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.
However, Stokes' law is only valid when the velocity of the gas at the surface of the particle is zero. 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: .
This allows us to calculate the terminal velocity of a particle undergoing gravitational settling in still air.
Neglecting buoyancy effects, we find: . 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: . 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: . Neglecting the slip correction, the particle settles at the terminal velocity proportional to the square of the aerodynamic diameter, d a : .
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.
An increasing interest in studies of atmospheric aerosols in the context of their impact on the formation of climate, heterogeneous chemical reactions in the. Atmospheric Aerosol Properties: Formation, Processes and Impacts (Springer Praxis Books / Environmental Sciences) [Kirill Ya. Kondratyev, Lev S. Ivlev.
Pharmaceutical companies typically use aerodynamic diameter, not geometric diameter, to characterize particles in inhalable drugs. The previous discussion focussed 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 charges 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 called the Aerosol General Dynamic Equation GDE characterizes the evolution of the number density of particles in an aerosol due to these processes. 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.
The Knudsen number of the particle define three different dynamical regimes that govern the behaviour of an aerosol:. 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:. 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:. These equations do not take into account the heat release effect. 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: [ citation needed ]. 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.
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. The following formula gives relative humidity at equilibrium:. Kelvin equation for saturation vapor pressure above a curved surface is:. There are no general solutions to the general dynamic equation GDE ;  common methods used to solve the general dynamic equation include: .
Some devices for generating aerosols are: . 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.
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 techniques. Particles can deposit in the nose , mouth , pharynx and larynx the head airways region , deeper within the respiratory tract from the trachea to the terminal bronchioles , or in the alveolar region.
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 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 elutriators , and large pore membrane filters. Two alternative size-selective criteria, often used in atmospheric monitoring, are PM 10 and PM 2.
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. The atmosphere of Earth contains aerosols of various types and concentrations, including quantities of:. Aerosols can be found in urban ecosystems in various forms, for example:. The presence of aerosols in the earth's atmosphere can influence its climate, as well as human health. From Wikipedia, the free encyclopedia.
This article is about the physical properties of an aerosol. Atmospheric Aerosols field exepriment prgaramme. A history of studies - 2. Field observational experiments in America and Western Europe - 3.
Cloud Cover - 4. Processes of aerosol formation nucleation - 5. Aersol and chemical processes in the atmosphere - 6. Save to Library. Create Alert. Share This Paper.