Liquid Aerosols Mists and Fogs

The presence of charge on aerosol particles does have its important implications, however. As will be discussed in later sections, many practical applications of aerosols depend on the presence of charge, as does one of the most important process for destroying unwanted aerosols. 

Liquid aerosols may be formed by one of two processes, depending on whether the dispersed system begins as a liquid or undergoes a phase change 

The second class of mists or fogs is that produced by some process in which the incipient liquid phase is introduced as a vapor and forms droplets as a result of some equilibrium condensation process, or the liquid is produced as a result of some chemical reaction. The former mechanism includes, of course, cloud and fog formations, while the latter corresponds to some chemical fogs and mists. 

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Airborne partieulate matter may eomprise liquid (aerosols, mists or fogs) or solids (dust, fumes). Refer to Figure 5.2. Some eauses of dust and aerosol formation are listed in Table 4.3. In either ease dispersion, by spraying or fragmentation, will result in a eonsiderable inerease in the surfaee area of the ehemieal. This inereases the reaetivity, e.g. to render some ehemieals pyrophorie, explosive or prone to spontaneous eombustion it also inereases the ease of entry into the body. The behaviour of an airborne partiele depends upon its size (e.g. equivalent diameter), shape and density. The effeet of partiele diameter on terminal settling veloeity is shown in Table 4.4. As a result ... 

An aerosol is a suspension of solid or liquid particles in a gas such as air. Smoke, fumes and dusts are airborne suspensions of solid particles, i.e. solid aerosols. Mists and fogs are airborne suspensions of liquid particles, i.e. liquid aerosols. 

Pipe-type electrostatic precipitators are used to collect liquid aerosols (e.g., mists and fogs). They are also used in applications which require water flushing of collection electrodes. The diameter of precipitator pipes is typically in the range of 15 0 cm, and the length is in the range of 3-6 m. The number of pipes depends on the total gas flow. The gas-flow rates in pipe-type electro static precipitators is normally much lower than in duct-type precipitators. 

Dispersed System Disperse Phase Dispersing Agents and Dispersion Medium. Dispersed system is an apparently homogeneous, system which consists of a microscopically heterogeneous mixture of two or more finely divided phases, eg liquid-liquid (emulsions such as milk) liquid-solid (suspension, such as clay in water) gas-liquid (aerosols, such as fogs, clouds, mists) and gas-solid (such as smoke, dusts)... 

Mists and fog Aerosols produced by the disintegration of liquid or the condensation of vapor. Because liquid droplets are implied, the particles are spherical. They are small enough to appear to float in moderate air currents. When these droplets coalesce to form larger drops of about 100 jun or so, they can then appear as rain. 

An aerosol is a suspension of liquid or solid particles in a gas. Natural aerosols include smoke, mist, and fog, all particles suspended in air. Aerosols may be mechanically generated by making a solution, emulsion, or suspension of a liquid or solid in a compressed or liquefied gas in a sealed container. As the gas (the propellant) is released through a valve an aerosol or aerogel (foam) is generated. The pressurized containers holding these mixtures are known as aerosols or pressurized products. 

Mist and Fog Liquid-particle aerosols formed by condensation or atomization. Particles are spherical with sizes ranging from submicrometer to about 200 pm. 

Mists and fogs are colloidal dispersions of a liquid in a gas. They may therefore be regarded as being the inverse of foams. The interactions controlling their stability, however, are not generally the same as those involved in foam stabilization, because most mists and fogs do not possess the thin lamellar stabilizing films encountered in foams. In fact, the stability of liquid aerosols is usually more dependent on fluid dynamics than on colloidal factors, as illustrated below. 

Mists or fogs are typically formed either by the condensation of water or other vapors on suitable nuclei, giving a suspension of small liquid droplets, or by the atomization of liquids. Particle sizes of natural fogs and mists lie between 2 and 200 pm. Droplets larger than 200 pm are more properly classified as drizzle or rain. Many of the important properties of aerosols that depend on particle size are presented in Sec. 17, Fig. 17-34. 

Figure 2 This schematic diagram shows the absorption pathways and systems of distribution, metabolism, and elimination for potential toxins. Aerosols include dusts, other solid particulates (such as smoke), and liquid droplets (such as fog, mists, etc.). Distribution may involve deposition of a toxin within a target organ and/or metabolism with or without excretion of the toxin by the target organ (after Goyer and Clarkson, 2001).

Aerosol Colloidal dispersion of liquids or solids in a gas. Distinctions may be made among aerosols of liquid droplets (e.g., fog) and aerosols of solid particles (e.g., smoke and dust). See also Mist. 

Tiny liquid droplets are often called mist or fog and solid particles smoke, but for purposes of obscuration and signaling, we shall follow ordnance practice and call all of them smokes. These aerosolized substances exist in a wide range of particle sizes of about 1 to 10radius. In smokes for obscuration or as signals, we are concerned with 0.5—1.5 optimal size. 

The different types of dispersed systems are classified depending on the nature of the dispersed phase and the continuous phase. A solid or liquid dispersed in a gas is termed an aerosol smoke is a common example of the solid-in-air system. A liquid-in-air system is a mist or fog. Mayonnaise is an emulsion in which a liquid (vegetable oil) is dispersed in another liquid (water). Paints and inks are colloidal sols or dispersions and consist of solid particles dispersed in a liquid. 

A heterogeneous mixture in air is one where the contaminant is either a liquid or a solid. When the particle size ranges from about 0.01 to 100 pm, the mixture is called a colloid or an aerosol. (A colloid that is a heterogeneous mixture of one liquid in another liquid is called an emulsion for example, milk is an emulsion.) Some common aerosols are mists, fumes, and dusts. Mists are tiny droplets of liquid suspended in the air. Clouds and fog are mists of water droplets in air. Figure 3.1.1.1 shows the ranges of particle sizes. 

Other common apphcations of colloidal liquids include liquid aerosols, such as those occurring in the areas of environment (fog, mist, cloud, smog), agriculture (crop sprays), manufacturing (paint sprays), medicine (nasal sprays), and personal care (hair spray). 

When a liquid or solid substance is emitted to the air as particulate matter, its properties and effects may be changed. As a substance is broken up into smaller and smaller particles, more of its surface area is exposed to the air. Under these circumstances, the substance, whatever its chemical composition, tends to combine physically or chemically with other particles or gases in the atmosphere. The resulting combinations are frequently unpredictable. Very small aerosol particles (from 0.001 to 0.1 Im) can act as condensation nuclei to facilitate the condensation of water vapor, thus promoting the formation of fog and ground mist. Particles less than 2 or 3 [Lm in size (about half by weight of the particles suspended in urban air) can penetrate the mucous membrane and attract and convey harmful chemicals such as sulfur dioxide. In order to address the special concerns related to the effects of very fine, iuhalable particulates, EPA replaced its ambient air standards for total suspended particulates (TSP) with standards for particlute matter less than 10 [Lm in size (PM, ). 

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