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Physics and chemistry of particulate matter

Physics and chemistry of particulate matter - 3 PHYSICS AND...

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3-1 3. PHYSICS AND CHEMISTRY OF PARTICULATE MATTER 3.1 INTRODUCTION 3.1.1 Overview Atmospheric particles originate from a variety of sources and possess a range of morphological, chemical, physical, and thermodynamic properties. Examples include combustion-generated particles such as diesel soot or fly ash, photochemically produced particles such as those found in urban haze, salt particles formed from sea spray, and soil-like particles from resuspended dust. Some particles are liquid, some are solid; others contain a solid core surrounded by liquid. Atmospheric particles contain inorganic ions and elements, elemental carbon, organic compounds, and crustal compounds. Some atmospheric particles are hygroscopic and contain particle-bound water. The organic fraction is especially complex, containing hundreds of organic compounds. Particle diameters span more than four orders of magnitude, from a few nanometers to one hundred micrometers. Combustion-generated particles, such as those from power generation, from automobiles, and in tobacco smoke, can be as small as 0.003 μm and as large as 1 μm. Particles produced in the atmosphere by photochemical processes range in diameter from 0.003 to 2 μm. Fly ash produced by coal combustion ranges from 0.1 to 50 μm or more. Wind- blown dust, pollens, plant fragments, and cement dusts are generally above 2 μm in diameter. Particles as small as a few nanometers (Covert et al., 1992; Clarke, 1992) and as large as 100 μm have been measured in the atmosphere (Lin et al., 1993). Particles are ubiquitous in the atmosphere. The lowest concentrations are found in background marine environments, where particle number concentrations range from 100/cm to 3 400/cm . In background continental environments, particle concentrations vary from 100/cm to 3 3 5,000/cm ; while in urban areas of the United States concentrations may be as high as 3 4,000,000/cm (Willeke and Whitby, 1975; Whitby and Sverdrup, 1980). Particles account for a 3 mass of a few μg/m near the surface over dry continental areas to several hundred μg/m in 3 3 polluted urban areas.
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3-2 The composition and behavior of airborne particles are fundamentally linked with those of the surrounding gas. Aerosol is defined as a suspension of solid or liquid particles in air and includes both the particles and all vapor or gas phase components of air. However, the term aerosol is often used to refer to the suspended particles only. Particles may be solid or liquid or a mixture of both phases. Particulate is an adjective and should only be used as a modifier, as in particulate matter. Particulate material can be primary or secondary. Primary particles are composed of material emitted directly into the atmosphere. This includes material emitted in particulate form such as wind-blown dust, sea salt, road dust, mechanically generated particles and combustion- generated particles such as fly ash and soot. It also includes particles formed from the condensation of high temperature vapors such as those formed during combustion. The
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