An aerosol (abbreviation of "aero-solution") is a suspension
of fine solid
particles or liquid droplets
or another gas
[Hinds, 1999, p. 3]
Aerosols can be natural or anthropogenic
. Examples of natural aerosols are fog
, forest exudates
and geyser steam
. Examples of anthropogenic aerosols are particulate air pollutants
The liquid or solid particles have diameters typically less than 1 μm
; larger particles with a significant settling speed make the mixture a suspension
, but the distinction is not clear-cut. In general conversation, ''aerosol'' usually refers to an aerosol spray
that delivers a consumer product from a can or similar container. Other technological applications of aerosols include dispersal of pesticides, medical treatment of respiratory illnesses, and combustion technology.
[Hidy, 1984, p. 254.] Diseases can also spread
by means of small droplets in the breath
, also called aerosols (or sometimes bioaerosols
Aerosol science covers generation and removal of aerosols, technological application of aerosols, effects of aerosols on the environment and people, and other topics.
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.
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.
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
For a monodisperse aerosol, a single number—the particle diameter—suffices to describe the size of the particles. However, more complicated particle-size distribution
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 complete size range into intervals and finds the number (or proportion) of particles in each interval. One then can visualize these data 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
, one gets the frequency function:
is the diameter of the particles
is the fraction of particles having diameters between
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'':
[Hidy, 1984, p. 58]
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:
One also usefully can approximate the particle size distribution using a mathematical function
. The normal distribution
usually does not suitably describe particle size distributions in aerosols because of the skewness
associated a long tail 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, clearly not physically realistic. However, the normal distribution can be suitable for some aerosols, such as test aerosols, certain pollen
grains and spore
[Hinds, 1999, p 90.]
A more widely chosen log-normal distribution
gives the number frequency as:
is the standard deviation
of the size distribution and
is the arithmetic mean
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
Terminal velocity of a particle in a fluid
For low values of the Reynolds number
(<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
, always greater than 1. Including this factor, one finds the relation between the resisting force on a particle and its velocity:
is the resisting force on a spherical particle
is the dynamic viscosity
of the gas
is the particle velocity
is the Cunningham correction factor.
This allows us to calculate the terminal velocity
of a particle undergoing gravitational settling in still air. Neglecting buoyancy
effects, we find:
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
with an ''e''-folding time equal to the relaxation time:
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:
is the dynamic shape factor
The aerodynamic diameter of an irregular particle is defined as the diameter of the spherical particle with a density of 1000 kg/m3
and the same settling velocity as the irregular particle.
[Hinds, 1999, p. 53.]
Neglecting the slip correction, the particle settles at the terminal velocity proportional to the square of the aerodynamic diameter, ''da
= standard particle density (1000 kg/m3
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.
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
, gravitational settling, and electric charge
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.
[Hidy, 1984, p. 60]
A differential equation
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
is number density of particles of size category
is the particle velocity
is the particle Stokes-Einstein diffusivity
is the particle velocity associated with an external force
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.
The Knudsen number
of the particle define three different dynamical regimes that govern the behaviour of an aerosol:
is the mean free path
of the suspending gas and
is the diameter of the particle.
For particles in the ''free molecular regime'', ''Kn
'' >> 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''∞
'' are the pressures far from the droplet and at the surface of the droplet respectively, ''kb
'' is the Boltzmann constant, ''T'' is the temperature, ''CA
'' 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 Kn
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'', ''MA
'' is the molecular mass of the particle ''A'', ''DAB
'' 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 ''PA∞
'' and ''PAS
'' 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 ''Kn
'' ≈ 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:
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.
Condensation and evaporation
Aerosol partitioning theory governs condensation
on and evaporation
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
. Net condensation of the vapor requires supersaturation, a partial pressure
greater than its vapor pressure
. 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.
[Hinds, 1999, p.288]
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.
Water coats particles in an aerosols, making them ''activated'', usually in the context of forming a cloud droplet. Following the Kelvin equation
(based on the curvature of liquid droplets), smaller particles need a higher ambient relative humidity
to maintain equilibrium than larger particles do. The following formula gives relative humidity
is the saturation vapor pressure
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.
for saturation vapor pressure above a curved surface is:
'' 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
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
instruments, performing research, and testing sampling equipment and air filters;
* to deliver deodorant
s, and other consumer products in sprays;
* for dispersal and agricultural application
* for medical treatment of respiratory disease
* in fuel injection
systems and other combustion
Some devices for generating aerosols are:
* Aerosol spray
* Atomizer nozzle
* 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.
Aerosol can either be measured in-situ
or with remote sensing
''In situ'' observations
Some available in situ measurement techniques include:
* Aerosol mass spectrometer
* Differential mobility analyzer
* Electrical aerosol spectrometer
* Aerodynamic particle sizer
* Aerodynamic aerosol classifier
* Wide range particle spectrometer
* Micro-Orifice Uniform Deposit Impactor
* Condensation particle counter
* Electrical low pressure impactor
* Aerosol particle mass-analyser
* Centrifugal Particle Mass Analyser
Remote sensing approach
Remote sensing approaches include:
* Sun photometer
* Imaging spectroscopy
Size selective sampling
Particles can deposit in the nose
(the head airways region), deeper within the respiratory tract (from the trachea
to the terminal bronchioles
), 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
for the pre-collector, but other techniques include impactors, horizontal elutriator
s, and large pore membrane filter
Two alternative size-selective criteria, often used in atmospheric monitoring, are PM10
is defined by ISO
as ''particles which pass through a size-selective inlet with a 50% efficiency cut-off at 10 μm aerodynamic diameter'' and PM2.5
as ''particles which pass through a size-selective inlet with a 50% efficiency cut-off at 2.5 μm aerodynamic diameter''. PM10
corresponds to the “thoracic convention” as defined in ISO 7708:1995, Clause 6; PM2.5
corresponds to the “high-risk respirable convention” as defined in ISO 7708:1995, 7.1. 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
(also known as fine particulate matter) in 1997.
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) containing activated drops and 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
s, or bacteria
products of combustion such as: smoke, ashes
Aerosols can be found in urban ecosystem
s in various forms, for example:
* Cigarette smoke
* Mist from aerosol spray
or fumes in car exhaust
The presence of aerosols in the earth's atmosphere can influence its climate, as well as human health.
*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.
*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 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, 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.
*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, gravitational 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
, 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.
* Aerosol spray
, the spraying device
* Atmospheric particulate matter
* Deposition (Aerosol physics)
* Global dimming
* Kulkarni, Pramod; Baron, Paul A.; Willeke, Klaus''Aerosol Measurement - Principles, Techniques, and Applications,''
2011 John Wiley & Sons, .
* Preining, Othmar and E. James Davis (eds.), ''History of Aerosol Science,'' Österreichische Akademie der Wissenschaften, (pbk.)
Aerosol Education Resources
American Association for Aerosol Research