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EOS 340 Atmospheric Science EOS 340 Atmospheric Science Variable Gases Vertical Structure of Atmosphere While the dominant gases are permanent, others “cycle” through the system (earth-ocean-atmosphere), and their concentration is variable. This variability exists both in space and time. The residence time (τ) is the “typical” time an individual gas may persist in the atmosphere, and tells us the scale (temporal) of processes which are important in influencing it’s concentration and distribution. τ= The atmosphere can be divided into distinct layers associated with variations in the content and related processes. The layers are “spheres”, while the interfaces between layers are “pauses”. Troposphere The lowest (8-16 km) layer of the atmosphere, containing most of the weather. Heating of the Earth’s surface and the greenhouse effect cause the surface to be warm. The temperature decreases with altitude at the lapse rates between 4-10°C per 1 km of altitude gain. Well mixed, except for water vapour. Capped by Tropopause, where air temperature is uniform with height. concentration consumption _ rate Gas N2 O2 H2O CO2 CH4 O3 Aerosols CFCs Stratosphere Residence Time 4,000,000 years 5000 years 10 days 150 years 10 years 5-30 days ? 100 years Layer with maximum Ozone concentration, which absorbs UV radiation and warms the air. Temperature increases with altitude throughout the stratosphere. Capped by the Stratopause, where the temperature is uniform with height (again). Maximum height is about 50 km. Mesosphere Air is low in density, and temperatures decrease with altitude. Radiation readily passes through this layer, capped at 80 km by the Mesopause. Thermosphere Consumption Examples: CO2: Produced by respiration, combustion, eruption; consumed by photosynthesis and dissolution. Photosynthesis: Radiation+6H2O+6CO2à C6H12O6+6O2 Respiration: 6O2+C6H12O6+enzymesà6CO2+6H2O+energy CH4: (Methane) Produced by biology (cattle, rice fields, wetlands), consumed by reactions with hydroxyl radicals. Air is extremely “thin”, tapering eventually to almost nothing at altitudes of >120km. However, since the air is so rarified, the small amount of radiation that does “hit” the rare air molecule, imparts significant kinetic energy. A long free-path means the kinetic energy is not distributed among many molecules. High molecular kinetic energy is high temperature, so the temperature rises throughout the thermosphere. 7 8 EOS 340 Atmospheric Science EOS 340 Atmospheric Science Ionosphere In the upper reaches of the atmosphere, solar radiation can strip off electrons from the air molecules (mostly N2 , O, and O 2), introducing free electrons and ion molecules and atoms. Between 80 and 100 km altitude (D), the ion concentration increases during the day (radiation), but at night, the air density is sufficient that electrons and ions will recombine to form neutrally charged molecules and atoms. At higher altitudes (>100km), the recombination rate is slow, and a near permanent ion layer exist (F). AM radio waves are absorbed and damped within the “D” layer of the ionosphere, limiting their effective range. However, at night, AM radio waves can propagate higher, and reflect off the “F” layer of the ionosphere, thus reaching listening sites at long ranges. Magnetosphere Still reaching further into outer space (~10,000 km), is the Earth’s magnetic field. In addition to radiation, the sun emits solar particles (the solar wind). The ions within the solar wind will be funneled by the Earth’s magnetic field. These energetic particles can excite molecules in the upper atmosphere, which then give off light upon returning to their normal energy state. The light thus given off can be seen at night as the aurora borealis and aurora australis. Some Fundamental Physics Energy is defined as “the ability to do work” and is measured in Joules (J). Ultimately, energy cannot be made or destroyed, but it can be transferred from one form to another. Power is the rate at which energy is transferred, in J per s or Watts [=Js -1]. There are three primary forms of energy: 1) Potential Energy (e.g. chemical and gravitational) 2) Kinetic Energy (e.g. heat and motion) 3) Radiation and Mass (Em=mc2, E r=hν, h=Planck constant) Heat: The total kinetic energy of a volume of molecules. Temperature: The average kinetic energy of the molecules in a region. Heat ≠ Temperature 9 10 EOS 340 Atmospheric Science EOS 340 Atmospheric Science Energy Transfer Solar and Terrestrial Radiation There are three primary ways to transfer energy: 1) Conduction: the diffusion of heat energy by direct molecular collision. 2) Advection/Convection: the transfer of energy by the movement of air (advection=horizontal, convection=vertical). 3) Radiation: the transfer of energy by electromagnetic (EM) radiation (light). The first is a local change, enhanced where large temperature differences between surfaces/air masses exists. Advection and convection are larger, both in net heat flux and physical scale, as large volumes of air move throughout the atmosphere. The third, em radiation, is important on the largest scales (up to global scales), and ultimately determines our net heating and cooling rates. Later we will discuss sensible heat and latent heat. These terms relate to heat exchanged by two different processes. Sensible heat exchange is another way of stating conduction (direct molecular contact). Latent heat exchange is associated with the evaporation of liquid water and the subsequent condensation of water vapour. Advection and convection are essential mechanisms for moving moist and dry air, aiding the exchange of latent heat. 11 The Sun is our (atmosphere, ocean, land) primary source of energy. This energy drives both the circulations in the atmosphere and oceans, as well as the life growing on Earth. We receive that energy in the form of electromagnetic radiation. Light has both wave and particle characteristics (duality). As a wave, light can diffract; as a particle (photon), light can impart momentum. Waves are characterized by a frequency (ν, the number of wave crests passing by per second), and a wavelength (λ, the distance between wave crests). The wave speed is given by: c=νλ [m/s] For light in a vacuum: c = 3 × 108 m/s The energy of a Photon: E=hν=hc/λ, Type of Radiation Gamma ray X ray Ultraviolet (UV) Visible Light Infrared (IR) Microwave Radio h=6.63×10-34 Js Wavelength (λ) <0.0001 µm 0.0001 – 0.01 µm 0.01 – 0.4 µm 0.4 – 0.7 µm 0.7 – 100 µm 100 µm – 1 m >1m 12