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 • Spatial variability of atmospheric composition • Light • Nature of light • Interaction of light and matter • Sunlight and its propagation through the atmosphere
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)
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)
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
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.
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.
Particular Matter Concentration Background PM: 300/cm3
PM Mass Concentration Background PM: 1 mg/m3
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
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
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)
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)
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?
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
The Electromagnetic Spectrum increasing energy decreasing energy
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)
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