Dust particles, often referred to as particulate matter PM , in the atmosphere arise from a wide variety of sources. Both the size and chemical composition vary widely in relation to the nature of the source and the history of the particles.
Coarse particulates can be regarded as those with a diameter greater than 2. Under humid conditions many particles attract water vapour and grow to form small droplets. The term 'aerosol' is often used for both solid particles and droplets suspended in air.
Particulate matter may be generated mechanically, for example by the wind, may be emitted directly to the atmosphere or may be formed by reactions in the atmosphere from precursor gases e. The three major sources, each forming their own size mode, are e.
Chemical reactions of gases in the atmosphere. These can result in the production of small particles of a few nm diameter 1 nm is one thousand millionth of a metre which grow relatively quickly by coagulation.
There are a number of formation paths:. Combustion processes. These include industrial and transport related combustion processes, which directly emit fine particles, typically in the size range 0. Coarse bigger particles, called PM 10 , can irritate your eyes, nose, and throat. Dust from roads, farms, dry riverbeds, construction sites, and mines are types of PM Fine smaller particles, called PM 2.
Particle pollution can affect anyone, but it bothers some people more than others. People most likely to experience health effects caused by particle pollution include:. Read more about the health impacts of PM. If you have asthma, particle pollution can make your symptoms worse. Carefully follow your asthma management plan on days when pollution levels are high.
If you have heart disease, breathing in particle pollution can cause serious problems like a heart attack. Symptoms include:. If you have any of these signs, contact your doctor. Be sure to let your doctor know if the symptoms get worse or last longer than usual.
Start by learning about the Air Quality Index from the U. It can be seen that the lowest deposited fraction is between 0. Fontan] The lung can be schematically considered as formed by more and more cylinders when one sinks into the respiratory system, with bifurcations that end in the cells schematized by spheres.
From such a model, we can calculate the deposition by inertia, diffusion and sedimentation. Figure 6 shows the retention of particles according to their size and the different parts of the respiratory system:.
The optical properties of particles make it possible to understand the visibility problems that accompany pollution, giving some qualitative information about the aerosol. They are used to measure the dimensions and concentrations of particles, particularly from satellite observations [7].
Aerosol particles diffuse light. When the particle size is small in relation to the wavelength of the radiation, i. Short-wavelength radiation is much more strongly scattered than long-wavelength radiation. For the same flow, blue is about eight times more widespread than red. The blue of the sky, of a calm sea, are a consequence of the Rayleigh diffusion on the molecules of air or water; likewise the bluish grey aspect of cigarette smoke or certain mists made up of very fine particles.
Figure 7. Sunrise on the sea. The light from the sun, still under the horizon, is reddish. Shorter wavelength light, blue in particular, was scattered. The rays that reach the observer contain a higher proportion of red light. It is less sensitive to the wavelength of radiation. A cloudy sky or one with particles larger than one micrometre becomes white or whitish.
The amount of scattered light does not vary as much with the size of the particle. Cigarette smoke illustrates the effect of particle size on the appearance of smoke.
Indeed, when it expires it becomes whiter in appearance, due to the increase of particles size with water vapour in the respiratory tract. This change in optical properties clearly shows the change in size of some particles with the humidity of the air. This point is important for measuring the mass concentration of particles read Diesel engines and wood combustion in charge.
Observation of the atmospheric aerosol gives an idea of the origin of the particles. If they come from a gas-solid transformation, the aerosol has a blue haze appearance.
If they come from wind erosion case of particles coming in Europe from Sahara the aerosol is whitish in colour see paragraph 7. At sunrise or sunset, the light coming from the sun is reddish in appearance Figure 7. This is due to the lack of short wavelengths blue for example that have been scattered and are then absent, leaving the emphasis on red light. When the particle size becomes comparable to the wavelength, or larger, the scattering is preferentially in the direction of the incident light.
Aerosol particles are also made up of substances such as carbon, which can absorb light. This phenomenon becomes prevalent with infrared radiation emitted by the ground or the atmosphere [8]. Figure 8. The light coming from the tree and reaching the observer, at the bottom right, may be scattered and not reach the observer.
On the other hand, rays from the sun, after scattering by particles, reach the observer who looks towards the tree. The contrast between the object and its environment is very disturbed. Fontan] In a polluted atmosphere, with particles scattering and absorbing light, an observer receives light from the object he is looking at which can be partially absorbed but also from the particles Figure 8. The contrast between the object and its environment decreases.
If the scattered light becomes too strong, the object can no longer be distinguished. The phenomenon is similar to that of fog and mists caused by the condensation of water vapour into fine droplets.
Figure 9. Visibility problems due to pollution in Santiago de Chile. It can also be observed from an aircraft flying above the atmospheric boundary layer around 1 kilometre above sea level or from a hill or mountain. In the Sahel, Africa, wind erosion that introduces large amounts of dust into the atmosphere in the dry season causes significant decreases in visibility, which can be reduced to about metres, severely affecting air traffic.
Aerosol particles play a complex role in the radiation balance of the atmosphere. This leads to a backscattering towards space and a cooling of the planet. In addition, particles also absorb a fraction of the infrared radiation from terrestrial terrestrial sources, which results in a contribution to the greenhouse effect with warming of the lower atmosphere. The combined effects of these two mechanisms depend on the characteristics of the atmospheric aerosol but also on the properties of the soil and its reflection coefficient of solar radiation called albedo.
It is generally accepted that particles of anthropogenic origin slow down the increase in temperature due to greenhouse gases. Cover image. Smog in a polluted city Cairo. Multiplying a quantity by 10, a number for which the decimal logarithm is the unit, then results in the addition of a unit to its logarithm.
Science Technology, 9, , The mechanics of aerosol, Mac Millan, New York Health Phys. Human respiratory tract model for radiological protection.
ICRP 24, , Boulaud D. Physics and metrology. Lavoisier TEC and Doc, The articles in the Encyclopedia of the Environment are made available under the terms of the Creative Commons BY-NC-SA license, which authorizes reproduction subject to: citing the source, not making commercial use of them, sharing identical initial conditions, reproducing at each reuse or distribution the mention of this Creative Commons BY-NC-SA license.
While the problem of air pollution is old Focus 1 , it is nevertheless very contemporary…. What are we talking about? Particles and aerosols Figure 1. The term smoke refers to generally finer particles resulting from combustion.
In English, fume also refers to very fine particles resulting from the condensation of a vapour. Fogs fog and smog are formed by a suspension of liquid droplets resulting from the condensation of a vapour, or from a spray. In the atmosphere near the ground, the term fog is used when visibility is reduced to less than 1 kilometre. Mists mist or haze , are made up of smaller droplets or solid particles. They correspond to a lower visibility disorder than fogs, except in exceptional cases of haze in desert or semi-desert regions.
Other terminologies Cloud Condensation Nuclei CCN nuclei are the germs around which water vapour condenses in the atmosphere. Particles in the atmosphere 2. Primary and secondary pollutants Particulate matter includes both primary and secondary pollutants read Air pollution.
Some effects on the atmosphere Particles modify the optical properties of the atmosphere by scattering or absorbing light.
How are they characterized? To know the effects of particulate pollution, depending on the objective pursued, it will be necessary to characterize more or less precisely: The concentration which, depending on the case, will be expressed in mass or surface area or in number per unit volume. The chemical nature of particulate matter , but in air pollution monitoring networks it is only the mass concentration that is systematically measured.
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