Aerosol

An aerosol [ aeʁozo ː l] ( portmanteau of the Latin aer " air" and solutio " solution " ) is a mixture (emulsion ) of solid or liquid suspended and a gas. The behavior of an aerosol always depends on the particles and the carrier gas. The suspended particles are called aerosol particles or aerosol particles. An aerosol is a dynamic system and is subject to change due to the condensation of vapors on the existing particles, evaporating liquid components of the particles, coagulation of small particles are too large or the separation of particles in surrounding objects.

• 4.1 Weather and Climate 4.1.1 effect on cloud formation
• 4.1.2 effect on the climate

Types, formation and occurrence

Classification

Aerosols can be classified in various ways into categories. Criteria may be the origin of the aerosol particles to be their material properties ( solid or liquid) or its effect ( condensation nuclei ). In principle, the transition between all such categories is fluid.

Aerosols, as well as dust created in many different ways. In meteorology, condensation aerosols are of great importance. Their particles are formed spontaneously by condensation or Resublimation from supersaturated gases.

Depending on the origin of the particles can be distinguished between primary and secondary aerosols. The particles of the primary aerosols mostly come from mechanical or thermal processes. In the secondary aerosols, the particles have formed from gaseous substances by chemical reaction and / or by addition of the reaction products of condensation nuclei.

Examples

Aerosols are found in many areas of our environment:

• Dust in the room air
• Cigarette smoke
• Mist from a spray can
• Soot or oil fumes from a car exhaust

Our Earth's atmosphere always contains aerosols and aerosol particles of different types and different concentrations. These include, for example:

• Natural organic components: pollen, spores, bacteria and viruses
• Natural inorganic shares: incurred by erosion Desert or mineral dust, volcanic ash and sulfur dioxide as well as sea salt
• Introduced by man shares: Combustion products such as smoke ( gas fire, smoke) or ash or dust, industrially manufactured nanoparticles

Dissemination

The aerosol particles are small particles, which occur everywhere in the air. They are so small that they are not visible with the naked eye alone. Visible them only if they occur in a very large concentration in the air, from about 1,000,000 particles per cubic centimeter. This one perceives as so-called smog. The diameter of the particles is between 0.5 nm and several ten microns. At the upper end of this range, for example, larger pollen.

Aerosol particles start from a certain humidity to form droplets, the water in the air condenses on the particles. The higher the humidity, the greater the droplets. At a high humidity so the droplets are larger and come together, it comes to cloud formation and ultimately to the rain. This is called aerosol particles therefore cloud condensation nuclei.

The concentration of particles varies depending on the place, it takes with height. In 10 km above the ground, you will find usually only one ten-thousandth of the value at the bottom, which is about 2 milligrams per kilogram of air aerosol particles. In particular, volcanic eruptions can increase the concentrations of aerosols in the atmosphere strong and so affect the weather. Largely unexplored is the influence of desert dust. First measurements over the Sahara showed, among other moderating, absorbing climate impacts due to relatively large particles and a clearly defined five km thick aerosol layer.

Just as the wind, especially when it forms turbulence, soil mobilized ( aeolian soil erosion ), it can also mobilize sunken to the bottom surface of aerosol particles constantly new. Depending on wind direction, wind speed and the entire meteorological situation the wind can disperse aerosols over a large area. In polluted soils so the contaminated area is extended and remote regions can also be contaminated. A dramatic example was the widespread distribution of radioactive aerosols after the Chernobyl nuclear disaster in 1986. They were transported by the wind in the clouds before it contaminated as fallout large parts of Europe.

Of particles and their origin

Aerosol particles have many different compositions, suggesting also on the properties and the origin of the particles. Individual molecules are the smallest particles that very rarely greater than 1 nm, and they usually result in burns, as well as a metabolite of plants and animals, such as terpenes. They react in the atmosphere quickly with other molecules, or even with larger particles. But molecules include only one of the aerosol particles when they are large enough and have a solid state. In order to fulfill this condition, several molecules have to connect to a particle.

Hochofenausstöße consist largely of soot, but also from various sulphates and nitrates. The size range of these particles is between 1 and 1000 nm arising, for example, in the smelting of metals or as emissions of coal-fired power plants, but also by car exhaust. The size spectrum of aerosol particles is so large, since combustion is never clean and it came also to relatively large soot particles. Consist Similar Hochofenausstößen smoke particles mainly of soot. They arise, among others, with open fires and forest fires.

Particles of mineral dust are mainly due to the erosion of rocks. Many mineral dust particles occur in the event sandstorms. Aerosol particles of sea salt are formed when small saltwater droplets are whirled up from the sea by the wind. The water subsequently evaporates, leaving behind a sea salt particles.

Biological aerosol particles ( bioaerosols ) are pollen, algae, fungal spores, bacteria and viruses, cell organelles and secretions. This type of aerosol particles covers a very wide range of sizes. While some pollen are so large that they are already visible to the naked eye, there are also micro viruses which are sometimes even less than 10 nm. There is also very small viruses and bacteria, which are as aerosol particles in the atmosphere. Their number is very small. It is also investigated whether these micro viruses and micro bacteria can affect the health of humans.

However, there are very many more types of aerosol particles. Some are radioactive, others are made of precious metals, and still others do not come again from our planet. To determine the origin of a given particle accurately, it requires a very precise analysis of the ingredients. During her time as an aerosol, the particles also change constantly. If water condenses on the particles, and the many small droplets formed increasingly many aerosol particles react with each other, or are chemical processes in air catalyzes which modify the compositions of the particles.

Properties

The property of particles, longer time to be able to be transported by gases is that they behave with decreasing diameter more and more like gas molecules. By air resistance Aerosols have a maximum rate of descent is achieved the balance of the force of gravity and air resistance. Halving the diameter of a particle means a reduction of the mass and the gravitational force by a factor of 8 and a reduction in the air resistance force in this relevant Stokes field by a factor of 2, since the air friction force in the Stokes region is linearly dependent on the velocity, it follows that that when halving the particle diameter, the settling velocity is decreased by a factor of 4.

The square relationship is valid only as long as the particles are much larger than the mean free path of the surrounding gas ( air in 68 nm). With decreasing amount of particle size, a transition from the continuum in the region of a molecular flow takes place, whereby the flow resistance of a particle falls more slowly than expected from the Stokes' law. The resulting settling velocity is therefore greater than that of the above context and used to calculate the Cunningham correction must be considered.

Measurement

Aerosol concentrations are determined with the so-called core counters. Here, you can, in the simplest case, a certain amount of air to a thin Vaseline act and evaluates hereafter microscopically from. A distinction is made depending on the grain size

• Aitken nuclei: 0.01 to 0.1 microns
• Large nuclei: 0.1 to 2 microns
• Giant nuclei: greater than 10 microns

Other measurement methods in which particles are deposited for weighing, are impactors or centrifuges. You can aerosol particles in an air stream but (usually krypton -85 or americium -241 ) defined electrically charged and in a differential mobility analyzer ( engl. differential mobility analyzer, DMA) detect sorted by size classes with the help of a radioactive source. The detectors come here either condensation particle counter ( engl. condensation particle counter, CPC) in question, in which the particles increased by heterogeneous condensation processes and connect be detected optically, or electrical detectors, such as the Faraday Cup Electrometer ( FCE).

In addition, aerosol particles can be measured by optical methods. The integrating nephelometer is used to detect the total of aerosol particles dispersed in a reference volume of light ( of a particular wavelength ), polar nephelometer analyzing the scattered light in addition, depending on the scattering angle. Single particle counter to analyze the scattered light of individual aerosol particles in an air stream and so can provide a size distribution.

LIDAR systems analyze the "light echo " of the transmitted laser pulses into the atmosphere. According to the intensity and the distance in time to the emitted light pulse can be analyzed in the spray layering the atmosphere for several kilometers.

The atmosphere over the entire integrated aerosol optical depth (AOD, Ångström function of the coefficients) can be mapped by various remote sensing methods of their spatiotemporal distribution. To assumptions concerning the reflective properties of the surface are to be taken (for example, lower reflection surfaces of clear water in the near infrared is equal to zero). Such methods are used in remote sensing to correct the recorded from the satellite images.

Importance

Weather and Climate

An important role for the weather play hygroscopic aerosol particles which act as condensation nuclei and thus stimulate Drop or cloud formation. There are also aerosol particles that serve as ice nuclei and lead to the formation of ice crystals (which can aerosol particles by bacteria be, how they are used in snow cannons ). Ice crystals in clouds, the initiator of precipitation (the principle is described by the Bergeron - Findeisen process). For this reason, we sat for a long time also silver iodide and other chemicals to produce artificial ice nuclei from raining clouds. The hail flyer to defuse this way particularly "dangerous" cloud formations Especially in danger of hail. The absence of aerosols is used in cloud chambers and here leads to supersaturation of water vapor up to 800 percent.

It is currently unclear the role of aerosols on the climate or climate change. Anthropogenic emissions mainly locally in some cases very large concentration increases and a comprehensive air pollution show (smog ). This can affect the radiation balance of the Earth directly or indirectly ( cloud formation ) and therefore is a current focus of many research projects.

As an aggravating factor for the formation of cloud condensation nuclei is thought by some scholars, the cosmic radiation. A research team led by the Dane Henrik Svensmark showed a strong correlation with the global cloud cover, but which is strongly challenged by other scientists. To investigate the influence of cosmic rays on aerosol formation in the atmosphere, the CLOUD experiment at CERN has been held since 2006, which is currently able to demonstrate a small enhancing effect on the formation of aerosols in upper atmosphere areas. But it also showed that the process of formation of cloud condensation nuclei can be explained still insufficient, which makes further research in this area is necessary.

Effect on cloud formation

Your most important role the aerosol particles occurs in the formation of cloud droplets. The ability to act as condensation nuclei, each particle, but the intensity of this capability is determined by the composition and size of the particle. The larger a particle is, the more water-soluble components contained in it. It is thus more hydrophilic mass present, the water vapor can condense on the particles. In aerosol particles, in which no hydrophilic components are contained, such as with carbon black, it depends on the surface of the particle, as well can condense water vapor in it. The larger the surface of the aerosol particle, the more water can condense on it. Larger particles form cloud droplets earlier than smaller ones. But it also depends on the composition of the particles. Cloud condensation nuclei of hydrophilic mineral salts, such as ammonium sulfate or ammonium nitrate can have from 70 % Humidity form droplets, while form droplets hydrophobic soot particles only at a humidity supersaturation, ie at about 100 % humidity. In general, all aerosol particles form from 103 % humidity droplets. Were it not for the aerosol particles, a super-saturation would be up to 300 % relative humidity necessary to induce droplet formation. Is usually in conjunction with aerosol particles always relative humidity of the speech. It is also investigated how the concentration of the particles affects the formation of clouds. When the condensation nuclei form of the cloud droplets decreases the humidity, since the water dissolved in the air has been condensed on the particles. Is the humidity so low, the droplets listen sometime to grow since there is not enough water in the air. So are little particles present in the air to form large droplets, which then collide with high probability also, it comes quickly to rain. But are very many particles present, only small droplets, the probability crash is low form. It forms a very big cloud, but hardly, if ever gives rain. This is often observed in forest fires. These so-called Pyrowolken sometimes grow up to the stratosphere.

Effect on the climate

At concentrations from an average of 10,000 particles per cubic centimeter of air aerosol particles have a large effect on the climate. However, they have nothing to do with the greenhouse effect, because are responsible for the greenhouse effect gases only. How do exactly aerosols on the climate has not yet been thoroughly explored, since they are effective in many areas. The most important climatological behavior of the aerosols is whether they absorb sunlight and release heat ( such as soot ), or whether they reflect the light or break, such as salt particles. This can also be examined in specific devices. The particles are irradiated with UV light, here, at the same time is measured how much light reflecting, break or absorb the particles (ie, transformed to heat ). But it is not only these properties per se, but also where it affects. In the troposphere soot particles for example, ensure temperature rise as they absorb sunlight and therefore emit thermal radiation. In the stratosphere, however, they begin their absorption from the light, so less UV light reaches the troposphere, the temperature decreases in the troposphere. The exact opposite of this effect is the case of mineral particles. They provide for their cooling in the troposphere, while they are responsible for their heating when they are in the stratosphere. Aerosol particles affect the climate in a very complicated system. But the property of the cloud formation has a very large impact on the climate. One could even argue that this system can also counteract global warming, because it can lead by heating to evaporate more water, it creates more clouds, which in turn cool the troposphere. Recent research could eliminate some contradictions in previous understanding of the effects of aerosols and significantly improve the state of knowledge.

Influence on the ozone hole

As is already known, the ozone hole materially from chlorofluorocarbons (CFCs ) is caused. These substances are very stable in the troposphere, but split up into the stratosphere, fluorine and chlorine radicals from which catalyze then the reaction of ozone (O3 ) to oxygen (O2). For this elimination of chlorine and fluorine radicals are responsible in the stratosphere aerosols, as the reaction can take place only on the surface of an aerosol particle.

Acid Rain

When acid rain is called rain, due to excessive acidity (mainly sulfuric acid ( H2SO4) and nitric acid ( HNO3) ) lowers the pH of precipitation water and influenced by the soil acidification thereby supported the Edaphon. Cause of the high acidity of certain aerosols, such as nitrates ( NO3 - R ), sulfates ( RSO4 ) and various nitrogen oxides. They react with other aerosols in the air, or during the formation of droplets of sulfuric acid and nitric acid. The main sources for such aerosols are the exhaust gases that are caused by people. In addition, soot filters were in the 1970s, when little was known about the formation of acid rain is not so much used in the chimneys of many factories. So then less visible soot was indeed released, the invisible nitrogen oxides and other acid forming aerosols were, however, further expelled. Since the soot particles react in alkaline atmosphere, so they have the ability to neutralize acids, the use of such filters has also contributed to the formation of acid rain.

Human Medicine

Aerosols are inhaled by humans, while a portion of the inhaled aerosol particles is deposited in the respiratory tract. Approximately 10 % of inhaled aerosol particles remain in the respiratory tract. Because the Abscheidewahrscheinlichkeit of a particle is strongly dependent on its size, this can only be a rough estimate. Particles that can penetrate at least into the bronchial region, called respirable. This includes all the aerosol particles having a diameter of below about 10 microns (PM10). Larger particles separate out already in the nose or throat or have not to inhale. Least to separate out particles with a diameter between 0.5 microns and 1 micron. This also means that they penetrate very deeply into the lungs. Significantly larger and smaller particles are deposited already in the upper regions more dependent, thereby penetrate a less deep and put a strain on the delicate alveoli less.

In the respiratory tract deposited aerosol particles remain there for some time. Your residence time depends on the particle material and the deposition site. The substance easily soluble particles is rapidly distributed to the entire organism. Chemically very poorly soluble particles may remain up to several years in the alveolar region. Nevertheless, the organism also combats these particles. Alveolar macrophages surround the particles and they can digest in some cases, or at least move to the lymph nodes. Cilia in the bronchial transport particles mechanically deposited there rather quickly from the respiratory tract out. With the statutory provisions for fine dust PM10 and PM2.5 will attempt to emulate the conditions in the respiratory tract to define the limits based on their harmful effect.

The effect of inhaled particles on humans range from poisoning through exposure to radioactive radon decay products to cause allergic reactions. Especially dangerous are fibrous particles, especially asbestos and nanotubes because fibers block the lung clearance by macrophages.

Aerosols also serve inhalation therapy. Inhalation devices atomize medications that receives the patient by inhalation into the body. Except for Bronchialbehandlung this path can also handle incompatibilities of tablets or injections. The biggest problem with this application is the correct dosage of a drug.

Application and use

Are prepared and used selectively aerosols to apply uniformly substances on surfaces, for instance when painting or applying of plant protection agents or lubricants. Spray cans with nasal spray, hair spray or room spray aerosols give that are to serve the well -being, but also have risks and side effects. Mist fountain produce an aerosol of air and water, to humidify the air due to evaporation of the droplets. Cold Spray cools by evaporation or athlete.