is the particle velocity
:
is the particle
Stokes-Einstein diffusivity
Diffusivity is a rate of diffusion, a measure of the rate at which particles or heat or fluids can spread.
It is measured differently for different mediums.
Diffusivity may refer to:
*Thermal diffusivity, diffusivity of heat
*Diffusivity of mass: ...
:
is the particle velocity associated with an external force
Coagulation
As particles and droplets in an aerosol collide with one another, they may undergo coalescence or aggregation. This process leads to a change in the aerosol particle-size distribution, with the mode increasing in diameter as total number of particles decreases. On occasion, particles may shatter apart into numerous smaller particles; however, this process usually occurs primarily in particles too large for consideration as aerosols.
Dynamics regimes
The
Knudsen number
The Knudsen number (Kn) is a dimensionless number defined as the ratio of the molecular mean free path length to a representative physical length scale. This length scale could be, for example, the radius of a body in a fluid. The number is name ...
of the particle define three different dynamical regimes that govern the behaviour of an aerosol:
:
where
is the
mean free path
In physics, mean free path is the average distance over which a moving particle (such as an atom, a molecule, or a photon) travels before substantially changing its direction or energy (or, in a specific context, other properties), typically as a ...
of the suspending gas and
is the diameter of the particle.
For particles in the ''free molecular regime'', ''K
n'' >> 1; particles small compared to the mean free path of the suspending gas.
In this regime, particles interact with the suspending gas through a series of "ballistic" collisions with gas molecules. As such, they behave similarly to gas molecules, tending to follow streamlines and diffusing rapidly through Brownian motion. The mass flux equation in the free molecular regime is:
:
where ''a'' is the particle radius, ''P''
∞ and ''P
A'' are the pressures far from the droplet and at the surface of the droplet respectively, ''k
b'' is the Boltzmann constant, ''T'' is the temperature, ''C
A'' is mean thermal velocity and ''α'' is mass accommodation coefficient. The derivation of this equation assumes constant pressure and constant diffusion coefficient.
Particles are in the ''continuum regime'' when K
n << 1.
In this regime, the particles are big compared to the mean free path of the suspending gas, meaning that the suspending gas acts as a continuous fluid flowing round the particle.
The molecular flux in this regime is:
:
where ''a'' is the radius of the particle ''A'', ''M
A'' is the molecular mass of the particle ''A'', ''D
AB'' is the diffusion coefficient between particles ''A'' and ''B'', ''R'' is the ideal gas constant, ''T'' is the temperature (in absolute units like kelvin), and ''P
A∞'' and ''P
AS'' are the pressures at infinite and at the surface respectively.
The ''transition regime'' contains all the particles in between the free molecular and continuum regimes or ''K
n'' ≈ 1. The forces experienced by a particle are a complex combination of interactions with individual gas molecules and macroscopic interactions. The semi-empirical equation describing mass flux is:
:
where ''I''
cont is the mass flux in the continuum regime. This formula is called the Fuchs-Sutugin interpolation formula. These equations do not take into account the heat release effect.
Partitioning
Aerosol partitioning theory governs
condensation on and
evaporation
Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. High concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when humidi ...
from an aerosol surface, respectively. Condensation of mass causes the mode of the particle-size distributions of the aerosol to increase; conversely, evaporation causes the mode to decrease. Nucleation is the process of forming aerosol mass from the condensation of a gaseous precursor, specifically a
vapor
In physics, a vapor (American English) or vapour (British English and Canadian English; see spelling differences) is a substance in the gas phase at a temperature lower than its critical temperature,R. H. Petrucci, W. S. Harwood, and F. G. Her ...
. Net condensation of the vapor requires supersaturation, a
partial pressure
In a mixture of gases, each constituent gas has a partial pressure which is the notional pressure of that constituent gas as if it alone occupied the entire volume of the original mixture at the same temperature. The total pressure of an ideal gas ...
greater than its
vapor pressure
Vapor pressure (or vapour pressure in English-speaking countries other than the US; see spelling differences) or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phas ...
. This can happen for three reasons:
# Lowering the temperature of the system lowers the vapor pressure.
# Chemical reactions may increase the partial pressure of a gas or lower its vapor pressure.
# The addition of additional vapor to the system may lower the equilibrium vapor pressure according to
Raoult's law.
There are two types of nucleation processes. Gases preferentially condense onto surfaces of pre-existing aerosol particles, known as heterogeneous nucleation. This process causes the diameter at the mode of particle-size distribution to increase with constant number concentration. With sufficiently high supersaturation and no suitable surfaces, particles may condense in the absence of a pre-existing surface, known as homogeneous nucleation. This results in the addition of very small, rapidly growing particles to the particle-size distribution.
Activation
Water coats particles in aerosols, making them ''activated'', usually in the context of forming a cloud droplet (such as natural cloud seeding by aerosols from trees in a forest). Following the
Kelvin equation
The Kelvin equation describes the change in vapour pressure due to a curved liquid–vapor interface, such as the surface of a droplet. The vapor pressure at a convex curved surface is higher than that at a flat surface. The Kelvin equation is de ...
(based on the curvature of liquid droplets), smaller particles need a higher ambient
relative humidity
Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present.
Humidity dep ...
to maintain equilibrium than larger particles do. The following formula gives
relative humidity
Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present.
Humidity dep ...
at equilibrium:
:
where
is the
saturation vapor pressure
Vapor pressure (or vapour pressure in English-speaking countries other than the US; see spelling differences) or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phase ...
above a particle at equilibrium (around a curved liquid droplet), ''p''
0 is the saturation vapor pressure (flat surface of the same liquid) and ''S'' is the saturation ratio.
Kelvin equation
The Kelvin equation describes the change in vapour pressure due to a curved liquid–vapor interface, such as the surface of a droplet. The vapor pressure at a convex curved surface is higher than that at a flat surface. The Kelvin equation is de ...
for saturation vapor pressure above a curved surface is:
:
where ''r
p'' droplet radius, ''σ'' surface tension of droplet, ''ρ'' density of liquid, ''M'' molar mass, ''T'' temperature, and ''R'' molar gas constant.
Solution to the general dynamic equation
There are no general
solutions
Solution may refer to:
* Solution (chemistry), a mixture where one substance is dissolved in another
* Solution (equation), in mathematics
** Numerical solution, in numerical analysis, approximate solutions within specified error bounds
* Soluti ...
to the general dynamic equation (GDE); common methods used to solve the general dynamic equation include:
* Moment method
* Modal/sectional method, and
* Quadrature method of moments/Taylor-series expansion method of moments, and
* Monte Carlo method.
Generation and applications
People generate aerosols for various purposes, including:
* as test aerosols for
calibrating
In measurement technology and metrology, calibration is the comparison of measurement values delivered by a device under test with those of a calibration standard of known accuracy. Such a standard could be another measurement device of know ...
instruments, performing research, and testing sampling equipment and air filters;
* to deliver
deodorant
A deodorant is a substance applied to the body to prevent or mask body odor due to bacterial breakdown of perspiration or vaginal secretions, for example in the armpits, groin, or feet. A subclass of deodorants, called antiperspirants, prevents ...
s,
paint
Paint is any pigmented liquid, liquefiable, or solid mastic composition that, after application to a substrate in a thin layer, converts to a solid film. It is most commonly used to protect, color, or provide texture. Paint can be made in many ...
s, and other consumer products in sprays;
* for dispersal and agricultural application
* for medical treatment of
respiratory disease
Respiratory diseases, or lung diseases, are pathological conditions affecting the organs and tissues that make gas exchange difficult in air-breathing animals. They include conditions of the respiratory tract including the trachea, bronchi, bro ...
; and
* in
fuel injection
Fuel injection is the introduction of fuel in an internal combustion engine, most commonly automotive engines, by the means of an injector. This article focuses on fuel injection in reciprocating piston and Wankel rotary engines.
All comp ...
systems and other
combustion
Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combusti ...
technology.
Some devices for generating aerosols are:
*
Aerosol spray
Aerosol spray is a type of dispensing system which creates an aerosol mist of liquid particles. It comprises a can or bottle that contains a payload, and a propellant under pressure. When the container's valve is opened, the payload is forced out ...
*
Atomizer nozzle
Atomization refers to breaking bonds in some substance to obtain its constituent atoms in gas phase. By extension, it also means separating something into fine particles, for example: process of breaking bulk liquids into small droplets.
Atomizati ...
or
nebulizer
In medicine, a nebulizer (American English) or nebuliser (British English) is a drug delivery device used to administer medication in the form of a mist inhaled into the lungs. Nebulizers are commonly used for the treatment of asthma, cystic fibro ...
*
Electrospray
The name electrospray is used for an apparatus that employs electricity to disperse a liquid or for the fine aerosol resulting from this process. High voltage is applied to a liquid supplied through an emitter (usually a glass or metallic capilla ...
*
Electronic cigarette
* Vibrating orifice aerosol generator (VOAG)
Stability of generated aerosol particles
Stability of nanoparticle agglomerates is critical for estimating size distribution of aerosolized particles from nano-powders or other sources. At nanotechnology workplaces, workers can be exposed via inhalation to potentially toxic substances during handling and processing of nanomaterials. Nanoparticles in the air often form agglomerates due to attractive inter-particle forces, such as van der Waals force or electrostatic force if the particles are charged. As a result, aerosol particles are usually observed as agglomerates rather than individual particles. For exposure and risk assessments of airborne nanoparticles, it is important to know about the size distribution of aerosols. When inhaled by humans, particles with different diameters are deposited in varied locations of the central and periphery respiratory system. Particles in nanoscale have been shown to penetrate the air-blood barrier in lungs and be translocated into secondary organs in the human body, such as the brain, heart and liver. Therefore, the knowledge on stability of nanoparticle agglomerates is important for predicting the size of aerosol particles, which helps assess the potential risk of them to human bodies.
Different experimental systems have been established to test the stability of airborne particles and their potentials to deagglomerate under various conditions. A comprehensive system recently reported is able to maintain robust aerosolization process and generate aerosols with stable number concentration and mean size from nano-powders. The deagglomeration potential of various airborne nanomaterials can be also studied using critical orifices. In addition, an impact fragmentation device was developed to investigate bonding energies between particles.
A standard deagglomeration testing procedure could be foreseen with the developments of the different types of existing systems. The likeliness of deagglomeration of aerosol particles in occupational settings can be possibly ranked for different nanomaterials if a reference method is available. For this purpose, inter-laboratory comparison of testing results from different setups could be launched in order to explore the influences of system characteristics on properties of generated nanomaterials aerosols.
Detection
Aerosol can either be measured
in-situ or with
remote sensing
Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object, in contrast to in situ or on-site observation. The term is applied especially to acquiring information about Earth ...
techniques.
''In situ'' observations
Some available in situ measurement techniques include:
*
Aerosol mass spectrometer
Aerosol mass spectrometry is the application of mass spectrometry to the analysis of the composition of aerosol particles. Aerosol particles are defined as solid and liquid particles suspended in a gas (air), with size range of 3 nm to 100 ...
(AMS)
*
Differential mobility analyzer
Ion mobility spectrometry (IMS) is an analytical technique used to separate and identify ionized molecules in the gas phase based on their mobility in a carrier buffer gas. Though heavily employed for military or security purposes, such as detect ...
(DMA)
*
Electrical aerosol spectrometer Electrical aerosol spectrometry (EAS) is a technique for measurement of the number-size distribution of aerosol using a combination of electrical charging and multiple solid state electrometer detectors. The technique combines both diffusion and fi ...
(EAS)
*
Aerodynamic particle sizer (APS)
*
Aerodynamic aerosol classifier An aerodynamic aerosol classifier (AAC) is an embodiment of a measurement technique for classifying aerosol particles according to their aerodynamic diameters.
The technique allows online size classification of particles without requiring them to ...
(AAC)
*
Wide range particle spectrometer
WIDE or Wide may refer to:
*Wide (cricket)
*Wide and narrow data, terms used to describe two different presentations for tabular data
*WIDE Project, Widely Integrated Distributed Environment
*Wide-angle Infinity Display Equipment
*WIDE-LP, a radio ...
(WPS)
*
Micro-Orifice Uniform Deposit Impactor(MOUDI)
*
Condensation particle counter
A condensation particle counter or CPC is a particle counter that detects and counts aerosol particles by first enlarging them by using the particles as nucleation centers to create droplets in a supersaturated gas. Aerosol Measurement: Princip ...
(CPC)
*
Epiphaniometer
*
Electrical low pressure impactor
Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described ...
(ELPI)
*
Aerosol particle mass-analyser (APM)
*
Centrifugal Particle Mass Analyser (CPMA)
Remote sensing approach
Remote sensing approaches include:
*
Sun photometer
A sun photometer is a type of photometer conceived in such a way that it points at the sun.
Recent sun photometers are automated instruments incorporating a sun-tracking unit, an appropriate optical
system, a electromagnetic spectrum , spectrally ...
*
Lidar
Lidar (, also LIDAR, or LiDAR; sometimes LADAR) is a method for determining ranges (variable distance) by targeting an object or a surface with a laser and measuring the time for the reflected light to return to the receiver. It can also be ...
*
Imaging spectroscopy
In imaging spectroscopy (also hyperspectral imaging or spectral imaging) each pixel of an image acquires many bands of light intensity data from the spectrum, instead of just the three bands of the RGB color model. More precisely, it is the simult ...
Size selective sampling
Particles can deposit in the
nose
A nose is a protuberance in vertebrates that houses the nostrils, or nares, which receive and expel air for respiration alongside the mouth. Behind the nose are the olfactory mucosa and the sinuses. Behind the nasal cavity, air next passes th ...
,
mouth,
pharynx
The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its st ...
and
larynx
The larynx (), commonly called the voice box, is an organ in the top of the neck involved in breathing, producing sound and protecting the trachea against food aspiration. The opening of larynx into pharynx known as the laryngeal inlet is about ...
(the head airways region), deeper within the respiratory tract (from the
trachea
The trachea, also known as the windpipe, is a Cartilage, cartilaginous tube that connects the larynx to the bronchi of the lungs, allowing the passage of air, and so is present in almost all air-breathing animals with lungs. The trachea extends ...
to the
terminal bronchioles
The bronchioles or bronchioli (pronounced ''bron-kee-oh-lee'') are the smaller branches of the bronchial airways in the lower respiratory tract. They include the terminal bronchioles, and finally the respiratory bronchioles that mark the start o ...
), or in the
alveolar region. The location of deposition of aerosol particles within the respiratory system strongly determines the health effects of exposure to such aerosols. This phenomenon led people to invent aerosol samplers that select a subset of the aerosol particles that reach certain parts of the respiratory system. Examples of these subsets of the particle-size distribution of an aerosol, important in occupational health, include the inhalable, thoracic, and respirable fractions. The fraction that can enter each part of the respiratory system depends on the deposition of particles in the upper parts of the airway. 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. The respirable fraction is the proportion of particles in the air that can reach the alveolar region. To measure the respirable fraction of particles in air, a pre-collector is used with a sampling filter. The pre-collector excludes particles as the airways remove particles from inhaled air. The sampling filter collects the particles for measurement. It is common to use
cyclonic separation
Cyclonic separation is a method of removing particulates from an air, gas or liquid stream, without the use of filters, through vortex separation. When removing particulate matter from liquid, a hydrocyclone is used; while from gas, a gas cyclone ...
for the pre-collector, but other techniques include impactors, horizontal
elutriators, and large pore
membrane filter
Membrane technology encompasses the scientific processes used in the construction and application of membranes. Membranes are used to facilitate the transport or rejection of substances between mediums, and the mechanical separation of gas and li ...
s.
Two alternative size-selective criteria, often used in atmospheric monitoring, are PM
10 and PM
2.5. PM
10 is defined by
ISO
ISO is the most common abbreviation for the International Organization for Standardization.
ISO or Iso may also refer to: Business and finance
* Iso (supermarket), a chain of Danish supermarkets incorporated into the SuperBest chain in 2007
* Iso ...
as ''particles which pass through a size-selective inlet with a 50% efficiency cut-off at 10 μm aerodynamic diameter'' and PM
2.5 as ''particles which pass through a size-selective inlet with a 50% efficiency cut-off at 2.5 μm aerodynamic diameter''. PM
10 corresponds to the "thoracic convention" as defined in ISO 7708:1995, Clause 6; PM
2.5 corresponds to the "high-risk respirable convention" as defined in ISO 7708:1995, 7.1. The
United States Environmental Protection Agency
The Environmental Protection Agency (EPA) is an independent executive agency of the United States federal government tasked with environmental protection matters. President Richard Nixon proposed the establishment of EPA on July 9, 1970; it be ...
replaced the older standards for particulate matter based on Total Suspended Particulate with another standard based on PM
10 in 1987 and then introduced standards for PM
2.5 (also known as fine particulate matter) in 1997.
Atmospheric
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
Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular formu ...
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
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, Rain and snow mixed, sleet, snow, ice pellets, ...
, solid and liquid, can be described as aerosols, a distinction is commonly made between such dispersions (i.e. clouds) containing activated drops and crystals, and aerosol particles. The
atmosphere of Earth
The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing fo ...
contains aerosols of various types and concentrations, including quantities of:
* natural
inorganic
In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemist ...
materials: fine dust, sea salt, or water droplets
* natural
organic materials: smoke,
pollen
Pollen is a powdery substance produced by seed plants. It consists of pollen grains (highly reduced microgametophytes), which produce male gametes (sperm cells). Pollen grains have a hard coat made of sporopollenin that protects the gametophyt ...
,
spore
In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, f ...
s, or
bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...
*
anthropogenic products of combustion such as: smoke,
ashes or dusts
Aerosols can be found in urban
ecosystem
An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syste ...
s in various forms, for example:
* Dust
* Cigarette smoke
* Mist from
aerosol spray
Aerosol spray is a type of dispensing system which creates an aerosol mist of liquid particles. It comprises a can or bottle that contains a payload, and a propellant under pressure. When the container's valve is opened, the payload is forced out ...
cans
*
Soot
Soot ( ) is a mass of impure carbon particles resulting from the incomplete combustion of hydrocarbons. It is more properly restricted to the product of the gas-phase combustion process but is commonly extended to include the residual pyrolysed ...
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
Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. The unde ...
and
hydrochloric acid
Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid
Acid strength is the tendency of an acid, symbol ...
into the atmosphere. These gases represent aerosols and eventually return to earth as
acid rain, having a number of
adverse effects
An adverse effect is an undesired harmful effect resulting from a medication or other intervention, such as surgery. An adverse effect may be termed a "side effect", when judged to be secondary to a main or therapeutic effect. The term complica ...
on the environment and human life.
[
]
*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
The greenhouse effect is a process that occurs when energy from a planet's host star goes through the planet's atmosphere and heats the planet's surface, but greenhouse gases in the atmosphere prevent some of the heat from returning directly ...
and therefore contributing to the global climate change.
::The ''indirect'' effects refer to the aerosols 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. On a global scale however, aerosol cooling decreases greenhouse-gases-induced heating without offsetting it completely.
*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 in the 20 μm range show a particularly long persistence time in air conditioned rooms due to their "jet rider" behaviour (move with air jets, gravitationally fall out in slowly moving air); as this aerosol size is most effectively adsorbed in the human nose,
the primordial infection site in
COVID-19
Coronavirus disease 2019 (COVID-19) is a contagious disease caused by a virus, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The first known case was COVID-19 pandemic in Hubei, identified in Wuhan, China, in December ...
, such aerosols may contribute to the pandemic.
*Aerosol particles with an effective diameter smaller than 10 μm can enter the bronchi, while the ones with an effective diameter smaller than 2.5 μm can enter as far as the gas exchange region in the lungs,
[
] which can be hazardous to human health.
See also
*
Aerogel
*
Aeroplankton
Aeroplankton (or aerial plankton) are tiny lifeforms that float and drift in the air, carried by wind. Most of the living things that make up aeroplankton are very small to microscopic in size, and many can be difficult to identify because of ...
*
Aerosol spray
Aerosol spray is a type of dispensing system which creates an aerosol mist of liquid particles. It comprises a can or bottle that contains a payload, and a propellant under pressure. When the container's valve is opened, the payload is forced out ...
, the spraying device
*
Bioaerosol Bioaerosols (short for biological aerosols) are a subcategory of particles released from terrestrial and marine ecosystems into the atmosphere. They consist of both living and non-living components, such as fungi, pollen, bacteria and viruses. Comm ...
*
Deposition (Aerosol physics)
In the physics of aerosols, deposition is the process by which aerosol particles collect or deposit themselves on solid surfaces, decreasing the concentration of the particles in the air. It can be divided into two sub-processes: ''dry'' and '' ...
*
Global dimming
Global dimming is the reduction in the amount of global direct irradiance at the Earth's surface that has been observed since systematic measurements began in the 1950s. The effect varies by location, but worldwide it has been estimated to be of ...
*
Nebulizer
In medicine, a nebulizer (American English) or nebuliser (British English) is a drug delivery device used to administer medication in the form of a mist inhaled into the lungs. Nebulizers are commonly used for the treatment of asthma, cystic fibro ...
*
Monoterpene
Monoterpenes are a class of terpenes that consist of two isoprene units and have the molecular formula C10H16. Monoterpenes may be linear (acyclic) or contain rings (monocyclic and bicyclic). Modified terpenes, such as those containing oxygen func ...
*
Particulates
Particulates – also known as atmospheric aerosol particles, atmospheric particulate matter, particulate matter (PM) or suspended particulate matter (SPM) – are microscopic particles of solid or liquid matter suspended in the air. The ter ...
References
Citations
Sources
; Works cited
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Further reading
*
*
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External links
International Aerosol Research AssemblyAmerican Association for Aerosol Research(see chapters on aerosol sampling)
{{Authority control
Colloidal chemistry
Colloids
Fluid dynamics
Liquids
Physical chemistry
Pollution
Solids