Chuong Hoa hoc khi quyen (2) pps

10.1 PARTICLES IN THE ATMOSPHERE Commonly called particulates Size of about 0.5 mm or less Terms pertaining to particles Aerosol: Colloidal-sized atmospheric particles Condensation aeros

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Chapter 10

PARTICLES IN THE ATMOSPHERE

Environmental Chemistry, 9th Edition

Stanley E Manahan Taylor and Francis/CRC Press

2010

10.1 PARTICLES IN THE ATMOSPHERE

Commonly called particulates

Size of about 0.5 mm or less

Terms pertaining to particles

Aerosol: Colloidal-sized atmospheric particles

Condensation aerosol: Formed from gas or vapors

Dispersion aerosol: Formed from grinding bulk solids or dispersion of liquids

Fog: High level of water droplets

Haze: Decreased visibility due to particles

Mists: Liquid particles

Smoke: From incomplete combustion of carbonaceous fuel

Particles may be

• Inorganic • Organic • Biological (pollen, microorganisms)

Figure 10.1 Bursting bubbles of seawater that produce aerosol droplets from which evaporation of water yields solid sea-salt nuclei

10.2 Physical Behavior of Atmospheric Particles (Figure 10.2)

Figure 10.3 Particle size distributions

10.3 Physical Processes for Particle Formation

Physical processes produce dispersion aerosols

• Generally larger above 1 µm

• Less respirable and less harmful than condensation aerosols

Natural sources include

• Sea spray • Windblown dust • Volcanic dust

Sources from human activities include

• Rock quarries • Disturbed soil

• Dust from crop harvesting

10.4 Chemical Processes for Particle Formation

Particles from chemical processes

• Generally smaller, below 1 µm

• More respirable

• Higher organic contents

• Higher contents of toxic substances

• Toxic elements (arsenic) • Carcinogenic organics

Inorganic Particles Formation

Oxides: 3FeS2 + 8O2→ Fe3O4 + 6SO2

Sulfuric acid: 2SO2 + O2 + 2H2O → 2H2SO4

Salts: H2SO4(droplet) + 2NH3→ (NH4) 2SO4(particulate)

Organic Particle Formation

Polycyclic Aromatic Hydrocarbon (PAH) Synthesis

10.5 The Composition of Inorganic Particles

Sources of Carbon in Inorganic Particles

• Al, Fe, Ca, Si: Soil erosion, rock dust, coal combustion

• C: Incomplete combustion of carbonaceous fuels

• Na, Cl: Marine aerosols, organohalide polymer burning

• Sb, Se: Very volatile, combustion of oil, coal

• V: Combustion of residual petroleum (Venezuelan oil)

• Zn: In small particles, from combustion

• Pb: Combustion of fuels and wastes containing lead, less now with phaseout of leaded gasoline

Fly Ash

• Residue from fossil fuel combustion

Asbestos

• Formerly a problem from sources such as wear on brake linings

• Now phased out of general use

10.6 Toxic Metals in the Atmosphere

• Fossil fuel combustion

• Formerly as lead halides from leaded gasoline combustion

Beryllium

• High toxicity

• Lowest allowable limits of all elements

• Very limited uses

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Some from cosmic rays acting on nuclei in atmosphere

• Example: 22Na

Most troubling from natural sources is radon gas

• Decays to polonium which adheres to particles

Much particulate radioactivity from 1986 Chernobyl fire

Formerly many radioisotopes from above-ground nuclear weapons testing

10.8 The Composition of Organic Particles

Hydrocarbons

• Long-chain alkanes cause few problems

• Aromatics more toxicc

Organooxygen particles

• Atmospheric oxidation products of hydrocarbons

• Aldehydes • Ketones • Carboxylic acids

Organonitrogen compounds

such as acridine

Hydrocarbons and their derivatives are bound to carbonaceous particles from diesel engine emissions

• Collected by filters and burned off

Polycyclic Aromatic Hdrocarbons and their Oxidation Products in Organic Particulate Matter

Oxidation by atmospheric chemical processes of

benzo(a)pyrene PAH

10.9 Effects of Particles

Particles are one of six Criteria Pollutants designated by the U.S Environmental Protection Agency

• Others are sulfur dioxide, carbon monoxide, nitrogen dioxide, and lead

Most obvious effect is reduction and distortion of visibility

• Most pronounced in 0.1-1µm range near wavelengths of visible light

Health effects from respirable particles

• Especially those less than 2.5 µm, pm2.5

Elevated levels of particles in the 1952 London air pollution episode

• 4000 more deaths than usual over 5 days

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between air and particles, which deposit along with their load of organics on

vegetation, water, soil, and anthrospheric structures

10.10 Water As Particulate Matter

Water droplets are widespread in the atmosphere

• In clouds • In fog

Water droplets in fog as carriers of pollutants

• Strong acid, especially H2SO4

• Corrosive salts, especially ammonium sulfate and nitrate

Most important effect is visibility reduction

Water droplets are important media for atmospheric chemical processes

• Oxidation of SO2 to H2SO4

• Reactions involving HO•

• Transferred from air to water

• Produced chemically or photochemically in water

10.11 Atmospheric Chemical Reactions Involving Particles Figure 10.7

Example of sodium sulfate forming on NaCl particles (from sea salt)

2NaCl + HO •→2NaOH + Cl2

Cl2+ 2NaOH →NaOCl+NaCl+ H2O

2NaOH + H2SO4→ Na2SO4+ H2O

SO2+ 2NaOH + 1/ 2O2→ Na2SO4 + H2O

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Sedimentation for larger particles

Inertial mechanisms that force particles onto a surface

• Dry centrifugal collectors (cyclones)

Fabric filters, Figure 10.8

Scrubbers

Uses liquids to wash particles from gas streams

Ionizing wet scrubbers place a charge on particles prior to scrubbing

Venturi scrubber, Figure 10.9

Electrostatic precipitator, Figure 10.10