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Particulate Pollutants

Particulate Pollutants. ENV 4101/5105 Elements of Air Pollution, Spring 2007. Prepared by: Ying Li. January 24, 2007. Introduction Atmospheric Aerosol Sources (natural vs. urban aerosol) Particle size (three modes) Particle concentration Chemical composition

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Particulate Pollutants

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  1. Particulate Pollutants ENV 4101/5105 Elements of Air Pollution, Spring 2007 Prepared by: Ying Li January 24, 2007

  2. Introduction • Atmospheric Aerosol • Sources (natural vs. urban aerosol) • Particle size (three modes) • Particle concentration • Chemical composition • Dynamic processes (formation, growth, removal) • Visibility Effect of Particulates • Light extinction • Visual range Particulate Pollutants Reading materials: Godish, Air Quality, Chp.2, pp 55-67. Valuable learning materials: Online Module of Atmospheric Aerosol http://aerosol_beta.ees.ufl.edu

  3. Particle vs. Aerosol • Aerosol: solid or liquid particles suspended in air • Primary vs. Secondary Particles • Primary particles – introduced directly into the atmosphere (e.g. smoke from combustion) • Secondary particles – formed by chemical reactions in the atmosphere (e.g. gas-to-particle conversion) • Natural vs. Anthropogenic Aerosol • Natural – dominates in rural (remote) areas • Anthropogenic – dominates in urban areas • Effects of Atmospheric Aerosol • Health, Welfare, Climate, Visibility Introduction

  4. Sources of Atmospheric Aerosol

  5. Stratospheric and tropospheric aerosol • Stratospheric aerosol • 11-50 km • Formation of sulfuric acid droplets by gas-to-particle conversion of SO2 injected into the stratosphere by major volcanic eruptions • Tropospheric aerosol • <11 km • Direct emissions from natural sources: desert, ocean, and vegetations • Gas-to-particle formation Natural Background Aerosol Eruption of Mt. Pinatubo (in the Philippines) in 1991 Do You Know… During the eruption of Mt. Pinatubo, 14-20 Tg (1 Tg= 106 tons) of SO2 was injected into the stratosphere, causing the aerosol concentration to increase from 2-5 µg/m3 to 20-100 µg/m3 Does tropospheric aerosol concentration vary with altitude? Why?

  6. Anthropogenic sources • Stationary sources: power plants, refinery plants, mines, etc. • Motor vehicles • Combustion – very important source • Particle concentration • A few tens of µg/m3 to 1 mg/m3 in heavily polluted areas Mumbai Guangzhou Urban Aerosol Table 2. Aerosol concentrations for various types of areas in the United States Location Mass concentration (µg/m3) Background 20 Rural areas 40 Urban areas Population < 105 86 < 105-106 104 > 106 154 Hazes produced by the urban aerosol in Mumbai, India and Guangzhou, China Data from: W.C. Hinds, Aerosol Technology, 2nd Edition

  7. Urban aerosol size distribution • Nuclei mode (0.001 – 0.1 µm) • Accumulation mode (0.1 – 2.5 µm) • Coarse-particle mode (2.5 – 100 µm) Urban Aerosol

  8. Nuclei Mode • Size: 0.001 – 0.1 µm • Combustion particles and gas-to-particle conversion • Location: near highways and combustion sources • High concentration • Rapid coagulation Animation – Nuclei Mode Aerosol Urban Aerosol You can also access this animation through: http://aerosol_beta.ees.ufl.edu/atmos_aerosol/section04-1.html

  9. Accumulation Mode • Size: 0.1 – 2.5 µm • Combustion particles, smog particles, and coagulated nuclei mode particles • Slow coagulation • Accounts for most of the visibility effects • Fine particles • Nuclei + Accumulation mode • PM2.5 Urban Aerosol Animation – Accumulation Mode Aerosol You can also access this animation through: http://aerosol_beta.ees.ufl.edu/atmos_aerosol/section04-2.html

  10. Coarse-Particle Mode • Size: 2.5 – 10 µm • Dusts, sea salts, particles from surface mining • Ready to settle down on the surface • PM2.5, PM10, and TSP • PM2.5: < 2.5µm • PM10: <10 µm • TSP: total suspended particles Animation – Coarse-Particle Mode Aerosol You can also access this animation through: http://aerosol_beta.ees.ufl.edu/atmos_aerosol/section04-3.html Urban Aerosol Which mode of aerosol has the longest lifetime in the atmosphere? Why?

  11. Dynamic Processes • Formation • Growth • Removal Animation – Dynamic Processes of Atmospheric Aerosol You can also access this animation through: http://aerosol_beta.ees.ufl.edu/atmos_aerosol/section05.html Atmospheric Aerosol What are the processes of atmospheric aerosol formation, growth, and removal, respectively?

  12. Chemical Composition • Fine particles: acidic; sulfate, ammonium compounds, elemental carbon • Coarse particles: basic; crustal materials and their oxides TABLE 3. Average composition of fine and coarse particles in µg/m3 at an urban and a rural site Atmospheric Aerosol • What are the three major components of fine aerosols? • What are the four major components of coarse aerosols?

  13. Visibility • Clarity with which distant objects can be seen • Visibility degradation by atmospheric aerosols • Light Extinction • Light Scattering • Light Absorption • Visual Range Visibility Effect of Particulate Pollutants Visibility degradation due to high concentration of aerosols in the atmosphere in Cairo, Egypt Interesting link: Sandstorm in China

  14. Beer-Lambert Law Animation – Light Extinction You can also access this animation through: http://aerosol_beta.ees.ufl.edu/atmos_aerosol/section09-1.html I0 = incident light intensity; I = light intensity traversing the aerosol; σe = extinction coefficient of the aerosol; L = path length of the light through the aerosol Light Extinction • Extinction Coefficient N = particle concentration (number per unit volume); Ap = cross-section area of a particle; dp = particle diameter (assuming spherical particles); Qe = single particle extinction efficiency, which is a complex function of particle size and refractive index

  15. Extinction Efficiency, Qe Light Extinction

  16. Example Problem A polluted air contains 0.3-µm-diameter particles at a concentration of 2×1010 /m3. What is the light intensity (as a fraction of the incident light) that traverses 1.0 km of such polluted air? Assume the particle refractive index (m) is 1.5. Light Extinction How do you calculate the light extinction when the aerosol has a size distribution (i.e. several sizes)?

  17. Visual Range • How far one can see in a given direction • Distance at which the object is barely discernible For a dark object viewed against the horizon during daylight, the visual range Lv can be obtained as a simple inverse function of extinction coefficient (assuming the threshold of brightness contrast is ε = 0.02): Visual Range What are the key parameters of an aerosol that influence the visual range?

  18. Atmospheric Aerosol • Sources of natural and anthropogenic aerosol • Particle concentration • Nuclei, accumulation, and coarse-particle mode aerosols • PM2.5, PM10, TSP • Chemical composition • Dynamic processes (formation, growth, removal) • Visibility Effect • Light extinction (Beer-Lambert law) • Visual range Summary Reading materials: Godish, Air Quality, Chp.2, pp 55-67. Valuable learning materials: Online Module of Atmospheric Aerosol http://aerosol_beta.ees.ufl.edu

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