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The Atmosphere

The Atmosphere. Composition and Structure. Outline. Intro to air pollution Atmospheric Composition Measures of concentration Concentration of gases Concentration and composition of PM Structure of the Atmosphere Thermal stratification Characteristic vertical and horizontal mixing times

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The Atmosphere

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  1. The Atmosphere Composition and Structure

  2. Outline • Intro to air pollution • Atmospheric Composition • Measures of concentration • Concentration of gases • Concentration and composition of PM • Structure of the Atmosphere • Thermal stratification • Characteristic vertical and horizontal mixing times • Spatial variability of atmospheric composition • Light • Nature of light • Interaction of light and matter • Sunlight and its propagation through the atmosphere

  3. Problems due to Air Pollution • Question • What are the major problems due to chemicals discharged into the atmosphere? List them. • Stratospheric ozone depletion (due to CFCs, HCFCs, etc) • Global climate change (due to GHGs, etc) • Acid deposition (SO2, NOx) • Smog (VOCs, NOx) • Particulates (PM, especially “fine PM”) • Other toxic air pollutants (eg, CO, Pb, Hg, PAHs and other toxic organics, etc)

  4. Lecture Question • What are the criteria pollutants? • Carbon monoxide, CO • Nitrogen dioxide, NO2 • Ozone, O3 • Lead, Pb • Particulates, PM10 and PM2.5 • Sulfur dioxide, SO2 • Lecture Question • What are the four most abundant components of dry air at sea level? • Nitrogen (N2), oxygen (O2), argon (Ar), carbon dioxide (CO2)

  5. Atmospheric Composition

  6. Measures of Concentration • Relative concentration: volume mixing ratio (VMR) • What is it? The mole (or volume) fraction. • Units: %, ppmv, ppbv, etc • Easy to understand • Constant with altitude for inert gases like N2

  7. Measures of Concentration • Lecture Question • List the current atmospheric concentration of CO2 in air in units of (a) % and (b) ppm. • Answer in book: • 375 ppm = 0.0375% by volume. • According to NOAA (Jan 2007): • Current average CO2 concentration (at Mauna Loa) is 383 ppm • 383 ppm = 0.0383% by volume.

  8. Measures of Concentration • Absolute concentration • Typical units • Mass/volume (eg mg/L) • Number density (cm-3), particularly for low concs • Pressure units (torr, atm, bar, etc) Note: diameter of the Earth is 6400 km.

  9. Particular Matter Concentration Background PM: 300/cm3

  10. PM Mass Concentration Background PM: 1 mg/m3

  11. Thermal Stratification of the Atmosphere • Lecture Question • List the major regions (layers) of the atmosphere, along with the typical altitudes for each region. • Troposphere (0 – 15km) • Upper limit (the tropopause) varies between 9 – 16 km depending on lattitude and season • Stratosphere (15 – 50 km) • Contains the stratospheric ozone layer, which (mostly) shields us from harmful uv light • Mesosphere (50 – 100 km) • Thermosphere (above 100 km) • Above 60 km is the ionosphere, where there is a significant concentration of ions and electrons

  12. Thermal Stratification of the Atmosphere • Troposphere is heated by the ground • Stratosphere and mesosphere are directly heated by ozone chemistry (solar energy) • Tropopause varies in height (9–16 km) depending on latitude and season • Vertical mixing in troposphere is rapid, but stratosphere is fairly stagnant • Lower 1 – 3 km called the planetary boundary layer (PBL), which is rapidly mixed and often topped by a local inversion

  13. Time Scales of Vertical Mixing

  14. Time Scales of Horizontal Mixing

  15. Mixing vs Removal • Two competing processes • Effect on atmospheric composition • Depends on relative rates of mixing and removal • Species with higher rates of removal are more concentrated near their sources • Removal mechanisms • Water scavenging • Dissolution into suspended water droplet (or water bodies on ground) • Rate depends on solubility • Reaction/decomposition • Rxn with a reactive species (often OH) • Photodissociation: absorption of light, followed by breaking of bond • Adsorption to solid surface • Either PM or surface on the ground • Mixing mechanisms • Buoyant mixing (vertical) • Atmospheric circulation – ie, wind (horizontal)

  16. Light: Electromagnetic Radiation • Plays a critical role • The vast majority of our energy arrives as sunlight • Drives global circulation of atmosphere and water • Many air pollution problems directly involve light • Ozone depletion • Global climate change • Photochemical smog • Drives atmospheric chemistry • Through photodissociation • Through generation of reactive species (OH, Cl, NO3, O3)

  17. Light: Electromagnetic Radiation • What is light? • Light acts like a wave • With oscillating electric and magnetic fields • Can propagate through a medium or through vacuum • animation here • Light also acts like a particle • Localized energy packet: a photon • Massless...but has momentum • Lecture Question • What is the relationship between wavelength, frequency, and the speed of propagation?

  18. Light: Electromagnetic Radiation • Lecture Question • What is the relationship between photon energy, wavelength, and frequency? • Photon energy • Is the minimum energy available in its interaction with matter

  19. The Electromagnetic Spectrum increasing energy decreasing energy

  20. The Electromagnetic Spectrum • Can further subdivide the UV region • UV-C is 200-280 nm (most energetic, completely blocked by ozone layer) • UV-B is 280-320 nm (ground-level UV-B intensity most affected by ozone depletion) • UV-A is 320-280 nm (least energetic, almost all gets through the ozone layer)

  21. Incident SunlightTop & Bottom of Atmosphere Lecture Question What fraction of incident sunlight is in the uv, visible and ir regions? • 8% ultraviolet • 39% visible • 53% infrared

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